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Elutriation of attrited carbon fines in the fluidized combustion of a coal
Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 1213-1221
ELUTRIATION OF ATTRITED CARBON FINES IN T H E F L U I D I Z E D COMBUSTION OF A COAL R. CHIRONE, A. CAMMAROTA, M. D'AMORE, L. MASSIMILLA Istituto di Chimica Industriale e Impianti Chimici, Universitd Istituto di Ricerche sulla Combustione, C.N.R. Piazzale Vincenzo Tecchio--80125 Napoli (Italy)
The relevance of coal fragmentation and bed emissions of combustible gases on the performance of the fluidized combustion of a coal and, in particular, on the elutriation rates of attrited carbon fines, is investigated for various coal feed sizes. Experimental data are compared with results of model calculations carried out with different assumptions as regards actual coal feed size distribution, because of fragmentation, and actual oxidizing conditions of the bed, owing to bed emissions of combustible gases subsequently burnt in the free board. Disregarding these phenomena results in model overestimations of elutriation rates of attrited carbon fines. Under the working conditions tested, discrepancies between calculation results and experimental observations are specially found when neglecting, at low excess air factors, the effects of bed emissions of combustible gases. 1. Introduction Recent works have pointed out that efficiency of fluidized combustion of coals might be affected by phenomena such as coal fragmentation, bed carbon attrition and emissions of combustible gas from the bed depending both on design and operation of fluidized combustors and on coal characteristics (Gibbs and Be6r, 1975; Andrei, 1978; Park et al., 1979; Be6r et al., 1980). Fragmentation, generally due to thermal shock and coal devolatilization, produces relatively coarse particles. Attrition, due to rubbing of carbon particles while burning in the bed, produces fines which, at the ordinary gas velocities of fluidized combustion, are rapidly elutriated from the combustor. Bed emissions of combustible gases, subsequently burnt in the free board, cause the bed to operate at oxygen concentrations higher than estimated from combustor outlet conditions. Attrition results in loss of carbon fines. Fragmentation and bed emission of combustible gases might indirectly influence such loss by affecting bed carbon loading and particle size distribution, which, in turn, are related to carbon attrition rates (Donsi et al., 1978; Donsi et al., 1981). The purpose of this work is outline the relationship between loss of attrited carbon, coal fragmentation and bed emissions of combustible gases. To this end, such emissions are measured together with elutriation rates of attrited carbon fines, bed carbon loading and particle size distribution during the fluidized combustion of a coal of known fragmen-
tation and attrition behaviour. Experimental data are compared with results of a fluidized combustion model, for some aspects simpler than others in the literature (Be6r and Sarofim, 1978; Wells et al., 1980; Saxena and Rehmat, 1980), but specifically structured to account for elutriation rates of attrited carbon and embody effects of both fragmentation and bed emissions of combustible gases. It is a development of a model previously used to evaluate carbon loading and gas composition in a fluidized bed combustor (Donsi et al., 1979). It should be noted that the influence of fragmentation on carbon loss in exit gases is subsequently.examined only from the stand point of its influence on carbon fines formation by attrition. Other contributions to carbon loss which might be enhanced by fragmentation, such as the possible formation of fines together with coarse fragments, and of fragments close in size to that elutriable, are negligible with the coal tested, so are not considered in the model. 2. Experimental Apparatus and Technique
Experiments are carried out with a 140 mm ID, 4.5 m high combustor, equipped with a coal screw feeder (Fig. 1). Air is injected into the bed through 19 pipes, each with 4 orifices 1.5 mm diameter. The bed, essentially made of 7 Kg of 200-400 I~m sand, is without overflow. Most ashes are in fact elutriated as fines. Additions of sand compensate for solids sampling from the bed. Bed temperature 1213
1214
COAL COMBUSTION TECHNIQUES
i
Bed solids are probe sampled. Water cooled, gas sampling probes are inserted into the bed and free board. During each run concentrations of oxygen and combustible gas, namely CO and CH4, are measured. H 2 is practically absent in bed outlet gases. As detailed elsewhere (Donsl et al., 1978), carbon loading is determined from carbon content in samples of bed solids and total bed inert material. Flow rates of elutriated carbon (mostly <150 Izm) are determined by accounting for solids separated at the cyclone per unit time and their carbon content. Because of the high combustor free board, elutriation of carbon coarser than d* = 180 tzm, the size at which particle terminal velocity is equal to gas velocity, is less than 1.5% of collected carbon. A South African 1-3, 3-6 and 6-9 mm size coal is used (Table I). Fragmentation characteristics of such coal when burnt in a fluidized bed were studied by Chirone (1980), with Andrei (1978) technique of collecting fragmented particles from the bed with a net basket. Fragmentation took place during coal devolatilization shortly after injecting coal particles into the bed, with results shown in Table II. A limit to this basket technique was the impossibility of providing information on carbon finer than the mesh. Tests made by Chirone (1980) showed that weight of carbon fragments of size lower than 0.8 mm left in the bed was about 0.5% of fixed carbon injected with coal. Attrition characteristics of the South African coal tested were studied by Donsl et al. (1981) in the same range of coal and sand sizes as the present work. They TABLE I Properties of the coal tested
FIG. 1. Experimental apparatus: 1) screw feeder; 2) start up burner; 3) pipe distributor; 4) bed solids sampling bottle; 5) manometer; 6) gas analyzer; 7) cyclone; 8) fines collecting bottle; 9) gas sampling probe; 10) cooling coil; 11) thermoeouple. is 850~ C; gas fluidizing velocity, 80 cm s -1 (at bed temperature). An expanded bed height about 50 cm is measured at this velocity from pressure drop profiles across the bed. For given air flow rate, runs at different excess air factors are made by changing coal feed rate. Elutriated carbon fines are collected together with ashes in a three stage cyclone. Ash content in cyclone material generally varies between 70 and 85%. The combustor free board is left practically unlagged. Bed and free board temperatures are measured along the axis of the combustor. Free board temperatures ensure postcombustion of combustible gases emitted from the top of the bed. Instead, postcombustion of carbon fines entrained in the exit gas is likely to be very limited.
South African coal Calorific value net, kcal Kg-1 Proximate analysis, % on dry basis: Ultimate analysis, % on dry basis:
Fixed carbon Volatile matter Ash Carbon Hydrogen Oxygen Sulphur Ash
6280 60 25 15 74.3 4.7 5.5 0.5 15.0
Free swelling index (ASTM
D720) Hardgrove Grindability Index (ASTM D409) Ash Fusion Temperature Reducing Atmosphere Carbolite Furnace (Pyramids)
1-2
Deformation Hemispherical Flow
51 1400~ C 1400~ C 1400~ C
ELUTRIATION OF ATrRITED CARBON FINES TABLE II Fragmentation behaviour of the coal tested (~ Feed coal
Fragmented coal
1--3 mm nominal size
Number of injected particles: 100 Size interval, izm >3000 3000-2000 2000-1400 1400-800 <800 3-6 mm nominal size Number of injected particles: . 100 Size interval, i~m >6000 . 6000-4760 4760-4000 4000-3350 3350-2800 2800-2000 2000-800 <800 6-9 mm nominal size Number of injected particles: 100 Size interval, p~m >9000 9000-6350 6350-5000 5000-4000 4000-3350 3350-2000 2000-800 <800
gr/gr -O.440 O.338 0.222 --
gr/gr -0.5368 0.2402 0.1852 0.0347 0.0031 ---
gr/gr -0.5720 0.3790 0.0365 0.0081 0.0044 ---
Number of collected particles: 150 gr/gr -O.400 O.300 0.300 -Number of collected patticles: 310 gr/gr 0.3543 0.2596 0.2092 0.0895 0.0364 0.0310 -Number of collected patticles: 700 gr/gr -0.3205 0.3476 0.1583 0.0823 0.0672 0.0241 --
(~ obtained by Chirone (1980) under the same conditions of the present work as regards bed temperature, fluidizing velocity and bed sand size. found that carbon attrition rates were proportional to bed carbon exposed surface and gas fluidizing velocity in excess of the minimum.
