ACTIVATED CARBONS FROM CO2 PARTIAL GASIFICATION OF EUCALYPTUS KRAFT LIGNIN

The behavior of eucalyptus kraft lignin as activated carbon precursor is investigated. The development of porosity upon CO2 partial gasification at 1073 and 1123 K of low-temperature precarbonized kraft lignin has been studied. Increase of micropore volume and broadening of micropore size distribution takes place as activation proceeds. At intermediate burn-off, corresponding to an overall yield of about 25%, a substantial development of mesoporosity is already observed. Macroporosity becomes important at higher activation degrees. The development of apparent surface area compares favorably, in an overall yield basis, with the reported for lignocellulosic materials upon physical activation. In the case of kraft lignin, the evolution of porosity shows a different pattern with a higher contribution of meso and macroporosity. The presence of S in the activated carbons is negligible, and the final ash content is easily reduced up to below 2% by simply washing with dilute H2SO4 aqueous solution. The reactivity curves for CO2 gasification of low-temperature precarbonized kraft lignin have been obtained at different temperatures within the 1023-1173 K range. The reactivity values are indicative of significant catalytic effect which can be attributed to the presence of finely distributed Na, the major metallic component of the inorganic impurities. This effect becomes more relevant at high conversion values where a higher Na/C atomic ratio is reached. No saturation is observed at least up to X approximately 0.95, most probably as the result of the aforementioned fine distribution of Na. Apparent activation energy values in the range of 212-239 kJ/mol have been obtained, the lower values corresponding to higher conversions.