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Release of volatiles from large coal particles in a hot fluidized bed
Release of volatiles from large coal particles in a hot fluidized bed John
F. Stubington
and Sumaryono
Department of Fuel Technology, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, PO Box 1, Kensington, NSW 2033, Australia (Received 27 June 1983)
Coal particles with diameters of 3-11 mm were injected into a small, hot bed of sand fluidized by nitrogen. Volatiles evolution was followed by sampling the exit gas stream and subsequent analysis by gas chromatography. Three Australian coals covering a range of volatile matter were studied and the effects of coal particle size and bed temperature were determined. The yields of gaseous components, char and tar are explained by consideration of the competitive reactions for coal hydrogen and oxygen and secondary reactions of the volatile species within the coal particle. The pore structure developed during devolatilization has a significant effect on the extent of these secondary reactions. It is concluded that heat transfer is the main process controlling the volatilization time in fluidized bed combustors. The time required for heat transfer into the coal particle, determined by calculation and experiment, agrees with the measured volatilization time. Significant factors are external heat transfer to the surface of the particle, internal conduction through the coal substance and radiation through the pores, and the counterflow of volatiles out of the coal particle. For different coals, variations in the volatilization time appear to be caused by the development of different pore structures, which affect radiant heat transfer through the pores. (Keywords:
coal; coal particle;
fluidizad
bad combustion;
In recent years, there has been a growing awareness of the importance of volatiles combustion in the fluidized bed combustion of coal’. Particularly for coals of high volatile matter, a significant proportion (up to ~50%) of the specific energy of the coal is released by combustion of the volatiles. Hence, a study of the processes leading to volatiles combustion is of vital significance to an understanding of fluidized bed combustion. The processes to be considered are :
volatilization)
EXPERIMENTAL The experiments were performed in a small stainless steel reactor (35 mm diameter x 200 mm high), shown in Figure 1. This contained an 80mm deep bed of sand which had been sieved to a narrow size range (SW-599 pm) and was supported by a perforated stainless steel distributor. The bed was fluidized at 1.5 times the measured minimum fluidization velocity by nitrogen preheated in a coil surrounding the reactor. The reactor was radiantly heated
(1)coal particle mixing on injection into the bed; (2)volatiles release from the coal particles; and (3) gas-phase mixing and combustion with oxygen. In this Paper, initial results from a study of the second process, i.e. the release of volatiles from coal particles in a hot fluidized bed, are reported. Relatively large coal particles (Z 1-15 mm in diameter) are burnt in fluidized bed combustors, so the extensive literature on volatilization of p.f.-sized coal particles is not relevant to this study. A limited number of studies of the volatilization of larger coal particles have been made’ - ‘, but the reported data on devolatilization behaviour vary widely. This may be attributed to differences in the experimental equipment and in the measurement techniques used. The mechanism controlling the volatilization of large particles of coal is still unclear; heat transfer4, kinetics’, mass transfer9 and evaporation3 have been suggested by various workers. In addition, the effect of coal properties on volatilization has not been resolved. Thus, a systematic study of large coal particle volatilization is needed to provide a better understanding of the process and its controlling parameters as well as fundamental data for modelling coal volatiles combustion in a fluidized bed combustor. 00162361/84/071013~7%3.00 @ 1984 Buttenvorth & Co. (Publishers) Ltd
Figure 1 Pyrolysis reactor: A, N2 fluidizing gas inlet; B, valve: C, delivery pipe for coal particles (12.5 mm i.d.); D. chromelalumel thermocouple connected to temperature controller; E, reactor; F, preheater tube (6.2 mm o.d.); G, sand bed; H, perforated plate distributor (0.5 mm openings); I, joint; J, first tar trap, immersed in dry ice; K, stainless steel perforated plate; L, second tar trap; M, outlet to gas sampling manifold; 0, glass wool; P, cotton wool; 0, anhydrone
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1984, Vol 63, July
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Volatiles in large coal particles: J. F. Stubington and Sumaryono
in a furnace constructed from cast refractory and internally wound with resistance heating wire. The bed temperature was measured by a chromel-alumel thermocouple inserted through the lid of the reactor and was controlled at 750, 850 or 950°C. The coal particles were shaped into a nearly spherical form, to eliminate the uncertainties associated with differences in shape between particles, For each experiment, either one or two particles (depending on size) were dropped through the ball valve into the hot fluidized bed. The volatiles were swept from the reactor by the fluidizing gas and the condensibles were collected in two tar traps. Samples of the product gas were taken in syringes from the manifold at various times after injection of the coal particle and were subsequently analysed by gas chromatography for hydrogen, methane, ethane, ethylene, propane + propylene, carbon monoxide and carbon dioxide. The remaining char was cooled in a flow of nitrogen, recovered from the bed and weighed. Since the accuracy of the determination of the gain in weight of the tar traps was poor, the yield of tar plus pyrolysis water was calculated as the difference between the original dry coal weight and the sum of the gas and char yields. Three Australian coals, covering a range of volatile matter, were chosen for the experiments. Their analyses are given in Table I. For each coal and temperature, three to six separate experiments were performed with particles ranging in diameter from 3 to 11 mm. Comparison of the diameter of the recovered char particle with the diameter of the original coal particle showed that only a slight amount of swelling (< 15 ~01%) occurred.
