Page 243 - Biofuels for a More Sustainable Future

218 Biofuels for a More Sustainable Future

For a given system that does not experience mass flow, exergy can be
defined as the maximum amount of work that can be extracted reversibly
from an energy flow with respect to a dead state (Beccali et al., 2003; Gokcen
and Reddy, 1996).
In this study, the selected functional unit is 1MJ of exergy and the energy
and environmental impacts of the examined supply chain are referred to this
functional unit considering the exergy production during the whole life
cycle of the plant, calculated as described in the next section.
The selection of the system boundaries is based on the “zero burden”
assumption: the production of biomass waste (agricultural waste and animal
dejections) in input to the plant does not cause any energy or environmental
impact. In detail, the system boundaries include
– The production of the plant components, including the raw materials,
energy sources, and materials supply.
– The operation of the plant, including the transport of the biomass waste to
the plant, the biogas production, the electricity and thermal/refrigeration
energy production from the biogas combustion, and the end of life of the
digestate.
– The end of life of the plant that includes the recycling of steel, aluminum,
and plastics and the landfilling of the other components.
The energy and environmental impact categories selected to calculate the
performance of the investigated functional unit are presented in Table 7.1.

Table 7.1 Energy and environmental impact categories
Impact category Acronym Unit of measure
Cumulative energy demand CED MJ
Acidification potential AP Mole of H+ eq
Ecotoxicity for aquatic fresh water E FW CTUe
Freshwater eutrophication FE kg P eq
Human toxicity cancer effects HT c CTUh
Human toxicity noncancer effects HT nc CTUh
Ionizing radiation IR kg U235 eq
Global warming potential GWP kg CO 2eq
Marine eutrophication ME kg N eq
Terrestrial eutrophication TE Mole of N eq
Ozone depletion OD kg CFC 11eq
Particulate matter PM kgPM2.5 eq
Photochemical ozone formation POF kg NMVOC
Resource depletion, fossil, and mineral RD kg Sb eq
Total freshwater consumption TFC UBP