Maintenance and useful life
In contact with water, the H2S content of biogas promotes corrosion. Consequently, adherence to
the prescribed oil-change intervals is very important (after each 100 operating hours or so for
vehicle spark-ignition engines). Dual-fuel engines should be started on pure diesel, with biogas
being added gradually after about 2 minutes. For shutdown, the biogas fraction should be gradually
reduced prior to stopping the engine. Any engine that has not been in operation for a considerable
length of time should first be flushed out with scavenge oil (50% motor oil, 50% diesel oil) and filled
with fresh oil. As long as extreme operating conditions are avoided, the engine can expected to
achieve its normal useful life.
Exhaust-heat utilization
Internal-combustion engines have efficiency levels of 25 - 30% (gasoline engine) and 33 - 38%
(diesel engine). A higher overall efficiency can be achieved by exploiting the heat content of the
cooling water and exhaust, e.g. by:
- an exhaust heat exchanger (danger of H2O-corrosion if the exhaust gas cools down to 150
°C or less)
- coolant heat exchanger (at coolant temperatures of 60 - 70 °C).
Fig. 5.37: Energy shares of an internal-combustion engine. 1
Energy input, 2 Dissipated energy (radiant heat and exhaust), 3
Useful exhaust energy, 4 Thermal energy in cooling water, 5
Mechanical power applied to crankshaft (Source: Mitzlaff 1986)
The recovered heat can be used for:
- heating utility water
- drying agricultural products
- space heating.
However, the requisite equipment/control effort makes heat recovery uneconomical except for large
heavy-duty engines.
Motor-generators
The most frequent use for biogas-fueled engines is the generation of electricity. Suitable
components include:
- asynchronous generators for system interconnection, i.e. the generator can only be operated in
connection with a central power network. If the network breaks down, the generator cannot stay
in operation. System control and network adaptation are relatively uncomplicated.
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