Mass and energy balances for black liquor gasification with borate autocausticization

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Gasification of black liquor is considered to be a promising alternative technique for recovery of black liquor in kraft pulping. Compared to the conventional recovery process, i.e. combustion of black liquor in a recovery boiler, the primary advantage of gasification is the potential to produce biofuels and chemicals. The causticizing demand is though higher for the gasification technology. If the existing lime kiln can not handle the extra load, partial borate autocausticizing may be an alternative. In this work, the energy balance for a kraft pulp mill using black liquor gasification and partial borate autocausticization is compared with a mill using gasification and extended lime causticizing and also with a mill having a conventional recovery cycle. Additionally, the chemical costs for the partial borate autocausticizing is compared to the investment cost of the larger lime kiln. Mass and energy balances are based on the updated reference mill in the MISTRA Research Program, "The Eco-Cyclic Pulp Mill", producing 2000 ADt of pulp per day. The results show that less energy is released to steam production in a gasifier than in a recovery boiler and instead energy is released in chemical form in the produced syn gas. Using partial borate autocausticizing, the demand of external energy is lowered in the lime kiln compared to extended lime causticization. The borate autocausticization, taking place in the gasifier with black liquor as energy source, only demands a third of the ene...