NISTIR 8128

Feasibility Studies for Technologies to 

Effectively Scrub Soot from SmallScale Fire Experiments

Anthony Hamins

Thomas Cleary

Matthew Bundy 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.IR.8128
NISTIR 8128

Feasibility Studies for Technologies to 

Effectively Scrub Soot from SmallScale Fire Experiments

Anthony Hamins 

Thomas Cleary

Matthew Bundy 

Fire Research Division 

Engineering Laboratory 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.IR.8128

May 2016 

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and 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.
ABSTRACT

Elimination of particulate from fire exhaust streams is a common issue for fire labs. Two series 

of fire experiments were conducted in the National Fire Research Laboratory (NFRL) to test 

technologies for possible use in the filtration of soot from the Building 224 fire research exhaust. 

The first experimental series used two large surface area pleated cartridge filters to capture 

particulate in the gas phase. The second series of experiments employed a wet scrubbing system. 

The experiments showed that neither technology provides adequate performance. The results 

showed that the first system rapidly clogged and the pressure drop across the filters did not 

efficiently “recover” to its pre-test value when pulsed. The particulate removal efficiency of the 

wet scrubbing soot filtration system was inadequate.

i
Introduction

During summer 2015, a survey was conducted of experimentalists in the National Institute of 

Standards and Technology (NIST) Fire Research Division on the amount of particulate 

generation from exhaust hoods in Building 224. The survey results are shown in Table 1. Based 

on recent history and projected experimental plans, it was determined that about 11 kg of 

combustion particulate, primarily soot, was expected to be generated on-average per year. The 

amount of soot generated varies depending on the amount and type of materials burned. A wide 

range of test materials were identified as seen in the table. While the historic data in the table is 

based on lab notes and is probably fairly accurate, the uncertainty of the projected values was 

difficult to estimate. i The estimated soot emission values listed in Table 1 were provided to the 

State of Maryland Office of Environmental Compliance,ii which subsequently provided an 

exemption to the requirement of no visible emission of particulate from fire research activities in 

Building 224. Although an exemption allows continuation of current fire research experiments, 

it does not guarantee that future needs with possibly larger soot emission will be allowed. For 

this reason, the experiments described below were conducted to test the performance of various 

technologies to filter soot from a fire product stream. 

Recently, the NIST Building 224 fire research exhaust was refurbished and fitted with a filter 

assembly that allows three layers of filtration. Each layer is composed of 12 planar filters - each 

with a 0.4 m2 (4 ft2) area, for a total of 4 m2 (48 ft2) total system area. Experience with this 

system showed that the first layer of 12 planar filters clogged (about 1 kPa or 4 in of water 

column pressure drop) within 15 min of burning of a 5 cm diameter toluene pool fire with 100 ml 

of toluene (87 g). With a soot yield of about 20 % [1], the toluene fires were estimated to have 

clogged the filtration system with about 16 g of soot - a soot mass loading on the filters of 

roughly 4 g/m2 (0.4 g/ft2). This soot filtration system is not cost effective and does not provide 

adequate mass collection for the needs of the fire research enterprise in Building 224. 

It should be noted that soot filtration is an issue facing many fire laboratories, including those in 

Maryland and Montgomery County. A number of fire labs do not scrub particulate from the 

exhaust of small (e.g., cone calorimeter at the Consumer Product Safety Commission Rockville 

facility, Montgomery County’s Fire and Rescue Department, and the Fire Protection Engineering 

Department at the University of Maryland at College Park) and even moderate-scale (order of 

500 kW) fire experiments (University of Maryland at College Park and the Fire Protection 

Engineering Department at Worcester Polytechnic University).[2, 3, 4, 5]. Thus, a cost-effective 

treatment process would be of general interest to the fire testing and research communities. 

Soot has unique properties – it is “fluffy” with a very low bulk density. It is an effective 

attenuator of visible light. When illuminated, it absorbs and scatters light. Even small amounts 

of soot can be visualized. The objective of these experiments was to test the performance of 

technologies that filter soot-laden flow such that no particle emission is visible, or at least 

remove a large fraction of the flow particulate. The fire source is a 20 cm diameter toluene pool 

fire. Toluene has a high soot yield and this fire is considered representative of a source that 

produces a challenging, yet plausible, amount of soot loading for the exhaust system in 

i. The projected values for 2016 appear to have been significantly over-estimated.

ii. through the NIST Safety Office.

