catalytic technology for soot and gaseous pollution control...today, pollutants are emitted...
TRANSCRIPT
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18Catalytic Technology for Soot and Gaseous Pollution ControlOlaf Deutschmann and Athanasios G. Konstandopoulos
18.1Introduction
Pollutant emissions from stationary sources have long been a public topic, with theBritish Alkali Act requiring soda ash plants to cut acid gas emissions by 95% as farback as 1863. Since then, the anthropogenic emission of pollutants has drasticallyincreased, leading to severe health risks and climate changes.With the general publichaving been aware of such risks for a long time, technologies have developedcontinuously to reduce the amounts of gases and particles emitted into the atmo-sphere, though these are often still enforced by legislation.
Today, pollutants are emitted continuously from both stationary and mobilesources worldwide. Catalysis has developed as a major technology also for thereduction of pollutant emissions. This chapter will present a survey of today�stechnology and challenges for the catalytic control of soot and gaseous pollutants,detailing mobile – and, to a certain extent, also stationary – sources. Topics related tothe reduction of CO2 emissions and catalytic combustion technologies as primarymeasures for reducing the formation of pollutants will not be included at this point.
18.1.1Pollutant Emissions from Stationary Sources
Stationary sources for soot and gaseous emissions range from private homes, smallindustrial sites and waste-incineration facilities to large, coal-fired power plants.Thesemay have no, little, or extensive flue-gas cleaning technologies, many of whichwill include catalytic processes. Themain pollutants are nitrogen oxides (NOx), sulfuroxides (SOx), particles/soot, carbon monoxide (CO), hydrocarbons (HCs), volatileorganic compounds (VOC), ammonia (NH3), heavy metals, and others. A survey ofthe typical ranges of gaseous emissions from stationary sources without flue gascleaning is provided in Table 18.1.
Emissions of NOx, SOx, and NH3 cause acidification of the environment and theformation of smog, while the greenhouse gases CO2, N2O, and CH4 are made
Handbook of Combustion Vol.2: Combustion Diagnostics and PollutantsEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1
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Table 18.1 Typical ranges of gaseous emissions from stationary sources without flue gascleaning [1].
Industry Source Fuel type Gasemissions
Levels/pp mva)
Heat and powergeneration
Boiler Coal NOx 150–170
SOx 300–2500Particlesb) 10mgNm�3
Oil/Petcoke
NOx 250–500
SOx 1000–5000Particlesb) 10mgNm�3
Biofuel NOx 100–300SOx 0–50
Gas turbine Gas NOx 15–50CO 1–200
Diesel engine Oil NOx 1000–1500SO2 100–2000CO 100–1000Hydrocarbons 50–500 ppm C1
Incineration Municipal waste,sewage sludge
NOx 150–300
SO2 10–100CO 5–20Dioxins 1–10ngNm�3
Particlesb)
Process industry Nitric acid plants NOx 100–2000N2O 100–1000HNO3 TraceNH3 Trace
Cementcalcination
Gas þ solid NOx 100–3000
Ethylene burners Gas NOx 10–100Smelters Gas SOx 20 000–100 000
NOx 10–100Heavy metals <1 (after scrubber)
Coal gasification Coal NOx 10–50Industry heaters Gas SOx 10–100Glass furnaces Gas NOx 200–500FCC catalystregeneration
Gas NOx 50–2000
Printing industry,etc.
Solvents VOC 1–10 gNm�3
a)Q1b)
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responsible for global warming. Several other gaseous emissions such as dioxins,VOCs, H2S, and heavy metals present direct health risks.
The topic of catalytic technologies for exhaust-gas after-treatment of stationarysources has also been described in many books, book chapters, and review articles.For further survey information, the reader is referred to reviews by Gabrielsson andPedersen [1], Spivey [2], and others.
