SCI论文 SCR技术的概述.pdf

Review of state of the art technologies of selective catalytic reduction of NOxfrom diesel engine exhaust Bin Guan a b Reggie Zhanb He Lina Zhen Huanga aKey Laboratory for Power Machinery and Engineering of Ministry of Education Shanghai Jiao Tong University Shanghai 200240 China bEngine Emissions and Vehicle Research Division Southwest Research Institute 6220 Culebra Road San Antonio TX 78238 USA h i g h l i g h t sg r a p h i c a la b s t r a c t The review of state of the art tech nologies of selective catalytic reduc tion of NOx The mainstream V based Cu and Fe zeolite and chabazite catalysts are illustrated The development of highly optimized hybrid integration SCR systems are analyzed The by products of SCR systems and thecorrespondingregulationsare discussed Thefutureperspectivesofthe advancedSCRtechnologiesare described a r t i c l e i n f o Article history Received 5 November 2013 Accepted 8 February 2014 Available online 18 February 2014 Keywords Emission legislations Selective catalytic reduction Catalysts performance Optimized hybrid integration system By products a b s t r a c t Increasingly stringent emission legislations such as US 2010 and Euro VI for NOxin mobile applications will require the use of intensifi cation of NOxreduction aftertreatment technologies such as the selective catalytic reduction SCR Due to the required higher deNOx effi ciency a lot of efforts have recently been concentrated on the optimization of the SCR systems for broadening the active deNOxtemperature window as widely as possible especially at low temperatures enhancing the catalysts durability and reducing the cost of the deNOxsystem This paper provides a comprehensive overview of the state of the art SCR technologies including the alternative ammonia generation from the solid reductants Vanadium based Cu zeolite CuZ and Fe zeolite FeZ based and the novel chabazite zeolite with small pore size SCR catalysts Furthermore the progresses of the highly optimized hybrid approaches involving combined CuZ and FeZ SCR passive SCR integration of DOC DPF SCR as well as SCR catalyst coated on DPF referred as SCRF hereinafter systems are well discussed Even though SCR technology is considered as the leading NOxaftertreatment technology attentions have been paid to the adverse by products such as NH3and N2O Relevant regulations have been established to address the issues 2014 Elsevier Ltd All rights reserved Corresponding author Engine Emissions and Vehicle Research Division Southwest Research Institute 6220 Culebra Road San Antonio TX 78238 USA Tel 1 210 522 2331 fax 1 210 522 3950 Corresponding author Key Laboratory for Power Machinery and Engineering of Ministry of Education Shanghai Jiao Tong University Shanghai 200240 China Tel 86 21 3420 7774 fax 86 21 3420 5553 Corresponding author Key Laboratory for Power Machinery and Engineering of Ministry of Education Shanghai Jiao Tong University Shanghai 200240 China Tel 86 21 3420 6379 fax 86 21 3420 5553 E mail addresses guanbin B Guan rzhan swri org R Zhan linhe H Lin z huang 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 Applied Thermal Engineering 66 2014 395e414 1 Introduction Criteria emissions such as oxides of nitrogen NOx 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 Euro VI as well as Tier 4 off road regulations which drives the development of highly effi 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 NOx For light duty vehicles the current standard of Tier 2 Bin 5 is 0 07 g mile NOxfor 120 000 miles However the proposed future standard is 0 03 g mile for NMOG non methane organic gases NOx SULEV 30 at 150 000 miles 2 3 There is a signifi cant improvement needed in NOxreduction deNOx effi ciencies for diesel vehicles to achieve the future standard The new regulation is setting new benchmark for the performance and durability requirements of the NOxreduction systems The application of the deNOxtechnique to the mobile sources has to overcome some diffi culties such as transient oper ating conditions broadening operating windows such as the improvementofthelow temperatureactivityandhigh temperature stability 4 5 Among the lean NOxaftertreatment technologies the selective catalytic reduction SCR technology shows impressive