Nonpremixed Approaches for Fuel Flexible, Low NOx Combustors in High-Efficiency Gas Turbines

Abstract Lean, premixed combustor designs have almost completely replaced non-premixed combustors for industrial gas turbine applications where NOx emissions are regulated. Nonetheless, these designs have also introduced turndown and fuel flexibility constraints and made combustion instabilities and flashback more problematic. Moreover, modern high-efficiency machines have high enough turbine inlet temperatures that additional fuel staging is needed to manage NOx. These issues are becoming increasingly problematic in a decarbonizing energy sector. First, combustors must also be able to accommodate a range of fuels, including hydrogen or ammonia, whose levels can vary from 0–100%, blended with natural gas. Second, operational flexibility for gas turbines will become increasingly important, as their role evolves from a primary provider of energy to one of providing capacity and resilience. Moreover, they must compete for these services with a host of new technologies, including energy storage and fuel cells, and these cost-pressures require higher-efficiency machines. This pushes pressures and flame temperatures higher, both of which accelerate NOx formation rates. In this environment, having systems with broad operational boundaries, ultra-low emissions, and extreme fuel flexibility will become increasingly important. The purpose of this paper is to propose nonpremixed, multistage designs for the next generation of high turndown, high fuel flexibility, low NOx combustion designs — referred to here as a Nonpremixed, Rich, Relaxation, Lean (NRRL) combustor. The key concept we explore is non-premixed combustion, followed by additional fuel mixing to locally fuel-rich conditions, a relaxation stage, and then a lean stage. This non-premixed approach can handle essentially any fuel composition, including pure hydrogen, liquid fuels, pure methane, pure ammonia, and any combination in between while breaking the NOx-CO tradeoff and reducing combustion instability risk. This paper provides chemical reactor network modeling calculations to identify key kinetic processes and time scales required for such a concept. This concept has completely inverted sensitivities from lean, premixed systems which prefer short residence times, low pressures, and low temperatures to minimize NO formation. This concept prefers long residence times, high pressures, and high temperatures, indicating a very different set of design trades for part load and off-design performance.