Environmental Engineering Reference
In-Depth Information
and deeper than that of maize. Sorghum plants have a capacity to remain relatively inactive during
drought and renew growth when conditions are favorable (Doggett 1988).
Because sorghum grown for biomass can be harvested before it is fully mature, it is possible
to grow it in a double crop sequence with a winter annual legume. Winter annuals are planted in
the fall, grow rapidly in the spring, and reach harvest anytime during late spring to early summer.
Sorghum, which is well adapted to germination under limited moisture, can be no-till planted into
the stubble of a winter annual crop. The primary advantages of a double crop sequence are to
maximize use of solar radiation, provide winter cover against wind and water erosion, and increase
biomass per hectare yields (Karpenstein-Machan 2001). Because sorghum and winter annuals have
differing cardinal temperatures for growth, the double crop sequence can take advantage of a longer
growing season than either crop alone.
Given that water availability is poised to become a major constraint to agricultural production in
coming years, cultivation of corn becomes difficult. Sweet sorghum would be a logical crop option
in lieu of corn in such situations. Sweet sorghum can be grown with less irrigation, rainfall, and
inputs compared with corn. In addition to sweet stalk, grain yields of 2-6 t/ha (which can be used
as food or feed) could be harvested from sweet sorghum.
A wide range of maturity classes is required to extend the harvest period to meet the requirements
of the processing factories. Sweet sorghum's energy-savings and value emerge in several ways.
• The crop only needs 12-15 in. of rain during the growing season to make a crop; therefore,
it is suitable for dry-land production or under limited irrigation. If the crop receives more
moisture, it will respond positively.
• It requires only 40-60 lb of nitrogen per acre. The crop is long-rooted and can extract
residual nitrogen left by previous crops or from nitrogen-fixing soybeans proceeding in
rotation.
• Sweet sorghum juice does not require the long fermentation and cooking time needed to
process corn ethanol.
• Some of the crop residue left after juice extraction (called bagasse) can be dried and burned
for fuel ethanol distillation. These residues can also be used for animal feed, paper, or fuel
pellets.
• The crop need not be grown on a farmer's best land which allows farmers to make use of
poorer ground.
• The simplicity of ethanol production from sweet sorghum could lend itself to on-farm or
small-cooperative efforts at fuel-making.
• Ethanol plants in the state could choose, with some additional equipment, to make sea-
sonal runs of sweet sorghum juice.
19.6 comParIson to other BIoenerGy croPs
Sorghum is relatively inexpensive to grow to obtain high yields (Chiaramonti et al. 2004). It can
produce approximately 30-50 dry Mg/ha of biomass per year on low-quality soils with minimal
inputs—fertilizer and water per dry ton of crop. When compared with the input requirements of
other crops, sorghum requires half of those needed by sugarbeets, and one-third of the requirements
of sugarcane or corn (Soltani and Almodares 1994; Renewable Energy World 2000). Sweet sorghum
juice is ideally suited for ethanol production given its higher content of reducing sugars compared
with the content of other sources, including sugarcane juice. These important characteristics, along
with suitability for seed propagation, mechanized crop production, and ethanol production capacity
comparable to sugarcane molasses and sugarcane, make sweet sorghum a viable alternative raw
material source for ethanol production. Furthermore, after the extraction of juice, sorghum stover can
be used as a fuel for ethanol distillation. The remaining stover can be used as fodder for animals or for
additional ethanol production through lignocellulose digestion. It can out-produce most other cereals