1
Table 1. Results from 2015 staff survey on historic and anticipated soot emission in Building 224. 

Year Number 

of Tests

Apparatus Estimated Fuel

Mass Loss/Test (g)

Material Soot Yield 

[1] 

Soot Emitted during 

Typical Test (g)

Estimated 

Soot (g)/year

2012 75 cone (vitiated) 200 Foam, wood, cable 0.05 10.0 750

2012 30 Cone 280 Polymer nanocomposites 0.02 5.6 168

2012 30 Cone 100 Polyurethane foam (PUF), 

polymethylmethacrylate (PMMA), 

Wood

0.03 3.0 90

2012 140 Cone 250 Cables 0.02 5.0 700

2012 40 Cone 100 Wood, Particle Board, Fabric 0.02 2.0 80

2012 50 Cone 30 Barrier Fabrics 0.02 0.6 30

2012 30 Cone 30 Foams 0.05 1.5 45

2012 180 Cone 100 Fire Retarded (FR) foam, control 

foam

0.1 10.0 1800

2012 15 Cone 20 Fire Retarded Foams 0.1 2.0 30

2013 190 Cone 20 Foams 0.02 0.4 36

2013 115 Cone 20 Cables 0.02 0.4 46

2013 50 Cone 20 Fabric 0.02 0.4 20

2013 10 Cone 300 Polystyrene 0.02 6.0 60

2013 74 Cone 300 Particleboard 0.02 6.0 444

2013 5 Cone 300 PMMA 0.02 6.0 30

2013 3 Cone 300 PMMA, Polyethylene (PE), 

Polystyrene (PS)

0.05 15.0 45

2013 6 Cone 300 Polyol 0.02 6.0 36

2013 65 Cone 100 Carpet 0.02 2.0 130

2013 15 Radiant Panel 500 Polyvinylchloride (PVC) 0.02 10.0 150

2013 200 Cone 100 PUF 0.10 10.0 2000

2013 100 Cone 250 PMMA 0.02 5.0 500

2014 100 LIFT 1000 Nylon carpet 0.02 20.0 2000

2014 200 TPP 10 Polyphenylene Ether (PPE) 0.02 0.2 40

2014 100 Cone 150 PMMA 0.01 1.5 150

2
Year Number 

of Tests

Apparatus Estimated Fuel

Mass Loss/Test 

(g)

Material Soot Yield 

[1]

Soot Emitted 

during Typical 

Test (g)