18.1.2Pollutant Emissions from Mobile Sources
Internal combustion engines in automobiles represent a major source for theemission of NOx, CO, and unburnt HCs, whereas diesel engines contribute alsoto the emission of soot. Themost appropriate way tominimize these air pollutants isto modify the combustion process. However, aside from those primary measures,current legislative emission standards can only be met by additional, secondarymeasures for exhaust purification by application of catalysts. The importance ofenvironmental catalysis will further increase in future due to the tightening ofemission limits, and an increasing number of automobiles. Already today, environ-mental applications exhibit a worldwide market share of 35% among all catalyticprocesses, whereas over 70 million automotive catalysts devices are produced peryear. The catalytic system used for after-treatment of the exhaust gas dependsprimarily on the fuel used (gasoline, diesel, biofuels) and the operating conditions.Typical engine raw emissions are listed in Table 18.2. In principle, there is a need todistinguish between stoichiometric-operated gasoline engines, lean-operated gaso-line engines, and diesel engines which exhibiting different major pollutants, namelyCO/NOx/HCs, NOx, and NOx/soot, respectively.
The topic of catalytic technologies for exhaust-gas after-treatment of mobilesources, automotive exhaust catalysis, has also been detailed in many books andreviews. Further survey information is available in the reviews of Lox et al. [4],Heck and Farrauto [5], Votsmeier et al. [6], Eigenberger et al. [7], and manyothers.
Table 18.2 Typical raw emissions of a spark-ignition (SI) (Otto) and a diesel internal combustionengine [3].
Diesel engine Otto engine
N2 (vol.%) 72.1 73.8O2 (vol.%) 0.7 9CO2 (vol.%) 12.3 8H2O (vol.%) 13.8 9NOx (vol.%) 0.13 0.17CO (ppm) 9000 80HC (ppm) 900 80
18.1 Introduction j467
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18.1.2.1 Spark-Ignition Internal Combustion EnginesThe concept and design of catalysts for gasoline engines have always been coupledwith changes in engine design and operation, and with legislation. The raw emissionof gasoline-fueled engines depends on the air–fuel ratio. In general, leaner conditionslead to higher NOx emissions, while richer conditions favor CO and HC emissions.The chemical composition of theHCs emitted is rather complex, and depends on thefuel composition, the air–fuel ratio, and the conditions during combustion. It is,therefore, not straightforward to find a model exhaust gas in laboratory studies thatcanmatch the properties of theHCraw emissions. Today,most studies use amixtureof propylene and methane, where the first component represents the fast-reactingHC species, and the latter component the rather more stable species.
Today, indirect-injection spark-ignition (SI) engines are operated at stoichiometricair–fuel ratios (i.e., l� 1), because the application of a three-way catalyst (TWC)requires such an operation. In fact, it was the development of highly efficient TWCexhaust purification systems that led to the coupling between exhaust-gas after-treatment and motor management.
The operation of a SI engine under lean conditions – that is, with excess air – leadsto a reduction in fuel consumption and, as a consequence, reduced CO2 emissions.Themajor pollutants here areNOx, primarily NO.During the past few years, theNOx
storage reduction catalyst (NSC) – which is also referred to as the NOx-Adsorber orlean NOx trap – has been the favored concept for the reduction of NOx emissionsfrom lean-operated gasoline-fueled engines. Yet, during the same time, the selectivecatalytic reduction (SCR) concept has also been under consideration for lean-operated SI engines.
18.1.2.2 Diesel-Operated Internal Combustion EnginesThe internal combustion engine requires ignition of the mixture of fuel and oxygen,either by SI (i.e., gasoline engines) or compression-ignition (CI); these includetraditional diesel and various premixed diesel combustion variants known as HCCI(homogeneous charge compression ignition) engines, PCCI (premixed chargecompression ignition) and LTC (low-temperature combustion). The diesel enginetakes in air, and utilizes the heat of compression to ignite a small quantity of fuel,which is injected into the cylinder. In the case of the diesel premixed mode engines,an air–fuel mixture is compressed until it reaches the point of autoignition. Thispromises not only to result in lower emissions, but also to exhibit an improved fueleconomy.Unfortunately, these new diesel engine technologies are often very difficultto control at higher engine loads, and it is for this reason that practical development isfocusing on the production of �mixed-mode diesel combustion� engines that operatewith a premixed fuel–air charge at low engine loads, but under conventional(nonpremixed) diesel combustion at higher loads.