Time
40
RESULTS AND DISCUSSION
60 Time fs)
80
loo
Figure 3 Hydrogen and methane evolution from Great Northern coal at 950°C. 0, CH,; 0, H,
Gaseous evolution Figure 2 shows a typical gaseous evolution curve for Bulli coal at 850°C in which the main components are methane and carbon oxides. In some experiments, the carbon oxides were observed to evolve over a long period at low concentrations. This tailing is attributed to the slow release of oxygen, some of which remains in the residual char even after volatilization to 1000”C’“~ll. Hydrogen was determined separately from the other components, and Figure 3 compares the hydrogen and methane release from Great Northern coal at 950°C. In contrast to the
Table 1 Analyses of coal samples Great Bulli
Northern
Greta
Proximate analysis (wt% air-dried basis) : Volatile matter Ash Fixed carbon Moisture
19.4 11.9 67.3 1.4
28.4 18.6 50.0 3.0
43.5 5.4 48.9 2.2
Ultimate analysis (wt% air-dried basis): Carbon Hydrogen Nitrogen Sulphur Oxygen (by difference)
75.3 3.7 1.4 0.5 5.8
63.0 3.6 1.3 0.6 9.9
72.0 5.0 1.5 1.1 12.8
Crucible swelling no. fB.6 1016: Part 12)
1
1014
(s)
Gas evolution from Bulli coal at 85o’C (particle diameter=7.5 mm). 0, CH,; Cl, CO,; n , C,H,; A. C,H,; 0, CO; n, C,‘s Figure 2
FUEL, 1984, Vol 63, July
0.5-l
2
single peak of methane, hydrogen evolved in two peaks, and this was particularly noticeable at high temperatures (850 and 950°C) and for large particles (> 6 mm). The yield of each component gas was obtained by integrating the evolution curves (e.g. as shown in Figures 2 and 3) and the total gas yield was calculated as the sum of the component yields. The measured total gas yields and char yields are given in Tables 2,3 and 4 for Bulli, Great Northern and Greta coals respectively. From the results a number of general trends have been identified. As the temperature increases, the char yield decreases; the total gas yield, CO + CO,, methane and ethylene increase; the tar + pyrolysis water and propane + propylene peak at ~850°C; and ethane is either constant or peaks at z 850°C. The effects of particle size on yields are not as clear as those of temperature, but the following trends can be observed as the size increases: the char yield is constant or increases at 750 and 850°C and increases at 950°C; the total gas and CO + CO, decrease at 850 and 950°C; and the tar +pyrolysis water is constant or decreases. The results may be explained by consideration of two processes: (1)competitive reactions for the hydrogen and oxygen contents of the coal; and (2) secondary reactions of reactive volatile species within the coal particle. The primary volatilization process starts at about 300400°C and continues to temperatures above 1OOOC.at
Volatiles in large coal particles: J. F. Stubington and Sumaryono Table 2 Volatilization
yields from Bulli coal Yields (wt% of air-dried coal)
Pyrolysis temperature (“C)
Particle diameter (mm)
Char
Total gases
CO+ CO2
Tar + pyrolysis H2O
750
4.0 6.8 8.3
87.3 83.9 85.7
3.3 3.3 3.3
1.1 0.5 9.4
8.1 11.6 9.7
850
3.1 4.3
80.2 -
5.3 6.7
2.4 2.6
13.2 _
4.3 6.1 7.6 9.0
81.9 82.5 81.6
5.0 7.3 5.5 -
1.6 3.7 2.3 _
11.7 10.6 -
11.0
81.6
6.0
2.3
11.0
2.8 4.6 4.6 6.0 7.7 10.0
73.2 75.8 80.4 80.8 80.4 82.9
9.8 9.8 8.3 8.0 8.8 5.7
4.9 5.2 4.3 3.8 3.6 2.6
15.5 13.0 10.0 9.9 9.5 10.1
950
Table 3 Volatilization
yields from Great Northern coal Yields (wt% of air-dried coal)
Pyrolysis temperature W)
Particle diameter (mm)
Char
Total gases
co+ CO2
Tar + pyrolysis H2O
750
3.5 6.0 7.0 8.2 9.6
72.3 71.1 76.5 67.6 77.3
4.3 5.0 5.6 7.0 6.4
1.6 1.9 2.5 2.5 3.5
20.4 20.9 14.9 22.4 13.3
850
3.4 3.5 5.3 5.3 5.3 8.1 10.0
_ 49.7 69.7 67.8 71.9 75.2
15.2 14.0 13.6 14.9 8.7 9.2 -
8.1 7.1 7.2 8.2 4.2 4.3 -
33.3 12.3 20.5 14.9 -
950
2.7 2.7 4.2 7.8 11.0
54.2 70.8 73.6 73.4
15.6 17.4 12.3 9.7 7.5
9.2 10.6 7.8 6.0 4.6
25.4 13.9 13.6 16.1
Table 4 Volatilization yields from Greta coal Yields (wt% of air-dried coal) Pyrolysis temperature (W
Particle diameter (mm)
Char
Total gases
750
3.9 5.3 5.3 6.4 7.8
58.6 58.1 57.7 58.9
10.2 9.5 10.6 9.7 9.9
2.8 2.8 4.8 2.1 2.6
29.0 30.2 29.5 29.0
850
3.3 6.1 8.2 9.0
46.2 49.9 50.9 53.3
23.9 17.9 15.0 -
6.4 4.3 4.1 -
28.6 29.9 31.9 -
950
3.7 5.3 6.3 8.6
39.6 45.8 47.0 49.7
35.8 25.9 27.8 25.0
20.5 16.6 11.1 10.2
22.4 26.0 23.0 23.1
co + CO2
Tar + pyrolysis H2O
rapid heating rates”. As part of this process, the hydrogen in the coal may combine either with oxygen from the coal to produce pyrolysis water or with carbon to produce hydrocarbons. In the latter case, the coal oxygen becomes available to combine with carbon and produce carbon monoxide or carbon dioxide, thus decreasing the weight of remaining char. Tables 24 show such an increasing yield of carbon oxides and an associated decreasing char yield as the temperature increases and as the particle size decreases, i.e. as the heating rate increases. Hence, the formation of carbon oxides is favoured at high temperatures and high heating rates, whereas water formation is favoured at low temperatures and low heating rates. In this respect, the present results for bituminous coals are similar to those of Suuberg et al.” for lignite pyrolysis. The primary volatilization products travel through the pores of the coal particle to its surface, where they are released into the surrounding gas stream. During this mass transfer through the particle, the more reactive volatile species can undergo secondary reactions (e.g. cracking, condensation, polymerization). As a result of these secondary reactions, larger hydrocarbon molecules (e.g. tars) are cracked to form smaller hydrocarbon molecules, hydrogen is produced and some carbon from the volatile components is deposited on the walls of the pores, thus increasing the char yield. The extent of these secondary reactions is determined by the temperature and the residence time of the volatiles within the coal pores. This residence time increases with increasing particle size and decreases with increasing pore size: in larger pores, mass transfer out of the coal particle is faster. As the temperature increases, the increasing extent of secondary reactions (particularly cracking reactions) causes the yield of higher hydrocarbons (e.g. tar, propane + propylene) to peak and the yields of methane and ethylene to increase. As the particle size increases, the increased volatiles residence time within the coal particle allows secondary reactions to occur to a greater extent, leading to higher char yields at the expense of gas and tar yields. The effect of pore size on the extent of secondary reaction may be seen by comparing the char yields (Tables 24) with the proximate analysis char yields (ash +tixed carbon), which are 79.2,68.6 and 54.3 wt% (air-dried basis) for Bulli, Great Northern and Greta coals, respectively. For Bulli and Great Northern coals. the char yields from the fluidized bed nearly always exceeded the proximate yield, whereas the char yield from Greta coal was nearly always less than the proximate yield. The extent of secondary reaction (and carbon deposition) within the Greta coal particles was much smaller because of the formation of larger-diameter pores during volatilization. This effect was verified by measuring the pore size distributions of the recovered char particles (see appendix for details). For Greta char, 50% of the pore volume occurred in pores with diameters > 48 pm, whereas the corresponding diameters were 9 pm for Bulli char and 5 pm for Great Northern char. Although further experiments are necessary for verification, it is suggested that the differences in pore sizes may be attributable to differences in the plastic behaviour of the original coals. When tested in the Gieseler plastometer, Bulli and Great Northern coals exhibited negligible plasticity with maximum creep rates of 0.2 and 0.4 divisions per minute, whereas the Greta coal had a maximum fluidity of 585 divisions per minute, at 412°C. The internal pressure arising from volatiles release within the Greta coal particle was
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Volatiles in large coal particles: J. F. Stubington and Sumaryono
probably sufficient to force the gas out through the plastic coal, generating larger pores. The evolution of hydrogen as two peaks may also be explained by the processes above. The first peak is due to the primary decomposition reactions, particularly from hydrogen formation at high temperatures close to the particle surface. In addition, the decomposition of reactive volatile species (tars and other unstable hydrocarbons) produces some hydrogen. These secondary reactions peak later because the temperature inside the particle increases relatively slowly, so thecombination of residence time and temperature required for secondary reaction occurs after much of the primary volatilization is completed. Hence, the second peak is more obvious for larger coal particles, at higher temperatures and for Bulli and Great Northern coals-exactly the conditions which enhance secondary reactions.
