Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.In title on title page, double underscored "x" appears as subscript.Includes bibliographical references (p. 15).A laboratory test rig was designed and built to easily test SCR (Selective Catalytic Reduction) technology. Equipped with three 6 kW heaters, connections for liquid N2 and an assortment of test gases, and a connection with an MKS NOx Analyzer, the rig allows for a vast range of SCR test conditions, and can easily be adapted for degreening, aging, and the implementation of other technologies onto the rig. To calibrate the test rig, baseline parameterization of a Cu-based zeolite (aluminosilicate) core was performed, including a temperature sweep and an NO/NO2 concentration sweep, while always maintaining a 1:1 ratio between NOx and NH3 reductant. The catalyst was found to have a peak deNOx efficiency of 99.8% between 250 'C and 300 'C, and maintained 98% efficiency with NO/NO2 < 1, while minimizing pollutant N20 generation.by Sebastián Castro Galnares.S.B

Castro Galnares, Sebastián (Castro Galnares Wright Paz)

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Calibration and Performance of a Selective Catalytic Reduction (SCR) Bench Rig forNOux Emissions ControlbySebastian Castro GalnaresMASSACHUSETTS INSTITUTEC)r TECHNOLOGYAUG 1 4 2008U.BRARIESSUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING INPARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OFBACHELOR OF SCIENCE IN MECHANICAL ENGINEERINGAT THEMASSACHUSETTS INSTITUTE OF TECHNOLOGYJUNE 2008©2008 Sebastian Castro Galnares. All rights reserved.The author hereby grants to MIT permission to reproduce and to distribute publicly paperand electronic copies of this thesis document in whole or in part in any medium notknown or hereafter created.Signature of Author:All, '- Department ofiMe~Tiaical EngineeringMay 9, 2008Certified by:Victor W. WongPrincipal Rsse*t-&eientist, Manager of Sloan Automotive LaboratoryAccepted by:_John H. Lienhard VProfessor of Mechanical EngineeringChairman, Undergraduate Thesis CommitteeARCHNES^ "Calibration and Performance of a Selective Catalytic Reduction(SCR) Bench Rig for NOx Emissions ControlbySebastiain Castro GalnaresSubmitted to the Department of Mechanical Engineering on May 9, 2008 in partialfulfillment of the requirements for the Degree of Bachelor of Science in MechanicalEngineeringABSTRACTA laboratory test rig was designed and built to easily test SCR (Selective CatalyticReduction) technology. Equipped with three 6 kW heaters, connections for liquid N2 andan assortment of test gases, and a connection with an MKS NOx Analyzer, the rig allowsfor a vast range of SCR test conditions, and can easily be adapted for degreening, aging,and the implementation of other technologies onto the rig.To calibrate the test rig, baseline parameterization of a Cu-based zeolite (aluminosilicate)core was performed, including a temperature sweep and an NO/NO2 concentrationsweep, while always maintaining a 1:1 ratio between NOx and NH3 reductant. Thecatalyst was found to have a peak deNOx efficiency of 99.8% between 250 'C and 300'C, and maintained 98% efficiency with NO/NO2 < 1, while minimizing pollutant N20generation.Thesis Supervisor: Victor W. WongTitle: Principal Research Scientist, Manager of Sloan Automotive LaboratoryIntroductionAs it becomes more apparent that greenhouse gases are contributing to globalwarming, reducing the emission of gases from vehicle exhaust becomes paramount. Inthe automotive industry, NOx gases produced in most combustion processes as a result ofelevated temperature have been shown to contribute significantly to the greenhouseeffect. Originally designed for use in heavy industry [6], selective catalytic reduction(SCR) provides a means of converting NOx gases to harmless N2 and H20 componentsthrough the use of a reductant. In the last decade, adapting SCR technology for use in thediesel automotive industry has been an important topic of research; several technicalbarriers prohibit the direct transplant of the technology. In industrial SCR applicationsanhydrous or aqueous NH 3 is used, both of which require large storage vessels that wouldbe difficult to reduce to the appropriate sizes for automotive applications. Additionally,NH3 or in solution can be extremely toxic and corrosive [4]; therefore, an alternatereductant which can be easily stored on a vehicle platform, can be metered out withrelative ease, and most importantly, can be reduced to NH3 in situ to be used as the mainreductant of NOx gases is needed. Most studies focus on the use of urea as this reductant.Chemistry of urea decomposition and NOx reductionTo obtain the NH3 necessary for the catalytic reduction of NOx, urea decomposesas shown in the following reactions:(NH2)CO - NH3 + HNCO (1)HNCO + H20 --+ NH3 + CO2 (2)A necessary condition for these reactions to go to completion is a sufficientlyelevated temperature of the exhaust