NIST Technical Note 1481

Ignition of Cellulosic Fuels by Heated 

and Radiative Surfaces

William M. Pitts 

NIST Technical Note 1481 

Ignition of Cellulosic Fuels by Heated 

and Radiative Surfaces

William M. Pitts 

Building and Fire Research Laboratory 

March 2007 

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

Technology Administration 

Robert Cresanti, Under Secretary of Commerce for Technology 

National Institute of Standards and Technology 

 William Jeffrey, Director 
Certain commercial entities, equipment, or materials may be identified in this 

document in order to describe an experimental procedure or concept adequately. Such 

identification is not intended to imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor is it intended to imply that the 

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1481 

Natl. Inst. Stand. Technol. Tech. Note. 1481, 106 pages (March 2007) 

CODEN: NSPUE2 
Abstract 

Experiments designed to characterize the ignition behavior of typical outdoor fuels by heated 

mufflers and catalytic converters found on outdoor power equipment are described. Ignition by 

direct contact with a heated surface and by radiation from a heated surface were considered. For 

the first scenario the fuel was placed in contact with an electrically heated surface, and the times 

required for ignition measured as a function of surface temperature. The effects of a wind on 

heated plate ignition were studied by passing air flows of 1.1 m/s and 2.5 m/s over the fuel bed. 

Fuels tested included shredded newsprint, four types of grass, pine needles, a grass/pine needle 

mixture and three types of dry leaves. Both glowing combustion and flaming were observed. 

The transition to flaming required glowing combustion to be present. Ignition times generally 

increased with decreasing surface temperature until a temperature was reached where ignition 

was no longer observed. Ignition times for given applied wind and surface temperature 

conditions and transition to flaming behavior were fuel dependent. An applied wind generally 

reduced the time required for ignition, with the reduction being greater for the higher velocity. A 

commercially available cone calorimeter operated in the non piloted ignition mode was used to 

impose known radiative heat fluxes on the fuel surface, and ignition times were measured as a 

function of applied heat flux. Shredded newsprint, two types of grass, pine needles, and non fireretarded flexible polyurethane foam were studied. Ignition times for radiative heating increased 

with decreasing applied heat flux until levels were reached where ignition did not occur. The 

polyurethane foam had a distinctly different behavior than the cellulosic fuels. Comparison of 

the heated plate and radiative heating experiments suggests that they can be related using the 

black body temperature corresponding to the applied radiative heat flux. 

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Table of Contents 

Abstract..........................................................................................................................................iii 

Table of Contents........................................................................................................................... iv 

List of Tables.................................................................................................................................. v 

List of Figures................................................................................................................................. v 

Executive Summary........................................................................................................................ x 

1. Introduction............................................................................................................................. 1 

2. Important Concepts and Literature Review............................................................................ 3 

3. Experimental Systems and Procedures................................................................................... 8 

3.1. Ignition by Thermal Conduction from a Heated Surface................................................ 8 

3.1.1. Experimental System .............................................................................................. 8 

3.1.2. Visual Recordings of Ignition Experiments.......................................................... 15 

3.1.3. Experimental Procedure........................................................................................ 15 

3.2. Ignition by Thermal Radiation...................................................................................... 17 

3.2.1. Cone Calorimeter.................................................................................................. 17 

3.2.2. Experimental Procedure........................................................................................ 20 

3.3. Fuels Investigated ......................................................................................................... 21 

3.3.1. Shredded Newsprint.............................................................................................. 21 

3.3.2. May Tall Fescue Grass.......................................................................................... 22 

3.3.3. August Tall Fescue Grass..................................................................................... 23 

3.3.4. Cheat Grass........................................................................................................... 23 

3.3.5. Fine Florida Grass................................................................................................. 24 

3.3.6. Pine Needles.......................................................................................................... 25 

3.3.7. May Tall Fescue/Pine Needle Mixture ................................................................. 25 

3.3.8. Preliminary Tests with Leaves.............................................................................. 26 

3.3.9. Polyurethane Foam ............................................................................................... 26 

3.3.10. Fuels Summary ..................................................................................................... 26 

4. Results................................................................................................................................... 28 

4.1. Some General Observations from Heated Plate Ignition Studies................................. 28 

4.2. Shredded Newsprint...................................................................................................... 32 

4.2.1. Heated Surface Ignition Results............................................................................ 32 

4.2.2. Radiative Heating Ignition Results....................................................................... 36 

4.3. May Tall Fescue............................................................................................................ 39 

4.3.1. Heated Surface Ignition Results............................................................................ 39 

4.3.2. Radiative Heating Ignition Results....................................................................... 44 

4.4. August Tall Fescue ....................................................................................................... 49 

4.5. Cheat Grass................................................................................................................... 54 

4.5.1. Heated Surface Ignition Results............................................................................ 54 

4.5.2. Radiative Heating Ignition Results....................................................................... 56 

4.6. Fine Florida Grass......................................................................................................... 58 

4.7. Pine Needles.................................................................................................................. 62 

4.7.1. Heated Surface Ignition Results............................................................................ 62 

4.7.2. Radiative Heating Ignition Results....................................................................... 64 

4.8. May Tall Fescue/Pine Needle Mixture ......................................................................... 67 

4.9. Preliminary Tests with Leaves...................................................................................... 69 

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4.10. Radiative Ignition of Non Fire-Retarded Flexible Polyurethane Foam.................... 71 

5. Summary and Discussion...................................................................................................... 74 

6. Acknowledgements............................................................................................................... 88 

7. References............................................................................................................................. 89 

List of Tables 

Table 1. Summary of Fuels, Sample Mass Used, Nominal Fuel Bed Densities, and 

Measured Fuel Moisture ............................................................................................. 27 

List of Figures 

Figure 1. Schematic plan and side views are shown for the heated copper plate used for the 

ignition studies. All dimensions are in cm................................................................... 9 

Figure 2. This photograph shows the heated copper plate used for ignition measurements 

of cellulosic fuels in place on a ring stand. The ends of the four cartridge heaters 

and their leads can be seen on the two edges of the plate. The leads for three 

thermocouples imbedded in the plate are visible below. ............................................ 10 

Figure 3. The temperature measured at the center of the heated copper plate is plotted as a 

function of the current applied to the four cartridge heaters embedded within the 

plate. Filled circles are the experimental measurements and the solid curve is the 

result of a quadratic least squares curve fit to the data. .............................................. 11 

Figure 4. Photograph showing the sonic anemometer probe used to characterize the applied 

wind velocity in place 1.2 cm above the unheated copper plate................................. 12 

Figure 5. The wind speed above the copper plate along the primary flow direction of the 

blower recorded as a function of time is shown with the blower turned on and 

off................................................................................................................................ 12 

Figure 6. The blower with screens in place used to generate the “low” velocity air flow is 

shown. ......................................................................................................................... 13 

Figure 7. The wire cage with its removable steel-plate bottom in place is shown looking 

down from above. ....................................................................................................... 15 

Figure 8. This photograph shows the wire-screen sample cage with its steel-plate bottom in 

place on top of the heated plate. The cage is filled with cheat grass. The fuel is 

placed on the plate by withdrawing the bottom of the cage using the handle 

visible in the photograph............................................................................................. 16 

Figure 9. Temperatures recorded during experiments in which cellulosic fuels were place 

on a heated plate are plotted as a function of time. Zero time is defined as the 

time when the fuel was exposed to the surface........................................................... 18 

Figure 10. This schematic shows the principal components of the cone calorimeter used to 

investigate the ignition behavior of cellulosic fuels and polyurethane foam.............. 19 

Figure 11. A sample holder used during the radiative ignition experiments is shown filled 

with fuel. ..................................................................................................................... 20 

Figure 12. Photograph of shredded newsprint in wire-screen cage. ............................................ 22 

Figure 13. Photograph of May tall fescue in wire-screen cage.................................................... 22 

Figure 14. Photograph of August tall fescue in wire-screen cage............................................... 23 

Figure 15. Photograph of cheat grass in cone calorimeter aluminum pan. .................................. 23 

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Figure 16. A photograph showing an uncut sample of cheat grass............................................. 24 

Figure 17. Photograph of fine Florida grass in wire-screen cage................................................. 25 

Figure 18. Photograph of pine needles in wire-screen cage......................................................... 25 

Figure 19. Photograph of the May tall fescue/pine needle mixture in the wire-screen cage. ...... 26 

Figure 20. Photograph of polyurethane in a cone calorimeter sample holder.............................. 26 

Figure 21. The smoke plume rising from a pine needle fuel bed in contact with the heated 

plate held at a temperature of 380 °C is shown 213 s after application...................... 28 

Figure 22 Two frames taken one second apart from a video of a hot surface ignition 

experiment with May tall fescue are shown. A small area of glowing combustion 

had appeared during the one second period (compare the blown up areas prior to 

(left) and after (right) ignition). .................................................................................. 29 

Figure 23. The smoke generated by a mixture of May tall fescue and pine needles applied to 

the heated plate at 380 °C in the presence of a low wind is shown 175 s after 

application................................................................................................................... 30 

Figure 24. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for shredded newsprint fuel beds applied to a heated plate. Results 

are included for no wind, low wind, and high wind cases.......................................... 33 

Figure 25. Bottom (left) and top (right) views of a shredded newsprint fuel bed are shown 

after the fuel was removed from the heated plate held at 333 °C with a low wind 

applied. Glowing combustion was not observed. ...................................................... 35 

Figure 26. Bottom (left) and top (right) views of a shredded newsprint fuel bed are shown 

after the fuel was removed from the heated plate held at 332 °C with no wind 

applied. Glowing combustion was not observed. ...................................................... 35 

Figure 27. Values of time to glowing or flaming ignition and maximum observed heat 

release rate are plotted as functions of applied heat flux for shredded newsprint 

fuel beds...................................................................................................................... 36 

Figure 28. Heat release rates and sample masses are plotted as a function of time for three 

experiments with a 50 kW/m2

 heat flux applied to shredded newsprint fuel beds..... 37 

Figure 29. Heat release rates and sample masses are plotted as a function of time for three 

experiments with a 25 kW/m2

 heat flux applied to shredded newsprint fuel beds..... 37 

Figure 30. Heat release rates and sample masses are plotted as a function of time for two 

experiments in which a 15 kW/m2 heat flux was applied to shredded newsprint....... 38 

Figure 31. Heat release rates and sample masses are plotted as a function of time for an 

experiment in which a 10.5 kW/m2

 heat flux was applied to shredded newsprint. .... 39 

Figure 32. Photograph of the top surface of a shredded newsprint fuel bed following 

application of a 7.5 kW/m2 heat flux. ......................................................................... 40 

Figure 33. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for May tall fescue grass fuel beds applied to a heated plate. Results 

are included for no wind, low wind, and high wind cases.......................................... 40 

Figure 34. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 341 °C with no wind applied. 

Glowing combustion was observed after 235 s. ......................................................... 42 

Figure 35. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 336 °C with a high wind 

applied. Glowing combustion was observed after 134 s............................................ 42 

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Figure 36. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 311 °C with no wind applied. 

Glowing combustion was not observed. ..................................................................... 43 

Figure 37. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 311 °C with a low wind 

applied. Glowing combustion was not observed. ...................................................... 44 

Figure 38. Values of time to flaming or glowing ignition and maximum observed heat 

release rate are plotted as functions of applied heat flux for May tall fescue fuel 

beds. ............................................................................................................................ 45 

Figure 39. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 45 kW/m2 heat flux was applied to May tall fescue. ............ 45 

Figure 40. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 35.2 kW/m2 heat flux was applied to May tall fescue. ......... 46 

Figure 41. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 15.3 kW/m2

 heat flux was applied to May tall fescue. 

The arrows indicate the times when glowing combustion appeared. ......................... 47 

Figure 42. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 13.5 kW/m2

 heat flux was applied to May tall fescue. 

The arrows indicate the times when glowing combustion appeared. ......................... 48 

Figure 43. Heat release rates and sample masses are plotted as a function of time for two 

experiments in which a 7.5 kW/m2

 heat flux was applied to May tall fescue. ........... 48 

Figure 44. This slightly out of focus photograph shows the top surface of a May tall fescue 

fuel bed following application of an applied radiative heat flux that was 

insufficient to induce glowing combustion................................................................. 49 

Figure 45. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for August tall fescue grass fuel beds applied to a heated plate. 

Results are included for no wind, low wind, and high wind cases. ............................ 50 

Figure 46. Photograph showing flaming from an August tall fescue fuel bed on the heated 

plate held at 310 °C in the presence of a low wind. Flames appeared 289 s after 

the fuel was placed on the plate, and the photograph was taken 5 s later. Strong 

glowing combustion is present along the upstream edge of the fuel bed. .................. 51 

Figure 47. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown 

after the fuel was removed from the heated plate held at 300 °C with no wind 

applied. No glowing combustion was observed. ....................................................... 52 

Figure 48. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown 

after the fuel was removed from the heated plate held at 290 °C with no wind 

applied. No glowing combustion was observed. ....................................................... 53 

Figure 49. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown 

after the fuel was removed from the heated plate held at 310 °C with a low wind 

applied. Glowing combustion and flaming were observed........................................ 53 

Figure 50. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown 

after the fuel was removed from the heated plate held at 320 °C with a high wind 

applied. No glowing combustion was observed. ....................................................... 54 

Figure 51. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for cheat grass fuel beds applied to a heated plate. Results are 

included for no wind, low wind, and high wind cases................................................ 55 

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Figure 52. Bottom (left) and top (right) views of cheat grass fuel bed are shown after the 

fuel was removed from the heated plate held at 341 °C with no wind applied. No 

glowing combustion was observed. ............................................................................ 56 

Figure 53. Bottom (left) and top (right) views of a cheat grass fuel bed are shown after the 

fuel was removed from the heated plate held at 350 °C with a low wind applied. 

No glowing combustion was observed. ...................................................................... 57 

Figure 54. Values of time to flaming ignition and maximum observed heat release rate are 

plotted as functions of applied heat flux for cheat grass............................................. 57 

Figure 55. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 45 kW/m2

 heat flux was applied to cheat grass. ................... 58 

Figure 56. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for fine Florida grass fuel beds applied to a heated plate. Results are 

included for no wind, low wind, and high wind cases................................................ 59 

Figure 57. Bottom (left) and top (right) views of a fine Florida grass fuel bed are shown 

after the fuel was removed from the heated plate held at 321 °C with no wind 

applied. Glowing combustion was observed. ............................................................ 60 

Figure 58. Bottom (left) and top (right) views of a fine Florida grass fuel bed are shown 

after the fuel was removed from the heated plate held at 310 °C with no wind 

applied. Glowing combustion was not observed. ...................................................... 61 

Figure 59. Bottom (left) and top (right) views of a fine Florida grass fuel bed are shown 

after the fuel was removed from the heated plate held at 290 °C with a low wind 

applied. Glowing combustion was not observed. ...................................................... 61 

Figure 60. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for pine needle fuel beds applied to a heated plate. Results are 

included for no wind, low wind, and high wind cases................................................ 62 

Figure 61. Bottom (left) and top (right) views of a pine needle fuel bed are shown after the 

fuel was removed from the heated plate held at 290 °C with no wind applied. No 

glowing combustion was observed. ............................................................................ 63 

Figure 62. Bottom (left) and top (right) views of a pine needle fuel bed are shown after the 

fuel was removed from the heated plate held at 340 °C with no wind applied. No 

glowing combustion was observed. ............................................................................ 64 

Figure 63. Values of time to flaming ignition and maximum observed heat release rate are 

plotted as functions of applied heat flux for pine needles. ......................................... 65 

Figure 64. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 35 kW/m2 heat flux was applied to pine needles.................. 65 

Figure 65. Heat release rates and sample masses are plotted as a function of time for an 

experiment in which a 15 kW/m2

 heat flux was applied to pine needles. .................. 66 

Figure 66. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for May tall fescue/pine needle mixture fuel beds applied to a heated 

plate. Results are included for no wind, low wind, and high wind cases. ................. 67 

Figure 67. Bottom (left) and top (right) views of a May tall fescue/pine needle mixed fuel 

bed are shown after the fuel was removed from the heated plate held at 280 °C 

with no wind applied. No glowing combustion was observed. ................................. 69 

Figure 68. Bottom (left) and top (right) views of a May tall fescue/pine needle mixed fuel 

bed are shown after the fuel was removed from the heated plate held at 320 °C 

with a high wind applied. No glowing combustion was observed. ........................... 70 

viii
Figure 69. Values of time to flaming ignition and maximum observed heat release rate are 

plotted as functions of applied heat flux for polyurethane foam. Numbers in 

parentheses refer to the number of repeated measurements. ...................................... 71 

Figure 70. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 50 kW/m2

 heat flux was applied to polyurethane foam........ 72 

Figure 71. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 25 kW/m2

 heat flux was applied to polyurethane foam........ 73 

Figure 72. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 22 kW/m2

 heat flux was applied to polyurethane foam........ 73 

Figure 73. Data for time to glowing ignition with no applied wind are plotted as a function 

of heated plate temperature for seven fuels. Data near the upper temperature axis 

represent experiments for which ignition did not take place. The solid curve is 

the result of a least squares fit assuming an exponential dependence on heated 

plate temperature for all data where ignition was observed. ...................................... 75 

Figure 74. Data for time to glowing ignition with a low applied wind are plotted as a 

function of heated plate temperature for seven fuels. Data near the upper 

temperature axis represent experiments for which ignition did not take place. 

The solid curve is the result of a least squares fit assuming an exponential 

dependence on heated plate temperature for all data where ignition was observed. .. 77 

Figure 75. Data for time to glowing ignition with a high applied wind are plotted as a 

function of heated plate temperature for seven fuels. Data near the upper 

temperature axis represent experiments for which ignition did not take place. 

The solid curve is the result of a least squares fit assuming an exponential 

dependence on heated plate temperature for all data where ignition was observed. .. 78 

Figure 76. Results of exponential least squares curve fits of experimental ignition times as a 

function of heated plate temperature for seven fuels are shown for no wind, low 

wind, and high wind cases. ......................................................................................... 79 

Figure 77. Ignition times due to non piloted radiative heating in the cone calorimeter are 

plotted as a function of applied heat flux for the four fuels indicated. The solid 

line shows the results of a linear least squares curve fit to the combined data for 

shredded newsprint, May tall fescue, and cheat grass. ............................................... 81 

Figure 78. Temperatures are plotted as a function of radiative heat flux assuming that the 

flux is generated by an ideal black body..................................................................... 82 

Figure 79. Values of maximum heat release rate are plotted as a function of applied heat 

flux for the four fuels indicated. ................................................................................. 83 

Figure 80. The curve fit times to ignition for no wind, low wind, and high wind shown in 

Figure 76 are plotted as a function of the AHF calculated for the radiative heat 

flux from a black body held at the heated plate temperature...................................... 87 

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Executive Summary 

Recently implemented and proposed regulations for outdoor power equipment require reductions 

in hydrocarbon and nitrogen oxides to levels that require the use of catalytic converters. 

Concerns have been raised that catalytic converters can result in increased temperatures for 

exhaust gases and surfaces that could increase the risk of fires. In response to this concern, The 

International Consortium for Fire Safety, Health, and the Environment (ICFSHE), with funding 

provided by the Outdoor Power Equipment Institute, awarded a contract to the SP Swedish 

National Testing and Research Institute (SP) for a “Scientific Evaluation of the Risk Associated 

with Heightened Environmental Requirements on Outdoor Power Equipment.” As part of this 

study ICFSHE requested that the Building and Fire Research Laboratory of the National Institute 

of Standards and Technology (BFRL/NIST) provide experimental support to SP. A work 

statement was adopted that involved characterizing the ignition of typical outdoor fuels by 

ignition sources representative of those expected for outdoor power equipment exhaust systems. 

This report summarizes the findings of the BFRL/NIST investigation. One series of experiments 

was designed to simulate the ignition behaviors of fuels that come into direct contact with a 

heated surface. A series of experiments was run in which porous fuel beds were brought into 

contact with a 10.2 cm × 10.2 cm heated copper plate held at a constant temperature. The plate 

faced upward. A constant mass of the material to be tested was placed in a stainless steel wire 

screen cage to a depth of 2.5 cm, placed over the heated plate, and then exposed to the surface by 

removing a metal shutter from the bottom of the cage. The primary experimental measurements 

were whether or not glowing and/or flaming developed and, if so, the heating times required. 

Observations were made visually and by video recording of the experiments. Secondary 

observations included smoke behaviors, locations of initial burning, and the appearance of the 

fuel bed following an experiment. The effect of a wind on ignition behavior was investigated by 

running experiments with no wind and in the presence of low (1.0 m/s) and high winds (2.5 m/s). 

Heated plate temperatures were varied from temperatures in excess of 500 °C where ignition 

periods were short (a few seconds) down to temperatures where ignition of the fuels was not 

observed after many minutes. 

A second series of experiments was designed to characterize ignition of porous fuel beds by 

radiative heating. Such heating occurs when fuel is located near, but not in direct contact with, a 

heated surface. The experiments were made with a cone calorimeter, a standard instrument 

designed primarily for heat release rate measurements with an applied radiative heat flux. Cone 

calorimeter measurements usually utilize an electrical spark as an ignition source, but these 

experiments were non piloted, and the spark was not used. Exposed fuel bed surfaces had the 

same dimensions as for the heated plate experiments, but samples were placed in sample holders 

with solid walls and were irradiated from above by a radiant source that was shielded from the 

fuel by a cooled shutter until the start of an experiment. Glowing and flaming ignition times 

were determined for a range of applied heat fluxes, from levels sufficient to induce ignition 

within a few seconds down to levels where ignition was not observed after many minutes. The 

only air flow was that around the sample induced by the experimental system. Useful secondary 

measurements included the heat release rate and mass for the sample holder and fuel as functions 

of time. 

x
Heated plate ignition experiments were done for shredded newsprint, four grasses (two samples 

of tall fescue collected from the same location at NIST in May and August, cheat grass from 

California, and a sample of fine grass from Florida provided by a manufacturer of outdoor power 

equipment that was intended to simulate debris from the top of a mower housing), pine needles, 

and a mixture of May tall fescue, and pine needles. A limited number of tests were also run for 

boxwood, American elm, and pin oak leaves. Cone calorimeter tests were done for shredded 

newsprint, May tall fescue, cheat grass and pine needles. Samples of a non fire-retarded flexible 

polyurethane foam, representing a common plastic, were also tested in the cone calorimeter. 

Some general observations from the heated plate experiments include the release of smoke, most 

likely a combination of condensed water and organic pyrolyzate as fuels are heated. The smoke 

distribution for experiments without a wind indicated that circular buoyant plumes were formed 

within the porous fuel beds by the flows rising from the heated plate and oxidizing fuel surfaces. 

With a wind applied to the fuel bed, smoke escaped along a band extending across the fuel bed 

perpendicular to the wind direction. This smoke distribution resulted from the interaction of the 

wind entering the fuel bed, which was slowed by interaction with the fuel, and the buoyancy 

driven flow. Qualitative observation indicated that the amount of pyrolysis occurring within a 

fuel bed was related to the amount of smoke observed. Smoke levels were generally heavy 

immediately prior to glowing or flaming ignition. 

For heated plate temperatures around 550 °C all of the fuels ignited within a few seconds. As the 

plate temperature was reduced, the ignition times at first slowly increased until temperatures 

reached about 450 °C. Further reductions in temperature resulted in ignition times that increased 

more rapidly as the plate temperature was reduced. The dependence on temperature was shown 

to be captured well by fitting with exponential curves. Eventually, as the plate temperature was 

lowered, a point was reached for which a given type of fuel bed no longer ignited. 

The ignition described above includes both glowing combustion and flaming. There were 

conditions for which glowing was observed but flaming was not and fewer examples of flaming 

without observed glowing. Even though glowing was not observed in all experiments in which 

flames were seen, in most experiments it appeared earlier. Video recordings of the transition to 

flaming showed that flames ignited within regions surrounded by intense glowing. A blue flame 

would suddenly appear and then spread rapidly before yellow flames appeared. These 

observations suggest that the transition to flaming requires the presence of high temperature 

glowing areas to supply the ignition energy and premixed volumes of gaseous fuel pyrolyzate 

and air having concentrations within the flammability limits for the mixture. 

The general heated plate ignition temperature dependence discussed above was observed for the 

three wind conditions tested, but ignition times and fuel dependent transition to flaming 

behaviors depended strongly on wind condition. When a low wind was applied the ignition 

times were reduced from those for the same fuel without a wind, assuming ignition occurred for 

both conditions. Similar reductions in ignition times were observed when results for low and 

high winds were compared. For the lowest plate temperatures, the reductions in ignition times 

on going from no wind to high wind were on the order of a factor of two. Transition from 

glowing combustion to flaming was observed whenever glowing developed for shredded 

newsprint and pine needles. With a few isolated exceptions, none of the grass samples that 

xi
developed glowing in the absence of wind transitioned to flaming, while the August tall fescue, 

cheat grass, and fine Florida grass did transition to flaming when low and high winds were 

present. For most cases, glowing May tall fescue failed to transition to flaming with wind 

present. Note that the difference between the May tall fescue and August tall fescue implies 

seasonal differences for grass samples taken from the same location. The May tall fescue/pine 

needle mixture displayed flaming behaviors intermediate between those for the individual fuels, 

transitioning from glowing to flaming in over half of the experiments without wind and in all 

cases when wind was present. 

