Role of pH on CO2 sequestration in coal seams

The effect of acidic or basic pre-treatment on the adsorption capacity of CO2 on coals was investigated. Argonne Premium Pocahontas No. 3, Upper Freeport, Pittsburgh No. 8, Lewiston–Stockton, Blind Canyon, Illinois No. 6, Wyodak, and Beulah–Zap coals were washed in weak solutions of H2SO4 and NaOH to the pH values of 10, 7, and 2, after an initial washing in acidic water. Attempts to treat the Wyodak and Beulah–Zap coals were unsuccessful because the base treatment after the initial acid treatment resulted in a suspension which could be separated neither via filtration through a 45 μm filter nor centrifugation. Equilibration took several days in some cases, although the as-received coal had been ground to 150 μm. Acid washing preferentially removed Ca (calcite) and Mg. Aluminosilicate clays were not notably removed. Iron was removed in significant amounts only after base treatment, possibly after it was converted to hematite. The adsorption capacity of CO2 on the acid treated coals was higher than both the base treated and untreated coals. The difference in adsorption capacities for acid and base treated coals was related to the pore sizes and mineral matter removal from the coals, where the calculated average pore size was higher for acid treated coals than for the base treated coals. It is concluded that the pH decrease due to CO2 dissolution in cleat water is favored in coal seam sequestration, which resulted in an increase in storage capacity of coals.