Kinetical driving force analysis of unstable combustion behavior in ammonia/hydrogen blending system

Abstract

Unstable combustion states are frequently observed in ammonia/hydrogen blended combustion systems. This study aims to explore the underlying thermokinetic feedback driving forces responsible for the occurrence of unstable combustion states based on the identified dominant reactions. To achieve this, the newly developed functional weight analysis method is employed and compared with the combination of heat-release/temperature sensitivity anaylsis. Analysis conducted within several popular mechanisms over a wide temperature and pressure range reveals that the important reactions identified in different mechanisms consistently exhibit loop-formed kinetics with similar mechanism functions.These loops include a hydrogen chain-branching loop, a NH2 dehydrogenation-oxidation loop, and a NOx-DeNOx loop. All of these loops encompass a combination of exothermic and endothermic reactions, and they mutually promote each other due to their loop-formed kinetic. Consequently, when the heat release/heat absorption ratios within the loop kinetics meet specific quantitative criteria, they establish effective thermokinetic feedback. This feedback mechanism initiates and sustains non-constant heat release, serving as a monopole sound source that produces acoustic waves. It is worth noting that the discovery of NOx-DeNOx loop provides a compelling explaination for the frequently observed association between oscillatory combustion states and elevated NOx emissions.