Effects of inter-stage mixing on the NOx emission of staged ammonia combustion

Abstract

Rich-lean staged combustion is a promising method to achieve low NOx emission for NH3 fuel. The NOx emission of the rich-lean staged combustion heavily relies on inter-stage mixing, which has not been explicitly explored for NH3 fuel. Therefore, this paper presents a modeling study concerning the effects of the inter-stage mixing on the NOx emission of NH3 fueled rich-lean staged combustion. The rich-lean staged combustor is modeled by chemical reactor networks. The non-uniform mixing is modeled by assuming Beta distribution, which is a suitable model for the random behavior of percentages and proportions that is featured in the jet-in-cross-flow mixing of the secondary air and the primary-stage combustion products. The NOx emission is obtained at different inter-stage mixedness, ηl, and rich-burn stage equivalence ratios, ∅r, with constant combustor outlet temperature (constant global equivalence ratio for lean stage, ∅g). The major impact of inter-stage mixing on NOx for staged NH3 combustion is strongly dependent on ∅r. The most significant NOx increase caused by poor inter-stage mixing occurs at around ∅r=1.25 and is due to the increase of thermal NOx, which is similar to hydrocarbon fuels. This finding has important implications for practical applications because the ∅r=1.25 point has the lowest NOx emission and is considered to be the desired operating point for staged gas turbine combustor. Therefore, complete mixing of the primary-stage combustion products and the secondary air needs to be done to ensure low NOx emission for rich-lean staged combustion of NH3. In contrast, at ∅r>1.4, poor mixing may lead to NOx reduction due to the reduction in fuel NOx. However, the NOx at this point is still high and should be avoided in practical operation.