Effect of preheating on extending lean extinction limit of ammonia/air combustion in a two-section porous burner

Abstract

Ammonia utilization is promising way to reduce greenhouse gases emissions. We propose the concept of reactants preheating to achieve stable combustion of lean NH3 in two-layer porous burner, this concept is validated by numerical study with full kinetics. Calculations are conducted over wide range of preheating temperature (Tpre) and mass flow rate (m″) to gain insight in stable map, with special emphasis on effect of Tpre on extending lean extinction limit (LEL). It is shown that Tpre has vital effect in expending LEL. LEL is continually extended for different m″ with preheating, the LEL for NH3/air is successfully extended from equivalence ratio (φ) of 0.54 to 0.08 with increasing Tpre from 400 K to 1000 K. It is found that there exist three distinct mechanisms (Ⅰ, Ⅱ and Ⅲ) with respect to φ. When φ is close to one, self-sustaining combustion is realized without preheating (Ⅲ). The combustion is becoming unstable when φ is decreased, stable combustion can only be achieved with preheating (Ⅰ). Flame extinction takes place even with the preheating (Ⅱ) when the Tpre is lower than critical one for the given φ. The results present guidelines for burner operating with lean NH3 mixture over wide parameters range.