Methane emissions in biogas production - The Biogas: Science, Production and Applications - page 245

increasingly important. One of the requirements is the greenhouse gas
performance of the energy production pathway. For AD, three greenhouse
gases are important in this: carbon dioxide (CO 2 ), methane (CH 4 )and
nitrous oxide (N 2 O). Since biogas contains about 60% methane, this gas is
especially important in biogas production pathways. As a greenhouse gas,
methane is 25 times stronger than carbon dioxide, 2 so small leakages of
biogas have a strong negative effect on the total greenhouse gas
performance of the energy production pathway.
To date, there have been only a small number of detailed studies on
methane emissions from biogas production, distribution and utilization. In
this chapter, the results of these studies are summarized, their main
conclusions are assessed and gaps in the knowledge in this field are
identified.

11.2 Methane emissions in biogas production

In the open literature there are about ten key articles dedicated to emissions
from biogas production and biogas utilization, and a few additional
publications that address particular aspects of biogas production. In this
chapter, an overview of the production process is given first, indicating the
key steps with regard to emissions. Then each process step, as shown in Fig.
11.1, is described in detail.
The scope of this chapter covers methane emissions from the production
of biogas and the utilization of the biogas in processes that are typically
carried out at the same site as the biogas production. Therefore, the
following processes are excluded:

. emissions from the production of energy crops

. emissions from animal stables and manure storage at farms

. emissions from industrial processing facilities in cases where an emission

could be allocated to the co-product

￿ ￿ ￿ ￿ ￿ ￿

. emissions during transport of substrates

. emissions from the gas grid or gas stations after injection into the grid.

Numbers are presented as they are given in the literature so it is possible
to refer back to the original articles. In addition, all emissions are also
expressed as a percentage of the total methane that is utilized, to allow
comparison between steps.
In the interpretation of the results from the storage of manure and

2 The global warming potential of methane is 25, compared to 1 for carbon dioxide. This is
mass-based and for a 100-year time horizon. In recent years, three values have been used: 21
(IPPC Second Assessment Report, 1995); 23 (IPPC Third Assessment Report, 2001); 25 (IPPC
Fourth Assessment Report, 2007).