Environmental Engineering Reference
In-Depth Information
19.5 sWeet sorGhum as a BIoenerGy croP
Sweet sorghum yields high levels of fermentable sugars, together with grain and lignocellulose.
Hunter and Anderson (1997) indicated that the sugar produced in sweet sorghum has a potential
ethanol yield up to 8000 L/ha, or about twice the ethanol yield potential of maize grain. In addition
to producing large amounts of sugar-rich biomass, hybrids can be developed from crosses between
grain-type seed parents and sweet-type pollen parents (Hunter and Anderson 1997). The product of
these crosses typically increase biomass yields and sugar content when compared with the original
grain-type seed parents. Such hybrids can co-produce grain at levels approaching the yields of the
grain-type seed parent (Miller and McBee 1993).
Sweet sorghum has already been identified as a preferred biomass crop for fermentation into
methanol and ethanol fuel. Its processing takes ethanol and its derivatives from the direct fermentation
of sugars present in the stem juices, followed by processing of the bagasse (the remaining part of
the stems after juice extraction), to pyrolytic oils, quality fuels, pellets of carbon, synthesis gas,
and lignocellulosic materials. An alternative energetic application of the bagasse may be electricity
production through combustion of total biomass. In addition, the stillage from sweet sorghum, after
extraction of juice, has a higher biological value than the bagasse from sugarcane when used as
fodder for animals because of higher levels of micronutrients and minerals. It is also processed as
a feed for ruminant animals. Apart from these, the stillage contains levels of cellulose similar to
those in sugarcane bagasse, suggesting that it has good potential as a raw material for pulp products.
Sweet sorghum offers several important advantages for bioenergy; remarkable drought tolerance,
productivity as a hybrid crop, amenability to genetic transformation, and extensive genomic
resources available for its study. Because of its high yield in biomass and fermentable sugars, sweet
sorghum can be converted into energy carriers through one of two pathways: biochemical and
thermochemical. Through biochemical processes, the crop sugars can be converted to biofuels
(ethanol, butanol, and hydrogen). Thermochemical processes such as combustion and gasification
can be used for the conversion of the sweet sorghum bagasse to heat and electricity (Table 19.1).
Pulp for paper, compost, and composites materials are some other products that can also be derived
from sweet sorghum bagasse.
Sorghum's adaptation to drought stress allows it to grow in some of the world's less favorable
climates. Morphological and physiological responses under drought stress make this plant unique
among the cereals. Sorghum's capacity to fold (rather than roll) its leaves and the deposition of a
heavy layer of wax over the leaves reduce evapotranspiration. The root system of sorghum is bigger
taBle 19.1
chemical analysis, Burning Profile, Biomass Production, Potential ethanol, and total land
needed for different Bioenergy crops to reach the 35 Billion Gal u.s. reachable Goal
total
Biomass
(mg/ha)
total land to
Produce
35 Billion Gal
hhv
(Btu/lb)
ethanol
(gal/ha)
crop
carbon
nitrogen
sulfur
ash
Tropical maize
49.00
0.97
0.09
4.78
8058
35.0
3500
10.0 a
Silage corn
47.42
0.58
0.05
6.84
7718
11.0
1100
31.4
Bioenergy millet
49.18
0.56
0.08
4.02
8161
36.0
3600
9.7
SS M 81 E
46.97
0.35
0.08
4.50
8031
35.0
3500
10.0
Switch grass
42.00
0.59
0.10
7.09
7590
10.4
1040
33.7
Corn stover
43.60
0.83
0.09
6.90
7782
7.4
740
47.2
Miscanthus
41.00
0.39
0.09
5.65
7750
28.0
2800
12.5
SS, sweet sorghum; HHV, high heating value.
a Million hectares.
 
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