part 1: overview of cogeneration and its status in asia - Fire

State of art review of cogeneration 25
enhance the effect of insulation. The wall temperature of the combustion chamber is
maintained above 1,000ºC, which helps to reduce the heat transfer from the combustion gas
to the wall. Such a structure eliminates the need for a cooling system and renders the engine
very compact. High efficiency is achieved by both diesel cycle combustion and the energy
recovery unit where exhaust energy from the heat insulation is recovered and converted into
electricity by a turbo compound system, an ultra high speed generator, and a highly efficient
converter. As for the ceramic gas turbine, the target is to develop units having efficiencies of
42 per cent or more.
The thermal efficiency of an Otto cycle engine is a function of the difference between the
maximum combustion temperature and the exhaust gas temperature. The maximum
combustion temperature in an engine increases with a higher compression ratio while the
exhaust gas temperature decreases with a lower expansion ratio. But the compression and
expansion ratios of an Otto cycle engine are the same and the engine is adjusted for a lower
compression ratio to avoid knocking. In a Miller cycle, the expansion ratio can be set larger
than the compression ratio by adjusting the intake timing, and this results in an improved
efficiency as well as improved durability due to the lower exhaust temperature.
The gas injection diesel engine can now attain an electrical efficiency of 45 per cent, which is
the highest among commercialized gas engines. The engine no longer requires pilot oil and
glow plugs be used to ignite natural gas ignited into the cylinder at 25 MPa.
R&D efforts are also on going to develop solid oxide fuel cells to exploit the excellent
properties of ceramic materials and achieve efficiencies in the range of 50 per cent. Once
these technologies are commercialized, cogeneration promotion can get a further boost as
an energy saving and environmentally sound technology.
2.11 Cogeneration and the Environment
The high efficiency of cogeneration and efficient use of fuel guarantee a significant reduction
of CO2 emission. However, cogeneration can have environmental implications in the form of
CO, SO2 and NOx emissions to the atmosphere. The quantity of each of the pollutant
generated depends largely on the type of fuel used and the characteristics of the
cogeneration technology adopted.
CO is a poisonous gas produced due to incomplete combustion and can be reduced to
negligible levels by assuring satisfactory air-fuel ratio control. SO2 is an acidic gas produced
when sulphur-containing fuels such as oil or coal are burned. Its emissions cause acid rain.
Sulphur-containing exhaust gases are the main cause of corrosion of heat recovery devices
when the SO2 in the gas is cooled below its condensation temperature. NOx is a mixture of
nitrogen oxides produced due to the combustion of a fuel with air, and its formation is a
function of the combustion condition, characterized by the air-fuel ratio, combustion
temperature, and residence time. It also causes acid rain and can result in ozone and smog
after undergoing several chemical reactions in the atmosphere.
Technologies which have undergone rapid development are those based on spark and
compression ignition engines and gas turbines, primarily using natural gas as the fuel.
Natural gas is considered the cleanest among the fossil fuels as it does not practically
contain any sulphur, nitrogen and is free of dust particles. However, the emission of NOx is
greater, particularly for the prime movers operating at high temperatures.