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Smoke Emission Factors fromMedium-Scale Fires: Part 1Nancy J. Brown a , R. L. Dod a , Frederick W. Mowrer a

, T. Novakov a & Robert B. Williamson aa Lawrence Berkeley Laboratory , University ofCalifornia , Berkeley, CA, 94720Published online: 07 Jun 2007.

To cite this article: Nancy J. Brown , R. L. Dod , Frederick W. Mowrer , T. Novakov &Robert B. Williamson (1989) Smoke Emission Factors from Medium-Scale Fires: Part 1,Aerosol Science and Technology, 10:1, 2-19, DOI: 10.1080/02786828908959216

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Smoke Emission Factors from Medium-Scale Fires: Part 1

Nancy J. Brown, R. L. Dod, Frederick W. Mowrer*, T. Novakov, and Robert B. Williamson Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720

The concept of "nuclear winter" has been postulated with a number of assumptions regarding the smoke pro- duced by post-nuclear-exchange fires. Whether nuclear winter would occur in the aftermath of a nuclear ex- change depends largely on the quantity and character of smoke generated, its distribution in the atmosphere, and its optical characteristics. Since most of the current information is derived from small-scale experiments with little or no correlating data for larger scales on the order of actual buildings, a series of 11 medium-scale experi- ments with representative urban fuels, such as wood and asphalt roofing shingles, have been conducted. The ef- fects of different combustion conditions resulting from differences in fuel composition, geometry, and ventilation have been investigated. The present state of knowledge regarding smoke production from building fires is briefly reviewed and placed into the nuclear winter framework. Plywood sheets were burned in a parallel plate configu- ration, a geometry in which fuel specimens create their own environment independent of the larger surroundings. Wood cribs were burned in fully and underventilated

conditions. Asphalt roofing shingles were burned under conditions representing the burning of a roof with the impingement of an external flame. The conduct of the experiments is described where mass loss of fuel during burning, the rate of heat release by oxygen depletion calorimetry, and the effective heat of combustion were determined as a function of time. The mass of smoke and the particle size distribution in each experiment were determined by sampling the aerosol particulate in the exhaust duct which captured all the effluent from the burning material. Reproducibility of the combustion pa- rameters measured during the experiments is good, and where possible, they compare favorably with values in the literature. The influence of the combustion conditions on soot loading and character is discussed. The implications of the measurements are discussed in the context of nuclear winter to relate the results of medium-scale fire experiments to characteristics of fires after nuclear blasts. The measurement of the smoke particulate and its char- acterization are described in an accompanying paper.

INTRODUCTION

The concept of "nuclear winter" has been posutlated with a number of assumptions regarding the smoke produced by post- nuclear-exchange fires (Turco et al., 1983, 1984). The general premise is: after the use of nuclear weapons, sufficient smoke would be generated from fires and deposited in the atmosphere to cause a decrease in the inci- dent solar energy reaching the Earth's sur- face. Such a change in the net radiative

*Current address: Department of Fire Protection Engineering, University of Maryland, College Park, MD 20742

balance could cause global cooling. Whether nuclear winter will occur in the aftermath of a nuclear exchange depends largely on the quantity and nature of smoke generated, its distribution in the atmosphere, and its opti- cal characteristics. The concept of nuclear winter was reviewed by a committee of the National Research Council (NRC, 1985). In general, the case put forward by Turco et al. (1983, 1984) was supported with the strong recommendation that the uncertainties be reduced by further research directed toward achieving an understanding of the basic pro- cesses that are important to the problem.

In order to develop possible nuclear win- ter scenarios, numerous assumptions have

Aerosol Science and Technology 10:Z-19 (1989) 0 1989 Elsevier Science Publishing Co., Inc.

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Smoke Emission Factors from Medium-Scale Fires 3

been made regarding smoke production from fires ignited and sustained as a result of nuclear blasts. Turco et al. (1983, 1984) used data from small-scale fire experiments (Tewarson, 1982) to estimate the smoke emission factors (i.e., the mass of smoke produced by the burning of a unit mass of a fuel). Penner (1986,1987) has reviewed many of the uncertainties in the smoke "source term" in which she considered the estimated fuel loading, smoke emission factors, the op- tical properties of the smoke, injection height of smoke, and the fraction of smoke scav- enged by precipitation.

