BOILER EFFICIENCY
DEFINITION
Boiler efficiency is defined as the ratio of heat absorbed by the working fluid and the heat
released by the fuel i.e.
Boiler efficiency =
Heat absorbed by working fluid
Heat released by fuel in furnace
FACTORS WHICH EFFECT BOILER EFFICIENCY
Loss due to the heat carried away
by hot flue gases.
Incomplete combustion.
Excess air.
Heat carried away by moisture.
Radiation.
Blow- down and leakages.
STACK LOSS
Heat carried away by flue gases or stack loss is the major cause for the reduction of boiler
efficiency. It is calculated as,
Stack Loss
Flue gas flow specific
Of flue gas (flue gas
exit temperature ambient temperature)
FACTORS ON WHICH STACK LOSS IS DEPENDENT
Sulpher content of fuel.
Fouling of the furnace or air preheater.
Excess air.
Leakages into the furnace.
High Burner tilt.
Higher level elevations in service.
Auxilliary damper positions.
HEAT CARRIED AWAY BY MOISTURE
Heat absorbed by the moisture in flue gases constitutes a loss because, water vapour is not
condensed in the boiler and thus substantial amount of latent heat (Approx..540 Kcal / kg at
atmospheric pressure) is lost.
There are two sources of moisture in flue gases viz.
Moisture in fuel.
Combustion product of hydrogen
content in fuel.
SOURCE OF MOISTURE IN FLUE GASES
Moisture in fuels such as coal
and gas.
Fuels containing high hydrogen
Such as natural gas generate
Large amount of water vapour
When burned.
2H2 + O2
2H2O
i.e . 1 kg of hydrogen, when burnt, produces 9kg of water vapour .
In addition to heat loss, presence of moisture promotes corrosion in presence of SO2.
LOSS
LOSS
INCOMPLETE COMBUSTION
Incomplete combustion means partial burning of fuel. Part of the fuel may remain unburned or
carbon in fuel may burn partiaaly to form carbon monoxide.
Causes of incomplete combustion are:
Inadequate air.
Improper distribution of air.
Fuel not properly pulverized or
atomized.
Low furnace temperatures.
Moisture in fuel.
Low secondary air temperatures.
WHY INCOMPLETE COMBUSTION IS A LOSS
Unburned fuel is obviously a loss
because it releases no energy, but
is paid for.
Partially burned carbon releases
much less energy than when burnt
fully.
2C + O2
2CO + 4,400 BTU.
C + O2
CO2 + 14,600 BTU.
As is evident partial burning of carbon generates carbon monoxide, a toxic gas and releases
much less heat and thus constitutes a huge loss.
LOSS DUE TO EXCESS AIR
Quantity of air supplied over and above the minimum required amount of theoretically
determined air (stoichometric) is called excess air. Certain quantity of excess air needs to be
supplied, because it is practically not possible for every molecule of fuel to come in contact
with every molecule of oxygen supplied in the limited time the mixture remains in boiler with
no excess air, resulting in improper combustion and consequent losses.
However, too much excess air increases the amount of flue gases. Since, the temperature of exit
gas temperature is constant , increase in flue gas flow means increase in stack loss.
DEFINITION
Turbine cycle efficiency is defined as the ratio of power developed by the turbine and the heat
added to working fluid in the boiler.
Mathematically,
Turbine cycle efficiency =
Power developed by turbine
Heat added to water & steam in boiler.
Factor on which turbine cycle
Efficiency is dependent
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Turbine back pressure (cont.)
Turbine back pressure has a major effect on cycle efficiency. As much as 50% or more of the
heat supplied to the cycle is typically lost in condenser. From the figure shown in previous
slide, the effect of increase in back pressure can be inferred. Increase in back pressure means
increase in exhaust steam temperature (since it is usually at saturated temperature) and hence
heat lost to circulating water is more. Conversely, decrease in back pressure will increase the
enthalpy drop across the turbine and will usually improve turbine cycle efficiency.
Turbine back pressure (cont.)
Condenser vacuum depends on the
following :
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Turbine back pressure (cont.)
Under most operating condition, turbine cycle efficiency improves with decrease in
back pressure. However, under some conditions, efficiency may actually decrease
due to the following reasons:
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Also, power consumed by CW pump increases with flow.
Why dose superheater spray affect efficiency adversely? This may be answered in
two different ways:1) Superheater spray water contributes to steam flow, but unlike feedwater, the
spray water bypasses the HP heaters and thus the benefit of extraction steam heating
is forfeited.
2) Thermodynamics laws state that a reversible process is the most efficient.
Spraying water into steam is a mixing processes are not reversible processes.
Hence, the loss in efficiency.
Why dose reheater spray affect efficiency adversely?
Reasons are the same as given for superheater spray.
However, reduction of efficiency due to reheater spray is much more than an equal
amount of superheater spray, as reheater spray water not only bypasses the HP
feedwater heaters, but also increases HP turbine back pressure, thus reducing its
efficiency.
Hence minimizing spray (either superheater or reheater spray) is one way to
minimize loss.
Why do feedwater heaters improve efficiency? This question can be answered in
two ways, too.
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2)Another explanation for the improvement in efficiency when feedwater heaters
are used is based on thermodynamic laws. Thermodynamic laws state that
reversible could be approached by minimising the temperature difference between
the heated and heating media. Hence, it is more efficient to use low temperature
extraction steam to heat feedwater than high temperature flue gases in the furnace.
