柴油机SCR技术NO_x转化效率影响因素及催化剂温度特性研究

发布时间：2018-04-14 18:34

本文选题：SCR + 催化剂温度　；参考：《北京工业大学》2016年硕士论文

【摘要】：选择催化性还原(Selective Catalytic Reduction,SCR)系统是目前在柴油机应用最广泛的后处理技术之一,SCR系统性能的优化,有利于提高NOx转化效率以满足日趋严格的排放标准。排气温度是NOx转化效率的重要的影响因素之一,直接影响着催化箱内部温度变化和催化剂温度;而催化剂温度直接影响SCR尿素喷射控制策略中对尿素喷射量的计算与修正。所以了解催化箱内部温度变化特性、准确估算催化剂温度,将会为SCR系统控制策略提供更加精确尿素喷射量,有效提高NOx转化效率,满足严格的排放标准。本文主要就SCR系统NOx转化效率及NH3排放和载体下游温度滞后时间的影响因素进行探究。本文搭建了柴油机SCR系统实验台架,进行不同工况的SCR系统温度特性及NOx转化效率特性测试,SCR系统台架测试结果表明:稳态工况下,催化箱出口温度和入口温度相差不大,但催化箱出口温度变化相对于入口温度存在25~40s的时间滞后。根据催化箱内主要化学反应、传热学及化学反应动力学,对SCR系统整体进行物理及化学能量变化过程进行分析。催化箱物理过程主要包括尿素水溶液的汽化吸热、催化箱与废气的对流换热、催化箱与外部环境的辐射及对流换热过程等;化学反应吸放热过程主要包括尿素的热解和水解反应吸放热、主要SCR催化还原反应吸放热及NH3的吸附和解吸附过程的吸放热过程。其中化学反应及废气与催化箱内壁及载体间的强制对流换热量对载体温度有较大影响。基于SCR能量过程的分析搭建了SCR系统一维模型,仿真结果表明排气温度、氧气浓度、尿素的质量分数、尿素的喷射量、NO/NOx比值和排气流量等因素对NOx转化效率和NH3排放有明显的影响;同时,对催化箱载体后端面的中心点温度滞后时间的影响因素进行分析发现,催化箱载体长度、载体直径和载体目数对此滞后时间影响相对较大,载体直径对催化箱下游温度滞后时间的影响最为显著,载体长度次之,最后是载体目数。
[Abstract]:Selective Catalytic reduction system is one of the most widely used reprocessing technologies in diesel engines. It can improve the efficiency of NOx conversion to meet the increasingly stringent emission standards.Exhaust temperature is one of the important factors affecting the conversion efficiency of NOx, which directly affects the temperature change in the catalyst box and the catalyst temperature, while the catalyst temperature directly affects the calculation and modification of urea injection amount in the urea injection control strategy of SCR.Therefore, understanding the internal temperature variation characteristics of the catalytic tank and estimating the catalyst temperature accurately will provide a more accurate urea injection for the SCR system control strategy, effectively improve the conversion efficiency of NOx and meet the strict emission standards.In this paper, the factors influencing the conversion efficiency of NOx and the emission of NH3 and the temperature lag time downstream of the carrier in SCR system are discussed.In this paper, the experimental bench of diesel engine SCR system is built, and the temperature characteristics of SCR system and the conversion efficiency characteristics of NOx system are tested under different operating conditions. The results show that under steady condition, the difference between outlet temperature and inlet temperature of catalytic box is not different.However, the temperature change at the outlet of the catalytic tank lags behind the inlet temperature by 25 ~ 40 s.According to the main chemical reaction, heat transfer and chemical reaction kinetics in the catalytic tank, the physical and chemical energy variation process of the SCR system is analyzed.The physical process of catalytic tank mainly includes vaporization heat absorption of urea aqueous solution, convection heat transfer between catalytic tank and waste gas, radiation and convection heat transfer process between catalytic tank and external environment, etc.The heat absorption and exothermic process of chemical reaction mainly includes the heat absorption and release of urea and hydrolysis reaction, the heat absorption and exothermic process of SCR catalytic reduction reaction and the adsorption and desorption of NH3.Among them, chemical reaction and forced convection heat transfer between exhaust gas and the inner wall of catalyst box and carrier have great influence on carrier temperature.Based on the analysis of SCR energy process, the one-dimensional model of SCR system is built. The simulation results show that the exhaust temperature, oxygen concentration, urea mass fraction,The factors such as no / no _ x ratio and exhaust flow rate of urea have obvious influence on NOx conversion efficiency and NH3 emission, meanwhile, the influence factors of temperature lag time at the center of the front surface of the catalyst box are analyzed, and the length of the catalyst box carrier is found.The influence of carrier diameter and number of carrier mesh on the hysteresis time is relatively great. The diameter of carrier has the most significant effect on the temperature lag time of the lower reaches of the catalytic tank, the length of the carrier is the second, and finally the number of the carrier mesh.
【学位授予单位】：北京工业大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TK421

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