Effects of hydrogen addition on NOx formation in a non- premixed methane low-NOx combustor

Hydrogen is a promising carbon-free fuel, but nitric oxides (NOx) emissions are significantly increased with higher temperature in hydrogen combustion. Internal flue gas recirculation (IFGR) is one of the most effective NOx reduction techniques in boilers. Previous research has widely reported NOx generation with hydrogen enrichments from a chemical kinetic perspective, however, the NOx formation principles in non-premixed methane/air combustion using IFGR with hydrogen addition are still unclear. This work aims to investigate the effects of hydrogen addition on NOx formation in a non-premixed methane low-NOx combustor using IFGR. The Reynolds-averaged Navier-Stokes (RANS) simulations were conducted on a 5 kW non-premixed combustor with IFGR rate of 17.7%, which generated 11.3 mg/m3 of total NOx generation in methane combustion. A series of three-dimensional computational fluid dynamics (CFD) simulations with detailed mechanisms was tested on four hydrogen power fractions. The reaction intensities of the main NOx formation pathways were analyzed on reaction rates. The results show that 70% hydrogen power fractions lead to significantly greater NOx concentration to 677.8 mg/m3, as high as 60 times of the original concentration. With no additional techniques, the original methane low-NOx combustor is only allowed for hydrogen addition of less than 20% power fraction. The NNH route becomes the second dominant pathway in hydrogen-added flames. Influenced by the growing hydrogen contents, the generation of the NNH route is largely generated from 2 mg/m3 to 65 mg/m3. This work connects the hydrogen addition and the NOx formation pathways in non-premixed methane combustion, and highlights the importance of eliminating thermal NOx and the NNH route to achieve low-NOx combustion rather than sole NOx suppression method, which can provide a reference for designing low-NOx techniques.