Article pubs.acs.org/EF
Experimental Study on Pyrolysis Behaviors of Coal in a Countercurrent Downer Reactor Pengfei Dong,†,‡ Ze Wang,† Zhengjie Li,†,‡ Songgeng Li,† Wei gang Lin,† and Wenli Song†,* †
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China ‡ Graduate University, Chinese Academy of Sciences, Beijing, China ABSTRACT: Experiments on pyrolysis of Chinese Fugu coal were carried out with a downer reactor, and the effects of temperature and coal feeding rate on product distribution were investigated. It was found that the gas yield increases with increasing temperature, and the yield of tar increases with increasing coal feeding rate. When temperature is 650 °C with coal feeding rate of 12 kg/h, the yield of tar reaches the highest value of 10.29 wt % (dry coal basis). Compared to cocurrent downer reactor, the countercurrent downer reactor favors higher yields of C2 + C3 and tar.
1. INTRODUCTION
2. EXPERIMENTAL SECTION
Coal is the main energy in China, which accounts for about 61% of the total primary energy consumption in 2010. Lignite and sub-bituminous coal with higher volatile and moisture account for 55% of the total coal reserve.1 So, it is advantageous to convert part of coal to gas, liquid fuels, and fine chemicals by pyrolysis. Meanwhile the char can be still used as fuel in boilers, with reduced emissions of SO2 and NOx.2,3 The yields of tar and gas are affected by several parameters. Among the parameters, the most significant are temperature, heating rate, residence time of gas, gaseous environment, particle size, and pressure.4,5 Commonly, higher temperature is favorable for increasing the release of volatiles and yield of gas;6−8 higher heating rate, shorter residence time of gas, and hydrogen-rich environment promote the yield of tar.8−13 Wang et al.14,15 applied a cocurrent downer reactor (CoCDR) as pyrolyzer and studied the effects of temperature and coal particle size on the product distribution and composition. It was found that the downer reactor is suitable for the pyrolysis of coal and has a higher tar yield than moving bed and spoutedentrained bed, with a higher heavy tar content. Compared with gas−solid cocurrent flow, gas−solid countercurrent flow enhances gas−solid contact and increases the residence time of particles.16 It is interesting to investigate the pyrolysis of coal in a countercurrent downer reactor (CCDR). For coal pyrolysis in a CCDR, the pyrolytic volatiles flow upward and the coal drops downward and meanwhile pyrolyzes in a hydrogen-rich gas environment. In the top of the CCDR, some heavy volatiles are cooled by cold coal particles, are condensed on the surface of the particles moving down, and pyrolyze again. So, the coal pyrolysis process with CCDR may be favorable for obtaining higher yields of light tar and gas. Presently, no reports have been published on coal pyrolysis with CCDR. In this paper, a downer reactor is used for coal pyrolysis with both CCDR and Co-CDR operations. The research focuses on the effects of temperature and coal feeding rate on product distribution of gas and liquid, and the comparison between Co-CDR and CCDR.
