Chemical reduction of sulphur dioxide to free sulphur with lignite and coal. 2. Kinetics and proposed mechanism

Abstract

Abstract The reactivity of lignite and different ranks of coal with sulphur dioxide has been investigated in a corrosive-gas, thermogravimetric reactor system. With all coals, the reaction occurred in two distinct stages. A rapid initial stage was controlled primarily by the devolatilization rate of the coal. The second stage limited the overall rate and was controlled by surface properties of the coal char. The portion of lignite associated with the second stage of reaction exhibited a much higher rate of SO 2 reduction than the corresponding material from all other coals. Correlation of the data showed an inverse relation between the reactivity of coal chars and the relative rank of the parent coal. Activation energies associated with the reduction of SO 2 by the coal chars increased slightly from 134 kJ mol −1 for lignite char to 150 kJ mol −1 for HVB bituminous coal char. The higher reactivity of lignite or lower-rank coals was due in part to entropy factors or available catalytic sites on the surface of coal. Formation of a thermally stable CS complex on the surface of coal appeared to poison the surface and thus limit further reaction. Alkali and alkaline earth metals in lignite served as active sites for catalysing the reaction of SO 2 with the CS complex and thus enhanced the rate of SO 2 reduction with lignite.