The terms heat of combustion and enthalpy of combustion are used interchangeably due to the First law of thermodynamics and the relationships between heat at constant pressure (qP), the change in internal energy (ΔU), and the change in enthalpy (ΔH). [3][4] The equation for the change in internal energy is ΔU = qP - PΔV. If the equation is rearranged, then qP = ΔU + PΔV. The equation for the change in enthalpy is ΔH = ΔU + PΔV + VΔP. The term VΔP cancels because there is no change in pressure so ΔH = ΔU + PΔV. As previously stated, qP = ΔU + PΔV. Therefore, qP = ΔH. Heat of combustion measurements are most common for the combustion of organic hydrocarbons, compounds composed of carbon and hydrogen, but can include other atoms found in organic compounds such as nitrogen, phosphorous, sulfur and especially oxygen. Heat of combustion values are most widely used for determining if a substance is an effective fuel source.[5] Many organic compounds can be found in Heat of Combustion Tables The units for heat of combustion can be varied, but are always reported as a unit of energy per mole or per unit of mass or volume depending on the method used to report the values. The heating value (or energy value, calorific value, heat of combustion) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The enthalpy of combustion is the same value expressed as an enthalpy, where release of heat is described as negative number. The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities: energy/mole of fuel energy/mass of fuel energy/volume of the fuel There are two kinds of heating values, called high(er) and low(er), depending on how much the products are allowed to cool and whether compounds like H2O are allowed to condense. The high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the standard enthalpies/heats of formation ΔH⦵f of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion). By convention, the (higher) heat of combustion is defined to be the heat released for the complete combustion of a compound in its standard state to form stable products in their standard states: hydrogen is converted to water (in its liquid state), carbon is converted to carbon dioxide gas, and nitrogen is converted to nitrogen gas. That is, the heat of combustion, ΔH°comb, is the heat of reaction of the following process: Water-only LHV One definition is simply to subtract the heat of vaporization of the produced water from the higher heating value. This treats any H2O formed as a vapor that is released as a waste. The energy required to vaporize the water is therefore lost. It assume that the water component of a combustion process is in vapor state at the end of combustion, as opposed to the higher heating value (HHV) (a.k.a. gross calorific value or gross CV) which assumes that all of the water in a combustion process is in a liquid state after a combustion process. GPSA LHV This definition, used by Gas Processors Suppliers Association (GPSA), is the enthalpy of all combustion products minus the enthalpy of the fuel at the reference temperature (GPSA currently uses 60 °F (15+5⁄9 °C)), minus the enthalpy of the stoichiometric oxygen (O2) at the reference temperature, minus the heat of vaporization of the vapor content of the combustion products. It can be alternatively restated as the HHV at reference temperature minus the heat of vaporization of the vapor content of the combustion products. By specifying a reference temperature, all products can be taken into account. This work is based on American Petroleum Institute (API) research project 44, which used 25 °C (77 °F) as the reference. API LHV The API defines a net heat of combustion to correspond to LHV. It uses the reference temperature of 60 °F (15+5⁄9 °C), but with gaseous water. It is calculated as the gross heat of combustion (HHV at reference temperature) subtracted by the heat of vaporization of water at reference temperature (and at the vapor pressure corresponding to reference temperature). 150 °C LHV This LHV is the amount of heat released when the products are cooled to 150 °C (302 °F). 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 150 °C (302 °F) 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. Most applications that burn fuel produce water vapor, which is unused and thus wastes its heat content. In such applications, the lower heating value must be used to give a 'benchmark' for the process. However, for true energy calculations in some specific cases, the higher heating value is correct. This is particularly relevant for natural gas, whose high hydrogen content produces much water, when it is burned in condensing boilers and power plants with flue-gas condensation that condense the water vapor produced by combustion, recovering heat which would otherwise be wasted. Note There is no difference between the lower and higher heating values for the combustion of carbon, carbon monoxide and sulfur since no water is formed during the combustion of those substances. BTU/lb