Environmental Engineering Reference
In-Depth Information
barriers are pretreatment technology, function and cost of hydrolytic enzymes,
mitigation of inhibitors, and fermentation of C6 and C5 sugars [11]. For the ther-
mochemical conversion process, the major technical barriers include understanding
the kinetics of gasification, syngas clean-up techniques, and advanced catalyst
development (selectivity and longevity) for the FT process [16].
5 Technical Details
5.1 Gasification of Lignocellulosic Biomass
5.1.1 Overview
Gasification is a process where carbonaceous feedstocks react with oxygen and
steam at elevated temperatures (500-1500
◦
C) and pressures (up to 33 bar or 480
psi) to yield a mixture of gasses. The mixed-gas product is called synthesis gas or
“syngas,” consisting primarily of hydrogen (H
2
) and carbon monoxide (CO), with
varying amounts of carbon dioxide (CO
2
), water (H
2
O), methane (CH
4
), and other
elements, depending on the feedstock, gasifier type and conditions [21].
5.1.2 Gasification Process
Depending on how heat is generated, gasification technology can be classified as
either directly- or indirectly-heated gasification. For directly-heated gasification,
pyrolysis and gasification reactions are conducted in a single vessel, with heat
arising from feedstock combustion with oxygen. The syngas generated from this
method has low heating values (4-6 MJ/m
3
or
100-140 Btu/ft
3
). For indirectly-
heated gasification, the heat-generating process (combustion of char) is separated
from the pyrolysis and gasification reactions, which generates high heating value
syngas (12-18 MJ/m
3
or
∼
300-400 Btu/ft
3
). Low heating value syngas is usually
used to generate steam or electricity via a boiler or gas turbine, while high heat-
ing value syngas can also be used as a feedstock for subsequent conversion to fuels
and chemicals [22]. According to the flow direction of the feedstock material and
oxidant, gasifiers can basically be classified into five types (Table 2, Fig. 3).
Although a portion of the feedstocks are converted to heat during gasifica-
tion, conversion efficiencies of biomass to syngas are relatively high: e.g. 50-75%
on weight basis [22]. This gasification efficiency is mainly due to the utilization
of lignin and other organic substances, which cannot be used directly in acid or
enzymatic hydrolyzing processes.
∼
5.2 Syngas Generation
Biomass gasification is basically a two-step process, pyrolysis at lower tempera-
ture followed by gasification at a higher temperature. Pyrolysis is an endothermic
process during which the biomass is decomposed into volatile materials (majority)
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