Estimated 

Soot (g)/year

2015 100 Radiant Panel 2500 PVC 0.04 100.0 10000

2015 100 Cone 20 PUF 0.10 2.0 200

2016 800 Cone 200 PUF (80%)/wood (20%) 0.05 10.0 8000

2016 30 Radiant Panel 100 Wood (red oak) 0.02 2.0 60

2016 30 Radiant Panel 50 Polystyrene (PS) 0.20 10.0 300

2016 30 Radiant Panel 50 PVC 0.10 5.0 150

2016 20 Flooring RP 100 Polypropylene 0.06 6.0 120

2016 20 Flooring RP 100 Nylon 0.08 8.0 160

2016 20 Flooring RP 200 Wood (red oak) 0.02 4.0 80

2016 20 LIFT 200 Wood (red oak) 0.02 4.0 80

2016 20 LIFT 100 PS 0.20 20.0 400

2016 20 LIFT 100 PVC 0.10 10.0 200

2017 1000 Cone 250 PS (unexpanded) 0.10 25.0 25000

2017 100 Detection Lab 50 Cooking stuff (food, oil) 0.02 1.0 100

2017 800 FE/DE 12.5 Propene 0.06 0.8 600

2017 500 textile testing 200 Fabrics 0.05 10.0 5000

2017 120 Radiant Panel 100 Wood (red oak) 0.02 2.0 240

2017 120 Radiant Panel 50 Polystyrene 0.20 10.0 1200

2017 120 Radiant Panel 50 PVC 0.10 5.0 600

2017 80 Flooring RP 100 Polypropylene 0.06 6.0 480

2017 80 Flooring RP 100 Nylon 0.08 8.0 640

2017 80 Flooring RP 200 Wood (red oak) 0.02 4.0 320

2017 80 LIFT 200 Wood (red oak) 0.02 4.0 320

2017 80 LIFT 100 Polystyrene 0.20 20.0 1600

2017 80 LIFT 100 PVC 0.10 10.0 800

Avera

ge

1028 

tests/yr

- 200 g/test - - 8.5 g/test 11 kg/year

3
Building 224. The ultimate interest is to provide a cost effective way to reduce the amount of 

particulate in the exhaust of experiments conducted by NIST’s fire research program in Building 

224 on the NIST campus. To this end, two series of experiments were conducted in the NIST 

National Fire Research Laboratory (NFRL) to test technologies for possible use in the filtration 

of soot. 

The first series of experiments in the NFRL used two large surface area pleated cartridge filters 

to capture the particulate in the gas phase. The second series of experiments employed a wet 

scrubbing system. These technologies were selected for testing based on recommendations in a 

report commissioned to evaluate alternative soot filtration technologies for possible 

implementation at the new Consumer Product Safety Commission (CPSC) laboratory in 

Rockville, Maryland by a fire protection engineer in consultation with industry particle filtration 

experts [6]. Obviously, there are many possible ways to scrub soot from combustion generated 

flows, including a baghouse, such as that implemented at the NIST NFRL. The technologies 

tested in this study were selected in terms of costs (initial cost and maintenance) and practicality 

(e.g., available space).

Cartridge Filtration Experiments

Dust collection cartridge filters are used in the biomass, gas turbine, and industrial boiler 

industries among others. The promise was that these are highly effective and when pretreated, 

able to accommodate high particulate loadings, and then through air pulsation, drop the collected 

particulate into a hopper installed below the filter. It should be noted that the Consumer Product 

Safety Commission (CPSC) was considering such a system in their new laboratory based on 

technical recommendations from experts in industry and the fire research community. A series 

of experiments were designed to test the performance of such a system. 

The first series of experiments involved nine tests conducted using 20 cm diameter toluene pool 

fire and two tests using polystyrene/heptane pool fires, all about 20 kW to 30 kW in measured 

heat release rate. A photo of the experimental set-up is shown in Figure 1. A round metal burner 

with an inner diameter of 21.5 cm (8.5 in) and 11.5 cm (4.5 in) deep was used for all experiments 

except the first two, when a 30 cm square metal burner was used. A total of about 1.4 kg of 

toluene was burnt with an estimated 20 % soot yield soot [Error! Bookmark not defined.] 

generated by the 9 toluene fires. Two cartridge filters were used to capture particulate in the 

exhaust stream. The filters, roughly cylindrical in shape, 1 m (39 in) long and 0.36 m (14 in) 

diameter, each had a highly pleated surface area of 39.5 m2 (425 ft2), or a surface area of 79.0 m2

(850 ft2) for the two filters. Not considering the pleats, the outer surface area of each of the 

cylinders was about 1.10 m2

 (11.8 ft2), yielding a ratio of pleated surface area to outer surface 

area of about 36. As recommended by the filter manufacturer, the filters were pre-treated with 

diatomaceous earth before each of the experiments. A larger amount was used initially, which 

was decreased for subsequent tests until no pre-coat was used for Tests 7 - 9, when the effect of 

not pretreating the filters was investigated. Not all the pre-coat stuck to the filters; some amount 

was observed on the bottom of the filter assembly. The filter assembly housed the two filters 

(flow in parallel through the filters) as seen on the left of the image. 

4
The fuel used in all tests was toluene, except Tests 1 and 2. The fuel used in Test 1 was 2 kg of 

Polystyrene (PS) plus 500 mL of heptane (340 g). The fuel used in Test 2 was 2 kg of PS plus 

250 mL of heptane (170 g). The polystyrene (PS) plus heptane fires burned for approximately 4 

min to 6 min until they were extinguished. Not all of the fuel was consumed during the first two 

tests. There appeared to be some amount of liquid, presumably heptane, on the bottom of the 

burner after the first experiment. Since the mass loss of heptane and PS are not similar, the mass 

loss and estimated soot yield are not accurate for polystyrene, since the fractions of heptane and 

PS burned are unknown. Subsequent experiments exclusively used toluene. 