Ignition times for the individual fuels displayed similar dependencies on heated plate 

temperature and wind as described above. By fitting the combined results for each wind 

condition to an exponential, it was possible to demonstrate that systematic effects of fuel were 

present that introduced ignition time variations that were larger than those observed for a given 

individual fuel. For a specified heated plate temperature and wind condition, the longest ignition 

times were found for the shredded newsprint and pine needles, while the grasses tended to have 

the shortest ignition times. The May tall fescue/pine needle mixture gave intermediate ignition 

times. 

Minimum heated plate temperatures required for a given fuel and wind condition can be 

estimated from the experimental measurements. Values ranged from 290 °C to 380 °C and had 

complicated dependencies on fuel type and wind condition. These dependencies are described 

in detail in the full manuscript. In general, when a wind was not present the grasses tended to 

have higher minimum heated plate ignition temperatures than the shredded paper and pine 

needles. When wind was introduced the minimum temperatures for the grasses decreased while 

those for the shredded paper and pine needles increased such that the lowest minimum 

temperatures with wind tended to be those for the grasses. Inspection of the grass fuel beds 

following experiments at temperatures lower than required for ignition with no wind provided 

insight into the reason for this behavior. It was found that non glowing smoldering took place in 

the absence of a wind down to temperatures on the order of 300 °C. Presumably, when a wind 

was applied more oxygen was available at the fuel surface and sufficient heat was generated for 

the non glowing combustion to transition to glowing. Similar non glowing smoldering was not 

observed for shredded paper and pine needles. It is likely that for these fuels the applied wind 

cooled the samples and resulted in the higher minimum ignition temperatures. The May tall 

fescue/pine needle mixture without wind yielded the lowest observed minimum ignition 

temperature of 290 °C. This suggests that the May tall fescue component of the mixture was 

able to begin non glowing smoldering at this temperature and that the heat generated was 

sufficient to ignite glowing combustion on the pine needle fraction. Transition to flaming was 

observed in this particular experiment. 

Limited experiments with a heated plate temperature of 380 °C indicated that leaves are likely to 

have similar ignition behaviors to those observed for the cellulosic fuels investigated more fully. 

The cone calorimeter radiative ignition (glowing and flaming) experiments were run without a 

lateral air flow. Applied heat fluxes were varied from a high of 50 kW/m2

 down to values where 

ignition was not observed. Plots of ignition time versus applied heat flux had similar 

appearances to the plots of ignition time versus heated plate temperature discussed above. For 

xii
the highest heat fluxes ignition times were on the order of a few seconds. As the applied heat 

flux was reduced to lower values, the ignition times increased slowly at first. Eventually, with 

further reductions, applied heat fluxes reached values where ignition times began to increase 

rapidly at rates that increased with decreasing flux. Ignition times of hundreds of seconds were 

observed before further reductions in applied heat flux resulted in no ignition. As for the heated 

plate experiments, the dependencies of ignition times on applied heat flux were well described 

by exponential curve fits. 

Results for three of the fuels, shredded paper, May tall fescue, and cheat grass, fell on very 

similar curves, with the rapid increases in ignition beginning around 20 kW/m2 and minimum 

applied heat fluxes required for ignition of approximately 10 kW/m2. The ignition time for the 

pine needles was slightly different, with the rapid rise starting around 30 kW/m2

 and a slower 

rate of increase with decreasing heat flux than found for the three other fuels. A minimum 

applied heat flux for ignition was not determined for the pine needles. 

Even though the ignition times as a function of applied heat flux were similar for the four fuels, 

transition to flaming behaviors were very different. Flaming was observed for all of the fuels 

with applied heat fluxes of 40 kW/m2 and higher, and maximum observed heat release rates were 

similar. However, as the applied heat flux was reduced below 40 kW/m2 only glowing 

combustion developed for the two grasses, while the shredded newsprint and pine needles 

continued to transition to flaming. These ignition behaviors were observed over applied heat 

fluxes in the 10 kW/m2 to 40 kW/m2 range. Observed time behaviors for heat release rate and 

mass loss were consistent with these observations. The highest heat release rates and mass loss 

rates were observed during flaming, while the values were much reduced during smoldering. 

The measurements for the glowing grass indicated that there were two distinct types of 

smoldering having different effective heats of combustion. There seemed to be an initial 

oxidation that converted a part of the grass to a high energy char, which then smoldered slowly 

with much slower mass loss but a comparable heat release rate. 

The cone calorimeter results are in good agreement with the heated plate results with regard to 

flaming behavior for the grasses. They indicate that with no wind present it is very difficult to 

generate flaming, consistent with the absence of flaming in the heated plate experiments. The 

shredded newsprint and pine needles transitioned to flaming in all cases where glowing 

developed, again consistent with the heated plate experiments. Measurements of ignition times, 

heat release rates, and mass loss were similar for the May tall fescue and cheat grass. Recall that 

these two grasses had different behaviors with regard to transition to flaming in the presence of a 

wind. The similarity of the results suggests that the cone calorimeter is not able to distinguish 

fuels that have different flaming behaviors in the presence of wind. 

Radiative ignition experiments were performed for polyurethane foam. The ignition time 

behavior was very different from those observed for the cellulosic fuels, and the minimum 

ignition applied heat flux was 23 kW/m2. Heat release rate curves had two peaks corresponding 

to initial burning of the foam that generated a liquid component, which then burned as a pool 

fire. At the lowest applied heat fluxes the foam formed the liquid without oxidation, and if 

flaming ignited, it was only for the liquid fraction. 

xiii
In order to determine if the response of a fuel to a radiative heat flux could be related to its 

response on a heated plate, black body temperatures corresponding to given applied heat fluxes 

were calculated. Comparisons of heated plate temperatures and the corresponding black body 

temperatures from the cone calorimeter results indicated that values for minimum ignition 

temperature and the temperature where the sharp increases in ignition times began were slightly 

higher for the black body temperatures, but were close enough to justify the approach. This is 

particularly true when it is noted that the application of heat to the bottom of the fuel beds and 

open walls of the fuel holder for the heated plate experiments should result in slightly lower 

ignition temperatures than the radiative heating experiments where heat is applied to the top of 

fuel beds enclosed on the sides. 

A review of the literature identified very few experiments that could be compared directly to the 

current findings. Limited numbers of measurements of ignition times versus surface temperature 

were consistent for similar fuel beds. A review of experiments that identified minimum ignition 

temperatures yielded a wide range of values that were of little use for comparison. No 

experiments were identified that investigated non piloted radiative ignition of these types of 

fuels. 

The experimental results are subject to uncertainties associated with such parameters as 

temperature measurement, identification of ignition time, and stochastic variations of fuel 

behavior and packing. Due to the limited number of measurements possible, it was not feasible 

to determine probability curves for such variables as ignition temperature as a function of heated 

plate temperature or applied heat flux. In spite of these uncertainties the results demonstrate that 

smoldering and flaming ignition of cellulosic fuel beds exposed to conditions chosen to be 

representative of those generated on heated exhausts of outdoor power equipment vary with 

surface temperature or applied heat flux, fuel type, wind, and season. In addition to these 

parameters, there are a number of others that would be expected to affect ignition behavior but 

were not varied in this study. These include fuel moisture, fuel bed exposed surface area, fuel 

bed thickness, fuel bed porosity, spatial orientation of the heat source and the fuel bed, and the 

accessibility of the interior of a fuel bed to an applied wind. 

Keeping in mind the experimental uncertainties and additional parameters that could affect 

ignition behavior, it is possible to provide some general guidance with regard to expected 

ignition behaviors of cellulosic fuels when subjected to contact with a heated surface or exposure 

to thermal radiation. Curve fits to exponentials based on data for multiple fuels have been made 

that provide reasonable approximations for the observed variations in ignition times with heated 

plate temperature and applied heat flux. For the heated surface, these fits are available for three 

wind conditions. While clearly not representative of worst case conditions, these curves should 

be useful for assessing the likelihood of ignition and the time required for a generic cellulosic 

fuel subjected to conditions similar to those investigated here. 

It is common to introduce safety factors when making engineering estimates having safety 

implications. For the ignition of cellulosic fuels by contact with a heated surface the following 

assumptions are appropriate: 1) ignition is possible for temperatures of 290 °C and higher, 2) 

any ignition that takes place will result in flaming, and 3) ignition times estimated from the 

exponential curve fits should be divided by a factor of two. The available experimental data 

xiv
cover a range of winds from 0 m/s to 2.5 m/s, and the fits can be used to estimate ignition times 

over this range. Caution should be exercised when extending the estimates to higher wind cases. 

An exponential curve fit of ignition times versus applied heat flux has been determined from the 

radiative heating experiments. Similar assumptions to those above would allow this curve to be 

utilized for estimating ignition times for cellulosic fuel beds exposed to a given level of heat 

flux. However, based on the similar ignition time dependencies observed between heated plate 

temperatures and black body temperatures derived from the applied heat fluxes, it is 

recommended that black body temperatures based on the applied heat fluxes be used in 

conjunction with the exponential curves derived from the heated plate experiments to estimate 

ignition times for radiative heating. This has the advantage of providing estimates for ignition by 

radiative heating in the presence of a wind. 

Suggestions are provided for improvements to the experimental system and for additional 

experiments. 

xv
xvi
1. Introduction 

Concerns about the levels of hydrocarbons and nitrogen oxides released from the exhausts of 

outdoor power equipment (OPE) led the California Air Resources Board to approve regulations 

in September, 2003 requiring reductions in these species to levels which require the use of 

exhaust catalytic converters. Having received a required US Environmental Protections Agency 

(EPA) waiver, these regulations went into effect in 2007. The EPA is currently considering 

extending a similar regulation to the entire country. [1] 

Exhaust catalysts operate by lowering the temperatures required to oxidize pollutant species. 

Since oxidation reactions are exothermic, heat is generated, and there is the potential for exhaust 

gas temperatures to increase when a catalyst is present. The use of a catalyst may raise the 

temperatures of the gases exhausted by the engine as well as the temperatures of exposed 

surfaces that form the exhaust system. 

It is known that the temperatures generated by existing exhaust systems on OPE are high enough 

to be potential fire ignition sources for typical organic fuels. Several groups, including the 

Outdoor Power Equipment Institute (OPEI) and the National Association of State Fire Marshals 

(NASFM), questioned whether higher temperatures associated with the use of catalytic 

converters on OPE might result in an increased risk of unwanted fire. 

In response to these concerns and at the request of NASFM, The International Consortium for 

Fire Safety, Health, and the Environment (ICFSHE), with funding provided by OPEI, awarded a 

contract to the SP Swedish National Testing and Research Institute (now renamed SP Technical 

Research Institute of Sweden, referred to as “SP”) for a “Scientific Evaluation of the Risk 

Associated with Heightened Environmental Requirements on Outdoor Power Equipment.” 

Phase I of this investigation included a literature review and development of an experimental 

research plan for studying this problem. The findings of the Phase I study are available. [2] 

ICFSHE requested that the Building and Fire Research Laboratory of the National Institute of 

Standards and Technology (BFRL/NIST) provide experimental support to SP as part of the 

Phase II effort. A work statement entitled “Ignition of Fuels by Conductive, Radiative, and 

Conductive Heating” was prepared and submitted to ICFSHE and SP for consideration. The 

work statement proposed to experimentally characterize the ignition behaviors of a variety of 

natural and man made organic fuels when exposed to heat sources believed to be representative 

of those generated by the exhausts of OPE. This work statement was accepted and research 

funding was provided to BFRL/NIST by ICFSHE with technical management and oversight 

provided by SP. 

The original work statement included ignition studies using three types of heat sources—heated 

plate (conductive), heated flow (convective), and radiative. A variety of materials were to be 

investigated including a grass, pine needles, leaves, sawdust, oil, and gasoline. Tests were 

proposed to characterize the effects of spatial orientation, an imposed wind, and an uneven heat 

distribution on the heated plate. 

1
Initial testing showed the ignition of grasses was dependent on the type of grass and the presence 

or absence of a wind played a dominant role in ignition behavior. Based on these observations 

and with the agreement of SP, the test matrix was modified to focus more attention on the effects 

of grass type and wind condition. It was decided that characterizing ignition by a heated plate 

and by radiation was more important than characterizing ignition due to a convective heat flow, 

and this heat source was dropped from the investigation. Ultimately, ignitions studies on a 

variable-temperature heated plate were carried out for shredded newsprint, four different grass 

samples, pine needles, and a grass/pine needle mixture. Three wind conditions—no wind and 

two imposed wind velocities of 1.1 m/s and 2.5 m/s—were used. A limited number of tests were 

done with three types of leaves. Radiative ignition studies were performed as a function of 

applied heat flux for shredded newsprint, two types of grass, pine needles, and non fire-retarded 

flexible polyurethane foam. 

The following section, Section 2, provides an overview of relevant past studies and introduces 

some concepts useful for the later discussion. The experimental systems and procedures are 

discussed in Section 3. Section 4 presents the experimental findings for the investigation. The 

final section, Section 5, summarizes the findings along with the implications for possible ignition 

by heated plates or radiative surfaces. 

2
2. Important Concepts and Literature Review 

The focus of this experimental program is the characterization of ignition for various fuels by 

heat sources deemed to be representative of those expected for exhaust systems on OPE such as 

lawn mowers. Two potential scenarios are emphasized. In the first it is assumed the fuel comes 

into direct contact with a heated surface. Since exhaust temperatures can vary over a wide range, 

the response of the fuel will be investigated as a function of surface temperature. In the second 

scenario the fuel is located close to, but sufficiently far from, a heated surface that heating of the 

fuel is dominated by radiative heat transfer. In this case the important experimental parameter is 

the radiative heat flux (kW/m2) impinging on the fuel surface. Since the heat flux depends on the 

radiating surface temperature to the fourth power, the fuel response to a range of heat fluxes is 

considered. 

Since by definition OPE is operated outdoors, ignition of natural fuels such as grasses, pine 

needles, and leaves are considered to be particularly important. It is also possible that heated 

exhaust surfaces will come in contact with a variety of man-made fuels during storage. As a 

result there is a nearly infinite number of potential fuels that could be considered relevant to this 

problem. For this study the ignition behaviors of shredded newsprint, four types of grass (May 

tall fescue, August tall fescue, cheat, and fine Florida), pine needles, various types of leaves, a 

May tall fescue/pine needle mixture, and polyurethane foam were investigated. While chosen to 

be representative of possible fuels, care should be exercised in extrapolating the findings to 

additional fuels. 

The focus of this investigation is ultimately fire safety. From this standpoint, “flaming 

combustion” in which gasified fuel molecules undergo a self sustaining reaction with oxygen in 

the air to release heat and light is considered to represent the most dangerous type of combustion. 

The natural fuels chosen for study as well as the shredded newsprint contain high concentrations 

of cellulose along with other natural polymers such as hemicellulose, and lignin. Materials 

containing these polymers are known to be susceptible to a type of surface combustion known as 

smoldering. In his review, Ohlemiller defines smoldering as “a slow, low-temperature, flameless 

form of combustion, sustained by heat evolved when oxygen directly attacks the surface of a 

condensed-phase fuel.” [3] 

Smoldering usually involves both endothermic and exothermic processes. Moisture removal 

from the fuel and gasification of low molecular weight fuel molecules (known as pyrolysis) 

generally require energy input. As described by Ohlemiller, fuel pyrolysis often leads to the 

formation of a poorly characterized carbon-enriched material referred to as char. Oxidative 

surface reactions with char are exothermic and provide the heat necessary to sustain smoldering. 

[3] Note that fuel pyrolysis is possible without oxidation. Local surface oxidation can also 

occur near a heated surface without the development of self-sustaining smoldering. 

Once initiated, in most cases the smoldering reaction rate is limited by the amount of oxygen 

available at the fuel surface. Therefore smoldering behavior is generally sensitive to parameters 

that affect the amount of oxygen reaching the surface. An example of such a parameter is the 

3
porosity of a smoldering fuel bed. The more open the fuel bed, the more easily oxygen will be 

able reach a given fuel surface and support smoldering. The presence of an imposed or natural 

(buoyancy-driven) flow strongly influences the amount of oxygen reaching a smoldering fuel 

surface. Everyone is familiar with the increase in intensity that occurs when one blows on a 

piece of paper or wood with visible smoldering. The fuel surface-to-volume ratio is also an 

important parameter controlling smoldering behavior with high ratios favoring smoldering. 

Most studies of smoldering consider such properties as the spread rate in a fuel bed. The study 

of Palmer, who investigated smoldering spread through a variety of materials and configurations, 

is a classic example. [4] Studies that consider ignition of smoldering are much rarer. In his 

comprehensive review on ignition, Babrauskas includes a section on smoldering in general and 

smoldering ignition in particular. [5] Many of the studies he summarizes involve determination 

of the minimum energy necessary to ignite smoldering. Of direct relevance to this study, 

Ohlemiller showed that the lowest temperature of a heated surface capable of igniting 

smoldering in a bed of cellulosic insulation varied with the size of the heater. [6,7] For a 0.48 

cm diameter wire a minimum temperature of 380 °C to 385 °C was required, while the limit 

dropped to 255 °C to 260 °C for a 200 cm × 200 cm plate. Times required for ignition with these 

low temperatures were on the order of hours. Babraukas also discusses a limited number of 

studies in which the time to ignition was determined as a function of applied heat flux. Materials 

studied included cotton fabrics and polyurethane foam. [5] 

A number of parameters have been identified as important in controlling smoldering behavior in 

porous fuel beds such as those considered in this study. [3,5] The fuel must be capable of 

forming a rigid char during pyrolysis. Char formation varies widely in solid fuels. Charring and 

char oxidation rates in cellulosic fuels are known to be sensitive to the concentration of various 

inorganic salts, metals, and metal hydroxides. The amount of moisture absorbed in the fuel is a 

very important parameter. Physical properties of the fuel beds such as size, thickness, and 

porosity also play roles. 

Babrauskas notes that there are two smoldering modes that are characterized by whether or not 

oxidizing surface temperatures are high enough to emit visible light. [5] The type of smoldering 

in which light is emitted is often referred to as “glowing combustion.” Drysdale includes a short 

discussion of glowing combustion as part of a discussion of smoldering in his book. [8] In the 

current work smoldering in which there is no visible light produced will be referred to as “non 

glowing combustion.” It appears that little previous research has considered the conditions 

necessary for the development of glowing combustion or its general behavior. 

It is well known in the fire community that under certain conditions smoldering will 

spontaneously transition to flaming combustion. [5,8] While the general requirements for 

ignition in the gas phase are known, there is little practical understanding of the transition to 

flaming from smoldering solids. At a minimum, it is necessary that a solid fuel pyrolyze with a 

sufficient rate to generate a premixed fuel/air mixture that lies between the lean and rich 

flammability limits. Such a mixture can ignite in two well defined ways—piloted and nonpiloted. In “piloted ignition” a source of heat and free radicals, such as a flame or spark, are used 

to initiate the combustion reactions, which then become self sustaining. In non-piloted ignition 

the flammable fuel/air mixture must be heated to a point where free radicals are generated within 

4
the mixture, and rapid chain branching chemical reactions lead to sustained combustion. 

Typically, higher temperatures must be present for non piloted ignition than for piloted ignition. 

In the current work a cone calorimeter will be used for the radiative ignition studies. Babrauskas 

and Parker have described the use of the instrument for this purpose. [9] Most ignition 

experiments that have been done in the cone utilized an electric discharge as a pilot for the flame. 

It is possible to operate the system without a spark in order to study non piloted ignition. 

A primary focus of this work is the ignition behavior of cellulosic fuels. Babrauskas included a 

review of ignition of forest materials, vegetation, and hay in his comprehensive monograph on 

ignition. [10] His general comments concerning such measurements are relevant. He points out 

that there are numerous measurements of ignition temperatures in the literature, but that the 

results vary widely. Potential sources of these variations include fuel composition and moisture 

content, but Babrauskas argues that variations due to such factors should be much less than 

observed, and he attributes much of the variability to experimental protocols and operator 

inexperience. 

Many of the ignition studies cited by Babrauskas involve determining a lower temperature limit 

for combustion. A variety of heating approaches were employed with the most prevalent being 

immersion in air held at a constant temperature in an oven. Minimum ignition temperatures 

reported for a variety of natural fuels ranged from 166 °C to as high as 675 °C. There is no 

indication of the times required for ignition or the type of combustion observed. 

In a number of studies relatively large heated bodies were deposited on various types of porous 

natural fuel beds. Ignition temperature tended to be somewhat higher than reported for other 

ignition sources, with minimum values ranging between 450 °C and 650 °C. 

More recently, several studies of radiant ignition of forest fuels have appeared. Most of these 

tests have utilized either the ISO 5657 [11] or cone calorimeter [12,13,14] tests. ISO 5657 and 

the cone calorimeter are similar tests, but differ in the sample size and configuration and in the 

type of igniter used in piloted ignition studies. Most of these tests were designed to measure the 

flammability of natural fuels and usually utilized piloted ignition. Some findings are 

summarized by Babruaskas, but it is difficult to draw conclusions relevant to the current 

investigation. [10] 

In a book on the investigation of wildfires, Ford includes a useful summary of selected studies on 

wildland fuel ignition. [15] He discusses similar large variations in minimum ignition 

temperatures to those cited by Babrauskas, but notes wildland physicists have settled on a 

temperature of 320 °C as an arbitrary average. Ford summarizes the results of an extensive 

investigation of ignition by heated surfaces carried out at the Oregon State Engineering Research 

Station in 1949. Lower ignition temperatures for a wide range of woods and papers fell in a 

range of 220 °C to 300 °C. 

There is an extensive literature available concerning the pyrolysis and burning behavior of 

cellulose and cellulosic fuels. The observed behaviors are quite complex and a detailed 

discussion is not appropriate for the purposes of this investigation. However, it is possible to 

5
provide some guidance for the general effects of fuel composition on burning behavior. The 

recent review of Plucinski is useful for this purpose. [16] Quoting this author, “Plant tissue is 

approximately 50 % carbon, 44 % oxygen and 5 % hydrogen by mass and varies between 41-53 

% cellulose, 16-33 % hemicellulose, and 16 – 33 % lignin. Cellulose tends to burn by flaming 

combustion and lignin by glowing combustion.” 

In the following, several investigations are briefly summarized because they used experimental 

approaches similar to those of this study or considered similar fuels. 

In a study primarily concerned with the temperatures generated by automobile exhaust systems, 

Harrison reported ignition times for a dry annual grass and dry pine needles as functions of the 

temperature of a heated barbeque charcoal igniter placed on top of piles of the fuels. [17] For 

both fuels the ignition times increased with decreasing temperature, with the times becoming 

longer as the temperature was lowered. For the grass the minimum ignition temperature was 400 

°C and for the pine needles the corresponding temperature was 350 °C. Both fuels required 

about 4 min for ignition at these minimum temperatures. Ignitions at lower temperatures were 

not observed in experiments lasting six minutes. A 0.9 m/s (2 mph) wind had a noticeable effect 

on grass ignition time, reducing the ignition time for a 400 °C surface to 1.3 min. 

In a particularly relevant investigation, Kaminski simulated the ignition of punky (decayed, 

crumbly, and dry) wood, cheat grass, saw dust, and tree moss by a chain saw muffler by 

attaching a wire heater to an actual muffler. [18] It was found that punky wood would smoke 

and glow at temperatures as low as 270 °C. The presence of a low wind reduced the time 

required for ignition and increased the intensity of the glowing combustion. Glowing for cheat 

grass was observed at 330 °C, but sawdust only browned under these conditions. The tree moss 

was reported to have a behavior similar to the punky wood. 

In a series of reports Stockstad reported on non piloted and piloted ignition studies of rotten 

wood [19], cheat grass [20], and pine needles [21] in which minimum heat flux intensities for 

ignition, time to ignition, and temperatures at times of ignition were given. Small individual 

pieces of the samples were heated by immersion in a furnace. A variety of rotten woods were 

tested. Minimum temperatures for ignition were on the order of 270 °C to 300 °C. Times 

required for glowing combustion were on the order of 1 min. For cheat grass the minimum 

temperature for which non piloted ignition was observed was 450 °C with ignition times on the 

order of 20 s. Ignition of pine needles occurred for temperature as low as 365 °C, with glowing 

appearing after roughly 60 s. 

Recently, Manzello et al. have investigated the ignition of shredded paper, pine needle, grass, 

and hardwood mulch fuel beds by glowing and flaming firebrands deposited on top of the beds. 