3. Model The following assumptions are based on the operative conditions of the combustor and on the behaviour of the coal tested. i) Two phase fluidization holds, with plug flow of the bed bubble phase and complete gas and
1215
solids mixing in the particulate phase. Interphase exchange is characterized by the number of transfer units X. ii) Swelling is negligible. Release of volatiles is fast, and their burning, although not fully achieved inside the bed, occurs uniformly throughout the particulate phase. Fragmentation is also fast, so that either size distribution of fragmented or feed coal may be taken as input to particle population balance equation (Levenspiel et al., 1968). Coal and bed carbon are made of spherical particles. iii) Combustion and attrition of bed carbon are parallel processes. Ash and carbon fines respectively generated by combustion and attrition are entrained in the fluidizing stream and separated at the cyclone. For a carbon particle of size d and density Pc, combustion pelt/6 (-dd3/dt)c and attrition pc~r/6 (-dd3/dt)a rates are both proportional to the exposed particle surface. Combustion rate depends on oxygen diffusion and reaction kinetics in series. The latter is first order in oxygen concentration. Attrition rate is proportional to (U - Uo), the excess gas velocity above the minimum fluidizing velocity, being: pelt/6 (-dd3/dt)a = k'a 7r d ~ (U -
Uo).
iv) The elutriated carbon rate Ec is essentially made of attrited carbon. Feed and fragmented coal, in fact, do not contain particles finer than d*. Moreover, the contribution to elutriation of unburnt residues of carbon particles, when reduced from the original to the elutriable size d*, is negligible. For each carbon particle of initial size d it is of the order of (d*/d) 3 times the mass of the particle. According to i), oxygen concentration in the bed particulate phase cp is obtained by means of an oxygen balance extended to such phase. Campbell and Davidson (1975) equation is modified to account for loss of attrited carbon fines and bed emissions of combustible gases. It is:
cp = c ~ -
Fo~,,~ + (Vc - Ec)/M c - F~)z A IV - (C - Co)e -x]
(1)
where, c i is the inlet oxygen concentration, A the combustor cross section, Fo2,v the oxygen molar flow rate required for complete combustion of volatiles, Fc the mass flow rate of fixed carbon charged with coal, Ec the elutriated carbon mass flow rate and F~)2 the oxygen molar flow rate whose consumption is delayed inside the free board to burn combustible gases emitted from the bed. Considering ii) and iii), (-dd/dt), the overall shrinkage rate of bed carbon particle of size d is made up of the contribution of shrinkage rates due to combustion and attrition being:
1216
COAL COMBUSTION TECHNIQUES
:
(2) C
a
Using Eqs. (6) and (7), bed carbon particle size distribution on a weight basis Pw(d) is obtained, being
(3)
c
(-
=
Pw(d) (4)
-
a
Eq. (3) is obtained as in Levenspiel (1972), with (d pc/2McSh Dg) and (pc/2Mc ks) accounting for diffusional and kinetic resistances to the combustion of the carbon particle, k a = 2k'a/Pc is the attrition rate constant of the carbon particle when burning in a fluidized bed, referred to Eq. (4). Without bed overflow, and negligible elutriation of particles coarser than d*, particle population balance equation reduces to:
--~
~r
6 Pc d3 P(d)
d O_.________L_+__~ p_.__._x___~ 2MeSh Dg 2M ck~
P(d)= _~_Jd
~
(5)
6 Pco where: P(d) is the bed carbon particle size distribution on a numerical basis, G co and Pco, respectively, the mass flow rate and the density of coal feed particles, and pw(d) the particle size distribution on weight basis of fragmented coal, with d varying between dmax and d*. Feed coal particle size distribution p .... (d) is taken instead of pw(d) when neglecting fragmentation. Substituting (-dd/ dt) from Eqs. (2-4) in Eq. (5) gives
Wc
(9)
Oxygen balance referred to the whole bed gives oxygen concentration at bed outlet: (%O2)/,,o = 21
9 [1 - F~
+ (FC'uA.ciEc)/nc- F*2]
(10)
Numerical solution of the system of Eqs. (6), (7), (8), (9)and (10)gives the unknowns W c, Ec, P(d), Pw(d) and (% O2)b,o when necessary input data are available, i.e. U, Uo, A, G~o, from which both Fo,~ and Fc are determined, Pw co(d), Pw(d) and 1 ' o f The kinetic constant k s has been obtained from Ayling and Smith (1972) reaction rate constant equation. Values of X = 3.5 and k~ = 0.7 • 10-7 are taken. The number of transfer units X has been determined to the best of present knowledge by means of a procedure substantially similar to that used by Donsl et al. (1979). All 76 distributor orifices are assumed to operate simultaneously. Davidson and Harrison (1963) correlation for rate of mass exchange between the bubble and particulate phase is modified as suggested by Sit and Grace (1980). The attrition rate constant k a has been calculated from data by Donsl et al. (1981). Note that, known Ec, the ratio Ec/F c gives the
Gco f l ~m=_p~(d) _ dd ,IT
d~
-~ Pco
(6) P(d) = [ci _ F ~ ] U "-~ -~, (U ---AU--~-e--~) o5 d p~ 2MeSh Dg Then, bed carbon loading W c and rate of elutriated carbon fines E c are:
"IT ~dmax Wc = ~ o~
d ~ P(d) dd
2M c k s
unburnt fraction of fixed carbon charged to the combustor which is elutriated as attrited carbon fines. 4. Results
f~maxt ~.~dd3~ ,
+ka(U - Uo)
(7)
./d*
E~ =
Pc
-P~ 6 dt / a P(d)dd
I)
-~,, ka(U - Uo)Tr
d2P(d)dd (8)
Numbers of fragmented particles per coal feed particle and their size distributions in Table II suggest that fragmentation of the coal tested varies with feed size, being more significant for coarser feed. Working out data in this table provides par-
ELUTRIATION OF ATrRITED CARBON FINES tide size distributions of feed and fragmented coal, pw,co(d) and pw(d), respectively, required for model calculations. CO and CH4 concentrations, at both bed and combustor outlets, are reported in Fig. 2 as a function of the excess air factor e = UAcJ(Foz,,~ + Fc/Mc). In this and following illustrations, note that the combustor could operate at steady state conditions even when e was less than 1, due to loss of attrited carbon. Bed emissions of combustible gases increase by a factor of four as e decreases from about 1.3 to 1. At the given fluidizing velocity, this decrease corresponds to an increase of coal feed rate by a factor of about t.~. l~ed emissions of combustible gases are somewhat affected by coal feed size, consistently increasing as coal size increases. This might appear contrary to expectation since smaller particles devolatilize faster, and it would be more likely that a "volatile plume" forms in the bed with the consequence of increased combustible gas emissions. Probably, the plume does not develop over the injection point under the conditions tested because of the limited combustor cross section and coal feed rates. Differences in devolatilization times of coal particles of various sizes in respect to the time required for coal to approach the top of the bed may be responsible for the observed trend. Using
024
bed
outlet , ~ 1 - -
-
~.x,~ ,-...
outlet
o
r
6 - 9 mm
__
t Jb,~-
c,;i
outlet
size 1-3 mm
! v
.-...\\\\~ I ~"%'Q;
,-,,
,7
3-6,, "79 "" 0
_
ecmbd,, outlet 9 9
*
_
I
0,
0 1.G
1.1
600
r
"%
6- 9 mm coal
506
4
2011
7ool---I
7 ' ,,a,,,"2-
,'5,.
