NOx Emission of Ammonia Combustion: Analyzing the Impact of Mixing With a Novel Monte Carlo Model

Abstract Ammonia is attracting a high level of interest as a green fuel produced by regenerative H2 and N2 using the Haber Bosch process. The transport of ammonia from areas featuring good regenerative sources to areas where power will be produced using gas turbines is straightforward. However, burning ammonia in conventional gas turbine combustors optimized to burn natural gas will generate NOx emissions of several 1000 ppm. Staged combustion systems have been proposed to keep NOx emissions below 200 ppm enabling further NOx emission reduction via conventional exhaust catalysts to acceptable levels. In a staged combustor the ammonia is burnt under fuel rich conditions at high temperatures towards water, hydrogen and nitrogen. The final dilution with excess air to meet the target turbine inlet temperature is provided in the secondary zone. Important factors influencing NOx generation are the fuel/air mixing in the primary zone of a staged combustor and dilution with secondary air in the second stage of the combustor. To investigate the mixing impact, a Monte Carlo method has been added to a reacting flow network model. This paper outlines the Monte Carlo technique and demonstrates the impact of mixing on NOx emissions for a staged combustion system burning ammonia and blends of ammonia with hydrogen or natural gas. Considering the mixing effects in the primary and secondary stage of the combustor an increase of NOx emissions by 100% has been shown. To obtain the low NOx emission minimum, the primary zone needs to be operated at equivalence ratios near to 1.2 over the whole gas turbine operation range. This requires additional measures on air management like combustor air bypass.