Page 104 - Materials Chemistry, Second Edition

90 A.-S. Nizami and I. M. Ismail

cellulose structure and second in a dissolved form where it forms carbonic acid.
Carbonic acid is a weak acid that dissolves hemicellulose without toxicity and
corrosivity to the AD process. These processes result in porous cellulosic surfaces,
which are easily accessible to enzymatic and microbial activity (Zheng et al. 1998;
Kim and Hong 1999, 2000, 2001). Thus, the use of CO 2 in biogas production has
untapped potential that will not only enhance the efficiency of the process with
reduced economic and energy requirements but will also decrease GHG emissions
tremendously. Nevertheless, the use of CO 2 as a pretreatment option is limited to
the ethanol industry in a supercritical CO 2 explosion form, where it increases
glucose yield by 50 % and overall ethanol yield by 70 % (Zheng et al. 1998).

4.2 Digester Configurations and GHG Savings

Continuous stirred tank reactors (CSTR) are widely used to digest slurries and
represent a simple and robust technology (Smyth et al. 2009; Mähnert et al. 2005).
The addition of a separate preprocessing tank with chopper pump, screw-feeder,
and flushing system (Weiland 2003) does increase the energy demand when using
for lignocellulosic biomass. Therefore, the values for GHG emission of CSTRs
will be higher than other digester configuration such as a dry batch digester or
leach beds digester coupled with an up flow anaerobic sludge blanket (UASB)
reactor (Nizami et al. 2009). In dry batch, leach beds, and UASB, there is less
requirement for mechanical or electrical feeding and mixing (Köttner 2002; Niz-
ami and Murphy 2010). This comparison of anaerobic digester configurations will
assist developers and farmers in selecting digester types and digester processes
suitable to digest lignocellulosic biomass with least GHG emissions.

4.3 Biogas Losses and Engine Efficiency

On average, the rate of biogas loss from AD to enriched biomethane production is
7.41 %, which accounts for indirect GHG emissions between 8.44 and
8.86 kg CO 2 m -3 of biogas (Power and Murphy 2009). Nizami et al. (2009),
suggested a closed-loop monitoring system equipped with sensory devices for
anaerobic digester (Nizami et al. 2009). The application of nanotechnology to
identify, monitor, and record these losses using sensory chips and devices is at the
infancy stage in the scientific community. Moreover, comparing the GHG emis-
sions of various digester configurations will assist the development of different
component of the digester as we attempt to reduce energy loss. Above all, vehicle
engines must be improved, as existing engines are less efficient in utilizing bi-
omethane and greater in their release of GHG (Power and Murphy 2009).
According to Korres et al. (2010), an improvement of 18 % can be achieved by the
improvements in engine efficiency to a similar km MJ -1 as diesel.