Experimental And Numerical Studies On Diluted Syngas Diffusion Flames

Combustion with diluted syngas, as one of the most efficient low-NO_x technologies, is important for integrated gasification combined cycle (IGCC) system that attains high efficiency and low pollutant emissions. Flame structure, stability, and emission characteristics were experimentally and numerically studied in various fuel diluted syngas flames. The purpose of this paper is to better understand the behavior and mechanism of fuel diluted combustion and to provide fundamental data base for the development of syngas combustion techniques.Experiments were conducted by using co-flow jet diffusion flames in a model combustor. To investigate the dilution effects on the flame structure, stability and pollutant emissions, flame size, exhaust temperature and emission concentration were measured. It was found that dilution reduced fuel concentration, made the flame surface move inward to the fuel side, and increased the jet velocity, enhanced the combustion process, thus resulting in a reduction in the flame length. In flame stability, when the air flow rate kept constant, the maximum dilution amount increased with the decrease in fuel flow rate (or with the increase in excessive air coefficient), and increased with the increase in the initial fuel temperature and with the H₂ concentration in the fuel stream. In the three diluted flames, flame diluted with H₂O had a relatively highest stability, while flame diluted with CO₂ had a lowest one. In pollutant emissions, due to the unique advantage of fuel dilution, the diluted flame can both reduce NO formation and also control CO emission in a low level. From the pictures of PLIF, it can be found that OH radical is more sensitive to the flame temperature than the diluents' chemical effect. Its concentration goes down as the temperature decreases.Numerical simulations were performed in 2-D jet flame model and 1-D counterflow flame model respectively, In 2-D simulation, the results demonstrated the dilution effects that by reducing peak flame temperature, residence time and NO formatting area, NO formation can be greatly suppressed. In the discussions of 1-D flame model, it was found that under the same peak flame temperature, dilution H₂O promoted CO oxidation by increasing OH radical, and dilution N₂ and CO₂ promoted NO formation by increasing N and O radicals. It was believed that O radical plays a significant role in the NO formation.After the research work of the present study, the characteristic of diluted diffusion flames has been well known. The results of this study showed that fuel dilution combustion can help reduce the flame size, promote the combustion intensity, be propitious to the reduction of combustor size. And its high flame stability and low pollutant emissions made this technique an extensive application in the development of gas turbine.