Effects of mixture composition on oxidation and reactivity of DME/NH3/air mixtures examined by a micro flow reactor with a controlled temperature profile

Abstract

Effects of mixture composition and equivalence ratios on oxidation and reactivity of dimethyl ether (DME)/ammonia (NH3) mixtures are investigated based on weak flame responses observed in a micro flow reactor with a controlled temperature profile (MFR). For observations, the NH3 fractions in fuel blends are varied as 0–50%. Three equivalence ratios (ϕ = 0.5, 1.0 and 1.2) are examined. A combined chemical reaction model for DME/NH3 mixtures is developed and used to simulate oxidation of DME/NH3 mixtures in MFR. Under the stoichiometric conditions, the DME oxidation is promoted and the mixture reactivity is enhanced as the NH3 fractions in fuel blends increase from 0 to 15%. As the NH3 fractions further increase from 15 to 50%, CO oxidation is inhibited; the mixture reactivities are suppressed strongly. Chemical reaction analyses indicate that NOx produced by the NH3 oxidation converts less-reactive radicals into more-reactive radicals (HO2→OH and CH3→CH3O) through the NO–NO2 catalytic loop at the intermediate temperatures of 800–1000 K. However, with larger NH3 fractions in fuel blends, the OH radical consumption by NH3 increases, thereby inhibiting CO oxidation. Moreover, when the equivalence ratios are varied, mixture reactivity is more inhibited as equivalence ratios increase, although they change only slightly irrespective of the equivalence ratios in neat DME cases. With richer equivalence ratios, more OH radicals are consumed by DME, CH2O, NH3, and H2. Moreover, the CO oxidation is inhibited in the intermediate temperature zone when NH3 is blended because CO has lower priority for OH radical consumption among these species as a result of its high activation energy. Results of this study demonstrate the strong potential of NH3 to control reactivity of hydrocarbon mixtures over a wide range by changing the mixture composition and equivalence ratios.