[22,23] Experiments were done with single and groups of four firebrands of two sizes 

(diameters of 25 mm and 50 mm). Imposed air flows of 0.5 m/s and 1.0 m/s were used. The 

effect of moisture level was tested by using 0 % and 11 % fuel moisture levels. Ignition by 

glowing brands is most relevant to the current study, and these findings can be summarized as 

follows. The only fuel that was readily ignited by a single firebrand was the shredded paper. 

Only glowing combustion was observed. When 4 brands were used the pine needle fuel beds 

ignited only with the 50 mm-diameter brands in the presence of the 1 m/s wind. In this case 

6
smoldering ignition with transition to flaming was observed for both fuel moistures. The larger 

firebrands induced smoldering in the dried hardwood mulch with both airflows, with the 

smoldering transitioning to flaming for the higher air flow. Grass only ignited when dry and 

exposed to four brands and the higher air flow. Combustion began as smoldering and 

transitioned to flaming. 

White and coworkers have described on an effort to develop an approach for flammability 

measurements of vegetation using a cone calorimeter. [24,25,26] Measurements have been 

typically made with applied heat fluxes of 25 kW/m2

 or 50 kW/m2 and piloted ignition. Changes 

in flammability due to seasonal variations for several species of ornamental vegetation [25] and 

for cheat grass grown in the presence of varying CO2 concentrations [26] have been reported. 

7
3. Experimental Systems and Procedures 

3.1. Ignition by Thermal Conduction from a Heated Surface 

Ignition of cellulosic fuels as the result of thermal conduction from a heated surface was 

investigated by placing loosely packed samples of the material to be tested in contact with a 

uniformly heated surface. The surface was oriented facing upward. The response of the sample 

to heating was observed while maintaining a constant surface temperature. The dependence of a 

given fuel response on surface temperature was determined by repeating the experiment for a 

range of surface temperatures. Experiments were done with and without a lateral air flow 

applied to the fuel bed. 

3.1.1. Experimental System 

3.1.1.1. Heated Plate 

The heated plate was fabricated by inserting four Watlow* 300 W cartridge heaters (Model 

number E4A30) into a 10.2 cm × 10.2 cm square plate having a thickness of 1.9 cm. Machinable 

copper was used for the plate due to its high thermal conductivity, which minimized temperature 

gradients within the plate. Each heater was 10.2 cm long and had a diameter of 0.63 cm. The 

four heaters were inserted into four parallel 0.64 cm-diameter holes, drilled horizontally through 

the plate at the vertical center, and positioned 0.95 cm, 3.5 cm, 6.7 cm, and 9.2 cm from the 

perpendicular edge of the plate. The heaters were equipped with male National Pipe Thread 

(NPT) fittings and were inserted firmly into machined female NPT fittings in the copper plate. A 

schematic of the plate showing planar and side views is shown in Figure 1. 

Two power supplies were utilized to provide electric current for the heaters. One of the sources 

was a Kepco Model ATE 100-2.5M capable of supplying up to 2.5 A at a maximum of 100 V 

DC. The second power supply was a Kepco Model ATE 150-7M that could provide up to 7 A 

with a maximum voltage of 150 V DC. One of the center cartridge heaters was connected to the 

smaller power supply, while the three remaining heaters were wired in parallel to the larger 

supply. Assuming that the four cartridge heaters are operated at the same current, this 

arrangement could provide a maximum current of 2.33 A to each heater. 

The temperature of the plate was monitored using three Type K thermocouples†

, which were 

press fit into 0.16 cm-diamter holes drilled from the back side of the plate to within 0.32 cm of 

 

*

 Certain commercial equipment, instruments, or materials are identified in this document. Such identification does 

not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it 

imply that the products identified are necessarily the best available for the purpose. 

†

 As explained in Section 5, the estimation of standard uncertainties (for time to glowing or flaming ignition and for 

maximum observed heat release rate) requires a greater number of experiments than could be performed within the 

scope of this project. In the absence of such data a qualitative approach was adopted in which the degree of 

collapse of experimental data onto well-defined curves was adopted as a qualitative basis fro assessing uncertainty; 

therefore, this report does not provide estimates of standard uncertainties, and graphs do not display error bars. 

8
Figure 1. Schematic plan and side views are shown for the heated copper plate used for the 

ignition studies. All dimensions are in cm. 

the top heated surface. Two of the mounting holes were located on opposite corners of the plate 

at distances of 1.91 cm from the edges parallel to the heaters and 2.54 cm from the edges 

perpendicular to the heaters. The third thermocouple was located at the center of the plate. The 

thermocouple leads were connected to Omega DP41-TC thermocouple readers that provided 

visual readouts of the temperatures at the three locations. 

The heated plate was placed on a ring stand. Figure 2 shows a photograph of the plate with the 

heaters and thermocouples in place. Initial testing of the heated plate in ambient air showed that 

setting the two power supplies to the same voltage resulted in a current flow for the larger supply 

9
Figure 2. This photograph shows the heated copper plate used for ignition measurements of 

cellulosic fuels in place on a ring stand. The ends of the four cartridge heaters and 

their leads can be seen on the two edges of the plate. The leads for three 

thermocouples imbedded in the plate are visible below. 

that was three times larger than recorded for the smaller. This is the expected behavior if the 

four cartridge heaters have the same resistance values. As the temperature of the plate was 

increased, the three thermocouple outputs provided temperature readings that agreed within 1 °C, 

indicating good uniformity of the temperature field within the plate. For recording purposes, the 

temperature of the thermocouple at the center of the plate was used. 

Figure 3 shows the plate temperature recorded as a function of the current applied to each of the 

cartridge heaters as solid circles from an initial calibration. The result of a quadratic least 

squares curve fit is shown as a solid line for comparison purposes only. Over the course of the 

experiments small variations in temperature for a given current were observed that were most 

likely due to changes in the emissivity of the copper surface. Some changes in the physical 

appearance of the plate were apparent when the operating temperatures were varied. 

3.1.1.2. Air Flows 

Experiments were performed with and without air flows over the heated plate. Air flows were 

generated using a Dayton Model 4C443A blower delivering a nominal volumetric flow rate of 

10
Figure 3. The temperature measured at the center of the heated copper plate is plotted as a 

function of the current applied to the four cartridge heaters embedded within the plate. 

Filled circles are the experimental measurements and the solid curve is the result of a 

quadratic least squares curve fit to the data. 

219 L/s. The output of the blower was mounted at the same height as the heated plate and 

directed towards the plate from a distance of 134 cm. The air velocity at the plate was 

characterized by recording the output from an Applied Technologies, Inc. Model SPAS/24 sonic 

anemometer with a portable computer. The anemometer measures wind velocity in two 

dimensions. The probe was placed 1.2 cm above the plate such that one of the velocity 

components was aligned along the primary direction of the flow generated by the blower, as 

shown in Figure 4. 

Figure 5 shows an example of the wind speed recorded by the sonic anemometer. Initially the 

blower was off. When the blower was turned on the air speed increased rapidly to values on the 

order of 2.5 m/s. Rapid fluctuations in speed are evident during the just over 300 s the fan was 

on. When the fan was turned off the speed dropped rapidly to that characteristic of the quiescent 

laboratory. 

Averaging the data plotted in Figure 5 yielded means and root mean square (rms) values of 0.09 

m/s ± 0.04 m/s and 2.5 m/s ± 0.3 m/s for the wind speeds with the fan turned off and on, 

respectively. The low wind speed recorded with the fan off is reasonable since the plate was 

located under an open exhaust hood. The mean wind speed recorded with the fan turned on 

should be indicative of the applied wind speed, but it likely that the fluctuations are 

underestimated due to the averaging inherent in recording over the 15 cm path length between 

11
Figure 4. Photograph showing the sonic anemometer probe used to characterize the applied 

wind velocity in place 1.2 cm above the unheated copper plate. 

Figure 5. The wind speed above the copper plate along the primary flow direction of the blower 

recorded as a function of time is shown with the blower turned on and off. 

12
Figure 6. The blower with screens in place used to generate the “low” velocity air flow is 

shown. 

the sonic transmitter and receiver. A more accurate value for the rms would have required 

measurements with much higher spatial resolution, which were not done. 

Velocity measurements such as those shown in Figure 5 were repeated six times over a two 

month period. The average of the six mean measurements with the fan turned on was 2.53 m/s 

with individual standard deviations comparable to that for the data in Figure 5. The measured 

ratios of rms to the average varied from roughly 8 % to 11 %. 

A wind speed of 2.5 m/s corresponds to 5.6 mph. It became clear early in the project that lower 

wind speeds affected the ignition behaviors of fuels placed on the heated plate when compared to 

cases without wind, and that it was desirable to apply a wind with a lower speed. Instead of 

generating the lower velocity by controlling the blower output, the velocity was reduced by 

leaving the flow unchanged and placing a series of four screens with an open aperture of roughly 

5.3 cm × 11.4 cm in the flow path. The screens were placed on a holder that could be easily 

placed into and removed from the blower stream in a repeatable manner by attachment with a 

clamp to a horizontal rod supported by a vertical mount. Figure 6 shows a photograph of the 

blower with the screens in place. 

The wind speed with the screens in place was measured in the same way described above. Four 

measurements yielded an average value of 1.1 m/s. The rms values for individual measurements 

varied from 13 % to 20 %, somewhat higher than observed for the higher speed flow. As 

discussed above, it is likely that the measurements underestimate the actual fluctuations. 

13
In this report experiments with an applied wind of roughly 2.5 m/s will be referred to as “high 

wind” cases, while those using 1.1 m/s will be denoted as “low wind” cases. Note that these 

designations simply refer to the relative magnitudes of the flows used in this study and have no 

relevance to potential magnitudes of outdoor winds. 

3.1.1.3. Wire-Screen Cage 

The purpose of these experiments is to characterize the ignition behavior of typical outdoor 

cellulosic fuels when placed in contact with a heated surface. Common outdoor fuels such as 

grasses and pine needles consist of small pieces that form porous fuel beds. These materials had 

to be contained in some manner in order to create fuel beds that could be reproducibly brought in 

contact with the heated plate. It was viewed as desirable to allow the fuel beds to have free 

access to air from the sides and top. As a compromise for these competing requirements, the 

fuels were contained within a cage fabricated from stainless steel wire screen. The screen was 

formed from wires with a diameter of 0.71 cm and a wire spacing of 0.32 cm. 

The wire-screen cage was designed so that the fuel beds would have nominal dimensions of 10.2 

cm × 10.2 cm × 2.5 cm when the cage was placed over the heated plate. An open-bottom wirescreen cage was formed by starting with a 10.2 cm-square section of the wire screen that served 

as the top of the cage and attaching four wire-screen sidewalls with fine wire. Three of the 

sidewalls had widths of 10.2 cm and depths of 4.2 cm. Two slots that lined up with the threaded 

heater mounts visible in Figure 2 were cut in each of the two opposed sidewalls. These slots 

were wide enough to just allow the cage to slide down pass the mounts while the slot depths of 

1.1 cm provided an open height above the plate of 2.7 cm when the cage rested on the mounts. 

The fourth side wall of the cage was formed from a 10.2 cm wide length of screen with a depth 

of 2.5 cm. When placed on the cage there was a narrow open gap of approximately 0.013 cm 

between the bottom of the short side wall and the top of the plate. 

In order to apply the fuel within the wire-screen cage to the heated plate it was necessary to 

provide a bottom of the cage that could be removed. The bottom section that served this purpose 

is shown in place on the inverted wire-screen cage in Figure 7. This section was cut from a 

0.008 cm-thick steel plate. The rectangular portion of the plate has dimensions of 10.2 cm ×10.8 

cm and was designed to slide between the two deep sidewalls on opposite sides of the cage. The 

2.54 cm × 1.27 cm tab on the right side of the bottom section in Figure 7 slides through a slot cut 

2.5 cm from the top of the third deep sidewall and rests on top of the short sidewall on the 

opposite side of the cage. The narrow section on the left side served as a handle that allowed the 

bottom plate to be slid along the top of the plate and removed to expose the fuel to the plate. 

The fuel to be tested was placed in the upside down wire screen cage to a depth of 2.5 cm. The 

bottom plate was then inserted into the slot as shown in Figure 7. By grasping the handle, the 

wire-screen cage was carefully inverted and place over the heated plate. The bottom was then 

rapidly removed by sliding it away from the cage. Note that the bottom plate insulated the fuel 

from the heated plate until it was rapidly removed, thus providing a well defined time of fuel 

application. 

14
Figure 7. The wire cage with its removable steel-plate bottom in place is shown looking down 

from above. 

It should be noted that the wire-screen cage provided resistance to the air flow into the fuel bed 

that varied with the velocity passing through the screen. A test with a single flat section of the 

screen placed upstream of the test plate indicated that the high wind flow velocity was reduced 

by 32 % by passing through the screen. 

3.1.2. Visual Recordings of Ignition Experiments 

The principle experimental diagnostic used for the heated plate ignition experiments was 

videography. Each ignition experiment was recorded using a Sony Model DCR-PC100 mini-DV 

video camera. The taped results were subsequently analyzed to determine the time when the fuel 

was placed on the plate and the times when glowing combustion and flaming were observed. 

Other behaviors such as the location and amount of smoke released could also be tracked. The 

temperature of the plate was monitored by reading the temperature recorded by the center 

thermocouple in the plate out loud so that it was recorded on the audio track of the video. 

Subsequent analysis of the video tape allowed the temperature to be determined as a function of 

time. Other visual observations were also recorded in this way. 

3.1.3. Experimental Procedure 

The various fuels investigated during the study are described below. For each fuel the mass of 

material required to fill the approximately 262 cm3

 volume of the wire-screen cage was 

determined, and this mass was used for all experiments with that fuel. In filling the cage no 

attempt was made to order or compress the fuel. The resulting fuel beds were typically loosely 

packed and porous. The density of the fuel bed was calculated by dividing the fuel mass by the 

nominal fuel bed volume. 

15
Figure 8. This photograph shows the wire-screen sample cage with its steel-plate bottom in 

place on top of the heated plate. The cage is filled with cheat grass. The fuel is placed 

on the plate by withdrawing the bottom of the cage using the handle visible in the 

photograph. 

The pre-determined mass of fuel was then loosely evenly distributed into the inverted wirescreen cage through the open bottom. Once the fuel was distributed, the tab on the plate that 

formed the bottom of the cage was inserted into the slot in the wire-screen side wall and allowed 

to rest on the opposite side wall. 

The first step in an experiment was to heat the plate to the desired temperature by adjusting the 

equal currents supplied to each heater by the two power supplies. When an experiment was to be 

run the camera was first started and identifying information such as the date and test conditions 

were recorded audibly. The wire-screen cage was then carefully inverted and placed over the 

heated plate as shown in Figure 8. Within a few seconds the plate was extracted from the bottom 

of the cage, and the fuel was applied to the plate. If a wind was to be applied, the fan was turned 

on immediately prior to or just after the application of the fuel. 

The application of the fuel and wind changed the heat losses from the plate and adjustments to 

the currents were required to maintain a constant temperature. For instance, applying the fuel in 

the absence of the wind typically caused the temperature to increase, indicating that heat transfer 

to the fuel was more than offset by reduced heat losses from the plate. In this case the currents to 

the heaters needed to be reduced in order to maintain a constant temperature. On the other hand, 

applying a wind without adding the fuel caused the temperature to drop due to increased 

16
convective heat transfer to the air from the plate surfaces, and it was necessary to increase the 

currents to maintain a constant temperature. 

By trial and error the amount of current change required to maintain a nearly constant 

temperature for a given fuel and wind condition was determined. As soon as the fuel was 

applied and the wind (if applicable) was turned on, the current would be manually adjusted to the 

appropriate level. During the remainder of the experiment the plate temperature would be 

monitored and small current adjustments would be made as necessary to hold the temperature 

variation within a small range. The actual current changes were not recorded, but it was noted 

that there was a correlation between the required current and the behavior of the fuels. For 

instance, when glowing combustion was observed, it was typical to have to reduce the current 

slightly in order to maintain the temperature. 

Experiments showed that it was normally necessary to reduce the currents substantially when 

fuels were applied without a wind, relatively small changes in current were required when 

experiments were done with a low wind, and increased currents were needed when a high wind 

was used. The changes in temperature were relatively slow, and it was generally possible to hold 

the recorded temperatures within ± 2 °C. Occasionally larger excursions were observed, but 

variations were always much smaller than the smallest step size (typically 10 °C) used for plate 

temperature settings. Figure 9 shows a plot of representative temperature histories recorded for a 

range of temperature settings and wind condition (none, low, high). Times are generally shorter 

at higher temperatures since an experiment was halted when flaming was observed or visual 

observation (e.g., smoke no longer visible) suggested no additional reaction was taking place. 

At the conclusion of an experiment the video camera was shut off. At a later time the video tape 

was replayed in order to record the plate temperature readings as a function of time, ignition 

times for glowing and flaming combustion, and other experimental observations such as smoke 

behavior. 

3.2. Ignition by Thermal Radiation 

Ignition of cellulosic fuels as the result of exposure to thermal radiation was investigated by 

placing loosely packed samples of the material to be tested in a cone calorimeter operated in a 

non piloted ignition mode. The exposed surface of the fuel bed was oriented facing upward. 

The response of the sample to an imposed heat flux was observed while maintaining a constant 

heat flux. The time required to ignite the fuel was recorded. The dependence of the ignition 

time on heat flux level for a given fuel was determined by repeating the experiment for a range 

of imposed heat fluxes. Other parameters available from the experiments, in addition to time to 

ignition, included time behaviors of heat release rate, mass, and smoke optical density. 

3.2.1. Cone Calorimeter 

The cone calorimeter is a widely available standard instrument used primarily to measure the 

heat release rate of small samples when subjected to a uniform radiative heat flux. Heat release 

rate is measured using the oxygen depletion approach. The cone calorimeter has been adopted 

17
Figure 9. Temperatures recorded during experiments in which cellulosic fuels were place on a 

heated plate are plotted as a function of time. Zero time is defined as the time when 

the fuel was exposed to the surface. 

as a standard method by ASTM International (ASTM E-1354) [12], the International 

Organization for Standardization (ISO 5660) [13], and the National Fire Protection Association 

(NFPA 264A) [14]. 

While primarily designed for heat release rate measurements, the cone calorimeter can be used to 

study radiative ignition since it provides a nearly uniform and easily varied source of radiative 

18
Figure 10. This schematic shows the principal components of the cone calorimeter used to 

eat flux at the sample surface. Babrauskas and Parker describe the use of the cone calorimeter 

igure 10 shows a schematic diagram of a cone calorimeter. The principal components are a 

s the 

ty 

he cone calorimeter used for these experiments was manufactured by Atlas Combustion 

ting 

e 

imported into spreadsheet files and displayed using a standard plotting software package. 

investigate the ignition behavior of cellulosic fuels and polyurethane foam. 

h

for ignition studies. [9] 

F

conical resistance heater wound in the form of a truncated cone with temperature control 

achieved using 3 type K thermocouples as inputs for a controller that automatically adjust

current supply, cooled shutter to protect the sample before the start of a test, a specimen holder 

placed on top of a load cell for mass measurement, a spark igniter used for piloted ignition of 

fuel vapors, a hood and blower driven exhaust system equipped with an orifice plate for veloci

measurements, a gas sampling port used to extract gases for oxygen (paramagnetic analyzer) and 

other molecular species concentration measurement, and a laser system for monitoring smoke 

obscuration. 

T

Analysis Systems. It was subsequently modified by replacing the original software for 

controlling the experiment and performing analysis with software developed by Fire Tes

Technology Ltd. The results of measurements include observed ignition times and heat releas

rate per unit area, sample and sample holder mass, and smoke optical density as a function of 

time, along with such derived properties as maximum heat release rate, time to maximum heat

release rate, total mass loss, effective heat of combustion, etc. The experimental results were 

19
Figure 11. A sample holder used during the radiative ignition experiments is shown filled wit

fuel. 

h 

3.2.2. Experimental Procedure 

The ex ing the standard cone calorimeter 

 and were contained within a sample 

ze, 

y 

ing 

e current to the cone heater until the desired level was measured with a calibrated gauge. At 

orate a 

 from 

r flaming. In practice, the measured heat release behaviors also provided good indications for 

periments were performed while generally follow

protocols. Sample sizes were 10.2 cm ×10.2 cm × 2.54 cm

holder formed from a double-layer of aluminum foil, which was open at the top. Note that 

unlike the experiments on heated surface ignition, the sides of the sample holder were closed. 

The sample holders for the plate heating and cone calorimeter experiments were the same si

and the mass of a particular fuel used was also the same. Figure 11 shows one of the sample 

holders filled with fuel. No external wind was imposed during these measurements, and the onl

air flow was that induced by the cone calorimeter blower and natural convection from the 

samples. The separation between the top of the fuel bed and base of the cone was 2.5 cm. 

Prior to an experiment the radiative heat flux at the location of the sample was set by adjust

th

this point the water-cooled shutter was positioned between the cone and the sample area to limit

preheating of the sample. After the sample holder and fuel were placed in position and 

background measurements were completed, the shutter was rapidly removed to expose the 

sample to the imposed heat flux. This defines time zero. Cone experiments often incorp

spark igniter above the sample to provide a pilot for the flammable gases that are generated

the fuel. Since the purpose of these experiments was to investigate non piloted ignition, the 

igniter was withdrawn from above the sample and was not activated during these measurements. 

Ignition times were determined visually by noting the appearance of either glowing combustion 

o

ignition times. An experiment was allowed to continue until obvious changes in fuel appearance

were no longer apparent and the time rates of change for mass and heat release rate were small. 

20
,100

,

, , × −

−

pdf

dfmf

mm

mm

quickly reweighed after removal from the desiccator in order to limit the uptake of moisture

the dried sample. The free moisture percentage was calculated using the following equatio

(1) 

3.3. Fuels Investigated 

A v redded newsprint, four grasses, and pine needles were 

inv rements were carried out on three types of dried leaves. 

he ignition behavior of a common plastic fuel, non fire-retarded flexible polyurethane foam, 

ttempts were made to control free moisture, defined as 

e percentage mass of water relative to the dry material, in this study beyond allowing the fuels 

f 

. 

. 

moved from the oven and allowed to cool in a desiccator. The cooled fuel sample and pan was 

 by 

n, 

where mf,m is the measured ma oisture, mf,d is the mass of the pan 

dried fuel, and mp is the mass of the empty pan. 

Commercial newsprint was obtained from a local publisher as remnants of large 61.0 cm wide 

rolls. M ations on a small area of the newsprint 

 cm. Sections of the newsprint were run through a shredder that 

2 shows a photograph of the shredded newsprint in the wirereen cage. 

d. Two independent measurements yielded values of 6.2 % and 6.4 %. 

ariety of cellulosic fuels including sh

estigated. In addition, limited measu

T

was investigated in the cone calorimeter. 

Free moisture absorbed in the fuel is known to be an important parameter for the ignition and 

burning behavior of cellulosic fuels. No a

th

to come into equilibrium with the surrounding laboratories, which had nominal temperatures o

20 °C with variable relative humidity that did not exceed 50 %. Free moisture in the fuels was 

measured using the procedure described by Manzello et al. [22,23] in which the material to be 

tested is placed in an oven held at 105 °C for three hours and the amount of mass loss quantified

These authors showed that no further changes in mass occurred after 3 hours for these thin fuels

Several grams of the material were placed in a small tared aluminum pan and weighed to ± 

0.0001 g. The pan and grass were then placed in the oven. After three hours the pan was 

re

ss of the pan and fuel with m and 

3.3.1. Shredded Newsprint 

easurements with a micrometer at several loc

-3 yielded a thickness of 6.4 × 10

generated strips that were roughly 0.4 cm wide by 4 cm long. There were substantial variations

in the lengths of individual pieces. 

Fuel beds were created by loosely packing 6.9 g ± 0.1 g of the shredded newsprint into the wirescreen cage sample holder. Figure 1

sc

The free moisture percentage of the shredded newsprint was measured well after ignition testing 

was complete

21
Figure 12. Photograph of shredded 

newsprint in wire-screen cage. 

Figure 13. Photograph of May tall fescue in 

wire-screen cage. 

3.3.2. May Tall Fescue Grass 

A sample of tall fescue grass (Festuca arundinacea) was collected on the NIST campus (GPS 

location of 39.08.082 N, 77.12.709 W) on May 25, 2006. The site was the outfield of a softball 

field that was planted in tall fescue about six years ago. The grass had been cut within the past 

two days. Clippings that had been cut and expelled from the mower and were resting on top of 

the growing grass were collected. The grass clippings felt dry and light to the touch. The 

weather over the previous few days had been nearly ideal for drying with no rain, a temperature 

range of roughly 7 °C to 21 °C, and dew point temperatures on the order of 4 °C. 

Spring is a time of rapid grass growth in this region. Significant rain had fallen within the past 

two weeks. The uncut grass was thick and dark green. As a result of the conditions, the grass 

had been fairly long when cut, with the clippings having a range of lengths varying from 8 cm to 

15 cm. A small amount of the grass was topped with seeds. Even after drying the grass 

clippings retained a light greenish color. Much of the collected grass consisted of thin blades. 

