Environmental Engineering Reference
In-Depth Information
Biogas
Air
Chimney
Turbo
charging
Heating
water
Heating
water
Mixture
air/biogas
Heat
exchanger 3
Exhaust gases/
heating water
Exhaust
gases
Heat
exchanger 1-
(cooler for air/gas)
Electric
generator
G
Internal
combustion
gas engine
Heat
exchanger 2/1
Engine cooling
water/
heating water
Motor
oil
Heat
exchanger 2/2
Motor oil/
engine
cooling water
Heating water
FIGure 32.10 (see color insert)
A CHP unit.
After treatment, biogas is used to produce energy. The most common way is to use all biogas in a
cogeneration plant (CHP unit) to produce power and heat simultaneously (Figure 32.10). In this case,
we can achieve maximum power production and enough excess heat to run the digesters. The energy
required for operation of the digester is also called parasitic energy. The anaerobic digesters require
heat for heating the substrate to operating temperature and compensating the digester heat losses.
The digester also requires energy for mixing, substrate pumping, and pretreatment. The largest
portion of heating demands in the digester operation is substrate heating. It requires over 90% of
all heating demands, and only up to 10% is required for heat loss compensation (Zupancic and
Ros 2003). In mesophilic digestion, a CHP unit delivers enough heat for operation, whereas in
thermophilic digestion, additional heat is required. This additional heat demand can be covered with
heat regeneration form substrate outflow to substrate inflow (Figure 32.11). Usually, a conventional
countercurrent heat exchanger is sufficient; however, a heat pump can be applied as well.
Electric energy is also required for digester operation. It is required for pumping, mixing, and
control and regulation equipment. In practice, no more than 10-15% of electric energy produced
should be used for parasitic demands. Naturally, the pretreatment process may require also electric
or heat energy. Pretreatment in the rule enhances anaerobic digestion, producing higher biogas
production rates. However implications of pretreatment methods must be carefully considered. The
golden rule is that pretreatment should not spend more energy than the energy of the biogas it is
producing. If the energy use and production balances out, pretreatment may have benefits such as
