Novel Design of an Integrated Pulp Mill Biorefinery for the ...

2.2.1 Low-Temperature Black Liquor Gasification
Low temperature gasification occurs in indirectly heated fluidized beds at about
610°C[35]. Sodium carbonate particles comprise the fluidized bed for black liquor steam
reforming at low temperatures. Steam injected at the bottom of the vessel fluidizes the bed
particles and also provides a source of water needed for steam reforming[34]. Black liquor is
also introduced at the bottom of the vessel through a nozzle system. In a separate refractorylined
combustion chamber, a fuel is burned in a pulse combustion mode to produce hot
combustion gas. Pulsed heater bed tubes carry the hot gas to the bed, transferring heat through
the tube walls to the bed material where reforming occurs. The steam reacts endothermically
with the black liquor char to produce medium BTU synthesis gas with about 65% hydrogen[35].
The condensed phase material leaves the reactor as a dry solid; the sodium exits a sodium
carbonate. Bed solids are continuously removed and mixed with water to form a sodium
carbonate solution [35].
A gasification process that has been demonstrated for low temperature black liquor
gasification has been developed by Manufacturing and Technology Conversion International
(MTCI) and marketed by their ThermoChem Recovery International (TRI) subsidiary[34]. The
process can be used to produce syngas either for heat and power or for fuels. Full-scale
operation has been successful at two plants in North America, but these demonstration facilities
treat black liquor from a pulp mill using non- kraft processes. This black liquor is much lower in
sulfur than kraft black liquor, creating less severe conditions for the structural materials to
endure[35].
There are some advantages to using a low-temperature gasification system. In general,
lower temperature gasifiers is carried out in a less severe environment, which can help to reduce
problems with materials in the gasifier. The biomass pretreatment requirements are also not as
rigorous. This is more important for gasification of extra biomass than for black liquor
gasification, because black liquor gasification does not require extensive pretreatment in
entrained flow reactors (the alternative to low-temperature gasification). Publication materials
from ThermoChem Recovery Incorporated (TRI) claim that advantages to their low-temperature
system include a flexibility in the desired product, since the H2/CO ratio can be varied from 8:1
to 2:1, and flexibility in the feedstock, since the gasifier can handle virtually any type of biomass
[35]. Finally, the combustion chamber that burns fuel for indirect heating of the bed is fuel
flexible; it can burn natural gas, No. 2 fuel oil, pulverized coal, recycled product gas, etc.[35].
Despite these advantages and the less harsh environment that is kept at lower
temperatures, problems with materials still exist, especially for the refractory lining of the pulse
combustor chamber[36]. There is a high temperature environment (1300-1500 °C) on the inside
of the chamber, while on the outer surface materials interact with H2, H2S, steam, and movement
of particles in the reactor bed, also causing stress [36]. Another disadvantage is that low
temperature reactions allow tar formation, which must be destroyed with a high temperature
oxygen-blown tar-cracker at ~1300°C[37]. Cracking of tars is preferred over use of a catalyst to
eliminate tars because tars contain potential CO and H2[38]. At temperatures above 1200 °C,
tars are cracked with oxygen and steam (acting as a selective oxidant); no catalyst is needed [38].
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