After several weeks of storage in a general purpose laboratory, three independent measurements 

of free moisture percentage taken over several days yielded values of 12.8 %, 11.9 %, and 13.6 

%. These values are typical of those expected when the moisture in the grass has come into 

equilibrium with air in the surrounding laboratory. 

As collected, the tall fescue did not pack well in the sample holders due to its length. Scissors 

were used to cut bunches of the grass into lengths of 2.5 cm to 5 cm, which were used to form 

the fuel beds. A mass of 9.0 g of material was found to be adequate for forming a 2.5 cm deep 

bed when lightly packed into the sample holders. Figure 13 shows a sample of tall fescue grass 

in the wire-screen cage used for the heated plate experiments. For the remainder of the report 

these samples will be referred to as “May tall fescue.” 

22
Figure 14. Photograph of August tall fescue 

 in wire-screen cage. 

Figure 15. Photograph of cheat grass in 

cone calorimeter aluminum pan. 

3.3.3. August Tall Fescue Grass 

A second sample of recently cut tall fescue grass was collected from the same location as the 

May tall fescue on August 29, 2006. Samples of this grass will be referred to as August tall 

fescue. Once again, clippings created by mowing were collected from above the uncut grass. 

August in this region is typically hot and often dry. Weather records indicate that over the four 

prior weeks daytime high temperatures ranged from about 27 °C to 38 °C, with lows ranging 

from 13 °C to 27 °C. The last significant rain had fallen on August 7th. 

This grass had a very different appearance than that collected in May. It was tan colored and 

appeared to consist primarily of stalks as opposed to blades. Its length varied from about 2.5 cm 

to 7.5 cm, and it could be packed into fuel beds without cutting. 

Figure 14 shows a fuel bed formed from 7.0 g of August tall fescue. 

The grass felt dry to the touch when collected. A measurement of its free moisture percentage 

after two days in the laboratory yielded a value of 13.1 %. 

3.3.4. Cheat Grass 

Figure 15 shows one of the samples of cheat grass in an aluminum pan. Cheat grass (Bromus 

tectorum) is an invasive plant introduced into the United States in the 1890s. It has subsequently 

spread widely across the western United States. Cheat grass is a winter grass, growing rapidly in 

late winter and spring. In summer it dies off, leaving a dried grass that is widely regarded as 

highly flammable and a major contributor to wildland fires. 

23
The cheat grass for this investigation was provided by 

researchers from the United Stated Department of 

Agriculture, Agricultural Research Service Laboratory 

in Reno, NV. The grass was collected in the Honey 

Lake Valley of northeastern California (GPS 

coordinates 40.08.291 N, 120.04.672 W) on July 19, 

2006 and shipped to Gaithersburg, MD. During the 

week prior to collection high temperatures were around 

38 °C with relative humidity below 10 %. 

Figure 16 shows a portion of the cheat grass as 

received. The grass was cut near its base and had 

heights on the order of 45 cm. The grass felt dry to the 

touch and was golden in color. It consisted of very 

slender stalks topped with branching seed structures 

known as panicles. The free moisture percentage of a 

sample of the grass was measured to be 11.2 %. 

As received, the grass could not easily be packed into 

the fuel beds. The grass was cut into lengths of 2.5 cm 

to 5 cm using scissors. No special care was taken to 

isolate the stalks and panicles, and both were present in 

the fuel beds. A mass of 8.0 g was sufficient to fill the 

sample holders. 

Figure 16. A photograph showing an 

 uncut sample of cheat grass 

3.3.5. Fine Florida Grass 

A grass sample collected at a lawn mower manufacturer’s test site near Tampa, FL was provided 

for the study. The grass had been removed from the tops of mower decks and was intended for 

testing the debris that collects on mowers. The sample had been collected at least a year earlier 

and was stored in bags under dry conditions. The grassy material was cut finely and felt fluffy 

when unpacked, having a consistency similar to sawdust. The material was brown in color. No 

additional information was available, including the type of grass that had been cut or weather 

conditions. This material will be referred to as fine Florida grass. 

An 8.0 g sample of the fine Florida grass just filled the wire-screen cage. Figure 17 shows a 

photograph of the cage filled with fire Florida grass. Two measurements of fuel moisture 

percentage recorded well after the ignition experiments were completed yielded values of 10.1 % 

and 9.6 %. 

24
Figure 17. Photograph of fine Florida grass 

in wire-screen cage. 

Figure 18. Photograph of pine needles in 

wire-screen cage. 

3.3.6. Pine Needles 

Pine needles were obtained from a commercial operation (Florida Pine Straw Company of Mayo, 

FL) that distributes pine needles for use as mulch. These needles come from loblolly pine trees 

(Pinus taeda). They are collected by machines that sweep up pine needles that have fallen to the 

ground. They were delivered in bales that were opened and left exposed to the laboratory. 

The pine needles were dark brown and dry to the touch. The fuel moisture was measured to be 

13.7 %. They had lengths that generally varied from 15 cm to 23 cm with rectangular cross 

sections having rough dimensions of 0.5 mm × 1.4 mm (dimensions were highly variable). 

Individual needles were brittle and easily broken. Often two or three pine needles were 

connected together at their base by a needle sheath, forming a fascicle. Since the needles were 

typically too long for the sample holders, they were cut into lengths between 2.5 cm and 5 cm 

using scissors for testing purposes. No attempt was made to separate the sheaths from the 

needles. Small debris, most often small pieces of pine wood, was removed when encountered. 

The cut pine needles formed a dense bed, with 16.0 g used to fill the sample holders. Figure 18

shows a photograph of a bed of pine needles in the wire-screen cage. 

3.3.7. May Tall Fescue/Pine Needle Mixture 

A mixture of May tall fescue grass and pine needles was tested using materials processed as 

described above. The masses required to fill the cage for the grass (9 g) and pine needles (16 g) 

were quite different. The masses of each required for the mixture were calculated for a 50 %/50 

% mixture by mass assuming the volumes of each component would be independent of mixing. 

The result was 5.8 g for a total mass of 11.6 g, with grass making up 64 % of the volume and the 

pine needles 36 %. The two materials were weighed independently and then mixed by hand until 

25
Figure 19. Photograph of the May tall 

fescue/pine needle mixture in 

Figure 20. Photograph of polyurethane in a 

cone calorimeter sample holder.

the wire-screen cage. 

they appeared to be evenly distributed. The mixture was then placed in the wire-screen cage. 

igure 19 shows a photograph of the resulting mixture. 

3.3.8. Preliminary Tests with Leaves 

A limit er sing leaves collected locally. These 

cluded leaves from boxwood (Buxus), American elm (Ulmaceae Ulmus Americana) and pin 

3.3.9. Polyurethane Foam 

Flexibl e calorimeter. The samples were obtained from 

 commercial supplier and were not fire retarded. The exact composition of the material is not 

3.3.10. Fuels Summary 

Table 1 ensity, and measured moisture content for all of the 

els used during the current investigation. Missing data represent cases for which the amount of 

F

ed numb of heated-plate experiments were done u

in

oak (Quercus palustris). 

e polyurethane foam was tested in the con

a

known. The sample thickness was 5.1 cm. Sample masses varied over a range of 11 g to 12 g. 

Figure 20 shows a photograph of a sample of the polyurethane surrounded by the aluminum foil

used as the holder for the cone experiments. 

 includes the mass used, nominal d

fu

fuel available was limited or measurements were not made. 

26
Table 1. Summary of Fuels, Sample Mass Used, Nominal Fuel Bed Densities, and Measured 

Fuel Moisture 

Fuel Sample Mass (g) Nominal Density (g/cm2) Fuel Moisture (%) 

Shredded Newsprint 6.9 0.026 6.2 

6.4 

May Tall Fescue 9.0 0.034 

12.8 

11.9 

13.6 

August Tall Fescue 7.0 0.027 13.1 

Cheat Grass 8.0 0.031 11.2 

Fine Florida Grass 8.0 0.031 10.1 

9.6 

Pine Needles 16.0 0.061 13.7 

May Tall Fescue/Pine 

Needle Mixture 11.6 0.044 -- 

Boxwood Leaves 8.0 -- -- 

American Elm Leaves 5.5, 5.5 -- -- 

Pin Oak Leaves 6.0, 9.4 -- -- 

Polyurethane Foam 

(5.1 cm thickness) 11 to 12 0.042 to 0.046 -- 

27
Figure 21. The smoke plume rising from a pine needle fuel bed in contact with the heated plate 

held at a temperature of 380 °C is shown 213 s after application. 

4. Results 

4.1. Some General Observations from Heated Plate Ignition 

Studies 

A variety of qualitative observations provide insight into the characteristics and response of the 

porous fuel beds when placed on a heated plate. First, cases without an applied wind will be 

discussed. This will be followed by observations when low and high winds were used. 

When the temperature of the heated plate was high enough, all of the fuels tested generated a 

white “smoke” that would flow from the top of the fuel bed. The composition of the smoke is 

unknown, but it is likely that it contained varying proportions of condensed water vapor and 

organic pyrolyzate. At times, particularly after it first appeared, the smoke would rise from 

localized sections of the fuel bed surface. More commonly, the smoke would appear over a large 

fraction of the surface area. In the latter case, the smoke immediately above the fuel bed 

typically assumed a circular shape such that there was a smoke-free area around the perimeter of 

the square fuel surface. Above the surface, the smoke would rise and its area would decrease so 

that the smoke near the surface adopted a conical shape. As the smoke moved further away from 

the plate it would develop the fluctuating shape characteristic of a buoyancy-dominated 

fluctuating flow. Figure 21 shows an example of this type of smoke flow for a bed of pine 

needles placed on the plate held at 380 °C. 

28
Figure 22 Two frames taken one second apart from a video of a hot surface ignition experiment 

with May tall fescue are shown. A small area of glowing combustion had appeared 

during the one second period (compare the blown up areas prior to (left) and after 

(right) ignition). 

The appearance of smoke at the top surface of a fuel bed indicates that the bed is porous to the 

buoyant flow of heated gases generated within the bed by contact with the heated plate and any 

heat release taking place within the fuel bed. The characteristic shape of the smoke at the fuel 

surface suggests that the buoyant flow within the bed acts much like a buoyant plume in open air, 

entraining air radially inward that has passed through the sides of the fuel bed. 

The amount of smoke released from a fuel bed (qualitative description, estimated visually) varied 

widely with time, the fuel tested, and plate temperature. The level of smoke observed ranged 

from just barely visible to quite heavy as seen in Figure 21. It was clear the smoke density was 

related to the amount of fuel pyrolysis that was taking place. Particularly for the lower plate 

temperatures where relatively long periods were required for glowing combustion and/or flaming 

to appear, the smoke would tend to increase with time, becoming quite heavy before glowing or 

flaming appeared. 

During the experiments, the times for the appearance of glowing combustion and flame were 

noted visually and recorded audibly on the video tape. The times were determined a second time 

based on a visual review of the video tape. The video camera used for the investigation responds 

to light in the near infrared, and was particularly useful for detecting the onset of glowing 

combustion as long as it is not hidden from view within the fuel bed. For experiments without 

an applied wind, glowing combustion was usually observed on the video tape before it was seen 

visually. The sensitivity possible using the video camera is demonstrated in Figure 22, which 

compares two frames taken one second apart from a video of an ignition experiment for May tall 

fescue with a plate temperature of 381 °C and no wind applied. The appearance of the glowing 

combustion is easily identified by comparing the two enlarged areas. Glowing combustion 

would generally appear at a small single point in the fuel bed and then spread to cover larger 

areas. 

The glowing combustion was not generally hidden within the fuel bed for cases without an 

applied wind because there was a high likelihood that it would appear on the side of the fuel bed 

facing the camera. The most likely reason for this side to be favored was the narrow open slit 

29
Figure 23. The smoke generated by a mixture of May tall fescue and pine needles applied to the 

heated plate at 380 °C in the presence of a low wind is shown 175 s after application. 

between the heated plate and the base of the wire-screen sidewall. As discussed earlier, 

smoldering development is very dependent on the amount of oxygen reaching the fuel surface. 

Apparently, the small slit allowed more air to enter the cage from this side than passed through 

the screens on the other three sides, with the result that the initial surface heating required to 

ignite glowing combustion was more likely near the slit. The practical effect of this preference is 

likely a short decrease in the time required for glowing combustion to develop compared to that 

which would have been observed if the screen wall extended below the heated plate. 

The observations changed substantially when a wind was applied. For both low and high wind 

cases, smoke that came from the top of the fuel bed tended to lie along a narrow band oriented 

perpendicular to the flow direction. Figure 23 shows an example of this behavior for the May 

tall fescue/pine needle mixture with a plate temperature of 380 °C and a low wind applied. 

This photograph was taken 175 s after the fuel was placed on the plate. Note that the band of 

smoke across the top of the fuel bed also extends down the side of the cage. 

While the amount of smoke generated varied with fuel type, surface temperature, and time after 

fuel application, the formation of a smoke band was normally observed with an applied wind. 

The location of the smoke band relative to the upstream edge would often shift somewhat during 

an experiment. It was also observed that for a given fuel type, the band would be located further 

downwind for the higher wind case. The upstream edge of the band tended to appear anywhere 

from upstream of the center of the fuel bed to locations reaching at least 4/5 of the way across 

from the upstream side. 

30
As the wind passed through the upstream side of the wire-screen cage, it was certainly attenuated 

in the manner shown experimentally above. The porous fuel bed would slow down the air flow 

further. It is the interaction of this attenuated wind with the buoyant flow rising from the heated 

plate and fuel that leads to the formation of the smoke band. The higher velocity flow is able to 

penetrate further, thus explaining the downstream shift observed for the higher wind velocities. 

The presence of an air flow also affected observed ignition behaviors. The wind causes two 

competing effects that can affect surface ignition. On one hand, the flow convectively cools the 

fuel and reduces fuel surface reaction rates. On the other hand, it substantially increases the 

amount of oxygen reaching the fuel surface, which enhances surface reaction rates. For most of 

the experiments described below, the latter effect was dominant since both glowing and flaming 

combustion appeared at locations towards the upstream sides of the fuel beds. 

One result of applying a wind was to slightly increase uncertainties associated with 

measurements of glowing ignition time. Unlike for no-wind cases, glowing ignition was often 

observed at locations well away from the fuel bed side facing the camera. This is another 

indication that wind is enhancing surface reactions, since it suggests it is overwhelming the 

effect of increased air flow through the narrow gap in the wires-screen cage on the camera side. 

The nascent ignitions with a wind were often hidden from the camera by the fuel bed, and it was 

common for the earliest observation of glowing combustion to be made visually by the observer. 

This effect is likely exacerbated when the ignition is location further within the fuel bed away 

from the observer. It is difficult to estimate how much earlier glowing combustion may have 

been present before it was detected, but it is likely to have involved relatively short periods since 

glowing combustion tended to spread rapidly in the presence of a wind, making it more apparent 

to both the observer and the video camera. 

Once glowing combustion appeared, it tended to spread more rapidly and become more intense 

than in the absence of wind. Generally, the glowing would move downstream away from the 

location where it was first observed. Often a band of unburned fuel was observed along the 

upstream edge of the bed. Strong temporal oscillations in glowing intensity with periods on the 

order of a few seconds were frequently observed. It was unclear if these oscillations were due to 

variations in the wind velocity or some instability associated with the combustion behavior. 

Some qualitative observations concerning the development and behavior of glowing combustion 

and the transition to flaming both with and without wind are relevant. Glowing combustion 

tended to appear at isolated small areas, here referred to as pinpoints, similar to those visible in 

Figure 22. In the absence of a wind the glowing pinpoints could usually be observed moving 

slowly over individual pieces of fuel. This seems to imply that glowing combustion is a distinct 

process from non glowing combustion that is self sustaining. As noted above, glowing is 

generally believed to be due to surface oxidation of char that had been formed by earlier fuel 

pyrolysis. 

In the presence of a wind, the intensity of glowing was enhanced substantially and at times 

would appear to be more wide spread than the pinpoints typically observed without wind, with 

glowing over longer lengths of individual fuel elements. 

31
A glowing fuel element appeared to be capable of inducing glowing in nearby fuel elements. As 

a result glowing combustion usually spread away from locations where it was initially noted. In 

this way volumes of glowing combustion would develop. Since the number of glowing fuel 

elements was increasing, the amount of heat release associated with glowing would increase 

during these glowing periods. 

In numerous experiments the transition to flaming was captured on video. A blue flame would 

appear over a small volume within the fuel bed and then spread rapidly to other locations. At 

times, these flames moved with speeds on the order of tens of cm/s. The location where the 

initial flame appeared was always surrounded by an area of extensive glowing. Within a short 

period of time yellow flames would appear, and the flaming became self sustaining. 

The observations concerning transition to flaming suggest that gaseous combustion developed in 

regions within the fuel beds where a flammable premixed gaseous fuel/air mixture had 

accumulated and eventually ignited. A rapidly spreading blue flame is characteristic of a 

premixed flame. The gaseous fuel was generated by pyrolysis of the solid fuel and mixed with 

air diffusing or flowing into the fuel bed. The initial rapid spread of the flames indicates that 

flammable mixtures were present over extensive volumes. Heat and/or free radicals are 

necessary to ignite a premixed fuel/air mixture. The fact that ignitions were observed in areas of 

intense glowing suggests that sufficient heat was present to ignite the locally flammable gas 

mixture. While not conclusive, the observation that transition to flaming occurred in areas of 

intense glowing suggests that glowing is required to ignite flaming in these pyrolyzing fuel beds. 

Once a premixed flame developed, the increased heat release rate rapidly pyrolyzed additional 

fuel, which then burned as a diffusion flame. The appearance of yellow flames, due to soot 

formation and subsequent irradiance from within the flame, is indicative of diffusional flame 

burning. 

4.2. Shredded Newsprint 

Both heated plate and cone calorimeter ignition experiments were done for this fuel. 

4.2.1. Heated Surface Ignition Results 

Figure 24 shows heated-plate results for the times required for the onset of glowing combustion 

and flaming as functions of surface temperature and wind condition for shredded newsprint. The 

symbols and protocols used in this figure will be used throughout the remainder of the report. 

The wind condition is indicated by both symbol and color as follows: no wind—red circle, low 

wind—green square, and high wind—blue triangle. Filled symbols represent results for glowing 

combustion, while open symbols are used for the onset of flaming. As the temperature was 

lowered, temperatures were eventually reached where neither glowing combustion nor flaming 

were observed after long periods. These experiments were allowed to proceed until it became 

clear (no indication of additional reaction) that ignition was unlikely. Instead of showing the 

actual experimental times, these results are indicated by plotting the appropriate filled symbol 

near the upper temperature axis. 

32
Figure 24. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for shredded newsprint fuel beds applied to a heated plate. Results are 

included for no wind, low wind, and high wind cases. 

While there is some scatter in results, clear trends emerge from the measurements shown in 

Figure 24. As expected, the times required for the appearance of glowing combustion and 

flaming increased as the plate temperature decreased, with data for all of the conditions falling 

on well defined curves. For temperatures around 500 °C glowing ignition appeared within a few 

seconds. With a wind applied the glowing combustion rapidly transitioned to flaming, while 

periods ranging from 26 s to 50 s were required when no wind was present. The time required 

for the appearance of glowing combustion increased to more than 400 s at temperatures around 

340 °C. One of the three tests around this temperature did not transition to flaming, but the other 

two did, requiring 38 s and 137 s after glowing first appeared. A review of the data shows that 

for the 30 experiments where glowing was observed without an applied wind, only four failed to 

transition to flaming. These four had heated plate temperatures of 342 °C , 375 °C, 429 °C, and 

469 °C. The wide temperature range suggests that the failure to transition was somehow 

dependent on a property of the fuel and not directly related to the surface temperature. 

For temperatures between 380 °C and 500 °C the effect of applying a wind was to reduce the 

times required for glowing ignition to appear as compared to cases without wind. These 

reductions approached 50 % for the lower part of this temperature range. The reductions were 

greater with a high wind than with a low, though the reductions were smaller than those between 

the no-wind and low-wind cases. 

33
The time required for transition to flaming was much shorter with wind applied than for the nowind cases. In all cases with an applied wind, if the shredded newsprint began to glow, it 

transitioned to flaming in a short period of time. 

When the heated plate temperature was lowered to around 350 °C glowing combustion was no 

longer observed in the presence of the high wind, but ignition still occurred for the low wind and 

no wind conditions. Even though glowing combustion did not develop, there were indications 

that some pyrolysis of the fuel did occur. Smoke was observed rising from the fuel bed within a 

few seconds of being placed on the plate. However, after a short period of time the smoke died 

off, and there was no additional indication of reaction. 

As a test, when it was apparent that no further visual signs of reaction were present, the wind was 

turned off with the fuel bed still on the plate during one of the tests around 350 °C. The fuel 

began to smoke immediately, and flames appeared quickly when the wind was turned back on 

after two minutes. These observations indicate that for plate temperatures around 350 °C the 

high wind within the fuel bed convectively cooled the fuel to a point where it could not sustain 

glowing combustion. 

As the plate temperature was lowered from around 350 °C to around 340 °C, a similar behavior 

was seen with low wind flows, with only one of the three tests igniting at the lower plate 

temperature. As for the high-wind cases around 350 °C, the fuel bed began to smoke again when 

the wind was turned off. In one case, glowing combustion was observed when the wind was 

turned back on after a couple of minutes. 

Shredded newsprint placed on the heated plate at 333 °C and exposed to the low wind also failed 

to develop glowing combustion, but light smoke did appear shortly after the fuel was applied and 

then dissipated and died off. Figure 25 shows photographs of the bottom and top of this fuel bed 

after removal from the plate. Inspection shows that the shredded newsprint was heavily 

blackened on the downwind side of fuel bed base and that there was a light brown band evident 

across the top of the fuel bed perpendicular to the wind flow about 3⁄4 of the way across the bed 

from the upstream edge. The absence of blackening on the upstream portion of the fuel bed base 

provides additional support that the air flow cooled the fuel sufficiently that pyrolysis could not 

take place, while further downstream the cooling prevented the development of smoldering from 

the pyrolysis region in immediate contact with the heated plate. The brown band on the top is 

located at the location where smoke was observed coming from the fuel bed. It may be due to 

either deposited smoke coming from below or light pyrolysis of the fuel by the heated winddriven plume passing through the fuel bed. 

At temperatures around 340 °C glowing combustion was observed for all three experiments run 

without an applied wind. There was considerable scatter in the times required for glowing to 

appear. When the temperature was lowered an additional 10 °C glowing combustion did not 

develop, even though some smoke was released and blackening of the fuel bed base occurred. 

This blackening is evident in Figure 26, which shows the bottom and top of the shredded 

newsprint after removal from the plate held at 332 °C. The blackened newsprint extends across 

the bottom of the fuel bed except for the outer edges. On the top there is a roughly circular area 

that is browned. The degree of darkening and the diameter of the circle were found to increase 

34
Figure 25. Bottom (left) and top (right) views of a shredded newsprint fuel bed are shown after 

the fuel was removed from the heated plate held at 333 °C with a low wind applied. 

Glowing combustion was not observed. 

Figure 26. Bottom (left) and top (right) views of a shredded newsprint fuel bed are shown after 

the fuel was removed from the heated plate held at 332 °C with no wind applied. 

Glowing combustion was not observed. 

with fuel depth. Similar to the low wind case, this behavior suggests that the browning resulted 

from soot deposition from or light pyrolysis due to the thermal plume within the porous fuel bed. 

These images reveal that even though the plate surface temperature was high enough to cause 

some pyrolysis of the fuel in immediate contact, it was insufficient to allow self-sustained 

smoldering to develop and did not provide sufficient heat to ignite glowing combustion. 

35
Figure 27. Values of time to glowing or flaming ignition and maximum observed heat release 

rate are plotted as functions of applied heat flux for shredded newsprint fuel beds. 

4.2.2. Radiative Heating Ignition Results 

The ignition behaviors of shredded newsprint fuel beds subjected to various applied heat flux 

(AHF) levels of thermal radiation were characterized using the cone calorimeter. The most 

direct indicator of ignition behavior dependence on AHF is the time required for flaming 

ignition. Figure 27 summarizes the time to glowing or flaming observations for shredded 

newsprint. Neither glowing nor flames appeared with very low AHFs. These experiments are 

indicated by the values plotted near the upper AHF axis, actual experimental times were usually 

longer. The heat release rate (HRR) behavior is widely considered the most important fuel 

flammability parameter. Maximum observed HRRs are plotted as a function of AHF in Figure 

27. 

For AHFs ranging from 20 kW/m2 to 50 kW/m2 times to ignition decreased slowly from about 

20 s to around 4 s with increasing AHF. For AHFs below 20 kW/m2

 ignition times began to rise 

rapidly with decreasing AHF, approaching 350 s for 11 kW/m2. Flaming ignition did not occur 

for AHFs around 10 kW/m2

, but glowing ignition was observed, requiring between 250 s and 

400 s. Glowing combustion was not observed when the AHF was decreased to 7.5 kW/m2

. 

