Mechanism and kinetics of sodium release from brown coal char particles during combustion

A model for the release of sodium during the combustion of single Loy Yang brown coal char particles is presented. The model is combined with further analysis of recently published measurements of the release of sodium from single brown coal particles burning in a flat flame to estimate the rate constant for sodium release as a function of burnout time for these experiments. A char combustion and heat transfer model is also used to predict the char burnout behaviour and surface temperature of the particle as a function of time during combustion for each of the experiments. By combining the predicted time–temperature history of the particles with the estimated rate constant for sodium release, an Arrhenius expression for the release of sodium during char combustion is determined as:kNa=105.2±0.4exp(-214kJ/mol/RTc)A full mechanism for sodium release during the various stages of coal combustion is also proposed. Utilising the proposed mechanism, the rate-determining step for sodium release during char combustion is proposed to be the formation of a reduced form of sodium in the char which subsequently leads to the rapid loss of sodium from the particle.