Laminar burning velocities of CH4—air-graphite mixtures and coal dusts

Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 249-256

L A M I N A R B U R N I N G V E L O C I T I E S O F C H 4 m A I R - - G R A P H I T E M I X T U R E S A N D C O A L D U S T S

DEREK BRADLEY, S. EL-DIN HABIK a.,,'n J. R. SWITHENBANK Department of Mechanical Engineering,

University of Leeds, Leeds LS2 9JT.

Lean CH4--air flames have been seeded with different amounts of graphite and laminar burning velocities measured. Burning rates of the graphite particles, together with particle and gas temperatures also were measured through the flame. The profile of net chemical heat release rate against gas temperature was found from the kinetically modelled chemical heat release rate, assumed independent of the added graphite, and the heat transferred to the gas from the particles as a consequence of their oxidation by the gas phase species.

Laminar burning velocities were calculated from the net heat release rate profile from Spald- ing's expression, which involved eigenvalue evaluation from the centroid expression. The burning velocities so obtained were in good agreement with those measured. This suggests a means of predicting the burning velocities of pulverised coal.

The approach adopted for this rests upon rapid devolatilisation and mixing, the assumption that the volatiles are entirely composed of CH4, and that the presence of char does not change the gas phase composition and kinetics. Char oxidation rates were found from the gas phase species concentrations. Radiative loss from the particles also was computed as they moved through the flame. From such considerations the net heat release rate profiles could be evaluated and, from these, the associated values of burning velocity.

The values of burning velocities of pulverised coal mixtures found in this way were compared with those of experiment. Agreement was good, provided devolatilisation and mixing with air were sufficiently rapid and the char did not create a heat sink sufficient to perturb the assumed uncoupled gas phase kinetics. In practice, these conditions seemed to be fulfilled for particles sizes of about 10 txm diameter or less, with volatile matter content greater than about 25%.

1. Introduction

T h e concept o f a l aminar b u r n i n g velocity has been used to descr ibe the bu rn ing rate o f p r e m i x e d f lammable dusts and a i r l -8 , and dusts in gaseous fuel-air mix tures 9-a3. Whereas with p r e m i x e d gases the b u r n i n g velocity is a physico- chemical constant that d e p e n d s upon mix ture composi t ion , t e m p e r a t u r e and pressure, when particles are bu rn t the b u r n i n g velocity addi- t ionally depends u p o n the rate o f evolut ion o f volatiles, the coupl ing o f par t ic le and gas phase oxida t ion kinetics, the size, shape and physical na tu re o f the particle, as well as the radiat ive ene rgy exchange with the sur roundings .

T h e p resen ted studies cover bu rn ing velocity measu remen t s o f lean C H 4 - - a i r flames with added graph i te and an associated simplified theory for the b u r n i n g velocity. This theory is tentat ively ex t ended into cer ta in r~gimes o f pulver i sed coal combus t ion where pred ic ted values o f bu rn ing velocity are found to be in good a g r e e m e n t with those measured . This

app roach is o f fe red as an in te r im contr ibut ion until a fuller, in tegra ted kinetic approach has been deve loped . T h e simplif ied theory is ne- vertheless, in some regimes , a good guide to the factors inf luencing the b u r n i n g rate.

2. Experimental Apparatus and Results

Lean CH4- -a i r mix tu res were seeded with g raph i te powder o f app rox ima te ly 4 txm diameter and burn t at 0.142 atm in a laminar flat f lame on a matr ix b u r n e r o f 76 m m diameter . Lamina r bu rn ing velocities were measured for d i f f e ren t amounts o f a d d e d graphite. Full details o f the appara tus and m e a s u r e m e n t techniques are given in Refs. 9 - 1 1 . Apar t f rom the radiat ive ene rgy loss f r o m the particles, the flames were adiabatic. Gas t empera tu res Tg, were measu red by silica coated p l a t i n u m - - 2 0 % r h o d i u m vs p l a t i n u m - - 4 0 % r h o d i u m thermo- couples, f o rmed f r o m wires o f 25 Ixm diameter , with al lowance m a d e for radiat ive loss in the