Maximum HRRs showed a comparable dependence on AHF. For AHFs between 20 kW/m2 and 

50 kW/m2

 maximum HRRs increased slowly with increasing AHF. This is the expected 

behavior and is commonly observed in cone calorimeter experiments since the rate of fuel 

pyrolysis is expected to decrease at lower AHFs. For AHFs below 20 kW/m2 the maximum 

HRRs dropped faster, approaching the noise floor of the experiment for an AHF of 7.5 kW/m2

. 

Additional details concerning the response of shredded newsprint to a radiative heat flux can be 

obtained by considering the time behaviors of the HRR and mass loss behavior. Figure 28 shows 

plots of these variables as a function of time for three experiments with AHFs of 50 kW/m2

. The 

results show that the HRRs began to increase immediately, reaching maximum values in 16 s, 

while the samples mass loss began with the application of the heat flux. These observations are 

36
Figure 28. Heat release rates and sample masses are plotted as a function of time for three 

experiments with a 50 kW/m2

 heat flux applied to shredded newsprint fuel beds. 

Figure 29. Heat release rates and sample masses are plotted as a function of time for three 

experiments with a 25 kW/m2

 heat flux applied to shredded newsprint fuel beds. 

consistent with rapid flaming observed with this AHF. Most of the samples HRR and mass loss 

were complete after 90 s. 

Three similar plots are shown in Figure 29 for an AHF of 25 kW/m2

. The major difference 

between Figure 28 and Figure 29 is the presence of substantial induction periods lasting 

approximately 11 s for the 25 kW/m2 AHF cases during which there were low mass loss rates 

and extremely low HRRs. The absence of a measurable HRR during the induction periods 

indicates that oxidation and heat generation were not occurring even though fuel moisture 

removal or slow non oxidative pyrolysis, indicated by the mass loss, was taking place. Once 

oxidation and heat release began, as evidenced by a measurable HRR, the HRR grew rapidly, 

and flaming was observed within a short period of time, similar to the behavior observed with 

the 50 kW/m2 AHFs. This suggests that once smoldering, and likely glowing combustion, 

developed there was a rapid transition to flaming. 

37
Figure 30. Heat release rates and sample masses are plotted as a function of time for two 

experiments in which a 15 kW/m2 heat flux was applied to shredded newsprint. 

Figure 30 shows similar plots of HRRs and sample masses for two experiments with an AHF of 

15 kW/m2. The observed data differ markedly from those with higher AHF values described 

above. As for an AHF of 25 kW/m2

, there is an induction period following the application of the 

heat flux during which mass loss is observed, but there is no measurable HRR. For 15 kW/m2

this period lasted about 75 s. After this time the HRR began to increase slowly for both 

experiments, and there is a marked increase in the mass loss rate. The relatively small values 

and initial slow growth of the HRRs suggest that smoldering had developed and was spreading 

within the fuel beds. 

The two experiments had different behaviors once smoldering developed. For one, the HRR 

seemed to grow more quickly, and there was a distinct transition to flaming roughly 25 s after 

smoldering developed that resulted in a sudden large increase in HRR by nearly 100 kW/m2, 

with a substantial increase in the mass loss rate at the same time. For the second experiment the 

smoldering phase lasted much longer (roughly 75 s), and the HRR due to smoldering was much 

higher. When flaming did develop around 150 s, the HRR increase and overall peak values were 

considerably reduced compared to the other case. Apparently, the longer smoldering time 

resulted in a reduction in the rate of fuel pyrolysis during flaming as well as the fuel available to 

support flaming. The mass loss behavior also reflects this difference, and the mass loss rate was 

lower during flaming for the second experiment. 

For AHFs as low as 11 kW/m2

 the HRR and mass behaviors were similar to those in Figure 30. 

For 11 kW/m2

 there was an induction period that lasted roughly 200 s, followed by a period of 

smoldering during which the HRR and mass loss slowly increased with time, followed by a short 

flaming period that appeared roughly 125 s after smoldering first started. It is the sum of the 

induction period followed by the smoldering period that combine to give the observed flaming 

ignition times. 

As seen in Figure 27, flaming ignition was not observed for AHFs of 10.5 kW/m2

 or less. HRR 

measurements with AHF around 10 kW/m2 did show that there was still an induction period 

during which mass loss took place, but there was no measurable HRR. The induction period was 

38
Figure 31. Heat release rates and sample masses are plotted as a function of time for an 

experiment in which a 10.5 kW/m2

 heat flux was applied to shredded newsprint. 

followed by a period during which smoldering developed and was marked by an initially 

increasing HRR and accelerated mass loss rate. Figure 31 shows an example of this behavior 

with an AHF of 10.5 kW/m2. Note that nearly 300 s was required for smoldering to develop. 

When an AHF of 7.5 kW/m2 was used there was a very small amount of mass loss immediately 

following the application of the heat flux. After this, the mass no longer changed. The HRR 

remained very close to zero over a 600 s period. 

Figure 32 shows the appearance of a newsprint fuel bed after applying 7.5 kW/m2 for several 

minutes. There is some light browning on the top of the bed, but no indication of a self-sustained 

reaction. The non uniform browning may be associated with small spatial variations in the heat 

flux from the cone heater or to variations in cooling associated with natural flow from the heated 

fuel surface. 

4.3. May Tall Fescue 

Both heated plate and cone ignition experiments were done for this fuel. 

4.3.1. Heated Surface Ignition Results 

Figure 33 summarizes the temperature dependence of the times required for glowing combustion 

and flaming following the application of May tall fescue fuel beds to the heated plate. The most 

striking feature of the data is the nearly complete absence of flaming combustion. For the 28 

cases in which glowing combustion was observed, only three resulted in flaming. It should be 

noted that even though a transition to flaming did not occur, the glowing combustion could be 

quite intense, particularly with an applied wind. When flaming did appear, it was weak and short 

lived. There is no apparent temperature dependence for whether flaming occurred or not. The 

flaming behavior for May tall fescue contrasts with that found for shredded newsprint, where 

39
Figure 32. Photograph of the top surface of a shredded newsprint fuel bed following application 

of a 7.5 kW/m2

 heat flux. 

Figure 33. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for May tall fescue grass fuel beds applied to a heated plate. Results are 

included for no wind, low wind, and high wind cases. 

40
flaming combustion to bustion was 

observed. 

Even though there is scat es required for 

conditions. The tim nd 525 °C to periods 

approaching 300 s for temperatur

newsprint, the ignition tim

the shredded newsprint, the low wind cases to high 

wind cases. 

aller than for the shredded 

newsprint. At the lower no wind to 

high wind appears to be greater than

There are other differen

though the sh rass, the May 

tall fescue seems to s es required for 

glowing combustion at a given te r the grass, and noting 

the 

shredded ne

temp

newsprint, com ed, with the 

lowest tem

eratures 

surface ign

im Figure 34

placed on th oldering 

side of fact, the 

rema

igure 35 shows photographs of the bottom and top of a fuel bed following removal from the 

eated plate held at 336 °C with a high wind applied. Glowing combustion was observed at 

ok place in nearly every experiment in which glowing com

ter in the glowing combustion data, it is clear that the tim

ignition fall on three distinct bands corresponding to no wind, low wind, and high wind 

es required for ignition increased from near zero arou

es around 325 °C. Similar to observations for the shredded 

es for a given temperature decreased with increasing wind. Unlike for 

largest reductions occurred on going from

The proportional change in going from no wind to low wind seems sm

 temperatures the reduction in ignition times on going from

 a factor of two for the grass. 

ces between the May tall fescue and shredded newsprint results. Even 

redded newsprint transitions to flaming much more readily than the g

molder more easily. This can be seen by comparing tim

mperature, which are generally lower fo

that glowing combustion develops for lower plate temperatures (as low as 310 °C) than for 

wsprint (as low as 340 °C). 

Another difference between the shredded newsprint and May tall fescue concerns the lowest 

eratures for which smoldering or flaming was observed for the three wind conditions. For 

bustion appeared at lower plate temperatures as the wind was decreas

peratures capable of igniting the newsprint being observed for the no-wind condition. 

The opposite is true for the data shown in Figure 33, with the lowest ignition temp

observed with wind present. This observation suggests that for the low temperature end of the 

ition curve the development of glowing combustion for May tall fescue is more 

sensitive to the amount of air supplied to the fuel surface than is shredded newsprint, since the 

additional air effectively counteracts the additional cooling resulting from the air flow. 

The appearance of the fuel beds following an experiment provides additional insights into the 

portance of air flow on the reaction behavior of the May tall fescue fuel beds. 

shows bottom and top views of a fuel bed that had developed glowing combustion after being 

e heated plate held at 341 °C without an applied wind. It is clear that the sm

spread through the entire bed. There is a narrow region of unburned fuel around the three edges 

of the fuel bed with wire-screen extending below the plate. No unburned fuel is evident on the 

 the cage with the narrow open slit at the base of the wire-screen side wall. In

ining fuel at this location seemed to be grayer, suggesting more complete combustion. 

These observations agree with the earlier discussion that the air inflows on the cage side wall 

with the open slit differs from that through the sides with full wire-screen sidewalls. 

F

h

41
Figure 34. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after the 

fuel was removed from the heated plate held at 341 °C with no wind applied. 

Glowing combustion was observed after 235 s. 

Figure 35. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after th

fuel was removed from the heated plate held at 336 °C with a high wind applied. 

Glowing combustion was observed after 134 s. 

e 

effects of the air flow are marked. The fuel bed has three distinct bands that run perpendicular to 

134 s following application to the plate and was allowed to proceed for a period of time. The

42
Figure 36. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after the 

fuel was removed from the heated plate held at 311 °C with no wind applied. 

Glowing combustion was not observed. 

e wind direction and extend from the top to the bottom of the bed. There is an area of 

pparently, cooling associated with the air flow 

ng combustion into this area. Near the center 

lete com

intense reaction due to a greater availability of ai

tall fescue is simply blackened, suggesting incomp

pattern is typical of those observed with an applied wi

upstream portion was narrower, and the gray band

It was noted above that glowing combustion for the May tall fe

°C in the absence of a wind. However, the appear

heated plate at lower temperatures indicates th

much lower plate temperatures. Figure 36

at 311 °C without an applied wind. The bottom of

entire extent. On the top there is circle of sim

unburned grass. Such blackening is

observations indicate that a non glowing smolder 

upward to the top surface of the fuel bed. The mo

For the low ind, 302 °C, the fuel bed had a 

milar appearance to that seen in Figure 36. 

th

unburned grass on the upstream side of the bed. A

was sufficient to prevent propagation of the glowi

there is a grayish area indicating more comp bustion. This is likely the result of more 

r. On the downstream side of the fuel bed the 

lete reaction of the fuel. This type of burning 

nd, but with the low wind the unburned 

 was located closer to the upstream edge. 

scue was not observed below 340 

ance of the fuel beds following exposure to the 

at non glowing smoldering was still taking place at 

 shows the bottom and top of a fuel bed that was held 

 the fuel bed is heavily blackened across its 

ilar blackened material surrounded by a band of 

 not expected in the absence of fuel surface oxidation. These 

wave started near the heated plate and passed 

st likely reason for the area of unburned grass 

around the outside of the top surface is that this grass was cooled by air being entrained in the 

thermal plume rising from the heated surface and smoldering fuel. 

est plate temperature tested without an applied w

si

43
Figure 37. Bottom (left) and top (right) views of a May tall fescue fuel bed are shown after the 

fuel was removed from the heated plate held at 311 °C with a low wind applied

Glowing combustion was not observed. 

. 

n 

, 

 With one exception, flaming 

ure 

aming combustion to appear following application 

f the radiative heat flux as a function of the AHF. Similar to the results for shredded newsprint 

ee Figure 27), the ignition times increased very slowly as the AHF was reduced from 

Figure 37 shows similar photographs for a fuel bed held at the same surface temperature, 311 °C, 

as in Figure 36, but with a low wind applied. Most of the bottom is blackened with the exception 

of a narrow band along the upwind (right hand side in photograph) edge. On the top surface of 

the fuel bed there is a brown band that extends across the bed perpendicular to the wind directio

and just downstream of the center. Deeper within the bed the band is much darker. Clearly there

had been some pyrolysis of the fuel in contact with the plate and in the vicinity of the line plume 

formed by heated gases rising within the fuel bed. Smoldering does not seem to have occurred

since there is no indication that areas of pyrolysis moved away from the heated areas. 

Tests with low and high winds with plate temperatures near 300 °C showed partial blackening of 

the bottoms of the fuel bed, but there were larger areas of unblackened fuels along the upstream 

edges. For the high wind case, this area extended nearly half way across the bed. For both low 

and high wind cases there were light brown bands across the tops of the fuel bed, with the band 

for the high wind case located further downstream. 

4.3.2. Radiative Heating Ignition Results 

The heated plate experiments for May tall fescue showed that these fuel beds were unlikely to 

transition to flaming, even when glowing combustion was present. 

was only observed in the radiative ignition experiments for AHFs equal to or greater than 40

kW/m2. On the other hand, glowing combustion was observed with much lower AHFs. Fig

38 shows the times required for glowing or fl

o

(s

44
Figure 38. Values of time to flaming or glowing ignition and maximum observed heat release 

rate are plotted as functions of applied heat flux for May tall fescue fuel beds. 

ven though the ignition time plots for shredded newsprint and May tall fescue have similar 

appearances, distinct differences in burning behavior appear in the plots of maximum HRR 

versus ax ined high for AHF as low as 11 

W/m2

, while maximum HRRs for the May tall fescue dropped to low values for AHFs lower 

aming 

Figure 39. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 45 kW/m2

 heat flux was applied to May tall fescue. 

50 kW/m2 to 20 kW/m2. For still lower AHFs the ignition times began to increase rapidly with 

decreasing AHF before ignition was no longer observed with AHFs near 10 kW/m2

 and lower. 

E

AHF. M imum HRRs for the shredded newsprint rema

k

than 40 kW/m2. The abrupt drop in HRR for the May tall fescue around an AHF of 40 kW/m2 is 

due to the absence of flaming combustion at lower AHFs. 

Plots of HRR and fuel mass versus time provide additional insights into the ignition behaviors of

the May tall fescue fuel beds. Figure 39 shows three sets of results for an AHF of 45 kW/m2

. 

After a brief period during which there was a relatively slow loss of fuel mass and low HRR, 

both the HRRs and mass loss rates increased rapidly, reflecting the onset of flaming. Fl

45
Figure 40. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 35.2 kW/m2 heat flux was applied to May tall fescue. 

after the flaming period since measurable HRRs 

Flaming was not observed when an AHF of 35.2 kW

Figure 40 shows HRRs and masses for three experim

induction period lasting about 10 s during which there is little

place. After this time, the HRRs began to increas

kW/m2. HRR values remained nearly constant for 

The fuel mass loss rate abruptly accelerated at th

near the time when glowing combustion first 

spreading over the fuel bed is responsible for 

the same time that the HRR began to drop slowly th

ass loss, with the mass loss rate decreasing abruptly. 

s 

rate, it is cle . These 

bservations suggest that at least two types of fuel surface oxidation reactions were taking place. 

lasted for about 50 s to 60 s before dying down. Apparently, smoldering combustion continued 

and low mass lost rates were still present. 

/m2

 was applied to the May tall fescue. 

ents. Similar to Figure 39, there is a brief 

 HRR, but mass loss is taking 

e rapidly, rising to values on the order of 32 

about 50 s before beginning to fall slowly. 

e same time the HRR first begins to rise. This is 

appeared, suggesting that glowing combustion 

the increased HRR and mass loss rate. At roughly 

ere was a distinct change in the slope of the 

m

When the effective heat of combustion (EHC) is defined as the HRR divided by the mass los

ar that the EHC abruptly increased at the time when the mass loss rate drops

o

The first has a relatively low EHC and likely corresponds to oxidative surface reaction of easily 

pyrolysed fuel components. Initial pyrolyis of cellulosic fuels is known to form an enrichedcarbon char that has a higher EHC that reacts more slowly. The oxidation of the char formed 

during the initial more rapid oxidative pyrolysis of the grass is most likely responsible for the

period of relatively high HRR with relatively low mass loss rate. 

In Figure 39 it is evident that the HRR and mass loss behavior following the flaming periods are 

similar to those observed at the longest times in Figure 40. This suggests that the flaming 

observed for the May tall fescue with the highest AHFs also formed a high energy containing 

char that then oxidized more slowly. 

Comparison of the HRR and mass loss behaviors observed for AHFs over the range of 20 kW/m2

to 35 kW/m2

 showed that they were similar to those in Figure 40 with some variation in slopes 

46
Figure 41. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 15.3 kW/m2

 heat flux was applied to May tall fescue. The 

arrows indicate the times when glowing combustion appeared. 

nd transition times. The relatively short times required for glowing combustion to develop for 

n 

er 

ns are consistent with findings from the 

eated plate experiments that showed that non glowing smoldering occurred at lower 

re 

h the HRR remained close to 

ero but during which there was a slow mass loss. After the induction period the HRR began to 

wing 

ss 

tained HRR and reduced mass loss rate at longer times 

gain indicates that an initial pyrolysis produced a char that then oxidized with a higher effective 

a

these AHFs (see Figure 38) suggest that glowing combustion rapidly developed and spread over 

the fuel bed, leading to the formation of chars that then oxidized more slowly. 

For AHFs less than 20 kW/m2 the time required for glowing combustion to develop rapidly 

increased. Figure 41 shows three sets of measured HRRs and masses as a function of time for a

AHF of 15.3 kW/m2

. Color-coded arrows have been added to the plots indicating the times 

when glowing combustion was observed. The data show that low HRRs developed shortly aft

the fuel was exposed to the AHF. Relatively slow mass loss rates appeared around the same 

time. Around 50 s after exposure there was a clear shift in the mass behavior, with the mass loss 

rate increasing. This change is most likely associated with the development of sustained 

smoldering. Interestingly, glowing combustion did not appear until about 20 s after smoldering 

developed. This suggests that the initial smoldering is non glowing and that transition to 

glowing required a short period of time. These conclusio

h

temperatures. 

The observation of nearly constant HRRs while there is a distinct reduction in the mass loss rate 

around 175 s indicates that two distinct EHCs occurred for this lower AHF. This behavior is 

similar to that observed with AHFs covering the 20 kW/m2

 to 35 kW/m2 AHF range. 

Figure 42 shows similar plots for three experiments with an AHF of 13.5 kW/m2

. For each the

was a short induction period at the start of the heating during whic

z

rise, but the increases were much slower than when 15.3 kW/m2 was used. At roughly 75 s there 

was a distinct increase in the mass loss rate suggesting that smoldering had developed. Glo

combustion was not observed until much later when a significant fraction of the more rapid ma

change had already occurred. The sus

a

47
Figure 42. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 13.5 kW/m2 heat flux was applied to May tall fescue. T

arrows indicate the times when glowing combustion appeared. 

he 

tion. The much longer periods required for each of these stages as compared to 

ases with AHFs of 15.3 kW/m2

 requires a strong dependence on AHF. 

2

. 

 an AHF 

f 7.5 kW/m2. After the heat fluxes were applied there were long periods lasting several hundred 

Figure 43. Heat release rates and sample masses are plotted as a function of time for two 

experiments in which a 7.5 kW/m2 heat flux was applied to May tall fescue. 

heat of combus

c

Glowing combustion was not observed for experiments with AHFs of 10 kW/m2

 and 7.5 kW/m

Figure 43 shows plots of HRR and mass as functions of time for two experiments with

o

seconds during which a very slow mass loss occured without significant HRR. The mass lost 

could be due to non oxidative pyrolysis or moisture removal. During one of the experiments the

mass loss rate accelerated around 400 s. There seems to have been a very small increase in the 

measured HRR at the same time, suggesting limited oxidation was taking place. Interestingly, 

both the HRR and mass loss rate seemed to decrease back to the lower levels after about 100 s, a 

time when a large fraction of the original mass remained. This suggests that even though some 

48
Figure 44. This slightly out of focus photograph shows the top surface of a May tall fescue fuel 

diative heat flux that was insufficient to 

stained smoldering. There is a hint that the 

e after 600 s, just as the experiment ended. 

ilar to those for 7.5 kW/m2 with a nearly linear mass 

ed mass loss rate lasting about 100 s. There 

ped at roughly the same time as the change in 

bed following application of an applied ra

induce glowing combustion. 

non glowing oxidation took place, there was no su

HRR for the second run may have started to increas

Results with AHFs of 10 kW/m2 were sim

change over the first 200 s followed by an increas

seemed to be a very small HRR that only develo

mass lost ra

ome blackening of the grass was observed at the lower AHFs. Figure 44 shows an example of 

Some 

over 

 the fuel bed that is similar to that seen for the shredded newsprint in Figure 32. As 

te. After 600 s about 20 % of the original mass remained. 

S

a fuel bed at the conclusion of an experiment in which glowing combustion did not occur. 

blackening of the surface is evident. The blackening is not uniform, but has a distribution 

the top of

before, this distribution could be due to the air flow distribution or small variations the cone 

thermal radiation distribution. 

4.4. August Tall Fescue 

Only heated plate ignition experiments were performed for August tall fescue. Figure 45 shows 

the measured ignition times for glowing combustion and flaming as a function of the heated plate

temperature. The results will be compared with the corresponding results for May tall fescue 

shown in Figure 33. 

49
Figure 45. n as a function of 

flaming tim les 

exposed to the wind tran se flames were robust and lasted for many 

evelop along the upstream edge with an applied wind is also visible in this image. 

ven though flaming was more likely for August tall fescue than for May tall fescue, the August 

the 

e glowing combustion was observed for temperatures of 370 °C 

nd higher, while it was absent for temperatures of 360 °C and lower, suggesting a 30 °C 

difference b e lowest temperatures required to induce glowing 

com

Ignition times for glowing combustion and flaming are show

temperature for August tall fescue grass fuel beds applied to a heated plate. Results

are included for no wind, low wind, and high wind cases. 

The most dramatic difference between the two fuels is that the August tall fescue was much more

likely to transition to flaming in the presence of a wind. Only three such transitions with limited 

es were observed with the May tall fescue, while all of the August tall fescue samp

sitioned to flaming. The

seconds. Figure 46 shows an example of the flames for an experiment with the heated plate held 

at 310 °C in the presence of the low wind. Flames were observed 289 s after the fuel was 

applied to the plate and the photograph was taken 5 s later. The strong glowing that tended to 

d

The transition to flaming for August tall fescue required the presence of a wind. Flames were 

not observed for the five experiments without wind in which glowing combustion developed. 

E

tall fescue required higher heated plate temperatures to induce glowing combustion in the 

absence of wind. With the one exception of a measurement with the plate temperature held at 

380 °C, glowing combustion for the May tall fescue was observed for plate temperatures at or 

above 340 °C and was absent for plate temperatures around 320 °C. Both responses were 

observed with the plate temperature held near 330 °C. For the August tall fescue, again with 

exception of one experiment, th

a

etween the two grass samples for th

bustion when wind was absent. 

50
Figure 46. Photograph showing flaming from an August tall fescue fuel bed on the heated plate 

held at 310 °C in the presence of a low wind. Flames appeared 289 s after the fuel 

was placed on the plate, and the photograph was taken 5 s later. Strong glowing 

combustion is present along the upstream edge of the fuel bed. 

Despite the fact that the transition to flaming behaviors for the two grasses were very different in

e of a wind, the dependencies of glowing combustion ignition times on the plate 

 were similar. For both the lowest plate temperature for w

the presenc

temperature hich glowing combustion 

eveloped in the presence of a low wind was around 310 °C, and the times required for ignition 

ded 

e, 

a low wind. 

or the May tall fescue glowing combustion, ignition was observed for plate temperatures as low 

t. 

g 

 

d

were similar, with roughly 275 s being required around 310 °C. The data for the August tall 

fescue indicate that the glowing combustion ignition times in the presence of a low wind are 

reduced from those in the absence of the wind, in a manner similar to those for the May tall 

fescue. Recall that the effect of the low wind was somewhat less than observed for the shred

newsprint. 

