Heat Rate of Thermal Generating Stations

Give Above is Heat Rate for Thermal Generating Stations as defined by ASME, Performance Test Code-6,  and CERC(Central Electricity Regulatory Commission, Delhi)

The Unit of Heat rate can be Kcal/Unit, or KJ/ Unit, or Btu/Unit, the Unit here in the denominator refers to Kw-hr of electricity produced at Generator Terminal.

Thus an Electric Generator Producing 300 MW-hr (MegaWattHour), consuming 720 Million Kilocalories of Heat input will have ,

 Heat Rate =  2400 Kcal/Unit

The Heat Rate is generally categorized as:-

 Gross Station Heat Rate and Net Station Heat rate.

If the Power generated considered in the denominator does not include the In-house or Auxilliary Power Consumed by the station then it is Known as Net Station Heat Rate other wise it is known as Gross Station Heat Rate or Station Heat Rate, In above example if we consider that the Unit/ Station has :-
Gross Power Generated = 300 MW or 300 MWHr Considering an Hour of Power Generation
Auxilliary or In house Power consumption = 21 MW or 21 MWHr considering an Hour of Power Consumption
Net Power Available Ex-Bus for Export= 279 or 279 MWHr of  Net Power exported

Then Gross Station Heat Rate(GSHR)= 720 MlnKcal/300000 Units= 2400 kcal/Unit

Net Station Heat Rate(NSHR)=  720MlnKcal/(300000- 21000) Units= 2581 kcal/Unit

As per CEA(Central Electricity Authority India) Technical Standards 2010

Which Means that in case Boiler efficiency of a Supercritical unit is = 87.5%, GTCHR= 1850
and Piping lossess are 1%, then the Gross Station Heat Rate will be = 1850/(0.875x0.99)= 2135.64 Kcal/Unit

Over Heat Degree of Super Heat (OHDR)—and its Implications

Over Heat Degree of Super Heat (OHDR)—and its Implications

In Operation of Supercritical Boiler, many important Parameters are required to be monitored and controlled; Over Heat Degree of Super Heat (OHDR) is one of them.

Just to simplify, if in a supercritical Boiler, if the evaporator water temperature is maintained at 405 Degc at 25 MPA , then it will have a OHDR of 20 degc, as the critical temperature as per T-S curve for a pressure of 25 MPA is 385 Degc

Generally Boiler Manufacturers specify OHDR around 25 Degc, and Water to coal ratio of 6 to 7.

Higher OHDR than 25 Degc essentially means that the Heat absorbed in the evaporator section is more than designed as a result of which steam will be generated in in Straight Water walls , steam being a bad conductor of Heat will not be able to absorb heat from the Water wall Tubes, as a result of which the metal temperature of water walls will have a tendency to shoot up. This may lead to failure of water walls or supply tubes exposed to high heat radiant portions, but not able to cool down.

Higher OHDR also means that as compared to Water flow, coal consumed will be more, displaying lower Water to Coal ratio than specified. Low water to coal ratio will also happen in cases where GCV of coal is much lower requiring much more coal for generating same quantity of steam.

Lower OHDR than 25 DegC signifies that Water is not able to absorb required heat, thereby leading to evaporation of water post evaporator, Water to coal ratio will show higher value , establishing that water flow is higher than Coal, this will lead to drop in Water and eventually steam temperature. Low OHDR will lead to lowering of Steam generation and eventually MW generated.

So monitoring and maintaining OHDR and Water to Coal ratio are significantly important aspects for stable and efficient Operation of a Supercritical Boiler

Chetan Shrivastav