2.1. Coal Samples. Fugu bituminous coal (Fg-coal) is used in the experiments. The proximate and ultimate analyses of the coal are given in Table 1. The particle size distribution of coal is shown in Figure 1, and the mean diameter of the coal particles is 776 μm. 2.2. Apparatus and Procedure. As shown in Figure 2, the test system is mainly composed of screw feeder, downer reactor (inner diameter of 0.088 m and length of 6.7 m), char receiver, cyclone, and a multistage cooling system. The downer is heated externally by 12 electric heaters with a maximum capacity of 60 KW. The temperature profile along the downer is measured by six K-type thermocouples.14 The temperature of the downer reactor is set at 600−800 °C, and the temperatures of the cyclone and the top of the reactor are controlled at 400 and 500 °C, respectively. The multistage cooling system is composed of four cooler tanks. The first tank is cooled by hot water at 70 °C, and others are cooled by a ice−water mixture. The coal particles were first dried at 105 °C for 4 h before experiment. When the temperature of the downer reactor reaches the setting value, nitrogen (40 L/h) was introduced into the reactor from bottom of the reactor and stabilized for 4 h. Then, coal particles were fed into the downer from the top by a screw feeder with nitrogen carrier gas (80 L/h). The feeding rate of coal was obtained by weight difference of coal in the coal hopper before and after experiment. The released volatile matter flows upward to cyclone and condensing systems, while coal or char particles move downward to the char receiver. It runs for about 1 h for each experiment. The volume of the incondensable gas product was measured by wet gas flowmeter, and the liquids in different cooler tanks were collected and placed in refrigerator at 4 °C. The light tar is defined as the liquid above water, which is mainly obtained from the fourth liquid tank of the cooling system. Each experiment was repeated two or three times. The relative deviation was within +10%. 2.3. Analysis and Characterization. After the experiment, the gas product was analyzed by a three-channel microgas chromatography (GC) instrument (Agilent 3000) equipped with three thermal conductivity detectors (TCD). The volume percentage of H2, O2, N2, CH4, and CO were analyzed by channel 1 with 5A molecular sieve column, CO2, C2H4, and C2H6 were analyzed by channel 2 with plot U column, and C3H6 and C3H8 were analyzed by channel 3 with plot Q
© 2012 American Chemical Society
Received: April 17, 2012 Revised: July 11, 2012 Published: July 11, 2012 5193
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Table 1. Proximate and Ultimate Analysis of Fugu Coal proximate analysis [wt %, ad]
ultimate analysis [wt %, drf]
coal sample
V
A
FC
C
H
N
S
O
Fg-coal
35.91
3.71
60.39
78.14
4.72
1.20
0.25
11.98
detailed calculations are given as follows. The total volumetric rate of gas (including N2) was obtained by nitrogen balance: Vt = VN2/C N2
(1)
where Vt, VN2, and CN2 are the total flow rate of gas exited from the downer (m3/h), the flow rate of nitrogen fed into the downer (m3/h), and concentration of nitrogen in gas (m3/m3), respectively. The mass of the produced gas (N2 free) was calculated by Vi = CiVN2/C N2
(2)
Vi − ST = VT i S/ TA
(3)
Q gas =
∑ (Vi − STMi /22.4)
(4)
where Vi is the flow rate of component i in gas product (m /h), TS is the standard temperature of 273.15 K, TA is the ambient temperature (K), Vi−ST is the flow rate of component i in gas product under standard condition (m3/h), Ci is the concentration of component i in gas product (m3/m3), Mi is the molecular weight of component i (g/ mol), and Qgas is the mass yield of gas product (kg/h). The weight fraction of the gaseous products can be obtained by 3
Figure 1. Particle size distribution of Fg-coal.
ygas (%) =
Q gas Q coal
× 100 (5)
where ygas and Qcoal are gas yield (wt %) and feed rate of coal (kg/h), respectively. Ash balance was applied to calculating the char yield by assuming that the amount of ash remained the same in pyrolysis: Q coalAcoal = Q charAchar
(6)
where Acoal, Achar, and Qchar are ash content in coal (wt %), ash content in char (wt %), and char mass yield (kg/h), respectively. The weight fraction of char is obtained by ychar (%) =
Q char Q coal
× 100 (7)
where ychar is char yield (wt %). The weight fraction of the liquid product was calculated as the difference:
yliquid = 100 − ygas − ychar
Figure 2. Schematic of countercurrent downer reactor.
