Calculation of heat rate & efficiency of the power plant

Heat rate is the amount of energy used by an electrical generator/power plant to generate one kilo Watt-hour (kWh) of electricity

Heat rate (HR) = Heat input / Power generation =Kcal / Kwh

Boiler calculations for Boiler operation engineer (BOE) exam

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Total heat input:

The chemical energy available in the fuel (coal, biomass, oil, gas etc) is converted into heat energy in Boilers, this process is called as oxidation. The heat available in the fuel is measured in terms of Kcal/kg, KJ/kg or BTU units. The part of this fuel is used as useful heat and rest is lost as dry flue gas loss, moisture loss, un  burnt loss, radiation/convection losses etc.Based on Boiler efficiency this heat energy from the fuel is utilised, generally fuel heat utilisation is in the range of 60 to 90%.

This heat generated in the boilers due to oxidation of fuel is used to generate high pressure & temperature steam. Thus generated steam is fed into the steam Turbine, where this heat energy also called thermal energy gets converted into Kinetic energy then into Mechanical energy in steam turbine, finally mechanical energy into electrical energy in Generator.

So total heat input to power plant =Chemical energy + Thermal energy + Kinetic energy+Mechanical energy

Output =Electrical power in Kwh

Heat rate =Heat input / Power generation

Efficiency calculation of power plant equipments

Efficiency:

Efficiency is nothing but the ratio of the useful work done to the heat provided. This means the friction and other losses are subtracted from the work done by thermodynamic cycles.

In Boilers efficiency = Heat out from the Boiler/Heat input to the Boiler

Heat output is Thermal energy in steam and heat in put is calorific value present in the fuel

In case of Turbine, Efficiency =860 X 100/Turbine heat rate

Case-1: Gross heat rate of Thermal power plant

In thermal power plants all the thermal energy generated from the steam generators/Boilers is used for only power generation.

Example: A 100 MW thermal power plant is running on 100% PLF, which consumes around 55 MT of coal having GCV 4500 kcal/kg per hour, then calculate the Gross station heat rate of the plant

 We have,

Gross station heat rate =Heat input to the plant / Power generation

                                    =Fuel consumed (MT)X GCV (kcal/kg)of fuel/Power generation/MWH

                                    = (55 X 4500)/100

                                    = 2475 kcal/kwh

Above problem can be solved by converting fuel consumed in kg/hr & power generation in

Kwh, then heat rate can be calculated as,

                                    =55 X 1000 X 4500/(100 X 1000) =2475 kcal/kwh

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Case-II

Station heat rate of Co-generation plant

In cogeneration plant thermal energy is used for process requirement and power generation. In cogeneration plant there are various sources of heat input & output to the station & from the station, where as in Thermal power plants heat input & output sources are only one.

Heat input to the station is in the form of heat energy present in fuel, make up water and return condensate from the process.

Heat output from the station is in the form of heat energy in process steam & power generation

Cogeneration heat rate = (Fuel consumed (MT) X GCV of fuel (kcal/kg + Quantity of return condensate from process (MT) X its enthalpy (kcal/kg) + Quantity of makeup water (MT) x its enthalpy kcal/kg)-(Process steam quantity (MT) X its enthalpy in kcal/kg) /Power generation in MW

Example: A process based cogeneration plant has following data on full day case study. Calculate the station heat rate

Sl No.	Particular	Value
1	Power generation	977 MW
2	Total coal consumption Q1	875 MT
3	Gross calorific value of coal G	5100 kcal/kg
4	Steam given to process-1 plant at 2 kg/cm2g and 135 0C Q2	3720 MT
5	Steam given to process-2 plant at 7 kg/cm2g and 175 0C Q3	192 MT
6	Return condensate from process-1 plant at temperature 120 0C Q4	3350 MT
7	Return condensate from process-2 plant at temperature 85 0C Q5	135 MT
8	DM water make up to boiler at temperature 25 0C Q6	490 MT

        

From above given data we have,

Enthalpy of steam given to process plant-1 h2= 666.71 kcal/kg……..Refer steam table

Enthalpy of steam given to process plant-2 h3= 651.68kcal/kg

Enthalpy of process-1 return condensate h4 = 120.3 kcal/kg

Enthalpy of process-2 return condensate h5 = 85 kcal/kg

Enthalpy of makeup water h6 = 25 kcal/kg

We have station heat rate = ((Fuel consumption X GCV +Heat content in return condensate + Heat content in makeup water-Sum of heat content in process steam))/Power generation.

