Flame structure of the Second Stage in Low-Emission Ammonia Rich-Quench-Lean Combustion

Ammonia is a promising carbon-free alternative to natural gas in dispatchable power generation. While carbon dioxide emissions are eliminated from the combustion process with ammonia, nitrogen oxide emissions (NOx) pose a significant challenge. One of the most promising strategies reducing such emissions in gas turbines is the rich-quench-lean (RQL) staged combustion system, where the combustion process is divided into two stages. With ammonia-based fuels, typically the first stage features a fuel-rich flame while the second stage comprises air injection to consume the remaining fuel. In this presentation, we will focus on the second stage combustion process by means of direct numerical simulations (DNS) of a simplified and downscaled combustor. The analysis reveals details of the interaction between the cold turbulent air jets and the hot hydrogen-containing flow from the first stage. We will discuss the impact of the air split ratio on the flame structure, relate the combustion process to local flame features and topologies, and shed light on processes such as preferential diffusion. We will also discuss NO and N2O emissions, in particular for an additional case with ammonia slip.