CN103391806A - 使用三个串联的scr催化剂区处理尾气中nox - Google Patents

使用三个串联的scr催化剂区处理尾气中nox Download PDF

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CN103391806A
CN103391806A CN2011800545031A CN201180054503A CN103391806A CN 103391806 A CN103391806 A CN 103391806A CN 2011800545031 A CN2011800545031 A CN 2011800545031A CN 201180054503 A CN201180054503 A CN 201180054503A CN 103391806 A CN103391806 A CN 103391806A
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T·阿斯拉姆
V·M·R·霍尔
R·R·拉什拉姆
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Johnson Matthey PLC
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Abstract

一种通过使用含烃的还原剂的选择性催化还原改善柴油发动机尾气中的NOx还原的方法，包括将尾气通过一连串的区，所述区包括一个第一区，其中置有促进HC SCR的第一催化剂，得到第一产品气体；一个第二区，第一产品气体通过该区，其中置有促进H2-促进HC SCR反应的第二催化剂，产生第二产品气体，以及第三区，第二产品气体通过该区，其中置有促进CO/H2SCR的第三催化剂，得到最终的贫NOx的排出气体，以及将含有CO和H2的重整气体与尾气、第一产品气体和第二产品气体中的一种或多种混合。串联催化剂的使用提供了一个替代基于尿素的SCR的方案，而不需要尿素的来源和存储。