= (( Q1X G + Q4 X h4+Q5X h5 + Q6X h6)-(Q2 X h2+Q3 X h3))/Power generation

= ((875 X 5100+3350 x 120.3+135 x 85 +490 x 25)-(3720 x 666.71+192 x 651.68))/977

= 2337.71 kcal/kg

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Turbine heat rate and efficiency:

Case-I: Turbine heat rate of a thermal power plant during performance guarantee (PG) test

Turbine Heat Rate (THR) = Steam flow X (Enthalpy of steam-Enthalpy of feed water)/Power generation

Case-II: Turbine heat rate of a thermal power plant during normal O&M condition

Turbine Heat Rate (THR) = (Steam flow X Enthalpy of steam-Feed water flow X Enthalpy of feed water)/Power generation

Turbine efficiency is given by

Turbine efficiency =860 X 100/Turbine heat rate

Example: A 22 MW Turbine has inlet steam flow 100 TPH at pressure & temperature 110 kg/cm2 & 535 Deg C respectively, then calculate the Turbine heat rate in both PG test case & O&M condition, also calculate Turbine efficiency in both the cases. Consider feed water temperature at economiser inlet is 195 deg c & flow is 102 TPH.

Solution:

Turbine inlet steam enthalpy at operating pressure & temperature H1 =824 kcal/kg

Feed water enthalpy =H2=198.15 kcal/kg

Steam flow =100TPH

Power generation =22 MW

Turbine heat rate of a thermal power plant during performance guarantee (PG) test

Turbine Heat Rate (THR) =(100 X (824-198.15)/22) =2844.77 kcal/kwh

Turbine efficiency =(860 X 100)/2844.77 =30.23%

Turbine heat rate of a thermal power plant during normal O&M condition

Turbine Heat Rate (THR) =(100 X 824-102 X 198.15)/22 =2826.25 kcal/kg

Turbine efficiency =(860 X 100)/2826.25 =30.42%

Case-III: Cogeneration Turbine heat rate

In case of Co-gen, Turbine heat rate is calculated by considering extractions and return condensate received.

Formula-1

Co-gen-THR =((Turbine inlet steam flow X its Enthalpy)-(Process steam flow X Enthalpy Exhaust steam flow X Enthalpy))Power generation

Formula-2

Co-gen-THR =((Turbine inlet steam flow X its Enthalpy + Process return condensate flow X its Enthalpy + Make up water flow X Its enthalpy)-(Process steam flow X Enthalpy + Feed water flow X Enthalpy)) / Power generation

Example:21 MW condensing cum extraction turbine has inlet steam flow 120 TPH at 88 kg/cm2g pressure and 520 0C temperature, it has two extraction first, at 16 kg/cm2g pressure and temperature 280 0C at flow 25 TPH and second at 2.5 kg/cm2g pressure and temperature 150 0C at flow 75 TPH.Remaining steam goes to condenser at exhaust pressure 0.09 kg/cm2a.Calculate the turbine heat rate and thermal efficiency by using both formulae. Consider steam given to process is 10 TPH less than each extraction, the return condensate from the process is 70 TPH at temperature 90 deg c, feed water flow 122 TPH at temperature 195 deg c & make up water flow 13 TPH at temp 28 deg c.

