Combustion of ammonia-rich NH3-H2-air mixtures: improvement of flame stability

Abstract

As a result of its high hydrogen density and extensive experience base, ammonia (NH3) has been gaining special attention as a potential green energy carrier, which can be used in power generation systems. Low flame speed is one of the main challenges of ammonia combustion for practical applications. In order to address this important matter, flame stability is experimentally studied in this paper, which focuses on premixed ammoniahydrogen-air flames under standard temperature and pressure conditions. We introduce the use of an inert silicon-carbide (SiC) porous block as a practical and effective medium for ammonia-hydrogen-air flame stabilization method, which enables stable combustion even at very high ammonia concentration levels in the fuel and over a wide range of operational conditions. Flames stabilized with the matrix of the porous media based burner have remarkably higher burning speeds and wider stability region compared to open flames or to flames held by other conventional flame holding methods. The relative effectiveness of the porous medium is believed to be related to the internal back flow of heat from the burned gases to the unburned gases by means of radiation and conduction. Effects of some major influential parameters including equivalence ratio, ammonia mixture fraction, and diameter of the burner on flame stability have been investigated. Measured temperature values at different equivalence ratios and mixture compositions are compared to the chemical kinetics predictions. Heat loss is proven to be the main reason for the deviation between the measured temperatures and the theoretical values rather than incomplete combustion. The remarkable capability of the porous medium burners to operate at very high ammonia concentrations in a wide range of equivalence ratios enables efficient combustion of ammonia in practical power generation applications. In the experiments power output densities as high as 1.1 kW/cm2 have been demonstrated.