Environmental Engineering Reference
In-Depth Information
externally to the fermenter. An internal heat supply is obtained with
stainless steel or polypropylene heat pipes located either in the fermenter or
in the fermenter walls. Electrical energy is required to circulate only the hot
water between the heat source and the fermenter. Depending on the
construction and the substrates used, heat transfer from the heat pipes into
the fermenter can be impeded by deposits on the pipes or the fermenter
walls; in this case, the fermenter must be opened to allow the pipes to be
cleaned. An external heat supply is obtained with external heat exchangers
in which water on the primary and secondary side must be circulated,
requiring more electrical energy than an internal heat supply system. The
advantage of external heating is the ability to clean and maintain the heat
exchangers without opening the fermenter.
Energy losses in the form of heat occur in hot water pipes, connectors and
heat exchangers, and are mainly determined by the quality of insulation.
Depending on the length of pipes and the technology used in the heat
exchangers, these losses can amount to up to 10% of the total heat supplied.
The choice of heat exchanger technology has a particularly significant
influence on heat losses.
9.3.3 Fuel supply
Fuel might be used to operate dual-fuel CHPs or external generators. In
some countries (e.g. Germany), legal requirements dictate that renewable
bio-oils should be used for dual-fuel CHP units. In most cases, fuel is
necessary to operate mobile equipment, generally wheel loaders, tractors
and transport vehicles. Fuels are usually bought locally and stored in tanks
at the biogas plant. Diesel (fossil diesel or biodiesel) fuels are used for most
applications. As an alternative, upgraded biogas can also be used for the
majority of applications. This requires an upgrading unit at the biogas plant
and high-pressure storage (200-300 bar), as well as a pump for filling
vehicles. The mobile equipment must also be able to run on natural gas
(upgraded biogas has to meet the same requirements as natural gas).
Unfortunately, only a few natural gas tractors and transport vehicles are
available commercially. Long-distance transportation and complex logistics
lead to high fuel consumption and should be avoided by careful planning
during the design stage. It is also important to avoid running equipment at
no-load.
9.4
Balancing energy flows
Due to the fact that efficient energy use is one of the key challenges for
biogas plant optimisation, a detailed analysis of the energy flows can show
possible areas for operational and technological improvements. An energy
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