For the control of pollutant emissions, mainly CO and NOx, the gas turbine manufacturers have upgraded the combustion systems to adopt lean premixed combustion (LPM) instead of the conventional non-premixed combustion. The Dry Low NOx (DLN) combustor is one of the greatest achievements classified under LPM combustion for significant control of NOx emissions. However, DLN combustors suffer from limited flame stability, especially under low load (near blowout) operating conditions, in addition to the difficulty of separating CO2 from the exhaust stream for reducing the gas-turbine carbon footprint. Modification of the combustors to handle oxy-fuel flames instead of the normal air-fuel flames can ease the processes of CO2 separation and capture form the exhaust stream for full control of emissions. However, oxy-fuel combustion technology has its own challenges that are associated with the high CO2 concentrations within the combustor, including, reduced rates of reactions, reduced combustion efficiency, and limited range of stable flame operation. To widen the operability, control the emissions, and improve the turndown ratio of oxy-fuel combustors, the concept of flame stratification, i.e., heterogenization of the overall equivalence ratio, is introduced here. This concept can be implemented through either combustion staging or splitting of the reactants into two coaxial streams of different equivalence ratios, which is also known as dual lean premixed (DLPM) combustion. In DLPM, the central stream is of near-stoichiometric equivalence ratio to act as a hot pilot that keeps the flame ignited even under low load conditions for achieving higher turndown ratio. On the other hand, the annular stream is ultra-lean to keep the emissions at the lowest level. The combination of oxy-fuel and DLPM combustion techniques in an oxidizer-flexible combustor can result in wider operability and greater turndown ratio of the combustor with emissions level approaching the zero level. Fuel flexibility is also an effective approach to improve flame stability near the blowout limit through fuel enrichment with chemically active species like hydrogen or through using synthetic fuels. Thus, the ultimate goal of the present study is the development of a fuel/oxidizer flexible DLPM combustor that can sustain stable flames of ultra-low emissions even at very low part loads. Special emphasis will be made on the performance of the combustor under stratified oxy-syngas conditions, motivated by the rising interest in this technology in combined cycle power plants. A DLPM burner will be designed, manufactured, and tested on a gas turbine model combustor to investigate combustor operability and emissions under such fuel/oxidizer-flexible stratified combustion conditions. Flammability limits, in terms of blow-out and flash back limits, and flame temperature and emissions will be recorded. Flame visualization study will be performed as well. Generalized combustor stability maps will be generated under different loading conditions. The stability maps will be presented on the contour plots of adiabatic flame temperature, combustor power density (power per unit volume), mixture mass flow rate, and inlet flow Reynolds number to identify the mechanisms of flame extinction. Detailed numerical simulations will be performed to characterize the different flames in detail and the results will be compared with the measured data. The obtained data will be analyzed to assess the positive effect of DLPM operation on oxy-fuel combustor performance.
|Effective start/end date
|1/04/21 → 1/04/23
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