NOx reduction effi ciency and can meet a new set of specifi cations which makes SCR become the dominant tech nology compared to alternatives 5 6 Generally the advantages of SCR are satisfactory in NOx reduction effi ciencies durable perfor mance with wide performance window reasonable cost and available infrastructure 7 8 NowadaystherequirementsfortheSCRtechnology are continuously increasing due to the fact that the emission legislation has been expanded into non road market The SCR technology has developed into its third or fourth generation since commercially introduced in Europe in 2003 At that time systems were removing upward of 75 NOxover the European heavy duty HD transient cycle i e European 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 NOxemissions 9 In Europe vanadium based SCR V SCR catalysts have already been widely used to meet Euro IV and V heavy duty diesel HDD emission targets and zeolite based SCR catalysts have been used to meet US 2010 HDD regulations To meet the emerging Euro VI regulations in 2014 the cycle average deNOx effi ciencies of 95 plus are realistic 10 11 Meanwhile with the stringent EPA 2015 reg ulations on large diesel engines for locomotive marine and sta tionarygenerator applications the need for NOxreduction via urea SCR catalysts arises given the proven performance of urea SCR in on highway and off highway applications 12 13 Effort is being made to develop high effi ciency SCR catalysts with broad temper ature windows with superb durability with precise control algo rithm incorporating ammonia storage capacity and with the entire aftertreatment system optimized to meet the current and emerging NOxregulations 14 15 As the vehicle fuel economy requirements continue to increase it is becoming more challenging and expensive to simultaneously improve fuel economy and meet emissions regulations Fuel con sumption a direct indication of CO2emissions has recently grown in importance due to the increasing focus on CO2emissions reduction 10 As a consequence of the chosen ambitious CO2 optimized combustion mode in the advanced engines and related technologies raw NOxemission increases the fuel economy im proves which also resulted in dropping the exhaust temperatures even further and the SCR technology is required to have higher NOx reduction effi ciency in combination with CO2minimization In addition cold start emissions reduction has become critical in the practical applications and the improved deNOxperformance in urban driving or other low load conditions As a result higher ac tivity of SCR catalysts at low temperatures is mandated to effec tivelyreducereal worldNOxemission whilefueleconomy 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 CO2engines only generate exhaust gas tempera tures below 200 C which is too insuffi cient for the conventional urea SCR based systems 7 9 Currently the state of art deNOx technology is urea SCRover primarily CuZ SCR with relativelywide temperature window better durability and low NH3slip 2 SCR chemistry For reasons of safety and toxicity urea is the preferred selective reducing agent for mobile SCR applications When urea is used as reductant a detailed understandingof its roles in the NOxreduction process is critical for optimizing the SCR process 16 It is generally accepted that if urea as an aqueous solution is atomized into the hot exhaust gas stream it decomposes in three steps that are physically separated in time and space evaporation the thermal decomposition of fi nely sprayed urea into ammonia NH3 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 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 NH2eCOeNH2 aqueous NH2eCOeNH2 molten H2O gas 1 Water vaporization heat is 2270 kJ kg Step 2 thermal decomposition i e molten urea will then heat up and thermally decompose to NH3and HNCO no catalyst NH2eCOeNH2 molten NH3 gas HNCO gas DH298 186 kJ 2 Equimolar amounts of NH3and HNCO are thus formed HNCO is very stable in the gas phase but hydrolyzes easily on many solid oxides with water vapor originating from the combustion process Step 3 hydrolysis i e isocyanic acid to hydrolyze to NH3and CO2 catalyst HNCO gas H2O gas NH3 gas CO2 gas DH298 96 kJ 3 The three steps given by reactions 1 e 3 correspond to