The random pore model with intraparticle diffusion for the description of combustion of char particles derived from mineral- and inertinite rich coal

An investigation was undertaken to determine the applicability of the random pore model with intraparticle diffusion for the determination of the reaction kinetics for the combustion of chars particles derived from coals rich in minerals and inertinites. The char particles which were pyrolysed at 900 °C consisted of a dense carbon-containing fraction originating from the inert macerals (mainly inertinites), a high concentration of minerals and carbominerites, pores generated by the devolatilisation of the reactive macerals and cracks as a result of the presence of minerals. Combustion experimentation was carried out with a thermogravimetric analyser using 1 mm particles with 20% (mole) oxygen in nitrogen at 287.5 kPa and low temperatures (450–600 °C) and with high gas flow rates. The random pore model with intraparticle diffusion (pores and cracks) was solved numerically according to a method consisting of a step-wise regression procedure. This was achieved by using carbon conversion and reduced time relationships to calculate the structural parameter and the initial Thiele modulus followed by determination of an initial lumped reaction rate and validation with conversion versus real time results. The model is characterised by a decreasing Thiele modulus (increasing porosity) occurring during the reaction period which gives rise to a transition to a chemically controlled reaction system. It was found that the initial overall reaction rate was controlled by intraparticle diffusion with an increasing influence with increasing temperature.