The effect of coal composition on carbon dioxide adsorption

Abstract

Greenhouse gas emissions due to fossil fuel combustion and gasification are recognized as the major contributor to climate changes. South Africa is ranked the 12th highest emitter of carbon dioxide in the world and accounts for 38 % of carbon dioxide emissions in Africa. Carbon dioxide sequestration in unminable coal seams is one of the options available for the reduction of carbon dioxide emissions in South Africa. The adsorption behavior of carbon dioxide on four parent coal samples from major coal fields in South Africa was studied. Heavy media separation was used to concentrate coal macerals and mineral matter. The effect of the coal macerals, mineral matter and coal rank on the carbon dioxide adsorption of the samples was evaluated. The coal samples covered a maturity range from 0.69 % to 2.28 %. The maceral compositions of the fractions varied from 92.8 % to 0.4 % vitrinite and 78.8 % to 0.4 % inertinite. Half of the samples had 0 % liptinite and the maximum liptinite obtained for the rest of the samples was 9.2 %. Mineral matter content ranged between 0.4 % and 93.0 %. Adsorption measurements were performed on 2.0 g samples with a grain size of 300 – 600 μm. The adsorption isotherms were measured at 303-304 K and a pressure range of 250kPa to 1500 kPa using the volumetric method. The volumetric system was specifically designed and constructed for this research work. The adsorption capacity was determined by fitting the data with a Langmuir equation. The model provided very good fits, the relative error between the measured value and the predicted value over the entire pressure range was less than 5.0 % for all the samples. The carbon dioxide adsorption capacity appears to positively correlate to the vitrinite content and negatively correlate to the mineral matter content. There appears to be no relationship between the inertinite content and gas adsorption capacity. The highest adsorption capacity obtained was for the highest rank vitrinite-rich coal sample, and the mineral matter-rich sample had the lowest adsorption capacity.