Two general fire types, urban and wild- land, are considered with respect to repre- sentative fuels and their smolung character- istics (Turco et al., 1983, 1984; NRC, 1985). Urban fires are composed of fuels derived from many types of materials found in the built environment, including primarily wood-based products and polymeric materi- als. Stores of liquid hydrocarbons and other organic chemicals are also potentially impor- tant fuels in urban settings. Wildlands are more homogeneous in fuel content, with vir- tually all combustible materials having a cel- lulosic molecular structure. This does not preclude, however, large differences in smoke production from individual rural fires, since smoke characteristics depend on a host of parameters in addition to fuel composition.

Previous investigations of nuclear winter (Turco et al., 1983, 1984; NRC, 1985) used baseline data on smoke emission factors and composition to reach their conclusions that disturbances to solar transmission in the wake of a nuclear exchange may be serious. Turco et al. (1983) used an average net smoke emission factor of 2.7%, with emission fac- tors of 1.1% for urban areas and 3.2% for wildfires which included the scavenging by rain. They also indicate that the hghly ab- sorbing sooty fraction, consisting principally of graphitic carbon, could comprise up to 50% by weight of the emission. The NRC (1985) study used a baseline urban smoke emission factor of 4% and a baseline forest fire emission factor of 3%. The urban smoke

emission factor is composed of two compo- nents: an emission factor of 3% for cellulosic materials and an emission factor of 6% for polymeric materials. An average graphitic fraction of soot of 20% was used in the NRC study as a conservative estimate, although it was noted that t h s fraction can range from 5% to 90% for polymeric materials, and is typically about 10% for forest fires.

Knowledge of the quantity and character of the smoke emitted by fires occurring in the aftermath of a nuclear exchange is cru- cial for predicting its climatic impact (Turco et al., to be published). Sooting is controlled by a complex interplay between chemical kinetics and fluid mechanics. Relative to our understanding of elementary reactions of small species, the understanding of the chemistry of larger species (soot precursors and soot) is in a very primitive state (Haynes and Wagner, 1981; Smith, 1981; Homann, 1985; Smyth and Miller, 1987). It is not possible to predict the soot quantity and character from our current understanding of the chemistry. Considerable problems of scale are also associated with identifying cru- cial heat transfer and turbulence parameters. The performance of smoke quantitation and characterization measurements over a range of scales will make positive and important contributions to ths most difficult and sig- nificant problem. These measurements should be accompanied by careful documen- tation of combustion conditions to enable researchers ultimately to employ the results obtained in controlled laboratory situations to extrapolate to conditions believed impor- tant in various post-nuclear-fire scenarios.

The organization of t h s first paper is as follows. The experimental facility and proce- dures are described, and the experimental results are given with emphasis on the com- bustion conditions. Background material is presented to allow the reader to put these results into the context of the nuclear winter hypothesis and the results are then discussed in that context. During the course of the various experiments, important combustion parameters, mass loss of the fuel, tempera-

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ture, rate of heat release (RHR), exhaust system flow rate, and 0, and CO concentra- tions, have been measured to provide a database of information on smoke relevant to the nuclear winter hypothesis. The sec- ond, accompanying paper presents and de- scribes the smoke characterization measure- ments and discusses them.

EXPERIMENTAL FACILITIES AND PROCEDURES

Eleven experiments are discussed in ths pa- per. In six of these Douglas fir plywood was

burned in a parallel plate fashion where two vertical faces are burned, as schematically shown in Figure 1. The fuel geometry, one of the two idealized cases presented by Babrauskas and Williamson (1978), repre- sents the condition in which fuel specimens create their own environment, independent of the larger surroundings. The nominal sep- aration between the sheets was 0.15 m. In expts. 1 and 2, a catch pan was not provided, thus allowing the burning mass that fell away from the load cell to give erroneous readings in the mass loss data. This was corrected by the addition of a wire mesh catch basket for

RACK I I I I

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I I 8

FIGURE 1. A schematic diagram of the vertical parallel plhte apparatus used to burn plywood. During the experiment two faces of the plywood are exposed to an igniting flame from a gas burner which is shut off after the plywood is burning in a self-sustaining manner.

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Smoke Emission Factors from Medium-Scale Fires

FIGURE 2. A schematic diagram of angled parallel slate appara- tus used to bum asphalt roofing shingles. As in the plywood experiments, the igniting flame of the gas burner is shut off after the shingles are burning in a self-sustaining manner.