Figure 1. Toluene pool fire burning during the first soot filtering experimental series conducted in the 

NFRL on July 10, 2015. Smoke emitted by the toluene fire is evident, while no smoke plume was 

observed at the illuminated vertical exhaust duct on the right side of the image.

As the test proceeded, soot was trapped by the filters and the pressure drop across the filters 

increased. After each test, the filters were rapidly pulsed as recommended by the manufacturer, 

to open the pleats and remove particulate material from the filter, permitting it to fall to the 

bottom of the assembly. This was repeated several times as the pressure drop was observed. 

After pulsing, the cartridge filter appeared covered in soot (see Figure 2) and some amount of 

light-color diatomaceous earth was observed to cover the bottom of the assembly and its wall 

surfaces.

Filter 

Assembly

Exhaust

Fan

5
Table 2 shows the fuel mass loss and the peak heat release rate (HRR) during the experiments. 

The HRR varied from about 30 kW to 50 kW. The toluene pool fires (Tests 3 - 9) were allowed 

to burn approximately 3 min to 6 min when the pressure drop across the cartridge filters 

increased to a point that the fan could not maintain flow. The mass loss for each test is listed in 

Table 2. The estimated soot in the flow (based on a 20 % soot yield) varied from 20 g to 70 g. 

As the tests were conducted, the filters clogged much more rapidly than expected as observed by 

the increasing pressure drop across the filters. Table 2 shows that during the toluene fuel tests 

(Tests 3 – 9), the pressure drop increased from about 300 Pa to 700 Pa (1 in to 3 in of water 

column) in about 3 min to 6 min of burning, capturing an estimated 25 g to 67 g of soot. 

Figure 2. Soot covered the cartridge filters, looking upwards within the filter assembly box (see Figure 1). 

Some amount of light-color diatomaceous earth can be seen on the assembly wall surfaces behind and to 

the left of the filter.

Although a relatively thin layer of particulate (pre-coat material and carbonaceous soot) was 

visible on the pleated surfaces, the outer portions of the filters were thick with particulate (see 

Figure 2). If the full pleated surface of the filters was effective, then there would be about 79 m2

(850 ft2) of effective filter surface area available and the filtration effectiveness as characterized 

by the soot mass loading would vary from about 0.3 g/m2 to 0.9 g/m2 (0.03 g/ft2 to 0.08 g/ft2) for 

the experiments. This result is 10 to 30 times smaller than expected when compared to the 

results found by CPSC during analogous experiments using the same toluene burner, with a 

6
measured value of 10 g/m2 (0.9 g/ft2

). iii[4] As expected, the CPSC result is the same order of 

magnitude as that found for the Building 224 planar filtration system as described above - a 

value of about 4 g/m2

 (0.4 g/ft2). This suggests that the effective surface area of the cartridge 

filter was closer to the 3.6 m2 (24 ft2) outer surface area of the cylindrical cartridges with a 

corresponding range of soot filtration effectiveness varying from 0.10 g/m2 (1.1 g/ft2) to 

0.26 g/m2 (2.8 g/ft2 ) as shown in Table 2. Based on these results, the cartridge filter technology 

was not seen as an attractive option for soot filtration. 

Table 2. Results from experiments using two large-area pleated filters.

Test Fuel Fuel 

Mass 

Loss 

(g)

Peak 

HRR 

(kW)

Estimated 

Soot (g) 

Precoat 

(kg)

Initial 

P (Pa)

∆P during 

burn (Pa)

Soot Mass 

over 850 ft2

(g/ft2

) 

Soot Mass 

over 24 ft2

(g/ft2

) 

1 Polystyrene

/Heptane

363 50 33 5 340 135 0.04 1.4

2 Polystyrene

/Heptane

235 23 22 2 570 365 0.03 0.9

3 Toluene 129 23 26 2 650 374 0.03 1.1

4 Toluene 170 25 34 2 550 452 0.04 1.4

5 Toluene 209 29 42 1 500 503 0.05 1.7

6 Toluene 333 33 67 0.5 415 578 0.08 2.8

7 Toluene 290 30 58 0 367 666 0.07 2.4

8 Toluene 209 32 42 0 387 627 0.05 1.7

9 Toluene 126 29 25 0 478 465 0.03 1.1

Wet Scrubbing Filtration Experiments

Wet scrubbers remove particulates from gas flows by means of inertial or diffusional 

interception of particulate matter with water droplets introduced in the flow. They include spray, 

entrainment, and Venturi scrubber designs to induce particulate/droplet interaction. Reported 

collection efficiencies are typically about 80 % or higher for particles with aerodynamic 

diameters 2.5 μm or greater.

