Page 103 - Materials Chemistry, Second Edition

Life-Cycle Assessment of Biomethane 89

4 Scenarios to Increase Sustainability of Biomethane
from Lignocellulosic Biomass

4.1 The Potential of CO 2 and GHG savings

A large portion of biogas consists of CO 2 (40–50 %), which is removed during
biogas upgrading to achieve enriched biomethane as a transport fuel. The range of
CO 2 removal during upgrading is 1.62–1.86 kg CO 2 m -3 (Power and Murphy
2009). This CO 2 removal is an additional source of GHG emission and thus can be
minimized by its use in the AD (Fig. 7). Using CO 2 as a pretreatment option to
accelerate the hydrolysis of cellulose (one of the major components in lignocel-
lulosic biomass) is observed and described by Zheng et al. (1995), 1998 and Clark
et al. (2006). The cellulose crystallinity, lignin sealing and cross-linkage of
hemicellulose around cellulose are barriers in the attachment of enzymes and
microbes to the cellulosic surfaces (Nizami et al. 2009; Fan et al. 1987). This is an
issue that impacts the efficiency of lignocellulosic biomass undergoing cellulose
hydrolysis (Kim and Hong 2001). The use of CO 2 as a pretreatment option in the
AD process is preferred due to less expensive, clean, less energy demanding, easy
to recover in a nontoxic manner and nonflammable properties in comparison with
the physical, chemical, thermal, and steam explosion pretreatments (Chahal et al.
1981; Zheng et al. 1995; Kim and Hong 2000, 2001). The CO 2 can be used in two
different forms: first in an explosive form at a high pressure where it disrupts the

Fig. 7 The CO 2 movement
through various subsystems
involved in the
lignocellulosic biomethane
system CO 2

CH 4

CO 2
Lignocellulosic Biomass CO 2

Biogas
production
Biofertilizer

Biomethane