New insight into NH 3-H 2 mutual inhibiting effects and dynamic regimes at low-intermediate temperatures

The hydrogen effect as a fuel enhancer on ammonia oxidation features is a relevant topic for ammoniabased 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, lowintermediate 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 thermokinetic instabilities from low-intermediate to high temperatures in a JSFR, parametrically varying the H 2 inlet concentration. Maps of combustion behaviours (T in - phi) are then drawn up, on the basis of experimental evidences, in the range 1200K < T in < 1350 K, and 0.2 < 1.2. Results show H 2 only moderately enhances the reactivity of the system for the investigated conditions. Consequently, dynamic regime areas in T in - phi maps are slightly shifted towards lower T in and restricted to a narrower phi range. Numerical simulations were able to predict the main NH 3 /H 2 oxidation features, albeit lowintermediate temperature oxidation chemistry description is very mechanism-dependent. Nonetheless, the H 2 -NH 3 mutual inhibiting interaction oxidation chemistry is congruently addressed: NH 3 acts as OH radical scavenger, thus partially inhibiting the direct H 2 oxidation, whereas H 2 re-coverts back NH 2 radicals to NH 3 , through the reaction NH 2 + H 2 = NH 3 + H. The same reaction produces the sole H radicals able to feed the high-temperature branching reaction of the H 2 /O 2 sub-system. Same concluding remarks on the NH 3 /NH 3 --H 2 oxidation chemistry open issues are then reported. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study investigates the effects of hydrogen on ammonia oxidation and thermokinetic instabilities, showing that hydrogen only moderately enhances the reactivity of the system. The mutual inhibiting interaction between NH3 and H2 is revealed through experimental tests and numerical simulations. However, there are still open issues regarding the NH3/NH3-H2 oxidation chemistry.