1、Review of state of the art technologies of selective catalytic reduction of NO x from diesel engine exhaust Bin Guan a,b , Reggie Zhan b, * , He Lin a, * , Zhen Huang a, * a Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, C
2、hina b Engine, Emissions, and Vehicle Research Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA highlights graphicalabstractThe review of state of the art tech- nologies of selective catalytic reduc- tion of NO x .ThemainstreamV-based,Cu-andFe- zeolite, and chaba
3、zite catalysts are illustrated.Thedevelopmentofhighlyoptimized hybrid integration SCR systems are analyzed.The by-products of SCR systems and the corresponding regulations are discussed.The future perspectives of the advanced SCR technologies are described. articleinfo Article history: Received 5 No
4、vember 2013 Accepted 8 February 2014 Available online 18 February 2014 Keywords: Emission legislations Selective catalytic reduction Catalysts performance Optimized hybrid integration system By-products abstract Increasingly stringent emission legislations, such as US 2010 and Euro VI, for NO x in m
5、obile applications will requirethe use of intensi cation of NO x reduction aftertreatment technologies, such as the selective catalytic reduction (SCR). Due tothe required higher deNO x ef ciency, a lot of efforts have recently been concentrated on the optimization of the SCR systems for broadening
6、the active deNO x temperature window as widely as possible, especially at low temperatures, enhancing the catalysts durability, and reducing the cost of the deNO x system. This paper provides a comprehensive overview of the state-of- the-art SCR technologies, including the alternative ammonia genera
7、tion from the solid reductants, Vanadium-based, Cu-zeolite(CuZ) and Fe-zeolite(FeZ) based, and the novel chabazitezeolitewith small poresizeSCRcatalysts.Furthermore,theprogressesofthehighlyoptimizedhybridapproaches,involving combined CuZ and FeZ SCR, passive SCR, integration of DOC (DPF, SCR), as we
8、ll as SCR catalyst coated on DPF (referred as SCRF hereinafter) systems are well discussed. Even though SCR technology is considered as the leading NO x aftertreatment technology, attentions have been paid to the adverse by- products, such as NH 3 and N 2 O. Relevant regulations have been establishe
9、d to address the issues. 2014 Elsevier Ltd. All rights reserved. * Correspondingauthor. Engine,Emissions, andVehicle ResearchDivision,SouthwestResearchInstitute,6220 CulebraRoad, SanAntonio,TX78238,USA.Tel.:1210522 2331; fax: 1 210 522 3950. * Corresponding author. Key Laboratory for Power Machinery
10、 and Engineering of Ministry of Education, Shanghai JiaoTong University, Shanghai 200240, China. Tel.:86 21 3420 7774; fax: 86 213420 5553. * Correspondingauthor.KeyLaboratoryforPowerMachineryandEngineeringofMinistryofEducation, ShanghaiJiaoTongUniversity, Shanghai200240,China.Tel.:86 21 3420 6379;
11、fax: 86 21 3420 5553. E-mail addresses: (B. Guan), rzhanswri.org (R. Zhan), (H. Lin), z- (Z. Huang). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: http:/dx.doi.org/10.1016/j.applthermaleng.2014.02.021 1359-4311/ 2014 Elsevier Ltd. All rights reserved. App
12、lied Thermal Engineering 66 (2014) 395e4141. Introduction Criteria emissions, such as oxides of nitrogen (NO x ), particulate matter (PM), carbon monoxide (CO),and hydrocarbons (HCs) from diesel vehicles, trucks and off-road machines are becoming increasingly stricter, represented by the US 2010 and
13、 Euro VI, as wellasTier4off-roadregulations,whichdrivesthedevelopmentof highly ef cient aftertreatment systems for diesel engine powered vehicles 1. For example, the U.S. federal vehicle emission stan- dards effective in 2007 require tight control of NO x . For light duty vehicles, the currentstanda
14、rd of Tier 2 Bin 5 is 0.07 g/mileNO x for 120,000 miles. However, the proposed future standard is 0.03 g/ mile for NMOG (non-methane organic gases) NO x (SULEV 30) at 150,000 miles 2,3. There is a signi cant improvement needed in NO x reduction(deNO x )ef cienciesfordieselvehiclestoachievethe future
15、 standard. The new regulation is setting new benchmark for theperformanceanddurabilityrequirementsof theNO x reduction systems. The application of the deNO x technique to the mobile sources has to overcome some dif culties, such as transient oper- ating conditions, broadening operating windows, such
16、 as the improvement of the low-temperature activity and high- temperature stability 4,5. Among the lean NO x aftertreatment technologies, the selective catalytic reduction (SCR) technology showsimpressiveNO x reductionef ciencyandcanmeetanewset of speci cations, which makes SCR become the dominant t
17、ech- nologycomparedtoalternatives5,6.Generally,theadvantagesof SCR are satisfactory in NO x reduction ef ciencies, durable perfor- mance, with wide performance window, reasonable cost, and available infrastructure 7,8. Nowadays the requirements for the SCR technology are continuouslyincreasingduetot
18、hefactthattheemissionlegislation has been expanded intonon-roadmarket. The SCR technology has developed into its third or fourth generation since commercially introducedinEuropein2003.Atthattime,systemswereremoving upward of 75% NO x over the European heavy-duty (HD) transient cycle, i.e., European
19、Transient Cycle, or ETC, to meet Euro IV regu- lations. In early 2010, most new medium- and heavy-duty diesel vehicles in the major international markets such as USA, Europe, and Japan have relied on the urea-based SCR technology in complying with the most stringent regulations on NO x emissions 9.