The high wind results for August tall fescue are limited. However, similar to the May tall fescu

the times required for glowing combustion ignition fall somewhat below those with 

F

as 317 °C, while the lowest ignition temperature for August tall fescue was 329 °C. Due to the

limited measurements, it is not possible to determine if this difference is statistically significan

The appearance of the fuel beds following experiments in which ignition was not observed 

provides additional insight in the smoldering of August tall fescue. Even though glowing 

combustion was not generally observed for temperatures below 360 °C without a wind, images

indicate that non glowing smoldering developed and spread partially through the fuel beds with

temperatures as low as 300 °C. Figure 47 shows the bottom and top of a fuel bed after bein

placed on the heated plate at 300 °C. The bottom of the fuel bed is blackened over a large 

fraction of its area. Three narrow bands of unblackened fuel are visible along the edges. The top

51
Figure 47. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 300 °C with no wind applied. No 

glowing combustion was observed. 

fuel bed and

cage. At hi s 

had similar appearan ere larger. 

rs 

e 

of 

 the 

 fuel bed that was placed on the heated plate held at 310 °C with a low wind applied is shown 

e 

hotograph) indicating that some ash formation had occurred. This suggests that complete 

ind. 

of the fuel bed is also blackened, indicating that smoldering had passed upward through the bed 

even though the smoldering seems to have progressed upward primarily on the right side of the 

 on the side facing the camera where the open slit was present in the wire-screen 

gher plate temperatures where glowing combustion was not observed, the fuel bed

ces, but the fractions of the bottom and top surfaces blackened w

Figure 48 shows the appearance of the bottom and top of an August tall fescue fuel bed after 

removal from the heated plate held at 290 °C without a wind applied. A blackened area cove

much of the bottom of the fuel bed, but on the top the only indication of the heating is a small 

circular brownish area near the center. As discussed earlier, this browning may be due to the 

deposit of smoke in the thermal plume coming from below or to light heating of the fuel by th

thermal plume. In either case, it is clear that smoldering did not propagate upward to the top 

the fuel bed from below. While sufficient to cause pyrolysis of the fuel in direct contact with

heated plate, this plate temperature was too low to generate sustained smoldering. 

A

in Figure 49. This is the same fuel bed shown in Figure 46. Both glowing combustion and 

flaming were observed. Much of the fuel bed is blackened, but there is an area of unburned grass 

on the downstream edge of the bed. This suggests that air entrained into the burning fuel bed 

was able to cool the fuel sufficiently to limit glowing combustion and flame spread. In the imag

showing the bottom of the fuel bed there is a small gray area on the upwind edge (right side of 

p

oxidation of the char formed by the initial fuel pyrolysis was aided by the presence of the w

52
Figure 48. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 290 °C with no wind applied. 

glowing combustion was observed. 

No 

ind applied. 

Figure 49. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown after 

the fuel was removed from the heated plate held at 310 °C with a low w

Glowing combustion and flaming were observed. 

The appearance of fuel beds that did not develop glowing combustion in the presence of a wind 

provides evidence for the important role that convective cooling played in hindering the 

development of smoldering at low plate temperatures. Figure 50 shows the bottom and top of 

the fuel bed following application to the heated plate held at 320 °C in the presence of a high 

wind. On the bottom there is a band of blackened fuel perpendicular to the wind direction that 

53
Figure 50. Bottom (left) and top (right) views of an August tall fescue fuel bed are shown a

the fuel was removed from the heated plate held at 320 °C with a high wind applied. 

fter 

No glowing combustion was observed. 

 side starting near the center of the bed. This 

nd passing through the upstream

air entrained into the resulting thermal plume from

the fuel to a level where significant pyrolysis coul

only a very narrow band of browning due either 

photographs show that significant smolderi

The fuel beds for the cases with low wind fo

similar appearances to those for the high wind case shown in 

blackening on the bottoms of the fuel surfaces and the degree of

the heated plate temperature. 

4.5. Cheat Grass 

Both heated plate and cone calorimeter ignition experim

Figure 51 sh

temperature for cheat grass fuel beds. As observed for the other fuels discussed thus far, even 

ough there is scatter in the experimental results, the data fall on three broad bands 

m

occurs for a

w-wind re

covers about 1⁄4 of the bed width on the downstream

distribution suggests that the wi side of the cage as well as the 

 the downstream side were effective in cooling 

d not take place. The top of the fuel bed shows 

to smoke deposition or light pyrolysis. These 

ng did not develop within the fuel bed. 

r which glowing combustion was not observed had 

Figure 50. The area subject to 

 browning at the top varied with 

ents were done for this fuel. 

4.5.1. Heated Surface Ignition Results 

ows a plot of times to glowing combustion and flaming as a function of heated plate 

th

corresponding to no wind, low wind, and high wind cases, with the ignition times increasing as 

the plate te perature decreases. Considering the results for glowing combustion, when ignition 

 given plate temperature, the data for no wind lies at longer times, followed by the 

lo sults, with the shortest times required for the high-wind cases. 

54
Figure 51. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for cheat grass fuel beds applied to a heated plate. Results are included

The general ansition to flaming was the same as 

bserved for the August tall fescue. Flaming was not observed when a wind was not applied to 

as reduced 

om around 380 °C to around 360 °C. However, the lowest temperatures for which glowing 

ting the tendency for cheat grass to develop glowing combustion 

erably lower than for the May tall fescue. 

The appearance of cheat grass fuel beds for which glowing combustion did not develop in the 

the tendency for the cheat grass fuel beds to 

cue. 

Figure 52 bustion when 

placed fuel bed is heavily blackened 

t 

for no wind, low wind, and high wind cases. 

 behavior of the cheat grass with regard to tr

o

the fuel bed, and transition to flaming occurred for all experiments in which glowing combustion 

developed in the presence of low or high wind. 

More detailed comparison with the results for August tall fescue suggests that cheat grass may 

have a slightly reduced tendency to develop glowing combustion. For both fuels the results 

suggest that glowing combustion no longer developed when the plate temperature w

fr

combustion were observed for the cheat grass were around 340 °C and 350 °C for low and high 

winds cases, respectively. The corresponding values for August tall fescue were 310 °C and 330

°C. Recall that these temperatures for the August tall fescue were already higher than observed 

for the May tall fescue, sugges

is consid

absence of a wind provides additional evidence that 

smolder (either non glowing or glowing) was reduced compared to the August tall fes

 shows photographs for a fuel bed that did not develop glowing com

on the heated plate held at 341 °C. The bottom of the

indicating significant pyrolysis took place. On the top of the fuel there is a darkened circular 

area near the center that indicates that non glowing smoldering spread upward through the fuel 

bed. This darkened area is surrounded by fuel that is not discolored and the darkened area is no

55
Figure 52. 

as observed for plate 

The appeara evelop glowing combustion in the 

resence of low and high winds was similar to those already described for other fuels, with 

 the 

asured for the May tall fescue, even though 

n 

Bottom (left) and top (right) views of cheat grass fuel bed are shown after the fuel 

was removed from the heated plate held at 341 °C with no wind applied. No 

glowing combustion was observed. 

as blackened as the bottom of the fuel bed, suggesting that smoldering was barely sustained. 

Similar fuel bed photographs for the heated plate held at 331 °C show that even though the fuel 

was pyrolyzed on the bottom, smoldering did not reach the top surface, which was barely 

browned near the center. For August tall fescue, non glowing smoldering w

temperatures down to 300 °C, but not for 290 °C. 

nce of the cheat grass fuel beds that did not d

p

blackening on the bottom primarily on the downstream side and a narrow brown band across

top perpendicular to the wind flow direction. Figure 53 shows an example of this for a heated 

plate temperature of 350 °C in the presence of a low wind. Once again the cooling effect of an 

applied wind has been shown to be important for the fuel reaction behavior at low heated plate 

temperatures. 

4.5.2. Radiative Heating Ignition Results 

Figure 54 shows plots of the times required for glowing or flaming ignition and maximum HRRs

as functions of AHF for the cheat grass. Similar to the results for May tall fescue (see Figure 

38), flaming was only observed for the highest AHFs used, but glowing combustion was seen for 

AHFs extending down to approximately 10 kW/m2

. The maximum values of HRR measured for

flaming cheat grass were roughly 80 % of those me

the initial mass of cheat grass was only 11 % less (8.0 g versus 9.0 g). Quantitative comparison 

shows that slightly more heat was released by the smoldering May tall fescue for given AHFs i

the 10 kW/m2

 and 35 kW/m2 range. 

56
Figure 53. Bottom (left) and top (right) views of a cheat grass fuel bed are shown after the f

was removed from the heated plate held at 350 °C with a low wind applied. No 

glowing combustion was observed. 

uel 

 no wind was applied, but that the grasses displayed very different transition to 

aming behaviors when wind was present. Since the cone calorimeter results are similar for the 

two fuels, it seems not sensitive to fuel behaviors 

hat dep ng

Figure 54. Values of time to flaming ignition and maximum observed heat release rate are 

plotted as functions of applied heat flux for cheat grass. 

The heated plate ignition experiments indicated that cheat grass and May tall fescue had similar

behaviors when

fl

 likely that radiative heating experiments are 

t end stro ly on the presence of wind. 

Figure 55 shows the time dependencies of HRR and sample mass measured for an AHF of 45 

kW/m2

. Figure 39 shows corresponding results for May tall fescue. The general behaviors are 

quite similar for the two grasses. 

57
Figure 55. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 45 kW/m heat flux was applied to cheat grass. 2

ormed and then slowly oxidized. 

set to 10 kW/m2. All three had HRRs and fuel variations

the May tall fescue with an AHF of 7.5 kW/m2

periods lasting about 200 s during which the mass 

followed by periods during which the ma

about 10 kW/m2. These observations suggest that non 

these times. Glowing combustion was observed for 

loss rate had decreased, indicating that the char fo

4.6. Fine Florida Grass 

plate temperature for no, low, and high wind cond

es and the high wind 

ases the shortest times. The reduction in ignition times was largest between the no wind and 

ith one exception, flaming was not observed in the absence of wind. In contrast, flaming 

transition from glowing combustion to flaming on wind condition is similar to those for August 

Results for HRR and mass loss versus time for experiments with glowing cheat grass had similar

time dependences to those for the May tall fescue shown in Figure 40 to Figure 42. Note that 

this indicates that smoldering cheat grass also has two distinct effective heats of combustion, 

implying that high energy char was f

Glowing combustion was observed for only one of three cheat grass samples run with the AHF 

 with time similar to those observed for 

 shown in Figure 43. There were long induction 

decreased but no heat was released. This was 

ss loss rate was noticeably faster and the HRR rose to 

glowing smoldering was taking place at 

one sample at 417 s. By this time the mass 

rmed earlier was being oxidized. 

Only heated plate ignition experiments were performed for the fine Florida grass. Figure 56

shows the times required for glowing combustion and flaming to appear as functions of heated 

itions. The results for the different wind cases 

fall on three distinct curves, with the no wind cases requiring longer tim

c

low wind conditions. 

W

occurred in each experiment with a low or high wind during which an ignition was noted. In 

Figure 56 many of the measurements with wind only show the times when flames appeared. For

most of these experiments glowing combustion was observed prior to the appearance of flames, 

but the time differences were too small to be resolved on the plot. The dependence of the 

58
Figure 56. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for fine Florida grass fuel beds applied to a heated plate. Results are 

included for no wind, low wind, and high wind cases. 

tall fescue (Figure 45) and cheat grass (Figure 51), but the times required for transition are 

generally shorter. These three types of grass all differ from May tall fescue, since it tended to 

not flame for any of the wind conditions. 

Even though the dependence of ignition behavior on wind condition for the fine Florida grass is 

the same as observed for August tall fescue and cheat grass, the lowest plate temperatures 

required for glowing combustion ignition are considerably lower. Ignitions were observed for 

plate temperatures as low as 321 °C, 301 °C, and 310 °C for the no, low, and high wind cases, 

respectively. Corresponding values for August tall fescue and cheat grass were 371 °C, 310 °C, 

and 331 °C and 381 °C, 351 °C, and 340 °C. On the other hand, the lower heated pl

mperature ignition limits for May tall fescue (330 °C, 310 °C, and 317 °C, respectively), which 

ate 

ing behavior, are similar to those for the fine Florida grass. 

ve g Florida grass have similar lower plate temperature limits 

n 

s 

te

had a different transition-to-flam

E n thou h May tall fescue and fine 

for the development of glowing combustion in the absence of wind, comparison of Figure 33 and 

Figure 56 shows that much longer periods were required for the appearance of glowing at a give

plate temperature for the fine Florida grass. This observation, along with the earlier findings that 

non glowing smoldering developed for August tall fescue and cheat grass for plate temperatures 

well below those for which glowing combustion occurred, suggests that each of these grasses i

capable of non glowing smoldering and that the lower plate temperature limit for glowing 

59
Figure 57. 

te 

corners of t

bottom to th blackening at other locations 

n the top surface suggests that the fuel bed was barely able to support smoldering at this heated 

 that the fuel in contact with the plate was 

Bottom (left) and top (right) views of a fine Florida grass fuel bed are shown after 

the fuel was removed from the heated plate held at 321 °C with no wind applied. 

Glowing combustion was observed. 

combustion depends on the ease with which non glowing smoldering transitions to glowing 

combustion. 

Figure 57 shows bottom and top views of a fine Florida grass fuel bed after it was removed from 

the heated plate held at 321 °C. A large fraction of the fuel bed is blackened on the bottom 

showing that the grass in contact with the heated plate was pyrolyzed. Two areas at opposi

he fuel are blackened on the top. This indicates that smoldering passed from the 

e top of the fuel bed at these locations. The absence of

o

plate temperature. Even so, glowing combustion was observed during this experiment. 

The corresponding photographs for a fuel bed placed on the heated plate at 310 °C are shown in

Figure 58. The blackening on the bottom shows

pyrolyzed. Unlike for the higher plate temperature cases shown in Figure 57, the top of fuel bed 

is only lightly browned in the center showing that smoldering for this fuel bed did not progress to

the top from the bottom surface. The light brown area is either due to deposited smoke or 

limited pyrolysis due to the thermal plume that developed within the fuel bed. 

The observations above suggest that if non glowing smoldering developed in these fine Florida 

grass fuel beds, it would transition to glowing combustion. This was not the case at the lower 

heated plate temperatures for the other grasses discussed above, for which non glowing 

combustion was observed without transition to glowing combustion. 

The tops and bottoms of fuel beds for which glowing combustion was not observed in the 

presence of a wind had similar appearances. Figure 59 shows an example for the heated plate 

60
Figure 58. Bottom (left) and top (right) views of a fine Florida grass fuel bed are shown after 

the fuel was removed from the heated plate held at 310 °C with no wind applied. 

Glowing combustion was not observed. 

Figure 59. Bottom (left) and top (right) views of a fine Florida grass fuel bed are shown after 

over the fuel and 

e thermal plume formed by the rising heated gases seemed to cool the fuel and limit reaction at 

the fuel was removed from the heated plate held at 290 °C with a low wind applied. 

Glowing combustion was not observed. 

held at 290 °C with a low wind. A rectangular area of the bottom fuel surface is blackened, with 

unblackened bands visible along the upstream (wider band on right side of image) and the 

downstream edges. As discussed earlier, convective cooling due to wind flow 

th

61
Figure 60. Ignition times for glowing combustion and flaming are shown as a function of 

luded 

unaffected b

ght pyrolysis by the thermal plume. 

4.7. Pine Needles 

Both heated plate and cone ignition experiments were done for this fuel. 

4.7.1. Heated Surface Ignition Results 

The times required for the appearance of glowing combustion and flaming are plotted as a 

function of heated plate temperature for pine needle fuel beds exposed to no wind, low wind, and 

high wind in Figure 60. Results for the three wind conditions fall on well defined curves with 

the data with no wind requiring longer times for a given plate temperature and the measurements 

with a high wind requiring the shortest periods. The relative separations of the curves are 

somewhat less than observed for the grasses discussed above, but similar to those for shredded 

newsprint (see Figure 24). 

temperature for pine needle fuel beds applied to a heated plate. Results are inc

for no wind, low wind, and high wind cases. 

the upstrea and downstream edges. The top of the fuel bed appears to be completely 

y the heating except for a narrow light brown band running perpendicular to the 

of the center. This band is 

m

wind flow direction just downstream 

li

either due to deposited smoke or 

62
Figure 61. Bottom (left) and top (right) views of a pine needle fuel bed are shown after the fuel 

was removed from the heated plate held at 290 °C with no wind applied. No 

glowing combustion was observed. 

There are several differences between the pine needle results and those for the grass samples 

discussed earlier. Perhaps the most important is that the transition from glowing combustion to 

flaming was observed for all samples that developed glowing combustion, irrespective of wind 

condition. All of the grasses were unlikely to flame without an applied wind, and the May tall 

fescue resisted flaming even in the presence of a wind. Interestingly, the shredded newsprint 

flaming behavior was similar to that for pine needles. 

In contrast to the grass findings where the fuel beds developed glowing combustion at lower 

plate temperatures when a wind was present, the high wind condition resulted in the highest plate 

temperatures for which the pine needles did not begin glowing. Glowing combustion for the 

pine needle fuel beds no longer occurred in the presence of a high wind for plate temperatures 

between 330 °C and 340 °C. Glowing ignition in the presence of a low wind was not observed 

below temperatures of about 310 °C, while plate temperatures had to fall below 300 °C before 

glowing combustion did not develop when no wind was present. Shredded newsprint is the only 

other fuel that showed a similar dependence on wind, but the range of temperatures over which 

the transition from glowing to non glowing occurred for the three wind conditions was somewhat 

smaller, taking place from about 350 °C to 330 °C. 

Another difference between the shredded newsprint and pine needle fuel beds and the grass fuel 

beds is the time required for glowing combustion to develop at the low ends of the heated plate 

temperature ranges. For the grasses these times ranged between 200 s and 600 s, with the 

Florida fine grass responsible for the longest times. For the shredded newsprint and pine needles 

the maximum periods observed approached 1000 s. 

Figure 61 shows photographs of the bottom and top of one of the pine needle fuel beds after 

removal from the heated plate held at a temperature of 290 °C in the absence of wind. The 

63
Figure 62. Bottom (left) and top (right) views of a pine needle fuel bed are shown after the fuel 

was removed from the heated plate held at 340 °C with no wind applied. No 

glowing combustion was observed. 

bottom of th

with the hea ot spread from the bottom to the top of 

e fuel bed since there is little, if any, discoloration of the fuel on the top surface. Since fuel 

ng 

eds, 

 a fuel bed that did not develop glowing combustion when 

laced on the heated plate held at 340 °C with the high wind. A narrow band on the bottom of 

the fuel bed is blackened on the downstream side. There is little indication of pyrolysis on the 

ps ion that convective cooling of the fuel has limited reaction at 

 the downstream edge. 

hotographs for other low and high wind cases where glowing combustion did not occur have 

similar oration on the tops of the fuel beds 

were m h those discussed earlier for other 

el beds that did not smolder in the presence of wind. 

e 

aming ignition was observed over this entire range of AHF. 

nfortunately, measurements are not available for lower AHFs. 

e fuel bed is blackened, indicating that some pyrolysis of pine needles in contact 

ted plate took place. Smoldering clearly did n

th

beds placed on the heated plate held at temperatures only 10 °C higher developed glowi

combustion and flaming, this suggests that if non glowing smoldering did start in these fuel b

it always transitioned to glowing combustion and ultimately flaming. 

Figure 62 shows the bottom and top of

p

u

th

tream side. This is an indicat

ese locations despite an increased air supply. Smoldering clearly did not develop within the 

fuel bed since the top of the fuel bed shows only a hint of discoloration at

P

 appearances, even though in some the bands of discol

ore pronouced. These observations are consistent wit

fu

4.7.2. Radiative Heating Ignition Results 

Plots of time to flaming ignition and maximum HRR as functions of AHF are included in Figur

63 from cone calorimeter measurements with pine needles. The data span a range of AHFs from

15 kW/m2

 to 50 kW/m2. Fl

U

64
Figure 63. Values of time to flaming ignition and m

plotted as functions of applie

creasing AHF were higher. 

dditional insight into the ignition behavior of pine needles is provided by considering the time 

behaviors of the HRR and mass loss. Figure 64 shows results for three experiments with an AHF 

aximum observed heat release rate are 

d heat flux for pine needles. 

Figure 64. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 35 kW/m2

 heat flux was applied to pine needles. 

Less than 30 s was required for ignition with AHFs equal to or greater than 35 kW/m2, and the 

ignition times varied little with AHF over this range. For AHFs around 30 kW/m2

 the ignition 

times began to increase more rapidly with decreasing AHF. The general dependence of ignition 

times on AHF is similar to those observed for shredded newsprint, May tall fescue, and cheat 

grass, but for these fuels the rapid increases in ignition times began when the AHF was on the 

rder of 20 kW/m2 o , and the rates of increase with de

Even though the dependence of the ignition times on AHF is highly non linear, the values of 

maximum HRR appear to have a nearly linear dependence, falling with decreasing AHF over the

entire AHF range. This behavior is expected for fuels where the pyrolysis rate during flaming 

combustion is proportional to the AHF. 

A

65
Figure 65. Heat release rates and sample masses are plotted as a function of time for an 

experiment in which a 15 kW/m2

 heat flux was applied to pine needles. 

ere 

ime plots for an AHF of 30 kW/m2

 have a similar appearance to those in Figure 64, with the 

nd 

 an AHF of 15 kW/m . 

here was a period of roughly 125 s at the start during which the HRR was very close to zero 

l 

 if glowing combustion developed in the 

one experiments, but observations from the heated plate experiments suggest that non glowing 

smoldering is not likely to play an important role for this fuel. For these fuel beds, transition to 

flaming alue of approximately 20 kW/m2

. 

of 35 kW/m2. Short induction periods were present at the beginning during which there were

relatively low mass loss rates and very little heat release. After the induction period, there w

abrupt increases in the rates of mass loss and the HRRs rose very rapidly to their maximum 

values. Flaming combustion lasted roughly 125 s, before the HRRs and mass loss rates 

decreased rapidly to levels expected for smoldering and then decreased slowly. 

T

exception that the induction periods increased substantially, varying from about 35 s to 50 s, a

the HRRs increased slowly to values on the order of 20 kW/m2 before the rapid increase 

associated with the transition to flaming. As the AHFs were reduced further the induction 

periods continued to increase. 

Figure 65 shows the HRR and mass loss curves for an experiment with 2

T

and mass was being loss relatively slowly. After the induction period the HRR began to increase 

slowly and the slope of the mass loss curve also increased slowly. After about 200 s the HRR 

had increased to roughly 20 kW/m2

, and there was a sudden transition to flaming. 

These data reveal that for low AHFs there are at least three distinct processes involved in the 

ignition of pine needles. The first is a period during which the fuel mass decreases, but there is 

little if any heat release. The mass loss is likely due to drying of the fuel and non oxidative fue

pyrolysis. During the second phase the fuel surface begins to oxidize and surface temperatures 

rise slowly. The increasing surface temperatures lead to more intense pyrolysis since the mass 

loss rates increase and smoldering spreads. It is unclear

c

 seems to take place when the measured HRRs reach a v

66
Figure 66. Ignition times for glowing combustion and flaming are shown as a function of 

temperature for May tall fescue/pine needle mixture fuel beds applied to a heated 

plate. Results are included for no wind, low wind, and high wind cases. 

4.8. May Tall Fescue/Pine Needle Mixture 

Only heated plate ignition experiments were performed for the May tall fescue/pine needle 

mixture. Fi lowing combustion and 

ing plotted as a function of plate temperature for no, low, and high wind cases. The results 

r the different wind conditions fall on well defined curves with ignition times that increase as 

n 

times were 

) 

 lower plate 

gure 66 shows the times required for the development of g

flam

fo

the plate temperature decreases. Generally, for a given plate temperature, the longest ignitio

observed for no wind cases and the shortest are for high wind cases. 

The two fuels in the mixture were chosen because they represented two extremes in the ignition

behaviors observed for the natural fuels considered in this study. May tall fescue (see Figure 33

had a low tendency to flame when placed on a heated plate for any of the wind conditions tested, 

while the pine needles (see Figure 60) transitioned to flaming whenever glowing combustion 

developed. The application of a wind resulted in larger reductions in glowing combustion 

ignition times with a given plate temperature for the May tall fescue. In the presence of a wind

the development of glowing combustion in the May tall fescue was observed for

temperatures than when a wind was not present. The opposite was true for pine needles. The

times required for the appearance of glowing combustion at a given plate temperature were 

typically much longer for the pine needles as compared to the May tall fescue. 

67
Comparison of Figure 66 with Figure 33 and Figure 60 suggests that a 50 %/50 % mixture by 

mass of May tall fescue and pine needles resulted in fuel beds with ignition behaviors that were 

intermediate between those for the individual fuels, but which also had some properties that 

might be considered less desirable than those for the individual fuels alone. 

