Environmental Engineering Reference
In-Depth Information
fermentation with a greater number of mixed and heated fermenters.
Substrates in liquid form, such as slurries, require less energy for pumping
and mixing than substrates with a high concentration of solids. This is
balanced out by the higher biogas yield per unit volume offered by a system
using more solid substrates.
Practical data obtained from German agricultural biogas plants that
process mainly energy crops and excrements and produce between 100 and
1000m 3 /h of biogas show variation in the electricity demand of the different
plants. In the case of liquid feedstock such as manure, the electricity demand
for biogas production (without gas utilisation) is between 15 and 23 kWh el
per MWh Hi of biogas produced. For higher solid content feedstock such as
energy crops, the electricity demand is slightly higher, at about 19-27 kWh el
per MWh Hi of biogas produced (VDI, 2011).
9.2.4 Substrate supply to the fermenter and fermentation:
heat demand
Heat input to the fermenters is required because the microorganisms are
active at defined temperatures mainly above ambient. Ambient temperature
changes with the seasons and thus the thermal energy input is variable. In
addition, the choice of technology influences the heat demand according to
the following parameters: use of mesophilic or thermophilic microorgan-
isms; ratio between surface and volume of the fermenters; thickness and
quality of insulation; and solids concentration in the fermenter. Under
climatic conditions found in Central Europe, for agricultural mesophilic
biogas plants using mixtures of manure, organic residues and energy crops,
the process heat demand is in the range of 5-15% of the energy available in
the biogas. When all biogas is locally used in a combined heat and power
(CHP) unit, typically 20-40% of the heat from the CHP unit is required to
heat the fermenters. During the planning stage, the thermal parasitic energy
demand of a biogas plant requires detailed analysis that takes into account
climatic conditions over the year. Figure 9.1 shows an example of the heat
demand of a biogas plant. In summer, the heat demand is significantly lower
than during the winter: in very cold climates the winter heat demand can be
as high as or higher than the total heat produced by a CHP unit. A detailed
analysis, and particularly one that takes account of the temperature of the
substrates before entering the fermenters, is essential for successful biogas
plant operation.
For biogas plants with high solid concentrations in the fermenters, self-
heating at high external ambient temperatures has been reported. It should
be borne in mind that anaerobic degradation is a slightly exothermic
biological process. More than one degree Kelvin self-heating can cause
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