Design and testing of a unique, compact gas turbine catalytic combustor premixer

Catalytic combustion systems represent a potentially significant technology development in low-NOx gas turbine technology. The challenges associated with the implementation of catalytic combustion in a gas turbine include the need to present both a uniform fuel/air mixture as well as a uniform approach velocity to the catalyst within a minimal volume. The effort described herein addresses the design of catalytic combustor premixer for use on a small frame industrial (5 MW) stationary gas turbine. The general requirements for the premixer were to provide velocity and fuel air mixture uniformity at the exit plane of ±10% of the mean and ±3% of the mean respectively at nominal idle and full load conditions while maintaining a pressure drop of 4% maximum. The target turbine's packaging and air-flow path presented an additional challenge requiring the flow to make a 180 degree flow reversal immediately upstream of the combustor. Computational fluid dynamics modeling was. utilized to iterate the mixer design. A unique solution was obtained utilizing an involute shaped contraction/throat/expansion scheme. The most promising design was fabricated and tested at atmospheric conditions. As compared to a "baseline" mixer, the final involute design improved upon the flow mixing and velocity uniformity. The novel premixer design also eliminated the use of fuel injection spokes and static mixers, relying upon fuel injection from the wall and a high turbulent kinetic energy (TKE) throat section to provide the mixing, thereby simplify manufacturing. To test both scaling issues in the mixer and to provide an opportunity to test the design at high pressure, a 25% scale (approximate) reduced size mixer was designed and fabricated. Subsequent atmospheric and elevated pressure (11 atm, 1110 kPa) testing confirmed that the mixture uniformity was maintained and the general concept was applicable at both a reduced scale and at elevated pressures. Finally the premixer was integrated with a catalyst and fueled to confirm the overall system performance. The emissions of NOx were <1ppm