The second series of experiments employed the same 20.3 cm diameter round pool burning 

500 ml of toluene as used during the first series of experiments. A two-stage horizontallyoriented wet-scrubbing system was used to filter the soot. A photo of the experimental set-up is 

shown in Figure 3. The first stage of the scrubbing system flowed the particle laden flow 

through a water-flooded packed media section designed to remove particulate greater than 

iii. The CPSC uses a commercial system to scrub soot. The fire exhaust flows through a filter media,

which traps the particulate. As the media becomes loaded with particulate, the pressure drop across the

filter increases and the system automatically indexes a new section of filter material into the flow stream.

7
40 μm. The structured packed bed scrubber was housed in a metal compartment (0.5 m wide, 

2.1 m long, and 1.4 m tall) and employed seven spray nozzles (similar to water sprinklers in 

design as seen in Figure 4) that provided water droplets that coated an offset vertical metal matrix

(denoted a mist eliminator by industry) shown in Figure 5. The nozzle was design to provide 

water droplets on the order of a millimeter in diameter with a spray pattern over a 15 cm round 

area. A self-priming circulation pump (500 L/min) provided continuous water to a shelf to which 

the nozzles were attached. The eliminator section in the scrubber was designed to strip-off the 

droplets and mist carried over from the scrubbing element, and as the droplets impinged against 

the mist eliminator, remove the droplets from the flow.

The second stage was a metal sieve tray scrubber housed in a metal compartment (0.5 m wide, 

2.1 m long, and 1.6 m tall) containing a series of water cascades and connected in series to the 

first stage. In one key experiment (Test 10), the water flow was turned off to investigate the 

importance of the water on the performance of the system. A number of instruments were used 

to characterize soot in the exhaust flow and thereby the effectiveness of the scrubbing system. 

Soot mass was determined gravimetrically and by laser light extinction. A cascade impactor was 

used to measure the particle size distribution in the flow. 

Bouguer's Law relates the ratio of the transmitted (I) and incident (Io ) intensities of light to the 

soot mass concentration (Ms), the pathlength through the smoke (L), and the mass specific soot 

extinction coefficient (σs ) through the following expression: I/Io = exp (-σs Ms L). Figure 6

shows the laser/detector system and soot sampling probe at the exhaust duct. The laser beam and 

detector were mounted on steel rods, ensuring laser alignment, and separated by 25 cm. The 

laser was positioned just above the exit of the duct, providing a measurement of light extinction 

(at 635 nm wavelength; visible light in the red part of the spectrum). 

Once soot was collected by the scrubbers, it appeared to stay suspended in the water. A 

pressurized filter apparatus shown in Figure 7 was used to remove the suspended solids from the 

liquid. Figure 8 shows a soot “cake” on the filter media after processing. This approach was 

quite effective as seen in the photos with the water very clear after treatment.

The water in the scrubbers was changed out once during the experiments. The water was rather 

dark with suspended soot. Mixing of the soot suspension was enhanced by adding a small 

amount of surfactant (about 50 ml) to the 500 L of water inside the scrubbers. 

The laser was allowed to warm-up before the experiment to minimize the rate of signal drift. 

Signal strength and background were measured before and after each experiment. Signal drift 

was removed by subtracting an assumed linear laser power drift. Figure 9 to Figure 11 show the 

laser extinction raw signal, corrected signal, and calculated soot concentration of the flow given 

the 25 cm path length across the duct, and the mass specific soot extinction coefficient 

(8.7 m2/g).[7] The detector signal upward drift in Figure 9 was due to a change in the laboratory 

ambient temperature caused by the inflow of cold outside make-up air just prior to and during the 

experiment inducing a gradual increase in the laser power. Had the laser been given time to 

8
Figure 3. Toluene pool fire burning during the second soot filtering experimental series conducted in the 

NFRL on November 24, 2015. A visible smoke plume was observed at the illuminated vertical exhaust 

duct as seen on the right side of the image. 

