Environmental Engineering Reference
In-Depth Information
Box 5.4 Parasitic demand of dry continuous digestion of OFMSW at
thermophilic temperature
Thermal demand
Specific heat capacity of water 4.184 kJ/kg/8C
Water content of feedstock 700 kg/t
Thermal demand of feedstock 4.184 kJ/kg/
8
C
6
0.7water/solids = 2.93 kJ/
kgOFMSW/
C
Temperature rise 45
8
C) = 131 kJ/kg = 36.6 kWh/t OFMSW
200m
n
3
biogas generated per tonne of OFMSW or 4.5GJ/t OFMSW
Parasitic thermal demand 131MJ/t = 131/4500 = 2.9% of energy in biogas
Electrical demand
Parasitic electrical demand of 85 kW
e
h/t (Murphy and McCarthy, 2005) =
306MJ/t
Parasitic electrical demand = 306/4500 = 6.8% of energy in biogas
If electricity generated at 35%
8
C (15-60
8
e
then demand = 306/(0.35)
6
4500 = 19% of
η
energy in biogas
reciprocal of 0.85, to 3.4%. Obviously the source of external heat source is
important to the overall system. Thermal energy may be generated from
fossil fuel combustion, biomass combustion or from combustion of the
biogas itself. If biogas is used for combined heat and power production then
the heat produced may be utilised to satisfy thermal demand. On the other
hand, if biogas is upgraded to biomethane then the biogas may be
considered too valuable to burn as a source of heat. Sustainability criteria
may suggest combustion of biomass rather than fossil fuel to satisfy thermal
demand.
5.6.2 Electrical parasitic energy demand
The electrical parasitic demand is 6.8% based on 85 kW
e
h/t; this is
associated with the pretreatment, pumping and mixing of OFMSW in a
dry digestion process (Murphy and McCarthy, 2005). The electrical demand
is significantly less for a dry batch process where the waste is loaded and
unloaded using standard diesel-fuelled equipment. Electricity in a small-
scale combustion process is typically generated at an efficiency of
approximately 35%. Thus, the parasitic electrical demand could rise to
around 19% of the energy contained in the biogas.
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