Given that,

Power generating capacity of turbine = 21 MWH

Q1 = 120 TPH

Enthalpy h1 at 88 kg/cm2g and 5200C = 820.66 kcal/kg

Q2 = 25 TPH

h2 at 16 kg/cm2g and 2800C  = 715.88 kcal/kg

Q3 = 75 TPH

h3 at 2.5 kg/cm2g and 1500C  = 658.40 kcal/kg

Condenser steam flow Q4 = Q1-Q2-Q3 = 120-25-75 = 20 TPH

h4 at exhaust pressure =  44.06 kcal/kg

Formula-1

Cogen-Turbine heat rate (THR) = (Input heat to turbine- Sum of extraction and exhaust heat)/Power generation

                                     = ((Q1 X h1)-(Q2 X h2+Q3 X h3 +Q4 X h4))/Power generation

                                     = ((120 X 820.66)-(25 X 715.88 +75 X 658.40 + 20 X 44.06))/21

                                     = 1443.85 kcal/kwh

Turbine thermal efficiency = (860 X 100)/Turbine heat rate

                                          = (860 x100)/1443.85

                                          = 59.56%

Formula-2

Co-gen-THR =((Turbine inlet steam flow X its Enthalpy + Process return condensate flow X its Enthalpy + Make up water flow X Its enthalpy)-(Process steam flow X Enthalpy + Feed water flow X Enthalpy))Power generation

THR =((120 X 820.66 +90 X 90 +13 x 28)-(15 X 715.88 +65 X 658.40 + 120 X 198.15))/21

THR =1495.73 kcal/kwh

Turbine thermal efficiency = (860 X 100)/Turbine heat rate

                                            = (860 x100)/1495.73

                                            = 57.49%

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Opportunities for energy conservation in power plants

   

Energy conservation in Boilers:

• Control excess air in combustion which can lead to huge dry flue gas loss. Install online O2 analyzer for better controlling of excess air.

• Control Air pre-heater (APH) out let flue gas temperature, it is said that on every 22 °C rise in boiler outlet flue gas temperature leads to decrease in boiler efficiency by 1%.

• Optimize fuel moisture, which can lead to loss due to moisture.

• Installation of fuel (bagasse) driers for recovering potential heat from chimney inlet flue gas.

• Control unburnt in bed ash and fly ash, which can lead heat and fuel loss.

• Maintain maximum possible feed water inlet temperature at economizer inlet. On 6–7 °C rise in temperature of feed water at economizer inlet leads to 1% fuel saving. And 15 °C rise can lead to increase in overall thermal efficiency by 3%.

• Utilize unburnt from Economiser, bank zones by incorporating Cinder reinjection (CR) system

• Optimize boiler continuous blow down to reduce heat loss with hot water

• Create the ideas for utilizing blow down water for heating of combustion air, or feed water and even boiler blow down water can be used for cooling tower make up by reducing its temperature

• Utilize blow down flash steam for heating feed water in deaerator. Continuous blow down (CBD) from steam drum operating at 115 kg/cm2 releases 42–45% flash steam.

• Reduce boiler pressure if permissible, for process, boiler pressure can be maintained as per process requirement. This is the most effective way of fuel saving.

• Incorporate VFD to all boiler auxiliaries like fans and fuel feeding systems.

• Instead of throttling discharge air dampers ,provide inlet guide vanes to fans to control the air flow.

• Arrest all boiler steam, flue gas and water leakages.

• Operate boiler on optimum loads, boilers give more efficiency at 65–85% of full load.

• Inspect air distribution nozzles, dampers regularly.

• Try to get maximum temperature of combustion air by incorporating Steam coil air preheaters (SCAPH).It is said that on every 20 °C rise in combustion air temperature leads to increase in boiler efficiency by 1%.

• 10–12% of hot air recirculation into FD fan suction can maintain rated APH inlet air temperature, which can lead to elimination of SCPAH thereby reducing LP steam consumption.

• Utilize DG exhaust flue gas temperatures for heating systems like combustion air, feed water etc.

• Operate the soot blowers regularly to remove soot formed on heating surfaces. It is estimated that 2.5% of fuel consumption will increase on deposition of 3 mm soot on heating surfaces of pressure parts.

• Operate boiler feed pumps at rated or maximum possible suction pressure to reduce auxiliary power consumption.

