Rapid hydropyrolysis of softening coal particles—A modeling study. Part I: Model formulation and parameter values

Abstract

A mathematical model is developed for rapid hydropyrolysis of softening coal particles. It is aimed to represent previously measured effects of pressure and particle size on product yields. The model includes a global chemical reaction scheme combined with diffusional transport of hydrogen dependent on the transient physical properties of the coal. Model predictions are compared with experimental data in a companion paper. The objective of this study was to define and formulate a mathematical model for rapid hydropyrolysis of softening coal particles consistent with the drastic changes in physical properties associated with softening and resolidification. Rapid hydropyrolysis, where coal particles are heated at rates of 100 to 10,000 K/s under high Hz pressure, is of interest not only as a process for converting coal into CHcrich gas and liquids but also in coal gasification with steam where significant partial pressures of Hz occur. Previous experimental studies (Anthony et al., 1976;Suuberg et al., 1980) indicate that the coal's transient fluidity plays an important role with respect to transport mechanisms involved. For example, a particle size effect on integral product yields had been found with Pittsburgh Seam bituminous coal, a strongly caking-i.e., softening-coal, but not with a lignite which retains a porous structure throughout the experiment. Previous modeling work has either concentrated on hydropyrolysis of coals which retain their porosity (Russel et al., 1979), or-in the case of softening coals-on pyrolysis, i.e., thermal decomposition in the absence of Hz. The resulting models therefore could not adequate represent the experimental results with softening coals. GEORG SCHAUBThis modeling study takes advantage of kinetic information from previous investigations of coal hydrogasification and pyrolysis. The study addresses the questions of intra-and extraparticle transport limitations for hydrogen and develops a tentative description of the coal's transient fluidity and its implication for hydrogen transport. The model leads to predictions of particle size and pressure effects which are compared with trends previously measured in a screen heater apparatus. CONCLUSIONS AND SIGNIFICANCEA mechanistic concept has been formulated for the chemical reactions and mass transport processes involved in rapid hydropyrolysis of softening coal particles. A global chemical reaction scheme borrowed from previous coal pyrolysis and hydrogasification studies is combined with a concept of intraparticle hydrogen transport, seen as diffusion of either hydrogen dissolved in the coal melt during the plastic period or of gas phase hydrogen in a reconstituted pore matrix after resolidification. The transient coal fluidity is expressed in terms of appearance and disappearance of a plasticizing material called metaplast, which is generated during the coal's pyrolysis reactions. Transport of metaplast away from the coal particle to Ceorg Schauh is currently with Lurgi GmhH, 6ooo Frankfurt 11, West Germany yield tar is modeled as...

Review of 30 Years of Research Using the Chemical Percolation Devolatilization Model

“…The small species formed in the metaplast then have a high enough vapor pressure to be released as tar. Some of the original data for coal hydropyrolysis was obtained by Anthony et al and Suuberg et al with subsequent development of a hydropyrolysis model . Recently, Guan et al developed a simple modification to the CPD model to describe coal hydropyrolysis.…”

“…Some of the original data for coal hydropyrolysis was obtained by Anthony et al 194 and Suuberg et al 195 with subsequent development of a hydropyrolysis model. 196 Recently, Guan et al 197 developed a simple modification to the CPD model to describe coal hydropyrolysis. The hydropyrolysis and crosslinking reactions are assumed to both decrease the amount of metaplast in the coal during pyrolysis according to…”

An experimental and modeling study of softening coal pyrolysis

“…Drawing on the work of Van Krevelen (1961) and Neavel ( 1 982) and studies in this laboratory (Schaub et al, 1985a, b;Fong et al, 1985Fong et al, , 1986a, it was concluded that softening occurs when a critical temperature and concentration of a thermally-derived plasticizing agent, known as metaplast, are reached. Some metaplast is assumed to preexist in the coal as a bitumenlike material.…”