Flame and emission characteristics from NH3/CH4 combustion under ultrasonic excitation

Abstract

As a high energy density hydrogen-rich carrier, ammonia (NH3) is a highly promising carbon-free fuel. The large-scale industrial application of NH3 is limited by its low reactivity and high nitrogen oxide (NOX) emission. In this work, the flame and emission characteristics of ammonia/methane (NH3/CH4) non-premixed combustion were investigated under ultrasonic excitation. An experimental system was designed and built, including non-premixed combustion system, loading ultrasonic system, deflection tomography temperature measurement system and flue gas measurement system. Combustion and measurement experiments at different ultrasonic frequencies and NH3/CH4 blending ratios were carried out. Flame images and flue gas species concentrations under ultrasonic excitation were acquired. The 3D temperature field was reconstructed. The influence of ultrasonic excitation at different frequencies on flame characteristics, flame temperature field and emission characteristics of the combustion process was analysed. The mechanism of NH3/CH4 combustion enhancement and emission reduction was revealed when the flame was excited by ultrasonic waves. Results showed that part of the hydrocarbon fuels was replaced by NH3 to reduce carbon dioxide (CO2) emission. The height and colour of the NH3/CH4 flame were changed and the high-temperature area of the flame gradually expanded as ultrasonic acted on the flame. As ultrasonic frequency increased, the emission concentrations of unburned CH4, unburned NH3 and NO decreased significantly. The flame was excited by ultrasonic waves, which reduced its local equivalent ratio, improved combustion efficiency and suppressed NOX generation.