the overall urea decomposition shown in reaction 4 NH2eCOeNH2 H2O 2NH3 CO2 4 in which 1 mol of urea would generate 2 mol of NH3 The NH3 NO stoichiometric molar ratio in typical SCR reactions is 1 1 As a B Guan et al Applied Thermal Engineering 66 2014 395e414396 consequence the stoichiometric molar ratio urea NOxof 1 2 can be referred to stoichiometric balance It is well known that NOxin diesel exhaust is usually composed of 90 NO Therefore the main reaction of SCR with NH3will be 4NH3 4NO O2 4N2 6H2O 5 The so called standard SCR reaction 5 implies a 1 1 stoichi ometry for NH3and NO and the consumption of some oxygen The reaction 6 consuming no oxygen is much slower and is therefore not relevant in lean combustion gases 4NH3 6NO 5N2 6H2O 6 On the other hand the reaction rate of fast SCR reaction 7 with equimolar amounts of NO and NO2 is much faster than that of the standard SCR reaction 5 4NH3 2NO 2NO2 4N2 6H2O 7 It should be mentioned that the reaction 8 with pure NO2is again slower than reactions 5 and 7 8NH3 6NO2 7N2 12H2O 8 Regarding the side reactions the commonly used catalysts tend to form nitrous oxide N2O at high temperatures 400 C One of the possible reactions leading to N2O is described in reaction 9 18 4NH3 4NO 3O2 4N2O 6H2O 9 At temperatures higher than 500 C the undesirable oxidizing properties of the SCR catalysts become pronounced as shown in reaction 10 which represents the oxidation of NH3to NO thus limiting the maximum NOxconversion 18 19 4NH3 5O2 NO 6H2O 10 Besides at lower temperatures below 200 C in the NOx feed containing NO2 ammonium nitrate NH4NO3 will be formed ac cording to reaction 11 18 2NH3 2NO2 NH4NO3 N2 H2O 11 3 Urea and alternative NH3generation from the solid reductants Current SCR systems are being designed to use a solution of urea 32 5 wt dissolved in water UWS32 or Diesel Exhaust Fluid DEF or AdBlue as the NH3source 20 However the urea solution which is used widely has a number of drawbacks It is a corrosive liquid with that freezes below 11 C decomposes slowly in the tank at temperatures above 50e60 C crystallizes easily and has to be made from rather pure urea and distilled water to avoid a buildup of impurities on the SCR catalyst and DEF injection system When sprayed into the exhaust line the evaporation of the water decreases the exhaust gas temperature by some 10e15 C 21 This cooling has an infl uence on activity especially in the low temperature region where the conversion is limited by intrinsic kinetics Moreover there is solid deposit formation in the exhaust and diffi culties in dosing at exhaust temperatures below 200 C 22 Additionally creating a uniform NH3concentration can be problematic complicating exhaust packaging and usually requiring a discrete mixer and careful design Furthermore with uncertain amounts of urea converting to NH3under various exhaust condi tions the controls for urea injection can be complicated 22 For the temperaturewindowof SCR the NOxremoval by SCR has the larger temperature window by far However with urea as the dosed reductant there exists an additional constraint in the lower temperature range because the conversion of urea to NH3in the exhaust line does not take place completely below 200 C The limited conversion of isocyanic acid by hydrolysis is perhaps the biggest obstacle facing the use of urea SCR for achieving high NOx conversion in urban areas 21 During urban driving the temper ature of the exhaust at the urea injection point can often be below 200 C While the SCR catalyst still has activity down to at least 150 C the urea dosing has to be stopped to avoid formation of deposits and undesired by products If gaseous NH3could be made available it would be possible to obtain NOxconversion throughout a greater span of the exhaust temperature window 21 All of these issues are improved or eliminated if NH3gas itself is injected into the exhaust Therefore the relevant properties of a range of NH3source materials for their ability to provide NH3for use in SCR have been examined Some important properties including volumetric effi ciency moles of NH3 liter mass effi ciency moles of NH3 gram and NH3release temperatures were evalu ated A number of materials provide an equivalent number of moles of NH3to AdBlue in roughly