Large Eddy Simulation of a Multiple-Injection Dry Low NOx Combustor for Hydrogen-Rich Syngas Fuel at High Pressure

Abstract

The successful development of coal-based integrated gasification combined cycle (IGCC) technology requires gas turbines capable of achieving the dry low-nitrogen oxides (NOx) combustion of hydrogen-rich syngas for low emissions and high plant efficiency. Therefore we have been developing a multiple-injection burner for hydrogen-rich syngas fuel in order to achieve high efficiency and low environmental load. This burner consists of a perforated plate with multiple air holes and fuel nozzles. The multiple air holes and the fuel nozzles are arranged coaxially. The burner is based on the concept of premixed combustion configured by mixing fuel and air in the each air hole rapidly and dispersing fuel with multiple fuel-air jet. This rapid mixing can reduce NOx emissions by getting homogeneous lean premixed combustion, and preventing flashback despite the high flame speed for hydrogen-rich syngas fuels. The unsteady phenomena that occur in the combustion field should be understood in detail in order to confirm this burner concept. However, their measurement under high pressure is difficult. Meanwhile computational fluid dynamics (CFD) is able to investigate the detailed distributions of various emissions and temperature even though under combustion fields of high pressure and high temperature. The purpose of this paper is to validate this concept of the multiple-injection burner by using CFD. The burner can change the combustion form between premixed and non-premixed combustion by controlling the mixing, so the combustion field coexisting with premixed combustion and non-premixed combustion is complicated. Therefore, we have developed a hybrid turbulent combustion (HTC) model applicable to both non-premixed and premixed flames. The HTC model is hybridized with the flamelet progress variable (FPV) model and a flame propagation model. The FPV model is based on the laminar flamelet concept. The flame propagation model considers the flame stretch effect, diffusion enhancement effect, and increasing rate of flame surface area. The turbulent flow model adopts large eddy simulation (LES) with a dynamic sub-grid scale (SGS) based on the local inter-scale equilibrium assumption (LISEA4). Both the turbulent combustion model and turbulent flow model were programmed into a simulation tool based on the OpenFOAM library. We validated the concept of this burner for hydrogen-rich syngas fuel by using the simulation tool. The simulation results showed the rapid mixing of fuel and air in the air holes, and by using HTC model we confirmed that premixed combustion is the combustion configuration of this multiple-injection burner. In addition, the multiple-injection burner has high flame stability. There is no zone of high temperature in the air hole and high temperature is maintained near the burner. The multiple-injection burner can thus maintain flame stability without any flashback.