20、InEurope,vanadium-based SCR (V-SCR)catalysts havealready been widely used to meet Euro IV and V heavy-duty diesel (HDD) emissiontargets,andzeolite-basedSCRcatalystshavebeenusedto meet US 2010 HDD regulations. To meet the emerging Euro VI regulationsin2014,thecycleaveragedeNO x ef cienciesof95%plus a
21、re realistic 10,11. Meanwhile, with the stringent EPA 2015 reg- ulations on large diesel engines for locomotive, marine, and sta- tionarygeneratorapplications,theneedforNO x reductionviaurea- SCR catalysts arises given the proven performance of urea-SCR in on-highway and off-highway applications 12,
22、13. Effort is being made to develop high-ef ciency SCR catalysts with broad temper- ature windows, with superb durability, with precise control algo- rithmincorporatingammoniastoragecapacity,andwiththeentire aftertreatmentsystemoptimizedtomeetthecurrentandemerging NO x regulations 14,15. Asthevehicl
23、e fueleconomy requirementscontinuetoincrease, it is becoming more challenging and expensive to simultaneously improve fuel economy and meet emissions regulations. Fuel con- sumption, a direct indication of CO 2 emissions, has recentlygrown in importance due to the increasing focus on CO 2 emissions
24、reduction 10. As a consequence of the chosen ambitious CO 2 optimized combustion mode in the advanced engines and related technologies, raw NO x emission increases (the fuel economy im- proves),whichalsoresultedindroppingtheexhausttemperatures evenfurther,andtheSCRtechnologyisrequiredtohavehigherNO
25、x reduction ef ciency in combination with CO 2 minimization. In addition, cold-start emissions reduction has become critical in the practical applications and the improved deNO x performance in urban driving or other low load conditions. As a result, higher ac- tivity of SCR catalysts at low tempera
26、tures is mandated to effec- tively reduce real-world NO x emission while fuel economy requirement has to be simultaneously met. A lot of effort has also been made, but the main obstacle is that during low-speed urban driving, the low-CO 2 engines only generate exhaust gas tempera- tures below 200C,
27、which is too insuf cient for the conventional urea-SCR based systems 7,9. Currently, the-state-of-art deNO x technologyisurea-SCRoverprimarilyCuZSCR,withrelativelywide temperature window, better durability, and low NH 3 slip. 2. SCR chemistry Forreasonsofsafetyandtoxicity,ureaisthepreferredselective
28、 reducing agent for mobile SCR applications. When urea is used as reductant,adetailedunderstandingofitsrolesintheNO x reduction processiscriticalforoptimizingtheSCRprocess16.Itisgenerally accepted that if urea, as an aqueous solution, is atomized into the hot exhaust gas stream, it decomposes in thr
29、ee steps that are physically separated in time and space: evaporation, the thermal decomposition of nely sprayed urea into ammonia (NH 3 ) and isocyanic acid (HNCO), hydrolysis, which would occur along the exhaust pipe prior to the catalyst, and the hydrolysis of isocyanic acid, which would occur on
30、 the catalyst surface 17. The above three steps are shown in the following reactions (1), (2), and (3), respectively 16,17. Step 1: evaporation of water from the droplets, thus leading to molten urea (no catalyst). NH 2 eCOeNH 2 (aqueous)/NH 2 eCOeNH 2 (molten) H 2 O (gas) (1) (Water vaporization he
31、at is 2270 kJ/kg) Step 2: thermal decomposition, i.e., molten urea will then heat up and thermally decompose to NH 3 and HNCO (no catalyst). NH 2 eCOeNH 2 (molten)/NH 3 (gas) HNCO (gas) DH 298 186 kJ (2) EquimolaramountsofNH 3 andHNCOarethusformed.HNCOis very stable in the gas phase, but hydrolyzes
32、easily on many solid oxides with water vapor originating from the combustion process. Step 3: hydrolysis, i.e., isocyanic acid to hydrolyze to NH 3 and CO 2 (catalyst). HNCO(gas)H 2 O(gas)/NH 3 (gas)CO 2 (gas)DH 298 96kJ(3) The three steps given by reactions (1)e(3) correspond to the overall urea decomposition shown in reaction (4). NH 2 eCOeNH 2 H 2 O/2NH 3 CO 2 (4) inwhich 1 mol of urea would generate 2 mol of NH 3 . The NH 3 :NO stoichiometric molar ratio in typical SCR reactions is 1:1. As a B. Guan et al. / Applied Thermal Engineering 66 (2014) 395e414 396
