Direct Numerical Simulations of Low-Emission Staged Ammonia 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 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. While such a strategy has been shown experimentally to significantly reduce NOx formation, there is no fundamental understanding on how the combustion process occurs and how pollutants form in such a configuration. We present Direct Numerical Simulation (DNS) results of a simplified and downscaled setup of the second stage derived from a real gas turbine combustor, which retains relevant timescales of the actual combustor. The results reveal details of the turbulent inverted diffusion flame forming as fresh air reacts with hot combustion products from the first stage containing unburnt hydrogen. We will also discuss how emission formation processes are affected by turbulent mixing and residence time.