Abstract
Selective catalytic reduction (SCR) is a prospective way to reduce NO_x emissions from combustion devices. The aim of this study is to develop a SCR catalytic filter device for diminishing NO_x and soot emissions simultaneously among exhaust emissions from various combustors. Catalysts are used to reduce NO_x emissions in a SCR catalytic filter device and catalytic filters inside it are specially designed and constructed to diminish soot particles. NO_x emissions react with ammonia on the catalyst surface of SCR device to produce a harmless matter in the range of reaction temperature. The amount of ammonia must be optimized with the amount of NO_x. If the amount of ammonia is much higher than that of NO_x, the ammonia slip will be formed. The ammonia to a certain amount in the atmosphere leads to some diseases for human. Hence, a flow uniformity of ammonia in front of SCR catalytic filter is a challenge to obtain a higher NO_x conversion and lower ammonia slip. In this study, three dimensional CFD simulations are performed to investigate the ammonia mass concentration (distribution) in a SCR catalytic filter device. Three catalytic filters with 120° intervals are vertically mounted on a horizontal plate. A heater is located at the bottom wall inside SCR system to heat up the gas for achieving the reaction temperature (temperature window). Ammonia is injected radially perpendicular to the inlet main flow and mixed with the exhaust gases. Ammonia is distributed convectively to the cavity of SCR catalytic filter device. It is found that the circulation causes ammonia concentration difference in the core. The high and low ammonia mass concentrations are observed in the core. This circulation restricts the transport of ammonia from the core to the outside. The mixing index decreases as increasing the heating temperature in the SCR system.
