Environmental Engineering Reference
In-Depth Information
glucose, and galactose. Because of its branched structure, it is relatively easy to
hydrolyze hemicellulose for fermentation processes.
Lignin is one of the most resistant materials on Earth and cannot be used in
fermentation processes. It is present in all biomass to a variable extent, depending
on its origin.
Grain and woody materials are examples of biomass with somewhat variable
compositions. For example, switchgrass contains 33% cellulose/glucan, 22%
xylan, and 18% lignin while aspen contains 53% cellulose/glucan, 19% hemicel-
lulose, and 19% lignin [28]. Aspen biomass has a slightly higher content of sugars,
making it valuable for ethanol production. Glucose and xylose are the main fer-
mentable sugars in all lignocellusosic materials, from rice to hardwoods. The only
exception is softwood, which is also relatively rich in 6-carbon sugar mannose. In
addition, pine has a high content of lignin, which can be used for pellet production.
Grasses meanwhile are relatively poor in lignin [29]. These differences in composi-
tion all impose different requirements on any potential ethanol production process.
It was shown by Huang
et al.
[28] that aspen could sustain a higher production of
ethanol than switchgrass or hybrid poplar. It was also pointed out that aspen could
contribute to wood chip production [30]. Poplar has also recently become a feed-
stock of interest because its genome has been sequenced and it can be genetically
engineered to have a sugar content that is more favorable for ethanol production.
7.3.5.2
Microbiological Ethanol Process: Pretreatment
The production of ethanol requires the pretreatment of biomass before fermenta-
tion. This may involve physical, chemical, or biological processing. Physical pre-
treatments include communition (dry, wet, or vibratory ball milling), irradiation
(electron beam irradiation or microwave heating), steaming, or hydrothermolysis
[28]. Chemical pretreatments include treatment with acids, alkaline substances,
solvents, ammonia, sulfur dioxide, or other chemicals. Biological pretreatments
involve the addition of decomposing microorganisms or enzymes. This makes
them environmentally friendlier than chemical or physical alternatives. However,
to date they have also proven to be slower and thus less economically viable.
Interestingly, however, the pretreatment of cellulose with certain enzymes can give
relatively high yields of ethanol. Depending on the source of the biomass to be
used for ethanol production, there is a wide range of different enzymes that can be
used for such pretreatments. In addition, the biomass must be prepared before it
can be pretreated with enzymes. This first step of pretreatment is often physical,
such as milling. This exposes the cellulose to the enzymes that will degrade it.
Cellulose is the main constituent of biomass from forests. Targeted enzymes are
selective for the conversion of cellulose to glucose, and therefore produce no
degradation by-products that may be encountered when using acid conversion
technology. For example, the Canadian company Iogen Ltd. initially subjects its
woody raw materials to a modified steam explosion process and then treats them
