Heat rate is the pulse rate of a power plant to know the health of the plant. Net heat rate is the single parameter that encompasses total performance indices of a power plant.

1. HEAT RATE-THE PULSE RATE OF POWER PLANT
PDMV Prasad ,P Koteswara Rao
Truth is ever to be found in simplicity…Sir Isaac Newton.
The following are the facts which make the understanding on heat rate simple and make
engineers feel the practicality and ensure team preparation for achieving what is possible.
Operating Heat Rate depends on three significant factors: Firing Boiler range coal, maintaining
high Loading factor and Operating the plant at design parameters.
Heat rate in simplicity is ratio between heat input and energy output. There are four
definitions.
1. Unit heat rate: - Heat input to boiler / gross electrical generation.
2. Net unit heat rate: - Heat input to boiler/net electrical generation
(Aux consumption must be subtracted from gross generation).
3. Actual unit heat rate :-Total heat input to boiler/ actual net generation of the period
( Including fuel burnt in unit offline period)
4. Design unit heat rate: - Design heat rate is the heat rate anticipated at the design
parameters at specific load like MCR (maximum continuous rating), VWO (valve wide
open operation) etc. with design efficiencies of equipments.
Now let us examine the order of significance in a power plant operation and performance.
1. Legal compliance. 2. Life of the plant. 3. Output of the plant. 4. Efficiency.
Firstly legal and environmental compliance is very important for continuing operations.
Secondly life of plant equipment is very important for it to live design expected life and thus
capable of producing power for its life time. Thirdly output is very important for sustaining
operations and ensuring accomplishment of purpose of plant. Efficiency comes in fourth
position and it will decide how well a power unit is performing in converting coal energy to
electrical energy. This order is written for guidance. For example, if life of plant is going to
decrease by operations as derived from efficiency lessons then higher life is to be preferred. For
example, if a reheater coil outlet metal temperature is going high reheater spray is to be given
even if reheater temperature is going to reduce below design temperature. Subsequently
problem is to be studied why such phenomenon is taking place in the unit.

2. Design Heat rate broadly depends on Rankine cycle –parameters of the unit and Design of
equipments and capacities.
Heat rate deviation occurs due to some or more of the following Equipment degradation /
ageing, Parameter deviations, Process Deviations and change in input conditions like fuel, CW
water etc.
RANKINE CYCLE
In selection of the unit various options are available which are dependent on Rankine Cycle
with variety of sets of parameters.
The higher the temperature and pressure parameters of main steam and reheat steam the
higher the cycle efficiency. The fixed cost of unit on per MW basis increases as higher
parameters are chosen due to usage of costlier metals for withstanding higher parameters. So
once design parameters are selected the heat rate limit is getting decided.
Design of equipments and capacities make the selected rankine cycle reality .The turbine
efficiency and condenser design, the boiler efficiency of heat absorption and converting into

3. steam etc achieved by designers and manufacturers decide the performance of equipment. The
designs are supposed to achieve the Rankine Cycle parameters, output etc.
The lapses in design cannot be covered up by operations on the plant. In design the heat rate is
not a static figure and not a constant for the unit. It is dependent on at what load unit is
operating. Design heat rate value is for 100% load operation. Once designs are completed and
equipments are supplied there is very little that can be done after unit is commissioned.
Knowing about cycle parameters and design of equipments is very important to operate
equipment correctly for getting primarily longer life and secondarily design heat rate.
Loading factor decides the upper limit of heat rate once a unit is in operation. For a selected
design set of parameters a unit gives power at different heat rates at different loads.
Turbine
MS Steam Boiler Unit Heat
Sl.No Operation MW Heat
Pressure Flow Efficiency Rate
Rate
MPa TPH kCal/kWh % kCal/kWh
1 VWO 643 16.67 2028 1933.25 87.8 2201.88
2 T MCR 600 16.67 1866 1943.06 87 2233.40
LOAD
3 80%sliding 480 14.82 1465 1978.71 87.6 2258.80
LOAD
4 60%sliding 360 11.11 1099 2052.87 86.8 2365.06
LOAD
5 40%sliding 240 7.41 752 2190.19 86.6 2529.09