1
":-. I N I oo "o- 2 - . '<.. \ l
,-,,.0,o,
L
3Oil 1 - 3 m! cool
15E
71 l.O
t.1
1.2
FlC. 3. Bed carbon loading as a function of the excess air factor: . . . . . chain line considering both coal fragmentation and bed emissions of combustible gases; - - - broken line neglecting fragmentation; - solid line neglecting coal fragmentation and emission of combustible gases from the bed. For 1-3 mm coal broken line is coincident with chain line.
9 9
_
I '~xxI v~'~
== 1.6
size
V
O08
2.4
combustor
1-3ram 3-6mm
v
616
coal
1217
12
Fro. 2. CO and CH, concentrations at the bed and combustor outlet as a function of the excess air factor.
data of Fig. 2, molar flow rates of oxygen FS~ which leave the bed unreaeted, but react in the free board with combustible gases emitted from the bed, are calculated, Experimental values of carbon loadings We, elutriation rates of attrited carbon Ec and oxygen coneentrations at bed outlet (%O2)b,o are respectively reported, as a function of e, in Figs. 3, 4 and 5 for the three coal sizes. Similarly to combustible gas emissions, W c and E~ also increase as a consequence of the decrease of e more than expected from the increase of coal feed rate. Cumulative bed carbon particle size distributions, for given e, are in Fig. 6. Cumulative feed and fragmented coal size distributions are also reported in this illustration. Comments follow for comparisons between experimental and model results under specific assumptions about coal fragmentation and bed emissions of combustible gases.
1218
COAL COMBUSTION TECHNIQUES
1.70
~
1~-----T~I02
t
6.Ol
36
t t
oL
6- 9 iam
o/1 1.70
.............. ..........
r
~
..... ~
LO
I-3
........ I
U
coal
.,
f am tool
"~.~.
1 4
v
1.2
I
~ ~ m m
coil
,e
FIC. 4. Elutriated carbon rate and fraction of unburnt fixed carbon as a function of the excess air factor: . . . . . chain line considering both coal fragmentation and emissions of combustible gases from the bed; - - - broken line neglecting fragmentation; -solid line neglecting coal fragmentation and bed emissions of combustible gases from the bed. For 1-3 mm coal broken line is coincident with chain line; . . . . . point line for Ec/Fc considering both coal fragmentation and emissions of combustible gases from the bed.
Neglect of Fragmentation and Bed Emissions of Combustible Gases (Solid Lines in Figs+ 3, 4, 5 and 6)
1.0
1.2 ,e
Fie. 5. Oxygen concentration at the bed outlet as a function of the excess air factor: . . . . . chain line considering both coal fragmentation and bed emissions of combustible gases from the bed; - - - broken line neglecting coal fragmentation; -solid line neglecting coal fragmentation and emissions of combustible gases from the bed. For 1-3 mm coal broken line is coincident with chain line9 I0( ~_ + ,,.
i
+.l
/
9
,-o .,,,.,J
,.,,,,+...]: 1141 gOll
/
J
+i!-
/
o.te7
,tltclr~n
Comparing solid lines with experimental data suggests that calculations carried out taking Pw co(d) instead of pw(d) and assuming F~>+ = O results in overestimation of W c at low e, particularly in the case of 3-6 and 6-9 mm feed coals (Fig. 3). More important, the shape of the We v s e curves predieted by the model under these assumptions is unsuitable to fit data, the calculated Wc increasing in fact as e decreases more than expected by experimental results. Underestimation of X could be responsible for the erroneous location of solid lines for We, and certainly a larger X would displace such lines downwards. However, the shape remaining substantially the same, a better agreement for some values of e would result in a larger discrepancy for other values of this variable. Moreover, an improvement of the agreement obtained for one of the coal sizes, by taking a larger X, does
1.1
e-I.O7
t~
§ llnll r 9 * ltlltmllll ecll / ] + c, s ' t P.tzb. ~ / - -
t --
t |
I. fll ,i.
L
I 62
15
d,t~g'l
0
3
6
d,mm
2.2 45
9 d,ttllTl
FIG. 6. Cumulative feed and fragmented coal, and bed carbon particle size distribution: . . . . . chain line considering both coal fragmentation and bed emissions of combustible gases from the bed; - - - broken line neglecting coal fragmentation; -solid line neglecting coal fragmentation and emissions of combustible gases from the bed. For all coal sizes, solid and broken lines are coincident.
ELUTRIATION OF ATI'RITED CARBON FINES
1219
not reflect a similar agreement for the other sizes. Consistent with comparison for W c in Fig. 3, overestimation in calculated Ec curves in respect to experimental results is shown by solid lines in Fig. 4. Solid lines in Fig. 5 indicate that ignoring emission of combustible gases from the bed results in an underestimation of (%O2)~ o- Finally, the comparison in Fig. 6 between solid lines of calculated cumulative bed carbon particle size distributions and experimental results shows rather large discrepancy for the coarser coal sizes.
Values of (1 - Er are the combustion efficiencies of fixed carbon charged into the combustor. They decrease as e decreases and, to some extent, as coal size increases. Neglecting bed emissions of combustible gases and fragmentation, i.e. basing Ec on solid lines rather than on chain lines would overestimate the fraction of unburnt carbon by about 30% at low values of e.
Effect of Allowing for Bed Emissions of Combustible Gases (Broken Lines in Figs. 3, 4, 5 and 6)
There is agreement between experimental data characterizing the performance of the fluidized combustion of the coal tested and results of model calculations carried out considering coal fragmentation and bed emissions of combustible gases. Neglecting these phenomena tends to overestimate bed carbon loading, elutriation rates of attrited carbon fines and, as a consequence, loss of combustion efficiency due to carbon attrition. Actually, using feed instead of fragmented coal size distribution slightly affects calculation results for the coal used. For other types of coal, or for coarser feed of the same coal, however, disregarding fragmentation might result in significant discrepancies between model predictions and experimental observations. Differences in calculation results depending on whether or not bed emissions of combustible gases are considered are relevant at low excess air factors under the experimental conditions of this work. Overestimations of bed carbon loading and elutriation rates of attrited carbon fines are likely to become even more important for larger combustors, when bed area served by each coal injection point increases, and tendency of combustible gases to escape from the bed also increases.