Description

使用三个串联的SCR催化剂区处理尾气中NOX
本发明涉及一种改进的排气后处理系统，特别是涉及一种用于柴油(压缩点火)发动机的后处理系统。
柴油发动机目前广泛使用于私家车及轻型商用车，以及更多的传统应用，如公共汽车和卡车中，因为其比以汽油为燃料的车辆具有更低的油耗。柴油发动机的工作方式导致排气中仍含有大量的氧气，以及燃烧产物和副产物包括CO2，H2O，NOX和颗粒物。还有少量的未燃烧的碳氢化合物(HC)和CO存在于排气中。
大多数国家已推出了车辆尾气排放法规，以改善空气质量，特别是在城镇和城市。规定的排放中，柴油机尾气中的NOX是特别难以处理的，因为在过量氧气的存在下排气中气体成分的化学还原很困难。NOx的某种程度的控制，可以通过发动机的设计实现，虽然通常以增加其他污染物作为代价，或通过NOX存储在催化剂组分上和发动机管理以提供丰富的偏移以释放存储的NOx并引起NOx的化学还原。目前最新的排气后处理系统包括选择性催化还原(SCR)阶段。SCR涉及添加还原剂，通常为在使用中会产生氨的尿素，以在气体通过SCR催化剂的同时将NOX还原为N2。避免了车载存储和供应，以及相关的基础设施的额外复杂性外，许多SCR催化剂仅具有相当窄的操作温度窗口。如果可以使用另一种还原剂，特别是柴油燃料本身，车载存储和供应以及相关的基础设施的问题也可以避免。这样的“HCSCR”已被提出，并建议包括使用沸石来存储未燃的HC用于在SCR催化剂的操作窗口中释放。尽管它有许多优点，HC SCR已经被证明，在现实生活中的发动机的工作情况下太难以操作。
烃重整形成合成气(H2和CO)，可以结合到水煤气变换反应中，以提高H2的产量，是一个众所周知的、在工业规模上实施的工艺。
已有一些建议涉及与汽车相关的燃料重整。可能最早的建议涉及制造H2用于给燃料电池汽车加装燃料。其复杂之处在于，需要避免大量的CO进给到燃料电池中，因为燃料电池中所用的Pt催化剂会被CO毒化。最近，已有建议出于以下一个或两个目的改善燃料和/或尾气：(a)回收热量并提高效率(伴随减少燃料消耗等)，通过将燃料和排气组分转换成更高热值的燃料，伴随着减少发动机排放；(b)生成H2和CO用于尾气的催化后处理，以减少规定的排放物。
来自两种主要类型的内燃发动机(火花点火或汽油燃料，以及压缩点火或柴油发动机)的尾气都含有大量的水蒸汽，但在其他方面有很大的不同。汽油发动机排出的尾气温度高，达到600～800℃，并含有相对较少的氧气。柴油机尾气温度低(在城市条件下操作的轻型柴油发动机有时低至150℃)，和相对高的含氧量。柴油尾气的低温对催化后处理设备提出了挑战，因为所有的化学反应的速度随温度变化，它证明“点亮”所有所需的不同反应的催化剂是困难的。“点亮”温度被认为是在该温度下发生50％的反应。
与温度和其它参数如生产量被控制的大型工业重整不同，汽车尾气的体积和物质处理量以及温度都可以在很宽的范围内变化。因此，已被证明设计和运行一个有效的柴油机后处理与尾气重整结合是非常困难的，虽然已经有一些，但基本都是学术性的建议。
已知的是，在发动机汽缸本身内，通过燃料注射和燃烧控制生产氢，以改善后续的后处理排放控制。
Energy&Fuels 2005，19，744-752公开了一种排气辅助柴油燃料重整系统。该文主要关心的是回收重整产物到发动机的入口侧。虽然在导言中提到了将重整产物用于后处理，但是没有详细给出如何有效地实现这一目标。
已知在相对低的温度下，氢气在Pt基催化剂下还原NO是有效的(这代表汽油冷启动条件)：J Catalysis 208，435-447(2002)。还已知添加氢到NO/O2/丙烷混合物中以在某些，但不是所有的，银基催化剂下辅助HCSCR：参见Applied Catalysis B：Environmental 51(2004)261-274。US5921076(戴姆勒-奔驰公司)描述了一种排气系统，其利用氢和/或烃类的添加，以协助柴油发动机尾气中的NOx的还原。虽然提到了一个或多个额外催化剂的可能性，而且预见该催化剂(们)可能储存和释放烃，但是不能认为公开或建议了在本发明的范围之内的任何实施方案。
仍然需要改进柴油发动机尾气的催化后处理，特别是为了满足在实际使用条件下推出的越来越具有挑战性的排放法规。
本发明提供了一种通过使用含烃的还原剂的选择性催化还原改善柴油发动机尾气中的NOx还原的方法，包括将尾气通过一连串的区，所述区包括一个第一区，其中置有促进HC SCR的第一催化剂，得到第一产品气体；一个第二区，第一产品气体通过该区，其中置有促进H2-促进的HC SCR反应的第二催化剂，产生第二产品气体；以及第三区，第二产品气体通过该区，其中置有促进CO/H2SCR的第三催化剂，得到最终的贫NOx的排出气体，以及将含有CO和H2的重整气体与尾气、第一产品气体和第二产品气体中的一种或多种混合。
令人惊奇地发现，这样串联的三个催化区能在现实生活操作条件下有效处理，实现良好的NOx还原。
因此，本发明还提供了一种催化剂体系，包括一连串的SCR催化剂，其包括促进HC SCR的第一催化剂，促进H2-促进的HC SCR反应的第二催化剂和促进CO/H2SCR的第三催化剂。
在本发明中，通过辅以燃料，优选辅以柴油燃料的发动机尾气重整形成的重整气体，与尾气、第一和第二产品气体中的一种或多种进行混合。在第一实施例中，所述重整气体与尾气在进入第一区前混合。在第二实施例中，通过将重整气体加入到所述第一和第二产品气体流中将重整气体与第一和第二产品气体混合。
将重整气体与尾气加入到第一区，被认为在某些情况下可以促进副反应，即O2与HC的氧化。建议在个别情况下进行常规实验，以确定这种副反应是否对具体的第一催化剂和第一区条件构成问题，经过这些测试后建立最终的设计。在下文中描述的实验中，重整产物与合成柴油尾气在进入第一区前进行混合。
重整产物的制造方法不是特别关键。然而，理想的是，由我们的共同未决申请(AA1876)中公开的装置中从柴油燃料生产。其公开将燃料与一小部分(低于10％，优选约1％至2％)尾气在通过重整催化剂之前进行混合。
第一区，在操作中，为高温HC-NOx还原区。催化剂如金属-沸石，例如Cu/ZSM5，是合适的。
第二区，接收从第一区排出的气体，优选混合了适量的重整气体，以达到优化的H2∶HC比，起到促进H2-HC-NOx还原的作用；合适的催化剂是基于Ag/Al2O3的。这样的催化剂通常容易受到HC限制，且易焦化，但H2的存在可以减少这样的问题。
第三区适宜地包含一个低温脱NOx催化剂，利用CO-H2促进NOx还原。这样的合适的催化剂可以是可能包括Pt或Pb的PGM催化剂。在此阶段注入相对高量的重整气体对于高NOx转化率可能是有利的。这样的Pt或Pb催化剂还可作为任何未转化的HC和CO的清理催化剂。
催化剂被方便地以传统方式涂覆在在一个单一的或分开的流通衬底上。现代涂层技术允许在金属或陶瓷基板上进行准确的分区沉积。
尾气重整器可进料到本发明的系统，或其输出物可在系统和被供给到发动机的入口的流体之间分离。
本发明现在将通过具体的实施例进行说明，但不应认为是仅限于此。
含有体积含量为8％O2，5％CO2，5％H2O和200ppmNOX的合成柴油车尾气，以及0.07ml/min的US06柴油燃料一起通过重整器，重整器在375℃和O/C比为3∶3.5下操作。重整催化剂的总体积为2.6cm3，以及由Pt-Rh/CeO2-ZrO2/Al2O3组成。重整器的输出物中含有约2％的H2。重整产物以2L/min的速率与另一种15L/min的合成尾气流混合，得到的气体混合物具有组成为8％O2，5％CO2，5％H2O，200ppmNOX，1000ppmH2，1500ppm CO和1000ppmCl HC。该气体混合物通入催化体系。催化体系之前没有再注入HC。
该系统使用第一Cu/ZSM5催化剂，第二Ag/Al2O3催化剂和第三市售Pd催化剂，沉积在具有相等长度为1in×3in(2.25cm×6.75cm)圆柱形衬底上。在200-450℃的温度范围测定NOx转化率，其范围从200℃的33％到300℃达到最大66％。
本系统在很宽的温度窗口实现有效的NOx转化。
一个最新的使用NH3作为尿素供应的常规SCR系统，在200到500℃之间能够实现接近100％的NOx转换，但尿素需要车载存储。

