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Variations in the temperatures of coal-char particles during combustion: A consequence of particle-to-particle variations in ASH-content
Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 1297-1304
VARIATIONS IN THE TEMPERATURES OF COAL-CHAR PARTICLES D U R I N G COMBUSTION: A C O N S E Q U E N C E OF PARTICLE-TO-PARTICLE VARIATIONS IN A S H - C O N T E N T REGINALD E. MITCHELL Combustion Research Facility Sandia National Laboratories Livermore, CA 94551-0969
Experimental evidence is provided that supports our hypothesis that the wide variations in the temperatures and burning rates of pulverized-coal char particles of nominally the same size are primarily a consequence of particle-to-particle variations in ash-content. In situ size, temperature, and velocity measurements on the char particles of two uncleaned, run-of-mine coals and on the char particles of their physically-cleaned products are presented. The cleaning process reduced the total mineral matter content of the coals. It is shown that the variations in particle temperatures are reduced with cleaning. The higher the mineral matter content of the coal, the wider the variations in the ash-content of individual particles and hence, the wider the variations in the temperatures of particles of nominally the same size. Wide variations in particle temperatures lead to wide variations in particle burning rates. It is also shown that the temperature-dependence of the chemical reactivity of the particle material remains unchanged throughout the early to late stages of burning. Further, the data indicate that catalysis by mineral matter is negligible at particle temperatures exceeding 1500 K.
Introduction Recently, we showed that at comparable residence times in a laminar flow reactor, the temperatures of char particles of nominally the same size can vary considerably. 1 For a narrow range of particle diameters (106 to 125/zm), at any given residence time, we observed particles that had temperatures several hundred degrees above the local gas temperature and particles that had temperatures close to that of the gas. We used mass, momentum, and energy balance equations to show that at comparable residence times, during the early to relatively late stages of burnoff, the large differences in the temperatures of char particles of the same size can be explained by particle-to-particle variations in ash-content and apparent density. 1,2 To test the effect of the mineral matter content of coal on the variations in the temperatures of particles, we made in situ size, temperature, and velocity measurements on the char particles of two uncleaned, run-of-mine coals and on the char particles of their physically-cleaned products. For each coal, both a medium-cleaned and a deep-cleaned product were examined. The cleaning processes employed yielded products having ash-contents reduced by 31% and 57% for one coal, and by 65% and 75% for the other. Here, in support of our theoretical explanation, we show that at comparable 1297
residence times, the standard deviations of the distributions of temperatures of char particles of the cleaned coals are reduced in comparison to the deviations exhibited by their parent, uncleaned coals. We show that the cleaning process had an insignificant effect on the chemical reactivity of the coal char and also that the temperature-dependence of the intrinsic reactivity of the particle material remains unchanged throughout the early to late stages of burning.
Experimental Procedure Uncleaned, run-of-mine (ROM) samples of Kentucky #9 and Pittsburgh #8 hva bituminous coals and their medium-cleaned and deep-cleaned products are used in the combustion tests. The coals were provided by Combustion Engineering's Kreisinger Development Laboratory in Windsor, Connecticut. In keeping with their notation, the uncleaned Kentucky #9 coal and its cleaned products are referred to as the baseline, the medium-cleaned, and the deep-cleaned coals; and the uncleaned Pittsburgh #8 coal and its cleaned products are referred to as the ROM, C1 (medium-cleaned), and C2 (deep-cleaned) coals. The Kentucky #9 cleaned coals were originally prepared at the Electric Power Research Institute's Coal Quality Development.,
MINERAL MATrER AND ASH
1298
Center (CQDC). The Pittsburgh #8 coal was cleaned at their Powhatten facility (C1) and at the CQDC (C2). Pertinent coal characteristics are shown in Table I. The principal influence of the physical cleaning processes was to reduce the total mineral matter content of the coals. As noted in the table, on a moisture-free basis, the mineral matter content of the Kentucky #9 coal was reduced from 15% for the baseline coal, to 10.3% and 6.4% for the medium- and deep-cleaned coals, respectively; and the mineral matter content of the Pittsburgh #8 coal was reduced from 33.7% for the ROM coal, to 11.8% and 8.7% for the C1 and C2 coals, respectively. A transparent, laminar flow reactor is used to provide the gaseous environments in which the combustion tests are performed. The flow reactor is described elsewhere, a-6 It is equipped with a particle-sizing pyrometer7 that allows in situ measurements of the sizes, temperatures, and velocities of individual particles. Tests with essentially ash-free (0.4% ash), spherical, carbon particles (Spherocarb) indicate that for individual particles, sizes are accurate to within 10/zm, temperatures to within 50 K, and velocities to within 5%. 8 The reactor is also equipped with an isokinetic solids sampling probe that allows particles to be extracted from the reactor at selected heights. Extracted samples are analyzed to determine extents of burnoff and apparent densities. With these measurements, we can determine kinetic parameters that describe the overall particle burning rates, the mode of particle burning, and the CO to CO2 production ratio at the particle surface. In the experiments discussed here, flow reactor environments containing 6% and 12% oxygen (by volume) are employed. The centerline gas temper-
ature profiles in each environment are similar, ranging from about 1750 K at the reactor inlet to about 1500 K, 30 cm above the inlet. At the inlet, pulverized coal particles in the 106 to 125/xm size range are injected along the centerline of the flow reactor at loadings low enough to assure that the gaseous reactor environments are not appreciably altered due to consumption of the coal volatiles and char. Size, temperature, and velocity measurements are obtained at heights above the reactor inlet of 12.7, 19.1, and 25.4 cm, which correspond to residence times of about 72, 95 and 117 ms, respectively, for a 100/xm diameter particle. At each measurement height, 200 to 300 particles are monitored by the particle-sizing pyrometer using filters whose center wavelengths are 800 and 500 nm with bandwidths of about 50 nm. For each of the uncleaned coals, samples are extracted from the reactor at the same heights at which the in situ measurements are made and at a height of 6.4 cm above the reactor inlet (about 47 ms). This measurement is made just subsequent to devolatilization, as evidenced by the disappearance of volatile clouds. 4 Analysis of this sample provides an estimate of the mass loss due to devolatilization and an estimate of the initial apparent density of the char. For each extracted sample, the extent of burnoff is calculated from the ratio m/mo, where m is the weight of char extracted at the measurement height, and mo is the weight of coal fed.