• Incorporate VFD to boiler feed pumps and reduce speed whenever possible

• Operate feed water control valve at maximum opening i.e. control valves should open 70–80% for normal operation. It will reduce the pressure drop in control valves and hence load on boiler feed pumps.

• Attend the the leakage of Boiler feed pumps ARC valves.

• Use Neutra pit water or ETP treated water for ash quenching and other services.

• Use Neutra pit water or ETP treated water for ash quenching and other services.

• Operate the pneumatic ash handling system in probe mode instead of timer mode to save the compressor power.If run in timer mode optimise ash conveying time & cycle time.

• Use high temperature coatings for valves, lines etc. o reduce heat loss.

• Optimize deaerator steam vent line and orifice size to avoid excess steam vent or utlise this vent out steam for heating combustion air.Deaerator steam vent loss is around 0.1 % of Boiler steam generation.

• Remove air from indirect steam using equipments, as 0.25 mm thick air offers the same resistance to heat transfer as a 330 mm thick copper wall.

• Maintain water quality to avoid boiler scaling. A 1 mm thick scale deposit in the water side could increase fuel consumption by 5–8%.

• Insulate all hot water storage tanks.

Energy conservation in Fuel handling & storage system:

• Always store fuel in fuel shed to avoid wetting and excess drying of fuel in rainy and summer seasons respectively.

• Store fuel on PCC (Plain cement concrete) bed to avoid carpet loss of fuel.

• Cover fuel yard and storage shed by trees (bamboo) to reduce handling and feeding loss of fuels during high wind speed.

• Conveyors should have seal proof hood covers to avoid flying/escaping of fuel during high wind speed.

Energy conservation in Turbine and its auxiliary system:

• Maintain maximum rated pressure at Turbine inlet to achieve less SSC.

• Maintain maximum possible vacuum to reduce SSC.

• Maintain optimum cooling water inlet and outlet temperatures for all heat exchangers like steam condensers, oil coolers and Generator air coolers

• Optimize cooling tower blade angles, more the angle more the power consumption.

• Reduce the clearance between CT fan blade and concrete hood or fiber hood; generally it should be up to 25–30 mm.

• Provide VFDs to cooling tower fans and Main cooling water pumps to control speed as per requirement.

• Replace all aluminum blades by energy efficient FRP blades.

• Clean cooling tower water spray nozzles regularly.

• Arrange drift eliminators and fills properly for uniform spreading of air and water.

• Clean condenser and ejector tubes yearly to achieve proper heat transfer

• Keep cooling tower surrounding free from structures, building, trees to enable free air movement.

• Ensure cooling water pump top casing crub level is below the normal operating level of cooling tower toavoid carrying of air in water or it may lead to cavitation.

• Make a provision for online backwash systems for surface condenser and oil coolers to improve heat transfer.

• Clean oil pumps filters regularly.

• Incorporate VFD to Condensate extraction pumps (CEPs).

• Install condensate recovery system from all process line drains and traps.

• Use CEP discharge water instead of BFP discharge water for desuperheating of low pressure process steam.

• Select optimum head condensate extraction pumps. If Deaerator working pressure is 2.75 kg/cm2A CEP of 80 meter discharge head considering all losses is sufficient.

• Try to get maximum percentage of condensate from processes.

Energy conservation in other balance of plant (BOP) areas:

• Replace all globe valves by ball valves of instrument pressurized air.

• Clean suction air filters of compressors regularly as 250 mmwc pressure drop in suction filter will lead to the reduction of compressor efficiency by 2%.

• Opening of air filters should be sufficient to suck enough air.

• Locate compressors at lower level as at higher elevation compressors consume more power.

• Install compressors in well ventilated areas.

• Ensure cool water and air for compressors air cooling system. At every 4 °C rise in inlet cooling water temperature will increase power consumption by 1%.

• Clean after and inter coolers regularly to get better heat transfer.

• Install auto moisture drains for air lines.

