Energy & Fuels 1987,1, 263-269
263
Pyrolysis and Combustion of Bituminous Coal Fractions in an Entrained-Flow Reactor Ching-Yi Tsai and Alan W. Scaroni* Combustion Laboratory, Fuel Science Program, The Pennsylvania State University, University Park, Pennsylvania 16802 Received August 29, 1986. Revised Manuscript Received December 29, 1986 The effects of particle size and maceral composition on the weight loss rate of coal particles during the ignition stage of a pulverized-coal flame in an entrained-flow reactor were investigated. The role of particle size in combustion was found to be coupled to associated changes in chemical composition and thermal history. In addition, the variation in particle size led to a difference in the radial dispersion of the coal particles in the reactor and consequently to a variation in the local air/fuel ratio. The role of maceral composition in combustion varied with the furnace temperature and particle size. A high concentration of liptinites significantly enhanced the ignitability and weight loss rate under fuel-rich conditions. The presence of inertinites showed a greater influence on delaying ignition at low temperatures. The data suggest that large particle size, high inerts concentration and low furnace temperature enhance the contribution of heterogeneous combustion to the weight loss observed during the ignition stage in a pulverized coal flame. 1. Introduction It is well-known that coal is heterogeneous in character and that combustion processes are complex in nature. The factors influencing the combustion behavior of pulverized-coal particles are numerous and include (1)particle size, chemical composition, mineral matter, and moisture contents, (2) the temperature and velocity of the combustion air, and (3) the fuel/air ratio and the mixing of the fuel with the oxidant. The control and design of pulverized-coal combustors to achieve an optimum combination of these factors is often less than an exact science. Successful operation of coal combustors is usually based on long-term operating experience. Recently, with the increasing demand for coal for pulverized-fuel combustors, reserves of coal that may be extremely different in chemical composition from the coal for which the plant was designed are being considered for use. Also, with the increasing stringency of antipollution legislation, the blending of coals with various sulfur contents is one way to comply with the pollution regulations. Information on the effect of variation in the organic constituents on the combustion performance, therefore, is important for attaining better control of combustors. The role that the organic constituents of the coal play in pulverized-coal combustion has been investigated by several researchers.’+ Finney and Spicer’ found that the ignitability of coal particles generally decreased with decreasing volatile matter content. Erasmus2 reported that, for coals of the same rank, the ignition temperature decreased as the tar yield increased. He also found that the removal of higher specific gravity fractions from the raw coal resulted in a lowering of the ignition temperature. Similar results were reported by Singer3 who found that deashed coal was more ignitable than coals containing some “ash”. Shiba0ka,~7~ on the basis of hot-stage mi(1) Finney, C. S.;Spicer, T. S. Pyrodynamics 1964, l , 231-249. (2) Erasmus, T. C. “The Ignition Properties of South African Pulverized Coal”; technical report, 1979; The Fuel Research Institute of South Africa. (3) Singer, J. M. Fuel 1968, 47, 223-234. (4) Shibaoka, J. J . Zmt. Fuel 1969, 42, 59-66. (5) Shibaoka, M. Fuel 1969,48, 285-295. (6) Street, P. M.; Weight, R. P.; Lightman, P. Fuel 1969 48,343-365. (7) Nandi, B. N.; Brown, T. D.; Lee, G. K. Fuel 1977, 56, 125-130. (8) Lee, G. K.; Whaley, H. J . Imt. Energy 1983, 56, 190-197. (9) Sanyal, A. J. Inst. Energy 1983,56, 92-95.
