Pilot Scale Gasification of Spent Cooking Liquor from Sodium Sulfite Based Delignification

This paper describes a pilot scale high pressure entrained flow gasification experiment with spent cooking liquor from a sodium sulfite based delignification process in the DP-1 black liquor gasifier in Piteå, Sweden. Approximately 92 tons of sulfite thick liquor were gasified during 100 h of operation without any operational problems despite the new feedstock. The syngas quality was found to be good for all operating points with the CH4 content below 0.3% and H2/CO ratio between 1.03 and 1.15. The experiment shows that the process capacity is limited by green liquor quality parameters primarily dependent on the presence of small amounts of unconverted carbon. The pilot plant capacity was found to be somewhat lower than for Kraft black liquor on mass basis but higher when measured as thermal load, due to the higher heating value of sulfite thick liquor. Mass and energy balances were made difficult by the unavailability of measured green liquor and syngas flow rates, which lead to the necessity of using alternative approaches for the estimation of these flows. Using these estimates, overall mass and energy balances were closed to within 5% for all operating points except one, and the process cold gas efficiency was 60–68% on sulfur-free lower heating value basis. Carbon balances indicate that 95–97% of feedstock carbon leaves with the syngas, mainly as CO and CO2 with the remainder being mostly green liquor carbonate. More than 95% of the feedstock sodium is found in green liquor, while 3–5% ends up in the gas condensate purge stream. The sulfur balance does not close as well as other elements but indicates that 70–73% of the feedstock sulfur ends up in the syngas as H2S and COS with the remainder being present in green liquor as dissolved sulfide salts.