Theoretical Procedure In determining the overall particle burning rates and associated chemical kinetic parameters from the
TABLE I Coal properties Kentucky #9 Coal Proximate Wt-%, dry Volatile Matter Fixed Carbon Ash
Baseline coal
Medium-cleaned coal
Deep-cleaned coal
38.6 46.4 15.0
40.9 48.8 10.3
42.9 50.7 6.4
Pittsburgh #8 Coal Proximate Wt-%, dry
ROM coal
C1 coal
C2 coal
Volatile Matter Fixed Carbon Ash
29.9 36.4 33.7
39.5 48.7 11.8
41.1 50.2 8.7
TEMPERATURE VARIATION OF COAL-CHAR PARTICLES measurements, the same mass, momentum, and energy balance equations described in earlier work9 are used. Given the overall particle burning rate as a function of diameter and temperature, the equations allow the prediction of the diameter, temperature, mass, and apparent density of a single particle as it burns in the laminar flow reactor. The overall particle burning rate per unit external surface area, q, is expressed in terms of a mass transfer coefficient, kd, and an apparent chemical reaction rate coefficient, ks, as
ka (i-'YPs/e~= k se~
q = - - In
"yeg/e/
(1)
1299
chemical reaction rate coemcients with temperature involves the integration of the mass, momentum, and energy balance equations. 9'1~ Starting values for the mass balance equation are derived from analyses of the char samples extracted at the 6.4 cm height in the reactor. Luminous emission from burning volatile clouds4 prevented reliable size, temperature, and velocity measurements to be obtained at this measurement height. Therefore, starting values for the temperatures of particle of various sizes at the height are estimated, assuming that the particles and gas are in thermal equilibrium.
Results and Discussion
where PMcDoxSh kd - - dpR'TMVo
(2)
ks = A , exp(-Ea/RTp)
(3)
and
This formulation allows for Stefan flow in the boundary layer surrounding the particle, a consequence of an increase in the number of moles at the particle surface due to reaction. The theoretical procedure used to determine the Arrhenius parameters, A~ and Ea, and the apparent reaction order, n, that best correlate the apparent
Table II shows the mean particle temperatures and the standard deviations of the distributions of temperatures of char particles at each measurement height in each gaseous environment for the uncleaned Kentucky #9 coal and for its cleaned products. Table III shows similar results for the uncleaned and cleaned Pittsburgh # 8 coals. Standard deviations as large as 50 K can be associated with variations in particle shapes and sizes, availability of internal surfaces, and possible reactivity differenoes of coal constituents. As observed with the coals previously studied, t once the standard deviations in the temperature distributions exceed about 50 K, they increase with increasing height above the reactor inlet and hence, with increasing particle res-
TABLE II Mean and standard deviations of the distributions of temperatures of the char particles of baseline Kentucky #9 and its cleaned products 6 mol-% 02 environment Medium-cleaned coal
Baseline coal
Deep-cleaned coal
Height (cm)
Temp (K)
qr (K)
Temp (K)
~rr (K)
Temp (K)
qr (K)
12.7 19.1 25.4
1769 1731 1665
46 63 82
1738 1697 1645
22 24 32
1720 1683 1632
24 25 32
12 mole-% O2 environment Medium-cleaned coal
Baseline coal
Deep-cleaned coal
Height (cm)
Temp (K)
trr (K)
Temp (K)
o'r (K)
Temp (K)
o'r (K)
12.7 19.1 25.4
1852 1737 1612
92 119 123
1867 1810 1698
45 64 100
1861 1819 1742
44 46 78
1300
M I N E R A L M A T F E R A N D ASH
TABLE I I I Mean and standard deviations of the distributions of t e m p e r a t u r e s o f the char particles of ROM Pittsburgh # 8 and its cleaned products 6 mole-% Oz e n v i r o n m e n t ROM
C1
C2
coal
coal
coal
Height (cm)
Temp (K)
err (K)
Temp (K)
Temp (K)
err
12.7 19,1 25.4
1683 1687 1630
110 47 44
1730 1699 1643
49 36 34
1733 1695 1641
40 34 34
12 mole-% O~ environment ROM coal Height (cm)
Temp (K)
12.7 19.1 25.4
1842 1759 1669
r
(K)
Tgal -- 1632 K
C2 coal
Temp (K)
Temp (K)
err (K)
1856 1809 1715
38 48 88
1858 1813 1740
53 53 77
80 88 103
1.0028.4 r
C1 coal
25.4 cm, Tgal -- 1632 K
2 6 . 4 c m , TOss
19.1 c m . T g a | -
19.1 cm. Toes -- 1591 K
-- 1632 K
~ 0.75~ 0.50z 0 ~
0.25-
0 t.O0
-
19.1 cm, Toe| -- 1691 K
1591K
~ O.76~ O.60z o ~
<
oum~
0.25-
0 , ,. 1.00
. . .
. . . .
12.7 crn, T o e s - - 1 8 4 6 K ~0.76
......
?omlo .....
-
12.7 c m . T o a l - - 1 6 4 6 K
12.7 c m , T g a | -- 1 6 4 6 K
-
~
0.60-
~
0.26-
0 , ., 1300
.
.
.
.
.
.
.
.
1600 1700 1900 2100 PARTICLE TEMPERATURE (K)
12 m o l e - % O x y g e n Baseline
Kentucky
#g
,,
1300
,,
,, ,, ,, .. ... .. . , , , , , , , 1500 1700 1900 2100 PARTICLE TEMPERATURE (K) ,,
12 m o l e - % O x y g e n Kentucky #9
Medium-cleaned
,
1300
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
1600 t700 1900 2100 PARTICLE TEMPERATURE (K) 12 m o l e - % O x y g e n Deep-cleaned Kentucky #9
Fro. 1. The distributions of the temperatures of the char particles of the Kentucky #9 coals burning in the 12 mole-% oxygen environment. For the baseline coal, burnoff is 66%, 76%, and 84%, 12.7, 19.1, and 25.4 cm heights in the reactor, respectively.
