Biomedical Engineering Reference
In-Depth Information
4. They have a large specific volume.
5. These materials are cheap and currently are considered as waste.
These characteristics indicate that natural lignocelluloses have a great potential to
solve some current problems, such as food shortages, energy crises, environmental
pollution, and other issues facing the world. However, they also make the biocon-
version of lignocellulose difficult.
Multiform utilization modes for straw exist in China, including utilization as
energy, fertilizer, industrial raw materials, and substrates for edible mushrooms.
Actually, the unitization mode of straw is differed from the economic development
degree and industrial structure. According to field investigation and document
analysis, in 2006, it is estimated that the main crop straw used for the consumption
of straw as energy for rural residents, for direct application, as a feed for livestock,
as a raw material for papermaking, as a substrate for edible mushrooms, and burned
or as waste was about 108, 130 (containing the part not been collected), 79, 20,
10, and 86 million tons, respectively, accounting for 25, 30, 18, 4.6, 2.3, and 20 %
of the theoretical amount of resources, respectively [
7
]. Thus, straw in China is
mainly used as fuel or burned directly in the field. A high percentage of waste and a
low percentage of use as industrial raw materials exists. This industry structure not
only destroys the ecological balance, sterilizes the soil, causes a vicious agricultural
cycle, but also results in serious environmental pollution and the potential for a
fire disaster. Meanwhile, the low utilization ratio of straw burned for heat (below
10 %) is also a great waste of resources. Biodecomposition and transformation are
effective for the high-value utilization of natural lignocellulose. They have great
practical significance for solving environmental pollution, food shortages, and the
energy crisis.
1.2.1
Composition Complexity of Lignocellulose
Cellulose, hemicellulose, and lignin constitute the main components of plant cell
walls [
8
]. They are the major components of natural lignocellulose. The sum of
these three components accounted for 80 % of the total weight of raw materials;
the contents of cellulose, hemicellulose, and lignin are 30-35, 25-30, and 10 %,
respectively. In addition to the three main components, lignocellulose contains
protein, lipid, ash, water, pectin, low molecular weight carbohydrate, and other
items. Ash content of straw is generally more than 5 % (up to 15 % of rice straw),
and most of the ash is silica. The content of crude protein is low, in the range of
2-8 %, and the lipid content accounts for 1-2 % of straw. In mature straw, almost
all vitamins are destroyed.
Cellulose is a macromolecular compound with the formula (C
6
H
10
O
5
)
n
,awater-
insoluble homogeneous polysaccharide consisting of a linear chain of several
hundred to over 10,000
(1-4)-linked
D
-glucose units. Cellulose molecules often
exist in the state of aggregation by arranging together. The structure of cellulose
usually includes a crystalline region and an amorphous region. As for the crystalline
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