Sulfur Distribution During Air Gasification of Kraft Black Liquor Solids in a Fluidized Bed of TIO2 Particles, 1998 International Chemical Recovery Conference Proceedings

The sulfur distribution was measured during atmospheric air gasification and pyrolysis of kraft black liquor solids in a fluidized bed of TiO2 particles at 700 - 900°C. The detected gaseous sulfur species are H2 S, COS, CH3 SH, C3 H7 SH, (CH3 )2 S and (CH3 )2 S2 : H2 S is the major gaseous sulfur species, which is at least one order of magnitude larger than the second major sulfur gas, COS; while C3 H7 SH is the dominant organosulfur species. The present gasification data indicate that the formation of H2 S by the H2 S stripping reaction (conversion of Na2 S by CO, and H2 O) is not important. Therefore, it is suggested that thermal cracking of CH3 SH and (CH3 )2 S is the major route for H2 S generation. The gaseous sulfur yields of H2 S, COS and organosulfur are found to decrease with increasing temperature for both gasification and pyrolysis. The mechanism of C3 H7 SH formation is proposed to be insertion of sulfur vapor, S,, in the propyl side chain of lignin fragments.

In the bed residual solids (BRS), the identified sulfur species are sulfide, thiosulfate, sulfite and sulfate. Sulfide is the major species. The total sulfur content of the BRS for both gasification and pyrolysis increases with increasing temperature, presumably because more H2 S is recaptured by Na2 CO3 , via the reverse of the H2 S stripping reaction.

The total sulfur reduction efficiency, which considers both Na2 S and H2 S, is evaluated for kraft black liquor gasification. Total sulfur reduction efficiencies of about 90% and 80% are conservatively estimated at 750 and 820°C respectively. The total sulfur mass balance closure, and the influence of air ratio and TiO2 particle size on the sulfur distribution are also discussed.