Lean Premixed Gas Turbine Combustor Research Articles

Fuel Effects on Lean Blowout and Emissions from a Well-Stirred Reactor

The design and development of low-emission, lean premixed aeroand industrial gas-turbine combustors is challenging because of a need to satisfy conflicting requirements of operability, combustion performance, and low emissions. A toroidal well-stirred reactor (WSR) is a laboratory idealization of a compact primary zone of a gas-turbine combustor. Such a WSR was designed, built, and operated at atmospheric pressure to study lean blowout and emissions of CO, unburned hydrocarbons, and NOV over a range of residence time and fuel-air ratios. A variety of normal and cyclic alkanes, aromatics, and a blend of pure hydrocarbon fuels were tested. Results showed that fuel type affects lean blowout limits, combustion efficiency, and pollutant emissions. These observations have important practical implications to the design of gas-turbine combustors.

Combustor Stability and Emissions Research Using a Well-Stirred Reactor

The design and development of low-emissions, lean premixed aero or industrial gas turbine combustors is very challenging because it entails many compromises. To satisfy the projected CO and NOx emissions regulations without relaxing the conflicting requirements of combustion stability, efficiency, pattern factor, relight (for aero combustor), or off-peak loading (for industrial combustor) capability demands great design ingenuity. The well-stirred reactor (WSR) provides a laboratory idealization of an efficient and highly compact advanced combustion system of the future that is capable of yielding global kinetics of value to the combustor designers. In this paper, we have studied the combustion performance and emissions using a toroidal WSR. It was found that the toroidal WSR was capable of peak loading almost twice as high as that for a spherical WSR and also yielded a better fuel-lean performance. A simple analysis based upon WSR theory provided good predictions of the WSR lean blowout limits. The WSR combustion efficiency was 99 percent over a wide range of mixture ratios and reactor loading. CO emissions reached a minimum at a flame temperature of 1600 K and NOx increased rapidly with an increase in flame temperature, moderately with increasing residence time, and peaked at or slightly on the fuel-lean side of the stoichiometric equivalence ratio. Finally, emissions maps of different combustors were plotted and showed that the WSR has the characteristics of an idealized high-efficiency, low-emissions combustor of the future.

A NOx Prediction Scheme for Lean-Premixed Gas Turbine Combustion Based on Detailed Chemical Kinetics

The lean-premixed technique has proven very efficient in reducing the emissions of oxides of nitrogen (NOx) from gas turbine combustors. The numerical prediction of NOx levels in such combustors with multidimensional CFD codes has only met with limited success so far. This is to some extent due to the complexity of the NOx formation chemistry in lean-premixed combustion, i.e., all three known NOx formation routes (Zeldovich, nitrous, and prompt) can contribute significantly. Furthermore, NOx formation occurs almost exclusively in the flame zone, where radical concentrations significantly above equilibrium values are observed. A relatively large chemical mechanism is therefore required to predict radical concentrations and NOx formation rates under such conditions. These difficulties have prompted the development of a NOx postprocessing scheme, where rate and concentration information necessary to predict NOx formation is taken from one-dimensional combustion models with detailed chemistry and provided—via look-up tables—to the multidimensional CFD code. The look-up tables are prepared beforehand in accordance with the operating conditions and are based on CO concentrations, which are indicative of free radical chemistry. Once the reacting flow field has been computed with the main CFD code, the chemical source terms of the NO transport equation, i.e., local NO formation rates, are determined from the reacting flow field and the tabulated chemical data. Then the main code is turned on again to compute the NO concentration field. This NOx submodel has no adjustable parameters and converges very quickly. Good agreement with experiment has been observed and interesting conclusions concerning superequilibrium O-atom concentrations and fluctuations of temperature could be drawn.

Stability and emissions characteristics of a lean premixed gas turbine combustor

The effect of incomplete fuel-air mixing on NOx emission and combustion stability has been studied in an atmospheric pressure, optically accessible, laboratory-scale, premixed dump combustor. The degree of fuel-dimensional acetone fluorescence. Combustion stability was characterized both in terms of the level of combustion-generated noise during combustion oscillations and the lean blowouts limit. The structure of the flame zone was characterized using two-dimensional OH fluorescence measurements that were phase locked with the combustion-generated pressure oscillations. As has been observed previously, it was shown that incompletes fuel-air mixing results in increased NOx emissions at overall fuel-lean conditions. The results, however, indicate that “reaction zone” NO mechanisms, enhanced by superequilibrinm O-atom concentrations, are in part responsible for more than 50% of the increased NO production. It was also shown that incomplete fuel-air mixing in a premixed combustor can significantly reduce the stable operating range of the combustor both in terms of increasing the tendency for combustion oscillation and raising the lean blowont limit. Lastly, it was observed that during unstable combustion, the flame moves in a well-defined periodic manner: however, the details of the flame behavior and instability mechanism can vary significantly depending on the overall equivalence ratio and the degree of fuel-air mixing.