Influence of ammonia cofiring ratio and injection mode on the NOx emission and control mechanisms of NH3-CH4 cofiring

Experimental studies were conducted in a one-dimensional (1D) reactor to investigate the NOx formation characteristics of NH3-CH4 cofiring and explore effective NOx control methods. The results showed that when NH3 was injected with CH4, the NO emissions increased and then decreased with the increase of NH3 cofiring ratio (RNH3), and air-staging was effective in NOx control only when RNH3 < 40 %. Whereas when NH3 was injected separately downstream of CH4, the NO emissions showed monotonical increase with the increase of RNH3 and air-staging became ineffective in NOx control. It was found that the NO in the reactor experiences a distinctive process of initial formation in the initial combustion zone (ICZ), reduction in the O2-deficient zone (ODZ), and secondary formation in the burnout zone (BOZ) forming an overall N-shape distribution of NO concentration along the reactor. Although air-staging could reduce the NO formation in the ICZ, the consequent increase in residual NH3 may cause significant NO formation in the BOZ. This is the primary cause why air-staging became ineffective in NOx control under many NH3 cofiring conditions. Thus, additional means needs to be considered to balance the reduction of NO formation in the ICZ and BOZ.