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

Abstract

With the goal of developing advanced hydrogen combustors for micro gas turbines with low NOx emission at high temperatures up to Tit=1700°C, single coaxial injectors with two-staged (RQL; rich-quench-lean) combustion method were built, based on the excellent combustion features of hydrogen. The injector consisted of an inner pipe issuing rich premixed gases, and an outer concentric annular hole injecting secondary air at high velocity. The combustion characteristics of small test combustors, which had the injector and a combustion tube of 30 mm diameter, were examined at room temperature and atmospheric pressure, concerning NOx emissions, temperature distributions, combustion efficiencies, total pressure losses and flame stability limits. The optimum equivalence ratio of the rich premixed inner flame was confirmed to be about 2.0 for the viewpoint of decreasing NOx emission and total pressure loss. The NOx concentrations were reduced by decreasing the area of the coaxial air hole, 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 hole, having 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 mixing in the second stage was so rapid. It was confirmed that this type of injector was suitable for two-staged RQL combustion.