Environmental Engineering Reference
In-Depth Information
to methane as fuel and valuable fertilizer from available resources that
otherwise would go unused. This is particularly the case when exclusively
by-products and waste are used as substrates for digestion. But even if
energy crops serve as the substrate for biogas production, the energy balance
is positive as Effenberger et al. (2010) were able to demonstrate with
scientific support from the example of ten biogas plants.
In addition to the energy balance, the carbon balance is an important
indicator assessing the environmental efficiency of a technology providing
electricity from renewable sources. The carbon footprint of a biogas life
cycle assessment should consider the carbon dioxide equivalents for the
construction and operation of the biogas plant, the feedstock, the
uncontrolled methane emissions from the plant and the digested residue,
and during spreading in the field. Methane emissions that may have been
avoided by the digestion of liquid manure, the emission of carbon dioxide
prevented by the use of the fertilizer and the credits for the supply of
electricity and heat improve the energy balance further. It goes without
saying that in calculating the footprint it is assumed that the generally
accepted rules of technology are observed along the entire production chain
(Bachmaier et al. 2009).
As methane has a 25 times higher impact on climate than carbon dioxide
(IPCC 2007), the causes of the uncontrolled release of methane in the
process of biogas production and utilization should receive particular
attention. These are, in a biogas plant, leaks from the digester, the gas
piping or the digestion residue store, as well as methane slip due to
incomplete combustion of biogas in the engine. Leaks are relatively easy to
detect by regular leak checks. Avoiding leaks also improves the financial
return of the process because emitted methane bears on the earnings
situation of the plant. The so-called methane slip is a function of the
cogeneration unit. Suppliers of cogeneration units are required to disclose
relevant data and every buyer should be aware of the methane slip
performance of the engine and insist that the slip is warranted. If an engine
fails to meet the specified methane slip prescribed in the country in which the
engine operates, lean gas combustion equipment may have to be installed to
ensure methane is not allowed to escape into the atmosphere. The
ecobalance of a biogas plant should also consider storage and the spreading
of the digested material in the fields. The best approach is to ensure that as
much methane as possible is released by digestion so that very little methane
will be set free if the digested material is stored without cover. As an
alternative solution, the digestate can be provided with a gastight cover.
This is the preferred solution when it cannot be ensured that the digestion
residue contains little methane. This is particularly the case when the flow of
material from digestion is irregular and the bacterial populations are
required to adapt to different substrate frequently.
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