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measurements of natural gas engines and 14 biogas engines. They found a
range for the methane emission of 1-6% of fuel input. However, this was
based on relatively old engines that have most likely been replaced by now.
An interesting conclusion was that the methane emission was 40% lower for
the biogas engines compared to the natural gas engines. The authors of this
chapter believe that methane emission differences between biogas engines
and natural gas engines are not necessarily characteristic and cannot simply
be explained by differences in methane content. The difference can be
explained by the fact that the methane number of biogas is around 130,
allowing for high compression ratios and early spark timing. Also, the
capacity of the engines and the number of measurements will both influence
the final results (see also Chapter 17 on CHP for further information).
Biogas upgrading
The previously mentioned work of Liebetrau et al. (2011a, 2011b) is the only
study among the original set of articles to report specifically on methane
emissions during gas utilization. Two of the ten biogas plants investigated
by Liebetrau et al. had a gas upgrading facility with pressurized water
scrubbing. From the two gas upgrading units, only the results of one of
them were presented. The reported emission of methane during the
regeneration part of the process was 1.5% of total methane production.
In Germany, from 2012 onwards, the maximum allowable methane slip in
upgrading installations is 0.2%. 3
In recent years, two biogas upgrading installations in the Netherlands
(Beverwijk and Mijdrecht) have been monitored and the results are reported
by Jonker (2010a, 2010b). For Beverwijk it was found that no methane slip
occurred. This is a direct result of the thermal treatment (combustion)
applied to the residual gas at the plant. This approach was chosen because
of the relatively high methane concentrations in this residual gas. Despite
the low efficiency of methane production at the Beverwijk plant, the
effectiveness of the combustion treatment process demonstrates very well
that essentially zero methane emissions can be achieved in practice.
In Mijdrecht, a state-of-the-art upgrading plant using a chemical washing
liquid is applied to remove carbon dioxide. This process is highly selective
and the residual gas from the regeneration of the washing liquid contains
only small amounts of methane. The methane slip that was derived from the
measurements was 0.2% of the input. The result from this plant indicates
that the future German requirements can be met with the application of
appropriate technology.
￿ ￿ ￿ ￿ ￿ ￿
3 According to the Gasnetzzugangsverordnung (GasNZV) (http://www.biogaspartner.de/index.
php?id=10141).
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