I I

the remaining experiments. In expt. 2 there thck. In expt. 10, three-ply plywood, which was only a 0.10-m separation between the had thicker layers of veneer, was burned. plates, and in expt. 5 the height and width This specimen is intermediate between the were reversed. Experiments 1-5 were con- samples used for expts. 1-5 and whole wood. cerned with the burning of five-ply CDX A propane-fired gas flame was used to ignite grade Douglas fir plywood whch was 1.3 cm the plywood faces. The gas supply to the

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ter the fuel speci-

used to ignite the

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Smoke Emission Factors from Medium-Scale Fires 7

The remaining three experiments dis- cussed in this paper used wood cribs as specimens. The cribs weighed 28 kg and were composed of Douglas fir whole wood sticks (3.6 cm X 3.6 cm X 0.45 m) arranged in 15 layers of six sticks. The spacing be- tween each layer was 4.8 cm to yield a lattice work box with rectangular faces of dimen- sions given in Table 1. Ignition of a crib was accomplished by heptane burning in a 0.3- m-diameter pan located on the platform floor beneath the crib. Smoke emission measure- ments were not conducted until the heptane was completely consumed. In one experi- ment a single wood crib was burned in the open, whereas in the other two, three wood cribs were burned under limited ventilation conditions. Table 1 summarizes the 11 exper- iments performed in this study.

The experimental facility used in t h s work is shown in plan and section in Figures 4 and 5. It consists of a 1100-m3 (38,000-cu ft) " high-bay'' room, a forced ventilation sys- tem, data acquisition facilities, and auxiliary sample preparation and storage area. The ventilation system is shown schematically in Figure 5. The 3.0-m X 3.0-m (10 ft X 10 ft) canopy hood is the first element in the venti- lation system whch is designed to capture the products of combustion. The experi- ments reported here were either conducted on a 2.4-m x 2.4-m (8 ft X 8 ft) platform

TABLE 1. Summary of Experiments

Specimen

Experiment Height Width no. Material Ventilation (m)

Plywood Plywood Plywood Plywood Plywood Plywood Asphalt Asphalt Wood crib Three wood cribs Three wood cribs

Open Open Open Open Open Open Open Open Open Room Room

directly under the hood or in a 2.4-m x 2.4- m X 2.4-m (8 ft x 8 ft x 8 ft) burn room which was connected by a 0.76-m x 1-m window to another 2.4-m x 3.7-m x 2.4-m (8 ft X 12 ft x 8 ft) antechamber whch had a doorway under the hood. The locations of the platform and the two rooms are shown in Figures 4 and 5. The experimental room and ventilation system are capable of sus- taining postflashover fires of several megawatts for up to one-half hour duration. The main purpose of the facility is to permit medium- to large-scale fire experiments to be conducted in whch all the effluents are col- lected in the ventilation duct.

The ventilation system is capable of trans- porting combustion products at a maximum rate of approximately 4 Sm3/s (8475 SCFM). The canopy hood is connected to the exhaust fan by 0.61-m (24 in) diameter stainless steel ducting. Straightening vanes are placed in the duct to minimize turbulence. The 0.92-m (36 in) diameter exhaust fan is powered by a 1.5-kW (2 HP) AC motor, and the flow rate of the exhaust system is controlled by a solid-state motor controller. This allows the exhaust rate to be operated at approximately 40% of its maximum capacity for low-inten- sity fires. During the performance of this set of experiments, the maximum flow rate was used. The smoke samples were withdrawn at the sampling ports shown in Figure 4. Spe- cific details of the sampling and analytical procedures used to analyze the smoke are given in the companion paper.

Measurements of the RHR were made using oxygen depletion calorimetry (ODC) (Parker, 1982). This method of determining the heat release rate is based on the assump- tion that all materials release approximately the same amount of heat per unit mass of oxygen consumed. Oxygen concentration measurements were performed in the ex- haust stream by extracting gas samples from the exhaust stream and analyzing them with a Beckman Instruments paramagnetic oxy- gen analyzer. Measurements of oxygen con- centration were recorded every 5 seconds by

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Smoke Emission Factors from Medium-Scale Fires 9

FIGURE 5. A schematic diagram of the ventilation system and data acquisition facilities used for all experiments. Note that the ventilation system is situated so that all of the effluent from the experiment can be monitored.

a laboratory minicomputer. Flow rates in the exhaust stream were determined using a bidirectional low-velocity probe, as de- scribed by McCaffrey and Heskestad (1976), and which is accurate to within 10%. Loss of fuel mass during burning was measured us- ing a Toledo scale model 8132 whch was placed under the platform or the assembly containing the fuel. The output from the scale was recorded in real time using the computer controlled data acquisition system. The effective heat of combustion for the various fuel assemblies was calculated as

dHc = Q / m , (1)

using the experimentally determined heat re- lease rates and fuel mass loss rates. In some cases, mass loss rate measurements were av- eraged over five time scans (25 seconds) to smooth the data.