Diesel engines are endowedwith a high energy efficiency, fuel economy, durabilityand, provided that an emission control technology is applied to reduce emissions ofNOx, HCs, CO, and particulatematter (PMor soot), the diesel engine can become thedominant automotive powertrain, not only for heavy-duty and commercial vehicles,but also for passenger cars, as recent trends in Europe have already demonstrated.
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To meet the ever-increasing, stringent diesel emission regulations, emissioncontrol technologies have been developed to address each pollutant. In the case ofNOx emission reduction there are several routes:
. Selective catalytic reduction (with ammonia resulting from urea injection anddecomposition in the exhaust).
. NOx-absorbing materials that store NOx and release them under appropriate (forthe reduction of NOx to nitrogen) conditions in the exhaust.
. Selective NOx reduction with the aid of HCs.
In the case of CO and HCs, diesel oxidation catalysts (DOCs) are used for theoxidation of these gases toCO2,while soot emissions are reducedby employingdieselparticulate filters (DPFs).
In this chapter, attention will be focused on catalytic technologies for emissioncontrol formobile applications. First, technologies for controlling soot emissionswillbe discussed, focusing on DPFs and DOCs, after which gaseous pollutants and theconcepts of TWCs, SCR, and NSCs will be discussed. A final section will detail thecontrol technologies used to reduce gaseous pollutants from stationary applications.
18.2Catalytic Technology for Soot Pollution Control
18.2.1Introduction
18.2.1.1 Diesel SootThe development of efficient particulate reduction technologies requires the priorcharacterization of soot particles emitted from diesel engines. Representative recentstudies conducted in this area [8, 9] have indicated that the characterization of the sootparticulates depends on several factors. From a control technology point of view, thestate of the soot particles in the raw exhaust upstream of the DPF is of great interest.The diesel particulates are a complex entity (see Figure 18.1) that contain carbona-ceous solid aggregates and organic fractions that may form nuclei particles oraggregates. Their state depends on the exhaust temperature (which may inducecondensation of the soluble organic fraction; SOF), on the sampling conditions(dilution process), and on various other parameters [9].
Most modern DPF systems are preceded by a DOC, the role of which is to oxidizeexhaust HCs (either injected into the engine cylinder, or in the exhaust), and thusraise the exhaust temperature to the levels needed for DPF regeneration (i.e., theprocess of removing the accumulated soot by oxidation; see below). The DOCeliminates almost completely any SOF that might be present on the soot particles.
The composition and morphology of diesel soot particles in the exhaust isvery important. Such morphology will affect the structure of any deposits [12]formed in the DPF (hence the engine backpressure), while their composition(predominantly carbonaceous) affects their oxidation potential and the ease of DPF
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regeneration [13–17]. The substructure of soot primary particles obtained by differentcombustion sources and fuels has been studied using high-resolution transmissionelectronmicroscopy (TEM) [13–15, 18, 19], and is known to be connected to reactivityof the soot.
Soot aggregates are assemblies of primary particles formed from carbon plateletsarranged in so-called turbostratic structures (Figure 18.2). More specifically, sootparticles are composed of a distribution of graphitic (G) and diamond (D) -like carbondomains (Figure 18.3) that correspond, respectively, to ordered and amorphouscarbon structures that, in turn, are largely responsible for the variation in thereactivity of soot particles.
Figure 18.1 Schematic of diesel particulate matter. Note: This pictorial representation appearedoriginally for the first time in Ref. [11], and has been adopted since then in several subsequentpublications, but without reference to the original source.
Figure 18.2 Primary carbon particle structure [12, 20].
