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.

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

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.

Sl.No Operation MW MS Pressure

Steam Flow

Turbine Heat Rate

Boiler Efficiency

Unit Heat Rate

MPa TPH kCal/kWh % kCal/kWh1 VWO 643 16.67 2028 1933.25 87.8 2201.882 T MCR 600 16.67 1866 1943.06 87 2233.40

3LOAD 80%sliding 480 14.82 1465 1978.71 87.6 2258.80

4LOAD 60%sliding 360 11.11 1099 2052.87 86.8 2365.06

5LOAD 40%sliding 240 7.41 752 2190.19 86.6 2529.09

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

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

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

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.

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

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.