ytar = yliquid ×
column. The liquids were analyzed by GC-MS (gas chromatographymass spectrometry, Varian CP-3800/300-MS) with column of FFAP (25 mm-0.25 mm-0.2 μm). The oven temperature starts at 40 °C (3 min) and then increases to 100 °C (3 min) by 4 °C/min, and finally, it increases to 240 °C (10 min) by 6 °C/min. The volatile content of char or coal was obtained by thermogravimetric analysis (Netzsch STA 449C). For each run, approximately 10 mg of each sample (d < 76 μm) placed in an aluminum oxide crucible were heated in heating rate of 40 °C/min in a nitrogen atmosphere from 25 to 110 °C. Then, the sample was held at 110 °C for 30 min. After this period, the temperature was increased to 910 °C at a heating rate of 40 °C/min and was held at 910 °C for 30 min. Finally, the atmosphere was switched from nitrogen to air at 910 °C and held for 30 min. 2.4. Data Treatment. The yields of the pyrolysis gas and char were obtained by nitrogen balance and ash balance, respectively.14,17 The liquid yield was calculated from the difference between the total and the summary yields of gas and char. The tar fraction was defined as the mass ratio of tar to total liquid (sum of water and tar). The
mtar mliquid
(8) (9)
where yliquid, ytar, mtar, and mliquid are liquid yield (wt %), tar yield (wt %), mass of tar by weighing (kg), and mass of liquid by weighing (kg), respectively
3. RESULTS AND DISCUSSION 3.1. Products Distribution. The yields of gas, liquid, and char products obtained under different temperatures with coal feeding rate of 6 kg/h are shown in Figure 3. It can be seen that the gas yield increases with increasing temperature, while the char yield decreases. With temperature increasing, the yield of water is lower than the yield of tar. The yield of tar is the highest at 650 °C. With temperature increasing, the amount of released volatiles increases, and thus, the yields of tar and gas increase; the further increase of temperature causes secondary reactions of 5194
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Figure 5. Influence of temperature on distribution of tar. Figure 3. Effect of temperature on yields of products.
compounds decreases significantly with increasing temperature first, and then remains about 4.5 wt % after 700 °C. The effect of temperature on coal pyrolysis with CCDR is complicated. On the one hand, the higher temperature promotes secondary cracking of the volatiles,19−24 and thus, the yields of components with lower stability such as phenols, aliphatic hydrocarbons, and oxygen containing components are decreased, while the more stable components such as polyaromatic groups and benzene derivatives are increased. On the other hand, with increasing temperature, the amount of released volatiles and the heating rate of particles increase. Thus, the residence time of released volatiles decreases, which decreases the opportunity for secondary cracking reaction of released volatiles. The counter effects of temperature make the component distribution with increasing temperature inconsistent. The influence of coal feeding rate on component distribution of the tar is shown in Figure 6. The contents of phenol
the volatiles, and thus, the tar yield decreases and the gas yield increases. It is in accordance to coal pyrolysis by other pyrolysis processes.14,15,17,18 Figure 4 shows the effect of coal feeding rate on the distribution of product yields at the temperature of 650 °C. It
Figure 4. Effect of feeding rate on yields of products.
can be seen that the yield of tar increases obviously with increasing coal feeding rate between 3 and 6 kg/h, while the gas, water, and char yields decrease. The increase of feeding rate leads to an increased amount of released volatiles, which brings about a shorter residence time of volatiles, a longer residence time of solid, and a faster rate of heat and mass transfer. The shorter residence time of volatiles reduces secondary cracking reactions of released volatiles; thus, the yield of tar rises, and the yield of gas is reduced. 3.2. Tar Compositions. Figure 5 shows the influence of temperature on the component distributions of tar. The content of phenol derivatives is the highest at 650 °C and has a lowest value at 750 °C. The distribution of aliphatic hydrocarbons is similar to that of phenol derivatives. The content of benzene derivatives increases with increasing temperature and reaches the maximum value at 750 °C. The distribution of polyaromatic groups is similar to that of benzene derivatives beyond 650 °C. The content of oxygen-containing
Figure 6. Influence of feeding rate on component distribution of tar.
derivatives, aliphatic compounds, and oxygen-containing compounds increase with increasing coal feeding rate, while the contents of benzene derivatives and polyaromatics decrease. With the increased feeding rate, the residence time of released volatiles reduces. The secondary cracking reactions of less stable components including phenols, oxygen-containing compounds, and aliphatics are lower.19 5195
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Figure 7 displays the compositions of the light tar obtained at 600 and 650 °C. The light tar is rich in aliphatic series. The
Figure 9. Effect of temperature on the yields of gas.