249

250 COAL COMBUSTION

usual way 14. Particle velocities, almost identical with those of the gas, were measured by laser Doppler velocimetry. The same system gave the particle size distribution from measurements of the modulation of the Doppler signal of the scattered light, as well as the particle concentrations. The temperature of the particles Tp, was measured by the two colour method at wave- lengths of 0.4264 and 0.563 bin1. Such measurements enabled the rate of burn ing of the graphite to be found at different axial positions through the flame.

Some results from a CH4--air equivalence ratio + of 0.65 and a graphite--CH4 mass ratio of 49.3% are shown on a time basis in Fig. 1, whilst Fig. 2, for the same value of ~, shows the variation of bu rn ing velocity ue with graphi te - - CH4 mass ratio. This velocity was that in the axial direction of the cold gas just above the burner matrix. Also shown is the measured %

~c

loc

8c

m

6O i

7"

4o

1J3

: ~ 2 0

01 ,

HEIGHT ABOVE BURNER TOP, x / ( m m ) 10 20 30 40

i 1 i

~-': '4="~ t ~ ,

O - - T p ~ , /

o /

rg~

j

5 to ;s TIME /(ms)

1.2

,0 �9

22

0.8

0.6

0 4

0.2

Fro. 1. Measured burn-up, particle and gas temperatures through CH4Iair flame, + = 0.65, pressure = 0.142 atmos. Graphite/CH4 mass ratio of 49.3%, H is the volumetric energy transfer rate from particles to gas.

15- m-

0" 1.25 0 8

~- _j / . . . . . . . 1o ~o6 ~ \ q h

o 2, ~ % 2'~

Olo C~'APHIT E - ME THANE RATIO I 016 0'65 07' 075' 018

OVERALL E(~UIVALENCE RATIO, (~

21

19 3

%

4O ~

FIG. 2. Burning velocity, burn-up and maximum temperature for flame of Fig. 1, with different amounts of added graphite. Theoretical burning velocity ratio is Gh.

burn-up, B, of the added graphite at the maximum measured temperature Tb exp. The adiabatic temperatures Tb and TbB were calculated on the basis of complete burn-up and the experimentally measured burn-up, respectively. The temperatures Tb, were calculated for the experimental burn-up, with allowance for the radiative loss from the particles. Theoreti- cally, this curve should coincide with that of Tb ~xp. The fact that it was higher might be explained by an overestimation of the measured burn-up and by an increased radiation loss from the thermocouple due to some par- ticulate deposition upon it. The overall equivalence ratio based upon complete burn-up, "r%, is given on the abscissa.

The elevation of Tp above Tg is due to heat release on the graphite particle. For a chemical heat release rate per unit spherical external area of particle, q, the particle energy equation is

1 Or3pph ~ 3r 2 at (1)

where a is the convective heat transfer coeffi- cient, ~ the particle emissivity, assumed unity throughout, ~ the Stefan--Bol tzmann constant, r, pp and h the particle radius, density and enthalpy and t is the time. The equation neglects radiative absorption from the walls, gas and other particles. Evaluation of each term and, in particular, q, has been discussed previously 9-11.

LAMINAR BURNING VELOCITIES 251

3. Heat Release Rates and Burning Velocities for Graphited Flames

Only when the chemical heat that is released by the reaction of the solid is transferred to the gaseous phase can it be effective in enhancing the burning rate. Thus, with a particle number density n, the effective volumetric energy rate source term, additive to that in the gaseous phase, is given by

H = 4~rr2an (Tp-Tg) (2)

The value of a was obtained from the Nusselt number for a sphere with no slippage. Because at low pressure the mean free path of the gas is comparable to the size of the particles, allowance must be made for Knudsen number effects 11'15. In the present experiments this reduced the Nusselt number from a continuum value of 2 to one in the region of 0.5. Measured values of r, n, Tp and Tg enabled H to be evaluated. For the given experimental conditions, these are given in Fig. 1.