Figure 4. One of seven spray nozzles used to scrub soot in the first stage of the scrubbing assembly.

Exhaust

Wet scrubbing 

stages

9
Figure 5. Vertical metal matrix assembly used in first stage of the wet scrubbing system.

Figure 6. Exhaust duct instrumented with a red laser/detector system and soot sampling probes. Laser 

light scattered by soot in the flow is evident above the duct. 

Duct

Detector Laser

Soot sampling probe

Scattered laser light

10
Figure 7. Pressurized filter device used to remove soot from the water. Filtered water (soot removed) is 

seen in the bucket and flowing from the device.

v

Figure 8. Soot “cake” on TYVEC filter media extracted from water using the pressurized filter apparatus 

shown in Figure 7. 

11
stabilize to the ambient room temperature, the drift would have been much less severe, e.g. 

Test 9 with the signal shown in Figure 10. The soot mass flow rate in the exhaust was computed 

by multiplying the volumetric flow by the soot concentration. Integrating over the duration of 

the experiment provides an estimate of the total soot mass exiting the exhaust duct, which can be 

compared to the estimated soot yield from the toluene pool fire. The relative expanded 

uncertainty is on the order of 20% for the soot mass exiting the exhaust duct. Table 3 

summarizes the results from the laser experiments which showed that the percentage soot 

sampled in the exhaust stream ranged from about 67 % to 70 % of the expected soot yield, 

implying a system collection efficiency of about one-third. A comparison of the results from 

Test 10 to those of Tests 8 and 9 suggests that water in the scrubbing system had little effect on 

scrubber performance. Thus, much of the soot collection was apparently not due to water 

scrubbing, but simply soot losses on surfaces – either within the scrubber or external to it.

Table 3. Results from wet scrubbing experiments. 

Test Fuel Water 

Scrubbing

Fuel 

Volume (ml)

Fuel 

Mass (g)

Estimated

Soot Yield (g)

Mass (g) 

from Laser 

Measurement

Percentage Soot in 

exhaust at Sampling 

Location

8 Toluene On 500 433 87 61 70 %

9 Toluene On 500 433 87 60 69 %

10 Toluene Off 500 433 87 59 67 %

A 10-stage cascade impactor was assembled with greased aluminum foil impaction substrates 

and a (PTFE) after filter. Impaction substrates were coated, weighed and reweighed more than 

24 h later to establish stable coated substrate initial mass. The soot-laden flow before (Test 9) 

and after (Test 8) the wet scrubber was sampled iso-kinetically in two separate experiments. The 

soot sampling probe consisted of a 7.4 mm (internal diameter) copper tube aligned directly 

towards the incoming flow. The probe was approximately positioned in the middle of the duct, 

either near the duct exit or about 1 m before the first scrubbing stage. Sampling times were 200 s 

and 100 s during Test 8 and Test 9, respectively. The sample flowed through an assembly fitted 

with dried and weighed polytetrafluoroethylene (PTFE) filters to collect soot. The sample flow 

12
2

3

4

5

6

7

8

9

10

0

50

100

150

0 200 400 600 800 1000

Raw Signal

Corrected Signal

Soot Conc.

Detector Signal (v)

Soot Concentration mg/m3

Test Time (s)

Figure 9. Soot concentration determined from the baseline corrected laser signal measured in the fire 

exhaust duct exit after the wet scrubber during Test 8. 

5

6

7

8

9

10

0

50

100

150

0 200 400 600 800 1000

_

Raw Signal

Corrected Signal

Soot Concentration

Detector Signal (v)

Soot Concentration mg/m3

Test Time (s)

Figure 10. Soot concentration determined from the baseline corrected laser signal measured in the fire 

exhaust duct exit after the wet scrubber during Test 9.