The well-ventilated experiments, as indi- cated by "open" in Table 1, were conducted on the platform which was placed directly under the hood. Experiments 12 and 13 were conducted in the burn room, which is con- structed with a steel frame containing the

bounding surfaces of the room. The frame rests on a fire resistant floor whch is ele- vated above the laboratory floor to permit instrumentation (e.g., the Toledo scale) to be located below the burn room. The walls and ceiling of the room can be built to simulate different types of construction. For t h s se- ries of experiments, the small interior burn room was lined with ceramic fiber insulation blankets and the outer larger room was lined with gypsum wallboard. The fiber insulation blanket was used in the burn room to give more fire resistance and provide more insula- tion during the experiment. The entire room was only ventilated through a window open- ing (1 m x 0.76 m).

EXPERIMENTAL RESULTS AND DISCUSSION

The combustion conditions of the various fire experiments are described in thls section to enable the reader to understand better the smoke characterization results described i~ our accompanying paper. The wood crib ex-

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periments are discussed first since they rep- resent a wide range of ventilation conditions.

Burning of Wood Cribs

Experiments 11, 12, and 13 were concerned with the burning of wood cribs. Wood cribs are used primarily as a means of generating reliable and reproducible fire sources for re- search purposes. Heat confinement w i t h the wood cribs and cross-radiation between the burning surfaces allow sticks of substan- tial cross section to burn efficiently. Gross (1962) and Block (1971) have investigated wood cribs in detail and have summa- rized their combustion characteristics. Burn- ing rate and duration for cribs are deter- mined by fixing values of independent variables which include stick thckness, num- ber of layers, separation of the layers, and the length of the sticks.

Experiment 11 consisted of burning a sin- gle crib in the open under well-ventilated conditions. Figure 6 is a plot of the rate of heat release (kW) versus time, determined from oxygen depletion calorimetry. Ignition of the crib is accomplished in approximately

3 minutes. Sampling for smoke particles oc- curs between 7.5 and 16.5 minutes, and for most of this period, burning is nearly steady state with an average heat release rate of 230 kW. The steady-state nature of the burning is even more evident in Figure 7 where the mass of the specimen plus the platform are plotted as a function of time. The slope is nearly constant over the sampling period. The data shown in Figures 6 and 7 were smoothed and used to obtain the effective heat of combustion, as defined in Eq. (I), and plotted as a function of time in Figure 8, where the smoothed value varies 5% from its average value of 11 kJ/g. Although the ef- fective heat of combustion plotted in Figure 8 appears to decrease, the deviation from the average value is within the uncertainty of the measurements. The effective heat of combus- tion determined for this experiment, under well-ventilated conditions, yields a value similar to that given in the literature by Browne and Brenden (1964) for wood burn- ing (dry Ponderosa pine) in the first stage of devolatilization. At 600 seconds after igni- tion, 30% of the wood crib mass has been consumed, and t h s is characteristic of the

EXPERIMENT 11 - HEAT RELEASE RATE SINGLE CRIB

0 300 600 900

TIME FROM IGNITION (s)

FIGURE 6. Heat release rate (kW) as a function of time from ignition (seconds) for a single wood crib burning under well- ventilated conditions.

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EXPERIMENT 11 - MASS LOSS RATE SINGLE CRIB

TIME FROM IGNITION (s)

FIGURE 7. Mass loss rate (kg/s) as a function of time from ignition (seconds) for a single wood crib burning under well- ventilated conditions.

EXPERIMENT 11 - HEAT OF COMBUSTION SINGLE CRIB

17 - w 2- 5 15

z 8

13

B U Cr 11 0

CI 4 W m 9

d Cr W

7 300 350 400 450 500 550 600

TIME FROM IGNITION (s)

FIGURE 8. The effective heat of combustion (kJ/g) as a function of time from ignition (seconds) for a single wood crib burning under well-ventilated conditions.

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first stage of devolatilization. Wood burning occurs with the accumulation of a layer of char and a change in the composition of the volatiles. During burning, the C/H ratio of the volatiles increases with time. Thls change effects an increase in the heat of combustion during the course of burning, but ths is not found in exp. 11. The change in gas phase composition during the course of wood com- bustion has an effect on the characteristics of smoke.