4. At this time it is also to be noted that in our country general tariff conditions cover up to 6.5%
deterioration from design heat rate which happens at 60% loading factor. The loading factor is
so important that a super critical unit operating at 70%loading factor will be no better than a
subcritical unit operating at 100% loading factor. Once a power unit is established coal input to
plant, distributing capacities and customers indirectly decide loading factor. This has influence
on achievable heat rate.
COAL GCV IN BOILER FIRING RANGE
Here it is pertinent to mention that the quantity of coal to be fired for full load is a function of
coal quality i.e., GCV. But it does not mean that boiler can accommodate limitless quantity of
coal flow to meet load demand. Boiler is designed for a given range of coal between worst coal
and best coal. The boiler heat loading, heat absorption patterns, flue gas velocity patterns etc.
are designed in between the best and worst coal range. Generally the boiler and auxiliaries are
designed for BMCR condition with worst coal. The minimum amount of coal that can be fired is
corresponding to the best coal and the maximum amount is decided by the worst coal.
Therefore it is always advisable to fire coal within the range (in between worst and best coals).
However if the moisture content is more than design moisture, then by coal quantity,
equivalent to the difference in moisture can be increased. For example if a boiler is designed for
229 TPH with worst coal at 15% moisture and the actual moisture is say 18%, then without
exceeding the boiler heat loading we can feed 3% more coal, i.e. 235 TPH provided that margins
exist in mills, fans, ESP etc.
OPERATING AT NEAR DESIGN PARAMETERS
The last but most important controllable parameters come under “operations at design
parameters”. Please refer Annexure at the end for appreciating importance of operating at
design parameters. For deviation in parameters like main steam pressure, main steam
temperature, reheat steam temperature, condenser vacuum etc. heat rate deteriorates. So
design margins are essential for achieving condenser vacuum in all the life time. So condenser
on line tube cleaning system is very important. The other parameters are already limited due to
consideration of long life of equipment due to metallurgy considerations.
The O&M Employees of power plant shall ensure parameters at design value as far as possible
by appropriate operations suitable to the unit. For example Burner tilt, SH RH gas dampers
operation, RH spray, SH spray, soot blowing etc. The mapping of more than 50 parameters of
design and actual in operating unit and comparing them continuously will give guidance for
operations. For example, even the best boiler manufacturers can not design soot blower
frequency of operation or even the blowers to be operated. It depends strongly on soot

5. formation after combustion depending on coal. Whenever soot is formed these blowers need
to be operated at the required frequency. Spray indications are guiding factors. A power unit `s
continuous long run operations broadly indicate whether operations are matching to plant
equipment. Every parameter and every equipment has it`s own importance and has it`s own
influence on heat rate. So the mapping of parameters and continuous monitoring and
controlling bring out the best possible heat rate of the unit. The maintenance works like steam
leakage arresting, maintaining heaters availability, soot blower`s availability, high energy drains
passing elimination etc are of high importance in reaching the targeted performance of heat
rate. Some maintenance works are long term planning oriented like HIP turbine module
efficiency, LP turbine module efficiency etc which can be restored at best to design values in
long time overhauls.
AUXILIARY POWER CONSUMPTION AND NET HEATRATE
The net unit heat rate is an efficiency measure considering the auxiliary power consumption. In
a unit if auxiliary power consumption is reduced the output to customer increases for the same
fuel input to the unit. Since the tariff covers normative consumption any performance better
than normative consumption will result in substantial savings. The first level of achievement
shall be running only the minimum auxiliaries required to be running and only for required
time. More efficient drives will give less aux consumption which reflects in net heat rate, please
note that the features of design like cooling towers design IDCT/NDCT , motor driven boiler
feed pumps/turbo driven boiler feed pumps are factored in tariff systems. The cost of better
efficient technology is factored in fixed cost and in return on fixed cost recovery, the accepted
inefficient operating technology is factored in variable cost so that variable cost covers the cost
of design. Any performance better than design plus tolerance is benefit to plant and any
performance beyond tolerance limit has negative influence. Aux consumption is strong function
of loading factor. The design aux consumption is a percentage figure for unit operating at rated
load. The fans and pumps are designed for high performance at full load. In the unit operating
at partial load these equipments consume power disproportionately at higher levels, so aux
consumption will be higher. Reducing number of outages will not only reduce specific oil
consumption but also aux consumption considerably.
HEAT RATE CALCULATION SMETHODS
1. Direct Heat Rate 2. Loss method 3. Parameter deviation method.
Direct heat rate method uses coal quantity consumed, GCV and units generated. Coal quantity
consumed is accurately measured by gravimetric feeders within the specified accuracy. GCV is