An independent check on the value of H was obtained from the measured rate of burning of the graphite particles 9. I f q is small relative to the radiative energy loss rate, ~ cr Tp 4, the value of H becomes negative and the particle is a heat sink.

Figure 3 shows, plotted against the gas temperature, the gas phase volumetric heat release rate, ~, given by the chemical kinetic model of Dixon-Lewis and Islam z6'17, applied to the present CH4--air flame and used in a previous study 9. Also shown is the experimentally measured value of H from Fig. 1. Clearly, the gas phase reactions provide the overwhelm- ing heat release. It has been shown l~ that the reduction in burning velocity associated with the addition of an inert fine powder to a laminar CH4--air flame can be explained by the powder acting as a heat sink. This reduces the gas phase heat release rate that effectively propagates the flame. Theoretical demonstra- tion of this implicitly assumed that the CH4 oxidation kinetics were otherwise unaffected by the presence of the powder.

A similar approach was adopted for the present addition of reactive graphite powder to the CH4--air flame. I f it is assumed that the gas phase kinetics are unaffected by the presence of the graphite, then ~), and H can be added algebraically to give a total gas phase volumetric heat release rate, Q, shown by the dotted curve in Fig. 3. This assumption will become less valid as the C/CH4 ratio is increased, and the gas phase kinetics are significantly perturbed by the graphite. On the other hand, its validity is

50

4 5

4 0

35

30

i E

25 5

0 20

15

10

5 0 0 700

I

9 0 0 1100 1300 1500 1700

GAS TEMPERATURE "['9 / (K)

improved by the relatively slow rate of graphite oxidation, low radiation loss and the limited change in the final gaseous temperature with graphite addition. This is supported by some previous work 9. The validity of the algebraic separation of H and ~ is also likely to be greatest when the former is numerically small compared with the latter.

The area under a heat release rate--temperature profile is a principal determinant of the value of the burning velocity. Other factors being unchanged, the greater the area, the greater the burning velocity. Such profiles of Q against Tg were obtained from the experimental measurements associated with six different concentrations of graphite. The values of Q were employed in the analytical expression of Spalding is to evaluate the burning velocity. The expression allowed for the variation of gaseous thermal conductivity with temperature and involved the evaluation of the eigenvalue from the centroid of area. The enthalpy change at the maximum temperature was evaluated from the measured gas temperature and graphite burn-up. Without added graphite, the CH4--air mixture burning velocity derived from the heat release rate of the

Fzc. 3. Gas phase heat release rate, c), and heat release rate from particles to gas, H, for conditions of Fig. 1. Q =~ + H.

252 COAL COMBUSTION

chemical kinetic model agreed with that experimentally measured to within about 10%. The former also agreed with that predicted by the kinetic model to within about 1%.

Values of ue obtained in this way, normalised by the value without any graphite, are shown as O~h, by the dashed curve in Fig. 2. Values are close to the directly measured normalised values; labelled z~e. The agreement is satisfactory and the relatively small changes in both fhh and ~e underl ine the predominance of gas phase kinetics. Because the observed heating of graphite particles is in fair agreement with what is known of graphi te oxidation kinetics in such a flame~,11, the satisfactory agreement between uth and ~e suggests a simplified theory for the burning velocity of gaseous flames with added carbon. This might be useful until such a time as the oxidation kinetics of both phases have been fully integrated.

4. The Burning of Coal Dusts

The exper imenta l results show the burning velocity to be de te rmined predominant ly by the chemical heat release in the gas phase and that this can be est imated reasonably well on the assumption that the gas phase kinetics of the CH4--air flame are unchanged by the presence of the graphite. This suggests that, under certain conditions, it might be possible to estimate the burn ing velocities of coal dusts. For example, if it is assumed that the CH4 in the experiments represented the entire volatilised products of coal then the maximum g raph i t e - - CH4 ratio of 49.3% would be equivalent to a coal, albeit with an abnormally high mass proport ion of volatile matter of 67%.