daf,
at the
TEMPERATURE VARIATION OF COAL-CHAR PARTICI,ES idence time and extent of burnoff. Note that at comparable residence times (i.e., heights in the reactor), the standard deviations of the temperature distributions are reduced with coal cleaning. The reduction is to the 50 K (or less) range except at the 25.4 cm height in the reactor where particles are in their late stages of burning. Figures 1 and 2 show the measured temperature distributions for the Kentucky #9 and Pittsburgh #8 coal chars, respectively, in the 12 mole-% oxygen environment. The increases in the standard deviations with height in the reactor are readily apparent. With the baseline Kentucky #9 coal, as burnoff progresses, a shift from most char particles having temperatures from 200 to 300 K higher than the local gas temperature (e.g., at the 12.7 cm height) to most particles having temperatures near the local gas temperature (e.g., at the 25.4 em height) is clearly evident. The low temperatures are indicative of extinguished particles or particles near burnout. Few of the char particles of the cleaned coals have temperatures this low. Similar behavior is observed with the char particles of the Pittsburgh #8 coals (see Fig. 2). 1OO,
~
25.4 cm. T01o -- 1532 K
1301
Figures 3 and 4 show apparent chemical reaction rate coefficients determined for the char particles of the Kentucky #9 and Pittsburgh #8 coals, respectively, and the corresponding Arrhenius rate parameters and apparent reaction orders. The rate coefficients determined are considered to be preliminary since the effects of the CO to CO2 ratio and mode of particle burning have not yet been considered. In the calculations, CO is assumed to be the sole heterogeneous reaction product and the apparent density of the char is assumed to vary, with mass to the 0.25 power, l~ Since the data for each coal are analyzed in a consistent manner, the relative behavior between uncleaned and cleaned coals is accurately assessed. The lines in the figures represent the least squares fits to the coefficients for the uncleaned coals (from which Aa and Ea are determined from the intercepts and slopes). The value determined for the apparent activation energy of each coal is in the range expected for particle burning limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the particle material (Zone II burning). H The apparent reaction order deter-
25.4 cm, TOOl -- 1532 K
2 6 4 cm. T011 = 1532 K
075
050.
0.25'
=.: 0 1.00
,,,
,.Uuu!m,m,
......
....
....
19.1 cm, Tgls - 1591 K
19.1 cm, T o i l = 1591 K
.
.
.
.
.
.
.
.
.
.
19 1 cm. Tool -- 1591 K
035
=.. 0.50
~
O.25
....
0
1.00
-,,,,,!J,,,
.....
12.7 gin. TOOl -- 1645 K
.......
~
.....
12.7 cm, TOOl -- 1045 K
12.7 cm. Tool -- 1545 K
.J 0.75
~) 0 5 0 0.25 u. 0 1300
1600 1700 1900 21OO PARTICLE TEMPERATURE (K)
1300
15OO 1700 1900 21OO PARTICLE TEMPERATURE (K)
12 m o l e - % O x y g e n
12 m o l e - % O x y g e n
ROM P ~ t t s b u r g h # 8
CI P i t t s b u r g h # 8
1300
15OO 17OO 1900 2100 PARTICLE TEMPERATURE (K)
12 r n o l e ~
Oxygen
C2 P i l t s b u r g h # 8
FIG. 2. The distributions of the temperatures of the char particles of the Pittsburgh #8 coals burning in the 12 mole-% oxygen environment. For the ROM coal, burnoff is 86%, 96%, and essentially 100%, daf, at the 12.7, 19.1, and 25.4 cm heights in the reactor, respectively.
1302
MINERAL MATTER AND ASH
1~
KENTUCKY # 9 COAL. 12 m o l e - % OXYGEN
A
PITTSBURGH # 9 COAL, 12 m o l e - % OXYGEN
k= -- 3 7 2 . 3 e x p ( - 3 4 2 3 0 / R T p )
k s = 3.90
O l r e a c t i o n order: n -
0 2 r e a c t i o n order: n = 0 . 0 5
0.35
10"
o Baseline o Medium-cleener * DeeD-cleaned
" ~ o
exp(-21820/RTp)
o ROM n C1 , C2
E ~
r
to |
9
%
D o
eqi
o o
E
O~
10-=
10"2
o~ o
4~ 9 Oo 9
o
O
R e a c t i o n Product: 1 0 0 % CO
10"
500
5~o
R e a c t i o n Product: 1 0 0 % CO
ebo
1.0/Tp (K "1) "10 "a
10 50.0
5~o
8~o
1.0/Xp (K "1) "10 "a
FIG. 3. Chemical reaction rate coefficients determined for the char particles of the uncleaned and cleaned Kentucky #9 coals burning in the 12 mole% oxygen environment. The rate parameters and line reflect least squares fits to the rate coefficients of the baseline coal.
FIG. 4. Chemical reaction rate coefficients determined for the char particles of the uncleaned and cleaned Pittsburgh #8 coals burning in the 12 mole% oxygen environment. The rate parameters and line reflect least squares fits to the rate coefficients of the ROM coal.
mined for the char particles of the Pittsburgh #8 coal is lower than one would normally expect for such burning, however. It may suggest burning in the transition region between Zone I (chemical reaction-limited burning) and Zone II when the true reaction order is zero, i.e., when desorption of CO (or COz) limits the overall particle burning rate. Note that for each coal, the rate coefficients for the char particles of the uncleaned and cleaned samples are comparable (at any specific temperature) and exhibit the same temperature dependence. This indicates that the physical cleaning process had an insignificant effect on the chemical reactivity of the particle material. Also note that for both the Kentucky #9 and Pittsburgh #8 coals, the overall burning rates of each of their char particles appear to be governed by the same chemical reaction mechanism, as evidenced by the rate coefficients all exhibiting the same temperature dependence. Figure 5 shows predicted temperature profiles for Kentucky #9 char particles burning in the 12 mole% oxygen environment established in the laminar flow reactor. The rate parameters shown in Fig. 3 were used in the calculations. Two particle sizes (105 and 125 p,m) are considered at the 6.4 cm height and particles are assumed to have high (twice the
average), average (15%, dry), and low (half the average) ash-contents. The figure shows that particle temperatures decrease as the local gas temperature decreases and then rapidly drop to their no-burning values as burnout is approached. The height in the reactor at which burnout is reached depends upon the amount of carbonaceous material in the particle (the higher the ash-content, the lower the amount of carbonaceous material). Also shown in the figure are the spans (two standard deviations) in the measured particle temperatures at the heights in the reactor that measurements were obtained. The predicted ranges in the particle temperatures at the 12.7, 19.1 and 25.4 cm heights adequately reflect the measured temperature spans. This indicates that the low temperatures at each height represent high-ash particles near burnout and the high temperatures represent average to low-ash particles in their middle to relatively late stages of burnoff. The fact that the rate coefficients of particles in their final stages of burning (particles at the lowest temperatures) exhibit the same temperature-dependence as particles at earlier extents of burnoff suggests that the intrinsic chemical reactivity of the particle remains unchanged during the lifetime of the particle. Also, since all the particles appear to
TEMPERATURE VARIATION OF COAL-CHAR PARTICLES RAW KENTUCKY #9 COAL 2100-
1800
)ARTICLES
-
9
S ~
QAS
1700-
o
= ~,'o~,".,~
-h-~,
E 1500-
1300
o
12% OXYGEN i i
5
lO
1'~
i
20
2'5
3'o
35
Height in Reactor (cm)
FIG. 5. Predicted temperature profiles in the flow reactor for Kentucky #9 char particles having diameters of 105 and 125 gm at a height of 6.4 cm in the reactor. Particles having low (1, half the average), average (a, 15% dry), and high (h, twice the average) ash-contents are considered. The arrows represent two times the standard deviations of the measured temperatures at the measurement heights.