6. measured by sampling coal at bunker inlet or feeder inlet however the coal overtime in a day
also is not homogeneous due to various blending operations in coal yard.
Sometimes coal is directly sent into bunkers without storage in open stock yards and
sometimes it is stored for a longtime in stock yard where coal loses calorific value due to
smoldering fires. Water is sprayed to control smoldering. Rains in rainy season will increase
moisture in coal. Feeder’s weighment increases with moisture in coal due to rains. The
moisture in coal will take away part of useful heat while flowing through boiler. There will be
losses due to wind and transportation. So total coal weighment does not exactly match with the
coal received by power station. So CERC provides for 0.2% loss of coal quantity for pithead
power stations and 0.8% for non-pit head stations. GCV of sampled coal also will not match
with GCV of dispatched coal from mines due to deterioration in the coal yard. So heat rate
based on as received GCV will be higher in kCal/kWh when compared to as fired GCV based
heat rate (So CERC norms on as fired GCV). So it can be concluded that performance
assessment of power station reflected by heat rate of fired coal and not of receipt coal.
Gross heat rate by loss method is calculated from turbine heat rate and boiler efficiency found
by loss method. The loss method heat rate depends on measured GCV and on high accuracy PG
test instrumentation for evaluating turbine heat rate. The big advantage is that the calculation
is on unit basis i.e.: for 1 kg of coal. This eliminates any inaccuracies in flow measurements. Air
and gas quantities are determined on theoretical basis (Stochiometry) and from laboratory
analysis of the fuel. This is more accurate than the field flow meters. Since each loss is
separately calculated it is easy to identify problem areas. This method is used to demonstrate
equipment performance capabilities under defined conditions by equipment suppliers to
equipment customers. This is special testing method universally standardized for handing over
of the equipment to customers with assuring performance.
Controlling of parameters at design values will bring best performance out of the equipment
installed in the plant. So parameters deviations method is considered as the best method for
operating the plant efficiently. Equipment’s efficiency determination tests will help in
maintaining the equipment over long periods of time. Regarding calorific value of coal flowing
into the boiler at the instant ,a fair judgment can be given by operation department by
considering the coal flow (tons/hour)being fired for achieving the targeted load and they will
vary the coal flow to reach targeted load within boiler operation range. Similarly automation
also assesses calorific value and adjusts automatic response for calorific value changes from
time to time in a day. Offline calorific value measurements in labs for the coal received in
power station will help in coal customer confidence in the coal supplied by coal mines.
So, conclusively it can be said that team work of O&M can try to get highest possible
performance of the unit by microscopic identification parameter wise and improving it to meet

