### R EVIEW OF 

### P OTENTIAL 

### A IR 

### E MISSIONS FROM 

### B URNING 

### P OLYETHYLENE 

### P LASTIC 

### S HEETING WITH 

### P ILED 

### F OREST 

### D EBRIS



### F INAL 

### R EPORT

October 28, 2003

Prepared by:

Christopher Wrobel
Tim Reinhardt

URS Corporation
1501 4 th Avenue, Suite 1400 Seattle, WA 98101

Prepared for:

USDA Forest Service
Pacific Northwest Research Station

400 North 34th Street, Suite 201 Seattle, WA 981031 

### T ABLE OF 

### C ONTENTS

Page

1.0 I NTRODUCTION ........................................................................................................ 3

2.0 B ACKGROUND .......................................................................................................... 4

3.0 C HEMISTRY O F P OLYETHYLENE P YROLYSIS A ND C OMBUSTION M ECHANISMS ...... 5

4.0 C HEMICAL C OMPOUNDS E MITTED F ROM T HE P YROLYSIS A ND C OMBUSTION O F P OLYETHYLENE ....................................................................................................... 7

5.0 B IOMASS E MISSIONS .............................................................................................. 12

5.1 M ECHANISMS O F W OOD P YROLYSIS A ND C OMBUSTION ....................................... 12

5.2 Emissions From Wood Combustion ..................................................................... 14

6.0 D ISCUSSION ........................................................................................................... 21

7.0 C ONCLUSION .......................................................................................................... 23

8.0 R EFERENCES .......................................................................................................... 251 Executive Summary

URS reviewed the available scientific li terature regarding em issions from burning polyethylene (PE), and burning silvicultural p iles of woody debris. Few papers report measuring emissions from polyethylene burne d in an open pile, and none has assessed emissions from silvicultural piles with and without a PE covering. Thus the literature
review must draw from bench-scale studies of PE pyrolysis and combustion. These are unlikely to duplicate the actual emissions from operational bu rns of silvicultural piles.

Bench-scale research investig ating the pyrolysis and combus tion chemistry and emissions from pure PE has identified a broad range of emissions, many of which are also identified in emissions from burning woody silvicul tural debris. Carbon dioxide (CO 2 ), carbon monoxide (CO), water and particulate matter are major emissions from both sources. Polycyclic aromatic hydrocarbons (PAHs) are found in emissions from both materials, and at widely varying concentrations. Ma ny partially oxygenated products of incomplete combustion (PICs) have been quantified in woody debris emissions, and identified but not quantified in PE emissions—these along with PAHs and other PICs are known to
have adverse effects on human and nonhuman receptors when they are exposed at significant concentrations. There is no evidence that unique classes of chemicals are, or should be found in emissions from burning PE, in comparison to burning wood debris.

The relative impacts from PE and silvicultura l debris cannot be accurately estimated from these diverse data sources, but the expected ranges can be summarized. The literature suggests that the emissions to the atmosphere contributed by the sheet of PE covering are chemically similar to the emissions from the underlying pile of silvicultural debris. For many of these emissions, such as CO, CO 2 and particulate matter, the amount emitted from the woody debris will of course overw helm the contribution from the PE. Some
studies indicate that PE does not produce a great deal of PAHs , while others indicate that it may be an efficient producer of PAHs, and in theory could produce almost as much PAH as the silvicultural pile, but only when two unlikely assumptions are made: 1) that PE emissions from pellets in high-temperat ure furnaces replicate the emissions of PE covering a silvicultural pile where temperatures increase more slowly and half or more of the PE is likely to be volatilized or burned at lower temperatures before it reaches the high temperatures required to form PAHs, and 2) that the relatively complete spectrum of
individual PAHs identified in PE combustion st udies is directly comparable to the smaller subset of PAHs monitored from burning woody debris.

The literature, and anecdotal evidence, clearly indicates that silvicultural piles burn more efficiently and produce fewer PICs when they are allowed to cure to a dryness that readily supports combustion. Inasmuch as re gions in Oregon where silvicultural burning occurs are exposed to significant amounts of pr ecipitation, there is an overall emissions
reduction benefit from coveri ng silvicultural piles. Po lyethylene does not include chlorinated compounds or significant amounts of other chemicals likely to form uniquely toxic emissions, nor have these been demons trated in the literature. As a covering material, it is reportedly much less expensiv e than coated or uncoated kraft paper, an alternative product used to cove r piles in central and southern California at the request of2 local air quality management districts. Th ere are no literature studies of the emissions from coated kraft paper, but the cellulose should be similar to woody debris and the waxy coating, when melted, should burn very simila rly to melted polyethylene. In Oregon, some 314 grams of PE are burned on a typical pile covered with 4-mil PE sheeting. If 1 or 2-mil sheeting were used, the PE emissions would be cut by half or more. In terms of overall emissions from the pile and the cover, the benefit of cutti ng PE emissions by only covering the center of the pile may be offs et by higher emissions from uncovered wood which has attained higher fuel moistures during the curing period.

All the bench-scale studies suffer from a wea kness in extrapolating from very controlled combustion conditions to those found in opera tional burns. A definitive answer as to whether emissions from a PE-covered or partia lly-covered pile are measurably different from the uncovered or paper-covered pile can only be obtained from actual open burning of the piles. The availabl e literature does not support a contention that burning PE sheeting would produce unique chemicals or cla sses of chemicals that are not also found in emissions from burning wood debris.3 1.0 I NTRODUCTION

Polyethylene (PE) plastic sheeting has long b een used by the forest products industry to prevent the accumulation of excessive moisture within silvicultural debris piles as they
cure over a timeframe of several months to a year or more. The cu ring process dries the piled woody debris and enha nces the combustion process, making ignition easier and producing fewer products of incomplete combus tion. Timber harvest residues (branches and small woody fuels) and woody debris from silvicultural thinning and fire hazard reduction projects are collecte d, piled and partially covered with PE sheeting. The sheeting (typically six to ten f oot squares) is draped over a p ile and anchored with sticks, logs and rocks to prevent removal by the wind. The sheeting protects the piles from rain and snow until they cure. These piles are then ignited with a drip torch and burned, usually achieving 100% consumption of the piled debris. The PE sheeting is usually left on the pile, to burn, along with the woody debr is. Most regulatory agencies enforce ordinances against the combustion of plastics in silvicultural debris piles because of concerns that the emissions may have an adverse affect on air quality.

This paper reviews the available literature on the pyrolysis and combustion products of PE and assesses whether burning the typical amount of PE pr oduces significantly more or different emissions than the woody debris piles themselves. This work has been undertaken at the request of the Smoke Ma nagement Review Committee of the Oregon Smoke Management Plan Citizens Advisory Group. The Committee is a multiparty association comprised of parties interested in smoke management and forestry issues in Oregon, including the USDA-Forest Service, the U.S. Department of the Interior-Bureau of Land Management, the Oregon Department of Forestry, the Oregon Department of Environmental Quality, Oregon State Univer sity Cooperative Exte nsion, the Nature Conservancy, the Oregon Forest Industries Council, Lane Regional Air Pollution Control Authority, Jackson County, and small woodland owners and public representatives. The Committee has posed the following speci fic questions to the authors:

• Is there such a thing as pure PE? An d, if so, is pure PE readily available (commercially) and how would one tell PE from other polymer products on the
market that are known to contain toxic ma terials (PVC, styrene, etc.) and heavy
metals?

• What is the basic chemistry of PE and how does it combust and react within a pile
in an ambient environment?

