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

State of art review of cogeneration 13
CHAPTER 2: STATE OF ART REVIEW OF COGENERATION
2.1 Technological Advances in Cogeneration
Cogeneration plants benefit from many of the energy efficiency improvements that are
brought about in utility power generation because the same basic technology is employed in
both cases. However, cogeneration being more attractive for small-scale decentralized
applications, significant technological progress has been made in the development of
modular and packaged cogeneration systems of lower capacities. Moreover, as such
systems are being adopted in industrial zones and city centres, the stringent laws and
regulations put in place for protecting the local environment has obliged the cogeneration
technology providers to innovate incessantly. The greater availability of natural gas in many
parts of the world has helped in the maturing of gas turbine technology. In addition, the
possibility of using alternative fuels such as wood, agro-industrial residues, biogas, etc., for
powering small-scale cogeneration systems has led to further technological progresses by
taking the specific characteristics of the fuels into consideration. This section briefly
describes some of the developments in this domain.
2.2 Reciprocating Engines
Reciprocating engines are mostly employed in low and medium power cogeneration units.
The lower and upper limits of engine sizes are often a function of the fuel in use; these can
range from 50 kW to 10 MW for natural gas, from 50 kW to 50 MW for diesel, and 2.5 MW to
50 MW for heavy fuel oil. One of the major advantages of reciprocating engines is their higher
electrical efficiency as compared to other prime movers.
The two main types of internal combustion engines employed in cogeneration systems are
diesel engines and Otto engines. The characteristic feature of the Otto engine is that an
electric spark from a spark plug ignites a mixture of fuel and air, and this is thus known widely
as a spark-ignition engine. In power generation applications, the Otto engine may be either a
gasoline engine or a diesel engine converted to have spark-ignition operation. Gasoline
engines have the ratings ranging from 20 kW to 1.5 MW. The spark-ignition engines
converted from diesel engines and running on natural gas are available in ratings from 5 kW
to 4 MW. The Otto engines operate at speeds between 750-3,000 rpm and have the electrical
efficiencies of 25-35 per cent. These engines can run on different fuels such as gasoline,
natural gas, producer gas, and digester gas.
As opposed to Otto engines, fuel is injected into the diesel engine cylinders in which it mixes
with air and is ignited by the heat generated when the pistons compress the fuel/air mixture,
and this engine is often known as a compression-ignition engine. Diesel engines can
generally be classified into two main categories, i.e. two-stroke and four-stroke engines. The
two-stroke engine is also known as a low-speed engine, and is characterized by ignition
taking place once every revolution, and by the engine running at a speed below 200 rpm and
delivering an output of 1-50 MW at a high electrical efficiency of 45-53 per cent. In a fourstrike
engine, ignition takes place during every other revolution, and this engine can be
divided into two categories. Medium speed engines are those running at speeds between
400 and 1,000 rpm and can be designed for ratings between 0.5 and 20 MW with electrical
efficiencies of 35-48 per cent. High-speed engines are those operating at speeds between
1,000 and 2,000 rpm and with ratings between a few kW and about 2 MW with electrical
efficiencies of 35-40 per cent.