In order for both low and high temperature black liquor gasification technologies to become economically viable alternatives to the conventional recovery cycle, issues relating to causticization must be addressed. Direct causticizing processes, which involve adding a chemical agent to the black liquor that reacts preferentially with the sodium in the gasifier to form a salt and thus allow the carbon to be released as CO_2, offers a potential solution to this problem. Experiments on thermochemical conversion of black liquor in the presence of sodium tri-titanate or Mn_3O_4 were carried out in a pressurized, entrained-flow reactor simulating high temperature gasification conditions and in a fixed bed reactor simulating low temperature gasification. The experimental results showed that a degree of decarbonization up to 65% was obtained for titanate direct causticization during the studied low and high temperature gasification conditions. The experimental results for Mn_3O_4 direct causticization showed that this system is not possible in combination with high temperature black liquor gasification, since no carbonate conversion was obtained in these experiments. For the low temperature gasification of Mn_3O_4 doped black liquor a degree of decarbonization of 45% was obtained.
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