Application of the present approach to coal dust burning would be dependent upon: (a) rapid coal devolatilisation and mixing of known gases, (b) a chemical kinetic model of the gas phase heat release rate, (c) ability to evaluate the source term H for the char, and all of these (d) within the f ramework of a high ratio of~/H. These are examined in turn.

(a) Coal Devolatilisation and Mixing

These should be on a shorter time scale than the gase phase heat release rate. Experiments suggest this occurs with coals with a high proport ion of volatile mat ter and small particle diameter 19-22. Her tzberg et al. 21 suggest it exists for 16% volatile mat ter with the d iameter < 10b~m, for 35% < 40~xm and for 100% volatile matter < 901xm diameter . For the larger particle size r6gime the rate of devolatilisation

appeared to limit the overall rate of flame propagation.

A number of exper imental studies have measured the devolatilisation rate under conditions of rapid heat ing 23-27. In the present work the rate of devolatilisation was given by

d--t- = k~ exp ( V - V) (3)

where V -= mass of volatiles released

initial mass of coal

V ~ _~

maximum mass of volatiles which may be released

initial mass of coal

Johnson et al. 23 suggest a value ofko of 1013 s -1 and of E/R of 21649 K. The flame structure modelled by Dixon-Lewis and Islam 16-17 enabled dV/dt to be evaluated for different equivalence ratios of CH4--air , initially at 298 K and one a tmosphere pressure. From the heat release rate profiles th rough the gaseous flame the percentage of the total heat release that had been released for 90% devolatilisation was found. Results are shown in Fig. 4 for a coal of 37% volatile matter , five different particle diameters and two different values of E/R. Clearly, devolatilisation and mixing are likely to occur fur ther ahead of the region of maximum gas phase reaction when the d iameter and equivalence ratio are low. Mixing with the surrounding air is likely to be rapid for these conditions 2u-22.

(b) Chemical Kinetic Model

Wen and Dutta 25 have reviewed coal devolatilisation studies. The evolved gases are princi- pally CH4 and H2 and the proport ions depend upon the heat ing rate. The higher the heating rate, the greater the propor t ion of hydrogen. In principle, a chemical kinetic model could give the gaseous phase flame structure for any proport ions of CH4 and H2 in air. In the absence of more precise data the CH4--a i r flame structure was used in the present work and was generously supplied by Dixon-Lewis. In practice, any heavier volatiles probably form lighter components ra ther rapidly.

(c) Evaluation of H

This was found from Eqs. (1) and (2) and the flame structure profiles of CH4--a i r flames.


100

90

~0

70

9~6o ~o >

=~5o J

w 4 0 t ~

~230

20

10

0

20pro ~ /

/ /

_ 33pro ~ / / ~ / / ~

/ / 20pm ~ - - ~

_ / /

33prn ,i I 10 p m

I / 10 p m

/ \ / \

/ 5 pPq " \ ,

/g/ r / / . / . . . . . . . l~m . . . . . -~ . . . . i : - . r = - ~ - . . . . i . . . . r - - - i . . . . ~ . . . . J - -

0 I 6 0 7 0 I 8 0 9 10 11 12

METHANE-AIR EQUIVALENCE RATIO

F:o. 4. Percentage of gas phase heat released when 90% of volatiles have been released, for a coal with 37% volatile matter in CH4-air flames. Particle sizes given.

For the larger particle sizes, the accumulation term becomes significant. Evaluation of q re- quires a knowledge of the detailed flame composition, char kinetics, diffusion rates and char porosity, as well as of the endothermicity associated with devolatilisation.

