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Heating Value, Heat of Reaction, and Free Energy One of the issues that arises when discussing the calculation of the electrical efficiency of a fuel cell or an electrolysis cell is confusion about the terms heat of combustion (often called the heating value), the heat of reaction, the heat of formation, the free energy of reaction, and the free energy of formation. Hydrogen and hydrocarbon fuels also pose a source of confusion about whether the water vapor produced during combustion is condensed back into liquid water or is lost as a vapor diluted in the combustion products. Condensing the water vapor produces additional heat. Finally, there is confusion about standard conditions. Total energy is composed of both electrical and thermal energy known as enthalpy (H). The amount of electrical energy is known as the Gibbs free energy (G) and corresponds to the maxi- mum amount of useable electrical energy available when hydrogen recombines with oxygen. Irreversible energy or entropy (S) is the “cost of doing business” and is dependent on the temper- ature at which the reaction takes place. The loss due to entropy is similar to how a bouncing ball loses energy when hitting the floor, as friction from the action of bouncing causes a transfer of thermal energy to atoms in the floor. The energy transferred to those floor atoms dissipates and is not recoverable. Consequently, the change (Δ) in these quantities from a standard set of conditions follows the form shown below. ∆H = ∆G + T∆S Heat of Combustion and Heat of Reaction By definition, the heat of combustion is the amount of heat released when 1 gram molecular weight of a substance is burned in oxygen. (Heat of reaction is a more generic term and refers to the heat released in any chemical reaction, combustion or otherwise.) Heat of combustion measurements usually are made in a calorimeter under controlled conditions in which the water vapor, if produced, is condensed and the additional heat of condensation is included. The heat of combustion of methane, for example, is 890.3 kilojoule (kJ) per mole when measured at 25°C in a calorimeter. CH4 + 2 O2 → CO2 + 2 H2O(liquid) + 890.3 kJ/mole heat If the water vapor produced is not condensed, then the heat of combustion of methane is 802.3 kJ/mole, with the difference being the latent heat of condensation of the water vapor. The greater value—based on forming liquid water—is known in the industry as the higher heating value (HHV) of methane. The lesser value—based on forming water vapor—is the lower heating value (LHV) of methane. Most standard natural gas appliances (e.g., water heaters, gas furnaces, gas ranges) operate with an excess of air, and the water vapor does not condense but remains dissolved in the exhaust stream. This also is true of internal combustion engines and gas turbines. In the United States, however, the efficiencies of appliances and heat engines usually are rated based on the HHV, whereas in European communities LHV is used. The use of LHV in calculating heat engine efficiencies yields greater efficiency numbers than those using HHV. Standard Conditions The heat of combustion changes with temperature. For example, the heat of combustion of methane (LHV) at 1,000 K (727°C or 1,340°F) is 800.9 kJ/mole, which is slightly less than the LHV value of 25°C. It is helpful, therefore, to use a standard set of conditions for calculating 1PDF Image | Hydrogen Production: Fundamentals
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