Energy Fuels 2009, 23, 5322–5330 Published on Web 10/26/2009
: DOI:10.1021/ef900306w
Experimental Study of Combustion Characteristics of Bituminous Char Derived under Mild Pyrolysis Conditions Chunmei Shen,†,‡ Weigang Lin,‡ Shaohua Wu,*,† Xiaobo Tong,‡ and Wenli Song‡ †
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China, and State Key Laboratory of Multi-phase Complex System, The Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China
‡
Received April 7, 2009. Revised Manuscript Received July 31, 2009
The combustion characteristics of chars derived from a bituminous coal (Datung, also denoted as DT) at different pyrolysis temperatures were studied via simultaneous thermal analysis (STA). The chars were obtained at pyrolysis temperatures of 550, 650, 750, and 850 °C. The carbon structure of chars and the parent coal were characterized using X-ray diffraction. Pore structures of chars and parent coal were measured by nitrogen adsorption. Based on the thermogravimetric analysis results, the ignition behavior and reactivity of the chars and the parent coal, as well as those of Yangquan (YQ) anthracite, were compared. The ignition temperature (Ti) and weighted mean apparent activation energy (Em) were obtained and used to evaluate reactivity. The results show that the ignition type of the bituminous parent coal and chars derived at 550, 650, and 750 °C is predominately hetero-homogeneous, whereas that of char derived at 850 °C and the YQ anthracite is heterogeneous. Thermal deactivation was observed under the pyrolysis conditions in this work. The reactivity of all chars is lower than that of the parent coal but higher than that of Yangquan anthracite. Moreover, the char that was derived at 650 °C has the highest reactivity among all chars. Carbon structure ordering contributes more to the thermal deactivation at high temperatures than at low temperatures. Changes in pore structure and material composition affect the reactivity for chars derived from both low and high temperatures.
(CFB) system.2-4 In the system, the return leg acts as a pyrolyzer, from which volatile matters are released by mixing coal with a hot recirculating solid in the system. The remaining char goes back to the combustor (riser), together with the recirculating solid. The released volatile matters are separated with solid particles and quenched to obtain liquid and gaseous products. Such processes may be competitive technically and economically. However, the system presently can only applied in a CFB system, which limits its application to wider consumers of coal. By considering the fact that the majority of coal consumed in the power industry is burned in pulverized mode, the idea of combining the coal topping process with a pulverized coal boiler was proposed, from which the pulverized coal is pyrolyzed first to obtain a certain amount of liquid and gaseous products. The char will be injected to the burners and combusted in the pulverized coal furnace. However, the combustion behaviors of the char will govern the performance of the pulverized burner and furnace when a pulverized coal combustor is combined with the coal topping process. Unlike the CFB system, pulverized coal combustors are more sensitive to the type of coal.5 Thus, it is important to obtain a basic knowledge of how the pyrolysis conditions affect the char combustion characteristics, such as ignition temperature and combustion reactivity.
1. Introduction Coal is the major energy resource in China, which accounts for about three-quarters of the total energy consumption. Such an energy profile is caused by the distribution of the energy reserves, especially the fossil fuel reserves in China: this country is abundant in coal and limited in natural gas and petroleum. Thus, reasonable utilization of coal is a challenge. Coal is a mixture of organic compounds and a small amount of inorganic ingredients. During the thermal conversion, the volatile matters in coal will first release, which are mainly composed of CO, CO2, and hydrocarbons. The resources of high-volatile coal (bituminous coal and lignite) in China comprise up to 85% of the total coal reserves. The highvolatile coals may be utilized to extract aromatic hydrocarbons such as tar as liquid products under mild conditions before being further combusted for power generation by a hybrid process. The so-called “coal topping” process is such a process, which was proposed at the Institute of Process Engineering, Chinese Academy of Sciences (IPE, CAS).1 The system has been realized in a circulating fluidized bed *To whom correspondence should be addressed. Tel.: þ86-451-86412628. Fax: þ86-451-8641-2528. E-mail address:
[email protected]. (1) Kwauk, M. S. Coal Topping Process. In Selected Papers of the 9th Member Forum of Academia Sinica; Academic Press: Beijing, 1998; pp 202-204. (2) 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 (3), 463–470. (3) Yao, J. Z.; Wang, X. Q.; Lin, W. G., Coal Topping in a Fluidized Bed System. In The 16th International Conference on Fluidized Bed Combustion, Reno, NV, 2001. r 2009 American Chemical Society
(4) Wang, J. G.; Wang, X. Q.; Yao, J. Z., Coal Topping Process and its Prelimiminary Experiments. In Proceedings of the 7th China-Japan Symposium on Coal and Chemistry, 2001; pp 185-188. (5) Li, Z. Q.; Yang, L. B.; Qiu, P. H.; Sun, R.; Chen, Z. C.; Sun, S. Z. Experimental study of the combustion efficiency and formation of NOx in an industrial pulverized coal combustor. Int. J. Energy Res. 2004, 28 (6), 511–520.
5322
pubs.acs.org/EF
Energy Fuels 2009, 23, 5322–5330
: DOI:10.1021/ef900306w
Shen et al.