120
IA
w x -
80
I-
40
Volatilization time
The time taken for volatilization in a fluidized bed combustor is very significant, inasmuch as it determines the position at which the volatiles are released into the bed. In particular, the volatilization time must be compared with the particle mixing time in order to decide on the most appropriate of the various models suggested for volatiles combustion’. Most previous workers have inferred volatilization times from the occurrence of a volatiles diffusion flame around individual particles2~’ - ‘, but this method must underestimate the total volatilization time, since volatilization may still occur after flame extinction. Instead, volatilization times have been measured in the present work from the gas evolution concentration proliles4. Visual observation of the tar fog passing from the reactor to the tar traps indicated that tar evolution finished slightly earlier than gas evolution, so the volatilization time was assessed from the latter. In this work, the volatilization time was defined as the time corresponding to evolution of 95 wt% of the total gases. This was determined from the experimental results, the tailing of hydrogen and carbon oxides at very low concentrations over long periods being excluded. The correction for the time of travel from the bed to the sampling point, obtained by CO2 tracer gas injection into the bed, was found to be about 3 s. Since the spread of travel times was small compared with the measured volatilization times, no correction was made for the dispersion of gases during their travel from the fluidized bed to the sampling manifold. The measured volatilization times are shown in Figures 46 as a function of coal particle diameter at bed temperatures of 750, 850 and 950°C. The volatilization times for Greta coal are shorter than those for Bulli and Great Northern coals. This may be attributed to differences in the pore structure developed within the char particles, by one or both of the following arguments. First, the larger-diameter pores (see Appendix) in Greta char allow more rapid mass transfer of the volatiles through the particle to its surface. Second, the greater pore volume (see Appendix) in Greta char may enhance radiative heat transfer through the pores into the particle. This more rapid heating would be significant if heat transfer into the particle limited the volatilization rate. Both of these arguments are consistent with the explanations given above, under ‘Gaseous evolution’, since both lead to a
1016
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1984, Vol 63, July
1,
0
COAL PARTICLE DIAMETER (mm
1
Figure 4 Volatilization and heat transfer times at 75o’C. 0, Bulli; 0, Great Northern; n , Greta; -, time for particle centre to reach 740°C; - - - - -, results of Pilla? at 775°C
COAL PARTICLE DIAMETER [mm) Figure 5 Volatilization and heat transfer times at 850°C. 0, Bulli; 0, Great Northern; m, Greta; -, time for particle centre to reach 84OC; - - - - -, results of Pilla? at 875°C; ---, results of Ragland and Weiss’ at 815°C
shorter volatiles residence time within the Greta particles and hence less secondary reaction. With increasing temperature, the volatilization time for all three coals decreased slightly, as observed by previous workers6s7. A possible explanation for this effect may again be the enhanced radiative heat transfer through
Volatiles in large coal particles:
a
4 COAL
PARTICLE
and Sumaryono
coal particle surface and subsequent heat transfer into the coal particle, both by conduction through the coal substance and by radiation through pores and cracks. Since the coal particles were large relative to the bed particles, the rate of heat transfer to the coal particle surface should be the same as the rate of heat transfer to an immersed surface. Botterill et a1.l3 have given data for heat transfer to an immersed surface as a function of fluidizing velocity, particle size and temperature. From these data, heat transfer coefficients h ranging from 460Wm-*K-‘at750“Cto500Wm-*K-‘at950”C were deduced for the conditions used. Badzioch et al.‘* measured the effective thermal conductivity and specific heat of a range of coals up to 900°C. They found that the thermal conductivity k was approximately constant up to z 400°C and increased rapidly at higher temperatures, predominantly as a result of radiant heat transfer across pores and cracks in the carbonized coal; the volumetric specific heat (PC,) was approximately constant up to z 350°C and decreased at higher temperatures. The following expressions were fitted to their mean results for use in the present heat transfer calculations:
0
0; 0
J. F. Stubington
12
DIAMETER (mm1
Figure 6 Volatilization and heat transfer times at 95o’C. l , Bulli; 0, Great Northern; n , Greta; -, time for particle centre to reach 940°C
k=
0.23 W m-l K-’ for T<4OO”C 0.23+2.24x 10-5(7’-400)1~8 W m-l K-’ i for T>4OO”C 1.92x 106 J mm3 K-’
PC,=
pores at the higher temperatures (since radiation varies as T*). According to James and Mills”, diffusion out through the coal particle is not fast enough to explain the short devolatilization times measured for particles lO& 1000~ in diameter. They claim that a pressure-driven flow of volatiles out of the particle must occur, so the effect of temperature may not be explainable by an increase in the diffusion coefficient. Pillai’s results6 are shown in Figures 4 and 5 for Glen Brook coal with a proximate analysis close to that of the Great Nothem coal used here but with a crucible swelling number of 7. The other coals which he tested had shorter volatilization times, except for Pittsburgh No. 8. The results of Ragland and Weiss’ are also shown in Figure 5. The volatilization time was measured by flame extinction in both of these studies, so the shorter times noted are to be expected, since volatiles release continues even though the flame can no longer be supported. For very small particles, the rate of pyrolysis is controlled by chemical kinetics, so there is no effect of particle size on pyrolysis rate”. As the particle size increases, a critical size is reached at which heat and/or mass transfer becomes limiting, and this size appears to be < 2 mm for heating rates < 100 K s-l. Hence, under the present experimental conditions, heat and/or mass transfer is expected to control the volatilization time. A squarelaw relationship (t,ad*) between volatilization time t, and particle diameter d has been proposed for both of these processes, so the effect of particle size on volatilization time cannot be used to distinguish between heat and mass transfer. Heat transfer
To assess its importance in coal volatilization, the rate of heat transfer into the coal particle has been determined by calculation and by experiment. The processes considered were heat transfer from the fluidized bed to the
i 1.92 x 106-2.92x
for T6350”C
103(T-350) J me3 K-’ for T>350”C
(1)
(2)