• How will PE volatilize by itself, and as part of a slash pile?

• Are there any material safety data sheets that include a combustion test of PE?

• Is there a difference in PE emissions betw een a pile that is completely covered
and one that uses PE in the center to create a dry spot for ignition?

• Does it make a difference in the emissions if 1-mil or 4-mil PE sheeting is used?4 • Is there any benefit to requi ring the PE to sit in the environment for a period of time (weather) prior to burning?

• What are the alternatives to PE a nd what is the chemical makeup of “impregnated” paper product?

• Is the use of PE cost effective compared to alte rnative products?

2.0 B ACKGROUND

Polyethylene is the most comm only used plastic, worldwide. It is produced by the free radical polymerization of ethylen e, 1-propene, 1-butene, 1-hexe ne or 1-octene. There are three main types of PE: high-density PE (HDP E), low-density PE (LDPE) and linear lowdensity PE (LLDPE). The majority of PE produced is either HDPE or LDPE. HDPE
chains are not branched while LDPE cont ains about 50 branches for every 1000 carbon

atoms 1

.

Commercially manufactured PE is a relatively pu re material. Pure PE is translucent; the opaque black sheeting used to cover woody de bris piles is produced by adding carbon black to the polymer. Carbon black, an amorphous form of carbon, does not possess a long-range, or macrocrystalline structure. It does exhibit a short-range crystal structure
that deviates from both the diamond lattice and graphite lattice with respect to the interatomic distances and bond angles 2 . Water, polycyclic aromatic hydrocarbons (PAHs), metallic oxides, salts and other c ontaminants may be present in trace amounts
within carbon black 3

.

Small quantities of chemical additives are of ten used in order to enhance or attenuate some of PE’s characteristics. PE is easily oxidized at temperatures as low as 180 °C. To minimize oxidation during processing, either phenol or phosphite based antioxidant chemicals are commonly added. Fatty acid amid es are added to aid in the formation of plastic sheets. Polyethylene glycol este rs, glycerol monostearate and ethoxylated secondary amines are sometimes added to re duce PE’s propensity to collect a static charge 4 . The chemical composition and some of the physical properties of pure PE pellets are listed in Table 1.5 Table 1. Polyethylene – Chemical Composition and Physical Properties

Volatiles (wt.%) 100

Ash (wt.%) 0 Carbon (wt.%) 85.4 – 86.4 Hydrogen (wt.%) 13.5 - 14.3

Sulfur (wt.%) 0 - 0.08 Nitrogen (wt.%) 0 Oxygen (wt.%) 0 - 0.2 Chlorine (wt.%) 0 Heat Value (MJ/kg) 40.5

Density (g/cm 3 ) 0.910 - 0.940 Degree of Crystallinity (%) 45 - 55 Melting Point Range ( o C) 105 - 115 Molecular Weight (Daltons) 10,000 - 50,000 Reference: 4, 5, 6, 9,

Poly-America is a manufacturer of the LDPE sheeting used in Oregon as a moisture barrier on silvicultural debris piles. Although the material safety data sheet provided by the company identifies the product as HDP E and linear LLDPE, the manufacturer’s website, and subsequent phone conversations with a representative of Poly-America,
have confirmed that the material is LDPE. It would be difficult, if not impossible to differentiate among the three vari eties of polyethylene and most other types of plastics by
a visual examination of the sheet material.

The density of LDPE ranges from 0.910 to 0.925 grams per cubic centimeter. Guidance from the USDA-Forest Service stipulates that silvicultural debris pi les are to be covered with a sheet of 4-mil polyet hylene plastic of 6-foot by 6- foot minimum dimensions. A
sheet of LDPE with these dimensions w ould have an approximate mass of 314 grams (11.1 ounces).

3.0 C HEMISTRY OF P OLYETHYLENE P YROLYSIS AND C OMBUSTION M ECHANISMS

Polyethylene combustion is more akin to the combustion of a liquid, ra ther than a solid. PE melts and the liquid pyrolyzes, th en burns, producing a visible flame 7, 8 . The combustion of PE, and all polymers (including w ood) is a complex process that is still not fully researched. Pyrolysis and combustion reactions occur simulta neously in both the liquid and gas phases. Volatile components ca n continue to thermally degrade, oxidize
and/or react with other ch emical species to produce a complex mix of chemicals 9, 10

.

The first step in the pyrolysis of PE is random scission of the polymer into smaller subunits, which in HDPE produces a wide ra nge of primary radicals; because LDPE contains varying proportions of side chains, it can initially produce secondary and tertiary
radicals as well 1, 11 . Low reaction temperatures fa vor the formation of long-chain radicals, while a higher temperature leads to the formation of a grea ter number of shortchain radicals. Primary radicals can either undergo β -scission to produce ethene, or6 intramolecular hydrogen transfer to form more st able secondary radicals. The latter path dominates at low temperatures. High temperatur es also increase the rate of volatilization of longer chains, where they undergo β -scission at a rate much higher than that found in the liquid phase 12, 13

.

The rate of pyrolysis increases as the extent of the branching increa ses. LDPE pyrolyzes at a faster rate than that of HDPE, primarily as a result of the stability of the radicals formed during the initial step. The exposure of PE to ultraviole t radiation has been shown to lower its thermal stability. This trend is less pronounced for branched polymers, because they already exhibit significantly lower activation energies 11

.

The formation of aromatic rings in the PE pyrolysis/combustion gas is believed to occur at higher temperatures due to the rapid thermal degradation of the polymer. The cracking of heavy olefins produces gaseous compounds of lighter molecular weight, which at temperatures above 730 o C, can react to generate aroma tic compounds via the Diels Alder
mechanism 13, 14, 15

.

Chromatographic evidence has determined that at temperatures below 750 o C significant pyrolytic degradation of PE occurs, howeve r, complete combustion of PE would not

occur 9 . This may provide some insight into th e expected emissions from burning PE with a silvicultural debris pile. Immediately afte r pile ignition, especia lly considering the low thermal conductivity of plastics, the com bustion temperature would not be intense enough to heat the PE to a temperature high enough to initiate combustion. Because PE melts and thermally degrades at rela tively low temperatures (105 and 180 o C), pyrolysates would be formed and emitted befo re the temperature can rise high enough to ensure more complete combustion. At temperatures below 755 o C, as much as 18 to 41 percent of the mass of PE is lost and volatilized prior to particle ignition 16

.

In the context of a burning silvicultural debris pile, PE pyrolysis chemistry would be of far greater importance than combustion chemis try. The temperature of the pile surface
would rise from ambient to about 1000 o C over roughly one to five minutes. Thus, the PE is likely to undergo thermal degrada tion and melt during the early stage of combustion, when the pile temperature is between 250 and 600 o C. These temperatures are not high enough to allow PE combustion. Th e emissions from this early phase of the pile burn would contain a high percentage of aliphatic hydrocarbons and radicals, as well as a low percentage of aromatic and polyc yclic aromatic hydrocarbons (PAHs). As the biomass pyrolysis and oxidation rates accelerate, the temperatur e of the pile will increase high enough to make PE combustion an important process, but by this time most, if not all of the PE would have already been pyr olyzed and lost to the atmosphere. The temperature of the plume at the time of PE pyrolysis would be lower due to cooling by entrained air, precluding the downwind form ation of aromatic compounds because there is not enough heat to overcome the activ ation energy of the Diels Alder reaction.7 4.0 C HEMICAL C OMPOUNDS E MITTED FROM THE P YROLYSIS AND C OMBUSTION OF P OLYETHYLENE

Most of the data regarding the emissions from the pyrolsis/combustion of PE were measured at temperatures ranging from 500 to 1000 o C. The experiments were conducted in various apparatus’ that coul d be described as horizontal la minar flow reactors, vertical tube-type furnaces or fluidized bed reactors. In many cases the device had a secondary combustion chamber and exhaust samples were collected and analyzed from both the primary and secondary stages. Only results reported from a primary combustion stage are discussed here, as they are more comp arable to the conditions under which PE sheeting would burn on a silvicul tural debris pile. There is one major difference between the pile and the furnace conditions; the temperat ure of the furnace is maintained at a set temperature, while the silvicultural debris p ile begins at ambient temperature. This means that the pyrolysates and combustion products in a furnace at 800 o C are exposed to that temperature immediately upon initiation of the experiment until the samples are collected. In contrast, a silvicultural debris pile will exhibit a temperature gradient. Compounds emitted from the thermal degrada tion of PE will not remain at that temperature indefinitely.