A fair agreement between model and experimental results is obtained if bed emissions of combustible gases are considered, with values of F~)s as taken, for given e, from Fig. 2. The shape of such lines for Wc is more adherent to experimental results throughout the entire range of e and, on average, for the three coal feed sizes. Analysis of the dependence of P(d) on F~)~ explains the better agreement of broken in respect to solid lines. With F~)s = O, the term in square brackets at the denominator of the second member of Eq. 6 might become very small as e decreases, resulting in too high values of P(d) and, in consequence, of W c. This tendency is corrected by consideration of F~)s, which, according to Fig. 2, reaches its higher levels right at the lower values of e. The fair agreement between data and broken lines for W~ reflects a similar trend for E~ and (%O~)~,~o. Bed carbon particle size distribution, instead, is little affected by considering or not bed emission of combustible gases, and in fact in Fig. 6 solid and broken lines are coincident.
5. Conclusions
Effect of Also Allowing for Coal Fragmentation (Chain Lines in Figs. 3, 4, 5 and 6) A better agreement between model and experimental data is found as regards carbon particle size distributions for 3-6 mm and 6-9 mm coal sizes in Fig. 6. Concerning Wc and E c in Figs. 3 and 4 the improvement is rather limited. Oxygen concentration at the bed outlet is slightly affected by considering fragmentation. Chain lines are coincident with broken lines in Figs. 3-5 with reference to all model results for 1-3 mm coal, due to the negligible fragmentation for this size.
Carbon Combustion Efficiency Additional point lines in Fig. 4 give fractions Ec/ F c of unburnt carbon as obtained from model calculations, as a functions of e, considering both fragmentation and bed emissions of combustible gases.
Nomenclature A
cl Cp d d* dmax Da e Er Fc Fo2.v F~)2
bed cross section inlet air oxygen concentration particulate phase oxygen concentration carbon particle diameter diameter of carbon particle of terminal velocity equal to gas velocity feed or fragmented coal maximum particle diameter oxygen diffusivity air excess factor elutriation rate of attrited carbon fixed carbon feed mass flow rate oxygen molar flow rate consumed for complete combustion of volatiles oxygen molar flow rate consumed in the free board for combustion of bed emissions of combustible gases
COAL COMBUSTION TECHNIQUES
1220
COO
k~,k 'a ks Mc
(% O2)b,o
p,~(d) p~,~o(d) P(d)
P~(d)
Sh t
U Uo
w~ x pc Oco
coal feed mass flow rate attrition rate constants surface reaction rate constant carbon atomic weight oxygen concentration in gases at bed outlet fragmented coal particle size distribution on a weight basis: grams of fragmented coal of size between d and d + dd per gram of fragmented coal feed coal particle size distribution on a weight basis: grams of feed coal of size between d and d + dd per gram of feed coal bed carbon particle size distribution on a numerical basis: number of bed carbon particles of size between d and d + dd bed carbon particle size distribution on a weight basis: grams of bed carbon particles of size between d and d + dd per gram of bed carbon particles Sherwood number time fluidizing velocity minimum fluidizing velocity bed carbon loading number of transfer units carbon density coal density
Acknowledgments This study has been carried out under the sponsorship of the Progetto Finalizzato Energetica, financed by the C.N.R., Roma. Authors are indebted to Prof. G. Donsl, for useful discussion, and to Ing. D. Paulillo who helped in performing the computation work.
REFERENCES 1. AYLING, A. B., AND SMITH, I. W., Combustion
and Flame, 18, 173, 1972. 2. ANDREI, M., "Time Resolved Burnout in the
Combustion of Coal Particles in Fluidized Bed," S.M. Thesis in Chemical Engineering, M.I.T., Cambridge, Massachusetts, 1978. 3. BEI~R, J. M., SAROFIM, i . F., Seventeenth Symposium (International) on Combustion, p.
189, The Combustion Institute, 1978. 4. BEI~R, J. M., MASSIMILLA,L., ANn 8AROFIM, A. F., "Fluidised Coal Combustion: The Effect of Coal Type on Carbon Load and Carbon Elutriation," International Conference on Fluidised Combustion: Systems and Applications, Institute of Energy Symposium Series No 4, London, November 1980. 5. CAMPBELL, E. K., AND DAVIDSON, J. F.: "The Combustion of Coal in Fluidised Beds," Institute of Fuel Symposium Series No 1, London, September 1975. 6. CHIRONE, R., "Coal Fragmentation in Fluidised Bed Combustion," Thesis in Chemical Engineering, University of Naples, 1980. 7. DAVIDSON, J. F., AND HARRISON, D., Fluidized Particles, Cambridge University Press, England, 1963. 8. DoNSl, G., MASSIMILLA, L., RUSSO, G., AND STECCONI, P., Seventeenth Symposium (International) on Combustion, p. 205, The Combustion Institute, 1978. 9. DONSI, G., MASSIMILLA,L., MICCtO, M., Russo, G,, AND STECCONI, P., Combustion Science and Technology, 21, 25, 1979. 10. DONSI, G., MASSlMILLA,L., AND MICCIO, M., Combustion and Flame, 41, 57, 1981. 11. GIBI~S, B. M., AND BEI~R, J. M., "'A Pilot Plant--Study of Fluidised Bed Coal Combustors," Institute of Chemical Engineering Symposium Series No 43, paper 23, p. 1-11, Harrogate, England, 1975. 12. LEVENSPIEL, O., KUNII, D., AND FITZGERALD,
T., Powder Technology, 2, 87, 1968-69. 13. LEVENSPIEL, O., Chemical Reaction Engineering, p. 346, Wiley, New York, 1972. 14. PARK,D., LEVENSPIEL,O., AND FITZGERALD,T., "A Model for Large Scale Atmospheric Fluidized Bed Combustors," paper 26c, 72nd Annual AIChE Meeting, S. Francisco, November 1979. 15. SAXENA, S. C., AND REHMAT, A., "'A Mathematical Model for Char Combustion in a Fluidized Bed," 6th International Conference on Fluidized Bed Combustion, Atlanta, Georgia, 1980. 16. SIT, S. P., AND GRACE, J. R., Chemical Engineering Science, 33, 327-335, 1980. 17. WELLS, I. W., KIRSHNAM,R. P., AND BALL, G. R., "A Mathematical Model for Simulation of AFBC System," 6th International Conference on Fluidized Bed Combustion, DOE, Atlanta, Georgia, 1980.
ELUTRIATION OF ATTRITED CARBON FINES
1221
COMMENTS Y. Levy, Technion, Israel. The gas flow in the freeboard has a large velocity fluctuation. As d* depends on velocity, did you consider the fluctuation effects in your model? Author's Reply. No, d* is the elutriable carbon particle size based on average velocity. In general, evaluation of contribution of u n b u r n t residues to carbon elutriation can be certainly improved, by accounting for velocity fluctuation. In the experimental conditions tested, however, possible changes in d* are small when compared to coal feed size and, as a consequence, such contribution is in any case negligible.
K. Jung, CSIRO, Australia. (i) In 1977 I developed a wire gauze basket sampler to study drying and devolatilization behavior of wet brown coal particles in a fluidized bed of sand. The aperature size of the sampler
was 2 mm and the size distribution of residual particles collected indicated underestimation of fine fractions generated by attrition. (ii) The mathematical model given applies to the first order kinetics. For the case where kinetic resistance in the overall combustion rate is large and the order of reaction is not the first order, the model's prediction on the shrinkage rate may be considerably different.
Author's Reply. (i) The aperture size of the basket sampler was 0.6mm in our case. It has been shown that residual fragmented carbon particles left in the bed under this condition is negligible (Arena et. al. "Carbon attrition in the fluidized combustion of a coal," to be puslished in A1ChE Journal). (ii) It is likely not to be the case for the relatively coarse (1-9 mm) coal particles tested, where the kinetics contribution to the overall combution rate is small.
