An experimental investigation on the design and implementation of the mixing chambers for improving the thermal-decomposition of the urea

Abstract

Paper presented at the 7th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Turkey, 19-21 July, 2010.

Urea-SCR has been reported to be the most promising method for meeting NOx emissions regulation. In the urea-SCR process, NH3 is generated by urea thermolysis aod hydrolysis and used as a reductant of NOx. Consequently, to improve NOx convention efficiency of urea-SCR, it is required to enhance the thermal-decomposition and hydrolysis rate upstream of SCR catalyst. In the present work, experiments are carried out by varying inlet velocities (4-12 m/s) and temperatures (350--500"C) to analyze the characteristics of urea thermal-decomposition. Two types of the mixing chambers are designed and fabricated to maximize the thermal-decomposition of urea. The thermal­-decomposition characteristics of urea to isocyanic acid (HNCO) and ammonia (NH,) are experimentally examined without and with mixing chamber. The thermal-decomposition of the urea is greatly enhanced at low velocities and high gas temperatures. At inlet velocity of 6 m/s and inlet temperature of 400 "C, NH3 concentration with A-type mixing chamber is maximally about 200% greater than that without mixing chamber. When the inlet velocity and temperature with B-type mixing chamber are 4 m/s and 400 "C, respectively, NH3 concentration is maximally about 187% greater compared to that without mixing chamber. It is noted that as the mixing chamber is implemented the conversion of urea to NH3 is significantly enhanced because the residence time of urea in the exhaust pipe increases and the mixing between urea and exhaust gas is improved.