The US EPA lists emission factors for some PAHs due to the burning of agricultural plastic 17 . Specifically, Chapter 2.5.2.4 of the widely used document “AP-42” discusses the burning of “agricultural plastic film.” Th e type of plastic is not expressly stated, however, agricultural plastic film is almost always black PE. These experimental
conditions under which these emission factors were calculated are mo re similar to those expected in silvicultural applications than those obtained under furnace conditions. The
AP-42 emission factors are rated a “C.” This indicates that there is no known statistical bias present in the data, however, the data may not represent a random sample population.

AP-42 emission factors are based on the research performed by Linak et al. at the US EPA’s Air and Energy Engineering Res earch Laboratory. The experimental methodology used kerosene to initiate the pyrol ysis of 1-lb samples of used and unused PE. The clean unused PE was balled in a manner similar to the way in which used agricultural film is prepared for incineration in the field. The used PE samples contained varying quantities of soil, biomass and water 18

.

The thermal decomposition and combustion of PE, as with the combustion of any carbonaceous fuel, produces carbon monoxide (CO), carbon dioxide (CO 2 ), water (H 2 O) and a plethora of alkanes, alkenes and aromatic compounds 9 . At least 140 chemical species have been tentatively identified in the emissions from the pyrolysis and gasification of PE 19 . The pyrolysis energy of activati on is approximately 260 kilojoules (kJ)/mole, and a range of reaction orders between 0.5 and 1.5 has been reported 12, 20

.

Some general trends regarding the types a nd quantities of emission products from the
combustion of PE have been identified. CO 2 emissions are maximized at around 600 to
700 o C, while the minimum concentrations tend to occur near 1000 o C 5, 21 . CO and8 particulate matter concentrations exhib it little variability over the 500 to 1000 o C temperature range and CO concentrations peak at around 800 o C. Particulate matter
emissions are almost identical at both e nds of the temperature range (500 and 1000 o C),
with little variation found at the temperatures betw een, although the chemical mechanisms and compounds responsible for this observation are different. As much as 70 to 97 percent of the particulate matter formed was found to have a mean aerodynamic diameter of 2 micrometers (μm) or less 22

.

The proportions of organic chemicals emitted can vary widely with the PE combustion temperature, although most chemicals can be found at some concentration at all temperatures 23 . Emissions of benzene and its derivatives are maximized around 700 to
800 o C. From 800 to 900 o C, alkenes, naphthalene, fluor ene and phenanthrene were the predominant organic compounds detected. At a combustion temperature of
950 o C, the production of light hydrocarbons wa s reduced, while PAH concentrations
increased 5 . PAH concentrations were found to increase along with combustion temperature 5, 19, 14 . Of those PAHs analyzed for; naphthalene was found in the largest concentrations followed by acenaphthylene, phenanthrene, pyrene, fluoranthene, fluorene, biphenyl, anthracene, cyclopenta[ c , d ]pyrene, benzo[ a ]pyrene, acephenanthrylene, benzo[ b ]fluoranthene, chrysene, benzo[ g , h

, i ]fluoranthene, benzo[ g , h

, i ]perylene, and perylene 5

.

Hydrocarbon emissions tend to exhibit a tre nd opposite to that of PAHs. Hydrocarbon emissions peak in the temp erature range of 600 to 800 o C 19, 14 . The concentration of ethylene was the greatest, due to the β -scission of primary radicals. The mole fractions of methane, ethane, and ethylene all reache d maximums around 700 °C. At temperatures greater than 700 °C the concen trations of methane, ethane, and ethene all decrease 5

.

The research suggests that s ynthetic polymers produce as much as one to two orders of magnitude more soot than cellulosic material at typical heating rate s (10 –100 K/s). Soot production has been found to increase with th e combustion temperat ure for all plastic polymers. PAH concentrations in the soot vary, however, soot from PE was found to have the greatest concentration of benzo[a]pyrene (1.5 mg/g) among the common plastics. Soot production was found to be very low in experiments using very high heating rates (~10,000 K/s) and combus tion temperatures in excess of 1000 o C 24, 22

.

Oxygenated organic compounds have been identified in the co mbustion gases of PE. The species positively identified include a cetaldehyde, acrolein, propionaldehyde, acetone, methyl ethyl ketone and acetic acid 23, 24

.

Emission factors for those compounds quantif ied during the pyrolysis/combustion of PE have been compiled in Tables 2 and 3. The minimum and maximum emission factors found in the literature are reported. It is im portant to note that the emission factors reported were determined under controlled laboratory conditions . The procedures generally involved dropping pure PE pellets into a furnace maintained at a known temperature. Experimental conditions and s ources of error should be expected to vary among the cited experiments. The overall c onditions at which the emission factors were9 obtained may provide values that differ from those produced under the uncontrolled, open burn conditions expe rienced in the field.

Some additional chemical species that have be en qualitatively identified are listed in Table 4. A large number of organic chemical s have been reported in PE combustion emissions, however, only those chemicals confirmed to be emitted from the pyrolysis/combustion of PE are listed.

Tables 2 and 3 also list the anticipated range of each chem ical quantified. These values were calculated using the minimum and maxi mum emission factors for each chemical compound and assuming a sheet of PE with a mass of 314 g (6-foot by 6-foot by 4-mil) is burned. The mass emissions are only intended to provide an estimate of the range and magnitude of the emissions produced from the combustion of the PE sheet. As discussed previously, the values from among those studies that reported emission rates were used in the calculations. The accuracy of the emi ssion rates is not known, and too few studies have been conducted to allow for a meaningf ul statistical analys is of these data.