0887-0624/87/2501-0263$01.50/0
croscopy work, found that sporinite and cutinite took longer to become softened and form vesicles but burned more quickly than vitrinites. Street et a1.6 concluded that vitrains tended to form cenospheres thinner walled than those formed in durains. Also, vitrains gave chars having more internal structure as the rank decreased. Nandi et al.’ and Lee and Whaley8 recently reported that combustion efficiencies were inversely related to the inerts content of various coals in a pilot-scale pulverized-coal combustor. Sanyalgalso reported that inert-rich coals took longer for combustion to complete. These results indicate that there is a strong relationship between the organic constituents and the ignitability, structural change, and burnout rate of pulverized-coal particles. However, since combustion of pulverized coal is coupled to mass- and heat-transfer processes, both physical and chemical processes are important in controlling the combustion behavior of coal particles. The effect of chemical composition on combustion behavior, therefore, varies when the combustion conditions or the physical properties of the coal change. This study was directed toward elucidating the pyrolysis and combustion behavior of pulverized-coal particles in an entrained-flow reactor. The primary goal was to examine the role of particle size and maceral composition during the initial stages of pulverized-coal combustion and to identify the various physical transport processes that are coupled to the changes in particle size and maceral composition. 2. Experimental Section 2.1. Sample Preparation. In this study coal samples with various particle sizes and with various chemical constituents were obtained by a fractional separation of two coals. The fractionation involved dry sieving and specific gravity separation. The gravity separation, a sink and float procedure, was conducted in a centrifuge by using ZnClz solutions with specific gravities of 1.25, 1.35,and 1.55 as the suspension media. Following centrifugation, the samples were boiled in, and washed with, distilled water to remove the chloride ions. Table I shows the petrographic, ultimate, and proximate analyses of the samples. The liptinite concentration was determined by blue light microscopy,under which the liptinite macerals become fluorescent and both vitrinite and inertinite are black in color. The vitrinite and inertinite concentrations were calculated from their relative ratios by conventional white light microscopy. The data indicate that the liptinite concentration was the highest 0 1987 American Chemical Society
Tsai and Scaroni
264 Energy & Fuels, Vol. 1, No. 3, 1987
Table I. Analyses of Coal Samples PSOC-363 (hvA Bituminous) 100 X 200 mesh A4 A2 A3
200 X 270 mesh B2 B4
270
1.35 1.25
1.55 1.35
1.35 1.25
1.25
1.55
1.55
1.35 1.25
1.6 4.3 46.8
1.8 3.1 42.9
1.6 40.0
0.6 52.3 41.2
2.6 2.9 40.0
2.0 45.7 38.7
2.3 5.6 42.2
2.5 3.1 40.4
81.0 6.0 1.8 0.9 0.89
81.6 5.7 1.9 0.8 0.84
81.7 5.6 1.7 0.7 0.82
78.7 5.2 1.7 1.3 0.79
81.1 4.8 2.1 0.8 0.71
84.2 4.8 2.0
81.4 5.2 1.9 0.8 0.77
81.3 5.2 1.9 0.8 0.77
18 62 20
11
71 18
14 53 33
16 66 18
7 80 13
A1 SP gr flaat sink proximate anal., wt % moisture ash (dry) VM" (daf) ultimate anal., wt % (daf) C H N S H/C atomic ratio maceral comp, vol % liptinite vitrinite inertinite Darticle size. mesh proximate anal. wt % moisture ash (dry) VM" (daf)
1.25
11.2
1.1
0.68
9 74 17
PSOC-122 (hvA Bituminous) (-1.25 sp . gr) _ 50 X 100 100 X 140 140 X 200 0.8 8.6 53.2
0.9 7.5 50.1
200
1.0 6.8 49.8
X
X
c1
270
240 mesh c2
270
1.0 6.3 46.4
X
400
1.0 6.2 44.4
