In the Kraft recovery unit, the black liquor passes first through a
set of multiple-effect evaporators, in which it is concentrated from
15% to about 70–75% of solids to become an adequate fuel. Before
entering into the boiler, this liquor is generally mixed with the sodium sulfate to adjust the inorganic ion contents. The Kraft recovery boiler works as a chemical reactor (producing Na2S(l) and
Na2CO3(l)), as a steam generator (using the heat of combustion of
organic materials to produce vapor) and also as a residue incinerator. The molten inorganic salts produced (Na2S(l) and Na2CO3(l))
flow from the furnace (lower boiler region) to the dissolving tank,
in which they are mixed with the weak white liquor to generate
the green liquor. This green liquor is clarified and causticized to recycle the calcium carbonate and to regenerate the white liquor,
which returns to the process [1]. Although, in the soda recovery
unit, the bamboo black liquor can be concentrated and burned,
its high viscosity limits the solid concentration up to 50%, witch
makes the bamboo black liquor recovery more difficult to perform