Direct Causticizing for Pressurized Black Liquor Gasification, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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Direct causticizing with titanates has already been shown to hold great promise as an alternative to the costly lime cycle when used in conjunction with black liquor gasification (BLG). It has been proven in the lab to produce high conversions of carbonate to hydroxide in situ during BLG, and has been shown to be economically competitive with the use of conventional lime kilns with BLG. Also worth noting is that the causticizing load increases when switching from a recovery boiler to a gasifier; therefore an existing lime kiln may no longer be adequate and require upgrades. Direct causticizing would be another alternative to the resolving the capacity issue. Direct causticizing chemistry is cyclic. Sodium trititanate reacts with sodium carbonate in the gasifier to produce sodium pentatitanate, thus tying up sodium and allowing carbonate to form CO₂. When the smelt leaves the gasifier it is hydrolyzed (leached) where the pentatitanate returns to trititanate and sodium hydroxide is released into solution to form white liquor directly. Previous work has shown the process to achieve high yields of hydroxide and suitable white liquor with respect to pH and sulfidity. The problem is that CO2 inhibits the reaction in the gasifier, limiting the process to about 2 bars total pressure for oxygen-blown BLG. However the economics of BLG favor pressures of 20-30 bar. In this work we explored ways of increasing the pressure limitation as much as possible. A process scheme was developed wherein we achieved suitably high causticizing yields at 10 bars pressure, which is a substantial improvement but fell short of our target pressure. The economics of the process, which included an acid wash to purge NPE’s, were evaluated relative to a reference case of using BLG with an existing lime kiln. The key factor in the economics was the price of natural gas or fuel oil to fire the kiln. At $8/MMBtu, the NPV was $10MM with IRR of 17%. However at $14/MMBtu, the NPV rose to $62MM with IRR of $46%.