The next sequence of experiments (expts. 12 and 13) involved burning three wood cribs in an interior room ventilated only through a window. This type of combustion is characteristic of a postflashover fire. The reproducibility of the combustion conditions in the two experiments is extremely good. Measurements of mass loss of the fuel dur- ing both these experiments indicate that the burning is steady state in each. Mass loss during a 25-minute period from ignition was [73 kg] and this included the mass of the cribs and the heptane ignition fluid. There was less than 1% variation in the burning rates for the two experiments. After the com-

bustion of the heptane, the rate of heat re- lease increased from 600 to 1000 kW. There was a spike in the rate of heat release for each experiment which corresponded to the burning of paper from the gypsum wall board in the exterior room, and this was repro- ducible in time as well. The effective heat of combustion was determined for both these experiments for the period 5-10 minutes af- ter ignition, and was found to increase lin- early from 11 to 13.6 kJ/g for each, as shown in Figures 9 and 10. This increase is typical for the burning of wood where the composition of the volatiles changes with time with a resulting increase in the effective heat of combustion. Browne and Brenden (1984) investigated the combustion charac- teristics of Ponderosa pine and indicated that the heat of combustion during the first stage of wood burning was 11.0 kJ/g and for the second, when the weight loss is approxi- mately 60%, 14.2 kJ/g. In our experiments, the weight loss corresponding to an effective heat of combustion of 13.6 kJ/g was approx- imately 50%. What is somewhat different in these experiments is that sampling for smoke

EXPERIMENT 12 - HEAT OF COMBUSTION

i 300 350 400 450 500 550 600

TIME FROM IGNITION (s)

FIGURE 9. The effective heat of combustion (kJ/g) as a function of time from ignition (seconds) for three wood cribs burning (expt. 12) with underventilated conditions.

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EXPERIMENT 13 - HEAT OF COMBUSTION THREE CRIBS - UNDERVENTILATED

15

w 2- 5 14

i: 2 13 m 3 U

Er 12 0

C W 3: 11

ir Er W

10 1 I I I I I 300 350 400 450 500 550 600

TIME FROM IGNITION (s)

FIGURE 10. The effective heat of combustion (kJ/g) as a function of time from ignition (seconds) for three wood cribs burning (expt. 13) with underventilated conditions.

was performed at slightly different stages in the burning. In expt. 12, sampling with the filter began 3.5 minutes after ignition and continued for 11 minutes while for expt. 13 sampling with the filter began 4 minutes after ignition and continued for a 10.5- minute period. These different sampling pe- riods correspond to different heat release rates, and corresponding changes in the ef- fective heat of combustion. In expt. 12 sam- pling occurred over the entire period when the heat release rate was increasing, and in expt. 13, the sampling period did not include the last 2-3 minutes when the rate of heat release increased from 900 to 1000 kW. It would be interesting to repeat these experi- ments, which are so very reproducible with regard to their combustion characteristics, for sampling times (measured as time follow- ing ignition) and sampling periods treated as experimental variables.

Burning of Parallel Plywood Sheets

Experiments 1-5 and 10 were concerned with burning plywood sheets of dimensions given

in Table 1. Each ply is attached to the other by a layer of adhesive that burns differently from the wood. Frequently during the burn- ing, there is a decrease in the heat release while another of the plies is igniting from the backside of the most recently burning veneer layer. A typical heat release rate curve for expt. 3 is shown in Figure 11. Note that the decrease in heat release rate at 11 minutes occurs during the period when the second ply is igniting. The effective heat of combus- tion for plywood burning determined from Eq. (1) was approximately 12 kJ/g, and this is in accord with values reported for wood burning in the earlier stages. A typical effec- tive heat of combustion plot is shown in Figure 12 for expt. 5, and the line drawn from 250 to 650 seconds is the least squares fit through the data. The fit gives an initial value of 12.3 kJ/g and the effective heat of combustion decreases slightly during burn- ing. The effective heat of combustion for expt. 10 increased slightly during the burn- ing from 11 to 11.5 kJ/g. Combustion in expt. 10 is intermediate between that ob- served for the well-ventilated wood crib and

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EXPERIMENT 5 - RATE OF HEAT RELEASE PARALLEL PLYWOOD PLATES

TIME (minutes)

FIGURE 11. Heat release rate (kW) as a function of time (minutes) for the burning of parallel plywood plates.

