摘要
以氨气(NH3)为还原剂的选择性催化还原(SCR)技术因催化剂成熟且抗硫中毒性能强等优点成为颇受关注的柴油机NOx催化还原技术,是最有希望在规定时间内使柴油机NOx排放达到欧Ⅴ及以上排放标准的技术之一。但是,柴油车用SCR技术需要解决低温工况下NOx去除效率低下的问题。为此,国外采用在SCR反应器前增加预氧化装置(DOC)使NOx低温还原效率低的问题得到缓解。但是,DOC通常所用的贵金属催化剂容易硫中毒,因此,SCR技术的应用受到柴油含硫量的制约。
本文针对SCR系统存在的技术问题提出新的解决方案,研究开发适用于高硫份柴油车尾气的新型柴油机SCR系统——等离子体辅助选择性催化还原氮氧化物。本文将进行模拟柴油机排气试验研究和柴油机台架试验研究,主要研究内容包括以下四个方面:
(a)自行搭建模拟柴油机排气NH3-SCR试验台架,考察了混合气组分、反应温度以及空速等因素对SCR系统NOx去除效率的影响。研究发现,反应温度对NOx去除效率影响非常大。提高NO2的量对于低温工况下能大幅度提高SCR还原NOx效率,当[NO2]/[NO]=1时可以使SCR在各低温工况下去除效率最高。
(b)采用介质阻挡放电(DBD)型式的等离子体发生器作为NO的预氧化反应器,研究了DBD过程中NO的预氧化,并考察了混合气组分、温度以及能量密度等因素对NO预氧化性能的影响。研究发现,在合适的O2体积分数下,通过放电产生的等离子体可以有效地将NO部分氧化为NO2。研究还发现,C3H6在等离子体预氧化过程中起了关键的作用。
(c)进行了DBD反应器与SCR反应器耦合系统试验,考察了混合气组分、反应温度以及能量密度对NOx去除效率的影响,并比较了SCR系统与DBD-SCR耦合系统在去除NOx方面的性能差异。研究发现,C3H6的加入可以大幅度提高NOx去除效率,但过多的C3H6同时又会产生大量的CO。与直接加入NO2实现快速SCR相比,使用DBD辅助SCR有更高的NOx去除效率。
(d)基于YC6G柴油机,搭建了带有DBD辅助SCR系统的试验台架。改变发动机工况,分别考察了SCR系统与DBD-SCR耦合系统去除NOx的性能。研究发现DBD-SCR耦合系统能显著提高发动机台架试验中低温工况下的NOx去除效率,但是输入DBD反应器的能量密度较大。
Due to the advantage of mature catalyst performance and strong resistance to sulfur poisoning, the selective catalytic reduction (SCR) with ammonia (NH3) as the reducing agent received much research interest, and is considered to be one of the most promising methods to meet the Euro V and above emission regulations within the specified time. However, the low NOx removal efficiency of SCR at low temperature condition is a big problem for diesel vehicle application. For this reason, partial pre-oxidation of NO into NO2 is needed to achieve the fast-SCR, which process is active at low temperature. Pre-oxidation of NO raise another challenge that the sulfur matters in the diesel exhaust easily devastate the precious metal catalysts usually used for NO oxidation.
In order to solve the problems mentioned above, a new approach, plasma-enhanced selective catalytic reduction of NOx was proposed. In this paper, a novel SCR system combined with plasma reactor was established. Both simulated gas experiment and diesel engine bench test were investigated in this study which mainly consists of four sections as followed.
(a) An experimental system of NH3-SCR for simulated gas was set up and on which, a series of experiments aiming to study the NOx removal efficiency of SCR system under different reaction conditions (component and concentration of gas mixture, reaction temperature and space velocity, etc.) were carried out. The results demonstrate that NOx removal efficiency was influenced by reaction temperature greatly. NOx removal efficiency increases with the increasing concentration of NO2 in the gas mixture, especially at low temperature. Peak NOx removal efficiency occurs when the ratio of [NO2]/[NO] reaches 1 at low temperature levels.
(b) A dielectric barrier discharge (DBD) reactor was designed to partially oxidize NO to NO2. Several factors, including gas component, temperature and energy density were investigated on the NO pre-oxidation performance of DBD reactor with the presence of appropriate amount of O2. It was found that hydrocarbon species greatly affect NO pre-oxidation in plasma process by promoting NO coversion to NO2.
(c) Removal of NOx in the system combing DBD reactor with SCR reactor was tested under different conditions (gas component, reaction temperature and energy density, etc.). The performance difference between SCR system and Plasma-SCR system was analyzed. The results reveal that with the addition of C3H6, the NOx removal efficiency can be improved significantly, but high C3H6 concentration leads to large amount of CO emission. The Plasma-SCR system has much higher NOx removal efficiency than that of fast-SCR process achieved by adding NO2 directly.
(d) An engine test bench with Plasma-SCR system was setup based on YC6G diesel engine. NOx removal in both SCR system and Plasma-SCR system was tested under different conditions (engine speed, torque). At low temperature condition, NOx removal efficiency can be improved significantly by coupling SCR reactor with DBD reaction. However, the consumed energy in engine bench test is much higher than that of simulated gas experiment.
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