"VM = volatile matter. in the lightest fraction, whereas the inertinite concentration was the highest in the 1.35-1.55 gravity cuts. The +1.55 gravity fractions had the highest mineral matter content as indicated by the proximate ash analyses. The data also indicate that the liptinite-rich fractions had higher volatile matter and hydrogen contents than the other fractions. Furthermore, the large particle sizes generally had higher volatile matter contents than smaller particles. 2.2. Apparatus. The pyrolysig and combustion of the coals were performed in a n entrained-flow reactor operated so as to produce a rapid heating rate in a dilute-phase condition. Details have been provided elsewhere.loJ1 The reactor consista essentially of a vertical tube furnace heated electrically and designed to inject a particle-laden primary gas stream at room temperature into the center of a preheated secondary gas stream. In the current study, helium was used as the primary gas and N2and air were used as the secondary gas to study pyrolysis and combustion, respectively. The diameter of the primary gas stream (0.3 cm) is small compared to that of the secondary gas stream (5.1 cm). The volumetric flow rate of each stream was set to give a gas velocity of 125 cm/s at the tip of the injector probe. A coal loading of 0.01 g/s was used to ensure that combustion occurred in a dilute phase with minimal interaction between particles. A water-cooled sampling probe, injected up the axis of the furnace, collected isokinetic and quenched particles that had passed vertically down the reactor. Residence time changes were made by raising and lowering the sampling probe. Char particles were removed from the gas stream in a cyclone separator. 2.3. Estimation of the Time-Temperature History of the Particles. Residence times and temperatures of the particles in the reactor were estimated from a detailed mathematical m~de1.ll-l~Residence time calculations involved predicting the (10) Maloney, D. J.; Jenkins, R. G. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 1435-1443. (11) Tsai,C. Y.; Scaroni, A. W. Twentieth Symposium (International) on Combustion, The Combustion Institute: Pittsburgh, PA, 1984; pp
1455-1462. (12) Tsai, C. Y.; Scaroni, A. W. Trans. Am. Inst. Min., Metallo., Pet. Eng., SOC.Min. Eng. AIME, in press. (13) Tsai, C. Y. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1985. (14) Morgan, M. E. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1983.
60
-I
000
A1
005
010
015
025
020
Residence Time,
030
035
s
Figure 1. Effect of maceral composition on weight loss during pyrolysis at 1200 K A l , liptinite-rich particles; A3, inertinite-rich particles. gas velocity profile in the entrance of a cylindrical tube and the particle slip velocity in the center region of the gas flow where most of the particles reside. Energy equations for cylindrical and shperical systems were solved numerically to obtain the gas and particle temperatures for given initial and boundary conditions. The temperature calculations take into account heat removed from the gas to heat the particles and the reaction heats involved in the pyrolysis and combustios. The estimated heating rate of the coal particles, under the operating conditions, is on the order of lo4 K/s, but the rate decreases with increasing particle size and coal feed rate. The total residence time of the coal particles in the reactor is 0 . 3 0 . 3 5 s, large particles having shorter residence times due to higher slip velocities between the gas and the particles.
3. Results and Discussion 3.1. Pyrolysis Behavior of Coal Particles. Figure 1 illustrates the weight loss vs. residence t i m e curves for
Pyrolysis and Combustion of Bituminous Coal
Energy &Fuels, Vol. 1, No. 3, 1987 265
w 60-l
M M
40
50
bQ
ASTM VM, %(dof)
Figure 2. Variation of asymptotic weight loss of coal particles during pyrolysis at 1200 K with ASTM volatile matter.
