Studies On The Evaporation,decomposition,mixing And Crystallization Of The Urea Spray In Diesel SCR Systems

Urea selective catalytic reduction?Urea-SCR?is a very effective technology of exhaust gas treatment to reduce NO_x emissions from diesel engines.In this paper,studies on the decomposition and mixing of urea spray,the mixing of NH₃ with exhaust in SCR systems and the urea crystallization in exhaust pipes and urea nozzles were conducted.In this paper,the evaporation and decomposition of urea spray and the mixing of NH₃ produced by urea decomposition with exhaust gas in the case of different spray nozzles and different spray angles were studied.The results show that the urea decomposition rate and NH₃ distribution uniformity are significantly higher in the case with four nozzle holes than that of single nozzle hole.The effect of spray angle on urea decomposition rate and NH₃ distribution uniformity was small.The urea crystallization in the exhaust pipe with different nozzles and different spray angles was studied.The results show that the amount of urea crystallization is almost 0g in the exhaust pipe installed the nozzle with single nozzle hole whose spray angle is 0°.The amount of urea crystallization is 7.1g in the pipe installed the nozzle with four nozzle holes whose spray angle is 25°.The amount of urea crystallization in the pipe is 39.3g in the pipe installed the nozzle with four nozzle holes whose spray angle is 45°.The urea crystallization of nozzle with and without heat shield was studied.The maximum temperature in the nozzle without heat shield is up to 188?,and the nozzle holes were blocked in the experiment.However,the maximum temperature in the nozzle heat shield is up to 146?,and the nozzles were not blocked.Hence the use of heat shield can effectively reduce the maximum temperature inside the nozzle and improve the anti-crystallization ability of the nozzle.The effects of the using of mixers on the evaporative decomposition of urea spray,distribution uniformity of NH₃ at the catalyst inlet and the of NO_x catalytic conversion efficiency were studied.The study shows that the use of the mixer can improve the temperature distribution in the exhaust pipe,the evaporation rate of spray droplets,and the secondary atomization of spray.The results also show that the SCR system can achive the highest NO_x catalytic conversion efficiency when the mixer was placed 1D position away from the nozzle.The SCR system can achieve the relatively high NO_x catalytic conversion efficiency and NH₃ distribution uniformity when the mixer was placed 3D away from the nozzle.