Environmental Engineering Reference
In-Depth Information
and lignin polymers. Milling also requires long processing times with high capital
and operating costs, thus it is not economical and has not been pursued in scale-up
operations [50, 54]. Radiation pretreatment utilizes gamma rays, electron beams, or
microwaves to react to weaken and break the chemical bonds between hemicellu-
lose and lignin through chemical reactions such as chain scission [55]. However,
the high consumption of energy and capital costs makes this process economically
unviable.
Dilute-acid pretreatment is a chemical process that increases the solubility of
hemicellulose to 80-100%, extensively redistributes the lignin, and depolymerizes
some of the cellulose [53]. The process soaks the biomass in a dilute solution of
sulfuric, hydrochloric, or nitric acid and then raises the temperature by injecting
steam to enhance the pretreatment method [50]. Autohydrolysis generates acids by
the introduction of saturated steams into the biomass to breakdown the hemicellu-
lose and lignin [50]. The pressure is rapidly released resulting in the breakup of the
biomass due to the instant vaporization of the trapped water. This process is known
as steam explosion pretreatment and results in 80-100% solubilization into a mix-
ture of monomers and oligomers of hemicellulose. It also redistributes the lignin,
and depolymerizes some of the cellulose [53]. Similar to steam explosion, ammonia
fiber explosion pretreatment (AFEX) uses high temperature and pressure ammonia
to de-crystallize cellulose, and increase the solubility of lignin by 10-20%, and of
hemicellulose up to 60% while hydrolyzing about 90% to oligomers [53].
Other chemical pretreatment methods include “hydrothermal” processes using
liquid hot water, supercritical carbon dioxide, “organosolv” processes that involve
organic solvents in an aqueous medium, concentrated phosphoric or peracetic acid
treatment, and strong alkali processes using sodium hydroxide or lime [50, 53]. A
biological pretreatment process utilizes fungi, such as white rot, brown rot, and soft
rot, to hydrolyze the cellulose component of biomass. Filamentous fungi, typically
Trichoderma
and
Penicillium
species, can be used directly for cellulose hydroly-
sis because of the greater capacity for extracellular protein production than that of
cellulolytic bacteria [56]. However, it requires a three-fold reduction in cost for com-
mercialization and the reaction rates for the hydrolysis of cellulose are relatively low
in comparison to chemical pretreatment methods [56].
Enzymatic saccharification utilizes enzyme blends for recovering carbohydrates
from the hydrolyzate generated after pretreatment [51]. Commonly, cellulase and
hemicellulase enzymes are used as a “cocktail” with other enzymes to enhance
yields and reduce enzyme costs. The products of enzymatic saccharification - the
process of breaking a complex carbohydrate into its monosaccharide components -
severely inhibit cellulases and hemicellulases [57]. To overcome this difficulty,
Simultaneous Saccharification and Fermentation (SSF) of the pretreated hydrolyzate
is preferred. Once the structure of the biomass is disrupted, the cellulose and
hemicellulose is enzymatically converted to sugars by the saccharification process.
During the fermentation process, yeasts such as
Saccharomyces cerevisiae,
con-
vert the sugars to ethanol. The advantage of SSF over Separate Hydrolysis and
Fermentation (SHF) is higher yields of ethanol but SSF requires more than dou-
ble the fermentation time [58]. However, the hydrolyzate also contains acetic acid
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