gas [3], as inefficiency results in unhydrolized ureadepositing itself on the catalyst surface, blocking reaction sites and decreasing theefficiency of the overall catalytic activity. A major part of catalyst design for SCRapplications is determining the temperature ranges at which the urea is fully decomposed,and avoiding the passing of excess uncatalyzed reductant, or slip, of both urea and NH3through the catalyst.With this in mind, consider the following reactions of NH3 and NOx gases:4NH3 + 4NO + 02  4N2 + 6 H20 (3)2NH3 + NO + NO2 -- 2N2 + 3H20 (4)8NH3 + 6NO 2 --+ 7N 2 + 12H20 (5)The rate of these reactions and their prevalence are determined in large part by theratio of NO2/NO gases in the exhaust stream. Reaction (3) dominates when no NO2 ispresent. Reaction (4) dominates when NO2/NO < 1 and is a fast reaction, while reaction(5) dominates when N0 2/NO > 1, and is a slow reaction. Secondary, pollutant-formingreactions (or reactions that prevent NOx from decomposing to N2 and H20) can alsooccur:4NH 3 + 4 0 2 -+ 2N 20 + 6 H20 (6)2NH 3 + 2NO2 - N20 + N2 + 3H20 (7)4NH 3 + 4 NO + 3 0 2 -- 4N20 + 6 H20 (8)4NH3 + 302 - 2N 2 + 6H20 (9)4NH3 + 502 - 4NO + 6 H20 (10)4NH3 + 702 - 4N0 2 + 6H20 (11)The actual chemistry on the catalytic surface is complex and can involve theevolution of phosphates, sulfates, and other species in the catalyzed exhaust both fromthe urea itself and from any impurities in the exhaust of a real engine [7]. However,because the metering of urea into an exhaust stream requires bulky, closed loopcontrolled equipment, simulated exhaust gas is often used and NH 3 is injected into thismixture to establish the baseline behavior of new catalysts. Thus, the above formulaeprovide a useful theoretical basis for analyzing the baseline behavior of catalysts tested inthis manner.Test Rig SpecificationsMuch of the work of this thesis focuses on the creation of a practical test rig forthe testing of the baseline performance parameters of SCR catalysts. As shown in Figure1, the developed rig is well suited for this goal, but can also be easily adapted to rundifferent types of catalyst de-greening procedures.Fig 1. Catalyst bench rig schematic [8]Because of the high space velocities that many catalyst testing proceduresimplement (i.e. above 30k hr -1), a dedicated, insulated input line is used to draw nitrogengas from a 250 L liquid N2 dewar pressurized at 230 psig. This input line runs through a 6kW Sylvania heater, which operates at 208 V and can draw up to 60 amperes of current.This heater is used to heat the N2 gas to about room temperature before it enters theexhaust gas-mixing manifold. Five input ports are used to feed cylinder gases into themixing manifold, and both these ports and the insulated line have flow meters used tocontrol the volume of gas per unit time being mixed into the system. Just downstream ofthe mixing manifold are two additional 6 kW Sylvania heaters in series, which are used toheat the exhaust mixture to the desired testing temperature. An Athena Series 16C and anExtech 48VTR temperature controller are used in conjunction with a power relay tocontrol the heaters, with thermocouples placed just downstream of the N2 input line andjust upstream of the catalyst section. An additional thermocouple was placed justdownstream of the catalyst section, but was not used for temperature control of thees~r~~ ~_Ea~Bexhaust gas. The catalyst section itself has sampling ports controlled through ball valvesjust upstream and downstream of the position of the catalyst. These sampling ports leadto an MKS Multigas 2030 FTIR (Fourier transform infrared spectroscopy) NOx analyzercapable of discerning all major exhaust gas species (with the exception of diatomicspecies, although for 02 there is an built-in dedicated sensor) at a maximum rate of onesample per second. Once exhaust goes through the catalyst section, it is carried through agate valve that is used to generate a pressure differential to ensure proper gas flow, andsubsequently through an exhaust port that removes the gases from the system.ExperimentalCatalyst preparation: For the initial test rig experiment, a proprietary copper-based formulation on a zeolite (hydrated aluminosilicate) matrix was obtained throughsuppliers at Ford. From the original sample, a 2.5" diameter, 5" long piece was coredwith a diamond bit. The catalyst was characterized by 400 cpsi (cells per square inch).Testing parameters and procedures: The base exhaust mixture used in allexperiments was 14% 02, 1500 ppm NH3, and the balance in N2. Before each testprocedure, the system was purged with pure N2 gas at 1500 C for 20 minutes. Twodistinct experiments were carried out.To analyze NOx conversion as a function of catalyst inlet temperature, atemperature sweep scheme was performed. Outlet composition data was taken until theoutlet compositions reached a steady state, starting at 150 oC catalyst inlet temperatureand increasing the