A1 and A3 samples for pyrolysis at 1200 K. The devolatilization of the coals follows the general trend of loss of weight slowly at first, followed by a rapid weight loss and then a slow approach to an asymptotic value. A comparison between the weight loss curves and the predicted particle temperaturel2J3 indicates that a t the positions where the pyrolysis occurs the particles are still heating up. The positions at which the weight loss curves start to level off and where the particle temperatures reach the final wall temperature are approximately the same (about 0.15-0.20 s). The results suggest that the weight loss of the coals during pyrolysis in the entrained-flow reactor o c c w under nonisothermal conditions and is consequently governed by the heat-transfer rate to the particles. The weight loss data at positions before reaching the asymptotic value, therefore, are sensitive to the heating rate and the operating conditions of the reactor. This has been supported by experimental data of a comparison between He and N2as the primary gas. The position where active pyrolysis occurs in the reactor was found to shift to a longer residence time when Nzreplaced He as the primary gas stream. A lower thermal diffusivity for Nz than He and consequently a lower heating rate of coal particles is responsible for this behavior. Figure 1 also indicates that the weight loss difference between the two fractions reaches a maximum during the stage where the coals approach their final asymptotic pyrolysis yields. This suggests that the effect of chemical composition on the weight loss rate before the asymptotic yield is reached is dominated by thermal processes. Consequently only asymptotic yields are meaningful in determining the influence of the chemical characteristics of a coal on its pyrolysis behavior. In this study asymptotic values, arbitrarily taken as the weight loss 0.3 s after injection (AWo,,), were used for the quantitative comparison of the pyrolysis yields. The pyrolysis results show that the maximum weight loss in the entrained-flow reactor has a ranking similar to that of the proximate volatile matter on a dry, ash-free basis. That is: liptinite-rich > vitrinite-rich > inertinite-rich = mineral-matter-rich particles. However, variations in A W, among the fractions are higher than for the ASTM volatile matter. Figure 2 shows the correlation between A W0,%and the ASTM volatile matter. The slope
0
IO
20
30
40
50
1
AVM, %(dol)
Figure 3. Variation of weight loss of partially pyrolyzed chars with change in ASTM volatile matter with respect to their parent coals.
of the linear regression line for the data is 1.39, which implies that weight loss differences about 40% greater than differences in the ASTM volatile matter can be expected in the entrained-flow reactor at a temperature of 1200 K. Enhanced weight loss in the flow reactor is similar to the concept of the Q factor, the slope of the linear regression line of the correlation between the weight loss of chars (AW) and the changes in the ASTM volatile matter content of the chars compared to their parent coals (AVM). It is attributed mainly to a lesser extent of secondary char formation by the primary volatiles released from the particles during pyrolysis in the flow reactor compared to that in the ASTM volatile matter test furnace. Q factors for particles with various maceral composition and for the overall samples were also determined. The limited data show that the Q factor for the pyrolysis of the vitrinite-rich fraction (1.67) is higher than those for the liptinite-rich (1.40)and inertinite-rich (1.44)fractions. This suggests that the primary volatiles generated from the vitrinites are sensitive to secondary char formation during the volatile matter test. Under the dilute phase and rapid heating conditions in the flow reactor, such secondary reactions are effectively reduced. On the other hand, both liptinites and inertinites are less sensitive to secondary char formation. For the liptinites, the primary volatiles are less susceptible to secondary reactions even under the fixed-bed conditions. For the inertinites, the primary volatiles produced either are too reactive toward secondary char formation to be effectively reduced by the rapid heating or are so inert that even slow fixed-bed pyrolysis conditions do not produce a significant amount of secondary reactions. Figure 3 presents the correlation between A W and AVM for all the pyrolysis chars generated in the current study. A detailed examination of the data reveals that the relationship can be divided into two stages. For the data at high weight loss, greater than about 40%, a low Q factor (0.99) compared to that for the data at lower weight losses (1.59)was found. This indicates that the pyrolysis behavior of bituminous coals should be mathematically modeled by at least a two-stage mechanism rather than by a single-stage scheme.
Tsai and Scaroni
266 Energy &Fuels, Vol. 1, No. 3, 1987
Size P
o 50 x 100 mesh 0 140 x 200 mesh
53.2 49.8
o 200 x 270 mesh
46.4
e 270 x 4 m mesh
44.1
-10
-
0.m 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Residence Time, s
Figure 6. Effect of temperature on weight loss of B2 fraction of PSOC-363 (200 X 270 mesh, 1.25-1.35 sp gr).
10
0 0.15
azo
0 0.25 0.30 0.35 Residence Time, s
0.40
0.45
Figure 4. Effect of particle size on weight loss of coal particles from PSOC-122 during pyrolysis at 1200 K.
