NO_2扩散作用对催化型微粒捕集器再生的影响

Effects of NO_2 Diffusion on Catalyst Diesel Particulate Filter Regeneration

针对催化型微粒捕集器被动再生反应机理的问题，采用试验和仿真研究了NO2扩散作用对壁面和碳烟层再生产生的影响。由于壁面层产生NO2，饼层消耗NO2，形成的浓度梯度作为驱动力产生反向扩散扩用，在多孔介质中发生孔扩散及努森扩散，NO2多次参与了碳烟的被动再生反应。研究结果反映考虑扩散作用的被动再生模型与试验值相比，再生模型反应压降偏差可控制在7.0%以内，NO2偏差小于7.3%。NO2扩散作用随入口温度的升高更加明显，载体入口气体温度增加100℃时的NO2增加率约为增加50℃时NO2增加率的1.90~1.95倍。425℃时，z/L=0.9，H=0.1829mm处NO2浓度最高，NO2质量分数最高为5.69×10-4。分析碳烟再生过程发现，排气温度提升增强了NO2扩散作用，进而促进了颗粒物再生。425℃条件下，1000s内完成了55.94%的碳烟再生，深床层完成了80.53%，总再生量是325℃条件下的3.68倍。NO2扩散作用有效提升了载体的被动再生能力，延长了主动再生周期，改善了载体耐久性能。

Aiming to explain passive regeneration reaction mechanism of catalytic diesel particulate filter, the diffusion mechanism of nitrogen dioxide is studied by experiment and simulation. Due to the wall layers are coated by precious metal, a chemical reaction is genesis in 300℃ between nitric oxide and oxygen. And reaction can produce nitrogen dioxide. In cake layer, passive regeneration consumes nitrogen dioxide. Therefore, concentration gradient of nitrogen dioxide is formed. The gradient comes into a driving force of the reverse diffusion. Diffusion mechanism includes pore diffusion and Knudsen diffusion in porous media of particulate filter. Because of back diffusion of nitrogen dioxide, it can participate in passive regeneration of soot many times. Research results show that the pressure drop of filter can be controlled within 7.0% offset and NO2 concentration can be controlled within 7.3% compared the experimental values with the passive regeneration model which considering the diffusion mechanism. The nitrogen dioxide diffusion effect is more obvious with the increase of inlet temperature. When inlet temperature is increased by 100℃,the increasing rate of NO2 concentration is 1.90~1.95 times of that when temperature is increased by 50℃. The maximum concentration of NO2 is 5.69×10-4 at the position of z/L=0.9, H=0.1829mm. Analysis of regeneration process of soot retained in the filter shows that a rise in temperature of inlet gas results enhancement of NO2 diffusion, and the passive regeneration rate is accelerated. Soot mass retained in the filter is decreased by 55.94% in the first 1000s at 425℃, and soot mass in the filter layer reacts 80.53%. The total decreased soot mass is 3.68 times of the reactive mass at 325℃. The variation of concentration for NO2 and the reaction rates of soot show that diffusion of NO2 can effectively increase the passive regeneration rate and extend the active regeneration period.