EXPERIMENT 5 - HEAT OF C'OMBITSTION PARALLEL PLYWOOD PLATES

15 - bQ '. 2 14 - Z C F 1" I/? 5 m X 12 S cr O 11

2 W 3:

10 d Cr W

TIME FROM IGNITION (s)

FIGURE 12. The effective heat of combustion (kJ/g) as a function of time from ignition (seconds) for the burning of parallel plywood plates.

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Smoke Emission Factors from Medium-Scale Fires 15

the five-ply sheets. With the exception of expt. 2 (where there was less ventilation), the emission factors for the first five experiments were remarkably similar and seemed quite independent of minute-to-minute variations in the heat release rates. Variations in the effective heat of combustion for these experi- ments were on the order of 10%.

Burning of Asphalt Shingles

Experiments 9 and 24 involved burning as- phalt shngles in a configuration designed to simulate roof burning ignited by the im- pingement of an external flame. These exper- iments were the most complex, in that pieces of the specimen often melted and dripped during combustion and burned in the catch pan underneath. Both samples ignited in three minutes, and each specimen underwent steady-state burning as evidenced by the nearly constant slopes of the individual mass loss curves. The two experiments did have different burning rates: expt. 9,0.36 kg/min; expt. 24, 0.45 kg/min. For expt. 9, sampling began at 4.5 minutes and continued for 12 minutes, and for expt. 24, it began at 5 minutes and ended at 12. The rate of heat release decreased during the experiments, and the effective heat of combustion for the 5-minute period beginning 5 minutes after ignition decreased from 46 to 41 kJ/g for expt. 9. The decrease is due, in part, to burning of the cellulosic material used to line the shngles.

Sooting Characteristics of the Experiments

The experiments conducted represent a rather large diversity of combustion condi- tions and these affect the soot volume frac- tions, particle density, particle size, and par- ticle composition. Wood materials have been burned with no adhesive, and with varying amounts of adhesive. Burning rates and rates of heat release have been affected by the adhesive and the geometry of the specimen during burning. Wood cribs have been

burned in fully and underventilated condi- tions. Asphalt wood shmgles, which are fuels of high aromatic content, have significantly different effective heats of combustion than cellulosic materials. Aromatic fuels also have a large tendency to soot when burned. The flow fields of all these experiments are char- acteristics of buoyancy driven flows, and in each of them there is a region where the flow field changes from laminar to turbulent flow. Fuel composition, ventilation conditions, and combustion intensity all affect the composi- tion/temperature/residence time hstory of a particle that is reacting in a combustion environment, and these, in turn, determine the sooting characteristics. Adequate resi- dence time at high temperatures tends to decrease the H/C ratio because it allows for significant surface growth. The ultimate soot loading from a fire results from the competi- tion between soot formation and destruction chemistries. The OH radical plays an impor- tant role in soot oxidation, and it is competi- tion for t h s by CO to form CO, and by particles which determines the balance. Oxi- dation requires adequate oxygen concentra- tions at sufficiently high temperatures in re- gions where fuel fragment and intermediate combustion product concentrations are high to produce the requisite oxygen-containing radical species. Underventilated fires result in less oxidation and more soot loading.

Elementary soot particles are spherical, about 200 A in diameter, and the data re- ported in the second part of t h s paper indicate that particles analyzed in our exper- iments were composed of up to 500 elemen- tary soot particles. Preliminary studies of these with electron microscopy indicate that they have chainlike structures. Particle growth consists (Haynes and Wagner, 1981; Smith, 1981) and as discussed in the accom- panying paper, of many stages: particle in- ception, surface growth, coagulation, and ag- glomeration. During the period of particle inception, surface growth and coagulation, the elementary soot particle maintains an approximate spherical shape and increases in

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size. When the surface growth rate declines due to the depletion of gas phase hydrocar- bons or loss of particle activity, agglomera- tion results in the formation of chains of spherical soot particles. Some investigators (Haynes and Wagner, 1981) believe that charges reside on the chains and that the chainlike shape can be attributed to the dis- tribution of charges. There is controversy regarding the relative roles of sampling arti- facts and agglomeration during combustion in determining the chainlike soot structures that are observed during electron micro- scopy. Optical evidence (Haynes and Wag- ner, 1981) does indicate that some of the growth is occurring in flames. Dobbins and Megaridis (1987) have demonstrated that ag- glomeration coexists with either particle in- ception, surface growth, or oxidation in a laminar coannular diffusion flame. The cir- cumstances requisite for agglomeration are particularly favorable for the case of the three wood cribs burning in underventilated conditions. The lack of oxygen increases the soot loading and particle concentration, which in turn, increase the rate of agglomer- ation (Haynes and Wagner, 1981; Smith, 1981).