The flame species composition profiles computed by Dixon-Lewis enabled the contribution of each species to the surface chemical heat release rate, q, to be found. For the rate of reaction of char with O2 the expression of Smith 28 for bi tuminous coal char was used, for the reaction rates with H20 and CO2, those of Dobner 29 and Wen 3~ respectively, were employed. For small particles Bradley et al. 9 have established free radical attack as the principal mode of carbon oxidation. In the flame, the rate of diffusion through the pore structure is slower than that of the surface reaction of such active species. For this reason, reaction was assumed to occur entirely at the external surface and the rates of reaction of O, H, OH were those used previously 9. In all cases, reaction rates were evaluated from the partial pressure of the species at the char surface. This pressure was found by equating the species

reaction rate per uni t external surface area to the diffusion flux to the surface. Each contribution to q was obtained by multiplying each reaction rate by the associated heat of reaction, with the values previously employed 9-11.

Devolatilisation leads to an endothermic contribution to q. To evaluate this, Eq. (3) was employed, with an endothermicity of 1724 kJ per kg CH4 released 4. The external surface area was required and this was obtained from the analysis of Baum and Street 31 which relates density, diameter and volatile release. A swelling factor of 1.2 was assumed and an initial coal density of 1300 kg m -3, The specific heat of coal was a function of devolatilisation and burn-up in the expression of Kiro@ 2.

(d) Comparison of ~ and H.

On the assumption that devolatilisation and mixing are rapid and that the evolved gas is entirely CH4, the energy source term due to char combustion was evaluted according to the approaches of Sections (a) to (c) and Eqs. (1) and (2). A computer program enabled Tp to be found through the flame. It was assumed that the gas phase composition and temperature were unchanged by the presence of the char.

Results are shown in Fig. 5 for a coal of 37c~ volatile matter and initial particle diameters of 1 and 10 Ixm. The gas phase equivalence ratio was 0.63, C/CH4 mass ratio 1.70 (or H/C mass ratio of 0.102) and initial coal concentration 0.117 kg m -3. As in Fig. 3, H is small compared with cl �9 For the 10 Ixm particles, H is predomi- nantly negative due to the endothermicity of devolatilisation and the radiative energy loss. These become relatively less important in comparison with the heat release by char reaction when the diameter is decreased to 1 txm and there is a significant exothermic contribution. This is in line with previous theoretical work li and the analysis of Ozerova and Stepanov ~:~ who showed the burn ing velocity to increase with decrease in particle size.

5. Coal Dust Burning Velocities

As with the graphite flames, burning velocities at atmospheric pressure and 298 K were computed from the profiles of Q against Tg and Spalding's expression. The enthalpy change was evaluated from the computed char burn-up and radiation loss. Results are shown, for a variety of conditions, with gas phase CH4--air equivalence ratios of 0.63 and 0.85 and a

254 COAL COMBUSTION

6OO

5 0 O

~ 4 0 0

�9 G 300 T

2 0 0

1 0 0

0

Q\

H ( l u m ) \

.,,.7" "~'-~ ~-- ~ " " " - - - ~ - - - / -

i i i i

4 0 0 8 0 0 1 2 0 0 1 6 0 0 G A S T E M P E R A T U R E , Tg/(K)

OVERALL EQUIVALENCE RATIO ,'(~o 0 6 10 20 30

(a) / ! - ' ~ i

/ \ j . jTo

~ ~ \ \ ~ Tbr

~ 2 0 I ~ 0 8 J ' .

VOLATILE MATTER I(~ ob6 o!~ 0'2 d4 , C O;~)6cA,::L:NM,TRATION /(kg rn- 3 )

03 O2 01 H/C MASS RATIO

OVERALL EQUIVALENCE RATIO , (Do 0 8 10 20 5 0

~o ~

~ Tb r - ~TbvTbB 20~-~

~ O0 10

o 071 6b 6'0 20 2'0 [ VOLATILE MATTER I(;I,) o b s o'i o!2 o'