The relative importance of external and internal heat transfer was deduced by calculating the Biot number (h/k, where I is the particle radius) using the constant values for the coal properties. Values of 0.3-2.8 were obtained and indicated that both external and internal heat transfer must be considered in the heat transfer calculations’5. Two types of heat transfer calculation were then made, first assuming constant values for the thermal properties of the coal (i.e. the low-temperature values) and then assuming variable properties (as given in Equations (1) and (2)). For the constant-property calculation, an analytical so1ution’6 was used, but a numerical solution’ ’ was necessary for the variable-property calculation. Figure 7 shows the temperature-time profile at the centre of a coal particle, calculated by each of these methods. As expected from the variation in thermal properties with temperature, the variable-property solution gives a significantly shorter time for the centre of the particle to reach bed temperature. Some experiments were performed to measure the temperature at the centre of a coal particle as a function of time. An ultrafine thermocouple (0.5 mm in diameter) was inserted into a coal particle, which was then placed in the hot fluidized bed. The temperature of the thermocouple was recorded and is reproduced in Figure 7 for comparison with the calculated profiles. It can be seen that the constant-property calculation gives a better approximation to the measured profile than does the variableproperty calculation. The maximum deviation between experiment and variable-property calculation occurs at about 45s roughly corresponding to the time for the maximum rate of release of volatiles from the coal particle. The difference between the results may be explained if mass transfer of volatiles out of the particle inhibits the
FUEL, 1984,
Vol 63, July
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Volatiles in large coal particles: J. F. Stubington and Sumaryono
800
2
600
F Z
O
0
80
40 TIME
120
is.1
Temperature rise at the centre of a Bulli coal particle at external temperature 833°C (particle diameter 8.0 mm). -, Calculation with variable coal properties; - - - -, calculation with constant coal properties; ---, experiment Figure 7
flow of heat into the particle, causing a slower rate of increase in the temperature at the particle centre than is predicted by the variable-property calculation. Hence the increased heat transfer rate due to the change in coal properties with temperature is offset by the inhibiting effect of volatiles flowing out of the particle, with the net result that the measured particle temperature is better approximated by the constant-property calculation. As a measure of the time required for heat transfer, the time for the particle centre to attain a temperature 10°C below bed temperature (since the curve is asymptotic to bed temperature) was arbitrarily chosen. The results of these calculations are plotted as solid lines on Figure 46 for comparison with volatilization times. To calculate these times, use was made of the analytical solution for heat transfer into a sphereI (including both external and internal resistances) with constant values of thermal coal properties, since this method gave the best agreement with measured temperature-time profiles. Comparison of volatilization and heat transfer times From Figures 4-6 it is clear that the 95% volatilization time is very close to the time needed for heat transfer into the centre of the particle. It is therefore concluded that the volatilization time is controlled mainly by the process of heat transfer to the centre of the coal particle. (It has already been shown that both external and internal heat transfer must be included in calculating this heat transfer rate.) This is in agreement with the conclusion of James
1018
FUEL, 1984, Vol 63, July
and Mills”, who suggested that the effect of particle size is mainly due to the slower heating rate rather than the larger residence time and surface area. This conclusion is also consistent with the other minor effects already noted under ‘Volatilization time’. The heat transfer calculations included external heat transfer to the particle, internal conduction within the particle and some contribution from radiation through pores (expressed as effective thermal conductivity by Badzioch et a1.14).In the present rapid-heating experiments, radiation through the pores would cause a greater rate of heat transfer at high temperatures than is predicted from the effective thermal conductivities measured under slow heating rates by Badzioch et al.14, owing to the larger temperature difference across the pores at the higher heating rate. Thus, the volatilization times tend to be less than the calculated heat transfer times at 950°C (Figure 6). The shorter volatilization time for Greta coal may also be a result of enhanced radiative heat transfer through the pores, which are both larger in diameter and occupy a greater fraction of the particle volume in Greta char than in Bulli and Great Northern chars. While heat transfer to and into the coal particle is the major process determining the volatilization time, the rate of this heat transfer is affected by the pore structure developed for each particular coal. The total pore volume within the char appears to increase with the volatile matter of the coal and the pore diameter appears to increase with the fluidity of the coal (see Appendix). The present results suggest that the variations in volatilization time from one coal to another are attributable to differences in the pore structure of the char and that these differences in pore structure may be correlated with the volatile matter and plasticity of the parent coal. Further experiments with a range of coal types are needed to test these suggestions. ACKNOWLEDGEMENTS The authors wish to acknowledge the financial support of the Australian Research Grants Scheme for this work, and Sumaryono wishes to thank the Australian Development Assistance Bureau for their support during his period of study in Australia. REFERENCES . 2 4 6
8
9 10
11 12 13 14 15
Stubington, J. F. J. Inst. Energy 1980, 53, 191 Essenhigh, R. H. ASME Trans.--J. Engng. Power 1963,85, 183 Peters, W. and Bertling, H. Fuel 1965,44,317 Morris, J. P. and Keaims, D. L. Fuel 1979,58,465 Jung, K. and Stanmore, B. R. FueZ 1980,59,74 Pillai, K. K. J. Inst. Energy 1981, 54, 142 Ragland, K. W. and Weiss, C. A. Energy 1979,4,341 Bywater, R. J. in ‘Proceedings of the Sixth International Coal Fluid&d Combustion Conference, Atlanta’, 1980, Vol. III, p. 1092 LaNauze, R. D. Fuel 1982,61,771 Howard, J. B. in ‘Chemistry of Coal Utilization, Second Sup plementary Voiume’(Ed. M. A. Elliott), Wiley, New York, 1981, Ch. 12 Suuberg, E. M., Peters, E. A. and Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37 James, R. K. and Mills, A. F. Lett. Heat Mass Transfer 1976,3, 1 Botterill, J. S. M., Teoman, Y. and Yiiregir, K. R. AlChE Symp. Ser. 1981, 77, 330 Badzioch, S., Gregory, D. R. and Field, M. A. Fuel 1964,43,267 Gelperin, N. I. and Einstein, V. G. in ‘Fluidization’ (Eds. J. F. Davidson and D. Harrison), Academic Press, London, 1977, p. 519
Volatiles in large coal particles: J. F. Stubington and Sumaryono 16
17
Wang, H. Y. ‘Heat Transfer for Engineers’, Longman, New York, 1977, p. 36 Sumaryono. MSc. Thesis, University of New South Wales, 1983
APPENDIX A few measurements were made of the pore structure in the char particles (6-10 mm diameter) recovered from the fluidized bed experiments at 950°C. Each particle was broken into a few fragments and stored in a desiccator to remove moisture. A char weight of 0.5-1.5 g was analysed by mercury porosimetry. The results are presented in Table 5. Insufficient data are available to ensure that these figures are representative, but the large differences between Greta and the other chars are certainly significant. The pore size distribution for Greta char shows that the pores are generally much larger than for Bulli and Great Northern chars. The total pore volume for Greta char is also significantly larger, and the measured pore volumes increase with increasing volatile matter of the coal. Indeed, the results in Table 5 suggest a direct relation between the pore volume (expressed as a percentage of the initial char volume) and the weight loss (expressed as a percentage of the initial coal weight). The percentage of volume occupied by pores is greater than the percentage