be governed by the same chemical reaction mechanism, the ash-content of each particle must be insignificant in influencing its kinetics. The chemical reaction rate coefficients of particles having very little ash exhibit the same temperature-dependence as particles that are almost entirely ash. There appears to be little effect of catalysis. During pulverized coal combustion, variations in the temperatures of char particles are expected due to (a) variations in the shapes and sizes of particles, (b) differences in the availability of the internal particle surfaces, (c) burnout of the most reactive coal constituents, (d) utilization (or loss) of active sites, and (e) catalysis by mineral matter. To this list, we add variations in ash-content. The data presented here clearly supports our hypothesis that the wide variations in particle temperatures are a consequence of the particle-to-particle variations in ashcontent. The variations due to ash-content overshadow the variations due to the other effects.
tent, theless carbonaceous material there is to burn and hence, the less time required for complete burnoff. In order to account for these variations in coal combustor models, it is necessary to allow for a distribution in ash-content for each particle size considered. The results of this investigation also lead us to conclude that for particle temperatures exceeding 1500 K, catalytic effects due to mineral matter on the burning rate of the carbonaceous matrix of the char are negligible. Further, the intrinsic chemical reactivity of the particle material remains unchanged during the course of burning. Based on this limited set of data, it is not necessary to postulate differences in the reactivities of coal constituents in order to explain the observed variations in particle temperatures or burning rates. Differences in the reactivities of coal macerals do exist, however, the effect these differences have on the particle temperatures and overall burning rates is overshadowed by the effect of variations in ash-content. Nomenclature apparent pre-exponential factor (gC/cmZ-s atm n) bulk oxygen diffusion coefficients (cm2/s) Dox particle diameter (cm) dv apparent activation energy (cal/mole) E, apparent chemical reaction rate coefficient ks (gC/cm2-s-atm") molecular weight of carbon (gC/mole carMc bon) apparent reaction order n total pressure (atm) P ambient oxygen partial pressure (atm) eg oxygen partial pressure the particle surface es (atm) overall particle burning rate per unit exterq nal surface area (gC/cm2-s) R, R' gas constant (cal/mole-K), (atm-cm3/mole-
Aa
K) Sh TM Tp
Conclusions
During the combustion of pulverized coals, large variations in the temperatures and burning times of char particles of nominally the same size arise because of the particle-to-particle variations in ashcontent. Low particle temperatures reflect high-ash particles in their final stages of burning and high particle temperatures reflect average to low-ash particles in their middle to relatively late stages of burnoff. The ash in the char primarily affects the time required for burnout. The higher the ash-con-
1303
Sherwood number (taken to be 2) arithmetic mean temperature between the particle and gas (K) particle temperature (K)
Greek 7 !)o
change in volume during reaction per unit volume of oxygen stoiehiometric oxygen coefficient for reaction at the particle surface (moles O2/mole C)
Acknowledgments This work was supported by the United States Department of Energy through the Pittsburgh En-
1304
MINERAL MATrER AND ASH
ergy Technology Center's Direct Utilization AR & TD Program and by the Electric Power Research Institute's Exploratory Research Program. The author extends thanks to Prof, Terry Wall (The University of Newcastle) for his contributions in determining the effects of ash on the heat capacities and emissivities of coal-char particles. Thanks are also extended to Scott Ferko and Wilson Ng for their work in the laboratory.
REFERENCES 1. MITCHELL, R. E., "The Thermal Influences of Ash on the Temperatures Attained by Burning Coal Char Particles," Paper No. 88-78, 1988 Fall Meeting of the Western States Section/The Combustion Institute, Dana Point, CA, October 17-18 (1988). 2. WALL, T. F., "rATE, A., BAILEY, J., AND MITCHELL, R. E., "The Temperatures Attained by Burning Coal Char Particles," Joint International Conference, Australia/New Zealand and Japanese Sections, The Combustion Institute, University of Sydney, Sydney, NSW, September 24-27, 1989. 3. HARDESTL D. R., POHL, J. H., AND STARK, A. H., "'The Rates and Mechanisms of Pulverized Coal Combustion," Sandia National Laborato-
4.
5.
6.
7.
8.
9.
10. 11.
ries Report SAND78-8234, Livermore, CA. (1978). MCLEAN, W. J., HARDESTY, D, R., AND POHL, J. H., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 1239 (1981). MCLEAN, W. J., POHL, J. H., AND HARDESTY, D. R., "Experimental Observations of the Early Stages of Combustion of Individual Coal Partides," Proceedings of the International Conference on Coal Science, Verlag Gluckauf GmbH, p. 708 (1981). MITCHELL, R. E. AND MCLEAN, W. J., Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 1113 (1982). TICHENOR, D. A., MITCHELL, R, E., HENCKEN, K. R., AND NIKSA, S., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 1213. (1984). NIKSA, S., MITCHELL, R. E., HENCKEN, K. R., AND TICHENOR, D. A., Combustion and Flame 60, 183 (1984). MITCHELL, R. E., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 69-78 (1988). MITCHELL, R. E., Combustion Science and Technology 53, 2-3, 165 (1987). MULCAHY,M. F. R. AND SMITH, I. W., Review of Pure and Applied Chemistry 19, 81 (1969).
