Reverse Flow Combustor Design for a Flexible Fuel Micro Gas Turbine

After the analysis of three tendencies in the energy field: substitution of liquid and solid fuels by gaseous fuels for electricity production; advent of distributed electricity generation; and the possibility of integration in fuel-cell hybrid systems for electricity production, the micro gas turbine comes as the most promising solution. Aiming at sustainable solutions, the micro gas turbine is intended for flexible fuel utilization, thus biogas is considered as well as natural gas. Biogas is produced via anaerobic digestion, the main types of reactors and feedstock are discussed. The resulting biogas composition is presented and besides a composition of 60% methane, 40% carbon dioxide, the purification for utilization in gas turbines is also featured. With the objective of designing a compact reverse flow annular combustor operating with lean pre-mixed flames (for reduced nitrogen oxides emissions), initially for a 100 kW micro gas turbine, a preliminary design phase has been carried out where the combustor main dimensions (diameters, lenghts and passage widths) were determined. Lately it was suggested by the research team the design of a similar combustor for a 1500 kW MGT, the preliminary design has been adapted and the refinement phase proceeded. The design refinement perfomance targets are: low pattern factor (obtained with proper dilution jets positioning and flow rate); contained liner temperatures (obtained with proper positioning of the splash rings that provide cooling air films) and low total pressure losses (obtained with a constant optimization of flow passages, avoiding recirculation and stagnation zones). The combustor should also be able to burn biogas or natural gas with a flame that does not touch the combustor liner or interferes with liner cooling. CH4 and CO emissions should also result low. The design methodology of the refinement phase included four main subjects, the improvement of a feature in one of them usually has brought a beneficial aspect in another, these main subjects are: injector improvement (for proper fuel pre-mixing); primary zone flow adequacy (for proper flame positioning and reduced recirculation zones in the combustion chamber); cooling adequacy (for the liner temperatures lying below the safety value of 1150 K) and dilution adequacy (for the pattern factor shall lie below 0.21). Tangential injectors were adopted since they allow for a compact combustor and a reduced number of injectors. The disposition adopted is three injector in an upstream plane (β1) and three injectors in a downstream plane (β2). CFD simualations models used are: energy equation; Reynolds-Averaged NavierStokes (RANS) with k − realizable model (since it deals with the presence of flows with complex secondary features), turbulent kinetic energy production limiter (avoid buildup in stagnation zones) and use of standard wall function (which limits total number of cells in the domain); Species transport with volumetric reactions with the turbulence-chemistry interaction modeled using Finite-rate/Eddydissipation, the mechanism chosen was methane-air 2-step. Simulations workflow consisted in non-reactive simulations where fuel-premixing in the injector could be studied and optimized followed by reactive simulations and reactive simulation with conjugate heat transfer analysis across the liner walls. A detailed analysis of results (total pressure, velocity, equivalence ratio, rate of reaction and temperature fields) for natural gas and biogas utilization has been produced. The total number of different combustors geometries is sixty-one. The final combustor model satisfies the project requirements in terms of performance and emissions while respecting the given geometric constrains (based on turbine dimensions) and boundary conditions.