ELUTRIATION OF ATTRITED CARBON FINES IN T H E F L U I D I Z E D COMBUSTION OF A COAL R. CHIRONE, A. CAMMAROTA, M. D'AMORE, L. MASSIMILLA Istituto di Chimica Industriale e Impianti Chimici, Universitd Istituto di Ricerche sulla Combustione, C.N.R. Piazzale Vincenzo Tecchio--80125 Napoli (Italy)
The relevance of coal fragmentation and bed emissions of combustible gases on the performance of the fluidized combustion of a coal and, in particular, on the elutriation rates of attrited carbon fines, is investigated for various coal feed sizes. Experimental data are compared with results of model calculations carried out with different assumptions as regards actual coal feed size distribution, because of fragmentation, and actual oxidizing conditions of the bed, owing to bed emissions of combustible gases subsequently burnt in the free board. Disregarding these phenomena results in model overestimations of elutriation rates of attrited carbon fines. Under the working conditions tested, discrepancies between calculation results and experimental observations are specially found when neglecting, at low excess air factors, the effects of bed emissions of combustible gases. 1. Introduction Recent works have pointed out that efficiency of fluidized combustion of coals might be affected by phenomena such as coal fragmentation, bed carbon attrition and emissions of combustible gas from the bed depending both on design and operation of fluidized combustors and on coal characteristics (Gibbs and Be6r, 1975; Andrei, 1978; Park et al., 1979; Be6r et al., 1980). Fragmentation, generally due to thermal shock and coal devolatilization, produces relatively coarse particles. Attrition, due to rubbing of carbon particles while burning in the bed, produces fines which, at the ordinary gas velocities of fluidized combustion, are rapidly elutriated from the combustor. Bed emissions of combustible gases, subsequently burnt in the free board, cause the bed to operate at oxygen concentrations higher than estimated from combustor outlet conditions. Attrition results in loss of carbon fines. Fragmentation and bed emission of combustible gases might indirectly influence such loss by affecting bed carbon loading and particle size distribution, which, in turn, are related to carbon attrition rates (Donsi et al., 1978; Donsi et al., 1981). The purpose of this work is outline the relationship between loss of attrited carbon, coal fragmentation and bed emissions of combustible gases. To this end, such emissions are measured together with elutriation rates of attrited carbon fines, bed carbon loading and particle size distribution during the fluidized combustion of a coal of known fragmen-
tation and attrition behaviour. Experimental data are compared with results of a fluidized combustion model, for some aspects simpler than others in the literature (Be6r and Sarofim, 1978; Wells et al., 1980; Saxena and Rehmat, 1980), but specifically structured to account for elutriation rates of attrited carbon and embody effects of both fragmentation and bed emissions of combustible gases. It is a development of a model previously used to evaluate carbon loading and gas composition in a fluidized bed combustor (Donsi et al., 1979). It should be noted that the influence of fragmentation on carbon loss in exit gases is subsequently.examined only from the stand point of its influence on carbon fines formation by attrition. Other contributions to carbon loss which might be enhanced by fragmentation, such as the possible formation of fines together with coarse fragments, and of fragments close in size to that elutriable, are negligible with the coal tested, so are not considered in the model. 2. Experimental Apparatus and Technique
Experiments are carried out with a 140 mm ID, 4.5 m high combustor, equipped with a coal screw feeder (Fig. 1). Air is injected into the bed through 19 pipes, each with 4 orifices 1.5 mm diameter. The bed, essentially made of 7 Kg of 200-400 I~m sand, is without overflow. Most ashes are in fact elutriated as fines. Additions of sand compensate for solids sampling from the bed. Bed temperature 1213
1214
COAL COMBUSTION TECHNIQUES
i
Bed solids are probe sampled. Water cooled, gas sampling probes are inserted into the bed and free board. During each run concentrations of oxygen and combustible gas, namely CO and CH4, are measured. H 2 is practically absent in bed outlet gases. As detailed elsewhere (Donsl et al., 1978), carbon loading is determined from carbon content in samples of bed solids and total bed inert material. Flow rates of elutriated carbon (mostly <150 Izm) are determined by accounting for solids separated at the cyclone per unit time and their carbon content. Because of the high combustor free board, elutriation of carbon coarser than d* = 180 tzm, the size at which particle terminal velocity is equal to gas velocity, is less than 1.5% of collected carbon. A South African 1-3, 3-6 and 6-9 mm size coal is used (Table I). Fragmentation characteristics of such coal when burnt in a fluidized bed were studied by Chirone (1980), with Andrei (1978) technique of collecting fragmented particles from the bed with a net basket. Fragmentation took place during coal devolatilization shortly after injecting coal particles into the bed, with results shown in Table II. A limit to this basket technique was the impossibility of providing information on carbon finer than the mesh. Tests made by Chirone (1980) showed that weight of carbon fragments of size lower than 0.8 mm left in the bed was about 0.5% of fixed carbon injected with coal. Attrition characteristics of the South African coal tested were studied by Donsl et al. (1981) in the same range of coal and sand sizes as the present work. They TABLE I Properties of the coal tested
FIG. 1. Experimental apparatus: 1) screw feeder; 2) start up burner; 3) pipe distributor; 4) bed solids sampling bottle; 5) manometer; 6) gas analyzer; 7) cyclone; 8) fines collecting bottle; 9) gas sampling probe; 10) cooling coil; 11) thermoeouple. is 850~ C; gas fluidizing velocity, 80 cm s -1 (at bed temperature). An expanded bed height about 50 cm is measured at this velocity from pressure drop profiles across the bed. For given air flow rate, runs at different excess air factors are made by changing coal feed rate. Elutriated carbon fines are collected together with ashes in a three stage cyclone. Ash content in cyclone material generally varies between 70 and 85%. The combustor free board is left practically unlagged. Bed and free board temperatures are measured along the axis of the combustor. Free board temperatures ensure postcombustion of combustible gases emitted from the top of the bed. Instead, postcombustion of carbon fines entrained in the exit gas is likely to be very limited.
South African coal Calorific value net, kcal Kg-1 Proximate analysis, % on dry basis: Ultimate analysis, % on dry basis:
Fixed carbon Volatile matter Ash Carbon Hydrogen Oxygen Sulphur Ash
6280 60 25 15 74.3 4.7 5.5 0.5 15.0
Free swelling index (ASTM
D720) Hardgrove Grindability Index (ASTM D409) Ash Fusion Temperature Reducing Atmosphere Carbolite Furnace (Pyramids)
1-2
Deformation Hemispherical Flow
51 1400~ C 1400~ C 1400~ C
ELUTRIATION OF ATrRITED CARBON FINES TABLE II Fragmentation behaviour of the coal tested (~ Feed coal
Fragmented coal
1--3 mm nominal size
Number of injected particles: 100 Size interval, izm >3000 3000-2000 2000-1400 1400-800 <800 3-6 mm nominal size Number of injected particles: . 100 Size interval, i~m >6000 . 6000-4760 4760-4000 4000-3350 3350-2800 2800-2000 2000-800 <800 6-9 mm nominal size Number of injected particles: 100 Size interval, p~m >9000 9000-6350 6350-5000 5000-4000 4000-3350 3350-2000 2000-800 <800
gr/gr -O.440 O.338 0.222 --
gr/gr -0.5368 0.2402 0.1852 0.0347 0.0031 ---
gr/gr -0.5720 0.3790 0.0365 0.0081 0.0044 ---
Number of collected particles: 150 gr/gr -O.400 O.300 0.300 -Number of collected patticles: 310 gr/gr 0.3543 0.2596 0.2092 0.0895 0.0364 0.0310 -Number of collected patticles: 700 gr/gr -0.3205 0.3476 0.1583 0.0823 0.0672 0.0241 --
(~ obtained by Chirone (1980) under the same conditions of the present work as regards bed temperature, fluidizing velocity and bed sand size. found that carbon attrition rates were proportional to bed carbon exposed surface and gas fluidizing velocity in excess of the minimum.
