High-Fidelity Design Support Tool for Supercritical CO2 Oxy-Combustors
- CRAFT Tech
- University of Central Florida
Oxy-combustors for direct fired SCO2 cycles operate at high supercritical pressures (~300 bar) with large amounts of diluent CO2. They present unique challenges for obtaining optimal performance both due to the large number of design parameters with potentially competing requirements as well as the lack of well-validated simulation tools at these conditions. To maintain relatively low turbine inlet temperatures (~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. Furthermore, for safe operation the oxygen in the injectors are diluted with CO2 to maintain oxygen concentrations in the range of 7-30%. The recycled CO2 diluent, that is split between the injector and the chamber wall for cooling, presents a wide design space for combustor operation. The fraction of CO2 that is mixed with the oxygen and flows through the injector is a primary design parameter and affects combustor performance directly through a number of inter-dependent effects. The injector stoichiometry directly impacts flame temperatures and flame stability. The adiabatic flame temperature controls the amount of CO that is produced as a non-condensable contaminant, e.g., for flame temperatures above 1800 °C, CO concentrations become significant and reduce cycle efficiency by about 0.75 percentage points per mole percent of CO in the exhaust. However, increasing the CO2 content in the injector to reduce flame temperatures could affect flame stability, but also reduce injector efficiency due to higher pressure losses arising from the higher velocities in the injector. Furthermore, impurities in the inlet feed can generate non-condensable products and reduce process efficiency. For example, pipeline natural gas can contain 1.6% nitrogen that can lead to NOX production despite pure oxygen being used as an oxidizer. Similarly, sulfur contamination in syngas is estimated to be 30 ppm and can lead to SOx production. In addition to reducing efficiency and increasing cost by requiring expensive mitigation in the exhaust, these contaminants (i.e., NOX and SOX) can also directly impact the flame stability by changing the ignition delay for example. As the discussion above indicates, the numerical simulation tool for providing design support would need to have sufficient fidelity for modeling complex kinetics and real fluid effects but must also be economical to provide quick turn-around that will permit parametric studies for design optimization to be performed. The development of this high-fidelity design framework within CRAFT Tech’s CRUNCH CFD® code has been the focus of the Phase I effort. To reduce computational cost of modeling large kinetic mechanisms required an MTS-FPV (multi-time scale flamelet/progress variable) formulation was employed where the chemical kinetics is represented in a parametrized form allowing the local species composition to be obtained from a table look-up approach. For contaminants such as NOX or SOX, which evolve at a much slower time scale, an overlaid finite-rate kinetic model has been developed that is applicable when the contaminants are dilute. A key developmental task undertaken in this effort was to extend the MTS-FPV framework to a “three-stream” problem to account for the diluent CO2 that is injected at the chamber wall. This “three-stream” will be further developed and matured in the Phase II effort by allowing the diluent CO2 to participate in the combustion process. The “three-stream” MTS-FPV framework was demonstrated on a preliminary sCO2 78MWth shear coaxial combustor designed by GTI to operate at 300 bar. The flame structure, CO concentration, NOX levels, as well as the efficacy of the diluent CO2 for film cooling and temperature modulation were simulated and the Phase II effort will involve refinement of this design based on these results. In addition to the numerical framework development, an integrated effort was initiated to generate experimental data in the relevant regime and mature our numerical framework by validating against this data. The test program is being conducted at UCF by our research partner Prof. Vasu and preliminary data has been generated for the following: a) chemical kinetics data at high pressure regime with large fraction of CO2 diluent and the presence of nitrogen and sulfur contaminants, and b) real fluid model calibration in the supercritical regime for natural gas combustion mixtures. The Phase II program will feature more extensive testing and the development of a well-validated kinetic model for combustion in the presence of impurities as well as potential corrections to analytical EoS models. The validated sub-models that will be developed will replace or enhance current models in CRUNCH CFD® code and thus provide a well-validated design tool.
- Research Organization:
- Combustion Research and Flow Technology
- Sponsoring Organization:
- USDOE Office of Science (SC) SC-1
- Contributing Organization:
- CRAFT Tech, UCF
- DOE Contract Number:
- SC0019640
- OSTI ID:
- 1575398
- Type / Phase:
- STTR (Phase I)
- Report Number(s):
- DOE-CRAFTTech-DE-SC0019640
- Country of Publication:
- United States
- Language:
- English
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