Optimisation of biogas yields from anaerobic digestion by feedstock type - The Biogas: Science, Production and Applications - page 131

farmer who grows and harvests a fodder crop once a year, for which he
receives no gate fee, and it costs him to produce this feedstock which he has
to store all year in order to feed the digester daily. In this case, the size of the
digester in relation to throughput may not be critical and a more important
consideration is that the process is capable of extracting the maximum
amount of energy from the substrate, as this is the only source of income. In
both cases, the plant owners wish to optimise the plant and in both it is
likely to be on economic performance: in the first against throughput and
gate fee income and in the second on exported energy. Even the second
approach may not maximise overall energy recovery, as the focus will
always be on the fraction of the energy output that generates the best
economic return.
At a higher level, governments and international organisations may also
be looking towards AD as a renewable energy source and see its value as a
second-generation process in achieving this from waste. Some have also
recognised that the process can give additional environmental benefits, for
example in minimisation and offsetting of greenhouse gas emissions,
through the use of digestate as a fossil fertiliser substitute and by improving
nutrient management, and as a means of facilitating more environmentally
sustainable management of animal manures and slurries through co-
digestion. This type of environmental optimisation, however, requires not
only consideration of the types, sizes and distributions of digestion plants
but also more coherent regional or national planning, a suitable regulatory
regime and careful selection of policy drivers and penalties needed to
incentivise the preferred outcome.

6.3 Basic definitions and concepts

6.3.1 Biogas and methane yield
A particular substrate has a maximum theoretical specific methane yield
based on its carbon content, and it is not possible to exceed this however the
plant is designed and operated. The basis for this theoretical value was first
explored by Symons and Buswell (1933) and is discussed in Chapter 5. In
practice, however, the theoretical value cannot be achieved, as a proportion
of the carbon along with the energy gained from the catabolic reaction is
used by the microorganisms themselves in maintenance and growth of their
own biomass. A proportion of the carbon may also not be converted
because it is not anaerobically biodegradable, being protected from
microbial attack because of its chemical structure or physical location
(e.g. within a lignin bundle). The maximum specific methane yield of a
substrate, expressed as m 3 CH 4 kg 1 VS, can, however, be experimentally
determined in a biochemical methane potential (BMP) test (see Chapter 3).

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