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

2.2.3 Black Liquor Gasifier Recommendation
The goal of this project is to maximize the production of transportation fuels. Based on
the advantages and disadvantages associated with high- and low-temperature gasification
systems for black liquor and the need for high-quality syngas for fuel production, an entrained
flow gasifier design should be the central focus of integrated pulp mill black liquor gasification
since very little tar is produced and its similarity to coal gasification can be useful in designing
an integrated process for gasification of coal and extra biomass to syngas. Finally, Oak Ridge
National Laboratory has concluded that for pulp mills using the kraft process, high-temperature
high-pressure systems are more efficient[42]. All of these reasons affirm that high-temperature,
high-pressure oxygen-blown entrained flow gasification is the best available option for a black
liquor gasification system.
An average Kraft pulp mill in the U.S. generates about 3420 tons of dry black liquor
solids per day. At this scale, estimations using Chemrec’s high-temperature black liquor
gasification process to synthesize DME predict a fuels yield of about 824 tons/day . An
economically efficient DME plant would ideally generate 1-2 million tons DME per year, which
would require a 3- to 7-fold increase in capacity. Increasing the yield of chemicals and fuels and
exploiting economies of scale calls for additional gasifier feedstock such as extra black liquor,
woody biomass, and/or coal. In areas of the southeast U.S. where many pulp and paper mills are
concentrated it may be possible to import more black liquor. In other locations, importing extra
black liquor and/or biomass results in transportation costs that often reverse the economies of
scale for large-scale gasification of this feedstock. Therefore, coal may be an attractive option
for gasification with black liquor.
To minimize costs, gasification of coal and black liquor should occur in the same reactor.
High-temperature entrained flow gasifiers with similar operating conditions have been designed
separately for coal and black liquor feedstocks. It may be possible to co-gasify black liquor and
coal to produce larger yields of high quality synthesis gas for further processing to Fischer-
Tropsch liquids and/or DME. Several existing designs for black liquor gasification can handle
other types of feedstocks, but to our knowledge, there is no current process design for the cogasification
of coal and black liquor to produce synthesis gas.
2.2.3 Coal Gasification Technology
High-temperature entrained flow gasification produces hot syngas and a molten slag of
fuel mineral matter. Three existing designs are popular for entrained flow coal gasification: the
Shell process, the GE/Texaco, and the Dow/Destec process. The Shell gasifier is a dry-fed
gasifier, while the Texaco and Destec designs are single-stage and two-stage slurry-fed gasifiers,
respectively. This project considered the popular and commercially-available technology of Shell
and Texaco gasifiers for co-gasification of black liquor and syngas. Similar downstream
processes can be used for the Shell and Texaco gasifier designs [43]. A Shell gasifier generates
syngas with a lower moisture content and lower H2/CO ratio than the Texaco design, as shown in
Table 3. A higher amount of moisture in the Texaco gasifier syngas and a greater overall syngas
volume corresponds to greater heat recovery in the downstream syngas cooler [43].
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