weight loss by 5-8 wt%, which is of similar magnitude to reported values lo of coal porosity ranging from 8 to 20%. Such a relationship would be affected by swelling of the coal, but the effect is minor for the low-swelling coals studied here. Table 5 Pore
structure of chars Great
Parent coal
Bulli
Northern
15 26 47 52 58 80
4 10 34 58 76 90
Greta
Pore volume distribution (56) >177 pm >50 pm >lO pm >l pm >O.l pm >O.Olpm
12 48 71 79 87 94
11 48 69 79 86 93
Total pore volume km3 g-1) (% of initial char volume)
28
33
57
60
Weight loss (% of initial coal weight)
20
27
52
52
0.22
0.30
0.68
FUEL, 1984, Vol 63, July
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1019
F. Stubington
and Sumaryono
Department of Fuel Technology, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, PO Box 1, Kensington, NSW 2033, Australia (Received 27 June 1983)
Coal particles with diameters of 3-11 mm were injected into a small, hot bed of sand fluidized by nitrogen. Volatiles evolution was followed by sampling the exit gas stream and subsequent analysis by gas chromatography. Three Australian coals covering a range of volatile matter were studied and the effects of coal particle size and bed temperature were determined. The yields of gaseous components, char and tar are explained by consideration of the competitive reactions for coal hydrogen and oxygen and secondary reactions of the volatile species within the coal particle. The pore structure developed during devolatilization has a significant effect on the extent of these secondary reactions. It is concluded that heat transfer is the main process controlling the volatilization time in fluidized bed combustors. The time required for heat transfer into the coal particle, determined by calculation and experiment, agrees with the measured volatilization time. Significant factors are external heat transfer to the surface of the particle, internal conduction through the coal substance and radiation through the pores, and the counterflow of volatiles out of the coal particle. For different coals, variations in the volatilization time appear to be caused by the development of different pore structures, which affect radiant heat transfer through the pores. (Keywords:
coal; coal particle;
fluidizad
bad combustion;
In recent years, there has been a growing awareness of the importance of volatiles combustion in the fluidized bed combustion of coal’. Particularly for coals of high volatile matter, a significant proportion (up to ~50%) of the specific energy of the coal is released by combustion of the volatiles. Hence, a study of the processes leading to volatiles combustion is of vital significance to an understanding of fluidized bed combustion. The processes to be considered are :
volatilization)
EXPERIMENTAL The experiments were performed in a small stainless steel reactor (35 mm diameter x 200 mm high), shown in Figure 1. This contained an 80mm deep bed of sand which had been sieved to a narrow size range (SW-599 pm) and was supported by a perforated stainless steel distributor. The bed was fluidized at 1.5 times the measured minimum fluidization velocity by nitrogen preheated in a coil surrounding the reactor. The reactor was radiantly heated
(1)coal particle mixing on injection into the bed; (2)volatiles release from the coal particles; and (3) gas-phase mixing and combustion with oxygen. In this Paper, initial results from a study of the second process, i.e. the release of volatiles from coal particles in a hot fluidized bed, are reported. Relatively large coal particles (Z 1-15 mm in diameter) are burnt in fluidized bed combustors, so the extensive literature on volatilization of p.f.-sized coal particles is not relevant to this study. A limited number of studies of the volatilization of larger coal particles have been made’ - ‘, but the reported data on devolatilization behaviour vary widely. This may be attributed to differences in the experimental equipment and in the measurement techniques used. The mechanism controlling the volatilization of large particles of coal is still unclear; heat transfer4, kinetics’, mass transfer9 and evaporation3 have been suggested by various workers. In addition, the effect of coal properties on volatilization has not been resolved. Thus, a systematic study of large coal particle volatilization is needed to provide a better understanding of the process and its controlling parameters as well as fundamental data for modelling coal volatiles combustion in a fluidized bed combustor. 00162361/84/071013~7%3.00 @ 1984 Buttenvorth & Co. (Publishers) Ltd
Figure 1 Pyrolysis reactor: A, N2 fluidizing gas inlet; B, valve: C, delivery pipe for coal particles (12.5 mm i.d.); D. chromelalumel thermocouple connected to temperature controller; E, reactor; F, preheater tube (6.2 mm o.d.); G, sand bed; H, perforated plate distributor (0.5 mm openings); I, joint; J, first tar trap, immersed in dry ice; K, stainless steel perforated plate; L, second tar trap; M, outlet to gas sampling manifold; 0, glass wool; P, cotton wool; 0, anhydrone
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1984, Vol 63, July
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Volatiles in large coal particles: J. F. Stubington and Sumaryono
in a furnace constructed from cast refractory and internally wound with resistance heating wire. The bed temperature was measured by a chromel-alumel thermocouple inserted through the lid of the reactor and was controlled at 750, 850 or 950°C. The coal particles were shaped into a nearly spherical form, to eliminate the uncertainties associated with differences in shape between particles, For each experiment, either one or two particles (depending on size) were dropped through the ball valve into the hot fluidized bed. The volatiles were swept from the reactor by the fluidizing gas and the condensibles were collected in two tar traps. Samples of the product gas were taken in syringes from the manifold at various times after injection of the coal particle and were subsequently analysed by gas chromatography for hydrogen, methane, ethane, ethylene, propane + propylene, carbon monoxide and carbon dioxide. The remaining char was cooled in a flow of nitrogen, recovered from the bed and weighed. Since the accuracy of the determination of the gain in weight of the tar traps was poor, the yield of tar plus pyrolysis water was calculated as the difference between the original dry coal weight and the sum of the gas and char yields. Three Australian coals, covering a range of volatile matter, were chosen for the experiments. Their analyses are given in Table I. For each coal and temperature, three to six separate experiments were performed with particles ranging in diameter from 3 to 11 mm. Comparison of the diameter of the recovered char particle with the diameter of the original coal particle showed that only a slight amount of swelling (< 15 ~01%) occurred.
