Mixing performance of preheated flue gas entering selective catalytic reactor under low loads

Abstract

The reduced inlet flue gas temperature of the selective catalytic reduction (SCR) denitrification device is a significant problem during the low-load operation of boilers. In this study, an experimental system for turbulent thermal mixing of high and low-temperature flue gases was built to study the thermal mixing method in bypass flue gas technology and the effect of the temperature difference and flow rate variation of high and low-temperature flue gases at low load on the thermal mixing characteristics. The experiments evaluated the thermal mixing characteristics by measuring the temperature field distribution, calculating the inhomogeneity parameters ζT, and investigate the multi-jet distribution characteristics. The investigation was carried out more accurately by combining the simulation results. The results show that the tube wall jet combined with the bottom jet method provides the highest warming effect and possesses the lowest mixing non-uniformity parameter. At the location which is 1.25 times the hydraulic diameter of the duct away from the jet pipe, the temperature difference between the mainstream and the jet drops from 270 °C to 180 °C, causing ζT to drop from 0.46 to 0.37. Decreasing the jet velocity and increasing the main flow velocity promote a decrease in ζT, indicating that the ratio of jet velocity to main flow velocity and the velocity ratio UJUm are crucial factors affecting the thermal mixing characteristics, which directly affects the heat transfer between hot and cold fluids. The ζT decreases from 0.52 to 0.42 as UJUm changes from 3.58 to 2.15. This study provides a reference for ensuring the safe and stable operation of SCR denitrification system during the low-load operation of coal-fired boilers.