VARIATIONS IN THE TEMPERATURES OF COAL-CHAR PARTICLES D U R I N G COMBUSTION: A C O N S E Q U E N C E OF PARTICLE-TO-PARTICLE VARIATIONS IN A S H - C O N T E N T REGINALD E. MITCHELL Combustion Research Facility Sandia National Laboratories Livermore, CA 94551-0969
Experimental evidence is provided that supports our hypothesis that the wide variations in the temperatures and burning rates of pulverized-coal char particles of nominally the same size are primarily a consequence of particle-to-particle variations in ash-content. In situ size, temperature, and velocity measurements on the char particles of two uncleaned, run-of-mine coals and on the char particles of their physically-cleaned products are presented. The cleaning process reduced the total mineral matter content of the coals. It is shown that the variations in particle temperatures are reduced with cleaning. The higher the mineral matter content of the coal, the wider the variations in the ash-content of individual particles and hence, the wider the variations in the temperatures of particles of nominally the same size. Wide variations in particle temperatures lead to wide variations in particle burning rates. It is also shown that the temperature-dependence of the chemical reactivity of the particle material remains unchanged throughout the early to late stages of burning. Further, the data indicate that catalysis by mineral matter is negligible at particle temperatures exceeding 1500 K.
Introduction Recently, we showed that at comparable residence times in a laminar flow reactor, the temperatures of char particles of nominally the same size can vary considerably. 1 For a narrow range of particle diameters (106 to 125/zm), at any given residence time, we observed particles that had temperatures several hundred degrees above the local gas temperature and particles that had temperatures close to that of the gas. We used mass, momentum, and energy balance equations to show that at comparable residence times, during the early to relatively late stages of burnoff, the large differences in the temperatures of char particles of the same size can be explained by particle-to-particle variations in ash-content and apparent density. 1,2 To test the effect of the mineral matter content of coal on the variations in the temperatures of particles, we made in situ size, temperature, and velocity measurements on the char particles of two uncleaned, run-of-mine coals and on the char particles of their physically-cleaned products. For each coal, both a medium-cleaned and a deep-cleaned product were examined. The cleaning processes employed yielded products having ash-contents reduced by 31% and 57% for one coal, and by 65% and 75% for the other. Here, in support of our theoretical explanation, we show that at comparable 1297
residence times, the standard deviations of the distributions of temperatures of char particles of the cleaned coals are reduced in comparison to the deviations exhibited by their parent, uncleaned coals. We show that the cleaning process had an insignificant effect on the chemical reactivity of the coal char and also that the temperature-dependence of the intrinsic reactivity of the particle material remains unchanged throughout the early to late stages of burning.
Experimental Procedure Uncleaned, run-of-mine (ROM) samples of Kentucky #9 and Pittsburgh #8 hva bituminous coals and their medium-cleaned and deep-cleaned products are used in the combustion tests. The coals were provided by Combustion Engineering's Kreisinger Development Laboratory in Windsor, Connecticut. In keeping with their notation, the uncleaned Kentucky #9 coal and its cleaned products are referred to as the baseline, the medium-cleaned, and the deep-cleaned coals; and the uncleaned Pittsburgh #8 coal and its cleaned products are referred to as the ROM, C1 (medium-cleaned), and C2 (deep-cleaned) coals. The Kentucky #9 cleaned coals were originally prepared at the Electric Power Research Institute's Coal Quality Development.,
MINERAL MATrER AND ASH
1298
Center (CQDC). The Pittsburgh #8 coal was cleaned at their Powhatten facility (C1) and at the CQDC (C2). Pertinent coal characteristics are shown in Table I. The principal influence of the physical cleaning processes was to reduce the total mineral matter content of the coals. As noted in the table, on a moisture-free basis, the mineral matter content of the Kentucky #9 coal was reduced from 15% for the baseline coal, to 10.3% and 6.4% for the medium- and deep-cleaned coals, respectively; and the mineral matter content of the Pittsburgh #8 coal was reduced from 33.7% for the ROM coal, to 11.8% and 8.7% for the C1 and C2 coals, respectively. A transparent, laminar flow reactor is used to provide the gaseous environments in which the combustion tests are performed. The flow reactor is described elsewhere, a-6 It is equipped with a particle-sizing pyrometer7 that allows in situ measurements of the sizes, temperatures, and velocities of individual particles. Tests with essentially ash-free (0.4% ash), spherical, carbon particles (Spherocarb) indicate that for individual particles, sizes are accurate to within 10/zm, temperatures to within 50 K, and velocities to within 5%. 8 The reactor is also equipped with an isokinetic solids sampling probe that allows particles to be extracted from the reactor at selected heights. Extracted samples are analyzed to determine extents of burnoff and apparent densities. With these measurements, we can determine kinetic parameters that describe the overall particle burning rates, the mode of particle burning, and the CO to CO2 production ratio at the particle surface. In the experiments discussed here, flow reactor environments containing 6% and 12% oxygen (by volume) are employed. The centerline gas temper-
ature profiles in each environment are similar, ranging from about 1750 K at the reactor inlet to about 1500 K, 30 cm above the inlet. At the inlet, pulverized coal particles in the 106 to 125/xm size range are injected along the centerline of the flow reactor at loadings low enough to assure that the gaseous reactor environments are not appreciably altered due to consumption of the coal volatiles and char. Size, temperature, and velocity measurements are obtained at heights above the reactor inlet of 12.7, 19.1, and 25.4 cm, which correspond to residence times of about 72, 95 and 117 ms, respectively, for a 100/xm diameter particle. At each measurement height, 200 to 300 particles are monitored by the particle-sizing pyrometer using filters whose center wavelengths are 800 and 500 nm with bandwidths of about 50 nm. For each of the uncleaned coals, samples are extracted from the reactor at the same heights at which the in situ measurements are made and at a height of 6.4 cm above the reactor inlet (about 47 ms). This measurement is made just subsequent to devolatilization, as evidenced by the disappearance of volatile clouds. 4 Analysis of this sample provides an estimate of the mass loss due to devolatilization and an estimate of the initial apparent density of the char. For each extracted sample, the extent of burnoff is calculated from the ratio m/mo, where m is the weight of char extracted at the measurement height, and mo is the weight of coal fed.