During pulverized coal combustion, combustion can be divided into two stages: (i) volatile release and combustion and (ii) char combustion. Devolatilization and volatile combustion is fast, compared to the char combustion. Thus, the characteristics of the char combustion play an important role in burnout processes. Extensive investigations have been conducted to determine the combustion reactivity of chars derived from high-temperature pyrolysis.6-13 With the development of coal poly-generation technologies, the reactivity of chars derived under relevant conditions was studied.14-16 The previous studies show that char reactivity is very sensitive to the pyrolysis conditions. In addition, chars derived from different pyrolysis conditions have significant differences, in physical and chemical properties.9-12 The pore structure evolution and the structural ordering of the carbon on a molecular level occur during pyrolysis. These two factors have a significant effect on the reactivity of the chars.6,8,9,12,13 Our previous work on the coal topping process was mainly focused on flash pyrolysis mechanisms of coal and the product distribution under different pyrolysis conditions.2-4,17-19 The combustion characteristics of the chars derived from the coal topping process have yet to be studied. The object of this work is to study the ignition and combustion characteristics of chars derived from the coal topping process under different pyrolysis temperatures to answer the following questions: (1) What is the difference between the parent coal and the char in combustion characteristics? (2) What are the reasons that cause such differences?
Figure 1. Schematic drawing of the pyrolysis apparatus.
(3) Is it possible for chars to have higher reactivity than anthracite, which may provide an opportunity for the coal topping process to combine with an anthracite-fired pulverized-coal combustor. The combustion of fuels is normally characterized by the ignition temperature and the combustion reactivity. Various experimental techniques were applied to determine the ignition temperature of coal. The coal ignition type was classified into three types: homogeneous ignition, heterogeneous ignition, and hetero-homogeneous ignition.20,21 In the present work, the ignition temperature of the coal and the derived chars, as well as ignition type and reactivity, are determined from the results obtained in simultaneous thermal analysis. The physical and chemical characteristics of the coal and chars, such as pore structure and carbon structure of chars, are measured and used to interpret the difference in reactivity.
(6) Lu, L. M.; Kong, C. H.; Sahajwalla, V.; Harris, D. Char structural ordering during pyrolysis and combustion and its influence on char reactivity. Fuel 2002, 81 (9), 1215–1225. (7) Alonso, M. J. G.; Borrego, A. G.; Alvarez, D.; Menendez, R. A reactivity study of chars obtained at different temperatures in relation to their petrographic characteristics. Fuel Process. Technol. 2001, 69 (3), 257–272. (8) Cai, H. Y.; J., G. A.; Chatzakis, I. N.; et al. Combustion Reactivity and Morphological Change in Coal Chars: Effect of Pyrolysis Temperature, Heating Rate and Pressure. Fuel 1996, 75 (1), 15–24. (9) Wells, W. F.; Soot, L. D. Relation between reactivity and structure for coals and chars. Fuel 1991, 70, 454–458. (10) Gale, T. K.; Bartholomew, C. H.; Fletcher, T. H. Effect of pyrolysis heating rate on intrinsic reactivities of coal chars. Energy Fuels 1996, 10, 766–775. (11) Shim, H.; Hurt, R. Thermal annealing of chars from diverse organic precursors under combustion-like conditions. Energy Fuels 2000, 14 (2), 340–348. (12) Russeell, N. V.; Gibbins, J. R.; Williamson, J. Structural ordering in high temperature coal chars and the effect on reactivity. Fuel 1999, 78, 803–807. (13) Best, P. E.; Solomon, P. R.; Serio, M. A.; et al. The Relationship between Char Reactivity and Physical Chemical Structural Features. Am. Chem. Soc., Div. Fuel Chem. 1987, 32, 44–51. (14) Chen, X. P.; Gu, X. B.; Duan, Y. F.; Zhao, C. S.; Wu, X. Investigation on the combustion characteristics of semi-coke at elevated pressure. J. Eng. Thermophys. 2004, 25 (2), 345–347. (15) Sheng, H. Z.; Liu, D. F.; Wei, X. L.; Huang, N. Characteristics of semi-cokes - the solid residues from coal partial gasification. J. Combust. Sci. Technol. 2004, 10 (2), 187–191. (16) Shen, S. Q.; Li, S. F.; Shi, Y. Experimental invenstigation on the ignition and combustion of semi-coke particles. J. Combust. Sci. Technol. 2000, 6 (1), 66–69. (17) Cui, L. J.; Song, W. L.; Zhang, J. Y.; Yao, J. Z.; Lin, W. G. Influence of the Gas and particle residence time on fast pyrolysis of lignite. Trans. ASME 2007, 129, 152–158. (18) 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. Chin. Univ. 2006, 22 (1), 103–110. (19) Cui, L. J.; Yao, J. Z.; Lin, W. G.; Zhang, Z., Product distribution from flash pyrolysis of coal in a fast fluidized bed. In Proceedings of the 17th International Conference on Fluidized Bed Combustion, Jacksonville, FL, May 18-21, 2003. (CD-ROM)
2. Experimental Section 2.1. Char Preparation. Datung (DT) coal, which is a bituminous coal, was used in the present study. The coal was milled to a particle size of