3. Model The following assumptions are based on the operative conditions of the combustor and on the behaviour of the coal tested. i) Two phase fluidization holds, with plug flow of the bed bubble phase and complete gas and
1215
solids mixing in the particulate phase. Interphase exchange is characterized by the number of transfer units X. ii) Swelling is negligible. Release of volatiles is fast, and their burning, although not fully achieved inside the bed, occurs uniformly throughout the particulate phase. Fragmentation is also fast, so that either size distribution of fragmented or feed coal may be taken as input to particle population balance equation (Levenspiel et al., 1968). Coal and bed carbon are made of spherical particles. iii) Combustion and attrition of bed carbon are parallel processes. Ash and carbon fines respectively generated by combustion and attrition are entrained in the fluidizing stream and separated at the cyclone. For a carbon particle of size d and density Pc, combustion pelt/6 (-dd3/dt)c and attrition pc~r/6 (-dd3/dt)a rates are both proportional to the exposed particle surface. Combustion rate depends on oxygen diffusion and reaction kinetics in series. The latter is first order in oxygen concentration. Attrition rate is proportional to (U - Uo), the excess gas velocity above the minimum fluidizing velocity, being: pelt/6 (-dd3/dt)a = k'a 7r d ~ (U -
Uo).
iv) The elutriated carbon rate Ec is essentially made of attrited carbon. Feed and fragmented coal, in fact, do not contain particles finer than d*. Moreover, the contribution to elutriation of unburnt residues of carbon particles, when reduced from the original to the elutriable size d*, is negligible. For each carbon particle of initial size d it is of the order of (d*/d) 3 times the mass of the particle. According to i), oxygen concentration in the bed particulate phase cp is obtained by means of an oxygen balance extended to such phase. Campbell and Davidson (1975) equation is modified to account for loss of attrited carbon fines and bed emissions of combustible gases. It is:
cp = c ~ -
Fo~,,~ + (Vc - Ec)/M c - F~)z A IV - (C - Co)e -x]
(1)
where, c i is the inlet oxygen concentration, A the combustor cross section, Fo2,v the oxygen molar flow rate required for complete combustion of volatiles, Fc the mass flow rate of fixed carbon charged with coal, Ec the elutriated carbon mass flow rate and F~)2 the oxygen molar flow rate whose consumption is delayed inside the free board to burn combustible gases emitted from the bed. Considering ii) and iii), (-dd/dt), the overall shrinkage rate of bed carbon particle of size d is made up of the contribution of shrinkage rates due to combustion and attrition being:
1216
COAL COMBUSTION TECHNIQUES
:
(2) C
a
Using Eqs. (6) and (7), bed carbon particle size distribution on a weight basis Pw(d) is obtained, being
(3)
c
(-
=
Pw(d) (4)
-
a
Eq. (3) is obtained as in Levenspiel (1972), with (d pc/2McSh Dg) and (pc/2Mc ks) accounting for diffusional and kinetic resistances to the combustion of the carbon particle, k a = 2k'a/Pc is the attrition rate constant of the carbon particle when burning in a fluidized bed, referred to Eq. (4). Without bed overflow, and negligible elutriation of particles coarser than d*, particle population balance equation reduces to:
--~
~r
6 Pc d3 P(d)
d O_.________L_+__~ p_.__._x___~ 2MeSh Dg 2M ck~
P(d)= _~_Jd
~
(5)
6 Pco where: P(d) is the bed carbon particle size distribution on a numerical basis, G co and Pco, respectively, the mass flow rate and the density of coal feed particles, and pw(d) the particle size distribution on weight basis of fragmented coal, with d varying between dmax and d*. Feed coal particle size distribution p .... (d) is taken instead of pw(d) when neglecting fragmentation. Substituting (-dd/ dt) from Eqs. (2-4) in Eq. (5) gives
Wc
(9)
Oxygen balance referred to the whole bed gives oxygen concentration at bed outlet: (%O2)/,,o = 21
9 [1 - F~
+ (FC'uA.ciEc)/nc- F*2]
(10)
Numerical solution of the system of Eqs. (6), (7), (8), (9)and (10)gives the unknowns W c, Ec, P(d), Pw(d) and (% O2)b,o when necessary input data are available, i.e. U, Uo, A, G~o, from which both Fo,~ and Fc are determined, Pw co(d), Pw(d) and 1 ' o f The kinetic constant k s has been obtained from Ayling and Smith (1972) reaction rate constant equation. Values of X = 3.5 and k~ = 0.7 • 10-7 are taken. The number of transfer units X has been determined to the best of present knowledge by means of a procedure substantially similar to that used by Donsl et al. (1979). All 76 distributor orifices are assumed to operate simultaneously. Davidson and Harrison (1963) correlation for rate of mass exchange between the bubble and particulate phase is modified as suggested by Sit and Grace (1980). The attrition rate constant k a has been calculated from data by Donsl et al. (1981). Note that, known Ec, the ratio Ec/F c gives the
Gco f l ~m=_p~(d) _ dd ,IT
d~
-~ Pco
(6) P(d) = [ci _ F ~ ] U "-~ -~, (U ---AU--~-e--~) o5 d p~ 2MeSh Dg Then, bed carbon loading W c and rate of elutriated carbon fines E c are:
"IT ~dmax Wc = ~ o~
d ~ P(d) dd
2M c k s
unburnt fraction of fixed carbon charged to the combustor which is elutriated as attrited carbon fines. 4. Results
f~maxt ~.~dd3~ ,
+ka(U - Uo)
(7)
./d*
E~ =
Pc
-P~ 6 dt / a P(d)dd
I)
-~,, ka(U - Uo)Tr
d2P(d)dd (8)
Numbers of fragmented particles per coal feed particle and their size distributions in Table II suggest that fragmentation of the coal tested varies with feed size, being more significant for coarser feed. Working out data in this table provides par-
ELUTRIATION OF ATrRITED CARBON FINES tide size distributions of feed and fragmented coal, pw,co(d) and pw(d), respectively, required for model calculations. CO and CH4 concentrations, at both bed and combustor outlets, are reported in Fig. 2 as a function of the excess air factor e = UAcJ(Foz,,~ + Fc/Mc). In this and following illustrations, note that the combustor could operate at steady state conditions even when e was less than 1, due to loss of attrited carbon. Bed emissions of combustible gases increase by a factor of four as e decreases from about 1.3 to 1. At the given fluidizing velocity, this decrease corresponds to an increase of coal feed rate by a factor of about t.~. l~ed emissions of combustible gases are somewhat affected by coal feed size, consistently increasing as coal size increases. This might appear contrary to expectation since smaller particles devolatilize faster, and it would be more likely that a "volatile plume" forms in the bed with the consequence of increased combustible gas emissions. Probably, the plume does not develop over the injection point under the conditions tested because of the limited combustor cross section and coal feed rates. Differences in devolatilization times of coal particles of various sizes in respect to the time required for coal to approach the top of the bed may be responsible for the observed trend. Using
024
bed
outlet , ~ 1 - -
-
~.x,~ ,-...
outlet
o
r
6 - 9 mm
__
t Jb,~-
c,;i
outlet
size 1-3 mm
! v
.-...\\\\~ I ~"%'Q;
,-,,
,7
3-6,, "79 "" 0
_
ecmbd,, outlet 9 9
*
_
I
0,
0 1.G
1.1
600
r
"%
6- 9 mm coal
506
4
2011
7ool---I
7 ' ,,a,,,"2-
,'5,.
