The interaction between NH3 and CH4 oxidation chemistry: A comprehensive study through combustion regimes and thermokinetic instabilities

The present work explores the interaction between CH4 and NH3 oxidation chemistry, with particular focus on the effect of NH3 on CH4 oscillation regimes and nitrogen oxides emissions. Experimental tests were carried out in a Jet Stirred Flow Reactor, for fuel-lean CH4-NH3/O2/N2 mixtures, at fixed equivalence ratio (0.8), dilution level (90 %) and residence time (0.4 s), changing the inlet temperature and the fuel-mixture composition. Experimental results revealed a different influence of NH3 on CH4 oxidation depending on the NH3 concentration in the fuel blend. In particular, mixture reactivity is slightly enhanced when NH3 is increased from 0 up to 40 %, while reactants conversion is shifted to higher Tin for NH3 > 40 %. Maps of instabilities were outlined for CH4-NH3 blends ranging from pure methane to pure ammonia. Both periodic and damped oscillations were detected, with different shapes, amplitudes and frequencies depending on the temperature and NH3 concentration. Numerical results showed that the selected kinetic scheme reproduced the overall effect of NH3 on mixture reactivity and NO emissions. Kinetic analyses suggested that the enhancing effect of NH3 at relatively low temperatures is mainly ascribable to the formation of NO and its interaction with CH3 radicals.