Showing papers by "Thomas H. Fletcher published in 1994"

Impact of coal pyrolysis on combustion

Abstract: The pyrolysis process has impacts throughout coal combustion. The roles of pyrolysis in various aspectsof the coal combustion process are described, including the devolatilization yield, nitrogen release, softening and swelling, soot formation, and char reactivity. These processes can be understood and quantitatively predicted using recently developed network pyrolysis models that describe the transformation of the coal's chemical structure. The models are described and examples of their predictive ability for important coal combustion phenomena are presented.

Devolatilization Rate Processes and Products

Abstract: Pyrolysis (devolatilization in an inert atmosphere) and hydropyrolysis (devolatilization in a hydrogen atmosphere) of coal are particularly dependent on the organic properties and structural characteristics of the coal. Many approaches are taken to characterize coal conversion and reaction processes. The most advanced of these approaches relate the observed coal conversion processes to coal structure and composition. Much evidence supports the hypothesis that the devolatilization of coal is a chemical reaction (e. g., Gavalas, 1982). With an increased availability of coal structure and characteristics by advanced methods as documented in Chapter 4, a monumental task lies in understanding the devolatilization mechanisms, relating coal structure to its devolatilization behavior, and developing accurate predictive capabilities. New devolatilization models are being developed which are based, at least in part, on the chemical structure of coals. Devolatilization, as the first step in thermally driven coal conversion and utilization processes, has a profound effect on course of combustion processes (Howard, 1981; Brewster et al., 1988; Nelson et al., 1988). Previous studies have demonstrated the key role of devolatilization processes in coal conversion and utilization through the use of comprehensive combustion codes (J. D. Smith et al., 1987, 1991; Brewster et al., 1988; Smith and Smith, 1990). In parametric sensitivity studies of a two-dimensional combustion model, J. D. Smith et al. (1987, 1991; Smith and Smith, 1990) showed that one of the most critical issues or critical subprocesses in the combustion code for both coal combustion and gasification is the devolatilization process.

Char Oxidation, Conversion, and Reaction Rate Processes

Abstract: The heterogeneous carbon oxidation and char gasification step is the second process to occur in the utilization of coal, and proceeds simultaneously or after devolatilization, depending on reaction conditions (Saito et al., 1991). The time required for the combustion of a char particle can be several orders of magnitude larger than that for devolatilization, ranging from 30 ms to over an hour, and is often the rate-determining step in the overall combustion of pulverized fuels (Essenhigh, 1981; I. Smith, 1982; Smoot and Smith, 1985). The processes of char oxidation are no less complex than those of devolatilization. The chemical structure of the coal does not control the reaction processes to the same extent as devolatilization, but, due to the high temperatures generally associated with char oxidation, pore diffusion and external diffusion often play a pronounced role. Thus, the physical structure of coal, including pore structure, surface area, particle size, and inorganic content, is important in understanding and modeling char oxidation processes. Intrinsic reactivity of char refers to the chemical reaction on the pore walls, after diffusion of gas through the pores inside the char. When intrinsic reactivity is the ratecontrolling process, oxygen migrates toward the center of the char particle, the particle size remains nearly constant during combustion, and the particle density decreases with conversion. However, if the reaction is fast, typically at high temperature, oxygen diffusion is the dominant process; the oxygen is consumed as it reaches the particle surface, and the density of the particle is near constant while the particle size decreases in a shrinking core mode.

Geochemistry and Macromolecular Structure of Coal

Abstract: Coal is a complex sedimentary rock derived from plant remains that underwent peatification and subsequent coalification. Coals originated as peat deposits formed in prehistoric swamps through the accumulation of plant substances whose components underwent differing degrees of chemical decomposition and polymerization. The most uniform microscopic constituents of coal, whose morphologically preserved or repolymerized materials retain distinct characteristics, are known as macerais. A brief study of the organic geochemistry of coal and its macerai characteristics will be helpful in clarifying the nature of coal structure and will give a basis for the understanding of coal structural characteristics. Given (1984b) states that even though the organic chemistry of coal has been studied for more than 100 years, the geological factors in coal chemistry have been almost totally ignored. According to Given (1984b), it is the geoscience of coals, or their geochemistry of origin and postburial geological history, that is responsible for the fact that coals are a very diverse set of materials. Consequently, it is only a study based on geological origins of coal chemistry that will enable coal researchers to rationalize the diversity in coal and to express this in a systematic manner.