0 3 0 2 01 H /C MASS RATIO

FIG. 5. Modelled gas phase heat release rate ~ and heat release rate from particles to gas, H. Coal concentration = 0.117 kg m -3, with 37% volatile matter. Particle sizes given.

particle diameter of 10 Ixm in Fig. 6. For each value of 6 a range of coal concentrations is expressed on the abscissa by four interrelated parameters: initial coal concentration in air, % volatile matter, H/C mass ratio and overall equivalence ratio, ~<o. Computed values of Tb, Tb~ and Tb, are shown, with no allowance for the endothermicity of devolatilisation. When such an allowance was made the temperature based upon the computed % burn-up, B, (also shown) was lowered still fur ther to Tbv.

Values of burn ing velocity were found as described above from the profile of ~ versus temperature for the CH4--air flame and that of H versus temperature computed for each coal concentration. Such values were normalised by that of the CH4--air mixture at the given value of 6 to give the values of fee shown in the figure. In general, for a given gas phase composition, an increase in coal concentration decreases the burning velocity, as well as B, Ultimately, at high coal concentration a propagating flame cannot be sustained.

This practical situation is reflected in the increasingly rapid decrease of Tbv with increasing coal concentration. A large decrease in the

FiG. 6. Theoretical burning velocity ratios, burn- ups and temperatures for coal dust of 10 ~tm diameter. (a) gas phase 6 = 0.63 with u~ = 0.105 m s -] (b) gas phase 0 = 0.85 with ue = 0.33 m s -l.

temperature makes the assumption of un- changing gas phase kinetics less valid. This is also epitomized by a decrease, in the ratio fH dTg/f (1 dTg with increasing coal concentration. At the lowest concentrations in Fig. 6 this ratio was about - 2 % , whilst at the highest concentrations it was about -14%. For particle diameters in the region of 1 Fm the ratio was positive, due to the exothermic solid phase reactions. It would appear that for 10 Ixm diameter particles the present theoretical approach might be valid for a ratio of the two integrals down to about -12%. In terms of the validity of the rapid devolatilisation and mixing assumption, Fig. 4 suggests this assumption is valid for the conditions of Fig. 6(a), but somewhat less so for those of Fig. 6(b).

Figure 7 shows experimental values of ue obtained by Smoot et al. 4 for 10 txm diameter coal particles with 29% and 37% volatile matter, burn ing with air. The curves on the figure were obtained for these conditions from the present theory down to a ratio of the two integrals of -12%. The agreement between experiment and theory is good. At the higher concentra-


0.4

~ ' 0 - 3 b E

U 0 0 2 ..J w >

(D z 7

m 0.1

0 . 0 0.1

i o

i I

I I I

PRESENT WORK

O'a,/-" / O / D O

[]

/

[]

O

EXPERIMENTAL

- - B I 2 9 " / ~ SEWELL O137~ PITTSBURGH

0 2 0 3 COAL CONCENTRATION / (kg m -3)

FIG. 7. Curves of theoretical values of burning velocity for coal-air, particle diameter of 10 I.tm. Experimental points taken from Ref. 4.

tions o f coal the kinetics probably are fu r the r compl ica ted by pyrolysis o f the volatiles.

A g r e e m e n t be tween the expe r imen ta l data o f Smoot et al. 4 for 33 I.tm particles with 37% volatile mat te r and the p resen t theory was not as good, probably because o f the low rate o f devolati l isat ion and mix ing associated with la rger particles. A g r e e m e n t also was not good with Smoot 's value for 10 I*m particles with 17% volatile matter . In this case the theoret ical b reakdown is associated with values o f the ratio o f the two integrals that a re eve rywhere less than - 2 0 % .

6 . C o n c l u s i o n s

(1) Lamina r bu rn ing velocities o f CH4- -a i r f lames with d i f f e ren t amoun t s of added g raph i t e have been measu red .

(2) T h e results suggest that, p rov ided the added graphi te does not significantly perturb the gas phase kinetics, the graphi te can be t reated as an i n d e p e n d e n t chemical heat source. O n this basis b u r n i n g velocities can be pred ic ted f rom the profi les o f heat release rate versus t empe ra tu r e .

