Determination of the kinetic models and associated parameters for the low temperature combustion and gasification of high-ash coal chars

Abstract

South Africa has large coal reserves and the power generation industry produces approximately 95% of the electricity from coal. Most of the high-grade coal is exported leaving behind a discard of high ash coal. For the power generation industry to sustain itself, some means of processing the high ash coal should be implemented. A fluidised bed gasification process is seen as the best alternative to conventional pulverised coal combustion process since it can handle a wide variety of feedstocks at low temperatures. The reaction kinetics becomes important for a gasifier design that can handle high ash feedstocks. This study is concerned with the determination of reaction kinetics of high ash South African chars suitable for the development of fluidised bed combustors and gasifiers. Combustion and gasification studies of two South African high ash chars (48% ash of 70 pm and 67% ash of 20 pm diameter) were carried out isothermally and non-isothermally in an atmospheric thermogravimetric analyser. In modelling the combustion experiments, it was found that the reaction mechanism follows the shrinking unreacted core model with surface reaction. This was attributed to the low porosities of both chars. Despite the high ash contents, the kinetics of the two samples were found to match well those of low ash chars. This was explained in terms of the mineral-carbon association in chars observed during detailed characterisation. A two-step regression method capable of evaluating parameters under non-isothermal experiments has also been developed. This method gave plausible results when compared with another non-isothermal method and was capable of predicting reaction kinetics under isothermal conditions. Gasification experiments conducted on the same chars with carbon dioxide and steam were described using a reaction controlled Langmuir-Hinshelwood rate equation. Reactivity profiles have been obtained in the temperature range from 800 to 950 OC at different steam and COz partial pressures. The shrinking unreacted core model with surface reaction was also found to provide the best fits for both chars. The obtained parameters match well with those of other chars with lower ash contents indicating that most of the carbon occurs with no mineral association for both chars investigated. Finally experiments were carried out under synthesis mixtures of CO/CO2/H2/H20 at equilibrium conditions and showed that the overall rate of reaction with CO2 and H20 proceed over different carbon sites.