The results in Figure 66 show that the fuel mixture was much more likely to flame than the May 

tall fescue. Transition from glowing combustion to flaming was observed in all cases with an 

applied wind and in just over half of the cases without wind. It is noteworthy that the tendency 

for the mixture to flame is greater than for all of the grasses tested. For the experiments without 

an applied wind, there is no clear correlation of the appearance of flames with plate temperature, 

suggesting that the probability of transition to flaming for this fuel mixture is related to the 

flammability of the gases generated by pyrolysis and not to the specific local ignition conditions. 

bustion 

required ov e May tall 

scue was between 250 s and 275 s. The ignition time for the mixture was just over 400 s. This 

e 

oldering, and 

ansition to flaming takes place. 

roduced a glowing and/or flaming ignition for all of the fuels 

 

st 

for 

r 

For the lower plate temperatures where both of the indivual fuels developed glowing 

combustion, the ignition times for the mixture tended to lie between those observed for the 

individual fuels, but to fall somewhat closer to those for the May tall fescue. As an example, 

consider the low-wind experiments with plate temperatures around 310 °C. Glowing com

er 600 s to develop for the pine needles, while the comparable time for th

fe

behavior suggests that the May tall fescue first develops non glowing smoldering with the heat

generated by the surface oxidation being transferred to both non smoldering May tall fescue and 

pine needles. Eventually surface temperatures on a portion of the mixed fuel reach a point where

transition to glowing combustion occurs. This glowing combustion spreads and increases th

heat release until it is sufficient to ignite the pyrolysis gases generated by the sm

tr

For the pine needles the lowest plate temperature where glowing and flaming ignition was 

observed was 300 °C without a wind present, and roughly 1000 s was required for glowing 

combustion to develop. For the same plate temperature the May tall fescue/pine needle mixture 

with a low wind required just under 500 s to begin glowing and rapidly transition to flaming. 

The lowest plate temperature that p

studied during this study was the 290 °C ignition of the fuel mixture in the absence of the wind.

As discussed earlier, non glowing smoldering was observed for the May tall fescue for the lowe

plate temperature, 302 °C, run without an applied wind. It is likely that non glowing smoldering 

of the May tall fescue within the fuel mixture generated the initial heat necessary to ignite 

glowing combustion in the fuel mixture. 

The conclusion that non glowing combustion of the May tall fescue component is responsible 

the initial heat release in the fuel mixture is supported by the photographs of the bottom and top 

of the fuel bed shown in Figure 67 for an experiment with the heated plate held at 280 °C and no

applied wind. No glowing combustion was observed in this experiment. The bottom of the fuel 

bed is blackened, indicating significant fuel pyrolysis had occurred. On the top there is a circula

dark brown area indicating that some pyrolysis of the fuel mixture took place here as well. 

Given the low plate temperature, pyrolysis on the top of the bed would not have been expected 

unless a non glowing smolder front had passed upward through the fuel bed. It has been shown 

that smoldering in the pine needle beds studies here always involved glowing combustion. 

68
Figure 67. Bottom (left) and top (right) views of a May tall fescue/pine needle mixed fuel bed 

are shown after the fuel was removed from the heated plate held at 280 °C with no 

wind applied. No glowing combustion was observed. 

t lower plate temperatures the effect of applying a wind to the fuel mixture was to decrease the 

 in the 

or 

pine needle r. 

he importance of convective cooling of the mixed fuel by an applied wind on the ignition 

behavior is demonstrated by the images shown in Figure 68 for a fuel bed removed from the 

hea t the bottom of the fuel bed there is 

 small blackened area on the downstream half of the bed. Fuel pyrolysis has clearly occurred 

 the 

4.9. Preliminary Tests with Leaves 

re 

 of 

 

A

likelihood of ignition since glowing combustion was observed for lower plate temperatures

absence of a wind than with wind present. This dependence on wind is the same as observed f

s and suggests this component of the fuel mixture dominated this particular behavio

T

ted pla e held at 320 °C in the presence of a high wind. On 

a

here. There is some discoloration of the fuel bed along a narrow band lying perpendicular to

wind direction near the downstream edge suggested smoke deposition from or some light 

pyrolysis by the thermal plume passing through the fuel bed. Similar observations for other fuels

have been attributed to convective cooling of the fuel near the heated plate. 

A limited number of ignition experiments were carried out on the heated plate using three types 

of dried leaves. All of these tests were run with heated plate temperatures of 381 °C. This 

temperature was chosen because the experiments discussed above indicated that this temperatu

is high enough to ensure that glowing combustion will develop in a reasonably short period

time. 

A single experiment was done for 8 g of boxwood leaves placed on the heated plate without a 

wind. Glowing combustion was observed 90 s after the leaves were applied, but transition to

69
Figure 68. Bottom (left) and top (right) views of a May tall fescue/pine needle mixed fuel bed 

are shown after the fuel was removed from the heated plate held at 320 °C with a 

high wind applied. No glowing combustion was observed. 

flaming did not occur. Photographs of the fuel bed showed blackening on the bottom, a circular 

blackened area on the top, and an area of unreacted fuel around the outside edge of the bed. 

Two experiments were run with 5.5 g of elm leaves. When a low wind was used, glowing 

t 2/3 

rved, 

ent showed that the bottom was 

clear that leaves can ignite and flame for plate temperatures around 380 °C. The results also 

combustion and flaming appeared nearly simultaneously after 93 s. Smoke was observed 

coming from the fuel bed along a line perpendicular to the wind flow direction located abou

of the way across the bed from the upstream edge. Photographs show that most of the fuel was 

blackened. For the second experiment no wind was applied. Glowing combustion developed 

253 s after fuel application, but no transition to flaming was observed. The glowing combustion 

spread quickly to cover most of the fuel bed. 

Two repeats of an experiment with the oak leaves in the presence of a low wind were done. For 

the first test 6.0 g of leaves were used. Neither glowing combustion nor flaming were obse

but heavy smoke was observed along a line perpendicular to the wind flow direction, indicating 

that pyrolysis was occurring. Photographs after the experim

heavily blackened, but the top appeared to be unaffected. This suggests that sustained 

smoldering did not occur. For the second experiment the mass of oak leaves was increased to 

9.4 g. Glowing combustion was observed after 216 s, and flames appeared 11 s later. Both 

ignitions occurred near the surface of the heated plate. The fuel bed was completely blackened, 

and there was some ash formation. 

The experiments with leaves are too few in number to draw many definite conclusions, but it is 

70
aximum observed heat release rate are 

flux for polyurethane foam. Numbers in 

4.10. Radiative Ignition of Non Fire-Retarded Flexible Polyurethane 

re observed for three 

sts. 

es e n times on AHF, there was a nearly linear 

2

 70 shows plots for an AHF of 50 kW/m2. Following a brief induction period during 

hich the HRR grew slowly and the mass loss rate was low, the HRR and mass loss rate grew 

Figure 69. Values of time to flaming ignition and m

plotted as functions of applied heat 

parentheses refer to the number of repeated measurements. 

suggest that the application of a wind will have similar effects in enhancing ignition to those for

the fuels discussed earlier. 

Foam 

Non fire-retarded flexible polyurethane was included in the study as an example of a plastic 

material that might be found in an area where outdoor power equipment is stored. Experiments 

were done only in the cone calorimeter. 

Figure 69 shows measured times to flaming ignition and maximum HRRs as a function of AHF. 

For AHFs between 30 kW/m2

 and 50 kW/m2 the times required for flaming ignition increased 

slowly as the AHF decreased. When the AHF was decreased to 25 kW/m2

 the ignition times 

suddenly jumped to values on the order of 250 s. When the AHF was reduced to 24 kW/m2

 the 

ignition times increased by about another 50 s. Flaming was observed in only 4 of 6 tests with a 

further reduction of 1 kW/m2

 in AHF. Maximum ignition times for the cases that flamed were 

oughly 325 s. When the AHF was set to 22 kW/m2 r no flaming ignitions we

te

D

d

pite th highly non linear dependence of the ignitio

ecrease in the maximum HRRs, with values dropping from about 1000 kW/m for an AHF of 

50 kW/m2

 to around 350 kW/m2 for the lowest AHFs where ignition occurred. 

Time resolved measurements of HRR and fuel mass reveal that the ignition and burning 

behaviors of the polyurethane foam were different from those observed for the other fuels. 

Figure

w

71
Figure 70. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 50 kW/m2

 heat flux was applied to polyurethane foam. 

rapidly to much higher plateau values that lasted about 25 s. About 40 s after exposing the 

sample to the AHF the HRR rate began to increase again, rising from about 300 kW/m2

 to a peak 

 zero, and the mass did not change appreciably. 

explanation am 

first darkened and began to shrink. A

ool 

 the bottom of the sample pan, which eventually ignited and resulted in the second HRR peak. 

2

el mass as a function of time for three experiments 

2

g 

e 

2

of around 1000 kW/m2. The mass loss rate during this second burning phase was much higher

than during the first. Following the second burning period the HRR fell rapidly to values close

to

Visual observations of flames spreading across a similar polyurethane foam provide a plausible 

 for the observed HRR behavior. As the flames spread onto unburned foam, the fo

s the foam continued to burn and shrink it generated a 

brown liquid that began to collect. Once the foam had fully collapsed, the liquid remained as a 

pool, which then burned as a pool fire. The first heat release peak in Figure 70 is likely 

associated with burning of the expanded foam. As the foam burned it was forming a liquid p

in

The HRR and fuel mass time behaviors for tests with AHFs between 30 kW/m2

 and 50 kW/m

had similar appearances to those in Figure 70. As the AHF was reduced the induction period 

tended to increase. For an AHF of 30 kW/m2

 the induction times began to vary significantly, 

ranging from 15 s to 35 s. 

Figure 71 includes plots showing HRR and fu

with an AHF of 25 kW/m . These curves have a very different appearance from those with an 

AHF of 50 kW/m2

 shown in Figure 70. There is a very long induction period during which a 

large fraction of the fuel mass is loss without significant heat release. This suggests that non 

oxidative pyrolysis of the foam was taking place. After periods ranging from 230 s to 250 s, the 

HRR began to rise quickly and flaming was present for a short period of time. During flamin

the mass loss rates increased substantially, having slopes comparable to those present during th

second burning phase with an AHF of 50 kW/m2

. Taken together, these observations suggest 

that the pyrolyzate generated directly from the foam was not ignited when a AHF of 25 kW/m

was used, but that the liquid pool that was generated by this pyrolysis did eventually ignite. 

72
Figure 71. Heat release rates and sample masses are plotted as a function of time for three 

experiments in which a 25 kW/m2

 heat flux was applied to polyurethane foam. 

Figure 72. Heat release rates and sample masses are plotted as a function of time for three 

2

of 24 kW/m2

 and the four experiments that ignited with an AHF of 23 kW/m2 are similar to 

 shows sim bserved. 

curring, 

experiments in which a 22 kW/m heat flux was applied to polyurethane foam. 

During these experiments there was concern that the ignition of the foam might be due to the 

presence of the cone heater acting as a pilot for the pyrolysis gases being generated. This 

possibility was checked by making a video recording of the ignition. It was clear from analysis 

of this video that ignition occurred within the sample holder and then spread rapidly upwards. 

Plots (not shown) of HRRs and fuel mass as functions of time for the three experiments with an 

AHF 

those shown in Figure 71 with induction times that determine the times to ignition shown in 

Figure 69. 

igure 72 ilar plots for an AHF of 22 kW/m2 F for which ignitions were not o

The HRR plots suggest that there were brief periods during which some oxidation was oc

but overall the HRRs were close to zero over the entire periods. On the other hand, a large 

fraction of the fuel mass was loss. This indicates that non oxidative pyrolysis was taking place in 

73
cases where no ignition was observed. At the end of these experiments the inside of the sample 

holder was covered with a thin layer of solid black m

liquid material that formed during th

Due to the complex reaction behavior of the foam 

left a hard-to-remove residue, the decision was m

heated plate. 

5. Summary and Discussion 

The purpose of this investigation was to provide e

aterial. This was likely the remnants of the 

e pyrolysis of the polyurethane foam. 

that led to the formation of a liquid and that 

ade to not risk testing this material on the 

xperimental findings that can be used to better 

 the 

d 

portant param

y 

ited number of experiments non glowing 

e 

g) 

estimate the potential for ignition associated with the heated exhaust surfaces present during

operation of OPE. Due to the application, emphasis was placed on natural fuels often found in 

the outdoor environment. Scenarios involving direct contact of fuel with a heated surface an

radiative heating of fuel placed near a heated surface have been considered. A number of 

im eters have been identified and varied during the investigation, including fuel 

type, heated surface temperature and intensity of applied radiative heat flux, and the absence or 

present of an imposed wind. 

An important characteristic of most of the fuels considered is the ability to smolder. The primar

source of information on smoldering behavior of the fuels used in this investigation is derived 

from the heated plate experiments. While for a lim

smoldering without a transition to glowing was observed (based on blackening of the fuel), it 

was far more common for glowing combustion to develop. As discussed earlier, the 

development of glowing appears to be a requirement for the ignition of flames in these fuel beds. 

The times required for the appearances of glowing and flaming are the primary data for the 

experiments reported here. 

As for all experiments, there are uncertainties associated with the measurements that need to be 

assessed. As discussed in Section 4.1, some uncertainty in ignition times was introduced during 

visual observation of glowing combustion. In some cases the initial glowing was hidden within 

the fuel bed and glowing was not observed until a later time. It is believed in these cases that th

time periods between initial glowing and the detection of combustion (either glowing or flamin

were relatively short and have little or no effect on the general conclusions drawn from the 

experiments. 

Due to the stochastic nature of the fuel beds and the natural variation in the tendency of 

individual fuel elements to ignite, uncertainties in measured ignition times for a given heated 

plate temperature or radiative heat flux are inevitable. The only effective way to quantitatively

characterize these variations would be to perform sufficient experiments to allow statistical 

analysis. It was not possible to perform the large number of experiments required within the 

scope of the current effort. In the absence of such data, a qualitative approach was adopted in 

which the degree of collapse of experimental data onto well defined curves is used as a 

qualitative basis for assessing uncertainty. 

74
Figure 73. Data for time to glowing ignition with no applied wind are plotted as a function of 

 Data near the upper temperature axis 

not take place. The solid curve is the 

an exponential dependence on heated plate 

where ignition was observed. 

inimum heated plate temperatures or 

 ignite a given type of fuel bed. A detailed study would be 

e in which the probability of ignition would 

heated plate temperature for seven fuels. 

represent experiments for which ignition did 

result of a least squares fit assuming 

temperature for all data 

Similar considerations apply to determinations of the m

radiative heat fluxes required to

expected to reveal an ignition probability curv

g out 

 discrete steps. Usually, but not in all cases, the step sizes were such that well defined 

temperature ic 

certainty of at least one step size to the results. 

el 

(2) 

increase from near zero to near one as temperature was increased over some range. Mappin

such a probability curve would have required far too many experiments. For this work an 

approach was adopted in which heated plate temperatures or radiative heat fluxes were reduced 

in

demarcations between experiments with and without ignition were identified. Lower 

 ignition limits can be identified from these experiments, but due to the stochast

nature of the processes it is prudent to assign an un

Results for individual fuels have been discussed in Section 4. In order to assess the roles of fu

type, heated plate temperature, and wind for the surface ignition experiments, ignition time data 

(the shorter of glowing or flaming) for all of the fuels tested have been plotted together on a 

single plot. Figure 73 shows the result for experiments without an applied wind. For 

comparison purposes, a least squares curve fit assuming the fall off of ignition times with 

increasing plate temperature has an exponential dependence was fit to all of the data for which 

ignition was observed. The result of the fit, 

729 , 0154.0

,,

Theated plate

nonefitglowing t ×= e

−

75
where tglowing,fit,none is the calculated time to ignition and Theated plate is the heated plate temperature

is shown as a solid curve in 

 

tial dependence on 

late temperature provides a good approximation for the temperature dependence. 

s that 

aller variations than the consolidated data. These relatively small variations show 

at variations in measurements for individual fuels are much smaller than those due to variations 

between fuels. Interestingly, data for all of the fuels seem to lie close together for plate 

e differences between fuels appearing for lower 

80 

, 

 The probability of transition to flaming in the absence of wind 

g 

 two 

ering was observed down to temperatures around 300 °C. The 

 heated plate ignition experiments with a low wind for the seven fuels are plotted in 

se for 

Figure 73 . The experimental data fall on a wide band on either side 

of the curve fit. 

Data for each fuel shows a similar dependence on plate temperature, with the ignition time 

increasing with decreasing plate temperature and the rate of increase accelerating. Comparison 

with the exponential curve fit included in Figure 73 shows that an exponen

p

Closer inspection of Figure 73 shows that data for individual fuels lie on well defined curve

show much sm

th

temperatures above 400 °C, with larger relativ

heated plate temperatures. 

Results for the May and August tall fescue grasses fall close together and lie well below the 

curve fit, data for fine Florida grass and the May tall fescue/pine needle mixture fall close to the 

curve fit, and the shredded newsprint and pine needle data appear well above the curve. It is 

difficult to assess the behavior of cheat grass because it did not ignite for temperatures below 3

°C. Recall that transition to flaming in the absence of a wind was unlikely for May tall fescue

August tall fescue, and cheat grass, while the May tall fescue/pine needle mixture transitioned to

flaming in over half of the tests, and the pine needles and shredded newsprint were likely to 

flame for all wind conditions.

seems partially correlated with the time-to-ignition variations observed between fuels, with a 

shorter time to ignition correlating with a lowered probability of transition to flaming. 

The data at the top of the plot in Figure 73 represents experiments for which glowing or flamin

ignition was not observed. Minimum ignition temperatures show distinct fuel-type 

dependencies. The highest minimum ignition temperature (380 °C ) was for cheat grass. This 

was followed by the August tall fescue (371 °C ) and May tall fescue (340 °C). Recall that even 

though glowing combustion was not observed just below the transition temperature for these

grasses, non glowing smold

minimum heated plate ignition temperature (340 °C) for shredded newsprint was similar to that 

for May tall fescue. Next was the fine Florida grass with a minimum ignition temperature of 

around 320 °C. The minimum ignition temperatures were 300 °C for pine needles and 290 °C 

for the May tall fescue/ pine needle mixture. In general, the four grasses seem to have the 

highest minimum ignition temperatures, but there is no clear correlation between the minimum 

ignition temperatures and the relative ordering of the ignition time curves. 

Results of the

Figure 74. The data fall within a band and have a similar temperature dependence to tho

the no wind case shown in Figure 73. A least squares curve fit to an exponential gave 

(3) 000,106 , 0176.0

,,

Theated plate

lowfitglowing t e

− ×=

76
Figure 74. Data for time to glowing ignition with a low applied wind are plotted as a function 

heated plate temperature for seven fuels. Data near the upper temperature ax

represent experiments for which

of 

is 

 ignition did not take place. The solid curve is the 

result of a least squares fit assuming an exponential dependence on heated plate 

and is show

fuels fall on

s for the no wind case, the data lie fairly close together for heated plate temperatures greater 

 tall 

the 

ed 

uels 

st tall 

) pine needles (310 °C), and the May tall fescue/pine needle mixture (300 °C) were similar to 

temperature for all data where ignition was observed. 

n as a solid curve in Figure 74. Similar to the no wind case, the data for individual 

 bands with fluctuations that are smaller than the differences between the fuels. 

A

than 400 °C and begin to diverge at lower temperatures. The data for May tall fescue, August

fescue, and fine Florida grass are close together and lie below the curve fit. The results for 

May tall fescue/pine needle mixture lie close to the calculated curve, while those for shredd

newsprint and pine needles fall above. As for the no wind case, it is difficult to assess cheat 

grass because it did not ignite below 380 °C. The locations of the curves for the different f

relative to the curve fit are similar to those for the no wind experiments, with the exception that 

the data for fine Florida grass lies somewhat closer to those for May tall fescue and Augu

fescue. 

Minimum heated plate ignition temperatures for cheat grass (380 °C), shredded newsprint (340 

°C

those observed without an applied wind. May tall fescue, August tall fescue, and fine Florida 

grass each ignited for plate temperatures as low as 310 °C. For all three grasses this value is 

lower than observed without a wind present. 

77
000,310 , 0148.0 Theated plate t e

− ×=

represent experiments for which ignition did not take place. The solid curve is th

result of a least squares fit assuming an exponential dependence on heated plate 

temperature for all data where ignition was observed. 

A similar plot of ignition times versus heated plate temperature is shown in Figure 75 for 

experiments with a high wind. For some of the fuels fewer measurements were done than for the 

no and low wind cases, but the results follow similar trends. The variations with plate 

temperature are similar, and a least squares curve fit to an exponential for all of the data with 

ignition yields

,, highfitglowing

temperature for seven fuels. Data near the upper temperature axis 

e 

(4) 

e 

ition did not occur. 

Figure 75. Data for time to glowing ignition with a high applied wind are plotted as a function

of heated plate 

which is shown in the figure as the solid curve. 

As for the no wind and low wind cases, the data for individual fuels fall on well defined curves 

having smaller fluctuations than the variations between different fuels. The results for May tall 

fescue, August tall fescue, and fine Florida grass fall close together and lie below the curve. 

Unlike earlier experiments where the cheat grass did not ignite with a plate temperature below 

380 °C, the lowest ignition temperature with a high wind was 350 °C. The data for the cheat 

grass fall close to the curve fit for all of the data, as do the measurements for the May tall 

fescue/pine needle mixture. As observed for the other two wind cases, longer ignition times 

were required for pine needles than indicated by the curve fit. It is not possible to assess the 

behavior of the shredded newsprint due to the gap of 30 °C between the lowest plate temperatur

where ignition was observed at 380 °C and the plate temperature where ign

78
Figure 76. Results of exponential least squares curve fits of experimental ignition times as a 

function of heated plate temperature for seven fuels are shown for no wind, low 

wind, and high wind cases. 

The highest value of minimum heated plate ignition temperature for these fuels was found for the 

shredded newsprint, lying between 380 °C and 360 °C. The next was the cheat grass with a 

minimum ignition temperature of 350 °C, which is about 30 °C lower than observed for this fuel 

with the other two wind conditions. The minimum ignition temperature for pine needles was 340 

fescue/pine

the lowest v

Figure 73 to Figure 75 show how ignition behaviors depend on the heated 

even 

pronounced over the lower portions of the tem

monotonic for the three conditions tested, suggesting that furth

would be likely at higher wind speeds. It should be noted that 

°C, but note that this material failed to ignite for some experiments at both 340 °C and 350 °C. 

The lowest minimum ignition temperatures for the three remaining grasses and the May tall 

 needle mixture fell between 310 °C and 330 °C, with the fine Florida grass having 

alue. 

The data plotted in 

plate temperature and fuel for the porous beds studied here. Additional insights into the effects 

of wind are obtained by plotting the curve fits given by Equation (2) to Equation (4) on a 

common plot, as shown in Figure 76. It should be kept in mind that these curve fits are for s

fuels and that there were systematic variations in the data with fuel. Nonetheless, given the 

similarities in the spreads of data and the similar effects of fuel for the three wind cases, it seems 

appropirate to use these fits to characterize the effects of wind on surface ignition. 

The feature that stands out in Figure 76 is the substantial reduction in ignition times with 

increasing wind speed for a given heated plate temperature. This effect is particularly 

The dependence on wind is 

er reductions in ignition times 

the two wind velocities used, 

perature range. 

79
1.1 m/ arable to 

environm

The reductio

ingful reaction rates 

are generally lim

ing ignition. 

A wind effe

inimum 

plate temp

the bottom

Presum eting effects 

he data for the different fuels fall on well defined curves which have a similar dependence on 

Figure 76). For the highest AHFs ignition occurs within a few seconds. As the AHF is reduced 

from the highest values there is a ran m reasing 

AHF. With further reductions in AHF, a poin

begin to ris

remaining t

higher for t than for pine needles. 

hese 

(5) 

 shown in Figure 77 as a solid line. The measurements for the three fuels fall close to the 

alculated curve. 

s and 2.5 m/s, correspond to 2.5 mph and 5.6 mph. These velocities are comp

speeds at which OPE operate, and could be induced by equipment motion in still air. Outdoor 

winds are frequently considerably higher than these levels, so substantial additional 

reductions in ignition times might be possible for conditions typically encountered in outdoor 

ents. 

ns in ignition time with increasing wind as well as qualitative observations 

concerning the ignition behavior such as initial ignition location and glowing intensity indicate 

that the primary effect of wind is to increase the amount of oxygen reaching the fuel surface. 

Since surface oxidation rates for temperatures high enough to generate mean

ited by the rate at which oxygen reaches the surface, the effect is to accelerate 

the surface reaction rate and decrease the ignition times for both glowing and flam

ct that is not revealed by the results plotted in Figure 76 is the potential to 

convectively cool the fuel and thus slow or prevent ignition. Observed changes in the m

eratures required for ignition with wind speed and the distributions of blackening on 

 of fuel beds that did not ignite in the presence of a wind provide evidence that 

convective cooling affected the surface reaction behavior of the porous fuel beds studied here. 

ably, the observed ignition behavior is determined by a balance of the comp

of increased oxygen availability and convective cooling. The need to consider these competing 

effects should be kept in mind when extending the results of this study to wind conditions that 

were not investigated. 

The non piloted thermal radiation ignition behaviors for four of the seven fuels investigated in 

detail using the heated plate were also studied with the cone calorimeter. Figure 77 shows plots 

of glowing or flaming ignition time versus AHF for shredded newsprint, May tall fescue, cheat 

grass, and pine needles. The data plotted near the upper axis represent experiments for which 

ignition was not observed. 