13
4

4.5

5

5.5

6

6.5

7

0

50

100

150

0 200 400 600 800 1000

Raw Signal

Corrected Signal

Soot Concentration

Detector Signal (v)

Soot Concentration mg/m3

Test Time (s)

Figure 11. Soot concentration determined from the baseline corrected laser signal measured in the fire 

exhaust duct exit after the scrubber (with the water not flowing) during Test 10.

was regulated with a rotometer with the flow adjusted such that it was held nearly constant 

throughout each experiment. The sampling time was adjusted to reduce particulate loading on 

the collection substrate, thus reducing sample collection biases. After each experiment, the 

substrates and back-up filter were weighed, then placed in a desiccator and re-weighed (more 

than 24 h) later to obtain a stable (dry) particulate mass. The mass did not change appreciably

indicating that the sample was mainly soot, not water. The relative uncertainty of the mass 

collected on each stage is estimated as 0.01 mg. 

Figure 12 shows the cascade impactor measurement results for Test 8 (sampling after the 

scrubber, at the exhaust duct exit) and Test 9 (sampling before the scrubber) plotted on a log 

probability scale, and Table 4 summarizes the cascade impactor measurements. The amount of 

soot mass sampled in Test 8 was higher than in Test 9 because the sampling time was longer. 

The collected particulate appeared blackish as expected – typical of soot. The particle size 

distribution is of particular interest and there is sufficient soot mass to compare the tests. There is 

little if any difference in the size distribution of the two samples. The wet scrubbing system did 

relatively little to remove large particles from the flow. 

14
0.01

0.1

1

10

.01 .1 1 5 10 20 30 50 70 80 90 95 99 99.9 99.99

Toluene 8 (after scrubber)

Toluene 9 (before scrubber)

50 % cut size 

(Aerodynamic Diameter 

m)

Percent Less Than Stated Size

Figure 12 Measured soot aerodynamic diameter before (Test 9) and after (Test 8) the wet scrubbing 

system for samples extracted from the exhaust of 500 ml toluene pool fires. 

Test # Sampling 

Location

Mass Median 

Diameter (μm)

Mass percentage from soot 

particles less than 2.5 μm

Mass collected

(mg)

8 after scrubber 0.65 69 % 8.9

9 before scrubber 0.75 69 % 5.1

Table 4. Summary of the cascade impactor measurements. 

Conclusions

Two series of fire experiments were conducted in NIST’s National Fire Research Laboratory to 

test technologies for possible use in the filtration of soot from the exhaust stream of 20 cm 

diameter pool fires burning 500 ml of toluene. The first experimental series used two large 

surface area pleated cartridge filters to capture particulate in the gas phase. The second series of 

experiments employed a wet scrubbing system. The experiments showed that neither technology 

provided adequate soot filtration. The gas phase filtering system rapidly clogged. The 

performance of the wet scrubbing system was found to be inadequate. The wet scrubbing system 

could possibly collect a significant portion of the exhaust particulate by significantly extending 

its length or putting several scrubbers in series; but its length would need to be prohibitively 

large, which is impractical in terms of available space. The use of other technologies to 

efficiently scrub soot ought to be investigated. 

15
Acknowledgements

The authors are grateful to Lauren DeLauter, Doris Rinehart and Mike Selepak for their expert 

help conducting the experiments and to Andrew Lock of CPSC and Josh Dinaburg of JensenHughes Associates for helpful discussions and assistance. 

References

1. Hurley, M., (Ed.), SFPE Handbook, Fifth Edition, Appendix 3, Society of Fire Protection

Engineers, 3437 (2015).

2. Personal Communication, Stanislav Stoliarov, the University of Maryland at College Park,

February 2016.

3. Personal Communication, Joshua Dinaburg, Hughes Associates, November 2015.

4. Personal Communication, Andrew Lock, Consumer Products Safety Commission, January

2016.

5. Personal Communication, Nicholas Dembsey, Worcester Polytechnic Institute, April 2015.

6. Jensen Hughes, Consumer Product Safety Commission Smoke Abatement Upgrade: Phase I –

Design Study and Construction Estimate, Project 1JBD00002.000, prepared for the Consumer

Product Safety Commission, May 15, 2015.7. Mulholland, G., and Coarkin, C., Specific

Extinction Coefficient of Flame Generated Smoke, Fire and Materials, 24, 227-230 (2000). o

16