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With regards to the size of the soot particles, a variety of measurements haveindicated that the majority of emitted solid diesel aggregate particles have electricalmobility and aerodynamic diameters in the range of 10–400 nm. The primaryparticle diameters are found to lie in the range of 8–40 nm [8, 9, 23]. Otherinvestigations into the effect of the different types of diesel engine, of fuels, andof engine-operating conditions on the size distribution of the soot particles havebeen conducted [24]. Notably, the soot aggregate size distributions were monitoredin the exhaust of five turbo-charged, direct-injection diesel engines (model years1997–2003), with displacements in the range of 1.9 to 2.4 liters and with advancedfuel injection systems [25, 26] The results showed that, for a constant shape, the sizedistribution was increased in line with the steady-state solution of the soot aggregatepopulation, achieving a balance that accounted for the coagulation and oxidativefragmentation processes [25]. Naturally, the simultaneous occurrence of these twoprocesses would lead to a distribution of soot aggregate morphologies. It was alsoshown how the fractal dimension of diesel soot aggregates (Figure 18.4) wasdistributed according to their electrical mobility diameter [27]. When these mea-surements were conducted on so-called Euro II and Euro III diesel engines operatingunder different conditions, there appeared to be a very robust pattern over thedifferent engines and operation conditions tested (for a review, see Ref. [10]). Here,Euro X refers to the series of automotive emission regulations specified by theEuropean Union (see Ref. [12]).
18.2.1.2 Diesel Particulate FiltersSeveral materials and configurations have been employed as DPFs [12, 29, 30], withthe combination of materials and geometries determining the performance of thefilter with regards to pressure drop evolution during soot loading,filtration efficiency,regeneration, and so on. Themost popular configuration for DPFapplications is that
Figure 18.3 Typical Raman spectrum of soot and its analysis in component peaks [21, 22].
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of the ceramic honeycomb wall-flow monolith, with alternately plugged channels.This functions primarily as a separator, collecting the soot particles inside and on thewalls of the channel, while allowing the gases to exit freely through the walls of thefilter. Its popularity is based mainly on its advantageous performance with respect topressure drop evolution during soot loading and particulate filtration efficiency, as itexhibits a compact arrangement that can achieve very high filtration efficiencies(>90%) with a small penalty in pressure drop.
18.2.2Soot Loading and Oxidation
One factor that determines the operating efficiency of a DPF is its permeability (i.e.,resistance to flow), which is in turn closely related to the geometric and microstruc-tural characteristics of the filters [10]. This resistance of the DPF to flow is expressedby its pressure drop, which can be very accurately described by summing theindividual pressure drop contributions (see Figure 18.5). Simulations of the evolu-tion of the pressure drop of the filter can provide a significant means of developingimproved configurations [10, 29, 31–36].
Konstandopoulos and Johnson [31], based on the fundamental principles of fluidmechanics and flow through porous media, published the first analytical solutionsfor flow fields and pressure drop of wall-flow monoliths in terms of filter micro-
1.20
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6005004003002001000mobility diameter (nm)
frac
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EURO III (2) 1400 rpm 3 barEURO III(2) 1500 rpm 3.5 barEURO III(2) 2500 rpm 6 bar
EURO III(1) 1500 rpm 2 barEURO III(1) 1500 rpm 5 barEURO III (1) 2500 rpm 6 barEURO III (1) 2000 rpm 13 barEURO II 1500 rpm 2 bar
EURO II 1500 rpm 5 barEURO II 2400 rpm 6 barEURO II 2000 rpm 13 barVan Gulijk et al (2003)-5.7 kW
Figure 18.4 Fractal dimension of soot aggregates with different mobility diameters obtained withdifferent engines and operation conditions [27, 28].
Color Fig.: 18.4
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structure and geometric configurations; these were subsequently validated experi-mentally for a particular extruded monolith design. The analytical model (extendedfor non-Darcian flow effects) was later shown to be in excellent agreement with three-dimensional (3-D) computationalfluid dynamics (CFD) simulations, andwas furthervalidated against a wider variety of filter media [32, 37, 38]. When the Konstando-poulos and Johnson [31] model was tested extensively against a range of filtersamples, it was reported to give excellent a priori predictions of pressure drop,thereby opening new development possibilities [39–41].
This approach was later extended to include the influence of the accumulated soot,which accounted explicitly for the soot layer microstructure and its dependence onthe operating conditions of the DPF [12, 42].
18.2.2.1 Soot Accumulation on the FiltersThe development of the pressure drop during soot accumulation on the filter ischaracterized by two filtration modes – deep bed and cake filtration – that depend onthe microstructure of the filter [36].