Claims (10)

1.一种通过使用含烃的还原剂的选择性催化还原改进柴油发动机尾气中的NOx还原的方法，包括将尾气通过一连串的区，所述区包括一个第一区，其中置有促进HC SCR的第一催化剂，得到第一产品气体；一个第二区，第一产品气体通过该区，其中置有促进H2-促进的HC SCR反应的第二催化剂，产生第二产品气体；以及一个第三区，第二产品气体通过该区，其中置有促进CO/H2SCR的第三催化剂，得到最终的贫NOx的排出气体，以及将含有CO和H2的重整气体与尾气、第一产品气体和第二产品气体中的一种或多种混合。
2.根据权利要求1的方法，其中第一催化剂是一种金属/沸石催化剂。
3.根据权利要求2的方法，其中第一催化剂是Cu/ZSM5催化剂。
4.根据前述任一权利要求的方法，其中第二催化剂是Ag/Al2O3催化剂。
5.根据前述任一权利要求的方法，其中第三催化剂是一种贵金属基催化剂。
6.根据权利要求5的方法，其中第三催化剂是一种Pd基催化剂。
7.根据权利要求7的方法，其中在进入第二区之前，重整气体与第一产品气体以能够达到用于H2-促进的SCR的最佳的H2∶HC比的量进行混合。
8.根据权利要求8的方法，还包括在进入第三区之前将重整气体与第二产品气体混合。
9.一种催化剂体系，包括一连串的SCR催化剂，其包括促进HC SCR的第一催化剂，促进H2-促进的HC SCR反应的第二催化剂和促进CO/H2SCR的第三催化剂。
10.根据权利要求9的催化剂体系，包括一连串的催化剂分区沉积在流通催化剂载体上。
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