Fuel Specific Efficiency and Heat Rate

Conversion from heat rate to fuel rate requires a knowledge of the energy density, or heating value, of the particular fuel. Liquid fuels generally are expressed on an energy per unit mass or energy per unit volume basis (Btu/lbm or Btu/gal). For gaseous fuels, values are expressed on a volume basis, and the reference conditions for the fuel volume measurements (pressure, temperature, and degree saturation with water vapor), as well as test conditions, have to be stated explicitly.

Care must be taken when converting heat rates to fuel rates to identify whether the fuel energy density refers to the higher heating value (HHV) or lower heating value (LHV) of fuel. During the combustion of hydrocarbon fuels, some of the oxygen is combined with hydrogen, forming water vapor that may leave the combustion device either in vapor or condensed to liquid state. When the latent heat of vaporization is extracted from the flue products, causing the water to become liquid, the fuel's energy density is identified as HHV. When the equipment used allows the water to remain in the vapor state, the energy density is identified as LHV. Since different fuels have varying amounts of hydrogen, the numerical relationship between LHV and HHV varies.

Prime mover (reciprocating engine and combustion gas turbine) performance is usually based on LHV, while fuel energy is often expressed in HHV. Thus, in order to convert heat rates specified in LHV to purchasable fuel units, one must know the energy density of the fuel and the ratio of LHV to HHV. Refer to chapter 5 for additional details.

To express true fuel-specific thermal efficiency, when heat added is expressed on an LHV basis, Equation 2-9 becomes:

Net work

Heat added (LHV)

To express true fuel-specific heat rate, when heat added is expressed on an LHV basis, Equation 2-11 becomes:

Work units of output

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Guide to Alternative Fuels

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