HYDROGEN ENRICHMENT IN OXY-FUEL COMBUSTORS: PREMIXED OR STRATIFIED?

Oxy-fuel combustion in the Allam cycle of supercritical-CO2 power plants must be highly diluted with recirculated CO2, because the temperature rise within the combustor is ~400 °C only. Highly diluted oxy-fuel flames have been proven in the literature to be susceptible to early blowout. Hydrogen enrichment is a viable solution to deter blowout and enhance combustor turndown, owing to the eminent combustion characteristics of hydrogen (H2). The author focused on the micromixer burner technology in gas turbines for its established superior flame stability. Two techniques were examined for introducing enrichment H2 in an oxy-methane micromixer-based model gas-turbine combustor: fully premixed with the oxy-reactants (i.e., CH4/H2/O2/CO2) and non-premixed/stratified (H2–CH4/O2/CO2). Since the onset of blowout in micromixer flame was observed to occur consistently at its circumference, the second (stratified) technique introduced H2 in non-premixed fashion from peripheral jets surrounding the premixed CH4/O2/CO2 core flame, with the aim to enhance combustor turndown by benefiting from inducing stratification. This approach thus attempted to use hydrogen to stabilize the peripheral region of the oxy-methane flame where blowout typically starts. It was found, however, that the blowout limit of fully-premixed (CH4/H2/O2/CO2) flame did not improve by stratification (H2–CH4/O2/CO2); it actually deteriorated slightly. The stability and CO emissions of the stratified flame as well as the interaction of H2 jets with core CH4/O2/CO2 flame in this technique were thus analyzed in detail, in order to explain this unexpected behavior and provide recommendations for how hydrogen enrichment should be introduced in oxy-fuel micromixer combustors.