Parametric Computational Studies to Decrease NOx Emissions by Sequential Combustion of Fuel with a Diffusion Second Zone

Abstract

This paper is the third in the series of publications devoted to computational studies on optimization of the combustion of a fuel-air mixture with the gas temperatures at the outlet of the combustion chamber reaching about 1600iC or more, while satisfying the harmful emissions requirements. The paper provides the calculation results related to the combustion of gaseous fuel in a combustion chamber (CC) consisting of two consecutively positioned combustion volumes, each of which is provided with its own burner unit (BU). The first BU is typical for low-emission combustion chambers (LECC) and contains swirlers and zoned mixture preparation, as well as pilot and main burners. This unit is fed with an ideally mixed fuel-air mixture (FAM). The second BU is positioned downstream of the first combustion zone. It is fed with gaseous fuel (natural gas) without pre-mixing with air. In the second zone, a diffusion combustion process is realized. Two BU designs were considered for the second zone. In the first design, the fuel is supplied from behind a bluff body (throat) serving to create a flame stabilization zone. It also contains a fuel manifold. The fuel is supplied through the special openings in the mouth of the throat. The second design represents an annular manifold positioned around the flame tube and fuel supply tube embedded by 15 mm. This design does not provide for a special arrangement of the flame stabilization zone. In both cases, the combustion of fuel of the second zone occurs in the oxygen- deficient environment and at high temperature. The obtained results demonstrate the optimized fuel distribution between the two combustion zones, as well as optimized number and diameter of the fuel supply openings and length of the second combustion zone to ensure the lowest NOx emissions at the exhaust gas temperature of 1700°C and high fuel combustion efficiency. The results allow estimating the minimum possible concentrations of nitrous oxides in the dual-zone combustion chamber with a diffusion second zone. A comparison of the dual-zone fuel combustion schemes during kinetic and diffusion combustion in the second zone is provided. It is shown that kinetic combustion provides a significant benefit in terms of NOx concentrations, both with respect to a single combustion zone and two zones with the diffusion second zone. The simplicity of the design of the diffusion second zone is attractive, however, it provides a smaller reduction in NOx concentration.