27-10-2012, 04:08 PM
PANKI THERMAL POWER STATION, KANPUR
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COAL FIRED THERMAL POWER PLANTS,
INTRODUCTION
Heat engines were developed around two hundred years ago to convert low grade
thermal energy into high grade mechanical energy. With invention of better materials
and development of theories to explain the working and evaluate the performance of
heat engines, it could become possible to convert more and more thermal energy into
mechanical energy. Coal fired power generating station is a sort of external combustion
heat engine, which operates on Rankine cycle with water as a standard working fluid.
Reheating and regenerative feed heating arrangements improve the performance of
power plants.
Conventional power plants are evaluated for performance through various
performance evaluation criteria (PEC), like Plant Load Factor (PLF), Loading Factor,
Partial Loading, Forced Outages, Auxiliary Power Consumption, Specific Oil
Consumption, Specific Coal Consumption etc. Most common criterion used for
performance evaluation is PLF, however the most important parameter from the point of
view of energy conversion is thermal efficiency (Heat Rate), which is calculated
frequently for boiler, turbine and for the entire plant.
HEAT ENGINE
A heat engine, in broader sense can be defined as a device which converts low
grade thermal energy into high grade mechanical energy. Internal combustion engines,
gas turbines, thermo electric power generators, steam turbine power plants, steam
engines etc. are the examples of heat engines. However in more restricted sense and
most commonly a heat engine is defined as a device that operates on thermodynamic
cycle and produces a net positive mechanical work through heat absorption from a high
temperature reservoir and heat rejection to a low temperature reservoir. The device
uses a working fluid, which goes under change of thermodynamic state to complete the
thermodynamic cycle during heat absorption and heat rejection processes. The
thermodynamic cycle employed for producing mechanical work is called “power cycle”.
COAL FIRED POWER PLANT
Conventional coal fired power generating plants operate on Rankine cycle with water
as standard working fluid. The most common design of these power-generating units
are 100/110 MW, 200/210 MW or 500 MW with features of reheating and regeneration.
In general, one reheating at reduced pressure and up-to main steam temperature is
provided. There are five to seven regenerative feed water heaters. The main steam at
the inlet of turbine is kept at 535-540 oC temperature and 13.0-14.0 MPa pressure. The
plants are equipped with pulverized coal combustion system.
Designed thermal efficiency of the coal plants installed during last 20 years is 36-
38%. However in India the old units commissioned during 70s, are still in use, and
comprise a considerable percentage of total installed capacity. Designed thermal
efficiency of these units is 31-33 %.
Most of the power plants proposed to be installed in future are based on Super
Critical Technology. These plants operate on thermal efficiency of about 41-42%. The
first Supercritical power generating unit in India started recently by Adani Power. Other
units are likely to be installed at Sipat & Barh Thermal Power Plants of NTPC.
RANKINE CYCLE
Cycle of operation of heat engines may be plotted on different thermodynamic planes
by assuming ideal or hypothetical reversible processes which represent the basic
desired operations. A Rankine cycle as shown on enthalpy-entropy (h-s) and
temperature-entropy (T-s) planes in Figure ‘2’ represents the operational cycle of a
simple steam power plant.
This Rankine cycle is an ideal reversible cycle consisting of reversible constant
pressure heating process in boiler (4-1) to convert water into steam, reversible adiabatic
expansion of steam in turbine (1-2) to convert thermal energy of steam into mechanical
work, reversible constant pressure heat rejection during condensation of steam in
condenser (2-3) and reversible adiabatic compression of feed water (saturated liquid) to
the boiler pressure by boiler feed pump (3-4).
REGENERATIVE FEED HEATING
To reduce the irreversibilities during heat addition in boiler, the feed water is supplied
in the boiler at elevated temperature. High feed water temperature at the inlet of the
boiler reduces the average temperature difference across which heat transfer takes
place in the boiler and thus reduces irreversibilities. In other sense there is an increase
in average temperature of heat addition, which results in higher thermal efficiency of
regenerative feed heating cycle. This improvement in thermal efficiency may also be
explained with the fact that heat rejection in condenser is reduced due to extraction of
steam from turbine for feed heating.
The feed water is heated in regenerative feed water heaters through steam extracted
from the turbine at one or more points after expansion through certain stages of turbine.
Feed water heaters may be an open type or a closed type heat exchanger. In an open
or direct contact type feed water heater, the extracted steam from turbine is mixed with
condensate (or feed water) to heat it up and condensate leave the heater at elevated
temperature. In a closed or surface type feed water heater, feed water flows through
the tubes and extracted steam is condensed in a shell outside the tubes. This
condensate, usually at the saturation temperature of extracted steam pressure (also
called heater drip) then passes to the next lower pressure heater or added to the
condensate leaving heater by means of a drip pump. The drip of heater if passes to a
lower pressure heater, is finally discharged at the hot well of the condenser or added to
the condensate by means of a drip pump from the lowest pressure heater.