The studied BLG systems differ in synthesis gas composition but
mainly in terms of chemical recovery. In conventional recovery
system, sodium and sulfur as molten sodium sulfide and
sodium carbonate are recovered at the bottom of the recovery
boiler. The smelt is then dissolved in weak wash to produce
green liquor. Before it can be reused for delignification, the
causticizing of green liquor is performed to produce calcium
carbonate and sodium hydroxide. However in the CBLG system,
a major part of sulfur in black liquor leaves the gasifier as H2S
and not as sodium sulfide (Na2S). This result in conversion of
only 45% of incoming sulfur in black liquor to Na2S and an
increase in causticizing need to 23% (i.e. increased load on lime
kiln fired with bark compared to conventional recovery system).
Estimated bark to lime kiln in the recovery boiler case is about
95 tonnes DS/day. However, in the CBLG system it is increased
to 132 tonnes DS/day. There is no direct causticization in the
CBLG system due to liquid phase and it is not possible to add
TiO2 to get causticization.