Experimental and numerical study on emission characteristics of NH3/DME/air flames in a premixed burner

Abstract

As a potential candidate for the use as a carbon-free fuel, ammonia (NH3) has an advantage over hydrogen in terms of storage costs but faced with two main issues, i.e., low burning speed and high NOx emission. In this study, dimethyl ether (DME) was used as a combustion promoter to improve the combustion characteristics of NH3, and the emission characteristics of NH3/DME/air premixed flames were investigated in a premixed burner. For the NO emissions, it was found that lean conditions and stoichiometric condition favored the formation of NO while rich conditions showed a negative impact on NO formation. Moreover, increasing of NH3 fraction from 50% to 90% at a volumetric ratio of fuel mixtures effectively decreased the NO emissions. Interestingly, the NO2 emission showed a similar trend to that of NO, but with only 1/10 of the relevant concentration, the underlying reason was ascribed to the similarity between NO and NO2 reaction pathways. In addition, the emission of CO and unburned NH3 increased sharply in fuel rich conditions due to the lack of oxygen. The simulations in the chemical reactor networks with a detailed mechanism were conducted to analyze the NO emission behaviors of the NH3/DME flames. Results showed that the two-element chemical reactor networks could capture the trend of NO emission accurately. Sensitivity analyses of NO concentration were conducted to identify and analyze the reactions responsible for NO formation. Results showed that the third body reaction CH3OCH3(+M)=CH3+CH3O(+M) has the greatest promotion effect on NO generation at the flame zone, while the reactions NH+NO=N2O+H and N+NO=N2+O become the major reactions of NO consumption at the post flame zone. DME has high reactivity, and a large number of active radicals produced by its oxidation could promote the generation reactions of NO and increase the production rate of NO. The NO reaction pathway analysis showed that the HNO pathway played a major role in NO formation in the main combustion zone.