The deep bed filtrationmode is characteristic of all filter structures, and is expressedby a nonlinear increase of the pressure drop (Figure 18.6). This is due to the initialdeposition of soot particles inside the porous structure of the filter wall, which blocklocally the flow paths. Depending on the microstructure of the porous filter, even asmall amount of deposited sootmay have a huge effect on the pressure drop, as itmayblock a disproportionately large part of the pore structure – hence the nonlinearcharacter of the pressure drop evolution.
As the porous wall becomes blocked by the deposited soot there will be a smoothtransition to the cake filtrationmode, where a macroscopic soot layer grows on top ofthe filter wall, characterized by a linear dependence of the pressure drop on theaccumulated particulate mass in the filter.
Konstandopoulos et al. [43] demonstrated for the first time that, during filterloading, the microstructure of the soot cake was determined by the relative strength
Figure 18.5 Schematic of filter inlet and outlet adjacent channels, depicting local pressure valuesfor the derivation of the effects of compressibility on pressure drop.
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of convective versus diffusive transport of the soot aggregates towards the deposit, asquantified by the dimensionlessmass transfer Peclet number (the ratio of convectiveto diffusive mass transfer) (see Figure 18.7). This showed that the properties of thesoot layer (i.e., packing density, permeability of the soot layer) were �dynamic�, thatthey depended on the deposit growth mechanism and its history, and that the sootdeposit microstructure may be deformed under sufficiently high pressure dropvalues [10, 42].
Figure 18.6 Filtration modes, from deep-bed to cake (surface) filtration.
Figure 18.7 Soot deposit growth mechanism [43].
Color Fig.: 18.7
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18.2.2.2 Soot Oxidation: DPF RegenerationAs soot accumulates on the wall of the filter, it alters the permeability of the filter andincreases the pressure drop, to a point where the backpressure would induce a veryhigh fuel consumption and hinder the engine�s performance. Therefore, the sootloaded filter must be �regenerated� by oxidizing the accumulated soot – a processtermed DPF regeneration. Unfortunately, the oxidation of soot must be initiated andmaintained at very high temperatures (550–650 �C) that do not normally occur in theexhaust of a diesel engine. However, several methods are available by which theoxidation of soot particles can be achieved at lower temperatures (200–550 �C). Thesemethods are classified as either �active� or �passive� [10, 12]. The passive approachinvolves the use of a catalyst, either in fuel-borne formor as a catalytic coating.Duringregeneration tests with an increasing temperature, the pressure drop of the soot-loaded filters was shown to decrease as the soot collected inside the filter wasoxidized, emittingCOandCO2 gases. Based on such gas evolution, the soot oxidationrate (Figure 18.8) can be calculated as a function of the temperature, thus providing ameans of evaluating the regeneration behavior of different DPF technologies. Thenormalized soot oxidation rate (s�1) is defined as:
_rsoot ¼ 1m0
dmdt
ð18:1Þ
wherem0 is the initial amount of sootmass collected. The soot consumption rate dm/dt is typically computed by summing the CO and CO2 produced during the oxidationin a synthetic exhaust gas stream which does not contain CO nor CO2 in order todetect the soot-derived CO/CO2. The evolution of the normalized soot oxidation rateas a function of temperature provides ameans to compare and evaluate differentDPF
Figure 18.8 Soot oxidation rate as a function of temperature.
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technologies with respect to their soot oxidation activity. The CO selectivity (Equa-tion 18.2) can be also calculated, which is an important parameter because it affectsthe total heat release during regeneration.
fCO ¼ COCOþCO2
ð18:2Þ
18.2.3Catalytic Diesel Particulate Filters (CDPFs)
Catalytic soot oxidation has a long history [44], the details of which have been recentlyreviewed [45, 46]. The CDPFs currently available commercially are coated with Pt-group metal (PGM) catalysts that aim at the indirect oxidation of soot by NO2
generated from the oxidation of NO (known as an NO2-assisted system [47]). Theoperation of the PGM-coated CDPFs depends on the balance between NO2 and sootin the exhaust.
In contrast, soot oxidation catalysts, which are based on metal oxides, are thoughtto act through redox and/or spill-over mechanisms [45]. These oxidize the sootdirectly and,when combinedwith aPGMcatalyst, can cover awide range of operatingconditions in the exhaust, thus broadening the soot oxidation temperature windowand increasing the catalytic reactivity of the CDPF.