• Avoid unnecessary use of compressed air for applications like cleaning, washing, pneumatic tools etc.

• Optimize discharge air pressure of compressor, on reduction of 1 kg/cm2 discharge air pressure leads to input power saving by 6–10% And on reduction of discharge air pressure by 1 kg/cm2 will reduce air leakage by 10%.

• Carryout periodic air leakages test.

• Prefer two stage reciprocating air compressors over single stage for same FAD and pressure as single stage compressors consume much power than multi stage.

• In centrifugal fans (ID, FD and SA fans) Maintain optimum cone clearance and overlap gap to reduce power consumption.

• Provide optimum height  silencers for high speed centrifugal fans and take care the silencer is free from all obstacles.

• Modify ducting system by removing all obstacles to avoid pressure drop.

• Incorporate VFDs to ventilation blowers to control speed during low temperature season (Winter and rainy seasons).

• Provide VFDs to all possible pumps.

• Replace all loose V belts as loose and damaged V belts conribute in more power consumption

• Timely checking of equipment vibrations and alignment to avoid unnecessary power consumption and machine failure.

• Avoid over greasing of bearings to reduce churning effect and energy consumption.

• Replace higher rated capacity pumps with rated to avoid running of higher capacity pumps under load.

• Replace all worn out wear rings of impellers and casing, as more clearance between wear ring and pump suction will contribute in more power consumption to maintain required flow.

• Instead of controlling discharge valve, impeller trimming is the best way to control flow and power consumption.

• Use booster pumps for small loads requiring higher pressure.

• Provide all measuring instruments like flow, pressures for pumps to monitor their performance regularly.

• Avoid bends and U seal pattern for suction line of pumps.

• Use mechanical seals over gland packings.

• Avoid connecting small size pump discharge line to higher size header, the ratio of header and discharge line should be in the range of 1.3 to 1.6 only. For higher size ratio power consumption will be more.

• Replace all conventional gear boxes like worm-worm wheel and helical by more efficient planetary gear box.

• Replace DOL starter motors of heavy equipments like chain conveyors with soft starters.

• Modify the chain conveyor system to reduce unwanted dead loads like slat weights, link size etc.

• Incorporate timers of street and plant lighting system.

• Replace all the plant lighting bulbs by high efficient LED bulbs.

• Maintain higher power factor at all the load ends by installing capacitor banks to reduce excess reactive power.

• Provide forced cooling system for higher capacity power transformers to reduce losses.

• Provide proper ventilation to the motors. For every 10 °C increase in motor operating 

• temperatures over recommended peak, the motor life is estimated to be halved.

• Synchronous motors are more suitable to improve power factor.

• Balance the three phase power supply, an unbalanced voltage can increase motor input power by 3–5%.

• Ensure the motor proper rewinding, an improper rewinding could lead to efficiency reduction.

• Ensure proper alignment between motor and load ends (fans, pump, gear box, blower etc.) to avoid more power consumption and failures.

• Optimize the quantity of lubricants in bearings and gearboxes to reduce unwanted load due to excessive lubricants.

• Provide an alternate arrangement for connecting river water directly into clarifier; it will save power consumption for lifting water from reservoir to clarifier.

• Prefer gravity make up system for condenser hot well make up, it will save pump power for hot well

For more>>>>read Powerplant and calculations

Introduction to power plant

A power plant also referred to as a power station and which generates power, it is an energy producing source and the final form of energy being converted is electrical energy. Power plant is one of the most vital sectors for any country or industry. In this competitive world nothing will move without the  electric power, be it an industry, home, office etc.So such an important sector needs to be operated and maintained very dexterously, efficiently and safely.

Based on the primary source of energy power plant are of following types;

Wind power plants: Primary source of energy is natural wind, the speed of win is being utilized to produce electric energy.

Solar power plants: Primary energy is the energy received from the sun that again is converted into electric energy.

