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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