Heat of combustion
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.That is, the heat of combustion, ΔH°comb, is the heat of reaction of the following process: Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and SO2 or SO3 gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids, respectively, when the combustion is conducted in a bomb calorimeter containing some quantity of water.Since the heat of combustion of these elements is known, the heating value can be calculated using Dulong's Formula: HHV [kJ/g]= 33.87mC + 122.3(mH - mO ÷ 8) + 9.4mS where mC, mH, mO, mN, and mS are the contents of carbon, hydrogen, oxygen, nitrogen, and sulfur on any (wet, dry or ash free) basis, respectively.The HHV is determined by bringing all the products of combustion back to the original pre-combustion temperature, including condensing any vapor produced.One definition of lower heating value, adopted by the American Petroleum Institute (API), uses a reference temperature of 60 °F (15+5⁄9 °C).The conventional power industry had used HHV (high heat value) exclusively for decades, even though virtually all of these plants did not condense exhaust either.For simply benchmarking part of a reaction the LHV may be appropriate, but HHV should be used for overall energy efficiency calculations if only to avoid confusion, and in any case, the value or convention should be clearly stated.Both HHV and LHV can be expressed in terms of AR (all moisture counted), MF and MAF (only water from combustion of hydrogen).