Theoretical Procedure In determining the overall particle burning rates and associated chemical kinetic parameters from the
TABLE I Coal properties Kentucky #9 Coal Proximate Wt-%, dry Volatile Matter Fixed Carbon Ash
Baseline coal
Medium-cleaned coal
Deep-cleaned coal
38.6 46.4 15.0
40.9 48.8 10.3
42.9 50.7 6.4
Pittsburgh #8 Coal Proximate Wt-%, dry
ROM coal
C1 coal
C2 coal
Volatile Matter Fixed Carbon Ash
29.9 36.4 33.7
39.5 48.7 11.8
41.1 50.2 8.7
TEMPERATURE VARIATION OF COAL-CHAR PARTICLES measurements, the same mass, momentum, and energy balance equations described in earlier work9 are used. Given the overall particle burning rate as a function of diameter and temperature, the equations allow the prediction of the diameter, temperature, mass, and apparent density of a single particle as it burns in the laminar flow reactor. The overall particle burning rate per unit external surface area, q, is expressed in terms of a mass transfer coefficient, kd, and an apparent chemical reaction rate coefficient, ks, as
ka (i-'YPs/e~= k se~
q = - - In
"yeg/e/
(1)
1299
chemical reaction rate coemcients with temperature involves the integration of the mass, momentum, and energy balance equations. 9'1~ Starting values for the mass balance equation are derived from analyses of the char samples extracted at the 6.4 cm height in the reactor. Luminous emission from burning volatile clouds4 prevented reliable size, temperature, and velocity measurements to be obtained at this measurement height. Therefore, starting values for the temperatures of particle of various sizes at the height are estimated, assuming that the particles and gas are in thermal equilibrium.
Results and Discussion
where PMcDoxSh kd - - dpR'TMVo
(2)
ks = A , exp(-Ea/RTp)
(3)
and
This formulation allows for Stefan flow in the boundary layer surrounding the particle, a consequence of an increase in the number of moles at the particle surface due to reaction. The theoretical procedure used to determine the Arrhenius parameters, A~ and Ea, and the apparent reaction order, n, that best correlate the apparent
Table II shows the mean particle temperatures and the standard deviations of the distributions of temperatures of char particles at each measurement height in each gaseous environment for the uncleaned Kentucky #9 coal and for its cleaned products. Table III shows similar results for the uncleaned and cleaned Pittsburgh # 8 coals. Standard deviations as large as 50 K can be associated with variations in particle shapes and sizes, availability of internal surfaces, and possible reactivity differenoes of coal constituents. As observed with the coals previously studied, t once the standard deviations in the temperature distributions exceed about 50 K, they increase with increasing height above the reactor inlet and hence, with increasing particle res-
TABLE II Mean and standard deviations of the distributions of temperatures of the char particles of baseline Kentucky #9 and its cleaned products 6 mol-% 02 environment Medium-cleaned coal
Baseline coal
Deep-cleaned coal
Height (cm)
Temp (K)
qr (K)
Temp (K)
~rr (K)
Temp (K)
qr (K)
12.7 19.1 25.4
1769 1731 1665
46 63 82
1738 1697 1645
22 24 32
1720 1683 1632
24 25 32
12 mole-% O2 environment Medium-cleaned coal
Baseline coal
Deep-cleaned coal
Height (cm)
Temp (K)
trr (K)
Temp (K)
o'r (K)
Temp (K)
o'r (K)
12.7 19.1 25.4
1852 1737 1612
92 119 123
1867 1810 1698
45 64 100
1861 1819 1742
44 46 78
1300
M I N E R A L M A T F E R A N D ASH
TABLE I I I Mean and standard deviations of the distributions of t e m p e r a t u r e s o f the char particles of ROM Pittsburgh # 8 and its cleaned products 6 mole-% Oz e n v i r o n m e n t ROM
C1
C2
coal
coal
coal
Height (cm)
Temp (K)
err (K)
Temp (K)
Temp (K)
err
12.7 19,1 25.4
1683 1687 1630
110 47 44
1730 1699 1643
49 36 34
1733 1695 1641
40 34 34
12 mole-% O~ environment ROM coal Height (cm)
Temp (K)
12.7 19.1 25.4
1842 1759 1669
r
(K)
Tgal -- 1632 K
C2 coal
Temp (K)
Temp (K)
err (K)
1856 1809 1715
38 48 88
1858 1813 1740
53 53 77
80 88 103
1.0028.4 r
C1 coal
25.4 cm, Tgal -- 1632 K
2 6 . 4 c m , TOss
19.1 c m . T g a | -
19.1 cm. Toes -- 1591 K
-- 1632 K
~ 0.75~ 0.50z 0 ~
0.25-
0 t.O0
-
19.1 cm, Toe| -- 1691 K
1591K
~ O.76~ O.60z o ~
<
oum~
0.25-
0 , ,. 1.00
. . .
. . . .
12.7 crn, T o e s - - 1 8 4 6 K ~0.76
......
?omlo .....
-
12.7 c m . T o a l - - 1 6 4 6 K
12.7 c m , T g a | -- 1 6 4 6 K
-
~
0.60-
~
0.26-
0 , ., 1300
.
.
.
.
.
.
.
.
1600 1700 1900 2100 PARTICLE TEMPERATURE (K)
12 m o l e - % O x y g e n Baseline
Kentucky
#g
,,
1300
,,
,, ,, ,, .. ... .. . , , , , , , , 1500 1700 1900 2100 PARTICLE TEMPERATURE (K) ,,
12 m o l e - % O x y g e n Kentucky #9
Medium-cleaned
,
1300
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
1600 t700 1900 2100 PARTICLE TEMPERATURE (K) 12 m o l e - % O x y g e n Deep-cleaned Kentucky #9
Fro. 1. The distributions of the temperatures of the char particles of the Kentucky #9 coals burning in the 12 mole-% oxygen environment. For the baseline coal, burnoff is 66%, 76%, and 84%, 12.7, 19.1, and 25.4 cm heights in the reactor, respectively.
