Energy Fuels 2010, 24, 6314–6320 Published on Web 11/08/2010
: DOI:10.1021/ef101026u
Pyrolysis Behavior of Macerals from Weakly Reductive Coals Yunpeng Zhao, Haoquan Hu,* Lijun Jin, Xinfu He, and Shengwei Zhu State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, 129 Street, Dalian 116012, People’s Republic of China Received April 5, 2010. Revised Manuscript Received October 20, 2010
Pyrolysis of the vitrinites and inertinites separated from three weakly reductive coals (WRCs) and one reductive coal were carried out by thermogravimetry coupled with mass spectrometry (TG-MS) and in a fixed-bed reactor to investigate the effects of the reductive degree on the pyrolysis behavior of macerals. TG/differential thermogravimetry (DTG) analyses showed that the vitrinites and inertinites from three WRCs have lower weight loss and weigh loss rate but higher peak temperature than those from Pingshuo (PS) coal. The pyrolysis process of the macerals can be described by three-step independent first-order kinetics models, and the macerals from three WRCs have lower average activation energy than those from PS coal. The differences in the evolution profiles of H2, CH4, C2H6, and CO2 between the macerals from three WRCs and from PS coal were also characterized using TG-MS. The results of coal pyrolysis in the fixed-bed reactor showed that the vitrinites and inertinites from three WRCs have lower tar yield but higher gas yield than those from PS coal and the chars from the macerals of three WRCs have higher combustion reactivity than those from PS coal. Vitrinite and inertinite are two typical coal macerals. With the development of maceral separation technologies, more attention has been paid to the structural characteristics and chemical reactivity of coal macerals.5-10 Although it is wellknown that WRCs exhibit different physicochemical properties from common coals, there is little information about the effects of the reductive degree on the properties of macerals. Zhao et al.11 studied the difference in chemical composition of the carbon-disulfide-extractable fraction between the macerals of WRCs and SRCs. It was found that the extract yield of the macerals from SRCs was much higher than those from WRCs and there was no remarkable difference in chemical composition from the extraction solutions between the vitrinite and inertinite of SRCs, whereas the difference between those of WRCs was obvious. Because pyrolysis is the primary step in the main coal conversion processes, such as gasification and liquefaction, the studies on the pyrolysis behavior of the macerals from these unusual coals are necessary to assess the effects of the reductive degree on the physicochemical properties of macerals and the potential of WRCs being used in certain coal use processes. Pyrolysis behaviors of the vitrinites and inertinites, separated from three Chinese typical WRCs, Shendong (SD) coal in Shaanxi province, Lingwu (LW) coal in Ningxia province, and Hami (HM) coal in Xinjiang province, were investigated using a thermogravimetry coupled with mass spectrometry (TG-MS) system and in a fixed-bed reactor. For comparison, one Chinese typical SRC, Pingshuo (PS) coal in Shanxi
Introduction Coal is a heterogeneous rock derived from plant debris, which has undergone complex changes during burial, and the differences in the depositional environment often result in the different composition and physicochemical properties of coals even with similar rank.1 The reductive degree of coal is related to the coal-forming environment, the embedding speed, and the chemical properties of peat bog during the ulmification process. In addition to coal rank and maceral composition, reductive degree is also a crucial factor that affects coal properties. Abundant Jurassic coals exist in the northwest of China and play an important role in the Chinese energy supply. These Jurassic coals were formed in the sedimentation environment of alluvium swamp facies, and their peat bog was covered with shallow water, frequently exposed to air, and subjected to a strong oxidative but weakly reductive effect during the accumulation process; therefore, these Jurassic coals were named as weakly reductive coals (WRCs).2 WRCs usually have lower ash, sulfur, and phosphorus contents but higher inertinite content and aromaticity than strong reductive coals (SRCs). Our previous studies have compared the composition and structural characteristics, pyrolysis, and extraction behaviors of WRCs to those of SRCs under a similar coal rank.2-4 The results indicated that WRCs have lower conversion, lower tar, and light hydrocarbon gas yields but higher oxygen-containing gas yields during pyrolysis and lower extract yield but higher oil yield during extraction with sub- and supercritical water than SRCs.