Table 2. Emission Factors and Mass Em issions from Polyethylene Covering a Typical Silvicultural Pile – Inorgani c, Alkane, Alkene, Aromatic and Total Suspended Particulate Matter

Emission Factor Mass Emitted 1 Chemical Minimum Maximum Units Minimum Maximum Units Reference CO 100 175 mg/g a 31 55 g 5

CO 2 400 1500 mg/g 126 471 g 5 Acetylene ND 2.9 mg/g ND 0.9 g 5 Methane 0.2 7 mg/g 0.1 2.2 g 5 Ethane 0.175 0.6 mg/g 0.1 0.2 g 5 Ethene 1.5 12 mg/g 0.5 3.8 g 5 Propene 0.4 1.5 mg/g 0.1 0.5 g 5 Benzene 1.23E-02 4.78E-02 μg/g b 3.86E-03 1.50E-02 mg 17 Toluene 3.30E-03 4.60E-03 μg/g 1.04E-03 1.44E-03 mg 17 Ethylbenzene 6.00E-04 1.20E-03 μg/g 1.88E-04 3.77E-04 mg 17 1-Hexene 1.00E-03 4.30E-03 μg/g 3.14E-04 1.35E-03 mg 17 TSP (soot) 8 36 mg/g 2.5 11.3 g 5, 22

ND = not detected

1 Mass emissions were calculated using the minimum and maximum emission factors for each chemical compound and assuming a sheet of PE with a mass of 314 g is burned.

a Milligrams of emission per gram of polyethylene

b Micrograms of emission per gram of polyethylene10 Table 3. Emission Factors and Mass Emissions from Polyethylene Covering a Typical Silvicultural Pile – Polycyclic Aromatic Hydrocarbons

Emission Factors (μg/g) a Mass Emitted 1 (g) Chemical

Minimum Maximum Minimum Maximum Reference

1-(2-Propenyl)naphthalene 6.2E-01 1.2E+02 1.9E-04 3.8E-02 14 1,1-Diphenylethylene ND 1.5E+01 ND 4.7E-03 14 1,2:7,8-Dibenzophenanthrene 2.1E+01 1.5E+02 6.6E-03 4.7E-02 14 1,2-Binaphthalene 1.7E+00 6.1E+02 5.3E-04 1.9E-01 14 1,6-Dimethylnaphthalene 3.4E+00 1.8E+02 1.1E-03 5.7E-02 14 1-Ethylidene indene 6.3E+01 1.5E+02 2.0E-02 4.7E-02 14 1-Ethylnaphthalene 2.0E+00 6.2E+01 6.3E-04 1.9E-02 14
1H-Phenalene 2.5E+01 7.4E+02 7.9E-03 2.3E-01 14 1-Methylanthracene ND 2.8E+01 ND 8.8E-03 14 1-Methylene indene 6.2E+01 1.2E+02 1.9E-02 3.8E-02 14 1-Methylfluorene 1.7E+01 1.2E+02 5.3E-03 3.8E-02 14 1-Methylnaphthalene 2.8E+01 1.5E+03 8.8E-03 4.8E-01 14 1-Phenylnaphthalene 2.8E+01 1.3E+03 8.8E-03 3.9E-01 14 1-Phenylphenanthrene 4.0E+01 8.4E+02 1.3E-02 2.6E-01 14 2,2-Binaphthalene 2.1E+00 4.1E+02 6.6E-04 1.3E-01 14 2,6-Dimethylnaphthalene ND 2.9E+00 ND 9.1E-04 14 2-Ethenylnaphthalene 7.5E+01 5.1E+03 2.4E-02 1.6E+00 14 2-Ethylnaphthalene 2.6E+00 9.0E+01 8.2E-04 2.8E-02 14 2-Methylanthracene 1.2E+00 4.1E+02 3.8E-0 4 1.3E-01 14 2-Methylbiphenyl 4.9E-01 1.9E+02 1.5E-04 6.0E-02 14 2-Methylfluorene 2.9E+01 5.3E+02 9.1E-03 1.7E-01 14 2-Methylnaphthalene 2.2E+01 1.8E+03 6.9E-03 5.6E-01 14 2-Methylphenanthrene 1.6E+01 6.2E+02 5.0E-03 1.9E-01 14 2-Phenylnaphthalene 7.9E+01 2.8E+03 2.5E-02 8.9E-01 14 3,4-Dihydrocyclopenta[cd]pyrene 2.7E+01 5.1E+02 8.5E-03 1.6E-01 14 3-Methylbiphenyl 2.5E+00 2.8E+02 7.9E-04 8.8E-02 14 3-Methylphenathrene 1.7E+01 7.2E+02 5.3E-03 2.3E-01 14 4H-Cyclopenta[def]phenanthrene 9.6E+00 3.5E+03 3.0E-03 1.1E+00 14 4-Methylbiphenyl 1.7E+01 5.5E+01 5.3E-03 1.7E-02 14 9-Ethenylanthracene 3.0E+00 2.8E+02 9.4E-0 4 8.8E-02 14 9-Methylanthracene 1.4E+00 2.3E+02 4.4E-0 4 7.2E-02 14 9-Methylphenanthrene 2.8E+01 4.7E+02 8.8E-03 1.5E-01 14 Acenaphthalene 3.1E+02 9.1E+03 9.7E-02 2.9E+00 5, 14 Acenaphthene 2.0E+01 1.2E+03 6.3E-03 3.7E-01 14 Acenaphthylene NO DATA Acephenanthrylene 1.3E+02 4.7E+03 4.1E-02 1.5E+00 5, 14 Anthracene 1.3E-03 3.1E+03 4.1E-07 9.6E-01 5, 14, 17 Benz[a]anthracene 1.4E-02 5.3E-02 4.5E-06 1.7E-05 17 Benzo[a]anthracene 8.5E+01 1.3E+03 2.7E-0 2 4.0E-01 14 Benzo[a]fluorene 6.2E+01 2.2E+03 1.9E-02 6.8E-01 14 Benzo[a]pyrene 7.5E-03 1.6E+03 2.4E-06 5.1E-01 5, 14, 17 Benzo[b]chrysene 2.6E+01 1.2E+02 8.2E-03 3.8E-02 14Table 3. (Continued)

11 Emission Factors (μg/g) a Mass Emitted 1 (g) Chemical

Minimum Maximum Minimum Maximum Reference

Benzo[b]fluoranthene ND 3.0E+02 ND 9.4E-02 5 Benzo[b]fluorene 2.2E+01 1.8E+03 6.9E-03 5.6E-01 14 Benzo[b]pyrene 9.3E-03 3.5E-02 2.9E-06 1.1E-05 17 Benzo[c]phenanthrene 3.6E+02 2.0E+03 1.1E-01 6.2E-01 14 Benzo[e]pyrene 9.7E-03 1.2E+03 3.0E-06 3.9E-01 14, 17 Benzo[g,h,i]perylene 1.5E-02 4.9E-02 4.7E-06 1.6E-05 17 Benzo[ghi]fluoranthene 4.7E+01 1.3E+03 1.5E-02 4.2E-01 5, 14 Benzo[ghi]perylene 3.2E+01 7.5E+02 1.0E-02 2.4E-01 5, 14 Benzo[j]fluoranthene 3.3E+01 2.5E+02 1.0E-02 7.9E-02 14 Benzo[k]fluoranthene 2.5E-03 4.7E+02 7.9E-07 1.5E-01 14, 17 Bibenzo[def,mno]chrysene 8.6E+00 3.3E+02 2.7E-03 1.0E-01 14 Biphenyl 7.9E+01 4.1E+03 2.5E-02 1.3E+00 5, 14 Chrysene 1.7E-02 2.8E+03 5.4E-06 8.7E-01 5, 14, 17