(3) This approach has been adap ted to predict the bu rn ing velocities o f coals. A g r e e m e n t with e x p e r i m e n t would a p p e a r to be good for part icle sizes o f about 10 txm d iamete r o r less, with volatile ma t t e r g rea te r than about 25%.

Acknowledgements

The SERC and Egyptian Education Bureau are thanked for their support.

REFERENCES

1. SMOOT, L.D. AND SMITH, P.J.: Coal Combustion and Gasification, (Luss, D., Ed.) p. l l 1, Plenum Press, New York, London, 1985.

2. SMOOT, L.D., HEDMAN, P.O. ANn SMITH, P.J.: Prog. Eng. Comb. Sci. 10, 359 (1984).

3. SMOOT, L.D. AND HO~TON, M.D.: Prog. Eng. Comb. Sci. 3, 235 (1977).

4. SMOOT, L.D., HORTON, M.D. AND Wn.LIAMS, G.A.: Sixteenth Symposium (International) on Combustion, p.375, The Cornbustion Institute, 1977.

5. SMOOT, L.D., HORTON, M.D., GOODSOX, F.P., WILLIAMS, G.A. AND HECKER, }V.C.: Measure- ment and Predictions of Laminar Flame Propa- gation in Methane/Coal~Air/Suppressant Sys- tems, AIAA 74-1112, 1974.

6. KRAZINSKI, J.L., BUCKIUS, R.O. AND KRIER, H.: Prog. Eng. Comb. Sci. 5, 31 (1979).

7. SLEZA~, S.E., FITCH, D.J., KRIER, H. AND BUCI4IL'S, R.O.: Combustion and Flame 54, 103 (1984).

8. ESSENmGH, R.H.: Sixteenth Symposium (Inter- national) on Combustion, p.353, The Combus- tion Institute, 1977.

9. BRADLEY, D., DIxo~,'-LEwIs, G., HABIK, S. EL-DIx AND MUSnl, E.M.J.: Twentieth Symposium (In- ternational) on Combustion, p.931, The Con> bustion Institute, 1985.

10. BRADLEY, D., HABIK, S. EL-DIN AND JA.X~EL, M.A.M.: The roles of Alumina and Graphite Dust Additives in Methane-Air Flames. First International Colloquium on Explosibility of Industrial Dusts, Part 1, p.4, Polish Academy of Sciences, 1984.

11. Dixox-LEwls, G., BRADLEY, D. AND HABIK, 8.

EL-DIN.: In preparation. 12. BRYANT, J.L.: Combustion Science and Technol-

ogy 2, 389 (1971). 13. CASSEL, H.M.: Some Fundamental Aspects of

Dust Flames, U.S. Bureau of Mines Report of Investigations 6551, 1964.

14. BRADLEY, D. AND MATTHEWS, K.J . : .J. Mech. Eng. Sci. 10, 299 (1968).

15. ANDREWS, G.E . AND BRADLEY, D.: In t . J . H e a t

Mass Transfer 15, 1765 (1972). 16. Dlxo.','-LEwIs, G. AND ISLAM, S.M.: Nineteenth

Symposium (International) on Combustion, p. 283, The Combustion Institute, 1982.

17. Dixoy-LEwIs, G.: Combustion Chemistry, (Gar- diner, w.C.Jr., Ed.) Cfi.2, Springer-Verlag, New York, 1984.

256 COAL COMBUSTION

18. SPALt)INC, D.B.: Combustion and Flame 1, 287 (1957).

19. HERWZBERC, M., CASHDOI.LAR, K.L. AND OPFER- MAN, J.J.: The Flammability of Coal Dust-Air Mixtures, U.S. Bureau of Mines Report of Investigations 8360, 1979.