one third the volume or one third the mass 21 22 After extensity investigation a number of other ma terials have beenproposed as alternative sources of NH3that would improve many of these issues materials such as solid urea ammonium salts like ammonium carbamate NH4COONH2 and ammonium carbonate NH4 2CO3 as well as metal ammine chloride salts like magnesium ammine chloride Mg NH3 6Cl2 calcium ammine chloride Ca NH3 8Cl2 and strontium ammine chloride Sr NH3 8Cl2 allow for the direct injection of NH3gas into the exhaust 22 3 1 Chemistry and physics of solid reductants Gaseous NH3can be generated from various solid reductants each associated with various decomposition characteristics The ammonium salts such as ammonium carbamate and ammonium carbonate are generally decomposed in two steps as in reactions 12 13 and 14 15 respectively 22 23 NH4COONH24NH3 HCOONH2 12 HCOONH24NH3 CO2 13 NH4 2CO34NH3 NH4 HCO3 14 NH4 HCO34NH3 CO2 H2O 15 Thermogravimetric analysis TGA was performed to determine the number of steps during decomposition as well as the temper ature range necessary for decomposition Besides the NH3partial pressures for both ammonium salts were measured as a function of temperature It was reported that the ammonium carbamate with the lowest decomposition temperature decomposes most readily also provides the most NH3on a mass basis and generates the highest NH3pressure and does not generate water which might lead to undesirable reactions it would appear to be the preferred candidate among the ammonium salts 22 Additionally ammo nium carbamate offers excellent NH3density requiring approxi mately 30 of the necessary packaging space when compared with that of liquid urea 24 B Guan et al Applied Thermal Engineering 66 2014 395e414397 Besides metal ammines are a class of well known materials called coordination complexes In the complex NH3coordinates to the central metal cation by donating both electrons from its lone pair to form the ligand bond When the materials are heated NH3 is released in a solid to gas transition without formation of a liquid phase The adsorption and desorption of NH3from this class of materials are completely reversible with forward reactions being exothermic and reverse reactions endothermic 22 Most metal ammines release NH3in a multi step process which is dependent on the coordination sites to which NH3molecules bind For magnesium ammine chloride for example four NH3 molecules attached to the equatorial sites release fi rst followed by the last two molecules bonded to apical sites Calcium and stron tium ammine chlorides can take up to eight NH3molecules with six and seven molecules leaving fi rst upon heating respectively The following reactions summarizes the desorption chemistry for Mg NH3 6Cl2 Ca NH3 8Cl2 and Sr NH3 8Cl2materials for deter mining in how many steps the material will release NH3 22 a Magnesium ammine chloride Mg NH3 6Cl24Mg NH3 2Cl2 4NH3 16 Mg NH3 2Cl24Mg NH3 Cl2 NH3 17 Mg NH3 Cl24MgCl2 NH3 18 b Calcium ammine chloride Ca NH3 8Cl24Ca NH3 2Cl2 6NH3 19 Ca NH3 2Cl24Ca NH3 Cl2 NH3 20 Ca NH3 Cl24CaCl2 NH3 21 c Strontium ammine chloride Sr NH3 8Cl24Sr NH3 Cl2 7NH3 22 Sr NH3 Cl24SrCl2 NH3 23 From a chemistry standpoint replacing DEF or AdBlue with a solid material that could be thermally decomposed to produce NH3 is evidently feasible The most likely candidate materials have an NH3density around 3 times that of AdBlue on a volume basis and 50 mass Past studies have been completed investigating various solid reductant sources narrowing selection criteria and nomi nating ammonium carbamate as the reductant of choice including comparisons with various salts and metal ammines 21e24 3 2 Applications of solid reductants NH3generation from solid reductants is attractive as a means to extend service interval range ease packaging limitations and improve low temperature SCR operation As a consequence a solid SCR SSCR system has been developed in the laboratory of Tenneco and FEV using ammonium carbamate as the solid reductant and its proof of concept is demonstrated not only in a small European passenger car but rather in something that is more representative of a typical American vehicle 24 In this SSCR system NH3is generated by heating the solid ammonium carbamate in a