CO2 may be formed from two processes of cleavage of carboxyl group and oxygen-containing heterocycle. The decomposition of the carboxyl group happens more easily under a low temperature, while the decomposition of oxygen-containing heterocycle occurs at high temperature.25−27 The distribution of CO is similar to that of CH4. J. J. Pis21 deemed that CO formation comes from the same precursors as methane. CO could be formed from the decomposition of phenolic groups.20,21 The different composition of the gases obtained under different temperature leads to different heating values of gas. The highest heating value (33.02 MJ/Nm3) appeared at 700 °C. The effect of coal feeding rate on the gas yields at 650 °C is shown in Table 2. It can be seen that the yields of H2, CH4,
Figure 7. Influence of temperature on distribution of light tar.
ratio of the light tar yield to the total tar yield is 8% and 12% at 600 and 650 °C, respectively. Above 650 °C, the yield of light tar decreases significantly with increasing temperature. The composition of the light tar under different feeding rates is shown in Figure 8. When the feeding rate is 12 kg/h, the content of aliphatics reaches to the maximum value (60.4 wt %) at 650 °C and the light tar accounts for 15 wt % of total tar.
Table 2. Effect of Feeding Rate on the Yields of Gas (wt %) at 650 °C feeding rate [kg/h]
Figure 8. Influence of feeding rate on component distribution of light tar.
Based on the analysis of the effect of temperature and feeding rate on the component distributions of the tar, it indicates that higher pyrolysis temperature and longer residence time of released volatiles favors secondary reactions of volatiles. Phenol derivatives, aliphatic compounds, and oxygen-containing compounds may be further converted to benzene derivatives, polyaromatics, and gases by secondary reactions. 3.3. Gas Composition. Figure 9 exhibits the effect of temperature on the yields of gas. Generally, all components of the hydrocarbons increase with increasing temperature. The yields of C2 + C3 alkenes are higher than those of C2 + C3 alkanes, especially under higher temperatures. Above 650 °C, the yield of H2 increases obviously with increasing temperature. The yield of CO2 increases with temperature in two stages, as
composition
3
6
12
H2 CH4 CO CO2 C2H4 C2H6 C3H6 C3H8 total hydrocarbons
0.039 1.548 1.566 2.317 0.660 0.740 0.956 0.270 4.174
0.022 1.098 1.487 2.589 0.548 0.569 0.764 0.232 3.211
0.020 1.018 1.412 2.640 0.401 0.521 0.686 0.218 2.844
CO, and total hydrocarbons decrease with increasing coal feeding rate, while the yield of CO2 increases. The result indicates that secondary cracking of the volatiles is favorable for the production of H2, CH4, CO, and hydrocarbons. The results are consistent with the previous report.19 3.4. Comparison with Cocurrent Downer Reactor. Coal pyrolysis process with Co-CDR at the same condition has also been studied. The distribution of products is compared between the process of Co-CDR and CCDR, as shown in Figure 10. The yield of tar by CCDR is higher than that of CoCDR, while the yields of char and water are lower. The difference in char yield between Co-CDR and CCDR is smaller with increasing temperature. The optimal temperature for tar yield is 650 °C for both processes. It can be seen that gas−solid 5196
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hot countercurrent volatiles contact cold coal in CCDR, some heavy tar condenses on the surface of coal.
4. CONCLUSION Coal pyrolysis by countercurrent fluidized downer reactor is studied. The following conclusions are found: (1) In the tested temperature range, gas yield increases with increasing temperature while char yield decreases. The yield of tar is highest at 650 °C. (2) Within the range of feeding rate between 3 and12 kg/h, the yield of tar increases with coal feeding rate, while the yields of char and gas decrease. (3) Higher pyrolysis temperature and longer residence time of volatiles favors secondary reactions of released volatiles. Phenol derivatives, aliphatic compounds, and oxygen-containing compounds are involved in secondary reaction to form benzene derivatives, polyaromatics, H2, CH4, and CO. (4) Compared with the process of Co-CDR, more volatiles are released and a higher yield of tar containing a higher content of aliphatic series and lower polyaromatic content by CCDR is obtained, attributed to a lower extent of secondary reactions of volatiles in CCDR and the removing of heavier volatiles by condensing on cold coal particles.