The value of close to unity for the Q fador for the chars with high weight loss suggests that the volatiles released in the secondary stage of pyrolysis are not subjected to a significant extent of secondary char formation, which is consistent with the general notion that these secondary products are mainly hydrogen and carbon oxides, which are relatively insensitive to secondary reactions. The effect of particle size on the pyrolysis yield in the flow reactor was found to be mainly due to the associated differences in chemical properties. Figure 4 shows that the larger particles produced higher final volatiles yields than the smaller particles, consistent with the order of the ASTM proximate volatile matter. This suggests that particle size per se plays a less important role than the associated variation in chemical composition in determining the maximum yield of volatiles in the reactor; otherwise, the larger particles would have had lower volatiles yields because of their lower heating rate. Nevertheless, during the heat-up stage the effect of particle size on weight loss rate is significant and dominated by the heating rate, which decreases with increasing particle size. An independence of final volatiles yield on particle size has also been reported by others for both bituminous coal and lignites.16J6 3.2. Combustion Behavior of Coal Particles. When coal particles are injected into a flow reactor with an oxidizing gas, in addition to the heat-up and devolatilization encountered in the pyrolysis process, coal particles are subjected to the further effects of heterogeneous oxidation and the intensive heat treatment caused by the temperature rise resulting from the ignition. These two processes are influenced by both the chemical and physical properties of the coal particles and the nature of the surrounding atmosphere. Figure 5 illustrates the weight loss vs. residence time curves of B2 particles during combustion at various temperatures. Each curve includes three distinct stages: (1) an initial stage with a low weight loss rate; (2) a rapid jump in the weight loss rate during a second stage; (3) a third stage involving a slow final weight loss rate. Compared to the pyrolysis curves, the rapid weight loss rate in the second stage occurs at a shorter residence time and has a greater slope. Thus the three stages may correspond to (1) preignition pyrolysis, (2) ignition and volatile matter (15)Anthony, D.B.;Howard,J. B.AZChE J. 1976,22,625-656. (16)Scaroni, A. W.;Walker, P. L., Jr.; Essenhigh, R. H.Fuel 1981,60, 71-76.
combustion, and (3) heterogeneous char combustion. The preignition time appears to decrease with increasing furnace temperature. The data also indicate that the weight loss rates during the rapid burning stage are similar for different furnace temperatures. In addition, the maximum weight loss achieved at the end of the volatiles combustion stage is almost independent of the fumace temperature. In other words, both the burning rate and the extent of weight loss by the coal during the volatiles combustion stage have a low dependency on the furnace temperature. The implication is that the weight loss behavior of particles during this stage is controlled by a physical process, either mass or heat transfer or a combination of both. Since the magnitude of the weight loss at the end of the rapid-burning stage is close to that observed at the end of pyrolysis in an inert atmosphere, the rapid weight loss can be ascribed predominantly to the thermal decomposition of the coal. The weight loss rate, consequently, is a function of the time-temperature history of the particles in the pulverized-coal flame, which is dominated by the flame temperature. It is well-known that temperatures of pulverized-coal flames are controlled by a combination of many variables such as oxygen/fuel ratio, furnace and gas temperatures, coal feed rate, and type of coal. The weight loss results indicate that the magnitude of the change in the furnace temperature in the current study does not produce a significant difference in the flame temperature. The oxygen/fuel ratio in the envelope surrounding the coal particles may be a sensitive parameter in determining the burnoff behavior of the coal particles during the rapid-burning stage. Most of the coal particles reside along the centerline of the reactor forming a pencil-like coal cloud.13 The oxygen/fuel ratio at the centerline is relatively low compared to that at the outer edge of the envelope. Although the overall air/fuel stoichiometric ratio is high, about 70070,the small extent of particle dispersion in the reactor produces a fuel-rich atmosphere during the volatiles combustion stage. Consequently, any parameter influencing the air/fuel ratio dominates the observed burnoff of the coal particles at postignition positions in the reactor. 3.2.1. Effect of Particle Size. Varying the particle size is commonly used to evaluate the role of physical processes in solid-gas reactions. However, for a heterogeneous material such as coal, changes in particle size are strongly coupled to changes in the chemical composition, making an analysis of the effect of particle size on combustion behavior therefore complicated. Figure 6 illustrates the effect of particle size on the weight loss behavior of coal particles during combustion at 1200 K. Except for the 50 X 100 mesh particles, all of the size fractions completed the volatiles combustion stage