Time
40
RESULTS AND DISCUSSION
60 Time fs)
80
loo
Figure 3 Hydrogen and methane evolution from Great Northern coal at 950°C. 0, CH,; 0, H,
Gaseous evolution Figure 2 shows a typical gaseous evolution curve for Bulli coal at 850°C in which the main components are methane and carbon oxides. In some experiments, the carbon oxides were observed to evolve over a long period at low concentrations. This tailing is attributed to the slow release of oxygen, some of which remains in the residual char even after volatilization to 1000”C’“~ll. Hydrogen was determined separately from the other components, and Figure 3 compares the hydrogen and methane release from Great Northern coal at 950°C. In contrast to the
Table 1 Analyses of coal samples Great Bulli
Northern
Greta
Proximate analysis (wt% air-dried basis) : Volatile matter Ash Fixed carbon Moisture
19.4 11.9 67.3 1.4
28.4 18.6 50.0 3.0
43.5 5.4 48.9 2.2
Ultimate analysis (wt% air-dried basis): Carbon Hydrogen Nitrogen Sulphur Oxygen (by difference)
75.3 3.7 1.4 0.5 5.8
63.0 3.6 1.3 0.6 9.9
72.0 5.0 1.5 1.1 12.8
Crucible swelling no. fB.6 1016: Part 12)
1
1014
(s)
Gas evolution from Bulli coal at 85o’C (particle diameter=7.5 mm). 0, CH,; Cl, CO,; n , C,H,; A. C,H,; 0, CO; n, C,‘s Figure 2
FUEL, 1984, Vol 63, July
0.5-l
2
single peak of methane, hydrogen evolved in two peaks, and this was particularly noticeable at high temperatures (850 and 950°C) and for large particles (> 6 mm). The yield of each component gas was obtained by integrating the evolution curves (e.g. as shown in Figures 2 and 3) and the total gas yield was calculated as the sum of the component yields. The measured total gas yields and char yields are given in Tables 2,3 and 4 for Bulli, Great Northern and Greta coals respectively. From the results a number of general trends have been identified. As the temperature increases, the char yield decreases; the total gas yield, CO + CO,, methane and ethylene increase; the tar + pyrolysis water and propane + propylene peak at ~850°C; and ethane is either constant or peaks at z 850°C. The effects of particle size on yields are not as clear as those of temperature, but the following trends can be observed as the size increases: the char yield is constant or increases at 750 and 850°C and increases at 950°C; the total gas and CO + CO, decrease at 850 and 950°C; and the tar +pyrolysis water is constant or decreases. The results may be explained by consideration of two processes: (1)competitive reactions for the hydrogen and oxygen contents of the coal; and (2) secondary reactions of reactive volatile species within the coal particle. The primary volatilization process starts at about 300400°C and continues to temperatures above 1OOOC.at
Volatiles in large coal particles: J. F. Stubington and Sumaryono Table 2 Volatilization
yields from Bulli coal Yields (wt% of air-dried coal)
Pyrolysis temperature (“C)
Particle diameter (mm)
Char
Total gases
CO+ CO2
Tar + pyrolysis H2O
750
4.0 6.8 8.3
87.3 83.9 85.7
3.3 3.3 3.3
1.1 0.5 9.4
8.1 11.6 9.7
850
3.1 4.3
80.2 -
5.3 6.7
2.4 2.6
13.2 _
4.3 6.1 7.6 9.0
81.9 82.5 81.6
5.0 7.3 5.5 -
1.6 3.7 2.3 _
11.7 10.6 -
11.0
81.6
6.0
2.3
11.0
2.8 4.6 4.6 6.0 7.7 10.0
73.2 75.8 80.4 80.8 80.4 82.9
9.8 9.8 8.3 8.0 8.8 5.7
4.9 5.2 4.3 3.8 3.6 2.6
15.5 13.0 10.0 9.9 9.5 10.1
950
Table 3 Volatilization
yields from Great Northern coal Yields (wt% of air-dried coal)
Pyrolysis temperature W)
Particle diameter (mm)
Char
Total gases
co+ CO2
Tar + pyrolysis H2O
750
3.5 6.0 7.0 8.2 9.6
72.3 71.1 76.5 67.6 77.3
4.3 5.0 5.6 7.0 6.4
1.6 1.9 2.5 2.5 3.5
20.4 20.9 14.9 22.4 13.3
850
3.4 3.5 5.3 5.3 5.3 8.1 10.0
_ 49.7 69.7 67.8 71.9 75.2
15.2 14.0 13.6 14.9 8.7 9.2 -
8.1 7.1 7.2 8.2 4.2 4.3 -
33.3 12.3 20.5 14.9 -
950
2.7 2.7 4.2 7.8 11.0
54.2 70.8 73.6 73.4
15.6 17.4 12.3 9.7 7.5
9.2 10.6 7.8 6.0 4.6
25.4 13.9 13.6 16.1
Table 4 Volatilization yields from Greta coal Yields (wt% of air-dried coal) Pyrolysis temperature (W
Particle diameter (mm)
Char
Total gases
750
3.9 5.3 5.3 6.4 7.8
58.6 58.1 57.7 58.9
10.2 9.5 10.6 9.7 9.9
2.8 2.8 4.8 2.1 2.6
29.0 30.2 29.5 29.0
850
3.3 6.1 8.2 9.0
46.2 49.9 50.9 53.3
23.9 17.9 15.0 -
6.4 4.3 4.1 -
28.6 29.9 31.9 -
950
3.7 5.3 6.3 8.6
39.6 45.8 47.0 49.7
35.8 25.9 27.8 25.0
20.5 16.6 11.1 10.2
22.4 26.0 23.0 23.1
co + CO2
Tar + pyrolysis H2O
rapid heating rates”. As part of this process, the hydrogen in the coal may combine either with oxygen from the coal to produce pyrolysis water or with carbon to produce hydrocarbons. In the latter case, the coal oxygen becomes available to combine with carbon and produce carbon monoxide or carbon dioxide, thus decreasing the weight of remaining char. Tables 24 show such an increasing yield of carbon oxides and an associated decreasing char yield as the temperature increases and as the particle size decreases, i.e. as the heating rate increases. Hence, the formation of carbon oxides is favoured at high temperatures and high heating rates, whereas water formation is favoured at low temperatures and low heating rates. In this respect, the present results for bituminous coals are similar to those of Suuberg et al.” for lignite pyrolysis. The primary volatilization products travel through the pores of the coal particle to its surface, where they are released into the surrounding gas stream. During this mass transfer through the particle, the more reactive volatile species can undergo secondary reactions (e.g. cracking, condensation, polymerization). As a result of these secondary reactions, larger hydrocarbon molecules (e.g. tars) are cracked to form smaller hydrocarbon molecules, hydrogen is produced and some carbon from the volatile components is deposited on the walls of the pores, thus increasing the char yield. The extent of these secondary reactions is determined by the temperature and the residence time of the volatiles within the coal pores. This residence time increases with increasing particle size and decreases with increasing pore size: in larger pores, mass transfer out of the coal particle is faster. As the temperature increases, the increasing extent of secondary reactions (particularly cracking reactions) causes the yield of higher hydrocarbons (e.g. tar, propane + propylene) to peak and the yields of methane and ethylene to increase. As the particle size increases, the increased volatiles residence time within the coal particle allows secondary reactions to occur to a greater extent, leading to higher char yields at the expense of gas and tar yields. The effect of pore size on the extent of secondary reaction may be seen by comparing the char yields (Tables 24) with the proximate analysis char yields (ash +tixed carbon), which are 79.2,68.6 and 54.3 wt% (air-dried basis) for Bulli, Great Northern and Greta coals, respectively. For Bulli and Great Northern coals. the char yields from the fluidized bed nearly always exceeded the proximate yield, whereas the char yield from Greta coal was nearly always less than the proximate yield. The extent of secondary reaction (and carbon deposition) within the Greta coal particles was much smaller because of the formation of larger-diameter pores during volatilization. This effect was verified by measuring the pore size distributions of the recovered char particles (see appendix for details). For Greta char, 50% of the pore volume occurred in pores with diameters > 48 pm, whereas the corresponding diameters were 9 pm for Bulli char and 5 pm for Great Northern char. Although further experiments are necessary for verification, it is suggested that the differences in pore sizes may be attributable to differences in the plastic behaviour of the original coals. When tested in the Gieseler plastometer, Bulli and Great Northern coals exhibited negligible plasticity with maximum creep rates of 0.2 and 0.4 divisions per minute, whereas the Greta coal had a maximum fluidity of 585 divisions per minute, at 412°C. The internal pressure arising from volatiles release within the Greta coal particle was
FUEL, 1984, Vol 63, July
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Volatiles in large coal particles: J. F. Stubington and Sumaryono
probably sufficient to force the gas out through the plastic coal, generating larger pores. The evolution of hydrogen as two peaks may also be explained by the processes above. The first peak is due to the primary decomposition reactions, particularly from hydrogen formation at high temperatures close to the particle surface. In addition, the decomposition of reactive volatile species (tars and other unstable hydrocarbons) produces some hydrogen. These secondary reactions peak later because the temperature inside the particle increases relatively slowly, so thecombination of residence time and temperature required for secondary reaction occurs after much of the primary volatilization is completed. Hence, the second peak is more obvious for larger coal particles, at higher temperatures and for Bulli and Great Northern coals-exactly the conditions which enhance secondary reactions.
