Higher heating value The higher heating value (HHV;
gross energy,
upper heating value,
gross calorific value GCV, or
higher calorific value;
HCV) indicates the upper limit of the available thermal energy produced by a complete combustion of fuel. It is measured as a unit of energy per unit mass or volume of substance. The HHV is determined by bringing all the products of combustion back to the original pre-combustion temperature, including condensing any vapor produced. Such measurements often use a standard temperature of . ; 150 °C LHV : This LHV is the amount of heat released when the products are cooled to . This means that the latent heat of vaporization of
water (and many other potential products) is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below cannot be put to use. The definition in which the combustion products are all returned to the reference temperature is more easily calculated from the higher heating value than when using other definitions. It will in fact give a slightly different answer.
Accounting for moisture Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen). AR, MF, and MAF are commonly used for indicating the heating values of coal: •
AR (as received) indicates that the fuel heating value has been measured with all moisture- and ash-forming minerals present. •
MF (moisture-free) or
dry indicates that the fuel heating value has been measured after the fuel has been dried of all inherent moisture but still retaining its ash-forming minerals. •
MAF (moisture- and ash-free) or
DAF (dry and ash-free) indicates that the fuel heating value has been measured in the absence of inherent moisture- and ash-forming minerals.
Gross heating value Gross heating value accounts for water in the exhaust leaving as vapor, as does LHV, but gross heating value also includes liquid water in the fuel prior to combustion. This value is important for fuels like
wood or
coal, which will usually contain some amount of water prior to burning.
End products for different elements Zwolinski and Wilhoit defined, in 1972, "gross" and "net" values for heats of combustion. In the gross definition the products are the most stable compounds, e.g. (l), (l), (s) and (l). In the net definition the products are the gases produced when the compound is burned in an open flame, e.g. (g), (g), (g) and (g). In both definitions the products for C, F, Cl and N are (g), (g), (g) and (g), respectively. There are many other definitions of "gross" and "net". == Estimating heating values ==