Coronene 4.7E-01 1.5E+02 1.5E-04 4.7E-02 14 Cyclopenta[cd]pyrene 1.7E+02 2.8E+03 5.3E-02 8.7E-01 5, 14 Dibenz[ah]anthracene NO DATA Dibenzopyrene ND b 2.4E+02 ND 7.5E-02 14 Dihydronaphthalene 4.4E+00 4.5E+01 1.4E-03 1.4E-02 14 Diphenylethylene 1.8E+01 2.4E+02 5.7E-03 7.5E-02 14 Diphenylethyne 2.3E+01 9.5E+02 7.2E-03 3.0E-01 14 Ethylbiphenyl 5.3E+00 3.9E+02 1.7E-03 1.2E-01 14 Fluoranthene 1.1E-01 8.6E+03 3.4E-05 2.7E+00 5, 14, 17 Fluorene 1.5E+02 3.7E+03 4.7E-02 1.2E+00 5, 14
Indene 3.9E+01 6.5E+02 1.2E-02 2.0E-01 14 Indeno[1,2,3-cd]pyrene 1.1E-02 8.5E+02 3.4E-06 2.7E-01 14, 17 Methylanthracene 7.9E-01 2.2E+02 2.5E-04 6.9E-02 14 Methylbiphenyl 2.5E+00 3.3E+02 7.9E-04 1.0E-01 14 Methylfluorene ND 2.6E+01 ND 8.2E-03 14 Methylpyrene 1.1E+02 1.2E+03 3.5E-02 3.7E-01 14 m-Terphenyl 5.5E-01 1.6E+02 1.7E-04 5.0E-02 14 Naphthalene 6.7E+01 9.0E+03 2.1E-02 2.8E+00 5, 14 Perylene 9.2E+00 3.5E+02 2.9E-03 1.1E-01 5, 14 Phenanthrene 2.4E-02 1.8E+04 7.6E-06 5.7E+00 5, 14, 17 Phenylindene 4.9E+00 3.3E+02 1.5E-03 1.0E-01 14
Picene ND 1.4E+02 ND 4.4E-02 14 Pyrene 5.9E-02 1.4E+04 1.8E-05 4.5E+00 5, 14, 17

Retene 1.9E-02 3.2E-02 5.9E-06 1.0E-05 17

Total PAH 0.77 39

ND = not detected

1 Mass emissions were calculated using the minimum and maximum emission factors for each chemical compound and assuming a sheet of PE with a mass of 314 g is burned.

a Micrograms of emission per gram of polyethylene

b Not Detected12 Table 4. Additional Chemical Compounds Id entified Qualitatively in Polyethylene Pyrolysis/Combustion Emissions

Chemical Reference Acetaldehyde 23
Acetic acid 23

Acetone 23
Acrolein 23 Methylethylketone 23 Propionaldehyde 23 4- and/or 9-Methylphenanthrene 9 4H-Benz[def]carbazole 9 4H-Cyclopenta[cd]pyrene 9 Acenaphthylene 9 Benzo[a]fluoranthene 9 Benzothiophene 9 Dibenz[a,h]anthracene 9
Dibenzofuran 9

5.0 B IOMASS E MISSIONS

5.1 M ECHANISMS OF W OOD P YROLYSIS AND C OMBUSTION

Forest (woody) biomass is mainly compri sed of cellulose, hemicellulose and lignin polymers that burn via a series of steps: 1) fuel heating and drying, 2) solid fuel pyrolysis to volatiles and char, 3) pyrolysis and oxida tion of the volatiles a nd 4) oxidation of the
char. 25 Heat and mass transport processes are cl osely linked to, and affect the chemical reactions that occur; these chemical reactions in turn release or absorb heat and reaction products, which then feed back to the heat and mass transport mechanisms and can cause
overall rates of combustion to increase or decrease.

After heat transfer drives off excess mois ture and the fuel has reached a high enough temperature (150-190 °C), cellulosic materi als decompose by two alternate pathways. 26 The first pathway is a reduction in the pol ymer length by bond scission and dominates at temperatures below 300 °C. The cellulosic poly mer of wood is broken down into shorter subunits by bond scission, releasing water, fo rming free radicals and carbonyls, carboxyl and hydroperoxide groups, which then l ead to decarboxylation and decarbonylation reactions. This first pathway converts the woody substrate into a reactive carbonaceous char and generates H 2 O, CO 2 and CO as the major products, along with the free radicals. The char is characterized by a high surface ar ea, a high degree of reactivity, and a large proportion of aromatic carbon. The aromatic cont ent of the char incr eases with increasing temperature.13 The second pathway, operating above 300 °C, breaks apart the remaining substrate molecules by transglycosylation, fission and disproportionation reactions. This second pathway produces tarry anhydro sugars (e.g., levoglucosa n, 1,6-anhydro-ß-Dglucofuranose, and randomly linked oli gosaccharides) and low molecular weight volatiles. At higher temperatures (300-500 °C), the tar-forming r eactions accelerate and overtake the production of char a nd gases. At such temperatur es, the heat of evaporation of levoglucosan is so endothermic that the py rolysis process appears to be controlled by
the rate of heat transfer rather than the chemical reaction kinetics.

Smoldering and glowing combustion are both ch aracterized by oxidation of the reactive
char created from the cellulosic substrate. Smoldering combustion operates at relatively low temperatures and involves the formation of reactive char by pyr olysis, chemisorption of oxygen on the char, production of CO and CO 2 , and generation of new reactive sites. Incompletely oxidized volatile pyrolysi s products are released, and smoldering combustion is characterized by a high propor tion of these products of incomplete combustion. Glowing combustion, which burns the char more rapidly and at higher temperature, is incandescent and requires more oxygen than smoldering combustion, but both are solid-phase combustion processes. 26

Flaming combustion is the result of oxidation of the gaseous volatiles, which are liberated
in the breakdown of the cellulosic structure. These volatiles then are combusted in the gas phase. Flaming combustion is dominate d by free radicals rapidly breaking apart larger molecules into simpler compounds a nd atoms. This involves a chain reaction mechanism that can be self-sustaining (e .g., regenerating the initiating radicals), or branching (generating more radicals). The re latively high rate of h eat release created by the combustion of volatiles provides the en ergy for gasification of the substrate and propagation of the fire.

The rate of combustion is strongly influenced by fuel-specific factor s, including the fuel bulk density, the arrangement (porosity) of the fuelbed (e.g., the deviation from the optimum ratio of air spaces to fuel material within the overall fuel array) and the fuel
moisture content. This last factor is the driving force behind the use of plastic sheeting and other materials to cover piled forest bioma ss. At higher fuel moistures, more of the
heat from a fire’s exothermic reactions is used to vaporize excess water from the intact fuel matrix, resulting in lower temperatures within the fuelbed and enhancing the fuel consumption by smoldering combustion. This produces a much higher proportion of incompletely combusted products (e.g., CO and particulate matter). Conversely, at lower fuel moistures, less of the heat released is us ed to turn water to steam, and thus more is available to enhance gasifi cation and liberation of vola tiles. The volatile compounds burn in the gaseous phase with more oxygen available than in smoldering combustion, resulting in cleaner combustion and producti on of more completely oxidized combustion products (e.g., CO 2 ).

In summary, the mechanism of wood combustion is in many ways similar to that of polyethylene combustion. Glowing and sm oldering combustion are wood-specific mechanisms that can produce significant products of incomplete combustion, whereas14 flaming combustion results in much more comple te, and cleaner, emissions. It is very difficult to precisely estimate emissions because they are dependent on the relative strengths of these various pyr olysis and combustion processes, which in turn depend on fuel moisture and pile arrangement.

