The challenge in the design of oxy-combustors for direct-fired supercritical CO2 (sCO2) cycles is in addressing disparate performance metrics and objectives. Typically, oxy-combustors feature a modest temperature rise in addition to the high combustor chamber pressure and near stoichiometric fuel/oxidizer ratio. To achieve a relatively low exit temperature from the combustor (operating at \~1150°C) a substantial fraction of the CO2 in the exhaust is recycled and injected back into the combustor as a diluent to modulate the temperature rise and control the turbine inlet temperature and combustion product species. Impurities in the fuel (natural gas) or oxidizer streams can enter the combustor to generate additional non-condensable products such as NO~x~ and SO~x~ and reduce process efficiency. Contaminants can also impact chemical kinetic processes. Key design parameters to consider include among others: injector design for mixing and flame stability, split of recycled CO~2~ diluent between injectors and cooling films, target flame temperatures to control non-condensable products, and strategies to inject the diluent CO~2~ for film cooling and thermal control. In order to support novel oxy-combustor designs, a high-fidelity yet numerically efficient modeling framework based on the CRUNCH CFD^®^ flow solver is presented, featuring key physics-based sub-models relevant in this regime.

For computational efficiency, a flamelet/progress variable (FPV) based tabulated-chemistry approach is utilized to (a) accurately model the flame structure based on chemical kinetic models with a large number of species and (b) capture turbulence-chemistry interactions. The FPV formulation has been upgraded to a three-stream framework to allow for the simulation of the CO~2~ film cooling stream (third stream) in addition to the fuel and oxidizer streams; the third stream accounts for the CO~2~ dilution and thermal effects. With a multi-timescale (MTS) approach, finite-rate chemistry effects are modeled in terms of multiple progress variables for the primary flame as well as for slower-evolving chemical species such as NO~x~ and SO~x~ contaminants. Real fluid effects are modeled using advanced equations of states, including NIST table lookup.  Validation test cases for the three-stream FPV formulation and MTS approach will be presented in the paper.

The predictive capabilities of this computationally-tractable design support tool are demonstrated on a conceptual injector design for an oxy-combustor operating near 300 bar. This conceptual design features multiple shear-coaxial injectors and operates with natural gas. Multiple simulations with increasing levels of fidelity will be presented to identify the effects of various physics being modeled.  Numerical results indicate the establishment of lifted flames. Finite-rate chemistry effects and ignition delay play a key role in the flame lift-off for the mass flow rates modeled.  Turbulence-chemistry interactions are shown to reduce the flame temperature but broaden the high temperature zone and increasing the heat flux to the chamber wall.  Simulations results provide quantitative feedback on the effectiveness of the film cooling as well as the level of contaminants (CO, NO, and N) in the exhaust due to impurities entering from the injectors. These results indicate that this framework would be a useful tool for refining the on-going efforts and optimizing the oxy-combustor designs.


A High-Fidelity Modeling Tool to Support the Design of Oxy-Combustors for Direct-Fired sCO2 Cycles

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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