However, one challenge that the direct soot oxidation catalysts face is the poor soot-to-catalyst contact. The importance of the contact of soot with the catalyst, in dieselemission control, was recognized almost three decades ago [44] as a barrier for activecatalytic filter development, and it has become popularized inmore recent laboratorystudies of powdered carbon black–catalyst mixtures (the introduction of �loose� and�tight� contact studies [45, 46]). Details of soot–catalyst contact effects on diesel sootoxidation in filters have been reviewed [48].
18.2.3.1 Direct Soot Oxidation CatalystsThe first step in the development of a soot oxidation catalyst is its synthesis, whichcan be conducted via different techniques employing either solid or liquidprecursors (e.g., solid-state reactions, sol–gel precipitation, combustion methods,aerosol methods).
The evaluation of the synthesized catalysts is conducted initially at the laboratoryscale, as mentioned before, by producing powder mixtures of soot with the catalystand employing, for example, a thermogravimetric analysis (TGA) technique. In thismethod, the loss in the weight of the mixture, caused by the oxidation of soot, ismeasured as a function of temperature; this change is interpreted, further, as the sootoxidation rate. The difference between the plain soot powder and the soot–catalystmixtures determines the reactivity of the catalysts (Figure 18.9).
However, as noted above, the performance of the catalysts measured in thesoot–catalyst mixtures on a laboratory scale, provides only a rough idea of thedifferences between the catalysts, which mostly are affected by the specific catalystchemistry.
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Under realistic conditions on the filters, the catalysts are affected also by thefilter�s geometry, as this will determine the degree of contact between the sootand the catalyst. A mathematical description of the incomplete soot–catalystcontact – the so-called �Two-Layer model [49]� – was introduced more than adecade ago, and has since been incorporated into state-of-the-art DPF simula-tors [50–53]. This model considers that the contact of soot with the catalyst isdetermined by the details of catalyst distribution in the filter (a type of �frozen�randomness), and the details of soot particle deposition and resultant depositmicrostructure, as well as soot deposit restructuring (a type of evolving random-ness). This forms the basic tool for the analysis of soot oxidation rates in CDPFs(Figure 18.10), and can be used to describe the soot oxidation on catalytic filtersto within �5% [10, 53].
0.0E+00
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Catalyst B
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soot
Figure 18.9 Effect of different catalyst formulations on the oxidation of soot [21].
Figure 18.10 Application of the two-layer model to experimental data for a catalyzed filter samplewith respect to (a) soot oxidation rate and (b) conversion [53].
Color Fig.: 18.10
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18.2.3.2 Deposition of Catalysts on FiltersIn order for the catalysts to be evaluated under realistic conditions in the exhaust of adiesel engine, they must be deposited on DPFs. Initially, segments of catalyst-coatedDPFs are evaluated on an engine test cell bench, employing side-stream reactortechnology [23, 29, 48]. The CDPF segment with the best catalyst formulation isfurther upgraded into a full-size CDPF, which is placed directly in the exhaust of adiesel engine and evaluated according to a certain methodology [10, 48].
Deposition of the catalyst on theDPFsegments can be conducted via impregnationin a slurry of the catalyst powder, in a catalyst precursor solution and subsequentfiring, or via in situ synthesis and deposition of the catalyst on thefilterwith an aerosolroute [54, 55]. In the latter case the catalyst is deposited on themonolith channelswiththe same mechanism as the soot particles, a feature which is expected to maximizethe contact of the soot particles with the predeposited catalyst sites.
18.2.3.3 Fuel-Borne CatalystsA different approach towards the geometry and soot–catalyst contact problem is theconcept of fuel-borne catalysts, also known as �fuel additives.� These are based on theformation of metal oxides, during combustion of the fuel in the engine, that caneither remove the precursors that may cause the formation of the particulate matteror, alternatively, inhibit the nucleation of such particle precursors [56, 57]. Some fueladditives can also formmetal oxides alongwith or inside the carbonaceous particulatematter, and thereby increase the soot reactivity for oxidation. In this way, they are inclose proximity to the soot particles, so that they achieve an increased contact andtherefore amore effective catalytic performance. One side effect of such a technology,however, is the formation of incombustible metal oxide deposits on the DPFs (ashaccumulation), which causes an irreversible increase of the pressure drop of the filterand affects its durability and lifetime. A second disadvantage is the possible emissionofmetal oxidenanoparticles in the atmosphere, that could pose a significant threat forhealth and the environment.However, no suchphenomenahave been reported in theopen literature to date.