5.2 Emissions from Wood Combustion

Burning forest biomass produces a diverse ar ray of many hundreds of chemical species, ranging from gases to liquids and solid partic ulate matter. Relatively small amounts of sulfur oxides (SO
x ) are formed due to the low sulfur content of forest fuels. Small amounts of oxides of nitrogen (NO
x ) can be produced from endogenous plant nitrogen (e.g., chlorophyll), but temperat ures are generally too low to dissociate atmospheric N 2

. 27 Table 5 lists maximum mixing ra tios for nitrogen species and SO 2 observed above a bench-scale burning system. 28

Table 5. Nitrogen and Sulfur Species Measured in Forest Biomass Smoke

Compound Maximum Mixing Ratio (ppm) Combustion Phase

NO

x 100-160 Flaming Nitrous oxide 2-19 Flaming Ammonia 5 Smoldering Hydrogen cyanide 4-30 Smoldering Methyl cyanide 2-10 Smoldering Sulfur dioxide 20-30 Flaming

Carbon by far dominates the emissions from forest biomass burning. The major carbonbearing pollutants can be broadl y broken down as shown in Table 6. 29

Table 6. Percentage of Carbon Emissions by Phase of Combustion

Carbon contribution (% by weight) Emission species Flaming phase Smoldering phase
Carbon dioxide 95.8 86.6
Carbon monoxide 3.0 10.1 Organic carbon particulate 0.7 2.3 Elemental carbon particulate 0.2 0.1
Methane 0.2 0.5 Nonmethane hydrocarbons 0.1 0.4

In flaming combustion, more of the fuelbed carbon is converted to CO 2 and elemental carbon in the particulate matter, while sm oldering combustion produces proportionately more CO, organic carbon, methane and nonmethane hydrocarbons. Most of the particulate matter (82%) is smaller than 1 μ m in aerodynamic diameter, and only 8% is15 above 5 micrometers. 30 Thus, measurements of fine particles less than 2.5 μ m in diameter (PM2.5) represent the bulk of par ticulate emissions. The production of CO is highly correlated to the producti on of particulate matter and PM2.5, with a coefficient of determination (r 2 ) between emission factors for these pollutants in the range of 0.8 to
0.84. 31 Combustion efficiency, a measure of how much of the carbon in the fuelbed is
converted to CO 2 , can range from less than 60% for smoldering-phase combustion to over 95% for flaming-phase combustion. Many of the pollutants emitted from forest fires
are well-correlated to combustion efficienc y, and algorithms have been developed to estimate their production. 32

Table 7 lists some of the major air toxics measured in emissions from burning forest biomass, and algorithms for calculating thei r production as a function of combustion efficiency or a ratio to commonly-measured em issions such as CO or particulate matter. 32

Table 7. Algorithms and Ratios for Estima ting Emissions of Air Pollutants from Prescribed Burning

Pollutant Regression or ratio Emission
Factor Units Mass Emitted 1 Units Reliability

Carbon dioxide η ×1833 a 1.7E+03 g/kg 1.0E+06 g ±5%
Carbon monoxide 961- η ×984 2.6E+01 g/kg 1.6E+04 g r 2 = 0.95
Methane 42.7- η ×43.2 1.7E+00 g/kg 9.9E+02 g r 2 = 0.77 Nonmethane hydrocarbons 0.76+EF CH4×0.616 b 1.8E+00 g/kg 1.1E+03 g r 2 = 0.69 Fine particles (PM2.5) 67.4- η ×66.8 1.3E+02 g/kg 7.8E+04 g r 2 = 0.74 Thoracic particles (PM 10) 1.18×EF PM2.5 1.5E+ 02 g/kg 9.2E+04 g ±30% Particulate Matter (TSP) 93.3- η ×90.5 7.3E+00 g/kg 4.4E+03 g r 2 = 0.54 Formaldehyde (HCHO) 0.0137×EF CO-0 .0179 3.4E-01 g/kg 2.0E+02 g ±30%
Acrolein 0.0029×EF CO+0.0699 1. 5E-01 g/kg 8.7E+01 g ±30% Acetaldehyde 0.315×EF HCHO 1.1E-01 g/kg 6.4E+01 g ±50% 1,3-butadiene 0.00213×EF CO 5. 6E-02 g/kg 3.3E+01 g ±40% Benzene 0.00592×EF CO 1.6E-01 g/kg 9.3E+01 g ±40% Toluene 0.00588×EF CO 1.5E-01 g/kg 9.2E+01 g ±40% o-Xylene 0.00089×EF CO 2.3E-02 g/kg 1.4E+01 g ±40% m,p-Xylene 0.00161×EF CO 4. 2E-02 g/kg 2.5E+01 g ±40%
n-Hexane 0.00017×EF CO 4.5E -03 g/kg 2.7E+00 g ±40% Benzo(a)pyrene 13×EF PM 9.5E+01 μ g/kg 5.7E+04 μ g ±50%
PAHs 345×EF PM 2.5E+03 μ g/kg 1.5E+06 μ g ±50%

1 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight feet (approximate mass = 598 kg).

a η = combustion efficiency (%)

b EF = emission factor

For woody fuels, piles have relatively hi gh combustion efficiencies, and thus lower emissions of products of incomplete combusti on, as compared to broadcast-burned slash. Table 8 lists emission factors for piled silvic ultural debris from the Pacific Northwest. 3316 Table 8. Emission Factors for Major Pollutants from Piled Silvicultural Debris

Emission Factor (pounds per ton of fuel consumed) Pile Type Phase PM PM10 PM2.5 CO CO 2 CH 4 NMHC Flaming 11.4 7.4 6.6 44 3492 2.4 2.2 Smoldering 25.0 15.9 14.0 232 3124 17.8 12.2Tractorpiled Fire-average 20.4 12.4 10.8 153 3271 11.4 8.0 Flaming 22.6 13.6 11.8 101 3349 9.4 8.2 Smoldering 44.2 33.2 31.0 232 3022 30 20.2Cranepiled Fire-average 36.4 25.6 23.4 185 3143 21.7 15.2

Other measurements of emissions from be nch-scale burning of forest biomass have identified a variety of PAHs; these are summ arized in Table 9, along with an estimated mass of emissions from a typica l silvicultural debris pile. 34 A list of quantified oxygenated organic chemicals a nd other various pollutants fro m bench-scale studies are presented in Tables 10 and 11. The quantitie s of each chemical species emitted are listed in Tables 9, 10 and 11. The mass range of each chemical was calculated from the widely-used Consume 2.1 fire consump tion and emissions model, assuming a composition of 60 percent Douglas fir and 40 percent western hemlock and assuming a dome shaped pile eight feet high and eight feet in diameter . A fuel bulk density of 27.5
lb/ft 3 was assumed, along with a fuelbed porosity of 20%. A pile with these characteristics was determined to have a mass of 598 kg.

Table 9. Emission Factors and Mass Emissions – Polycyclic Aromatic Hydrocarbons from Fir and Pine Burns

Emission Factor (μg/g) Mass Emitted 1 (g) Chemical

Minimum Maximum Minimum Maximum Reference

1-(2-Propenyl)naphthalene 1,1-Diphenylethylene 1,2:7,8-Dibenzophenanthrene 1,2-Binaphthalene 1,6-Dimethylnaphthalene 1-Ethylidene indene 1-Ethylnaphthalene
1H-Phenalene 1-Methylanthracene 1-Methylene indene 1-Methylfluorene 1-Methylnaphthalene 1-Phenylnaphthalene 1-Phenylphenanthrene 2,2-Binaphthalene 2,6-Dimethylnaphthalene NO DATA17 Emission Factor (μg/g) Mass Emitted 1 (g) Chemical

Minimum Maximum Minimum Maximum Reference

2-Ethenylnaphthalene
2-Ethylnaphthalene 2-Methylanthracene 2-Methylbiphenyl 2-Methylfluorene 2-Methylnaphthalene 2.265 2.575 1.35 1.54 34 2-Methylphenanthrene 2-Phenylnaphthalene 3,4- Dihydrocyclopenta[cd]pyrene 3-Methylbiphenyl 3-Methylphenathrene