RELATIONSHIP BETWEEN MEDIUM-SCALE FIRE EXPERIMENTS AND POST-NUCLEAR-EXCHANGE FIRES

The relationshp between the combustion en- vironment of medium-scale fire experiments and fire conditions in the aftermath of a nuclear blast is indeed complex. Even to attempt to relate the two environments, one must consider specific factors, such as scal- ing, geometry, and fuel composition and mixes of different fuels. Our understanding of post-nuclear-blast fires is founded, in part, on past observations and, in part, on speculation (Pittock et al., 1986). In the fol- lowing paragraphs, we briefly describe some of the factors that we believe to be impor- tant for placing our experimental results in the nuclear winter context.

The characteristics of urban fires that would follow from nuclear blasts and poten- tially lead to significant postnuclear effects are complex. In particular, the smoke emis- sion factors, particle size distribution, and optical properties must be determined under environmental conditions contrived to simu- late the characteristics believed to exist in postnuclear fires. Buildings in high-pressure blast zones may be reduced to rubble whose burning characteristics are very different from those of standing buildings located farther away from the blast epicenter. The simultaneous ignition of a multitude of buildings, with multiple ignitions in each building, presents complications not usually considered in the study of building fires. The potential for firestorms or conflagrations is pronounced. Many of the fires are likely to be ventilation controlled because air entrain- ment will not be adequate to ensure com- plete combustion of the fuel, but this is still an open question associated with the large- scale effects of the postnuclear environment.

Fires in buildings typically begin with the ignition of a single item. Initially, the fire grows on t h s item, and then, perhaps, ob- jects burn much as they would outdoors; there is little interaction with the surround- ing enclosure. As the fire develops, the role of the enclosure becomes important. A hot layer of buoyant fire gases and smoke devel- ops at the ceiling. This hot layer moves down to the level of the wall openings, and then the hot combustion products flow out- ward through the openings. The hot layer radiates heat down to other objects in the room, and their temperature increases. If the hot layer transfers a high enough radiant heat flux to the objects in the room, they, too, will ignite, and in many cases, almost simultaneously. This state in a compartment fire is frequently referred to as "flashover."

After flashover, the rate of pyrolysis of combustible objects in the room normally exceeds the rate at which oxygen can enter the compartment. Under these conditions, combustion within the burning compartment

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Smoke Emission Factors from Medium-Scale Fires 17

becomes ventilation controlled. Pyrolyzates that cannot burn within the compartment for lack of oxygen, termed excess py- rolyzates, bum in a buoyant diffusion flame at windows or other openings where the hot pyrolyzates mix with oxygen. In the postnu- clear environment one would expect building fires to be ventilation controlled, even though all the glass in the windows had been bro- ken, because the intensity and extent of fire would result in an oxygen-depleted atmo- sphere.

The spread of fire beyond the compart- ment of origin is usually a postflashover event: The flames burning excess pyrolyzates beyond the compartment of origin can ex- tend to adjacent floors or compartments, thus spreading the fire. Babrauskas and Williamson (1978,1979) present an extensive discussion of postflashover fires and the combustion of pyrolyzates within fire com- partments.

Building fires ignited by nuclear bursts would most likely differ from the typical scenario described above. Nuclear detona- tions from an air burst with the height of bursts (HOB) chosen for optimum city de- struction, would create a zone near the cen- ter of action of each weapon which would receive enough blast effect to be converted to rubble. There would also be sufficient ther- mal radiation to initiate extensive fires. Out- side of the central zone, a second zone would occur in which some structures would be converted to rubble, while others would re- main partially erect and would provide more traditional enclosure effects. In the second zone there would be sufficient thermal radia- tion to ignite fine fuels, such as cloth and paper, and it is llkely that in large areas of this zone, multiple ignitions would occur si- multaneously. It can be expected that these building fires would encircle the central rub- bilized area and may become ventilation controlled much earlier than typical building fires because the fire intensity would cause an oxygen-depleted atmosphere. As men- tioned above, these large-scale effects of con- flagrations are open to question.

It is also likely that urban fires are going to generate a significant percentage of the smoke produced by post-nuclear-exchange fires. The fire scenarios described above should apply to both urban and suburban areas, but it may be necessary to account for differences in construction in these areas.