120
IA
w x -
80
I-
40
Volatilization time
The time taken for volatilization in a fluidized bed combustor is very significant, inasmuch as it determines the position at which the volatiles are released into the bed. In particular, the volatilization time must be compared with the particle mixing time in order to decide on the most appropriate of the various models suggested for volatiles combustion’. Most previous workers have inferred volatilization times from the occurrence of a volatiles diffusion flame around individual particles2~’ - ‘, but this method must underestimate the total volatilization time, since volatilization may still occur after flame extinction. Instead, volatilization times have been measured in the present work from the gas evolution concentration proliles4. Visual observation of the tar fog passing from the reactor to the tar traps indicated that tar evolution finished slightly earlier than gas evolution, so the volatilization time was assessed from the latter. In this work, the volatilization time was defined as the time corresponding to evolution of 95 wt% of the total gases. This was determined from the experimental results, the tailing of hydrogen and carbon oxides at very low concentrations over long periods being excluded. The correction for the time of travel from the bed to the sampling point, obtained by CO2 tracer gas injection into the bed, was found to be about 3 s. Since the spread of travel times was small compared with the measured volatilization times, no correction was made for the dispersion of gases during their travel from the fluidized bed to the sampling manifold. The measured volatilization times are shown in Figures 46 as a function of coal particle diameter at bed temperatures of 750, 850 and 950°C. The volatilization times for Greta coal are shorter than those for Bulli and Great Northern coals. This may be attributed to differences in the pore structure developed within the char particles, by one or both of the following arguments. First, the larger-diameter pores (see Appendix) in Greta char allow more rapid mass transfer of the volatiles through the particle to its surface. Second, the greater pore volume (see Appendix) in Greta char may enhance radiative heat transfer through the pores into the particle. This more rapid heating would be significant if heat transfer into the particle limited the volatilization rate. Both of these arguments are consistent with the explanations given above, under ‘Gaseous evolution’, since both lead to a
1016
FUEL,
1984, Vol 63, July
1,
0
COAL PARTICLE DIAMETER (mm
1
Figure 4 Volatilization and heat transfer times at 75o’C. 0, Bulli; 0, Great Northern; n , Greta; -, time for particle centre to reach 740°C; - - - - -, results of Pilla? at 775°C
COAL PARTICLE DIAMETER [mm) Figure 5 Volatilization and heat transfer times at 850°C. 0, Bulli; 0, Great Northern; m, Greta; -, time for particle centre to reach 84OC; - - - - -, results of Pilla? at 875°C; ---, results of Ragland and Weiss’ at 815°C
shorter volatiles residence time within the Greta particles and hence less secondary reaction. With increasing temperature, the volatilization time for all three coals decreased slightly, as observed by previous workers6s7. A possible explanation for this effect may again be the enhanced radiative heat transfer through
Volatiles in large coal particles:
a
4 COAL
PARTICLE
and Sumaryono
coal particle surface and subsequent heat transfer into the coal particle, both by conduction through the coal substance and by radiation through pores and cracks. Since the coal particles were large relative to the bed particles, the rate of heat transfer to the coal particle surface should be the same as the rate of heat transfer to an immersed surface. Botterill et a1.l3 have given data for heat transfer to an immersed surface as a function of fluidizing velocity, particle size and temperature. From these data, heat transfer coefficients h ranging from 460Wm-*K-‘at750“Cto500Wm-*K-‘at950”C were deduced for the conditions used. Badzioch et al.‘* measured the effective thermal conductivity and specific heat of a range of coals up to 900°C. They found that the thermal conductivity k was approximately constant up to z 400°C and increased rapidly at higher temperatures, predominantly as a result of radiant heat transfer across pores and cracks in the carbonized coal; the volumetric specific heat (PC,) was approximately constant up to z 350°C and decreased at higher temperatures. The following expressions were fitted to their mean results for use in the present heat transfer calculations:
0
0; 0
J. F. Stubington
12
DIAMETER (mm1
Figure 6 Volatilization and heat transfer times at 95o’C. l , Bulli; 0, Great Northern; n , Greta; -, time for particle centre to reach 940°C
k=
0.23 W m-l K-’ for T<4OO”C 0.23+2.24x 10-5(7’-400)1~8 W m-l K-’ i for T>4OO”C 1.92x 106 J mm3 K-’
PC,=
pores at the higher temperatures (since radiation varies as T*). According to James and Mills”, diffusion out through the coal particle is not fast enough to explain the short devolatilization times measured for particles lO& 1000~ in diameter. They claim that a pressure-driven flow of volatiles out of the particle must occur, so the effect of temperature may not be explainable by an increase in the diffusion coefficient. Pillai’s results6 are shown in Figures 4 and 5 for Glen Brook coal with a proximate analysis close to that of the Great Nothem coal used here but with a crucible swelling number of 7. The other coals which he tested had shorter volatilization times, except for Pittsburgh No. 8. The results of Ragland and Weiss’ are also shown in Figure 5. The volatilization time was measured by flame extinction in both of these studies, so the shorter times noted are to be expected, since volatiles release continues even though the flame can no longer be supported. For very small particles, the rate of pyrolysis is controlled by chemical kinetics, so there is no effect of particle size on pyrolysis rate”. As the particle size increases, a critical size is reached at which heat and/or mass transfer becomes limiting, and this size appears to be < 2 mm for heating rates < 100 K s-l. Hence, under the present experimental conditions, heat and/or mass transfer is expected to control the volatilization time. A squarelaw relationship (t,ad*) between volatilization time t, and particle diameter d has been proposed for both of these processes, so the effect of particle size on volatilization time cannot be used to distinguish between heat and mass transfer. Heat transfer
To assess its importance in coal volatilization, the rate of heat transfer into the coal particle has been determined by calculation and by experiment. The processes considered were heat transfer from the fluidized bed to the
i 1.92 x 106-2.92x
for T6350”C
103(T-350) J me3 K-’ for T>350”C
(1)
(2)
The relative importance of external and internal heat transfer was deduced by calculating the Biot number (h/k, where I is the particle radius) using the constant values for the coal properties. Values of 0.3-2.8 were obtained and indicated that both external and internal heat transfer must be considered in the heat transfer calculations’5. Two types of heat transfer calculation were then made, first assuming constant values for the thermal properties of the coal (i.e. the low-temperature values) and then assuming variable properties (as given in Equations (1) and (2)). For the constant-property calculation, an analytical so1ution’6 was used, but a numerical solution’ ’ was necessary for the variable-property calculation. Figure 7 shows the temperature-time profile at the centre of a coal particle, calculated by each of these methods. As expected from the variation in thermal properties with temperature, the variable-property solution gives a significantly shorter time for the centre of the particle to reach bed temperature. Some experiments were performed to measure the temperature at the centre of a coal particle as a function of time. An ultrafine thermocouple (0.5 mm in diameter) was inserted into a coal particle, which was then placed in the hot fluidized bed. The temperature of the thermocouple was recorded and is reproduced in Figure 7 for comparison with the calculated profiles. It can be seen that the constant-property calculation gives a better approximation to the measured profile than does the variableproperty calculation. The maximum deviation between experiment and variable-property calculation occurs at about 45s roughly corresponding to the time for the maximum rate of release of volatiles from the coal particle. The difference between the results may be explained if mass transfer of volatiles out of the particle inhibits the