daf,
at the
TEMPERATURE VARIATION OF COAL-CHAR PARTICI,ES idence time and extent of burnoff. Note that at comparable residence times (i.e., heights in the reactor), the standard deviations of the temperature distributions are reduced with coal cleaning. The reduction is to the 50 K (or less) range except at the 25.4 cm height in the reactor where particles are in their late stages of burning. Figures 1 and 2 show the measured temperature distributions for the Kentucky #9 and Pittsburgh #8 coal chars, respectively, in the 12 mole-% oxygen environment. The increases in the standard deviations with height in the reactor are readily apparent. With the baseline Kentucky #9 coal, as burnoff progresses, a shift from most char particles having temperatures from 200 to 300 K higher than the local gas temperature (e.g., at the 12.7 cm height) to most particles having temperatures near the local gas temperature (e.g., at the 25.4 em height) is clearly evident. The low temperatures are indicative of extinguished particles or particles near burnout. Few of the char particles of the cleaned coals have temperatures this low. Similar behavior is observed with the char particles of the Pittsburgh #8 coals (see Fig. 2). 1OO,
~
25.4 cm. T01o -- 1532 K
1301
Figures 3 and 4 show apparent chemical reaction rate coefficients determined for the char particles of the Kentucky #9 and Pittsburgh #8 coals, respectively, and the corresponding Arrhenius rate parameters and apparent reaction orders. The rate coefficients determined are considered to be preliminary since the effects of the CO to CO2 ratio and mode of particle burning have not yet been considered. In the calculations, CO is assumed to be the sole heterogeneous reaction product and the apparent density of the char is assumed to vary, with mass to the 0.25 power, l~ Since the data for each coal are analyzed in a consistent manner, the relative behavior between uncleaned and cleaned coals is accurately assessed. The lines in the figures represent the least squares fits to the coefficients for the uncleaned coals (from which Aa and Ea are determined from the intercepts and slopes). The value determined for the apparent activation energy of each coal is in the range expected for particle burning limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the particle material (Zone II burning). H The apparent reaction order deter-
25.4 cm, TOOl -- 1532 K
2 6 4 cm. T011 = 1532 K
075
050.
0.25'
=.: 0 1.00
,,,
,.Uuu!m,m,
......
....
....
19.1 cm, Tgls - 1591 K
19.1 cm, T o i l = 1591 K
.
.
.
.
.
.
.
.
.
.
19 1 cm. Tool -- 1591 K
035
=.. 0.50
~
O.25
....
0
1.00
-,,,,,!J,,,
.....
12.7 gin. TOOl -- 1645 K
.......
~
.....
12.7 cm, TOOl -- 1045 K
12.7 cm. Tool -- 1545 K
.J 0.75
~) 0 5 0 0.25 u. 0 1300
1600 1700 1900 21OO PARTICLE TEMPERATURE (K)
1300
15OO 1700 1900 21OO PARTICLE TEMPERATURE (K)
12 m o l e - % O x y g e n
12 m o l e - % O x y g e n
ROM P ~ t t s b u r g h # 8
CI P i t t s b u r g h # 8
1300
15OO 17OO 1900 2100 PARTICLE TEMPERATURE (K)
12 r n o l e ~
Oxygen
C2 P i l t s b u r g h # 8
FIG. 2. The distributions of the temperatures of the char particles of the Pittsburgh #8 coals burning in the 12 mole-% oxygen environment. For the ROM coal, burnoff is 86%, 96%, and essentially 100%, daf, at the 12.7, 19.1, and 25.4 cm heights in the reactor, respectively.
1302
MINERAL MATTER AND ASH
1~
KENTUCKY # 9 COAL. 12 m o l e - % OXYGEN
A
PITTSBURGH # 9 COAL, 12 m o l e - % OXYGEN
k= -- 3 7 2 . 3 e x p ( - 3 4 2 3 0 / R T p )
k s = 3.90
O l r e a c t i o n order: n -
0 2 r e a c t i o n order: n = 0 . 0 5
0.35
10"
o Baseline o Medium-cleener * DeeD-cleaned
" ~ o
exp(-21820/RTp)
o ROM n C1 , C2
E ~
r
to |
9
%
D o
eqi
o o
E
O~
10-=
10"2
o~ o
4~ 9 Oo 9
o
O
R e a c t i o n Product: 1 0 0 % CO
10"
500
5~o
R e a c t i o n Product: 1 0 0 % CO
ebo
1.0/Tp (K "1) "10 "a
10 50.0
5~o
8~o
1.0/Xp (K "1) "10 "a
FIG. 3. Chemical reaction rate coefficients determined for the char particles of the uncleaned and cleaned Kentucky #9 coals burning in the 12 mole% oxygen environment. The rate parameters and line reflect least squares fits to the rate coefficients of the baseline coal.
FIG. 4. Chemical reaction rate coefficients determined for the char particles of the uncleaned and cleaned Pittsburgh #8 coals burning in the 12 mole% oxygen environment. The rate parameters and line reflect least squares fits to the rate coefficients of the ROM coal.
mined for the char particles of the Pittsburgh #8 coal is lower than one would normally expect for such burning, however. It may suggest burning in the transition region between Zone I (chemical reaction-limited burning) and Zone II when the true reaction order is zero, i.e., when desorption of CO (or COz) limits the overall particle burning rate. Note that for each coal, the rate coefficients for the char particles of the uncleaned and cleaned samples are comparable (at any specific temperature) and exhibit the same temperature dependence. This indicates that the physical cleaning process had an insignificant effect on the chemical reactivity of the particle material. Also note that for both the Kentucky #9 and Pittsburgh #8 coals, the overall burning rates of each of their char particles appear to be governed by the same chemical reaction mechanism, as evidenced by the rate coefficients all exhibiting the same temperature dependence. Figure 5 shows predicted temperature profiles for Kentucky #9 char particles burning in the 12 mole% oxygen environment established in the laminar flow reactor. The rate parameters shown in Fig. 3 were used in the calculations. Two particle sizes (105 and 125 p,m) are considered at the 6.4 cm height and particles are assumed to have high (twice the
average), average (15%, dry), and low (half the average) ash-contents. The figure shows that particle temperatures decrease as the local gas temperature decreases and then rapidly drop to their no-burning values as burnout is approached. The height in the reactor at which burnout is reached depends upon the amount of carbonaceous material in the particle (the higher the ash-content, the lower the amount of carbonaceous material). Also shown in the figure are the spans (two standard deviations) in the measured particle temperatures at the heights in the reactor that measurements were obtained. The predicted ranges in the particle temperatures at the 12.7, 19.1 and 25.4 cm heights adequately reflect the measured temperature spans. This indicates that the low temperatures at each height represent high-ash particles near burnout and the high temperatures represent average to low-ash particles in their middle to relatively late stages of burnoff. The fact that the rate coefficients of particles in their final stages of burning (particles at the lowest temperatures) exhibit the same temperature-dependence as particles at earlier extents of burnoff suggests that the intrinsic chemical reactivity of the particle remains unchanged during the lifetime of the particle. Also, since all the particles appear to
TEMPERATURE VARIATION OF COAL-CHAR PARTICLES RAW KENTUCKY #9 COAL 2100-
1800
)ARTICLES
-
9
S ~
QAS
1700-
o
= ~,'o~,".,~
-h-~,
E 1500-
1300
o
12% OXYGEN i i
5
lO
1'~
i
20
2'5
3'o
35
Height in Reactor (cm)
FIG. 5. Predicted temperature profiles in the flow reactor for Kentucky #9 char particles having diameters of 105 and 125 gm at a height of 6.4 cm in the reactor. Particles having low (1, half the average), average (a, 15% dry), and high (h, twice the average) ash-contents are considered. The arrows represent two times the standard deviations of the measured temperatures at the measurement heights.
