Effect and mechanism of combustion enhancement and emission reduction for non-premixed pure ammonia combustion based on fuel preheating

Abstract

Ammonia (NH3) is a hydrogen-rich carrier with high energy density and is a carbon-free energy source. NH3 is a new carbon-free alternative fuel with great attraction and prospect. NH3 is restricted to large-scale industrial application because of its low reactivity and high nitrogen oxide (NOx) emissions in combustion. This research evaluated non-premixed pure NH3 combustion enhancement method and emission law based on fuel preheating. An NH3 preheating device, non-premixed burner, flue gas analysis, and temperature measurement system were designed and constructed. Combustion characteristics of NH3 preheating were investigated under different preheating temperatures and NH3 flow rates. NH3 flame images were captured by a CCD camera. The temperature distributions of the NH3 flames were reconstructed by deflection tomography technology. In addition, the composition concentrations of flue gas were obtained by a multi-component gas analyzer. The influence of preheating temperature on flame characteristics, combustion temperature field, and pollutant emission were analyzed. A numerical simulation was performed under the same experimental conditions. The effects of NH3 preheating on flame and emission characteristics were analyzed by sensitivity analysis and reaction path analysis. The mechanisms of the combustion enhancement and emission reduction for preheating NH3 non-premixed combustion were explained. Results showed that stable non-premixed pure NH3 flame was generated at different experimental flow rates when the NH3 preheating temperature was higher than 593 K. The brightness, color, and structure of the flame varied significantly with increasing preheating temperature. Furthermore, the concentration of NOx decreased obviously, and the concentration of unburned NH3 was extremely low. The chemical reaction rate was tremendously promoted with increasing preheating temperature, and the reaction pathway of N radical was extremely related to nitric oxide (NO) production and consumption.