Based on its well-known merits ammonia has been gaining special attention as a potential renewable energy carrier which can be replaced in power generation systems. Considering its low flame speed and its potential for producing fuel NOx as the main challenges of combusting ammonia, flame stability, combustion efficiency, and NOx formation are experimentally investigated. Focus is on premixed ammonia-hydrogen-air flames with high mixture fractions of ammonia (60-90% by volume) under standard temperature and pressure conditions.
We introduce silicon-carbide (SiC) porous block as a practical and effective medium for ammonia-hydrogen-air flame stabilization which enables stable and efficient combustion of the mixtures in a wide range of conditions. Flames stabilized within the matrix of the porous media based burner has remarkably higher burning speeds and wider stability criteria than open flames and the flames held by many other conventional flame holding systems. This is due to the internal feedback of heat from the burned gases to the unburned gases by radiation and conduction through the porous medium. Effects of some major influential parameters on flame stability are inspected: equivalence ratio, ammonia mixture fraction, and size of the burner. Results indicate that acting as an effective means for elevating the low flame speed of ammonia doped fuel mixtures, the porous medium based burner noticeably boosts the blow-off and flashback limits of the flame making it an advantageous and reliable flame holding method applicable to ammonia based power applications.
Combustion efficiency is evaluated by comparing the measured temperatures with the data obtained from a simple volumetric heat release CFD model. Also, exhaust NOx concentration is measured in wide ranges of equivalence ratio and ammonia mixture composition. While the measured NOx profiles well capture the chemical kinetics predictions, very low levels of NOx (30-100 ppm) are observed for rich flames indicating the significant performance of the porous medium based burners in terms of combustion cleanness. The remarkable capability of the porous medium burners to efficiently operate at very high fractions of ammonia in a wide range of equivalence ratios provides a new achievability in more efficient and cleaner combustion in power generation applications.