FUEL, 1984,
Vol 63, July
1017
Volatiles in large coal particles: J. F. Stubington and Sumaryono
800
2
600
F Z
O
0
80
40 TIME
120
is.1
Temperature rise at the centre of a Bulli coal particle at external temperature 833°C (particle diameter 8.0 mm). -, Calculation with variable coal properties; - - - -, calculation with constant coal properties; ---, experiment Figure 7
flow of heat into the particle, causing a slower rate of increase in the temperature at the particle centre than is predicted by the variable-property calculation. Hence the increased heat transfer rate due to the change in coal properties with temperature is offset by the inhibiting effect of volatiles flowing out of the particle, with the net result that the measured particle temperature is better approximated by the constant-property calculation. As a measure of the time required for heat transfer, the time for the particle centre to attain a temperature 10°C below bed temperature (since the curve is asymptotic to bed temperature) was arbitrarily chosen. The results of these calculations are plotted as solid lines on Figure 46 for comparison with volatilization times. To calculate these times, use was made of the analytical solution for heat transfer into a sphereI (including both external and internal resistances) with constant values of thermal coal properties, since this method gave the best agreement with measured temperature-time profiles. Comparison of volatilization and heat transfer times From Figures 4-6 it is clear that the 95% volatilization time is very close to the time needed for heat transfer into the centre of the particle. It is therefore concluded that the volatilization time is controlled mainly by the process of heat transfer to the centre of the coal particle. (It has already been shown that both external and internal heat transfer must be included in calculating this heat transfer rate.) This is in agreement with the conclusion of James
1018
FUEL, 1984, Vol 63, July
and Mills”, who suggested that the effect of particle size is mainly due to the slower heating rate rather than the larger residence time and surface area. This conclusion is also consistent with the other minor effects already noted under ‘Volatilization time’. The heat transfer calculations included external heat transfer to the particle, internal conduction within the particle and some contribution from radiation through pores (expressed as effective thermal conductivity by Badzioch et a1.14).In the present rapid-heating experiments, radiation through the pores would cause a greater rate of heat transfer at high temperatures than is predicted from the effective thermal conductivities measured under slow heating rates by Badzioch et al.14, owing to the larger temperature difference across the pores at the higher heating rate. Thus, the volatilization times tend to be less than the calculated heat transfer times at 950°C (Figure 6). The shorter volatilization time for Greta coal may also be a result of enhanced radiative heat transfer through the pores, which are both larger in diameter and occupy a greater fraction of the particle volume in Greta char than in Bulli and Great Northern chars. While heat transfer to and into the coal particle is the major process determining the volatilization time, the rate of this heat transfer is affected by the pore structure developed for each particular coal. The total pore volume within the char appears to increase with the volatile matter of the coal and the pore diameter appears to increase with the fluidity of the coal (see Appendix). The present results suggest that the variations in volatilization time from one coal to another are attributable to differences in the pore structure of the char and that these differences in pore structure may be correlated with the volatile matter and plasticity of the parent coal. Further experiments with a range of coal types are needed to test these suggestions. ACKNOWLEDGEMENTS The authors wish to acknowledge the financial support of the Australian Research Grants Scheme for this work, and Sumaryono wishes to thank the Australian Development Assistance Bureau for their support during his period of study in Australia. REFERENCES . 2 4 6
8
9 10
11 12 13 14 15
Stubington, J. F. J. Inst. Energy 1980, 53, 191 Essenhigh, R. H. ASME Trans.--J. Engng. Power 1963,85, 183 Peters, W. and Bertling, H. Fuel 1965,44,317 Morris, J. P. and Keaims, D. L. Fuel 1979,58,465 Jung, K. and Stanmore, B. R. FueZ 1980,59,74 Pillai, K. K. J. Inst. Energy 1981, 54, 142 Ragland, K. W. and Weiss, C. A. Energy 1979,4,341 Bywater, R. J. in ‘Proceedings of the Sixth International Coal Fluid&d Combustion Conference, Atlanta’, 1980, Vol. III, p. 1092 LaNauze, R. D. Fuel 1982,61,771 Howard, J. B. in ‘Chemistry of Coal Utilization, Second Sup plementary Voiume’(Ed. M. A. Elliott), Wiley, New York, 1981, Ch. 12 Suuberg, E. M., Peters, E. A. and Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37 James, R. K. and Mills, A. F. Lett. Heat Mass Transfer 1976,3, 1 Botterill, J. S. M., Teoman, Y. and Yiiregir, K. R. AlChE Symp. Ser. 1981, 77, 330 Badzioch, S., Gregory, D. R. and Field, M. A. Fuel 1964,43,267 Gelperin, N. I. and Einstein, V. G. in ‘Fluidization’ (Eds. J. F. Davidson and D. Harrison), Academic Press, London, 1977, p. 519
Volatiles in large coal particles: J. F. Stubington and Sumaryono 16
17
Wang, H. Y. ‘Heat Transfer for Engineers’, Longman, New York, 1977, p. 36 Sumaryono. MSc. Thesis, University of New South Wales, 1983
APPENDIX A few measurements were made of the pore structure in the char particles (6-10 mm diameter) recovered from the fluidized bed experiments at 950°C. Each particle was broken into a few fragments and stored in a desiccator to remove moisture. A char weight of 0.5-1.5 g was analysed by mercury porosimetry. The results are presented in Table 5. Insufficient data are available to ensure that these figures are representative, but the large differences between Greta and the other chars are certainly significant. The pore size distribution for Greta char shows that the pores are generally much larger than for Bulli and Great Northern chars. The total pore volume for Greta char is also significantly larger, and the measured pore volumes increase with increasing volatile matter of the coal. Indeed, the results in Table 5 suggest a direct relation between the pore volume (expressed as a percentage of the initial char volume) and the weight loss (expressed as a percentage of the initial coal weight). The percentage of volume occupied by pores is greater than the percentage
weight loss by 5-8 wt%, which is of similar magnitude to reported values lo of coal porosity ranging from 8 to 20%. Such a relationship would be affected by swelling of the coal, but the effect is minor for the low-swelling coals studied here. Table 5 Pore
structure of chars Great
Parent coal
Bulli
Northern
15 26 47 52 58 80
4 10 34 58 76 90
Greta
Pore volume distribution (56) >177 pm >50 pm >lO pm >l pm >O.l pm >O.Olpm
12 48 71 79 87 94
11 48 69 79 86 93
Total pore volume km3 g-1) (% of initial char volume)
28
33
57
60
Weight loss (% of initial coal weight)
20
27
52
52
0.22
0.30
0.68
FUEL, 1984, Vol 63, July
0.72
1019