(5) Bryers, R. W. Fuel Process. Technol. 1995, 44, 25–54. (6) Sun, Q. L.; Li, W.; Chen, H. K.; Li, B. Q. Fuel 2003, 82, 669–676. (7) Das, T. K. Fuel 2001, 80, 97–106. (8) Strugnell, B.; Patrick, J. W. Fuel 1996, 75, 300–306. (9) Cai, H. Y.; Kandiyoti, R. Energy Fuels 1995, 9, 956–961. (10) Joesph, J. T.; Fisher, R. B.; Masin, C. A. Energy Fuels 1991, 5, 724–729. (11) Zhao, X. Y.; Zong, Z. M.; Cao, J.; Ma, Y. M.; Han, L.; Liu, G. F.; Zhao, W.; Li, W. Y.; Xie, K. C.; Bai, X. F.; Wei, X. Y. Fuel 2008, 87, 565– 575.
*To whom correspondence should be addressed. Telephone/Fax: þ86-411-39893966. E-mail:
[email protected]. (1) Borrego, A. G.; Marban, G.; Alonso, M. J. G.; Alvarez, D.; Menendez, R. Energy Fuels 2000, 14, 117–126. (2) Zhao, Y. P.; Hu, H. Q.; Jin, L. J.; Wu, B.; Zhu, S. W. Energy Fuels 2009, 23, 870–875. (3) Wu, B.; Hu, H. Q.; Huang, S. P.; Fang, Y. M.; Li, X.; Meng, M. Energy Fuels 2008, 22, 3944–3948. (4) Wu, B.; Hu, H. Q.; Zhao, Y. P.; Jin, L. J.; Fang, Y. M. J. Fuel Chem. Technol. 2009, 37, 385–392. r 2010 American Chemical Society
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pubs.acs.org/EF
Energy Fuels 2010, 24, 6314–6320
: DOI:10.1021/ef101026u
Zhao et al.
Table 1. Analyses of Macerals from Different Coals proximate (wt %)
ultimate (wt %, daf )
petrographic (vol %) a
sample
moisturead
ashd
volatile matterdaf
C
H
N
S
O
fab
PSV PSI SDV SDI LWV LWI HMI
4.59 3.02 9.77 6.53 11.60 12.82 7.16
4.56 21.87 1.77 3.72 2.68 5.53 2.93
40.22 32.55 41.17 27.08 39.50 29.26 24.67
81.67 80.15 77.93 82.08 77.42 78.77 82.99
5.08 4.49 4.71 3.68 3.96 3.36 3.38
1.35 1.33 1.00 0.78 0.81 0.71 0.77
1.08 0.81 0.18 0.29 0.07 0.11 0.07
10.82 13.23 16.18 13.18 17.74 17.05 12.79
0.79 0.83 0.80 0.91 0.86 0.92 0.94
a
vitrinite
inertinite
exinite
91.6 6.4 92.4 6.9 80.6 3.8 3.5
5.9 89.3 5.7 92.1 17.6 95.3 95.8
2.5 4.3 1.8 1.0 1.8 0.9 0.7
By difference. b fa, aromaticity index, calculated according to ref 13; fa = 1.22 - 0.58H/C.
province, which was formed in a marine-continent interactive sedimentary environment with its peat bog subjected to strong reductive effects during the coalification process, was also investigated. The combustion reactivity of chars obtained from pyrolysis in the fixed-bed reactor was measured with thermogravimetric analysis (TGA) based on a non-isothermal approach. Experimental Section Preparation and Analysis of Samples. The main maceral groups in SD, LW, and PS coals are vitrinite and inertinite, and the inertinite content in HM coal exceeds 90%; therefore, three vitrinite-rich samples (named PSV, SDV, and LWV) and four inertinite-rich samples (named PSI, SDI, LWI, and HMI) were obtained using the sink-float method combined with hand-picking from the three typical WRCs and PS coal.11,12 The samples were pulverized to below 200 mesh (SDV > LWV > PSI > LWI>SDI>HMI. The maximum devolatilization rate is in the order: PSV > SDV > LWV > PSI > SDI > LWI > HMI. The temperature of the maximum devolatilization rate (Tp) is in the order: PSV
