Toward ultra-low emission distributed combustion with fuel air dilution

Colorless distributed combustion (CDC) has been shown to offer enhanced combustor performance for stationary gas turbine application with near zero emissions, high combustion intensity and efficiency, thermal field uniformity, and enhanced stability. Mixture preparation to form hot and low oxygen concentration environment paves the path to achieve CDC conditions. In this paper, a new approach of air dilution in partially premixed combustion conditions is employed and the results compared to premixed and non-premixed injection of air and fuel. Portion of the fuel is introduced in the air stream and portion of the air is introduced in the fuel stream such that the local equivalence ratios for each stream is well outside the flammability limit to eliminate flashback and instabilities. The experimental data demonstrated ultra-low emissions with this injection scheme. At equivalence ratio of 0.6, NO emission was 63% lower than non-premixed combustion mode. Also NO emission was similar to the premixed combustion with the advantage of eliminating flashback and flame instabilities that often prevail in premixed combustion conditions. Dilution provided 50% CO reduction as compared to non-premixed combustion. Numerical simulations, validated through Particle Image Velocimetry, were performed to outline the mixing process in each of the three cases. The methane mixture fraction prior to ignition, determined numerically, was found to be one half of that for the non-premixed case and close to that of the premixed case. This enhanced mixture preparation, associated with the new air and fuel dilution technique, resulted in reduced emission. Also the jet momentum ratio (between both streams) is enhanced, mainly due to the air addition to the fuel stream, to result in better mixing and a better reaction distribution for ultra-low emissions. Further reduction of NOx is expected with improved distributed combustion condition.