7. the heat rate design value. Please note that coal based power technology had been in
continuous development all over the world in the last 125 years, hence operating better than
design heat rate is almost impossible.
The heat rate evaluation methods are direct heat rate method (with as fired GCV) best suited
for commercial purposes. However it has high uncertainty due to less accuracy in coal GCV
measurement (1%approx) and coal flow measurement (0.25% to 0.5%).The tariff systems take
as fired GCV measured value and estimate coal consumption for giving reimbursement of coal
purchase. This is based on heat rate norm of the unit which is presently 1.065 times design
value generally. GCV measurement accuracy is less however it has facility of cross checking at
different times by different agencies for confidence. Mass flow measurement by gravimetric
feeders is measurement with integration in time continuously. So cross checking is not
generally possible except flow rate calibration. Any water sprayed for coal fire quenching in the
yard can increase mass measurement in feeders. This will reduce coal combustion heat to boiler
also. So direct heat rate measured value can increase. This does not mean financial loss because
coal stock remains in the yard .While stock reconciliation mass balancing is generally done. For
mass loss norm is provided as 0.2%to 0.8%. Here it is important to note that there is no
compensation norm for coal quality degradation in the coal stock yard. It is by experience learnt
that coal coming from the mines which directly reaches the bunkers gives better heat rate than
the coal used after stocking two months in the yard. The thumb rule for coal firing is, the fresh
coal received must reach coal bunkers first for firing in the boiler.
The heat rate by parameters deviation method is the best method for controlling the process,
understanding the maintenance required for the equipment on day to day basis and to achieve
best performance from the plant. This method assumes machines efficiency at guaranteed
value.
CONCLUSION
This write up on heat rate is for engineers for beginning of a continuous journey and for
overview of heat rate. As sir Isaac Newton wrote one will find truth in simplicity, for better
contribution for heat rate from any engineer needs identification of self with any parameter
and continuously try to meet design performance. Many books and codes can be referred for in
depth understanding of every equipment performance. So in simplicity it can be concluded that
operational parameters maintaining will be responsibility of operation department through
automation and maintaining equipments efficiency is responsibility of maintenance
departments. Thus heat rate in simplicity a team performance of men and machines.

8. ANNEXURE
A. HEAT RATE DEVIATIONS WITH PARAMETER DEVIATIONS
1) MAIN STEAM PRESSURE - 14.54 kCal/kWh for 1 Mpa
2) MAIN STEAM TEMPERATURE - 0.38 kCal/kWh for 10C
3) REHEAT STEAM TEMPERATURE - 0.38 kCal/kWh for 10C
4) VACUUM – Standard – 89.50 Kpa
89 Kpa – 27.15 kCal/kWh
88 Kpa – 46.54 kCal/kWh
87 Kpa – 60.12 kCal/kWh
86 kpa – 75.63 kCal/kWh
85 kpa –89.21 kCal/kWh
93.85 Kpa – Improvement of 34.91 kCal/kWh
5) SUPERHEATER SPRAY - 0.28 kCal/kWh FOR 10 TONS
6) REHEATER SPRAY - 0.18 kCal/kWh FOR 1 TON
7) MAKE UP - 0.16 kCal/kWh FOR 1TPH
8) CONDENSER SUBCOOLING – 0.89 kCal/kWh FOR 10C
9) HPH HEATER TTD DEVIATION – 1.8 kCal/kWh FOR 10C
10) HPH HEATER DCA DEVIATION – 0.25 kCal/kWh FOR 10C
11) HP HEATER -1 OUT OF SERVICE – 23 kCal/kWh
12) HP HEATER -2 OUT OF SERVICE - 17 kCal/kWh
13) HP HEATER -3 OUT OF SERVICE - 17 kCal/kWh
14) HP/IP TURBINE CYLINDER EFFICIENCY 4 kcal/ %
15) EXCESS AIR IN BOILER – 7 kCal/kWh

9. 16) COAL MOISTURE – 2-3 kCal/kWh
17) BOILER EFFICIENCY – 22 kCal/kWh
18) UNBURNT CARBON / % - 10 – 15 kCal/kWh
B. COST OF HEAT RATE LOSS
Heat Rate Increase by 1 kcal/kwh
Total Generation in a day at 90 % PLF = 14.4 x 0.9 x 106 kWh = 12960000 kWh
So total extra coal consumed per day @ 4300 kCal/kg GCV = 3.013 MT
So cost of this extra coal per day @ Rs 6000/MT = Rs 18081
Cost per month = 18081 x 30 = Rs 5.4 Lacs PM.