Environmental Engineering Reference
In-Depth Information
losses; this must be designed for the real expected load that offers optimum
efficiency during operation. An existing transformer with low efficiency or a
very high load cannot be the best choice from an economic perspective.
9.3.2 Heat supply
A heat supply is necessary to maintain a constant temperature for
fermentation and for substrate pre-treatment processes such as sanitisation.
For these purposes, low temperatures of below 100
8
C are necessary and a
standard heat supply such as space heating is sufficient. For planning
purposes, the highest heat load must be determined from the heat demand
for
.
heating fresh substrates
.
compensation of heat losses from the fermenter surfaces
.
heating of buildings and biogas plant equipment (e.g. to avoid freezing).
This heat demand should usually be determined for the coldest season of the
year. Alterations in substrate composition must also be taken into account.
A continuous heat supply must be guaranteed so that the fermentation
temperature can be kept constant. The temperature should be kept at a
steady level, avoiding deviations of more than ±1K. A continuous
temperature change of more than ±1K over about ten days will have a
negative impact on the biological balance that drives the fermentation
process (VDI, 2011). The heat supply must therefore ensure that this type of
temperature fluctuation does not occur, although short interruptions in the
heat supply can be tolerated.
The heat supply itself can be based on the combustion of biogas or can
come from a local CHP unit in the form of waste heat. When either of these
two methods are employed, an alternative gas supply is required at the start
of the plant operation and at times of low biogas production: this can come
from natural gas or from bottled, compressed or liquefied gas. An
alternative method is for heat to be supplied from heating systems based
on renewable (e.g. pellets) or fossil (oil, coal) sources or via a connection
with a local heat grid.
In some cases, higher temperatures or steam may be required, for example
when a steam explosion process is applied for substrate pre-treatment or
thermal regeneration of a chemical adsorbent is required during biogas
upgrading. Steam can generally be supplied by direct biogas combustion
processes or by an exhaust heat exchanger from a specially equipped CHP
unit (often called de-coupling of heat). If this is impossible, then external
steam production is required. For detailed planning, the steam parameters
and the technology required for steam generation must be fine-tuned.
The heat for the fermentation processes can be supplied internally or
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