Influence of char formation conditions on pressurized black liquor gasification rates Original Research Article

Black liquor is a by-product which results from the pulping of wood in a solution of NaOH and Na2S, and accounts for a significant fraction of the total energy production in papermaking countries. Conventionally, the liquor is concentrated and burned in large recovery boilers, but pressurized gasification of black liquor is a promising alternative. A key concern for pressurized gasification is how char formation conditions affect its resulting reactivity. We studied this and found that prolonged exposure to high temperatures decreases the reactivity by two mechanisms, thermal annealing and, in the presence of CO, deposition of solid carbon. Increased pressure during char formation also resulted in lower gasification rates, with chars formed at 20 bar displaying reactivities less than half that of chars formed at atmospheric pressure. The results also suggest that kinetic behavior observed in previous studies of pressurized black liquor gasification has mistakenly been attributed to reaction kinetics alone, while in fact the char formation technique has influenced its reactivity.