Preparation of active carbons from coal Part II. Carbonisation of oxidised coal

Abstract

Four coals differing in origin, volatile matter (VM) content, plastic properties and degree of preoxidation have been carbonised in nitrogen up to 1123 K. VM and oxygen contents of the chars obtained from unoxidised coals are very low. The VM content of the chars generally increases with an increase in the degree of coal preoxidation but the oxygen content increases only at lower degrees of preoxidation. While the mercury density of the chars decreases, the helium density increases with the degree of coal preoxidation and is related almost linearly to the helium densities of the oxidised coals. Preoxidation of coal also influences the pore size distribution of chars. The pore size distribution, which is more favourable to macropores in the case of chars obtained from unoxidised coal, becomes more and more in favour of micropores as the degree of coal preoxidation is enhanced. The percentage of micropores increases from 30% to more than 70% after coal preoxidation. Unoxidised coal chars adsorb an insignificant amount of nitrogen at 77 K while an appreciable amount of CO 2 is adsorbed at 273 K. The large difference between N 2 and CO 2 adsorption on chars prepared from coals with low oxidation degree becomes smaller as the degree of coal preoxidation increases. There is a linear relationship between the total pore volume of the char and that of the corresponding oxidised coal, indicating that the chars produced by carbonisation of oxidised coal retain fingerprints of the pore structure of the precursor oxidised coal.