temperature by 50 'C for each data point, up to 500 'C. With eachsuccessive temperature increment, data was also taken from the catalyst inlet to ensurethat the desired gas composition of 750 ppm NO and 750 ppm NO 2 was maintained atsteady state.The second procedure focused on the effects of NOx composition itself. Keepingthe catalyst inlet temperature at a nominal 300 oC, 1500 ppm of NOx was mixed into theexhaust stream at a ratio of 0.5 NO/NO 2. After recording catalyst outlet data at steadystate, the ratio of NO/NO 2 was increased with steady state data taken for each additionalconcentration as shown in Table 1, always maintaining the total NOx levels at 1500 ppm.As in the first procedure, catalyst inlet data was taken between each data point to ensurethat the desired exhaust gas composition was maintained.NO/NO 2  [NO] ppm [NO 2] ppm0.5 500 10000.8 670 8301.0 750 7501.3 850 6502.0 1000 500Table 1. NO/NO 2 ratio sweep procedure exhaust stream compositionResults and DiscussionThe results for NOx conversion efficiency are shown in Figure 2. As thetemperature was ramped up from 150 oC, the NO, conversion efficiency increased andthen began dropping at higher temperatures. This kind of behavior is typical for manycatalysts [1], and although a peak efficiency of 99.8% is achieved between 250 'C and350 'C, it should be noted that as a baseline measurement, the catalyst has yet to bedegreened or loaded. Under loading, fuel and lubricant pollutants such as sulfates,nitrates, and heavy metals can block catalysis by reacting with the surface copper anddeactivating catalyst sites.10090807060Ss50z40302010n150 200 250 300 350 400 450 500Temperature, *CFigure 2. NO, conversion efficiency as a function of catalyst inlet temperatureTo ensure that the reductant is being consumed in its entirety, as given by thepredicted chemistry of reaction based on equation (4), the amount of NH3 goingunreacted through the catalyst, or NH3 slip, is plotted in Figure 3. Slip is highest when thetemperature is lowest, which is expected as there is not enough heat in the exhaust streamto activate the catalytic reactions and decompose the NOx. This slip, however, continuesto decrease and eventually levels out past the temperatures at which NOx conversion is atits highest.___160140120ES100x' 80z6040200150 200 250 300 350 400 450 500Temperature, 'CFigure 3. NH 3 slip as a function of catalyst inlet temperatureIt is theorized that the steady state achieved in slip at higher temperatures can beattributed to an adsorption/desorption process in which NH 3 adsorbs to the catalystsurface at lower temperatures, and then begins NO and NO2 forming reactions at highertemperatures once desorption begins [5]. The authors of this study suggest that when usedwith low to medium (200 oC - 350 oC) temperature transients such as those found inheavy duty cycles, this mechanism can be used to improve overall deNOx efficiency,provided that the temperature is carefully regulated to avoid the formation of additionalNOx as can be seen in reactions (9) - (11).Pollutant formation is also relevant to the efficiency tradeoffs of any catalystbaseline performance, particularly since the susceptibility to form N20 in the catalystoutlet stream is relatively simple, as seen in reactions (6) - (8). As shown in Figure 4,N20 formation is well matched to increase in deNOx conversion.riiiiiii350300E 250200Z 150100500OOv - -150 200 250 300 350 400 450 500Temperature, *CFigure 4. N20 formation as a function of catalyst inlet temperatureTransient imbalances in NO 2 concentration, along with elevated temperature, areparticularly prone to produce N20 in the catalyzed exhaust stream. However, once theexhaust temperature is sufficiently elevated, NO and NO2 formation is favored, as can beappreciated from the decrease in deNOx activity.Varying the NO/NO2 ratio is well established in the literature as a way of testingcatalyst behavior when exposed to different types of engine systems, and also when finetuning other technologies to work in conjunction with SCR, such as LNT (lean NOx trap)and DPF (diesel particulate filter) [7]. Figure 5 shows deNOx conversion when exposedto varying levels of NO/NO 2 but maintaining both the total NOx concentration and theamount of reductant in the exhaust stream.100999897960 95z94939291900.5 0.8 1 1.3 2NONO 2, ppm/ppmFigure 5. NO, conversion as a function of NO/NO 2 ratio in catalyst inletThe deNOx conversion percentage remains relatively constant under low tomoderate levels of NO concentration, but begins to fall dramatically once it ispredominant in the NOx stream. Studies show that high concentrations of NO willcompete with NH 3 for adsorption on copper catalysis sites on the catalyst and will resultin decreasing deNOx capability [2]. However at lower NO/NO2 ratio we incur a penaltyin the form of N20 production as shown in Figure 6.- - - - - - - - - - - -- . .... .....