Biomedical Engineering Reference
In-Depth Information
sugarcane (sugars). In China, fuel ethanol production reached 1.7 million tons in
2009, mostly using food crops such as corn, wheat, and rice as raw materials.
However, according to China's national conditions, on the one hand, vigorous
development of the fuel ethanol industry is necessary. On the other hand, fuel
ethanol should not compete for food with people and compete for fields with food.
Above all, increased efforts to develop lignocellulosic resources for fuel ethanol
production are of great value.
6.2.1
Use of Lignocellulose to Produce Bioethanol
Use of lignocellulose as a raw material to produce bioethanol can be completed by
both biological conversion and thermochemical conversion methods. Among these,
biological conversion contains a hydrolysis stage of substrates and a subsequent
fermentation stage; thermochemical conversion contains gasification of substrates
and subsequent fermentation or a catalytic reaction. They are respectively described
next.
6.2.1.1
Biological Conversion
Lignocellulose is composed of three main components: cellulose, hemicellulose,
and lignin. They tightly twist around each other. Thus, the structure of lignocellulose
is complex and must be changed into small molecules of sugar so it can be utilized
by microorganisms. Biological conversion of lignocellulose to bioethanol consists
of four major unit operations: pretreatment, hydrolysis, fermentation, and product
separation/distillation.
To increase the yield of hydrolysis, a pretreatment step is needed that softens the
biomass and breaks down cell structures to a large extent. A successful pretreatment
must meet the following requirements: (1) improve formation of sugars or the ability
to subsequently form sugars by hydrolysis; (2) avoid the degradation or loss of
carbohydrate; (3) avoid the formation of by-products inhibitory to the subsequent
hydrolysis and fermentation processes; and (4) be cost effective. There are many
types of processes to pretreat biomass. They are physical methods, physical-
chemical methods, chemical methods, and biological methods [
14
].
Acid hydrolysis and enzymatic hydrolysis are two types of processes to
hydrolyze cellulosic biomass. Acid hydrolysis is also divided into concentrated acid
hydrolysis and dilute acid hydrolysis. The principle of concentrated acid hydrolysis
is that crystalline cellulose can be completely dissolved in 72 % sulfuric acid, 42 %
hydrochloric acid, and 77-83 % phosphoric acid at a low temperature, resulting in
the homogeneous hydrolysis of cellulose. Among them, concentrated sulfuric acid
is used most commonly. The main advantage of concentrated acid hydrolysis is the
high yields (e.g., 90 % of theoretical glucose yield). The principle of dilute acid
hydrolysis is that dilute acid can cause the change of cellulose fine structure (such