Figure 10. Yields of products with different temperatures in CCDR and Co-CDR.
countercurrent flow favors the release of volatiles and thus increases the yield of tar, since gas−solid countercurrent flow increases residence time of particles, and the temperature of particles is higher than that with a gas−solid cocurrent flow at the same axial location. Tables 3 and 4 give the main components of tar and gas, respectively. The yields of C2 + C3 and CO2 by CCDR are
■
Table 3. Tar Distribution of CCDR and Co-CDR
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
composition [wt %)] components
CCDR
Co-CDR
phenol derivatives oxygen-containing benzene derivatives polyaromatic aliphatic series heterocyclic and other naphthalenol derivatives sum
33.18 12.89 13.28 15.26 17.72 4.25 3.42 100
32.06 9.29 16.79 30.49 5.21 4.65 1.51 100
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was financially sponsored by the joint project supported by the National Natural Science Foundation of China and Shen Hua Group Corporation Limited (51174284).
■
Table 4. Yields of Gases by CCDR and Co-CDR yields of gases (wt %) components
CCDR
Co-CDR
H2 CH4 CO CO2 C2H4 C2H6 C3H6 C3H8 C2+C3
0.022 1.098 1.487 2.589 0.548 0.569 0.764 0.232 2.113
0.033 1.245 1.662 2.218 0.306 0.305 0.244 0.097 0.952
REFERENCES
(1) Wang, Y. J. Development survey and application market analysis of semi coke industry. Chem. Ind. 2009, 9, 23−26 (in Chinese). (2) Thomas, K. M. The release of nitrogen oxides during char combustion. Fuel 1997, 76, 457−473. (3) Middleton, S. P.; Patrick, J. W.; Walker, A. The release of coal nitrogen and sulfur on pyrolysis and partial gasification in a fluidized bed. Fuel 1997, 76, 1195−1200. (4) Borah, R. C.; Ghosh, P.; Rao, P. G. A review on devolatilization of coal in fluidized bed. Int. J. Energy Res. 2011, 35, 929−963. (5) Christiansen, J. V.; Feldthus, Anders.; Carlsen, L. Flash pyrolysis of coals. Temperature dependent product distribution. J. Anal. Appl. Pyrolysis 1995, 32, 51−63. (6) Tyler, R. J. Flash pyrolysis of coal. 1. Devolatilization of a victorian brown coal in a small fluidized-bed reactor. Fuel 1979, 58, 680−686. (7) Xu, W. C.; Tomita, A. Effect of temperature on the flash pyrolysis of various coals. Fuel 1987, 66, 632−636. (8) Cui, Y. P.; Qin, L. L.; Du, J.; Chang, L. P. Products distribution and its influencing factors for coal pyrolysis. Coal Chem. Ind. 2007, 2, 10−15 (in Chinese). (9) Jon, G. M.; Rafael, K. Coal pyrolysis yields from fast and slow heating in a wire-mesh apparatus with a gas sweep. Energy Fuels 1988, 2, 505−511. (10) Griffin, T. P.; Howard, J. B.; Peters, W. A. An experimental and modeling study of heating rate and particle size effects in bituminous coal pyrolysis. Energy Fuels 1993, 7, 297−305.