4HCyclopenta[def]phenanthrene 4-Methylbiphenyl 9-Ethenylanthracene 9-Methylanthracene 9-Methylphenanthrene Acenaphthalene NO DATA

Acenaphthene 1.868 2.518 1.12 1.51 34 Acenaphthylene 1.41 2.418 0.84 1.45 34 Acephenanthrylene NO DATA
Anthracene 0.429 0.723 0.26 0.43 34 Benz[a]anthracene 0.114 0.25 0.07 0.15 34 Benzo[a]anthracene Benzo[a]fluorene NO DATA

Benzo[a]pyrene 0.019 0.694 0.011 0.415 34, 35 Benzo[b]chrysene NO DATA Benzo[b]fluoranthene 0.037 0.056 0.022 0.033 34 Benzo[b]fluorene Benzo[b]pyrene Benzo[c]phenanthrene NO DATA

Benzo[e]pyrene 0.024 0.054 0.014 0.032 34 Benzo[g,h,i]perylene Benzo[ghi]fluoranthene NO DATA

Benzo[ghi]perylene ND 0.002 ND 0.0012 34 Benzo[j]fluoranthene NO DATA Benzo[k]fluoranthene 0.039 0.136 0.023 0.081 34 Bibenzo[def,mno]chrysene Biphenyl NO DATA

Chrysene 0.1 0.217 0.06 0.13 34

Coronene Cyclopenta[cd]pyrene NO DATA

Dibenz[ah]anthracene ND ND ND ND 34 Dibenzopyrene Dihydronaphthalene NO DATATable 9 (continued)

18 Diphenylethylene Diphenylethyne Ethylbiphenyl
Fluoranthene 1.351 1.766 0.81 1.06 34
Fluorene 0.68 0.857 0.41 0.51 34
Indene NO DATA Indeno[1,2,3-cd]pyrene ND ND ND ND 34 Methylanthracene Methylbiphenyl Methylfluorene Methylpyrene m-Terphenyl NO DATA

Naphthalene 13.57 16.96 8.11 10.1 34 Perylene ND ND ND ND 34
Phenanthrene 2.594 3.938 1.55 2.35 34 Phenylindene
Picene NO DATA

Pyrene 1.066 1.469 0.64 0.88 34

Retene NO DATA

Total PAH 15.3 20.7

ND = not detected

1 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight feet (approximate mass = 598 kg).

Table 10. Emission Factors of Major Oxygenated Compounds Emitted from the Smoldering Combustion of Wood

Compound Emission
Factor (mg/kg) Mass Emitted 1 (g)

Acetic acid 4.40E+03 2.63E+03

Methanol 4.30E+03 2.57E+03 Vinyl Acetate 3.00E+03 1.79E+03 1-Hydroxy-2-propanone 2.00E+03 1.20E+03 Pyruvic Aldehyde 2.00E+03 1.20E+03

Formic acid 9.70E+02 5.80E+02 2-Furaldehyde 7.90E+02 4.72E+02 3-Oxobutanoic acid, methylester 7.00E+02 4.19E+02

2-Furanmethanol 2.80E+02 1.67E+02 2-Methoxy-4-methylphenol 2.60E+02 1.55E+02 Propanoic acid 2.50E+02 1.50E+02 2-(5H)-Furanone 2.20E+02 1.32E+02 2-Cyclopenten-1-one 1.90E+02 1.14E+02 2-Methoxyphenol 1.80E+02 1.08E+02 gamma-Butyrolactone 1.70E+02 1.02E+02 5-Methyl-2-furaldhyde 1.50E+02 8.97E+01Table 10 (continued)

19 Compound Emission
Factor (mg/kg) Mass Emitted 1 (g)

Phenol 1.10E+02 6.58E+01 4-hydroxy-3-methoxybenzaldehyde 7.60E+01 4.54E+01 2-Methoxy-4-(1-prop-2-enyl)phenol 6.80E+01 4.07E+01

Crotonic acid 6.80E+01 4.07E+01 2-Acetylfuran 5.40E+01 3.23E+01 m- and/or p-Cresol 4.40E+01 2.63E+01

o-Cresol 4.20E+01 2.51E+01

Benzoic acid 2.00E+01 1.20E+01 o-Hydroxybenzaldehyde 1.50E+01 8.97E+00

Reference: 36

1 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight feet (approximate mass = 598 kg).

Table 11. Emission Rates and Mass Em issions for Various Pollutants from Burning a Silvicultural Pile

Emission Rate (g/kg) Mass Emitted 1 (g) Compound
Minimum Maximum Minimum Maximum Reference

Acetic Acid 1.6 4.4 963 2631 37, 38, 36 Acetylene 0.2 0.9 120 526 37, 38 Ethene 1.1 3.3 661 1961 37, 38
Ethane 0.2 0.2 140 140 37 Methane 1.2 19.4 739 11601 37, 39, 38, 40, 35 Carbon monoxide 28.2 359.0 16834 214682 37, 39, 38, 40, 35 Carbon dioxide 1146 1771 685308 1059058 37, 39, 38, 40, 35 Formaldehyde ND 2.4 ND 1423 37, 39, 38 Formic acid 0.7 1.6 425 939 39, 38, 36 Hydrogen cyanide 0.6 0.6 365 365 38 Methanol 0.8 4.3 493 2571 39, 38, 36 Ammonia 0.3 1.5 169 891 39, 38 Nitric oxide 0.9 2.3 526 1357 37, 39, 38 Nitrogen dioxide 0.6 0.6 360 360 37
Sulfur dioxide 1.2 1.2 705 705 37 PM 6.0 34.2 3588 20452 33, 41, 42, 35, 43

PM10 6.2 12.8 3708 7656 28 PM2.5 2.8 17.6 1674 10525 33, 42, 35, 40, 43

ND = not detected

1 Mass emitted from burning one silvicultural debris pile with a diameter and height equal to eight
feet (approximate mass = 598 kg).

In summary, the burning of silvicultural debr is produces a wide range of air emissions, many of which have toxic or reactive propertie s. The mass of pollutants emitted is quite20 large for a typical pile, but piling has been shown to produc e fewer emissions per ton of silvicultural debris burned, because it optimizes combustion conditions when compared to broadcast burning of debris in-situ. 4421 6.0 D ISCUSSION

In this section, we specifically answer the questions posed by the Committee.

Is pure PE readily available (commercially ) and how would one tell PE from other polymer products on the market that ar e known to contain toxic materials (PVC, styrene, etc.) and heavy metals?

Pure PE is commercially available; however, most of the PE manufactured will contain various types and amounts of a dditives. The quantities of these chemicals added to the PE would be small and should not significantly affect the types or amounts of chemicals
emitted when PE is burned. The carbon black added to the polymer may contain trace amounts of PAHs, metal oxides and other cont aminants. Again, this is unlikely to produce any detectable increase in emissions as carbon black comprises a few percent of
the total mass of PE and the contaminants ar e present in only trace amounts in the carbon black itself. One study determined that ther e was no statistical difference in the heavy
metal concentrations found in the residual ash of a coal-fired boiler with, or without the
addition of PE waste to the fuel 45 . It would be difficult, if not impossible to differentiate among the three varieties of pol yethylene and most other types of plastics by a visual
examination of the sheet material.

What is the basic chemistry of PE and how does it combust and react within a pile in
an ambient environment?