T

AHF to the ignition time variations observed with heated plated temperature (see Figure 73 to 

es increase slowly with dec

t is reached where the measured ignition times 

e rapidly. For pine needles the rapid rise begins around 30 kW/m

ge where the ignition ti

2

 and for the

hree fuels around 20 kW/m2. The rate of increase with decreasing AHF is much 

he three remaining fuels

The data for the shredded newsprint, May tall fescue, and cheat grass fall close together. T

three sets of data were combined, and a least squares fit to an exponential was obtained. The 

result,

5540 , 260.0

,

AHF

fitignition t e− ×=

is

c

80
Figure 77. Ignition times due to non piloted radiative heating in the cone calorimeter are plotted 

o 

6)

where the R

temperature re plotted as a function of RHF in Figure 78. 

as a function of applied heat flux for the four fuels indicated. The solid line shows 

the results of a least squares exponential curve fit to the combined data for shredded 

newsprint, May tall fescue, and cheat grass. 

Due to the similarities in the response curves, it is worthwhile to consider whether the radiative

ignition results can be related to those for the heated plate in some way. One approach is t

compare effective temperatures for the radiant heat flux, determined by assuming the thermal 

radiation is generated by a black body, with corresponding heated plate temperatures. The 

radiative heat flux (RHF) from an ideal black body is given by 

− 413 (

HF has units of kW/m2 and TBB is the black body temperature in K. Values of 

 derived from Equation (6) a

,*1067.5 RHF ×= TBB

The results in Figure 77 indicate that ignition did not occur for AHFs of 7.5 kW/m2

 and that the 

minimum AHF required to ignite smoldering was around 10 kW/m2

 for shredded newsprint, May 

tall fescue, and cheat grass. The corresponding black body temperatures for these AHFs are 330 

°C and 375 °C. The minimum observed heated plate ignition temperatures for these fuels with 

no applied wind were 340 °C, 340 °C, and 380 °C. The close agreement between the minimum 

heated plate ignition temperatures and the black body temperature range over which radiative 

ignition no longer occurred suggests that the processes determining the minimum ignition 

conditions are similar for the two types of heating and that the fuel surfaces are being heated to 

comparable temperatures. 

81
Figure 78. Temperatures are plotted as a function of radiative heat flux assuming that the flux i

generated by an ideal black body. 

The results in Figure 73 show that ignition times increased slowly with decreasing plate 

emperature between 550 °C and 450 °C and then began to increase more rapidly with an 

s 

n slope 

e 

ment the radiative heating was applied to the top of a fuel bed that was 

 to the ignition behaviors of shredded newsprint, May tall fescue, and 

t

increasing slope as heated plate temperatures were lowered further. A similar change i

occurs around 20 kW/m2

 for the time to ignition data in Figure 77. The black body temperature 

for 20 kW/m2 is 498 °C. This temperature is somewhat higher than the 450 °C estimate from the 

heated plate experiments, but is close enough to support the conclusion that the response of thes

fuels to the two heating types is similar. This is particularly so when it is noted that differences 

in response should be expected based on configurational differences between the two 

experiments. The heated plate applied heat at the bottom of the fuel bed, and a buoyant plume, 

which entrained air through the sides of the fuel bed, was generated. This plume both heats fuel 

above the plate and provides additional oxygen for surface oxidation. On the other hand, in the

cone calorimeter experi

closed on the sides. In this case buoyancy-induced flow by heating near the top surface of the 

bed is expected to remove heat from the fuel bed, and the absence of air flow into the fuel bed 

from the sides results in a reduced surface oxidation rate compared to the heated plate 

experiments. Both of these effects are expected to lengthen ignition times. 

The above discussion refers

cheat grass. The agreement between experiments with the two types of heating is not as good 

when pine needles are considered. The radiative ignition times for this fuel began to increase 

around 30 kW/m2

. This corresponds to a black body temperature of 580 °C. Corresponding 

increases in ignition times for heated plate ignition did not appear until the plate temperature was 

reduced to around 450 °C. It has not been possible to provide an explanation for this difference. 

82
Figure 79. Values of maximum heat release rate are plotted as a function of applied heat flux

the four fuels indicated. 

he behavior of maximum HRRs determined in the cone calorim

 for 

eter experiments provide 

int, May tall fescue, cheat grass, and pine needles. The high maximum HRRs indicate 

that flaming was ignited for all of these fuels with AHFs of 40 kW/m2

 and higher. At these high 

AHFs the maximum HRRs were around 200 kW/m2

 for each of the fuels. 

ed 

cipitously to v oldering. The corresponding black 

ody temperature for an AHF of 40 kW/m2

 is 643 °C. This temperature is much higher than the 

fact that tra o 

wind was a

results sugg s for these grasses are only generated 

igh heating fluxes and/or high fuel surface temperatures. 

and pine needles for AHFs as low 10 kW 2 i

heated plate experiments, which showed that these two fuels transi

ot for the May tall fescue. 

T

additional information concerning ignition behavior, particularly with respect to transition to 

flaming. Figure 79 shows combined plots of maximum HRRs versus AHF for shredded 

newspr

As the AHF was reduced below 40 kW/m2 the two grasses no longer flamed, and the observ

HRRs dropped pre alues characteristic of sm

b

highest heated plate temperatures used, thus the cone calorimeter results are consistent with the 

nsition to flaming was not observed on the heated plate for these two grasses when n

pplied. Since smoldering was present at lower temperatures for both grasses, these 

est that flammable mixtures of pyrolysis gase

when the fuels are subjected to h

In contrast to the grass behavior, transition to flaming was observed for the shredded newsprint 

th the observations for the 

tioned to flaming in the 

absence of a wind whenever glowing was observed. 

The transition to flaming behaviors of May tall fescue and cheat grass were very different in the

presence of a wind, with transition observed for the cheat grass and n

/m . This is consistent w

83
The cone calorim

behaviors 

flux m

t 

samp

depending o r grass 

samp

The fact that flam HFs in 

the cone calorim

when placed

temp rass 

than fo

eter results for polyurethane foam were different from those observed for the 

ellulosic fuels. No smoldering was observed, and the material either flamed or did not ignite

or AHFs of 30 kW/m2

 or greater the ignition times were less than 50 s, increasing to several 

ith 

re 

As discussed in Section 2, only limited information is availa

with the results of this investigation. The most similar studie

he effect of a light wind was to reduce the ignition time. With the exception of 

unky wood, which ignited in several hundred seconds for a surface temperature of 270 °C, 

between 30 comparable to those reported here. Cheat grass 

as reported as requiring 120 s for ignition at 330 °C. The current measurements (see Figure 

as not 

eter results provide no indication of this difference since the observed ignition 

and maximum HRRs were similar for both. This suggests that while radiative heat 

easurements can provide insights into smoldering and flaming behaviors for fuels placed 

on a heated surface when no wind is present, they do not differentiate ignition differences tha

appear due to the presence of a wind. 

It is significant that similar differences in transition to flaming behavior were observed for the 

May tall fescue (did not flame) and August tall fescue (flamed) with wind present. These two 

les were collected from the same location and likely consisted of the same type of grass 

(there is a possibility that more than one type of grass was present and grew at different rates, 

n conditions). Even so, the results indicate that ignition characteristics fo

les from a given location can vary with seasonal changes. 

ing is observed for the May tall fescue and cheat grass with similar A

eter does provide a clue as to the reason for their different flaming behaviors 

 on the heated plate in the presence of wind. The observations suggest that the 

eratures developed on the fuel surfaces as a result of oxidation are higher for the cheat g

r the May tall fescue. 

The cone calorim

c . 

F

hundred seconds for AHFs between 23 kW/m2 and 25 kW/m2. No ignition was observed w

APHs of 22 kW/m2. This value is much higher than the minimum AHFs that ignited smoldering 

in the cellulosic fuels. Two distinct burning periods with different HRR behaviors we

observed in the HRR time histories for the polyurethane foam that were not characteristic of the 

other fuels. This was attributed to the formation of a liquid component during the foam burning 

followed by pool burning. The higher AHFs required for ignition and the burning behavior 

suggest that polyurethane foam placed in contact with or near a heated surface may not ignite as 

easily as shredded newsprint or the natural fuels that can smolder at lower temperatures. 

ble in the literature for comparison 

s were those of Harrison [17] and 

Kaminski [18], who reported limited measurements of ignition times for natural fuels when 

placed in contact with variable temperature heated surfaces. Harrison reported that pine needles 

and grass ignited in about four minutes with surface temperatures of 400 °C and 350 °C, 

respectively. T

p

Kaminski found that most of the fuels he investigated would ignite with surface temperatures 

0 °C and 330 °C. These results are

w

51) indicated that cheat grass did not ignite at temperatures below 380 °C when a wind w

present. 

Stockstad has reported the ignition behavior of small pieces of cheat grass [20], pine needles 

[21], and rotten wood [19] when exposed to heated air in an oven. Cheat grass ignition was 

84
observed to require 7 s for a temperature of 380 °C, while pine needles required about 15 s for 

eratures as low as 300 °C. These minimum ignition temperatures are in good agreem

s for the same fuels determined in the current study, but the observed ignition tim

ch shorter than those measured with the heated plate. 

ents were identified in the literature with which the radiative heat

could be directly compared. White and coworkers have employed the cone calorimeter to 

investigate piloted ignition for a variety of natural fuels including cheat grass, but m

have been made using only one or two AHFs. [24,25,26] 

Manzello et al. investigated the ignition behavior of similar fuel beds, including shredded 

newsprint, grass, and pine needles, in the presence of light winds due to contact with one or m

glowing fire brands placed on the top surface. [22,23] Their results indicated that shredded 

newsprint developed smoldering, grass did not smolder or flame, and pine needles developed 

oldering and transitioned to flaming only with four large glowing brands applied and with the 

d (1 m/s) used. These results suggest that glowing fire brands placed on top of a fuel 

bed are less likely to ignite smoldering or flaming than when similar fuels are placed o

heated plate. This is the case even though heated plate temperatures were likely mu

than the surface of a glowing fire brand. 

temp ent 

with value es 

are mu

No previous experim ing results 

easurements 

ore 

sm

highest win

n top of a 

ch lower 

centages 

e needles 

The results of this investigation have demonstrated that smoldering and flaming ignition of 

onditions chosen to be representative of those generated on OPE 

hese 

t 

o 

The ignition behaviors of the cellulosic fuels investigated can be classified into two broad 

groups: pine needles and shredded newsprint which developed smoldering and transitioned to 

flaming when no wind was present and the grass samples which developed smoldering, but did 

not transition to flaming without an imposed wind. Note that the May tall fescue/pine needle 

mixture had ignition behaviors intermediate between these two extremes. It is not possible to 

definitely identify the reasons for these differences without careful characterization of the fuels 

and their response to heating. However, it is possible to provide a plausible explanation. 

Newsprint is known to consist primarily of cellulose, while the natural fuels contain varying 

proportions of cellulose and lignin (see the discussion in Section 2). As discussed earlier, fuels

with high percentages of cellulose are more likely to flame, while fuels with higher per

of lignin tend to smolder. This suggests that the shredded newsprint and pin

investigated here tend to flame because they contain higher percentages of cellulose relative to 

lignin than the grasses that were studied. 

It is possible that the different ignition behaviors of May tall fescue (limited transition to 

flaming) and August tall fescue (transitioned to flaming) in the presence of a wind are due to 

differences in the cellulose/lignin ratios for the two grasses, with a higher ratio of lignin in the 

rapidly growing spring grass, or to some other type of seasonal compositional difference. 

cellulosic fuel beds exposed to c

heated exhausts vary with surface temperature, fuel type, wind, and season. In addition to t

parameters, there are a number of others that would be expected to affect ignition behavior bu

were not varied in this study. One of these parameters is fuel moisture. The grasses and pine 

needles used in this investigation had measured fuel moisture percentages ranging from 10 % t

14 %. This narrow range is consistent with the expected values for these types of dry fuels 

85
stored in a general purpose laboratory maintained near 20 °C. Such small variations are not 

expected to significantly affect ignition behavior. Much wider variations in fuel moisture are 

bserved in nature, with higher values often present in living and recently cut vegetation and 

ation 

an become self sustaining. 

 the 

 bed to an applied wind. The potential for 

hanges in ignition behavior due to these fuel bed properties must be kept in mind when using 

t 

 that were within 50 °C of this value. In general, these 

wer limits are in good agreement with the value of 320 °C recommended by Ford as being 

he 

er, when temperatures were 

creased by 100 °C or AHFs by 10 kW/m , these times dropped to tens of seconds and became 

ated 

 porous fuel beds brought into contact with heated surfaces having 

 temperature range of 290 °C to 550 °C. In order to provide a safety factor, the calculated 

e 

o

lower values found for dried vegetation located in higher temperature or low-humidity 

environments. 

Babrauskas provides good reviews of the effects of moisture content on ignition behavior in 

general and for outdoor fuels in particular. [5] The primary effect of moisture content on 

cellulosic fuels is expected to be a lengthening of ignition times since a fraction of the heat 

applied to the fuel will be used to remove fuel moisture at the expense of heating the fuel. Once

the fuel moisture has been removed, the fuel will begin to heat and eventually surface oxid

c

Other important parameters not varied in the current investigation include fuel bed exposed 

surface area, fuel bed thickness, fuel bed porosity, spatial orientation of the heat source and

fuel bed, and the accessibility of the interior of a fuel

c

the findings of this investigation to assess the potential for ignition of fuels by heated surfaces. 

Even though the findings of this investigation are subject to the limitations discussed above, it is 

possible to derive some guidance with regard to ignition behaviors expected for heated exhaus

surfaces of OPE. The results indicate that ignition of natural fuels can occur for heated surface 

temperatures as low as 290 °C. All of the cellulosic fuels studied had lower limit ignition 

temperatures for some wind conditions

lo

representative of the lower ignition temperature for natural fuels. [15] 

The temperature-ignition time curves observed in this study are likely to be important when 

considering potential scenarios involving ignition of fuels by contact with or radiation from 

heated surfaces. Several minutes were required for ignition (either glowing or flaming) near t

lower limit temperatures for all of the fuels investigated. Howev

2 in

very short when heated plate temperatures or AHFs were increased further. 

While clearly not representative of the worst possible cases, the ignition time curve fits to he

plate temperatures given by Equation (2) to Equation (4) should be useful for estimating times to 

ignition for general cellulosic

a

ignition times for a given surface temperature could be reduced by a factor of 2. It would also b

appropriate to consider the calculated ignition times to be the periods required for flaming to 

develop, even though in many cases only glowing combustion would occur. Additional 

reductions in calculated ignition times would be prudent if winds higher than 2.5 m/s are 

considered likely. 

The experimental findings suggest there is a correlation between the temperature dependence of 

ignition on the heated plate and black body temperatures corresponding to AHFs used for the 

86
Figure 80. The curve fit times to ignition for no wind, low wind, and high wind shown in Figu

76

re 

 on the different fuel bed holders and 

ed 

 are plotted as a function of the AHF calculated for the radiative heat flux from a

black body held at the heated plate temperature. 

radiative ignition experiments. For the fuel beds and heating configurations studied, the 

calculated black body temperatures were slightly higher for the lowest ignition temperature and 

the locations of the knees in plots of ignition time versus heated plate temperature and AHF. As 

noted, these differences are in the direction expected based

directions of heat application for the two experiments. 

A reasonable approach for estimating ignition times for cellulosic fuels subjected to a radiative 

heat flux is to use the time/temperature curves for heated surface ignition, and convert the heat

plate temperatures to AHFs assuming black body thermal radiation. Figure 80 shows the results. 

The curve for the no wind case has a similar appearance to those determined in the radiative 

ignition experiments. There are no experimental data for the effects of wind on the ignition of 

radiatively heated fuel beds. However, given the similarities of the no wind cases, it is 

reasonable to expect that the effects of applying winds are captured correctly. 

The determination of the ignition time/temperature and ignition time/applied heat flux 

relationships for cellulosic fuels was time consuming due to the large number of measurements 

required. It should be possible to learn a great deal concerning fuel effects with many less 

experiments at a single temperature. Based on the results of the experiments reported here, an 

appropriate plate temperatures for such tests would be 380 °C, with experiments run with and 

without an applied wind. Such experiments would provide relative indications for the propensity

of fuels to smolder and flame in the absence of a wind along with an indication of the effects of 

wind on these ignition behaviors. 

87
The experimental systems developed for this investigation have proven useful for the purpose of

characterizing the ignition of cellulosic fuels. There are a number of improvements that could be

made to the heated plate system. Perhaps the most important would be the use of a com

controlled system for maintaining a constant plate temperature while automatically recording th

plate temperature. It would also be possib

puter 

e 

le to record the currents required to maintain a 

onstant temperature. Qualitative observations indicate that this measurement would provide 

ll as an 

ch 

s 

ning 

d 

o 

ity, 

ickness that were not varied. 

g of 

n 

ne 

el bed ignition behaviors since surface reactions appear to play such 

rge roles. 

m Manzello of 

FRL/NIST provided the pine needles that were tested. 

c

insights into whether or not endothermic or exothermic processes were taking place as we

indication for the amounts of heat generated during surface oxidation. The development of su

a system should be straight forward and would alleviate the need to carefully review video tape

in order to record the temperatures. 

The current findings show that HRRs from the cone calorimeter provide information concer

smoldering and flaming of these fuels when subjected to radiative heat fluxes. The time 

behavior of the HRR provides a good indication for when ignition occurs. Due to the 

configuration, it is difficult to visually observe when glowing combustion develops within the 

fuel bed. It would be useful to modify the cone calorimeter in a way that would allow improve

detection of the onset of glowing combustion. 

In order to provide higher confidence in the findings of this investigation it would be useful t

make similar measurements for a wider variety of fuels. In particular it would be useful to 

consider a wider variety of grasses and to include various types of leaves. It would also be 

important to characterize the role of such parameters as fuel moisture and fuel bed dens

porosity, and th

The types of studies reported here could serve as the basis for a much improved understandin

the ignition behaviors of cellulosic fuels. It should be possible to correlate fuel compositio

(e.g., cellulose to lignin ratio) with ignition behavior. It would be also worthwhile to exami

the possibility that small scale heating experiments such as differential scanning calorimetry 

(DSC) and thermogravimetric analysis (TGA), which record the response of small samples to 

heating, correlate with the fu

la

6. Acknowledgements 

Many people contributed to the success of this effort. Funding was provided by The 

International Consortium for Fire Safety, Health, and the Environment (ICFSHE) with financial 

oversight by Karen Suhr. Technical oversight was provided by Petra Andersson and Margaret 

Simonson of SP Technical Research Institute of Sweden. 

Cheat grass was harvested and forwarded to NIST by Tye Morgan working under the direction

of Bob Blank, both with the United States Department of Agriculture Agricultural Research 

Service Laboratory in Reno, NV. Arrangements for acquisition of the fine Florida grass were

made by James McNew of the Outdoor Power Equipment Institute. Sa

B

88
ided the 

m

ith, who 

did an excellent job. 

[1] ent, 

[2] 

lusions from Phase I 

[3] T. J. Ohlem

ute of Standards and 

p. 833-

re Tests - Ignitability of Building Products Using a Radiant Heat Source, ISO 

[13] Reacti

Heat R 0, International Organization for 

Standardization, Geneva, Switzerland, 2002. 

d 

The heated plate was designed and assembled at BFRL/NIST. Bill Grosshandler prov

design and Mike Selepak, Michael Smith, and Ed Hnetkovsky ordered parts, did required 

achining, and assembled the plate. 

All experiments in the cone calorimeter were performed at BFRL/NIST by Michael Sm

7. References 

Control of Emissions From Nonroad Spark-Ignition Engines, Vessels, and Equipm

Docket EPA-HQ-OAR-2004-0008, Regulations.gov. 

P. Andersson and M. Simonson, Scientific Evaluation of the Risk Associated with 

Heightened Environmental Requirements on Outdoor Equipment—Conc

and Proposed Plan and Budget for Phase II, SP Technical Note 2006:02, SP Swedish 

National Testing and Research Institute, Borås, Sweden, 2006, 43 pp. 

iller, Smoldering Combustion, in SFPE Handbook of Fire Protection 

Engineering, 3rd Ed., P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. 

Custer, J. R. Hall, Jr,. and J. M. Watts, Eds., Society of Fire Protection Engineers, Bethesda, 

MD, 2002. 

[4] K. M. Palmer, Smoldering combustion in dusts and fibrous materials, Combustion and 

Flame, 1 (2), 129-154 (1957). 

[5] V. Babrauskas, Ignition Handbook, Fire Science Publishers, Issaquah, WA (2003), pp. 315-

320. 

[6] T. J. Ohlemiller, Smoldering Combustion Hazards of Thermal Insulation Materials. Interim 

Report. October 1, 1979-April 30, 1981, NBSIR 81-2350, National Instit

Technology, Gaithersburg, MD (August 1981). 

[7] T. J. Ohlemiller, Cellulosic insulation material III. Effects of heat flow geometry on 

smolder initiation, Combustion Science and Technology, 26 (1), 89-105 (1981). 

[8] D. Drysdale, An Introduction to Fire Dynamics, John Wiley and Sons, New York (1985), pp.

265-277. 

[9] V. Babrauskas and W. J. Parker, Ignitability measurements with the cone calorimeter, Fire 

and Materials 11 (1), 21-43 (1987). 

[10] V. Babrauskas, Ignition Handbook, Fire Science Publishers, Issaquah, WA (2003), p

847. 

[11] Reaction to Fi

5657, International Organization for Standardization, Geneva, Switzerland, 1997. 

[12] Standard Test Method for Heat and Visible Smoke Release Rates for Materials and 

Products Using an Oxygen Consumption Calorimeter, ASTM E 1354, American Society 

for Testing of Materials, Philadelphia, PA, 1997. 

on-to-Fire Tests -- Heat release, Smoke Production and Mass Loss Rate -- Part 1: 

elease Rate (Cone Calorimeter Method), ISO 566

[14] Standard Method of Test for Heat and Visible Smoke Release Rates for Materials an

Products Using an Oxygen Consumption Calorimeter, NFPA 271, National Fire Protection 

Association, Quincy, MA, 2004. 

89
[15] R. T. Ford, Investigation of Wildfires, Maverick Publications, 1995. 

] M. P. Plucinski, The Investigation of Factors Governing Ignition and Development of Fires 

y 

f Work, 

Center, San 

Dimas, CA, November 1974, 6 pp. 

earch 

ote 

30 (1), 77-87 (2006). 

] S. L. Manzello, T. G. Cleary, J. R. Shields, and J. C. Yang, Ignition of mulch and grasses 

nal 

 Fire 14 (3), 321-338 (2005). 

] R. R. Blank, R. H. White, and L. H. Ziska, Combustion properties of Bromus tectorum L.: 

[16

in Heathland Vegetation, Doctor of Philosophy Thesis, The University of South Wales, Jul

2003, 347 pp. 

[17] R. Harrison, Danger of Ignition of Ground Cover Fuels by Vehicle Exhaust Systems, 

ED&T Report 5100-15, U.S. Department of Agriculture, Forest Service Equipment 

Development Center, San Dimas, CA, November 1970, 39 pp. 

[18] G. C. Kaminski, Ignition Time Vs Temperature for Selected Forest Fuels, Report o

U.S. Department of Agriculture, Forest Service Equipment Development 

[19] D. S. Stockstad, Spontaneous Ignition of Rotten Wood, USDA Forest Service Res

Note INT-267, Intermountain Research Station, Ogden, UT, 1979, 12 pp. 

[20] D. S. Stockstad, Spontaneous Ignition of Cheatgrass, USDA Forest Service Research N

INT-204, Intermountain Research Station, Ogden, UT, 1976, 12 pp. 

[21] D. S. Stockstad, Spontaneous Ignition of Pine Needles, USDA Forest Service Research 

Note INT-194, Intermountain Research Station, Ogden, UT, 1975, 14 pp. 

[22] S. L. Manzello, T. G. Cleary, J. R. Shields, and J. C. Yang, On the ignition of fuel beds by 

firebrands, Fire and Materials

[23

by firebrands in wildland-urban interface fires, International Journal of Wildland Fire 15

(3), 427-431 (2006). 

[24] R. H. White, D. R. Weise, K. Mackes, and A. C. Dibble, Cone calorimeter testing of 

vegetation--an update, Papers presented at the Thirty-fifth International Conference on Fire 

Safety, Seventeenth International Conference on Thermal Insulation, Ninth Internatio

Conference on Electrical and Electronic Products, Columbus, OH, July 22 to 24, 2002 

Sissonville, WV, Products Safety Corporation, 2002, 12 pages. 

[25] D. R. Weise, R. H. White, F. C. Beall, and M. Etlinger, Use of the cone calorimeter to 

detect seasonal differences in selected combustion characteristics of ornamental vegetation, 

International Journal of Wildland

[26

Influence of ecotype and growth under four CO2 concentrations, International Journal of 

Wildland Fire 15 (2), 227-236 (2006). 

90