higher than those by Co-CDR, while yields of H2, CH4, and CO are lower. The oxygen-containing compounds, phenol derivatives, and aliphatic series by CCDR are higher than that by Co-CDR, while the components of polyaromatic and benzene derivatives are lower. It attributes to a shorter residence time of volatiles and a longer residence time of the coal particles in the reaction zone in the CCDR process, decreasing secondary cracking reactions. Additionally, when the 5197
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Article
(11) Hayashi, J. I.; Takahashi, H.; Doi, S.; Kumagai, H.; Chiba, T. Reactions in brown coal pyrolysis responsible for heating rate effect on tar yield. Energy Fuels 2000, 14, 400−408. (12) Liao, H. Q.; Li, B. Q.; Zhang, B. J. Co-pyrolysis of coal with hydrogen-rich gases. 1. Coal pyrolysis under coke-oven gas and synthesis gas. Fuel 1998, 77, 847−851. (13) Liao, H. Q.; Sun, C. G.; Li, B. Q.; Liu, Z. C. Coal pyrolysis under hydrogen-rich gases. J. Fuel Chem. Technol. 1998, 2, 114−118 (in Chinese). (14) Wang, J. G.; Lu, X. S.; Yao, J. Z.; Lin, W. G; Cui, L. J. Experimental study of coal topping process in a downer reactor. Ind. Eng. Chem. Res. 2005, 44, 463−470. (15) Wang, J. G.; Lu, X. S.; Yao, J. Z.; Lin, W. G.; Du, L. Total distribution and liquid composition of products from coal topping process. J. Process Eng. 2005, 5, 241−245 (in Chinese). (16) Luo, K. B.; Liu, W.; Zhu, J. X.; Beeckmans, J. M. Characterization of gas upward−solids downward countercurrent fluidized flow. Powder Technol. 2001, 115, 36−44. (17) Cui, L. J.; Lin, W. G.; Yao, J. Z. Influences of temperature and coal particle size on the flash pyrolysis of coal in a fast-entrained bed. Chem. Res. Chinse U. 2006, 22, 103−110 (in Chinese). (18) Zhang, M. D.; Wang, Z.; Zhang, X. W.; Wang, L. C.; Lin, W. G.; Song, W. L. Experimental study on fast pyrolysis of bituminous coals by solid heat carrier at low temperature. J.Process Eng. 2010, 10, 530− 535 (in Chinese). (19) Xu, W. C.; Tomita, A. The effects of temperature and residence time on the secondary reactions of volatiles from coal pyrolysis. Fuel Process. Technol. 1989, 21, 25−37. (20) Hayashi, J. I.; Amamoto, S.; Kusakabe, K.; Morooka, S. Evaluation of vapor-phase reactivity of primary tar produced by flash pyrolysis of coal. Energy Fuels 1995, 9, 290−294. (21) Arenillas, A.; Rubiera, F.; Pis, J. J.; Cuesta, M. J.; Iglesias, M. J.; Jimenez, A.; Suarez-Ruiz, I. Thermal behavior during the pyrolysis of low rank perhydrous coals. J. Anal. Appl. Pyrolysis 2003, 68−69, 371− 385. (22) van Heek, K. H.; Hodek, W. Structure and pyrolysis behavior of different coals and relevant model substances. Fuel 1994, 73, 886−896. (23) Collin, P. J.; Tyler, R. J.; Wilson, M. A. 1H NMR study of tars from flash pyrolysis of three Australian coals. Fuel 1980, 59, 479−486. (24) Ahmad, T.; Awan, I. A.; Nisar, J.; Ahmad, I. Influence of inherent minerals and pyrolysis temperature on the yield of pyrolysates of some pakistani coals. Energy Convers. Manage. 2009, 50, 1163−1171. (25) Siskin, M.; Scouten, C. G. Sources of carbon dioxide formed during coal pyrolysis. Energy Fuels 1989, 3, 427−428. (26) Xu, W. C.; Tomita, A. Effect of coal type on the flash pyrolysis of various coals. Fuel 1987, 66, 627−631. (27) Chen, J. C.; Castagnoli, C.; Niksa, S. Coal devolatilization during rapid transient heating. 2. Secondary pyrolysis. Energy Fuels 1992, 6, 264−271.
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