How PE will pyrolyze and combust within a slas h pile can only be surmised based on the available research regarding its behavior when burned in a furnace or alone using
kerosene as an accelerant. Several trends are evident in the pyrolysis/combustion behavior of PE. First, the production of PAHs will increase as the combustion temperature increases from 500 to 1000 o C. Second, the emissions of light hydrocarbon will peak around 700 o C. Third, particulate emissions ar e the highest at both low and high ends of the temperature range, although the em ission factor does not deviate considerably throughout the range of temperatures. Third, complete combustion of PE will not occur at temperatures below 750 o C.

How will PE volatilize by itself, and as part of a slash pile?

Based on the available data, it can be assumed that the volatilization of the PE will commence at a relatively low temperature. In the early stage of the pile burn, temperatures will quickly rise from ambient to around 350 to 400 o C. This is high enough to thermally degrade the PE, but not high enough to allow combustion. The pyrolysates will melt and drip into the hotter region of the fire and volatilize into the atmosphere. Since up to 41 percent of the PE is lost prior to ign ition at temperatures
lower than 750 o C, little PE will remain by the time the pile temperature becomes high enough to favor the formation of PAHs. This means the emissions of PAHs from the PE are likely to be low.22 Are there any material safety data sheet s that include a combustion test of PE?

Material Safety Data sheets do not contain detailed informa tion regarding the products of incomplete combustion of PE because the ra w material is not considered hazardous and burning is not an intended use of PE sheeting. The manufacturer provided material safety data sheet for the PE sheeting lists CO 2 and H 2 (O) as the compounds emitted during combustion and states CO would be produced under oxygen limiting conditions. The actual emissions from any fuel will vary c onsiderably with the conditions under which it is burned, but are almost never limited to these three chemicals.

Is there a difference in PE emissions between a pile that is completely covered and one that uses PE in the center to create a dry spot for ignition?

A smaller amount of PE would of course produce a proportionate ly smaller mass of emissions. From the pile as a whole, resear ch has documented that higher fuel moisture results in higher overall emissions due to the increased dominance of smoldering combustion processes. An uncovered pile that is left over a winter can take up to two drip torches of fuel to sust ain ignition—this would increase overall emissions due to the excess fuel used. However, when piled forest materials are dry enough to sustain ignition, the combustion of the smaller-diame ter materials in the fuelbed produce enough heat to drive off fuel moisture from the larg er fuel pieces. This reduces the consumptionlimiting effect of high fuel moisture, increa ses the combustion efficiency and consumes virtually all of the woody material in the pile.

Does it make a difference in the emissions if 1-mil or 4-mil PE sheeting is used?

The amount of PE used will proportionately affect on the overall amount of chemicals emitted from the PE. Using PE sheeting of only 1-mil thickness would cut the emissions by one quarter. If thinner PE is determin ed to work adequately, its use should be
considered. Thinner PE would be more cost-e ffective, and each roll would be easier to handle since it would weight less per square foot. However, if the thinner PE sheet is easily torn on the fuel pile , it may not be qui te as effective at drying the fuels beneath.

Is there any benefit to requiring the PE to sit in the environment for a period of time (weather) prior to burning?

There is no benefit to allowing the PE shee ting time to weather prior to burning. Exposure to ultraviolet radiation will begin to break the polymer down, but this will not have a significant affect on the overa ll combustion efficiency of the burn.23 What are the alternatives to PE and what is the chemical makeup of “impregnated” kraft paper?

The only alternative to PE used by the forest products industry is kr aft paper. The kraft paper is often treated with a waxy hydrocarbon (p araffin) to aid in its ability to resist water penetration. Anecdotal reports indicate some problems with the durability of the kraft paper, especially in high-precipitation climates. Paraffin is a long-chain hydrocarbon similar to some of the n-alkane emissions produced from the pyrolysis of PE. Kraft paper is made from cellulose and is therefore similar to the biomass contained in the silvicultural debris piles. The cellulose has been liberated from the lignin during the pulping process using heat, pressure, s odium hydroxide and sodium sulfide. The paper may contain trace amounts of residual sulfur compounds and could produce sulfurcontaining emissions upon combustion. The quantity of sulfur emitted is not expected to be large.

Is the use of PE cost effective co mpared to alternative products?

From an economic standpoint, PE is more cost-effective than paper. The paper used to cover silvicultural debris piles may or may not be coated with paraffin. The coated paper costs about $160 per 3 ft by 200 ft roll, which is 26.7 cents per square foot. PE sheeting comes in numerous sizes, although Grayback Fore stry in Oregon has indicated that they purchase the material in rolls 3 ft wide, weighing 22.9 lbs. A roll 3 ft by 400 ft would cost $14.40 (assuming at least 5000 lbs of PE is purchased per order) or about 1.2 cents per square foot.

7.0 C ONCLUSION

The benefit obtained from the increased co mbustion efficiency commensurate with dry
biomass fuel favors the use of some sort of moisture barrier to aid in the drying of logging slash piles. The articles reviewed provide no evidence that burning the PE plastic sheeting along with the slash pile woul d cause a significant im pact to air quality, however, the limited amount of information rega rding the pyrolysis/combustion of PE in conjunction with woody biomass pr ecludes an ironclad statement. It is, however, likely
that the emission factors for PAHs and single ring aroma tic compounds obtained for the burning of PE in furnace type conditions overe stimates the amount that would be formed during open burning. This is due to the f act that during an open burn, the pile temperature would be much lower than the 900 to 1000 o C temperatures that favor the formation of PAHs. By the time the higher te mperatures are reached, most of the PE will have thermally degraded and volatilized.

Because the conditions under which PE pyrolys is and combustion have been studied are so varied, it is difficult to quantify the expect ed emissions of hazardous air pollutants. At least two researchers agree on the expected concentration range of TSP (soot) in the smoke from PE pyrolysis/combustion (8 to 36 mg per gram of material). However, other articles provide ranges for PAH emissions th at are about 1000 times greater than those24 listed in AP-42. The emission factors for th e 77 PAHs found indicate that a sheet of 4 mil PE weighing 314 grams would produce between 2.5 and 11.3 grams of TSP and between 0.8 and 39 grams of PAH compounds when burned.

The literature is almost as ambiguous concer ning biomass emissions as it is for PE. Using the lowest and highest emission factors for PM found in the lite rature gives a range of emissions between 3.6 and 20.5 kg per 598 kg pile. Empirical data from Table 8 for crane-piled debris (similar to hand piling) would lead to an estimate of 10.9 kg of PM. Using the TSP emission factor from Table 8 (converting units--18.2 g/ kg) in the equation for PAHs listed in Table 7, returns a value of 3.8 grams of PAHs per slash pile. This mass of PAHs is considerably lower than the range of 15 to 21 grams per pile calculated using the emission factors for the 16 PAHs published by Jenkins et al . Further complicating the issue, because no published da ta exist for prescribed burning emissions of most of the 77 PAHs quantif ied in the PE research, a direct comparison of total PAH
emissions can not be made between the two sources.

Unfortunately, the literature has not provide d an unequivocal answer to whether burning PE plastic would have any greater impact on air quality than the burning of biomass. This is surprising given that AP-42 emission factors were determined in response to the agricultural industry’s practice of burning PE sheeting ma terial. If this is an ongoing practice across the country, then it seems contradictory to allow it for one industry, but not another. Unfortunately at this time, th e only way to be sure burning PE with logging slash would not be detrimental to air quality is to conduct an experi mental burn using the same type of fuel with and without PE sheeting and to quantify the emissions.25 8.0 R EFERENCES

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