Fate of carbon-containing compounds from gasification of kraft black liquor with subsequent catalytic conditioning of condensable organics

The effects of temperature, residence time, and gas environment on the evolution of carbon-containing products during gasification of black liquor are studied using a laminar entrained-flow reactor along with a basic study on the catalytic destruction of a model tar compound. The results suggest that the char residue yields depend mainly on the temperature and are not sensitive to heating rate. The effect of gasifying gases on char carbon and fixed carbon was significant at temperatures between 800–1000°C. Black liquor composition and gas atmosphere have little impact on the evolution of light gases during secondary reactions at 700–800°C but become more significant at higher temperature. Carbonaceous material from tar is converted to light gases through secondary reactions. The water-gas shift reaction may have a major role on the ratio of CO to CO₂ in the gas phase during secondary reactions at longer residence times and higher reactor temperatures.; Characterization of black liquor tars suggests that kraft lignin is the main source of these compounds. Low temperatures favor the gradual formation of diversified substituted aromatics. Higher temperatures result in more rapid formation then increased decomposition at longer residence times of these species. Non-substituted aromatics are more stable at higher temperatures and may be formed from these substituted aromatics. Oxidizing gases enhance both the formation and destruction of tars depending on the temperature and residence time. A network based model is successfully applied to predict the tar yield from black liquor pyrolysis. The model is an a priori simulation in which the input kraft lignin parameters are obtained from different resources. The predicted results agree very well with the experimental data. Catalytic tar destruction results indicated that supported Group VIII metal catalysts are active catalysts. However, deactivation by sulfur was observed for all the catalysts tested but at different rates. The source of catalytic deactivation was the formation of sulfate/sulfide metal on catalyst surface. There was no significant effect of adding Na₂CO₃ into the catalyst bed, while addition of Na₂S greatly reduced the catalytic activity.