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Effect of the Coal Particle Size on Pyrolysis and Char Reactivity for Two Types of Coal and Demineralized Coal Wenkui Zhu,†,‡ Wenli Song,*,† and Weigang Lin† State Key Laboratory of Multi-phase Complex System, the Institute of Process Engineering, and Graduate UniVersity, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed February 26, 2008. ReVised Manuscript ReceiVed April 29, 2008
A better understanding of the influence of particle size on pyrolysis and char reactivity is of crucial importance in optimizing the integrated process combining coal topping (coal fast pyrolysis) with char gasification. Different size fractions of two types of coal and demineralized coal were pyrolyzed in a spouted bed. The resulting chars were characterized by X-ray diffraction, and char reactivity was determined in a thermogravimetric analyzer (TGA). Within the range of particle sizes investigated, an increase in particle size results in an increase of char yield, which may be caused by the secondary reactions of volatile matters inside the coal particles. No significant difference in crystallinity was observed for the chars from different size fractions of the parent coals, while more crystallinity was observed for the chars from demineralized coals, which suggests that minerals in the coal play a role in the reduction of char crystallinity during pyrolysis. Char reactivity for raw coals decreased with the increasing of the particle size. For the demineralized coal samples, the change in char reactivity with the increasing of the particle size was reduced. Both the secondary reactions of volatiles and mineral distribution are believed to cause the influence of coal particle size on char reactivity.
Introduction Coal, as a traditional fossil fuel, will still be the major energy source in the foreseeable future, especially for China. In response to increasing environmental concern, the development of cleaner and more efficient coal conversion technologies is necessary. The multiproduct processing, which integrates different coal conversion technologies, has emerged as a clean and efficient way. In China, the resources of high volatile coal, such as bituminous coals and lignite, account for 75% of the coal reserves. Attention has been paid to recover liquid products in high yields by pyrolysis of these high volatile coals at mild conditions. For this purpose, a so-called “coal-topping” process, which is a flash pyrolysis process, has been developed.1,2 The main aim of the coal-topping process is to extract light oils and precursor fine chemicals by flash pyrolysis of coal before combustion of the residue. When the process is integrated with subsequent gasification of char, the solid fuel could be almost entirely converted to liquid products and gas. This multiproduct processing may supply a cleaner and more efficient method for use of the high volatile coal. In such systems, the influence of size of the coal particles is significant because of the variation of the heat and mass transfer inside coal particles during devolatilization,3 which will affect the product distribution from flash pyrolysis and the character* To whom correspondence should be addressed. Telephone: +86-1082627078. Fax: +86-10-62558065. E-mail:
[email protected]. † State Key Laboratory of Multi-phase Complex System, the Institute of Process Engineering. ‡ Graduate University. (1) Wang, J.; Lu, X.; Yao, J.; Lin, W.; Cui, L. Ind. Eng. Chem. Res. 2005, 44, 463–470. (2) Kwauk, M. S. Coal topping process. In Selected Papers of 9th Member Forum of Academia Sinica; Academic Press: Beijing, China, 1998; pp 202-204. (3) Wutti, R.; Petek, J.; Staudinger, G. Fuel 1996, 75, 843–850.
istics of char. A better understanding of the way the coal particle size affects the pyrolysis and subsequent gasification of char is important for optimizing the overall system. Our previous work revealed that an increase in particle size results in a decrease of liquid product yield of the “coal-topping” process.1,4 A recent work of Cousins et al.5 demonstrated a decreased char reactivity with an increase in coal particle size. However, an investigation of Hanson et al.6 showed that pyrolysis and gasification were relatively insensitive to particle size. These conflicting experimental results may be caused by the difference in coal type and reactor employed as well as the range of particle sizes used. The effect of minerals in the fuel structure on the pyrolysis of the coals and gasification of char (especially for low-ranked coal) has been revealed.7–9 In fact, these minerals in the coal always show a distribution with respect to the size fraction of pulverized coal. From this viewpoint, the effect of particle size may also be associated with the mineral segregation between size fractions of pulverized coal. However, in studying the effect of particle size on the pyrolysis and gasification, little attention was paid with respect to the mineral segregation. In the present paper, chars were prepared from two types of coal with and without demineralization with different size fractions. The reactivity of the chars was investigated to understand the effect of particle size on pyrolysis and char reactivity. Experimental Section Coal Samples. Two types of coal were chosen for this investigation, a bituminous coal (Shangwan) and lignite (Huolinhe). The coal particles were milled to a particle size below 250 µm and then (4) Cui, L.-j.; Lin, W.-g.; Yao, J.-z. Chem. Res. Chin. UniV. 2006, 22, 103–110. (5) Cousins, A.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2006, 20, 2489–2497. (6) Hanson, S.; Patrick, J. W.; Walker, A. Fuel 2002, 81, 531–537.
10.1021/ef800143h CCC: $40.75 2008 American Chemical Society Published on Web 06/19/2008
Coal Particle Size on Pyrolysis and Char ReactiVity
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Table 1. Proximate and Ultimate Analysis of Coal Size Fractions Shangwan (bituminous coal)
a
Huolinhe (lignite)
250-180 µm
180-120 µm
