Page 284 - Synthetic Fuels Handbook
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270 CHAPTER NINE
is normally grown for molasses production. Forage sorghum has been selected for high
yields of reasonably good quality animal feed. Sorghum varieties producing tall plants
with large stems make the best candidates for biomass production. Both sweet and forage
sorghum have a high potential for lodging. Lodging can result in harvest problems with
ensuing loss of yield from both initial and ratoon crops.
Sweet sorghums produce sugar syrups which could form the basis of fermentation pro-
cesses for methane or ethanol production and some of the forage types of the plant may be
suitable for biomass production.
9.1.9 Hemp
Hemp is an annual short day, C3 plant with a high cellulose and lignin content in its stems
and a high fat and protein content in its seeds. The entire plant consisting of bast fibers,
leaves, seeds, and processable remains can be used as a solid fuel when compacted. The
stalk contains a very strong and durable fiber. The average height is 7 ft but it can reach up
to 12 ft. For the energetic use the whole hemp crop is harvested.
Hemp can be easily planted and cultivated but its growing is prohibited in many coun-
tries as it can be used as a drug. The plants vegetative period is about 100 days, with the
main growth period in summer months followed by flowering in toward the end of summer.
The yields are high but the high plant moisture content can create storage problems. Two
different harvesting technologies are available for the energy use of the whole hemp crop.
The first is the whole-fiber-technology where, with a modified chopping technology,
the hemp culm is cut into 24- to 24-in lengths. The straw stays for 4 weeks on the field for
drying before the bales are pressed. Another technology is wet harvesting. This technology
includes the chopping of the crops followed by silaging. For combustion it is necessary to
press the hemp silage into bales to reduce the water content of the fuel.
9.2 PROCESSES
There are four basic types of processes used to generate energy from crops: direct combus-
tion, gasification, pyrolysis, and anaerobic digestion.
9.2.1 Direct Combustion
Direct combustion involves burning the energy crop and then using the resulting hot com-
bustion gases to raise steam. The steam is, in turn, used to drive a steam turbine which
drives a generator to produce electricity. The conversion efficiency from energy crop to
energy is fairly low, especially for small systems, but this is balanced by the relatively low
capital cost of direct combustion systems and the fact that the technology is tried and tested.
Furthermore, using the waste heat produces much better efficiencies and economics.
Combustion facilities can burn many types of biomass fuel, including wood, agricul-
tural residues, wood pulping liquor, municipal solid waste (MSW), and refuse-derived fuel.
Combustion technologies convert biomass fuels into several forms of useful energy for
commercial or industrial uses: hot air, hot water, steam, and electricity.
A furnace is the simplest combustion technology. In a furnace, biomass fuel burns in
a combustion chamber, converting biomass into heat energy. As the biomass burns, hot
gases are released. These hot gases contain about 85 percent of the fuel’s potential energy.
Commercial and industrial facilities use furnaces for heat either directly or indirectly
through a heat exchanger in the form of hot air or water.