The Structure and Stability of Premixed CH4, H2, and NH3/H2 Flames in an Axially Staged Can Combustor

Abstract

An experimental investigation of flame structure, stability, and emissions performance was conducted in a two-stage lab-scale generic combustor design operated with CH4, H2, and NH3/H2 fuel blends. The main flame zone features a premixed bluff body stabilized flame, with a secondary flame zone initiated downstream by injecting premixed air and fuel using two opposing radial jets. The total power and air flowrate are kept constant between the different fueling cases, while the air split between stages and equivalence ratios are varied to explore conditions relevant to gas turbine operation. Given the relative novelty of the configuration, special emphasis is given to analyzing the structure of the opposing jet flames in the secondary stage. In contrast to previous literature on reacting jets in cross flow, these interact significantly due to their proximity, leading to a merged flame zone at the impingement location in the center of the combustion chamber, and some flame propagation upstream of the jet location. As the jet-to-crossflow momentum ratio increases, the merged flame zone changes shape, reaching close to the walls for the methane cases but remaining very compact when operating with almost pure hydrogen. For the hydrogen flames, diverting more air to the second stage allows higher total thermal power conditions to be reached, while avoiding flashback, and eliminates thermoacoustic instabilities. For ammonia-hydrogen flames, air is diverted to the second stage, while a constant fuel flow is sent to the primary stage, resulting in some locally rich conditions in the primary flame. A local minima in terms of NOX occurs when the primary flame is operated at an equivalence ratio of 1.15. Analysis of the flame structure suggests that this state corresponds to almost complete combustion or pyrolysis of NH3 in the main flame, with the remaining hydrogen burned in an inverse diffusion flame in the secondary zone.