Environmental Engineering Reference
In-Depth Information
pressure control. The heat exchangers are subjected to high heat transfer rates at
high temperatures, but must maintain precise temperature control. The reaction
vessel requires accurate temperature, pressure, and flow control. The vessel must
seal reliably and be leak free each time it is used.
3.2.3
Chemical Pretreatment of Biomass
3.2.3.1
Acid Hydrolysis
Acid pretreatment of lignocellulosic biomass can generally be performed using
two approaches: either in concentrated acids at relatively lower temperatures or in
diluted acids at relatively higher temperatures. The overall goal for acid pretreat-
ment is to solubilize hemicellulose (primarily in the form of xylose), break the
lignin seal and decrease the crystallinity of cellulose so that the treated biomass is
more susceptible to subsequent enzymatic hydrolysis.
For concentrated acid pretreatment, concentrated sulfuric acid (H
2
SO
4
) of 72%,
hydrochloric acid (HCl) of 41%, or trifluoroacetic acid (TFA) of 100% can be
used [24]. During the treatment, the milled raw materials (e.g. corncob, switch
grass, or bagasse) are treated with pre-heated concentrated acid in a bioreactor
under 100°C. Since concentrated acids can efficiently remove lignin and hemicel-
lulose as well as hydrolyzing some cellulose into glucose; a variety of biomass
such as wood can be used as feedstock. However, the process also requires a mas-
sive use of concentrated acids which is highly corrosive. Acid recovery and the use
of special bioreactors are therefore necessary to make the process economically
feasible [25]. On the other hand, dilute acid (<4 wt%) hydrolysis is regarded as an
efficient and inexpensive pretreatment method [26, 27]. Lu
et al.
[28] reported the
pretreatment of corn stover with 2% H
2
SO
4
at 120°C for 43 min giving a maximum
xylose yield of 77%. The study indicated that the majority of hemicellulose is
degraded in the pretreatment. In addition to H
2
SO
4
, the use of other dilute acids,
including nitric, hydrochloric and phosphoric acids have also been reported [29].
Figure 3.3 depicts a pilot-scale reactor for acid hydrolysis of corn stover [30].
First, biomass is fed to the hopper and it is ground into small particles. Dilute acid
is then sprayed on the ground biomass. Finally, it is sent to the pretreatment
reactor for incubation at 150-200°C for 3-20 min in order to solubilize the
hemicellulose into sugars, while cellulose and lignin remains unaffected during
the process. At the end, the acid-pretreated biomass is recovered by filtration.
The major drawback of acid pretreatment is that the by-products formed during
the treatment may inhibit the downstream fermentation process, potentially causing
a reduction in product yield. These inhibitors can be categorized as organic acids,
furan derivatives, and phenolic compounds, which are derived from degradation of
hemicellulose, cellulose, and lignin (Figure 3.4) [31, 32]. If the concentration of
these by-products is too high, detoxification procedures for the removal of these by-
products might be required.