A number of factors are known to affect smoke production and smoke characteristics. Material composition, fuel surface-to-volume ratio, and ventilation rates are important factors. For wildland fires, where the mate- rial composition is fairly homogeneous, fuel loading, fuel moisture content, and fire slope have been identified as three parameters af- fecting the amount and type of smoke pro- duced (Patterson and McMahon, 1987).

Rasbash and Pratt (1979-80) and Rasbash and Drysdale (1982) commented on the importance and difficulty of quantita- tively relating smoke produced by con- stituent materials in bench-scale experiments to that produced under general fire condi- tions. Rasbash and Pratt developed empiri- cal formulas for deducing the smoking po- tential of materials, whch are based upon the measurement of optical density and a number of combustion parameters, many of which have been quantified in our own work, Smoke potential is given for a number of commonly employed building materials. Mulholland (1986) has noted that deriving a smoke emission factor from the data of Rasbash and Pratt is nontrivial, since their "smoke potential," Do, is presented in the little-utilized units of (obscura x m3/g). For full-scale, underventilated, wood-fueled, room fire experiments reported in the litera- ture, they use their formulas to determine smoke potentials that vary by a factor of 4. Under free burning conditions, the smoke emission is reduced to a value that is approx- imately one-half the smallest value reported for the underventilated wood burning sam- ples.

The major factors which have been identi- fied as having an influence on the physical characteristics of smoke produced by poly- meric materials have been classified into two

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groups: material variables and environmen- tal variables (Haynes and Wagner, 1981; Smith, 1981 ; Homann, 1985; Mulholland, 1986). Material variables include such pa- rameters as fuel type, exposed area, thick- ness, mass, density, orientation, and pres- ence of fire retardants or surface coatings. Environmental variables include enclosure volume, ventilation rate, oxygen partial pres- sure, space temperature, and heat flux trans- ferred to the material. Flaming combustion relative to the nonflaming type produces different quantities of smoke and the smoke particles have different optical properties. The coupling between the combustion chem- istry and turbulence leading to smoke pro- duction is not well understood (Haynes and Wagner, 1981). The effects of different scales of turbulence on smoke production should be investigated to improve our understand- ing of scaling.

SUMMARY

The concept of "nuclear winter" has been postulated with a number of assumptions regarding the smoke produced by post- nuclear-exchange fires (Turco et al., 1983). Whether nuclear winter will occur in the aftermath of a nuclear exchange depends largely on the quantity and character of smoke generated, its distribution in the at- mosphere, and its optical characteristics. Since most of the current information is derived from small-scale experiments with little or no correlating data for larger scales on the order of actual buildings, a series of 11 medium-scale experiments with represen- tative urban fuels, such as wood and asphalt roofing shingles, have been conducted. The effects of different combustion conditions re- sulting from differences in fuel composition, geometry, and ventilation have been deter- mined. The present state of knowledge re- garding smoke production from building fires is briefly reviewed and placed in the nuclear winter framework.

Plywood sheets were burned in a parallel plate configuration, a geometry in which fuel

specimens create their own environment in- dependent of the larger surroundings. Wood cribs were burned in fully and underventi- lated conditions. Asphalt roofing shingles were burned under conditions representing the burning of a roof with the impingement of an external flame. The conduct of the experiments is described where mass loss of fuel during burning, the rate of heat release (RHR) measured by oxygen depletion calorimetry (ODC), and the effective heat of combustion were determined as function of time. The mass of smoke and the particle size distribution in each experiment were determined by sampling the aerosol particu- late in the exhaust duct which captured all of the effluent from the burning material. The combustion parameters measured during the experiments show good reproducibility, and they compare favorably with literature val- ues. The influence of the combustion con- ditions on soot loading and character is discussed. The implications of the measure- ments are discussed in the context of nuclear winter to relate the results of medium-scale fire experiments to characteristics of fires after nuclear blasts. The measurements of the smoke particulate and its characteriza- tion are described in the second part of thls paper.

The authors are greatly indebted to William Mac- Cracken, Rene LaFever, and Charles Fleischmann for their assistance in performing the experiments. Cecile Grant's expert editorial assistance is warmly acknowl- edged. Financial support was provided in part from the Defense Nuclear Agency. The Lawrence Berkeley Labo- ratory is operated by the University of California for the Department of Energy under contract DE-AC03- 76SF00098.

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Received February 22,1988; revised and accepted April 14, 1988

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