20. HERTZBE~G, M., CASnDOLLAR, K.L. AND LAZZARA, C.P.: Eighteenth Symposium (International) on Combustion, p.717, The Combustion Institute, 1981.

21. HE~a-Z~ERG, M., CASHDOLLAR, K.L., NO, D.L. AND CONTI, R,S.: Nineteenth Symposium (Inter- national) on Combustion, p. 1169, The Combus- tion Institute, 1982.

22. SEEKER, W.R., SAMUELSEN, G.S., HEAP, M.P. AND TROLINGER, J.D.: Eighteenth Symposium (Inter- national) on Combustion, p.1213, The Combus- tion Institute, 1981.

23. Jouxso• G., ML'RDOCH, P.L. AND WILLIAMS, A.: To be published, 1986.

24. JAMALUDDIN, A.S., TRUELOVE, J.S. AND WALL, T.F.: Combustion and Flame 62, 85 (1985).

25. WE.',', C.Y. AYI~ DE"r'ra, S.: Energy Science and

Technology, (Wen, C.Y. and Stanley, L.E., Eds.) p.57, Addison Wesley, Reading, Mass., 1979.

26. JUNTGE.~, M. AND VAN HEEK, K.J.: Brennstoff Chemie 50, 172 (1969).

27. ANTHONY, D.B. ANO HOWARD, J.B.: AIChEJour- nal 22, 625 (1976).

28. SMIWH, I.W.: Combustion and Flame 17, 303 (1971).

29. DOBNER, S.: Modelling of Entrained Bed Gasifi- cation.: the Issues, EPRI, Paolo Alto, California, 1976.

30. WEN, C.Y.: Optimization of Coal Gasification Processes, R. and D. Res. Rep. 66, Vol. 1, p.74, 1972.

31. BAL'M, M.M. AND STREET, P.J.: Comb. Sci. Tech- nol. 3, 231 (1971).

32. Kmov, N.Y.: Specific Heats and Total Heat Contents of Coal and Related Materials at Ele- vated Temperatures, BCURA Monthly Bulletin 29, p.33, The British Coal Utilisation Research Association, 1965.

33. OZEROVA, G.E. AND STEPANOV, A.M.: Comb. Expl. Shock Waves 9, 543

COMMENTS

E. Suuberg, Brown University, USA. What composition and/or heating value was assumed for the volatiles in the modeling of Smoot's results? Will the flame behaviour not be sensitive to this assumption? The assertion that the approach will hold for high volatile coals is likely to be truly valid only for high rank, high volatile coals. The CO2 and H20 rich volatiles of high volatile but low rank coals will almost certainly lead to different behavior.

Author's Reply. The volatiles were assumed to be methane. The flame behaviour is, of course, sensitive to this assumption but, in principle, the present approach can be used for any know gaseous composition.

R. H. Essenhigh, Ohio State University, USA. The agreement between Smoot's flame speeds and your calculations are most impressive. However, your model was for a pre-mixed flame and Smoot's system was essentially a diffusion flame with volatiles released some distance behind the flame front and diffusing upstream. Do you really feel that you are

modeling the behavior actually present in Smoot's flames?

Author's Reply. The experimental measurements of Smoot, et al., for laminar burning velocity are commensurate with our definition of it, even though their theoretical model is different from ours. For heavier particles, allowance must be made for gas- solid slippage. This is a novel effect in defining burning velocity, not present for gaseous flames.

Q

V. Librovich, Institute for Problems in Mechanics, USSR. You have shown that at a pressure of one atmosphere the gaseous flame of volatiles is thick enough for all the coal particles to burn out. But at higher pressures the flame depth becomes thinner so one can look for the particles which will not be able to burn out. Is it so?

Author's Reply. At a pressure of ten (10) atmo- spheres, for a lean mixture, we compute a burn-up in the flame somewhat greater than that at one (1) atmosphere. This is because of changes in gas phase kinetics.

Documents

Laminar burning velocities of CH4—air-graphite mixtures and coal dusts