be governed by the same chemical reaction mechanism, the ash-content of each particle must be insignificant in influencing its kinetics. The chemical reaction rate coefficients of particles having very little ash exhibit the same temperature-dependence as particles that are almost entirely ash. There appears to be little effect of catalysis. During pulverized coal combustion, variations in the temperatures of char particles are expected due to (a) variations in the shapes and sizes of particles, (b) differences in the availability of the internal particle surfaces, (c) burnout of the most reactive coal constituents, (d) utilization (or loss) of active sites, and (e) catalysis by mineral matter. To this list, we add variations in ash-content. The data presented here clearly supports our hypothesis that the wide variations in particle temperatures are a consequence of the particle-to-particle variations in ashcontent. The variations due to ash-content overshadow the variations due to the other effects.
tent, theless carbonaceous material there is to burn and hence, the less time required for complete burnoff. In order to account for these variations in coal combustor models, it is necessary to allow for a distribution in ash-content for each particle size considered. The results of this investigation also lead us to conclude that for particle temperatures exceeding 1500 K, catalytic effects due to mineral matter on the burning rate of the carbonaceous matrix of the char are negligible. Further, the intrinsic chemical reactivity of the particle material remains unchanged during the course of burning. Based on this limited set of data, it is not necessary to postulate differences in the reactivities of coal constituents in order to explain the observed variations in particle temperatures or burning rates. Differences in the reactivities of coal macerals do exist, however, the effect these differences have on the particle temperatures and overall burning rates is overshadowed by the effect of variations in ash-content. Nomenclature apparent pre-exponential factor (gC/cmZ-s atm n) bulk oxygen diffusion coefficients (cm2/s) Dox particle diameter (cm) dv apparent activation energy (cal/mole) E, apparent chemical reaction rate coefficient ks (gC/cm2-s-atm") molecular weight of carbon (gC/mole carMc bon) apparent reaction order n total pressure (atm) P ambient oxygen partial pressure (atm) eg oxygen partial pressure the particle surface es (atm) overall particle burning rate per unit exterq nal surface area (gC/cm2-s) R, R' gas constant (cal/mole-K), (atm-cm3/mole-
Aa
K) Sh TM Tp
Conclusions
During the combustion of pulverized coals, large variations in the temperatures and burning times of char particles of nominally the same size arise because of the particle-to-particle variations in ashcontent. Low particle temperatures reflect high-ash particles in their final stages of burning and high particle temperatures reflect average to low-ash particles in their middle to relatively late stages of burnoff. The ash in the char primarily affects the time required for burnout. The higher the ash-con-
1303
Sherwood number (taken to be 2) arithmetic mean temperature between the particle and gas (K) particle temperature (K)
Greek 7 !)o
change in volume during reaction per unit volume of oxygen stoiehiometric oxygen coefficient for reaction at the particle surface (moles O2/mole C)
Acknowledgments This work was supported by the United States Department of Energy through the Pittsburgh En-
1304
MINERAL MATrER AND ASH
ergy Technology Center's Direct Utilization AR & TD Program and by the Electric Power Research Institute's Exploratory Research Program. The author extends thanks to Prof, Terry Wall (The University of Newcastle) for his contributions in determining the effects of ash on the heat capacities and emissivities of coal-char particles. Thanks are also extended to Scott Ferko and Wilson Ng for their work in the laboratory.
REFERENCES 1. MITCHELL, R. E., "The Thermal Influences of Ash on the Temperatures Attained by Burning Coal Char Particles," Paper No. 88-78, 1988 Fall Meeting of the Western States Section/The Combustion Institute, Dana Point, CA, October 17-18 (1988). 2. WALL, T. F., "rATE, A., BAILEY, J., AND MITCHELL, R. E., "The Temperatures Attained by Burning Coal Char Particles," Joint International Conference, Australia/New Zealand and Japanese Sections, The Combustion Institute, University of Sydney, Sydney, NSW, September 24-27, 1989. 3. HARDESTL D. R., POHL, J. H., AND STARK, A. H., "'The Rates and Mechanisms of Pulverized Coal Combustion," Sandia National Laborato-
4.
5.
6.
7.
8.
9.
10. 11.
ries Report SAND78-8234, Livermore, CA. (1978). MCLEAN, W. J., HARDESTY, D, R., AND POHL, J. H., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 1239 (1981). MCLEAN, W. J., POHL, J. H., AND HARDESTY, D. R., "Experimental Observations of the Early Stages of Combustion of Individual Coal Partides," Proceedings of the International Conference on Coal Science, Verlag Gluckauf GmbH, p. 708 (1981). MITCHELL, R. E. AND MCLEAN, W. J., Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 1113 (1982). TICHENOR, D. A., MITCHELL, R, E., HENCKEN, K. R., AND NIKSA, S., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 1213. (1984). NIKSA, S., MITCHELL, R. E., HENCKEN, K. R., AND TICHENOR, D. A., Combustion and Flame 60, 183 (1984). MITCHELL, R. E., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, p. 69-78 (1988). MITCHELL, R. E., Combustion Science and Technology 53, 2-3, 165 (1987). MULCAHY,M. F. R. AND SMITH, I. W., Review of Pure and Applied Chemistry 19, 81 (1969).