OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
and the H2O is removed . High – Btu gas is thus largely methane , with a heating value of 950 to 1000 Btu
/ scf , approaching that of natural gas , which is 950 – 1100 Btu / scf .It has all the characteristics of natural
gas but without the sulfur and other pollutants .Various methods under development for the production of
high Btu gas , is more complex and expensive than low – Btu gas . Purified low – Btu gas can be fired
directly in a conventional steam generator. It has the advantages of being sulphur and ash – free, thus
eliminating the need for precipitators and scrubbers. The lack of SO2 in the flue gases also permits lower
stack temperatures, which results in improved efficiency .
An attractive utilization of low – Btu gas for electric generation is a fuel for Combined – cycle power plant
.A combined cycle is one which uses a gas turbine at the high temperature end and a steam turbine at the
low temperature end .Low – Btu gasifiers operate at different pressures and temperatures , depending
upon the process used . They operate at exit temperatures between 1000degree F and 2000degree F . The
exit gas must be cooled for purification and cleaning .Normally this cooling represents a large loss of
energy to the environment . A combined cycle takes advantage of the high gasifier and recovers much of
that heat loss by a gas to gas heat exchanger .The gas turbine drives one of two electric generators and
the compressor. Superheated steam is generated in HRSG , expands through a steam turbine that drives the
second electric generator and exhausts to a condenser.
Gasification reactors are designed to suit coal characteristics. Three well-known configurations are the fixed bed,
fluidized bed, and entrained bed systems.
• Fixed Bed Gasifier reactor has different zones for each operation such as drying, devolatilizing, gasification,
and combustion. Coal of 10–50mm size is fed from the top and air or oxygen is blown through the fuel bed. The
crude gas leaves the gasifier from above. This type of gasifier obviates the need for a heat exchanger, has lower
oxygen consumption, and has the lowest energy requirement of all gasification processes. This technology has
been commercialized and about 200 fixed bed gasifiers are operating around the world[4].
• Fluidized Bed Gasifier works on the counter-current principle and allows coal particles to move vigorously.
It consists of a vertical cylindrical refractory lined vessel with recycle cyclone. The temperature in the reactor
goes up to 850–1,050°C. It is a non-slagging gasifier and has been tested with all types of coal and lignite, and is
found attractive for high ash coals as well as high reactive coals. The chemical reaction in this gasifier is
accelerated by turbulent mixing and close contact. There are no separate de-gasification and gasification zones.
Dust-laden gas leaves the reactor at the top, and is cooled and purified before use[14].
• Entrained Bed Process is the third configuration. Finely ground coal of 0.1 mm is entrained at high
temperatures of the order of 1,400 to 1,600°C. Coal gasifies instantly and volatile matter in coal also contributes
to the gas at the reaction temperatures. The product gas has almost 80 per cent of the energy of the feed coal.
The ash in the coal melts and runs down the refractory-lined walls.
2b(ii) Coal Liquefaction : It is the conversion of coal into a liquid fuel for direct energy production or a
liquid substitute for refinery feedstock from which other liquid fuels may be obtained. In coal
liquefaction, the long molecules are shortened by adding hydrogen. The needed hydrogen is generated , and
desulfurization is accomplished , in the same manner as for coal gasification. The Fischer – Tropsch process
first produces a mixture of CO and H2 from coal and steam.This is followed by catalytic reactions at
about 300 degree F and 150 bar , which yield a range of hydrocarbons from gaseous methane to higher
liquid hydrocarbons. These are then separated with methane going as pipeline gas and the rest going to
different liquid fuels.
2(c) Coal Firing / Combustion Technology
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