Hydal power plant: As the name indicates primary source of the energy is Hydro-water

Tidal power plant: Energy present in the tides is utilized for producing the electric energy

Geothermal power plant: Geothermal power plants use hydrothermal resources that have both water (hydro) and heat (thermal). Such type of  plants require  Hydrothermal resources having high-temperature that come from  dry steam wells or from hot water wells.

Thermal power plants: Primary source of the energy is fuel, where energy from the various fuel on oxidation is converted into heat energy, further, this heat energy is converted into high pressure and temperature thermal energy (Steam).This steam is made to fed in steam turbines where the thermal energy present in the steam is converted into kinetic energy in Turbine finally kinetic energy is converted into electric energy in Turbo generators.

Thermal power plants may be captive power plants, cogeneration plants and large power plants.

Power sector is matured sector and the plants are designed meticulously addressing the prior challenges faced. Ultimate automation is gradually being adapted to reduce the human error and to minimize the loss in efficiency.

Thermal power plant basically comprises of ;

Boiler, Steam Turbine, Alternator, Ash handling plant, fuel handling plant and water treatment plants.

Various fuels like coal, bagasse, wood, cane trash, rice husk, saw dust, gas, oil etc are used in Boilers to convert their chemical energy into heat energy.

In the present scenario most of the power plants are adopting the higher range of steam pressure and temperature to reduce the specific fuel consumption of the power plant.

The power plant auxiliaries include;

Air compressors: Different kinds of compressors like reciprocating compressors, screw compressors & centrifugal compressors are generally used in all power plants.

Boiler auxiliaries: like fans & pumps of different configuration are used .For Boiler feed pumps generally HT & LT type motors are used.

Ventilation and air conditioning systems: used for office, control rooms, electrical panel rooms.

Cooling water system: Includes cooling tower or chillers with auxiliary cooling water pumps.

Lube oil system: Includes lube oil storage tanks, pumps, filtering, cooling and purifying units.

Safety health and environment in power plants:

As the thermal power industry is related to high pressure and temperature fluid, high pressure compressed air, hot, height and confined  space related works, so it becomes very vital to follow all the operation and maintenance works with safely.

Safety objectives in power plant:

1.To create safe, environment friendly and healthy environment at the work place

2.Prevent accidents and harmful effects on workers health

3.To avoid injuries to workers and damages to the plant equipments

4.To ensure full compliance with health, environment and safety policies

5.To empower employees to stop unsafe practices in work

6.Investigating the accidents and report analysis

7.To perform work hazard analysis  and risk assessment

Responsibility of Employer for safety health and environment:

1.To provide safe and healthy environment for their employees

2.Providing proper training and Personal protective equipments to all employees

3.Providing proper tools tackles

4.To eliminate unsafe conditions in the plant

5.Providing proper first aids to all employees

6.Conducting medical examinations to all employees

7.Adopting & following all disposal methods

8.Adopting & following all pollution control norms

SIGNIFICANCE OF ENERGY CONSERVATION IN POWER PLANTS:

Energy is the prime mover of economic growth and is vital to sustenance of a modern economy. Energy efficiency has assumed a critical role in our pursuit of sustainable development. The gains from energy efficiency have been harnessed the world over and India in no exception. Studies carried out by various organizations reveal that World industry has the potential to save 20 to 30% of total energy consumption.

Power plant involves all types of energies namely Mechanical energy, chemical energy, pneumatic energy, hydraulic energy, thermal energy, kinetic energy and finally electrical energy, this implies that power generation sectors have huge scope to conserve and save the energy up to extreme zero levels. Now a days there is huge competition in reducing the auxiliary power consumption of power plants.Generally, around 6 to 10% of total power generated is consumed in house by plant auxiliaries like pumps, fans, compressors, fuel handling plant, ash handling plant, water treatment plant etc.

It is said that one unit power saved is equivalent to two units of power saving.

Main contents :
1.Power generation calculations in steam Turbine
2.Specific terms used in power plants
Heat rate in power plant
Opportunities for energy conservation in power plant
Best practices for reduction of power plant Auxiliary power consumption
Vibrations in machines
Constructional materials & Welding electrodes
ESP troubleshooting