Environmental Engineering Reference
In-Depth Information
9.1
Example of heat demand for fermentation in a mesophilic
fermenter: biogas is utilised in a CHP unit; substrates are manure and
agricultural residues at a scale of 10 000 t per year; average heat demand
is 45% of heat produced with a maximum of 62% in winter and a
minimum of 30% in summer. The CHP is sized at 190 kW
el
and 200 kW
th
.
reduced microbiological activity and hence decreased biogas yields. In
comparison with aerobic composting, only a small amount of energy is
released, as illustrated by the following formulae.
Composting: C
6
H
12
O
6
þ
6O
2
$
6CO
2
þ
6H
2
O
D
G
¼
1
:
100 kJ
=
kg
½
9
:
1
Anaerobic digestion: C
6
H
12
O
6
$
3CH
4
þ
3CO
2
D
G
¼
58 kJ
=
kg
½
9
:
2
Self-heating in a fermenter is caused firstly by the exothermic anaerobic
process from formula 9.2; however, it is very unlikely that the phenomenon
is caused by this process alone. Due to the fact that self-heating is related to
a high level of fermentation of solids, and to the feeding of dry substrates,
two additional effects are probably responsible. Within the dry substrates,
air is fed into the fermenter, causing aerobic processes with high heat
production, as described by formula 9.1. In addition, when there is a high
solid content in the fermenter, heat conductivity is low, which hampers heat
transfer from the fermenter to the outside. In the case of high fermentation
of solids, especially in hot seasons or regions, measures must be taken to
minimise self-heating. Such measures include compacting substrates before
feeding (to avoid oxygen entering the fermenter) and soaking the substrate
with fermenter liquids or water before feeding. The use of a thermophilic
process can also ensure a temperature gradient between the fermenter and
the external environment for cooling purposes. Finally, active cooling of the
fermenter can be an additional measure.
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