Study on Hydrogen Combustors with Two-staged Combustion Method for Micro Gas Turbines (Combustion Characteristics of Coaxial Type Injectors)

For the purpose to develop advanced hydrogen combustors for micro gas turbines with low NOx emission at high temperatures up to Tit=1700°C, single coaxial injectors with the two-staged (RQL; rich-quench-lean) combustion method were built based on the excellent combustion features of hydrogen. The injector was consisted of the inner pipe issuing rich premixed gases and the outer concentric annular port injecting secondary air with high velocity. The combustion characteristics of the small test combustors of 30 mm dia. installed the injector concerning NOx emissions, temperature distributions, combustion efficiencies, total pressure losses and flame stability limits were measured at room temperature and atmospheric pressure. The optimum equivalence ratio of the rich premixed inner flame was confirmed to be about 2.0 in the viewpoint of decreasing NOx emission and total pressure loss. The NOx concentrations were reduced with decreasing the area of the coaxial-air port and were in inverse proportion to the ratio of the momentum of the coaxial-air to the premixed gas. For the injector with the smallest area of air-port, having the pressure loss of below 4%, NOx emission levels were low enough and high combustion efficiencies over 99.95% were attained in the range from 0.3 to 0.6 of overall equivalence ratio since the mixing in the second stage was so rapid. It was confirmed that this type of the injector was suitable for two-staged of RQL combustion.