Energy & Fuels, Vol. 1, No. 3, 1987 267
Pyrolysis and Combustion of Bituminous Coal
1
50 x 100merh 100 x 140 mesh b 140 x 200 mesh o zoo x 270 mesh 270x 400 mesh
1
o
0
-/ 0.00
0.10
0 20 Residence Time, s
t
-
I
0.05 0.10
I
0.15
I
I
0.20 0.25
I
0.30
Residence Time, I
0.30
'
1 0.35
o.$o
Figure 6. Effect of particle size on weight loss of coal particles from PSOC-122 during combustion at 1200 K. within a relatively short residence time, less than 50 ms. The weight loss rate for the 50 X 100 mesh particles during the rapid-burning stage is slower than that of the smaller particles. A slower heating rate, and consequently a longer ignition delay, is believed to be responsible for this. Figure 6 also indicates that the larger particles reach higher weight losses in the postflame char combustion stage. This is consistent with the results observed in pyrolysis under an inert atmosphere shown in Figure 4. The results also confirm that the weight loss occurring during the volatiles combustion stage is mainly due to the thermal decomposition of the coal particles. However, the weight loss differences among the various size fractions appear to be much larger than those occurring during the pyrolysis process. A higher degree of radial dispersion, and consequently a higher oxygen/fuel ratio in the coal cloud, is believed to be responsible. The fact that the char combustion rates, defiied as the slope of the weight loss curves in the postflame region, are higher for the larger particles supports this hypothesis. This is deduced from the following: (1) theoretically, the larger particles should have lower mass transfer coefficients, and (2) the concentration of combustible material in the larger char particles should be lower than in the smaller ones because of a higher extent of burnoff in the former. To explore this point, computer modeling on char burnout in the entrained-flow reactor was conducted for the cases of 50 X 100 and 140 X 200 mesh particles. The equations used to predict the char combustion rate were essentially those described by Field,17Smith,18and Mulcahy and Smith.lg The rate of reaction per unit external geometric surface (R,) was calculated by the expression
where R,, and R, represent the limiting mass-transfercontrolled and chemically controlled rates, respectively. It was assumed that the carbon-oxygen reaction at the particle surface produces CO and the subsequent burning of CO occurs far away from the particle surface. Kinetic parameters derived from TGA studies20on the partially (17) Field, M. A. Combust. Flame 1969, 13, 237-252. (18) Smith, I. W.Nineteenth Symposium (International) on Combustion, The Combustion Institute Pittsburgh, PA, 1982; pp 1045-1065. (19)Mulcahy, M. F. R.; Smith, I. W. Rev. f i r e Appl. Chem. 1969,19, 81-108. (20) Tsai,C. Y.;Scaroni, A. W. Fuel, in press.
0.05 0.10
0.15
aiu
0.2s 0.30
o
5
Residence Time, s
Figure 7. Comparison of experimental data and predicted char burnoff based on given oxygen concentrations ranging from 5-20% for coal particles from PSOC-122 with various sizes: (a) 140 X 200 mesh; (b) 50 X 100 mesh ( ( 0 )experimental data; (-) prediction). burned chars were used to predict the char combustion rate under chemical control. Details of the char burnout kinetics have been described elsewhereS2l The chars collected at a residence time of 0.094 and 0.123 s, with corresponding weight losses of 55.4 and 55.9% in the entrained-flow reactor, were used as the starting point for the char burnout predictions for the 140 X 200 and 50 X 100 mesh particles, respectively. Oxygen concentrations ranging from 5% to 20% were used. Figure 7 indicates that for the case of the 140 X 200 mesh particles, the experimental data agree well with the predicted burnoff profile, assuming an oxygen concentration of 5 % in the postflame positions. For the larger particles, 50 X 100 mesh, the weight loss data fit better to the char burnout behavior predicted by assuming an oxygen concentration of 15-20%. This result suggests that particle dispersion in the reactor is particle size dependent, with the larger particles being exposed to higher air/fuel ratios. The importance of the air/fuel ratio in determining the weight loss at the end of the volatiles combustion stage implies that a portion of the weight loss observed can be attributed to heterogeneous char oxidation, and varying the particle size changes the extent of char oxidation at the end of the volatiles combustion stage. Due to the variation in the particle size related air/fuel ratio, the effect of furnace temperature on the weight loss behavior is particle size dependent. Figure 8 shows that the difference in the ignition delay time between lo00 and 1200 K is greater for the 50 X 100 mesh particles than for the 140 X 700 mesh particles. At 1200 K, both 50 X 100 (21) Scaroni, A. W.; Tsai,C. Y.; McIlvried, T. S.;Jenkins, R. G. Eighth International Symposium on Coal Slurry Fuels Preparation and Utilization: US.Department of Energy: Pittsburgh, PA, 1986;pp 409-421.