How many heaters should one plant have? There is really no limit,
thermodynamically. However, considering the fact that gains in efficiency diminish
with each heater, and the capital cost of each heater, practical limit is about eight
heaters.
What is the effect on efficiency, when one or more heaters is out of service?
If the last HP heater is out of service, turbine output would increase, because , more
steam flows through the turbine at constant valve opening. However, economiser
inlet temperature decrease too. As a consequence, exit gas temperature may go up,
spray water flows may increase and excess air may go up too. Also, considering the
irreversibilities introduced due to the loss of heater adiscussed earlier, both turbine
cycle and boiler efficiencies will decrease.
If an intermediate heater is taken out of service, the colder condensate/ feedwater
will draw additional steam from subsequent heaters and the change in economizer
inlet temperature would not be substantial. However, this may affect the plant in
two ways, viz.
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Steam temperature determine turbine cycle efficiency to a significant extent. At the
same pressure, higher the temperature of steam, larger is its energy content. Hence,
if the turbine back pressure remains same, larger fraction of heat is available for
performing work. Also, as steam temperature increases, wetness fraction of exhaust
steam decreases. This affects the cycle in the following ways:
1)
Heat content of exhaust steam increases, hence increase in efficiency is not as great
as expected.
2) Reduction in wetness of steam reduces erosion of last stage blades and is hence
beneficial.
Main steam & HRH temperature (cont.)
Internal efficiency of turbine
Internal or isentropic efficiency of the turbine is the capacity of the turbine to
convert available heat energy into mechanical energy. It is usually of the order
about 85% to 90%. It is determined by the following factors:
1) Accumulation of deposits on blades.
2) Leakage of steam through inter-stage seals.
3)Internal damage.
Deposits are formed on turbine blades due to carryover. Phosphate deposits are
usually formed on HP blades and silica deposits are on LP blades.
Internal efficiency of turbine (cont.)
Rate of silica vaporization varies directly water I saturated, temperature increases
with pressure and hence silica vaporization increases with pressure.
Deposits on turbine blading can restrict steam flow and cause decrease in turbine
efficiency & capacity.
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Reheating of steam is primarily carried out to maintain dryness of exhaust
steam. However , reheating may also affect efficiency. Efficiency
improvement due to reheating depends on reheat temperature and
pressure.
For reheating to improve cycle efficiency, the ratio of area 122-1 and 12- -2-1
should exceed the ratio of area 1245- and 1245- -1. That is reheating should be
carried out to adequate temperature at the right pressure.
Reheating (cont.)
Throttling
If throttle pressure (measured before turbine control valves) is more than the
required first stage pressure for maintaining a particular load, throttling becomes
necessary to regulate steam flow. Control valves close to maintain first stage
pressure. Closure of control valves results in a decrease of pressure and temperature
of steam downstream of the valves.
Throttling is a constant enthalpy process, with decrease in availability. This means
decrease in enthalpy drop across turbine and hence drop in efficiency. Also, BFP
power consumption increases and net efficiency drops further.
PREFORMANCE MONITORING
OBJCTIVES:
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Operation monitoring
Parameters
Boiler
Turbine
Excess air, exit gas temperature, combustibles in
Flue gas, boiler blow down, makeup, RAPH air leak
Fouling of heat transfer surface, spray (SH, RH),
SH/RH temperature, condenser vacuum
Feed water
Heater
Auxiliaries
water temperature, TTD
aux. power consumption
Heat exchangers fouling, terminal temp. diff. (condenser etc)
Tube cleaning syst availability & operation
Frequency
Filters
Pollution
Control
Equipment
no. of cleaning cycle, differential pressure across
Filters
FGD running hrs & efficiency
ESP availability of thyristors
ash clearance
Fly ash aggregate plant production
Ambient air quality (SO2, Nox etc)
Effluent quality (sewage, DM plant )
Trees planted
High pr valves passing, steam losses
Fuel
maintaining availability of fuel to meet generator
Requirement
Measures for performance improvement
Operating measures
Maintenance measures
Design measures
Refurbishment measures
1. Operating measures
Maintaining boiler heat transfer surfaces clean by soot
blowing .
Ensuring good combustion of fuels by optimising excess O2,
air
Distribution , atomising medium, scavenging burners, air
Temperature
Maintaining condenser tube cleanliness.
Minimising air ingress into condenser & condensate system
Maintaining all feed water heaters in service in optimum
condt.
Maintaining quality of various fluids-boiler water, primary
water
Control fluid, turbine oil, instrument air as per standards.
Monitor performance of each equipement & report deviations
Carry out performance audits for each vital resource
Carry out performance testing of eqpt regularly
Carry out safety interlock checks regularly & maintain
records
2. Maintenance measures
planning of outage on the basis of
operational performance requirement
(predictive maintenance).
Planning of adequate spares & services
for overhaul.
Attending passing valves &save steam,
Water, air & other precious medium
Leakages.
Tuning &calibration of instrument & control
Systems &maintain controls on auto.
3. Design & corrective measures
Failure & fault analysis-recommendation
design or O&M philosophy.
4. Refurbishment measures
for changes in
Long term measure for implementation during major
overhauls due to deterioration in
performance due to aging effects.
