Adsorption of Carbon Dioxide on Coals Titrated to Different pH Values

摘要

The effect of acidic or basic pre-treatment on the adsorption capacity of CO_2 on a coal was investigated. Argonne premium Pittsburgh No.8, Upper Freeport, and Pocahontas No.3 coals were washed in weak solutions of H_2SO_4 and NaOH to the pH values of 10, 7, and 2, after an initial washing in acidic water. Equilibration took several days in some cases, although the as -received coal had been ground to 150 mm. Acid washing preferentially removed Ca (calcite) and Mg. Aluminosilicate clays were not significantly removed. Iron was removed in significant amounts only after base treatment, possibly after it was converted to hematite. The adsorption capacity of CO_2 on the acid-treated coal was higher than both the base-treated and as - received Pocahontas coal. The accessible pore volume was larger in CO_2 than in He. The accessible volume was larger by 35 percent for acid -treated, 6 percent for base-treated, 30 percent for neutralized, and 10 percent for as-received Pocahontas No.3 coal when compared to the volume estimated by the helium expansion method. The accessible pore volume was the highest for the acid-treated coal. It is concluded that the positively charged layered structure of coal matrix swells more than the negatively charged structure. The increase in the accessible pore volume resulted in an increase in the adsorption capacity of acid-treated coal.