Tsai and Scaroni
268 Energy & Fuels, Vol. 1, No. 3, 1987
1
I
I
7o (a) 140 x 200 mesh
3
f
30
G
20
0
-
0
o.m
Liptinite Rich Vitrinite Rich
-a"
0.05 0.10 0.15 0.20 0.25 0.30 0.35 Residence Time, s
0.00
0,'lO 0.20 Residence Time, s
0.'30
50-
c
40-
j;
$ 30-
s
0-
1 0.W
0.00
0.10
0.20
0.30
Residence Time ,s
Figure 8. Effect of temperature on weight loss of fractions from
PSOC-122 with various particle sizes: (a) 140 X 200 mesh; (b) 50 X 100 mesh. and 140 X 200 mesh particles have similar ignition delays; however, at 1000 K, the ignition of the larger particles occurs at a longer residence time than that for the smaller particles. The results indicate that the effect of particle size on the ignition behavior of the coal is more significant at low temperature. The data also suggest that the ignition of the coal occurs during heat-up; consequently, the effect of particle heating rate, being associated with the particle size, is more pronounced at lower temperatures. 3.2.2. Effect of Maceral Composition. The effect of maceral composition on the weight loss behavior of coal particles during the initial stages of pulverized coal combustion appears to vary with the operating conditions and the physical properties of the coal particles. Figure 9 illustrates that the presence of a higher concentration of liptinite macerals in the 270 X 400 mesh particles from PSOC-363 coal enhances the ignitability and weight loss rate of the coal during combustion at lo00 K. The ignition delay for the liptinite-rich particles is about 50 ms shorter than that for the vitrinite-rich sample. The weight loss of the coals at the postignition positions is consistent with their pyrolysis behavior. The data suggest that the ignition and weight loss of the coals during the early stages of combustion are governed by their thermal decomposition behavior. In other words, the contribution of heterogeneous char oxidation to the observed weight loss is not significant. This may also be an indication that the dispersion of the small particles in the reactor is low and that the burning of the particles during the volatiles combustion stage is limited by oxygen transportation. Consequently, the pyrolysis of the coal dominates the observed weight loss. For a larger particle size, 100 X 200 mesh, variations in liptinite concentration similar to those in the smaller particles have relatively little influence on the ignition delay and final weight loss at 1000 K. This is illustrated
o
Liptinite Rich Vitrinite Rich
0 0.20 Residence T>\me, s
0.10
1 0 30
Figure 9. Effect of liptinite concentration on weight loss for coal particles with various sizes during combustion at lo00 K: (a) 270 X 400 mesh; (b) 100 X 200 mesh.
