Toward a second-law taxonomy of combustion processes

Abstract

Combustion is the source of heat—or sensible enthalpy—in a vast range of processes. Efforts directed at making the use of energy as effective as possible must include the attempt to match the quality of energy supplied to the quality of energy required. The extent to which that match is achieved is reflected in the second-law performance of the process. The thermodynamic function exergy offers a convenient means of quantifying that performance. The second-law performance of several combustion processes has been analyzed in terms of the exergy subsidy which they require. Sample calculations have been carried out for methane burning in stoichiometric proportions with air. To facilitate the computation of the exergies, the reference potentials have been tabulated in terms of the ambient partial pressures of CO 2, H 2O, and O 2, and the K p 's of formation. The adiabatic flame of CH 4-air with dissociation requires an exergy subsidy of 286 MJ/kmol of CH 4 for an effectiveness of 0.68. When dissociation of combustion products is neglected, the reference potentials cancel in the calculation of exergy subsidies. This simplified model is applicable to stoichiometric and lean mixtures. The non-dissociating adiabatic flame has an effectiveness of 0.718. Non-dissociating non-adiabatic flames require an exergy subsidy ranging from 235 MJ/kmol in the limit of zero heat release at 2325 °K to 809 when heat is supplied at 298 °K. The flame in a well stirred reactor at 1764 °K requires an exergy subsidy of 294 MJ per kmol of CH 4 burned in it. The open diffusion flame uses up all the fuel exergy and has zero effectiveness. The thermodynamic benefits of replacing a pure fuel with “low-Btu gas” are minor unless the gas is flared or otherwise vented. The effect of increasing the heat transfer coefficient between the combustion stream and the process stream in an isothermal furnace is always to improve the effectiveness.