Presentation Of A Dry Black Liquor Gasification Process With Direct Caustisization, 2004 International Chemical Recovery Conference

The main characteristics for this new BLG process is, that black liquor is injected into a circulating fluidized bed, with sodium titanate (Na2O.(TiO2)3) as the fluidizing medium. The organics are gasified by mainly steam reforming, as the temperature is relatively low (below 870 oC) and the water content of the black liquor relatively high (> 15 % water). The black liquor inorganics consist of mainly sodium carbonate and sodium sulphate. In the reactor, sodium carbonate is converted into Na2O.TiO2, which dissolved in water after removed from the bed, gives 4 NaOH + Na2O.(TiO2)3. After dewatering the titanate is reinjected into the fluidized bed. The sulphate is reduced to first Na2S, and then most of the sulphur is evaporated as H2S( Hydrogen sulphide), after reaction with CO2 + H2O.The hydrogen sulphide is absorbed in a scrubber from the exhaust gas, after filtering away dust. Sodium hydroxide and sodium carbonate is the absorbent, and the process equipment is designed in such a way, that H2S is selectively absorbed from CO2, giving a 1 M NaHS solution. The sulphide rich solution is used for preimpregnation of the wood chips, to give modified cooking, giving stronger and brighter fibres out from the digester. The NaOH from the leached bed material, with low sulphide content, is used in later stages of the cooking, and also in the bleach plant. The direct caustisization makes the lime kiln unnecessary. The gas is reheated after the dust removal, and then combusted in a gas turbine /steam turbine cycle, to produce 2-3 times more electricity than in existing steam cycles with conventional recovery boilers, that is an electrical efficiency of 30-35 % compared to typically around 9 - 14 %.