Performance Evaluation of Upward Swirl Combustor with Reverse Fuel Injector and Hydrogen Blending

Abstract

AbstractModern gas turbine combustors are required to provide high combustion efficiency, stability over wide range of operating conditions and fuel flexibility while meeting strict low emission norms, with other requirements such as low heat loads on combustor liner wall. One of the promising ways to meet such requirements is the provision of swirler. In conventional Can combustor, the swirler is positioned at upstream end of the primary region. This arrangement is termed as backward swirl arrangement. When swirler is placed between primary and secondary region, it is termed as upward swirl arrangement. Influence of key parameters on the performance of a Can-type upward swirl gas turbine combustor is reported in this chapter. First, a relative assessment of thermal and emission characteristics of conventional Can-type combustor and upward swirl Can-type combustor is presented. Test results of exit temperature profile, liner wall temperature and emission characteristics at exit of the combustor are compared for both the combustors. Results of large eddy simulation (LES) are compared to identify and understand the flow and combustion regimes using methane as a fuel. Temperature distribution obtained in conventional combustor indicates that backward swirl flow arrangement provided for flame stability and air–fuel mixing has drawback that it produces localized hot spots of high temperature gradients in the primary zone resulting in the formation of thermal NOx emissions. In addition, the thermal pattern created due to swirling of combustion products exerts high heat loads on the liner wall of combustor. When air is allowed to enter in the primary zone through upward swirl arrangement, it helps to confine the flame near the combustor axis. This peculiar flow regime set the liner wall at a low temperature compared to conventional combustion chamber. Comparison of emission characteristics demonstrates that NOx emissions are significantly low in upward swirl combustor compared to conventional combustor. However, low NOx levels are achieved by incurring unacceptable penalties in combustion chamber performance in terms of low combustion efficiency and high CO emission levels. To address the difficulties of upward swirl combustor, a modification to the fuel injection strategy is proposed. A new concept of reverse fuel injection is introduced. The existing upward swirl combustor has conical-shaped injector where fuel jets are arranged on the periphery of 90° cone. In reverse fuel injection strategy, the fuel exits the fuelling device in reverse axial direction towards the wall of hemispherical dome. The dome wall decelerates the fuel jet first, and then fuel flows in all directions. The injector length is chosen as variable for parametric investigation. Research indicates that the burned gas recirculation and highly turbulent shear flow caused by reverse fuel flow pattern improves fuel–air mixing and produces larger energy density during combustion. This improves the combustion efficiency and reduces CO emission levels remarkably. The length of injector has significant effect on increase in NOx levels and wall temperature. The reverse fuel injector with 5 mm injector length is identified as optimum configuration for achieving noticeable reduction in CO emission without any adverse effect on NOx levels and wall temperature. Chemistry between fuel and air is further augmented by blending methane with hydrogen. Results of experiments conducted for constant energy input condition are presented and discussed. For the same energy input, hydrogen addition to methane increases the mole fractions of free radicals H, O and OH, which promotes chemical reactions and produces intensive combustion. From flame visualization, it can be said that flames are relatively broader and shorter at higher hydrogen concentrations. Shorter flame obtained at higher hydrogen percentage indicates more rapid combustion and improved flame stability. Addition of hydrogen up to 5% in fuel blend contributes to additional reduction in CO emissions with marginal increase in NOx emissions.KeywordsUpward swirl combustorReverse fuel injectorHydrogen blendingCO and NOx emissionsCombustion efficiencyFlame visualization