Environmental Engineering Reference
In-Depth Information
When the biogas process feeds on an energy crop substrate, the energy
input and possible emissions of nitrous oxide (N
2
O) from the soil must be
considered. At present, there are no uniform standards for reliable
comparisons. Despite that, however, assumptions of possible N
2
O emissions
in connection with the cultivation of energy plants are included in the life
cycle assessment.
In the traditional process of biogas production in Europe today, energy
yields in the form of electricity and heat must also be included in the positive
side of the life cycle assessment. As a rule, the average CO
2
emission of the
power mix in a country serves as benchmark. Deducted from this is the CO
2
equivalent released in the production of biogas-based electricity and the
bottom line then indicates the amount of CO
2
release avoided by the
electricity from biogas.
The same procedure is applied when calculating credits for the use of heat
from cogeneration. As a result, a biogas plant in which the biogas is
converted to electricity and much of the heat coproduced can be used
distinctly improves the life cycle assessment of that plant. For this reason,
biogas plants operating in cogeneration mode and achieving a high degree of
heat utilization have a better life cycle assessment than biogas plants where
the gas is only converted to heat or plants whose gas is upgraded to
biomethane as fuel (Bachmaier et al. 2009). A similar analysis may be
undertaken for the use of biomethane as a transport fuel with petrol or
diesel as the displaced fuel (Korres et al. 2010).
A positive life cycle assessment is a priority target of any biogas plant.
The relatively high level of inputs required for the production of biogas, plus
the conversion of the gas into electricity, heat or transport fuel can only be
justified if this form of producing renewable energy also contributes to
reducing emissions of harmful climate gases. As regards biogas, it should be
noted, in addition, that biogas is the only versatile energy source and at
present the only available gas from renewable sources; renewable hydrogen
infrastructure is not widely in place and the technologies are not yet
commercially available. Another point in favor of biogas is that -
irrespective of whether it is converted into electricity at the place where it
is produced or at the end of the gas network - electricity and heat obtained
from it are available to meet demand. In this way, biogas can compensate
other fluctuating renewable power sources and thereby fill an important
function in the energy supply of the future. Thus, a life cycle assessment
alone cannot determine whether the production of biogas is viable
economically, but biogas should be considered as part of the whole system
of sustainable energy supply.
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