Insight into effects of pore diffusion on smoldering kinetics of coal using a 4-step chemical reaction model

Pore diffusion plays a crucial role in coal smoldering fires, however, its effects have been inadequately investigated. This paper aims to study influences of pore diffusion of oxygen (O2) on chemical reaction and apparent reaction rate of coal smoldering. Non-isothermal thermogravimetric (TG) tests for two bituminous coal samples at three heating rates (2, 5 and 10 K/min) were conducted, and a 4-step decomposition scheme was constructed. The decomposition schemes contained two competing paths: Coal oxidation → β-Char oxidation → Ash (Path One), and Pyrolysis → α-Char oxidation → Ash (Path Two). A number of kinetic triplets (reaction order, pre-exponential factor and activation energy) and stoichiometric coefficients of the developed decomposition schemes were optimized by a Genetic Algorithm (GA) tool. Emphasis was put on influences of pore diffusion on chemical reactions (e.g. pyrolysis, coal matrix's and chars' oxidation) and on competing paths. Research results show that Knudsen diffusion plays a dominant role in pore diffusion that retards coal oxidation and its successive char oxidation on the Path One, but basically enhances char oxidation on the Path Two. Pore diffusion may exert ignorable influence on pyrolysis as it takes place without O2. The investigation is beneficial for understanding mechanism of smoldering of coal piles and coal dust layers.