1
":-. I N I oo "o- 2 - . '<.. \ l
,-,,.0,o,
L
3Oil 1 - 3 m! cool
15E
71 l.O
t.1
1.2
FlC. 3. Bed carbon loading as a function of the excess air factor: . . . . . chain line considering both coal fragmentation and bed emissions of combustible gases; - - - broken line neglecting fragmentation; - solid line neglecting coal fragmentation and emission of combustible gases from the bed. For 1-3 mm coal broken line is coincident with chain line.
9 9
_
I '~xxI v~'~
== 1.6
size
V
O08
2.4
combustor
1-3ram 3-6mm
v
616
coal
1217
12
Fro. 2. CO and CH, concentrations at the bed and combustor outlet as a function of the excess air factor.
data of Fig. 2, molar flow rates of oxygen FS~ which leave the bed unreaeted, but react in the free board with combustible gases emitted from the bed, are calculated, Experimental values of carbon loadings We, elutriation rates of attrited carbon Ec and oxygen coneentrations at bed outlet (%O2)b,o are respectively reported, as a function of e, in Figs. 3, 4 and 5 for the three coal sizes. Similarly to combustible gas emissions, W c and E~ also increase as a consequence of the decrease of e more than expected from the increase of coal feed rate. Cumulative bed carbon particle size distributions, for given e, are in Fig. 6. Cumulative feed and fragmented coal size distributions are also reported in this illustration. Comments follow for comparisons between experimental and model results under specific assumptions about coal fragmentation and bed emissions of combustible gases.
1218
COAL COMBUSTION TECHNIQUES
1.70
~
1~-----T~I02
t
6.Ol
36
t t
oL
6- 9 iam
o/1 1.70
.............. ..........
r
~
..... ~
LO
I-3
........ I
U
coal
.,
f am tool
"~.~.
1 4
v
1.2
I
~ ~ m m
coil
,e
FIC. 4. Elutriated carbon rate and fraction of unburnt fixed carbon as a function of the excess air factor: . . . . . chain line considering both coal fragmentation and emissions of combustible gases from the bed; - - - broken line neglecting fragmentation; -solid line neglecting coal fragmentation and bed emissions of combustible gases from the bed. For 1-3 mm coal broken line is coincident with chain line; . . . . . point line for Ec/Fc considering both coal fragmentation and emissions of combustible gases from the bed.
Neglect of Fragmentation and Bed Emissions of Combustible Gases (Solid Lines in Figs+ 3, 4, 5 and 6)
1.0
1.2 ,e
Fie. 5. Oxygen concentration at the bed outlet as a function of the excess air factor: . . . . . chain line considering both coal fragmentation and bed emissions of combustible gases from the bed; - - - broken line neglecting coal fragmentation; -solid line neglecting coal fragmentation and emissions of combustible gases from the bed. For 1-3 mm coal broken line is coincident with chain line9 I0( ~_ + ,,.
i
+.l
/
9
,-o .,,,.,J
,.,,,,+...]: 1141 gOll
/
J
+i!-
/
o.te7
,tltclr~n
Comparing solid lines with experimental data suggests that calculations carried out taking Pw co(d) instead of pw(d) and assuming F~>+ = O results in overestimation of W c at low e, particularly in the case of 3-6 and 6-9 mm feed coals (Fig. 3). More important, the shape of the We v s e curves predieted by the model under these assumptions is unsuitable to fit data, the calculated Wc increasing in fact as e decreases more than expected by experimental results. Underestimation of X could be responsible for the erroneous location of solid lines for We, and certainly a larger X would displace such lines downwards. However, the shape remaining substantially the same, a better agreement for some values of e would result in a larger discrepancy for other values of this variable. Moreover, an improvement of the agreement obtained for one of the coal sizes, by taking a larger X, does
1.1
e-I.O7
t~
§ llnll r 9 * ltlltmllll ecll / ] + c, s ' t P.tzb. ~ / - -
t --
t |
I. fll ,i.
L
I 62
15
d,t~g'l
0
3
6
d,mm
2.2 45
9 d,ttllTl
FIG. 6. Cumulative feed and fragmented coal, and bed carbon particle size distribution: . . . . . chain line considering both coal fragmentation and bed emissions of combustible gases from the bed; - - - broken line neglecting coal fragmentation; -solid line neglecting coal fragmentation and emissions of combustible gases from the bed. For all coal sizes, solid and broken lines are coincident.
ELUTRIATION OF ATI'RITED CARBON FINES
1219
not reflect a similar agreement for the other sizes. Consistent with comparison for W c in Fig. 3, overestimation in calculated Ec curves in respect to experimental results is shown by solid lines in Fig. 4. Solid lines in Fig. 5 indicate that ignoring emission of combustible gases from the bed results in an underestimation of (%O2)~ o- Finally, the comparison in Fig. 6 between solid lines of calculated cumulative bed carbon particle size distributions and experimental results shows rather large discrepancy for the coarser coal sizes.
Values of (1 - Er are the combustion efficiencies of fixed carbon charged into the combustor. They decrease as e decreases and, to some extent, as coal size increases. Neglecting bed emissions of combustible gases and fragmentation, i.e. basing Ec on solid lines rather than on chain lines would overestimate the fraction of unburnt carbon by about 30% at low values of e.
Effect of Allowing for Bed Emissions of Combustible Gases (Broken Lines in Figs. 3, 4, 5 and 6)
There is agreement between experimental data characterizing the performance of the fluidized combustion of the coal tested and results of model calculations carried out considering coal fragmentation and bed emissions of combustible gases. Neglecting these phenomena tends to overestimate bed carbon loading, elutriation rates of attrited carbon fines and, as a consequence, loss of combustion efficiency due to carbon attrition. Actually, using feed instead of fragmented coal size distribution slightly affects calculation results for the coal used. For other types of coal, or for coarser feed of the same coal, however, disregarding fragmentation might result in significant discrepancies between model predictions and experimental observations. Differences in calculation results depending on whether or not bed emissions of combustible gases are considered are relevant at low excess air factors under the experimental conditions of this work. Overestimations of bed carbon loading and elutriation rates of attrited carbon fines are likely to become even more important for larger combustors, when bed area served by each coal injection point increases, and tendency of combustible gases to escape from the bed also increases.
A fair agreement between model and experimental results is obtained if bed emissions of combustible gases are considered, with values of F~)s as taken, for given e, from Fig. 2. The shape of such lines for Wc is more adherent to experimental results throughout the entire range of e and, on average, for the three coal feed sizes. Analysis of the dependence of P(d) on F~)~ explains the better agreement of broken in respect to solid lines. With F~)s = O, the term in square brackets at the denominator of the second member of Eq. 6 might become very small as e decreases, resulting in too high values of P(d) and, in consequence, of W c. This tendency is corrected by consideration of F~)s, which, according to Fig. 2, reaches its higher levels right at the lower values of e. The fair agreement between data and broken lines for W~ reflects a similar trend for E~ and (%O~)~,~o. Bed carbon particle size distribution, instead, is little affected by considering or not bed emission of combustible gases, and in fact in Fig. 6 solid and broken lines are coincident.
