Page 98 - Materials Chemistry, Second Edition

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

The environmental impact as a result of different operations is expressed in g CO 2
equivalent (CO 2 eq.) MJ -1 energy replaced.

2.3 System Boundaries

The system boundaries are determined initially by the goal and scope of the study.
They are further linked with energy inputs and outputs of unit processes, where all
of the direct and indirect emissions from agriculture, transportation and process are
calibrated (Singh et al. 2010a). The system boundaries for the GHG emission of
biomethane produced from lignocellulosic biomass are examined from cradle to
grave. The production of the lignocellulosic biomass is the cradle and enriched
biomethane as a transport biofuel is the grave. The EU directive 2009/28/EC,
Annex V, C-13 states that ‘‘… emissions from the fuel in use shall be taken to be
zero for biofuels and bioliquids’’ (EC 2009). Thus, emissions from biomethane
combustion (often taking place in vehicles) are not considered in LCA studies
(Korres et al. 2010, 2011).

2.4 Impact Category

To determine the potential impact of GHG emissions of CO 2 ,N 2 O, CH 4 , the term
GWP is used. GWP is defined as the collective outcome between the present
instant and a certain time in the future resulted in a unit mass of gas released in the
present (Casey and Holden 2005; Korres et al. 2010). A GWP of one (1) refers to
the release of 1 kg CO 2 (Korres et al. 2011). According to EC (2009), the GWP of
NO 2 and CH 4 on one (1) kg basis is 296 and 23, respectively. The following
formula is used to calculate the volume of GHG emission in terms of CO 2
(EC 2009).
GHG (t of CO 2 eq.) = CO 2 (t) ? 23 9 CH 4 (t) ? 296 9 N 2 O (t).

3 LCA of Biomethane from Lignocellulosic Biomass

3.1 Sustainability Criteria and Energy Efficiency

The energy efficiency of a biofuel source is determined by considering all energy
inputs and outputs over the entire product production cycle (Salter and Banks
2009). For example, biodiesel production in Europe is accomplished using rape
seed oil that covers about 80 % of the land set aside for nonfood energy crops
(Bauen 2005). Similarly, the rape seed biodiesel and wheat bioethanol both yield