in Figure 9b. Consistent with the data shown in the previous section, the larger particles probably burn under a relatively fuel lean condition where, after devolatilization has been completed and the released volatiles burned, a substantial amount of oxygen is still available to burn the char particles heterogeneously. The observed weight loss at the postignition stage, therefore, is a consequence of a combination of pyrolysis and char combustion. It is possible that heterogeneous char oxidation plays a more important role in determining the ignition behavior of larger particles than small ones; consequently, the effect of chemical composition is less significant. The increasing tendency for heterogeneous ignition to occur at low furnace temperatures and with increasing particle size has been reported by several investigator^.^^,^^ For the large particles at low temperatures, the pyrolysis rate is slow during the preignition and heat-up stage so that the oxygen concentration at the particle surface is sufficiently high to cause heterogeous ignition. The higher slip velocity for large particles may also sweep the released volatiles away from the particle and facilitate the exposure of the surface to oxygen. The role of inerts, inertinites and mineral matter, during the initial stages of pulverized coal combustion appears also to change with the combustion conditions. Figure 10 shows that the mineral-rich fraction (B4) has a longer ignition delay than the vitrinite-rich fraction (B2) at 1000 K. However, this difference does not exist at the higher temperature of 1200 K. Figure lob indicates that, at 1200 K, B4 particles have ignition delay times slightly shorter than those for B2 particles. One explanation is that B2 and B4 particles ignite by different mechanisms that have ______
~~
Gomez, C. P. M.S. Thesis, The Pennsylvania State University, University Park, PA, 1982. (23)Juntgen, H.; van Heek, K. H. Fuel Process. Technol. 1979, 2, 261-293. (22)
Pyrolysis and Combustion of Bituminous Coal
4
(a) 1000 K
i I
-
-10
0.m
0.10
0.20
0.30
Residence Time, s
9080
-
om
m) 1200 K
0.10
o 20
0.30
Residence Time, s
Figure 10. Effect of inerts concentration on weight loss of 200 X 270 mesh particles during combustion at various furnace temperature: (a) 1000 K; (b) 1200 K.
different temperature dependencies. The greater temperature dependency of the weight loss rate during the initial stages of combustion for the mineral-rich samples suggests a higher contribution from heterogeneous oxidation to the observed weight loss during ignition for the B4 particles than for the B2 fraction since the carbon-oxygen reaction usually has a higher global activation energy (approximately 30 kcal/mole) than the pyrolysis reactions (approximately 10 kcal/mole). The weight loss and char combustion rates for the mineral-rich fractions are also higher at postflame positions
Energy & Fuels, Vol. 1, No. 3, 1987 269
than those for the vitrinite-rich particles. This supports the hypothesis of an oxygen diffusion limitation at the end of the intensive volatiles combustion stage. Due to the presence of large amounts of noncombustible minerals, more than 45% (see Table I), the air/fuel ratio inside the coal cloud is higher for the combustion of the B4 fraction despite the fact that the feed rates for the coals were the same. Thus, the higher burnout for the mineral-rich particles on dry, ash-free basis at the postflame positions, as shown in Figure 10, is due to a higher oxygen concentration around the char particles. 4. Conclusions The weight loss of coal particles during the ignition stage of pulverized-coal combustion can be attributed to a combination of pyrolysis and heterogeneous oxidation. Lower furnace temperature, larger particle size, and a higher concentration of mineral matter in the particles tend to decrease the pyrolysis rate and increase the contribution of heterogeneous burning to the observed weight loss. As a consequence, the role of maceral composition in pulverized-coal combustion varies with the operating conditions in the reactor and the particle size. The presence of liptinites has a greater influence on the observed weight loss and ignitability under conditions when pyrolysis dominates. The presence of inerts delays the ignition for combustion at low temperatures. At higher temperatures, the ignition delay time for both vitrinite- and mineral-rich particles is similar. Variations in particle size are coupled to changes in the chemical composition of the pulverized-coal particles. The larger particles usually contain a higher concentration of liptinites and have higher yields of volatiles. In addition, larger particles experience wider radial dispersion in the flow reactor following injection, which consequently results in a higher air/fuel ratio for the intensive volatiles combustion.
Acknowledgment. Financial support for this research was provided by The Cooperative Program in Coal Research at Penn State. Coal samples were supplied by the Penn State/DOE Sample Bank.
