PREPARATION OF ACTIVE CARBONS FROM COAL .2. CARBONIZATION OF OXIDIZEDCOAL

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 unoxi dised coals are very low. The VM content of the chars generally increa ses with an increase in the degree of coal preoxidation but the oxygen content increases only at lower degrees of preoxidation. While the me rcury density of the chars decreases, the helium density increases wit h the degree of coal preoxidation and is related almost linearly to th e helium densities of the oxidised coals. Preoxidation of coal also in fluences the pore size distribution of chars, The pore size distributi on, which is more favourable to macropores in the case of chars obtain ed from unoxidised coal, becomes more and more in favour of micropores as the degree of coal preoxidation is enhanced, The percentage of mic ropores increases from 30% to more than 70% after coal preoxidation. U noxidised coal chars adsorb an insignificant amount of nitrogen at 77 K while an appreciable amount of CO2 is adsorbed at 273 K. The large d ifference between N-2 and CO2 adsorption on chars prepared from coals with low oxidation degree becomes smaller as the degree of coal preoxi dation increases. There is a linear relationship between the total por e volume of the char and that of the corresponding oxidised coal, indi cating that the chars produced by carbonisation of oxidised coal retai n fingerprints of the pore structure of the precursor oxidised coal. ( C) 1997 Elsevier Science B.V.