Numerical study of the influence of heterogeneous kinetics on the carbon consumption by oxidation of a single coal particle

Abstract

This work is devoted to the numerical study of the influence of heterogeneous kinetics on the oxidation rates of a single carbon particle in quiescent and non-quiescent environments. The coal particle is represented by moisture- and ash-free nonporous carbon while the coal rank is implemented using several kinetic rate expressions. The model includes six gaseous chemical species (O2, CO2, CO, H2O, H2, N2). Three heterogeneous reactions are employed (C+O2, C+CO2 and C+H2O), along with two homogeneous semi-global reactions, namely carbon monoxide oxidation and the water–gas shift reaction. Several semi-global reaction rate expressions taken from the literature are utilized. The Navier–Stokes equations coupled with the energy and species conservation equations are used to solve the problem by means of the pseudo-steady state approach. At the surface of the particle, the balance of mass, energy and species concentration is applied including the effect of the Stefan flow and heat loss due to radiation at the surface of the particle. The model and the code used are validated against an analytic two-film model. Good agreement is observed. The numerical simulations performed reveal that the origin of heterogeneous kinetics has a significant effect on the carbon consumption rates of the particle. In particular, the maximal discrepancy between results is achieved in a kinetically controlled regime and is proportional by a factor of 10 in respect to carbon mass flux on the particle surface. Additionally, the influence of the particle Reynolds number referring to the laminar flow regime, the ambient O2 mass fraction and the temperature on the regimes of combustion and gasification is discussed.