Page 261 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Sweet sorghum: a potential resource for bioenergy production 231

(Appels et al., 2011). Nitrogen-enriched slurry obtained after fermentation can be
used as soil conditioner/fertilizer in agriculture (Tambone et al., 2009). As biogas
technology is a well-established process, it is easy to adopt in any level of scale in
field. This helps in developing nations and rural places, where the energy require-
ment is not able to match demand and supply. Any organic matter, including dung
from cattle, agricultural residues, weeds from water bodies can be used for biogas
production to satisfy the energy requirements in rural households (Appels et al.,
2011; Tambone et al., 2009).
Fig. 10.7 represents the calculated and net energy from the biogas of different
feedstocks per annum. Sweet sorghum is a high-energy crop because of its photo-
synthesis efficiency and biomass yield per hectare area from its hybrids also
encouraging at all regions. It is mostly grown for using it in production of bioetha-
nol by crushing the sugar-rich stalks as a replacement to sugarcane in distilleries
and even the bagasse; residues left over after extraction of juice are also exploited
for bioethanol production. But anaerobic digestion is a simple process compared to
bioethanol fermentation process, hence becoming more attractive and valuable for
energy production (Martinez-Perez et al., 2007).
The utilization of sorghum stem for biogas production has been reported by many
researchers (Antonopoulou et al., 2012). Organosolv pretreatment (50% ethanol and
1% sulfuric acid) was carried out on sweet sorghum stalks, and after evaporating eth-
anol, the residues were subjected to anaerobic digestion for biogas production in
batch reactors and resulted in 270% more methane yield than the untreated stalks
(Ostovareh et al., 2015). Antonopoulou et al. (2012) used acidified sorghum extract
from a biohydrogen reactor and used it as a substrate for methane production in a
methanogenic reactor. The maximum methane yield obtained at 10 days was 35.2 L/kg
sweet sorghum biomass and methane production rate was 1.76 L/day. Coble et al.
observed that hay production from sweet sorghum is not a feasible option to use in
anaerobic digesters as 55.3% dry matter and 56.1% volatile solids losses were recorded
after 5.6 months of storage (Coble and Egg, 1987). Sambusiti et al. observed that there
was no significant different in methane yield on five varieties of sorghum on anaerobic
digestion of sodium hydroxide pretreated sorghum but increase in first order kinetic
constant, increased lignin removal with increase in alkali dosage. Even though ethanol
and methane are produced from sweet sorghum, it can also be used for production of
hydrogen gas, which is a clean gas with higher calorific value (Sambusiti et al., 2013).
Antonopoulou et al. studied the production of hydrogen and methane from sweet sor-
ghum in a two-stage process. Maximum hydrogen yield obtained from first stage was
10 L H 2 /kg of sweet sorghum biomass and the effluent of hydrogen reactor was
extracted for solid residues and leachates. Both were subjected to biomethanation and
yielded around 107 L CH 4 /kg sweet sorghum and around 78 L was achieved from solid
residues (Antonopoulou et al., 2008).

10.4.3 Biochar

Biochar is obtained through thermochemical conversion process, namely, pyrolysis,
gasification, torrefaction, and hydrothermal carbonization, where the lignocellulosic