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
All such stokers are designed to continuously feed coal into the furnace by moving it on a grate within
the furnace and also to remove ash from the furnace. Pulverised coal firing represented a major increase
in combustion rates over mechanical stokers. To prepare the coal for use in pulverised firing , it is crushed
and then ground to such a fine powder that approximately 70 percent of it will pass a 200 mesh sieve. It is
suitable for a wide variety of coal , particularly the higher grade ones. Advantages of pulverised coal firing
are the ability to use any size coal ; good variable load response ; lower requirement for excess air for
combustion , resulting in lower fan power consumption ;lower carbon loss ; higher combustion temperatures
and improved thermal efficiency ; lower operation and maintenance costs ; and the possibility of design
for multiple – fuel combustion.
Early, cyclone firing was introduced and became the major advance in coal firing. It is now widely used
though for a lesser variety of uses than is pulverised coal. These are the obvious savings in pulverising
equipment because coal need only be crushed , reduction in furnace size , and reduction in fly ash content of
the flue gases.Coal – size for cyclone – furnace firing is accomplished in a simple crusher and covers a
wide band , with approx. 95% of it passing a 4 – mesh sieve. In this type of firing , crushed particles of
coal are injected into the fluidised bed so that they spread across an air distribution grid.The combustion
air , blown through the grid , has an upward velocity sufficient to cause the coal particles to become fluidised ,
i.e. held in suspension as they burn. Unburned carbon leaving the bed is collected in a cyclone separator
and returned back to the bed for another go at combustion. The main advantage of fluidised bed
combustion is the ability to desulfurise the fuel during combustion in order to meet air quality standards
for sulphur dioxide emissions. Desulfurisation is accomplished by the addition of limestone directly to the
bed[5].
An integrated gasification combined cycle (IGCC) is a technology that uses a gasifier to turn coal and other
carbon based fuels into gas—synthesis gas (syngas). It then removes impurities from the syngas before it is
combusted. Some of these pollutants, such as sulfur, can be turned into re-usable byproducts. This results in
lower emissions of sulfur dioxide, particulates, and mercury. With additional process equipment, the carbon in
the syngas can be shifted to hydrogen via the water-gas shift reaction, resulting in nearly carbon free fuel. The
resulting carbon dioxide from the shift reaction can be compressed and permanently sequestered. Excess heat
from the primary combustion and syngas fired generation is then passed to a steam cycle, similar to a combined
cycle gas turbine. This results in improved efficiency compared to conventional pulverized coal.
Figure – 1 : Cost analysis for different power plants and its operations.
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