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 makeengineers feel the practicality and ensure team preparation for achieving what is possible.Operating Heat Rate depends on three significant factors: Firing Boiler range coal, maintaininghigh Loading factor and Operating the plant at design parameters.Heat rate in simplicity is ratio between heat input and energy output. There are fourdefinitions. 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 thuscapable of producing power for its life time. Thirdly output is very important for sustainingoperations and ensuring accomplishment of purpose of plant. Efficiency comes in fourthposition and it will decide how well a power unit is performing in converting coal energy toelectrical energy. This order is written for guidance. For example, if life of plant is going todecrease by operations as derived from efficiency lessons then higher life is to be preferred. Forexample, if a reheater coil outlet metal temperature is going high reheater spray is to be giveneven if reheater temperature is going to reduce below design temperature. Subsequentlyproblem 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 ofequipments 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, CWwater etc.RANKINE CYCLEIn selection of the unit various options are available which are dependent on Rankine Cyclewith variety of sets of parameters.The higher the temperature and pressure parameters of main steam and reheat steam thehigher the cycle efficiency. The fixed cost of unit on per MW basis increases as higherparameters are chosen due to usage of costlier metals for withstanding higher parameters. Soonce design parameters are selected the heat rate limit is getting decided.Design of equipments and capacities make the selected rankine cycle reality .The turbineefficiency and condenser design, the boiler efficiency of heat absorption and converting into
steam etc achieved by designers and manufacturers decide the performance of equipment. Thedesigns 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 isnot a static figure and not a constant for the unit. It is dependent on at what load unit isoperating. Design heat rate value is for 100% load operation. Once designs are completed andequipments 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 operateequipment 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 selecteddesign set of parameters a unit gives power at different heat rates at different loads. Turbine MS Steam Boiler Unit HeatSl.No Operation MW Heat Pressure Flow Efficiency Rate 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 LOAD3 80%sliding 480 14.82 1465 1978.71 87.6 2258.80 LOAD4 60%sliding 360 11.11 1099 2052.87 86.8 2365.06 LOAD5 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 isso important that a super critical unit operating at 70%loading factor will be no better than asubcritical unit operating at 100% loading factor. Once a power unit is established coal input toplant, distributing capacities and customers indirectly decide loading factor. This has influenceon achievable heat rate.COAL GCV IN BOILER FIRING RANGEHere it is pertinent to mention that the quantity of coal to be fired for full load is a function ofcoal quality i.e., GCV. But it does not mean that boiler can accommodate limitless quantity ofcoal flow to meet load demand. Boiler is designed for a given range of coal between worst coaland 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 aredesigned for BMCR condition with worst coal. The minimum amount of coal that can be fired iscorresponding 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 for229 TPH with worst coal at 15% moisture and the actual moisture is say 18%, then withoutexceeding the boiler heat loading we can feed 3% more coal, i.e. 235 TPH provided that marginsexist in mills, fans, ESP etc.OPERATING AT NEAR DESIGN PARAMETERSThe last but most important controllable parameters come under “operations at designparameters”. Please refer Annexure at the end for appreciating importance of operating atdesign parameters. For deviation in parameters like main steam pressure, main steamtemperature, reheat steam temperature, condenser vacuum etc. heat rate deteriorates. Sodesign margins are essential for achieving condenser vacuum in all the life time. So condenseron line tube cleaning system is very important. The other parameters are already limited due toconsideration 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 possibleby appropriate operations suitable to the unit. For example Burner tilt, SH RH gas dampersoperation, RH spray, SH spray, soot blowing etc. The mapping of more than 50 parameters ofdesign and actual in operating unit and comparing them continuously will give guidance foroperations. For example, even the best boiler manufacturers can not design soot blowerfrequency 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 needto be operated at the required frequency. Spray indications are guiding factors. A power unit `scontinuous long run operations broadly indicate whether operations are matching to plantequipment. Every parameter and every equipment has it`s own importance and has it`s owninfluence on heat rate. So the mapping of parameters and continuous monitoring andcontrolling bring out the best possible heat rate of the unit. The maintenance works like steamleakage arresting, maintaining heaters availability, soot blower`s availability, high energy drainspassing elimination etc are of high importance in reaching the targeted performance of heatrate. Some maintenance works are long term planning oriented like HIP turbine moduleefficiency, LP turbine module efficiency etc which can be restored at best to design values inlong time overhauls.AUXILIARY POWER CONSUMPTION AND NET HEATRATEThe net unit heat rate is an efficiency measure considering the auxiliary power consumption. Ina unit if auxiliary power consumption is reduced the output to customer increases for the samefuel input to the unit. Since the tariff covers normative consumption any performance betterthan normative consumption will result in substantial savings. The first level of achievementshall be running only the minimum auxiliaries required to be running and only for requiredtime. More efficient drives will give less aux consumption which reflects in net heat rate, pleasenote that the features of design like cooling towers design IDCT/NDCT , motor driven boilerfeed