New insight into NH3-H2 mutual inhibiting effects and dynamic regimes at low-intermediate temperatures

Abstract

The hydrogen effect as a “fuel enhancer” on ammonia oxidation features is a relevant topic for ammonia-based energy conversion systems. For the most, scientific literature is focused on high temperature ammonia-hydrogen oxidation chemistry, whereas few works focus on low-intermediate temperatures (900–1350 K), relevant for non-conventional low-temperature combustion processes. Recently, low-intermediate and the shift to high-temperature ammonia oxidation chemistry has been characterized through experimental tests in a Jet Stirred Flow Reactor (JSFR) by the same authors, with the identification of thermo-kinetic instabilities. In addition, the ammonia effect on hydrogen oxidation chemistry has been addressed through a mutual inhibiting interaction for low-intermediate temperatures.Given this background, this work investigates the hydrogen effects on ammonia oxidation and thermo-kinetic instabilities from low-intermediate to high temperatures in a JSFR, parametrically varying the H2 inlet concentration. Maps of combustion behaviours (Tin- ϕ) are then drawn up, on the basis of experimental evidences, in the range 1200K<Tin<1350 K, and 0.2 <ϕ<1.2. Results show H2 only moderately enhances the reactivity of the system for the investigated conditions. Consequently, dynamic regime areas in Tin-ϕ maps are slightly shifted towards lower Tin and restricted to a narrower ϕ range.Numerical simulations were able to predict the main NH3/H2 oxidation features, albeit low-intermediate temperature oxidation chemistry description is very mechanism-dependent. Nonetheless, the H2-NH3 mutual inhibiting interaction oxidation chemistry is congruently addressed: NH3 acts as OH radical scavenger, thus partially inhibiting the direct H2 oxidation, whereas H2 re-coverts back NH2 radicals to NH3, through the reaction NH2+H2=NH3+H. The same reaction produces the sole H radicals able to feed the high-temperature branching reaction of the H2/O2 sub-system. Same concluding remarks on the NH3/NH3--H2 oxidation chemistry open issues are then reported.