Calculating Fuel Requirements
Care must be taken in using published plant heat rates to calculate fuel requirements due to LHV and HHV difference.
Calculation of plant fuel requirements is very straightforward: simply multiply the heat rate (Btu/kWh) by plant rating (kW) and divide by fuel heating value (Btu lb or Btu mcf) and obtain hourly fuel flow.
Traditions and a disconnect.
Traditionally, gas turbine plant heat rates are specified on the basis of the low-er heating value (LHV) of the fuel, which defines the net amount of fuel energy available for use for conversion to electrical power.
But this causes a disconnect in the real world where fuel purchase con-tracts are traditionally based on the higher (or gross) heating value (HHV) of the fuel. As will be explained, this measure overstates the amount of fuel energy actually available and useable to generate power.
Therefore, before determining the amount of fuel to be purchased, one must be sure to be using an apples-to-apples relationship between the energy needed for plant operation and the energy available in a given quantity of fuel. To do this, conversion to one or the other heating value (HHV or LHV) must be correct.
The water vapor is the culprit.
The complexity here is in how the heating value of the fuel is measured and calculated. This has to do with whether the heat of vaporization of the water formed as a byproduct of combustion is included or not included.
The traditional gas turbine fuels, natural gas (mostly methane, CH4) and distillate fuel oil each contain a considerable amount of hydrogen by weight. This, in turn, results in a considerable amount of water vapor being formed during combustion.
The fuel’s HHV is calculated on the basis that all of the chemical energy in the fuel, including the heat of vaporization of this water formed during combustion, is available thermodynamically as heat input to the power generation cycle.
It assumes that the water vapor in the products of combustion condenses within the power cycle, and the heat released during that condensation is part of the available energy. (This is why HHV is often referred to as the “gross heat value” of a fuel.)
But, in reality, this does not happen since the water vapor escapes through the exhaust stack before it condenses; the amount of energy related to water vapor in the exhaust is actually lost and not usable in the power cycle.
The fuel’s LHV, on the other hand, measures the net useable energy avail-able to the gas turbine power cycle by excluding the heat of vaporization of the water in the combustion products.
This more accurately takes into ac-count the fact that this energy is lost as the water vapor condenses outside of the power cycle. (In some steam-powered plants this energy may be recovered in the air-preheater section where some condensation may take place be-fore the exhaust leaves the stack.)
Bottom line. How much fuel to buy for a gas turbine plant is determined by multiplying the fuel consumption based on the LHV heat rate by the HHV/LHV ratio to obtain the HHV fuel requirement. For distillate fuel this ratio equals about 1.06 whereas that for natural gas, due to its greater hydrogen content, is 1.11.
So, ultimately, these factors enable translation from gas turbine performance calculations (i.e, those done on the basis of LHV heat rate) to the amount and cost of fuel that will actu-ally be required to run the plant.
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