16-07-2013, 02:40 PM
Thermal Power Plants
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Power plant operation
• Rankine cycle with reheat – to reduce the formation of water
droplets in turbine, and to increase the efficiency of the cycle.
• In order to maximize the efficiency of a steam power plant it is
desirable to operate as high a temperature as possible in the
super-heater. However, above about 650oC various forms of
metal fatigue become significant due to very high temperatures
and pressures that the walls of the boiler tubes have to
withstand.
• A typical 500 MW coal-fired plant consumes around 250 tonnes
of coal an hour.
Efficiency of Thermal Power Plant
• The efficiency of a Rankine cycle is usually limited by the working
fluid. Without the pressure reaching super critical levels for the
working fluid, the temperature range the cycle can operate over
is quite small: turbine entry temperatures are typically 565°C
(the creep limit of stainless steel) and condenser temperatures
are around 30°C. This gives a theoretical Carnot efficiency of
about 63% compared with an actual efficiency of 42% for a
modern coal-fired power station.
• The power output or capacity of an electric plant can be
expressed in units of megawatts electric (MWe). The electric
efficiency of a conventional thermal power station, considered as
saleable energy (in MWe) produced at the plant busbars as a
percent of the heating value of the fuel consumed, is typically
33% to 48% efficient.
Fluidized Bed Combustion
• Fluidized bed combustion (FBC) reduces emissions of SO2 and
NO2 by controlling combustion parameters and by injecting a
sorbent (such as crushed limestone) into the combustion
chamber along with the coal.
• Coal mixed with the limestone is fluidized on jets of air in the
combustion chamber. Sulphur released from the coal as SO2 is
captured by the sorbent in the bed to form a solid calcium
compound that is removed with the ash. The resultant waste is a
dry, benign solid that can be disposed of easily or used in
agricultural and construction applications. More than 90 per cent
of the SO2 can be captured this way.
Supercritical Technology
• "Supercritical" is a thermodynamic expression describing the
state of a substance where there is no clear distinction between
the liquid and the gaseous phase (i.e. they are a homogenous
fluid). Water reaches this state at a pressure above 221 bar
• Up to an operating pressure of around 190 bar in the evaporator
part of the boiler, the cycle is sub-critical. This means, that there
is a non-homogeneous mixture of water and steam in the
evaporator part of the boiler. In this case a drum-type boiler is
used because the steam needs to be separated from water in the
drum of the boiler before it is superheated and led into the
turbine.
• Above an operating pressure of 221 bar in the evaporator part of
the boiler, the cycle is supercritical. The cycle medium is a single
phase fluid with homogeneous properties and there is no need to
separate steam from water in a drum. Once-through boilers are
therefore used in supercritical cycles.
Recent Coal based power plants
• All the recently commissioned coal-fired power plants of high
efficiency use pulverised coal combustion (PCC) with supercritical
(strictly, beyond the critical point of water, 22.1 MPa, 374°C)
steam turbine cycles.
• Among supercritical plants, those using the highest steam
temperatures (around 580°C and above) can be referred to as
ultra-supercritical, although that borderline is rather arbitrary.