Biomedical Engineering Reference
In-Depth Information
digestion of straw. Thus, crop straws are often used as ''auxiliary material''
instead of ''main material'' in anaerobic digestion process.
3. Straw biogas technology is in its infancy. At present, research on straw biogas
technology in China has just begun, and no straw biogas project has run for
more than 3 years so far. None of these processes, except two-phase anaerobic
digestion technology, has obtained identification and approval from the pro-
vincial science and technology departments. Moreover, there are some tech-
nical, economic barriers and management problems. The total operating
performance of biogas systems is not satisfactory and these processes are still to
be practically certified.
4. Compared to the animal manure biogas projects, straw-based biogas projects
need more work on material collection, storage and pretreatment, which all
increase the cost; the quality and availability of straw varies significantly with
the season, while purchasing straw directly from farmers may lead to poor price
controllability and further raise the cost. The existing straw biogas projects in
China mostly adopt a concentrated biogas supply mode, and their products are
biogas and fermentation residue. As a possible benefit, the biogas can be sold at
a low price to farmers as an energy source for cooking, while the fermentation
residues can be made into organic fertilizer. But the idea of using fermentation
residues as fertilizer has not been completely accepted by farmers, and thus the
residues are basically provided to them for free at the present stage. Besides,
clean production mechanisms have not been introduced, and the project benefit
is still not straightforward. In summary, high investment and running cost, and
low benefit from products, present significant barriers to the commercial
application of straw-based biogas technologies.
5. Straw biogas projects require more support at state policy level. In recent years,
especially since 2008, the Chinese government has put great emphasis on the
construction of biogas projects in rural areas. The investment funds from
central government, however, are mainly used to build straw biogas projects,
including construction and equipment investment, while less has spent on
improving the operational performance and the utilization of the final products.
The poor market profitability has severely weakened the operational perfor-
mance, so some projects are running below capacity or have even stopped
completely.
2.2.3 Livestock Waste
Since the mid-1990s, large-scale livestock farm pollution has gained increasing
attention. Especially in recent years, with rapid economic growth and improve-
ment in living standards, China's livestock industry has also seen a rapid devel-
opment. The number of cattle, pigs, sheep and poultry has shown an upward trend.
According to national statistics, in 2007 the total amount of livestock manure was
2.6 billion tons, and it is expected to reach 41 million tons by 2020, The manure,
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