pumps/turbo driven boiler feed pumps are factored in tariff systems. The cost of betterefficient technology is factored in fixed cost and in return on fixed cost recovery, the acceptedinefficient operating technology is factored in variable cost so that variable cost covers the costof design. Any performance better than design plus tolerance is benefit to plant and anyperformance beyond tolerance limit has negative influence. Aux consumption is strong functionof loading factor. The design aux consumption is a percentage figure for unit operating at ratedload. The fans and pumps are designed for high performance at full load. In the unit operatingat partial load these equipments consume power disproportionately at higher levels, so auxconsumption will be higher. Reducing number of outages will not only reduce specific oilconsumption 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 quantityconsumed 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 dayalso is not homogeneous due to various blending operations in coal yard.Sometimes coal is directly sent into bunkers without storage in open stock yards andsometimes it is stored for a longtime in stock yard where coal loses calorific value due tosmoldering fires. Water is sprayed to control smoldering. Rains in rainy season will increasemoisture in coal. Feeder’s weighment increases with moisture in coal due to rains. Themoisture in coal will take away part of useful heat while flowing through boiler. There will belosses due to wind and transportation. So total coal weighment does not exactly match with thecoal received by power station. So CERC provides for 0.2% loss of coal quantity for pitheadpower stations and 0.8% for non-pit head stations. GCV of sampled coal also will not matchwith GCV of dispatched coal from mines due to deterioration in the coal yard. So heat ratebased on as received GCV will be higher in kCal/kWh when compared to as fired GCV basedheat rate (So CERC norms on as fired GCV). So it can be concluded that performanceassessment 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 foundby loss method. The loss method heat rate depends on measured GCV and on high accuracy PGtest instrumentation for evaluating turbine heat rate. The big advantage is that the calculationis on unit basis i.e.: for 1 kg of coal. This eliminates any inaccuracies in flow measurements. Airand gas quantities are determined on theoretical basis (Stochiometry) and from laboratoryanalysis of the fuel. This is more accurate than the field flow meters. Since each loss isseparately calculated it is easy to identify problem areas. This method is used to demonstrateequipment performance capabilities under defined conditions by equipment suppliers toequipment customers. This is special testing method universally standardized for handing overof the equipment to customers with assuring performance.Controlling of parameters at design values will bring best performance out of the equipmentinstalled in the plant. So parameters deviations method is considered as the best method foroperating the plant efficiently. Equipment’s efficiency determination tests will help inmaintaining the equipment over long periods of time. Regarding calorific value of coal flowinginto the boiler at the instant ,a fair judgment can be given by operation department byconsidering the coal flow (tons/hour)being fired for achieving the targeted load and they willvary the coal flow to reach targeted load within boiler operation range. Similarly automationalso assesses calorific value and adjusts automatic response for calorific value changes fromtime to time in a day. Offline calorific value measurements in labs for the coal received inpower 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 possibleperformance 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 incontinuous development all over the world in the last 125 years, hence operating better thandesign heat rate is almost impossible.The heat rate evaluation methods are direct heat rate method (with as fired GCV) best suitedfor commercial purposes. However it has high uncertainty due to less accuracy in coal GCVmeasurement (1%approx) and coal flow measurement (0.25% to 0.5%).The tariff systems takeas fired GCV measured value and estimate coal consumption for giving reimbursement of coalpurchase. This is based on heat rate norm of the unit which is presently 1.065 times designvalue generally. GCV measurement accuracy is less however it has facility of cross checking atdifferent times by different agencies for confidence. Mass flow measurement by gravimetricfeeders is measurement with integration in time continuously. So cross checking is notgenerally possible except flow rate calibration. Any water sprayed for coal fire quenching in theyard can increase mass measurement in feeders. This will reduce coal combustion heat to boileralso. So direct heat rate measured value can increase. This does not mean financial loss becausecoal stock remains in the yard .While stock reconciliation mass balancing is generally done. Formass loss norm is provided as 0.2%to 0.8%. Here it is important to note that there is nocompensation norm for coal quality degradation in the coal stock yard. It is by experience learntthat coal coming from the mines which directly reaches the bunkers gives better heat rate thanthe coal used after stocking two months in the yard. The thumb rule for coal firing is, the freshcoal 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 achievebest performance from the plant. This method assumes machines efficiency at guaranteedvalue.CONCLUSIONThis write up on heat rate is for engineers for beginning of a continuous journey and foroverview of heat rate. As sir Isaac Newton wrote one will find truth in simplicity, for bettercontribution for heat rate from any engineer needs identification of self with any parameterand continuously try to meet design performance. Many books and codes can be referred for indepth understanding of every equipment performance. So in simplicity it can be concluded thatoperational parameters maintaining will be responsibility of operation department throughautomation and maintaining equipments efficiency is responsibility of maintenancedepartments. Thus heat rate in simplicity a team performance of men and machines.
ANNEXUREA. 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/kWhB. 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.