Models for Coal Kinetics

Abstract An important aspect in understanding coal/biomass combustion and gasification is the description of the thermal conversion behavior of the feedstock. Usually, this conversion (neglecting drying) is divided into pyrolysis and char combustion/gasification. The corresponding kinetics are highly feedstock-dependent, and a strong coupling exists to the surrounding gas phase. This work reviews the feedstock kinetics of both pyrolysis and char conversion looking at different levels of modeling complexity. The first step is the thermal decomposition of the large molecular structure during pyrolysis. A variety of coal pyrolysis models have been developed, and different levels will be discussed later in the text. In general, it can be concluded that with increasing complexity, the accurate prediction of the pyrolysis rate, the yield and its composition, the char formation rate, and the evolution of the coal morphology toward the initial char morphology become possible. This will be discussed, especially, for network models building upon the knowledge of the detailed coal structure and directly simulating the development of this structure including bridge fragmentation, metaplast formation, and evaporation. The subsequent process of char conversion is fundamentally different. Reactions of the remaining char (and hydrogen, albeit often neglected) with O2, CO2, and H2O take place on the particle surface. First, the reactant has to be adsorbed, and after the reaction, the product is desorbed. These reactions can take place on the entire particle surface of the porous char particle. Depending on the temperature and the gas composition, either the outer film diffusion, the pore diffusion, or the surface kinetics can be rate-limiting, illustrating that it is very important to consider the coupling between the heterogeneous reactions and the convective and diffusive transport processes. The kinetics strongly depend not only on the feedstock and the operating conditions (as in pyrolysis) but also on the preceding pyrolysis process, since the initial char structure and morphology and the initial reactivity are determined by them. Though pyrolysis and char conversion tend to occur sequentially in laboratory-scale environments, there is significant concurrency of these processes at higher temperatures. The model from Milan Technical University discussed in this article is a unified model of coupling pyrolysis and char conversion. Some of the noted models have been extended to biomass applications, but the scarcity of useful categorizations of biomass feedstocks analogous to, for example, coal rank is one of several impediments to progress toward kinetics modeling capabilities, comparable to those achieved for coal