-- - - - - -- - - -350300E 250100500A0.5 0.8 1 1.3 2NONOO2, ppm/ppmFigure 6. N20 formation as a function of NO/NO 2 in catalyst inletAgain, N20 pollution becomes an issue as the concentration of NO 2 decreases.The problem is simpler to adjust for here as better efficiencies for NOx reduction areachieved with less NO in the exhaust stream, which also produces fewer pollutants. Asmentioned previously, increasing NO2 concentration in the exhaust stream is beneficialfrom an outright efficiency standpoint, but also from an adsorption/desorption one [5], asNO2 that might adsorb to the catalyst is more easily released in a transient cycle anddecomposes more readily to N2; conversely, NO binds more readily to the Cu catalysissites and is more selective to oxidizing to NO2.__ _____________·IIUIIIIII··______-L··-·--l~·--·llllConclusionsThe tested Cu-based catalyst was found to have an appropriate baselineperformance in comparison with other catalyst types [1], and has the advantage of notpossessing any vanadium-containing compounds in its structure. Used extensively untilrecently, studies have shown health risks involved with breathing in released vanadiumwhen the catalyst core's integrity has been compromised [8].The true test of a catalyst lies in the preservation of its NOx reduction qualitieseven after extensive loading. The design of the test rig proves multifaceted in this regard:not only can one establish the baseline performance of the catalyst, but by mixing moredemanding mixtures of simulated diesel exhaust and submitting the catalyst to manyhours of duty simulating cycles, in situ performance can be characterized and comparedto the baseline.The literature has shown that much of the promise in SCR technology is itscombination with other mechanisms to further enhance the SCR reactions [7]. Anexample of this is with DPF technology; when placed upstream of an SCR, it can be asource of NO 2 emissions that in general increase the NOx reduction efficiency of the SCRreaction, although it should be noted that it can also provide higher temperature gaswhich can lead to lower efficiencies and durability issues with the catalyst itself. Theversatility of the SCR test rig is such that it could be easily adapted to 'piggy back' on amotor with DPF technology, or even adapt a DPF into the test rig itself, making the scopeof possible test procedures vast, and the rig itself a valuable tool.AcknowledgementsThe work for this thesis was performed at the Sloan Automotive Laboratory at theMassachusetts Institute of Technology. The author would like to acknowledge Dr. VictorWong for his providing a platform for this project to take place and for his expertise.Special regards to Mike Giansiracusa and Alex Sappok for their help in immersing theauthor in the workings of Sloan Auto Lab and the many discussions surrounding the topicof this thesis. The author also extends to many thanks to Thane DeWitt, Yuichi Kodamaand Simon Watson for their invaluable knowledge, patience, and assistance, withoutwhich this project would not have been possible. A heartfelt thank you to NataliaRodriguez for her insightful comments in editing this thesis' content.References[1] J. Huang, Z. Tong, Y. Huang, J. Zhang, "Selective catalytic reduction of NO withNH3 at low temperatures over iron and manganese oxides supported onmesoporous silica", Applied Catalysis B: Environmental 78 (2008).[2] L. Olsson, H Sj6vall, R.J. Blint, "A kinetic model for ammonia selective catalyticreduction over Cu-ZSM-5", Applied Catalysis B: Environmental 81 (2008).[3] Y. Cheng, J. Hoard, C. Lambert, J.H. Kwak, C.H. Peden, "NMR studies of Cu/zeoliteSCR catalysts hydrothermally aged with urea", Catalysis Today (2008).[4] J.A. Sullivan, J.A. Doherty, "NH 3 and urea in the selective catalytic reduction of NOxover oxide-supported copper catalysts", Applied Catalysis B:Environmental 55(2005).[5] L. Xu, R.W. McCabe, R.H. Hammerle, "NOx self-inhibition in selective catalyticreduction with urea (ammonia) over a Cu-zeolite catalyst in diesel exhaust",Applied Catalysis B: Environmental 39 (2002).[6] Wikipedia contributors, "Selective catalytic reduction," Wikipedia, The FreeEncyclopedia,http://en.wikipedia.org/w/index.php?title=Selective catalytic reduction&oldid=209435612 (accessed April 20, 2008).[7] W.A. Majewski, "SCR Systems for Mobile Engines", DieselNet Technology Guide,http://www.dieselnet.com/techlcat scr mobile.html (accessed April 20, 2008).[8] M. Giansiracusa, "Test-rig Evaluation and Catalyst Analysis of SCR Systems",Powerpoint presentation for the MIT Consortium to Optimize Lubricants andDiesel Engines for Robust Emission Aftertreatment Systems, November 2007.

Calibration and performance of a selective catalytic reduction (SCR) bench rig for NOx̳ emissions control

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Calibration and performance of a selective catalytic reduction (SCR) bench rig for NOx̳ emissions control

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