Modeling of decomposition and sulphation of oil shale carbonates on the basis of natural limestone

Abstract

Estonian oil shale is characterized by a high content of inorganic matter, calcium carbonate making up 57-75% of the total inorganic part. At combustion temperature the carbonates decompose, and the formed free oxides bind at pulverized firing about 80-85% of SO2 formed during combustion already in the boiler. At fluidized-bed combustion the conditions for SO 2 binding are close to optimum, and nearly full capture of SO 2 is expectable. Modeling of oxide formation and SO2 binding was the task of the present work. In order to obtain data for mathematical modeling of the decomposition and sulphation processes of oil shale carbonaceous part, series of thermogravimetric experiments accompanied by X-ray and SEM analysis, specific surface and porosity measurements were carried out with some natural limestone samples. Mathematically, the calcination process of limestone samples was satisfactorily described using a random pore distribution model, assuming that only chemical reaction limited the overall decomposition process. An extended unreacted shrinking core model with variable (conversion-dependent) effective diffusivity was applied to the sulphation data. From model calculations the kinetic parameters of the decomposition and binding reactions were determined.