5. Conclusions
Effect of Also Allowing for Coal Fragmentation (Chain Lines in Figs. 3, 4, 5 and 6) A better agreement between model and experimental data is found as regards carbon particle size distributions for 3-6 mm and 6-9 mm coal sizes in Fig. 6. Concerning Wc and E c in Figs. 3 and 4 the improvement is rather limited. Oxygen concentration at the bed outlet is slightly affected by considering fragmentation. Chain lines are coincident with broken lines in Figs. 3-5 with reference to all model results for 1-3 mm coal, due to the negligible fragmentation for this size.
Carbon Combustion Efficiency Additional point lines in Fig. 4 give fractions Ec/ F c of unburnt carbon as obtained from model calculations, as a functions of e, considering both fragmentation and bed emissions of combustible gases.
Nomenclature A
cl Cp d d* dmax Da e Er Fc Fo2.v F~)2
bed cross section inlet air oxygen concentration particulate phase oxygen concentration carbon particle diameter diameter of carbon particle of terminal velocity equal to gas velocity feed or fragmented coal maximum particle diameter oxygen diffusivity air excess factor elutriation rate of attrited carbon fixed carbon feed mass flow rate oxygen molar flow rate consumed for complete combustion of volatiles oxygen molar flow rate consumed in the free board for combustion of bed emissions of combustible gases
COAL COMBUSTION TECHNIQUES
1220
COO
k~,k 'a ks Mc
(% O2)b,o
p,~(d) p~,~o(d) P(d)
P~(d)
Sh t
U Uo
w~ x pc Oco
coal feed mass flow rate attrition rate constants surface reaction rate constant carbon atomic weight oxygen concentration in gases at bed outlet fragmented coal particle size distribution on a weight basis: grams of fragmented coal of size between d and d + dd per gram of fragmented coal feed coal particle size distribution on a weight basis: grams of feed coal of size between d and d + dd per gram of feed coal bed carbon particle size distribution on a numerical basis: number of bed carbon particles of size between d and d + dd bed carbon particle size distribution on a weight basis: grams of bed carbon particles of size between d and d + dd per gram of bed carbon particles Sherwood number time fluidizing velocity minimum fluidizing velocity bed carbon loading number of transfer units carbon density coal density
Acknowledgments This study has been carried out under the sponsorship of the Progetto Finalizzato Energetica, financed by the C.N.R., Roma. Authors are indebted to Prof. G. Donsl, for useful discussion, and to Ing. D. Paulillo who helped in performing the computation work.
REFERENCES 1. AYLING, A. B., AND SMITH, I. W., Combustion
and Flame, 18, 173, 1972. 2. ANDREI, M., "Time Resolved Burnout in the
Combustion of Coal Particles in Fluidized Bed," S.M. Thesis in Chemical Engineering, M.I.T., Cambridge, Massachusetts, 1978. 3. BEI~R, J. M., SAROFIM, i . F., Seventeenth Symposium (International) on Combustion, p.
189, The Combustion Institute, 1978. 4. BEI~R, J. M., MASSIMILLA,L., ANn 8AROFIM, A. F., "Fluidised Coal Combustion: The Effect of Coal Type on Carbon Load and Carbon Elutriation," International Conference on Fluidised Combustion: Systems and Applications, Institute of Energy Symposium Series No 4, London, November 1980. 5. CAMPBELL, E. K., AND DAVIDSON, J. F.: "The Combustion of Coal in Fluidised Beds," Institute of Fuel Symposium Series No 1, London, September 1975. 6. CHIRONE, R., "Coal Fragmentation in Fluidised Bed Combustion," Thesis in Chemical Engineering, University of Naples, 1980. 7. DAVIDSON, J. F., AND HARRISON, D., Fluidized Particles, Cambridge University Press, England, 1963. 8. DoNSl, G., MASSIMILLA, L., RUSSO, G., AND STECCONI, P., Seventeenth Symposium (International) on Combustion, p. 205, The Combustion Institute, 1978. 9. DONSI, G., MASSIMILLA,L., MICCtO, M., Russo, G,, AND STECCONI, P., Combustion Science and Technology, 21, 25, 1979. 10. DONSI, G., MASSlMILLA,L., AND MICCIO, M., Combustion and Flame, 41, 57, 1981. 11. GIBI~S, B. M., AND BEI~R, J. M., "'A Pilot Plant--Study of Fluidised Bed Coal Combustors," Institute of Chemical Engineering Symposium Series No 43, paper 23, p. 1-11, Harrogate, England, 1975. 12. LEVENSPIEL, O., KUNII, D., AND FITZGERALD,
T., Powder Technology, 2, 87, 1968-69. 13. LEVENSPIEL, O., Chemical Reaction Engineering, p. 346, Wiley, New York, 1972. 14. PARK,D., LEVENSPIEL,O., AND FITZGERALD,T., "A Model for Large Scale Atmospheric Fluidized Bed Combustors," paper 26c, 72nd Annual AIChE Meeting, S. Francisco, November 1979. 15. SAXENA, S. C., AND REHMAT, A., "'A Mathematical Model for Char Combustion in a Fluidized Bed," 6th International Conference on Fluidized Bed Combustion, Atlanta, Georgia, 1980. 16. SIT, S. P., AND GRACE, J. R., Chemical Engineering Science, 33, 327-335, 1980. 17. WELLS, I. W., KIRSHNAM,R. P., AND BALL, G. R., "A Mathematical Model for Simulation of AFBC System," 6th International Conference on Fluidized Bed Combustion, DOE, Atlanta, Georgia, 1980.
ELUTRIATION OF ATTRITED CARBON FINES
1221
COMMENTS Y. Levy, Technion, Israel. The gas flow in the freeboard has a large velocity fluctuation. As d* depends on velocity, did you consider the fluctuation effects in your model? Author's Reply. No, d* is the elutriable carbon particle size based on average velocity. In general, evaluation of contribution of u n b u r n t residues to carbon elutriation can be certainly improved, by accounting for velocity fluctuation. In the experimental conditions tested, however, possible changes in d* are small when compared to coal feed size and, as a consequence, such contribution is in any case negligible.
K. Jung, CSIRO, Australia. (i) In 1977 I developed a wire gauze basket sampler to study drying and devolatilization behavior of wet brown coal particles in a fluidized bed of sand. The aperature size of the sampler
was 2 mm and the size distribution of residual particles collected indicated underestimation of fine fractions generated by attrition. (ii) The mathematical model given applies to the first order kinetics. For the case where kinetic resistance in the overall combustion rate is large and the order of reaction is not the first order, the model's prediction on the shrinkage rate may be considerably different.
Author's Reply. (i) The aperture size of the basket sampler was 0.6mm in our case. It has been shown that residual fragmented carbon particles left in the bed under this condition is negligible (Arena et. al. "Carbon attrition in the fluidized combustion of a coal," to be puslished in A1ChE Journal). (ii) It is likely not to be the case for the relatively coarse (1-9 mm) coal particles tested, where the kinetics contribution to the overall combution rate is small.