18.2.4Assessment of DPF Technologies
18.2.4.1 Filtration EfficiencyThe filtration efficiency of a filter is the characteristic that determines its ability toremove soot particles from the exhaust stream. It is defined by measuring theconcentration and size distribution of the soot particles before (upstream) and after(downstream) thefilter [58, 59]. Thefiltration efficiency of aDPFmay changewith theaddition of the catalyst, as the latter alters the microstructure of the DPF (e.g.,blocking of the pores of the channel walls).
It has been shown that increasing the amount of catalyst on the filter increases, asanticipated, the initial filtration efficiency of the filter [59] (Figure 18.11). In all cases,the filtration efficiency of the uncoated and coated filters appears to increase as soonas a small amount of soot load accumulates on them (Figure 18.12).
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18.2.4.2 Soot LoadingThe overall pressure drop across a filter, during the accumulation of soot, can beformally decomposed into the sum of the pressure losses that occur due to the gasflow through the clean porous medium, and the additional pressure losses that are
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Figure 18.11 Particle size distribution downstream of the filters for the same soot mass load.
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Figure 18.12 Change in the filtration efficiency of filters with different catalyst amounts for a smallamount of soot mass load.
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induced due to the catalyst and soot deposition. The latter contains contributionsfrom the filter pore blockage during the deep-bed filtration mode, as well ascontributions from the layer of particles on its surface generated during the cakefiltration mode.
Both. the microstructure of the monolith (e.g., porosity, pore size), and the way inwhich catalyst is deposited on the monolith walls, affect the development of thepressure drop during soot accumulation. The conventional way to deposit a catalyston a filter is by using wet chemistry (sol–gel and slurry) techniques; in this way, thecatalyst is deposited mainly inside the walls of the filter, blocking the pores, andsubsequently causing a decrease in the permeability of the filter. This, in turn, willresult in an increase of the pressure drop of the filter, which is enhanced during sootaccumulation.
An example of the effect of different filter microstructure and different direct sootoxidation catalyst deposition techniques (e.g., wet chemistry or aerosol) on thedevelopment of the pressure drop of the filter during soot accumulation is givenin Ref. [60]. Here, it was shown that the way in which the catalyst is deposited on thefilter may even cause a lower pressure drop compared to an uncoated filter (as in thecase of the aerosol-coated filter) (Figure 18.13). The layer of the catalyst operates as a�filter,� onto which the majority of the soot is deposited. Consequently, the pressuredrop of the aerosol-coated filter will arise only from the resistance of the soot cake, thecatalyst layer, the soot inside the catalyst cake, and from a small contribution of thefilter wall. When the filter is not coated, or when the catalyst is not uniformly
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Figure 18.13 Effect of the coating technique on the development of pressure drop during sootloading on standard filters.
Color Fig.: 18.13
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deposited and the catalyst layer is not sufficiently porous that soot can enter inside thelayer, then the total pressure drop will be higher. In this case, the contribution of thepressure drop that has developeddue to soot being deposited inside thefilterwall, willcauses an increased resistance to flow. This can be more easily understood byconsidering the characteristics of cake versus deep-bed filtration, as modeled inRefs [29, 55]. The aforementioned advantageous effect of the aerosol-coated filter wasfurther investigated [60] for the case of an increase in the amount of a direct sootoxidation catalyst on the soot loading behavior. It was observed that, even with aquadruple amount of catalyst on thefilter, the pressure drop remained at a lower levelcompared to the uncoated filter (Figure 18.14).
18.2.4.3 Regeneration
18.2.4.3.1 Direct Soot Oxidation As noted above, the soot–catalyst contact has asignificant effect on the regeneration of thefilter. It has been shown that the increaseddifference between several catalytic formulations in the powder scale is eliminated atthe filter scale [21, 54, 60] (Figure 18.15). Whilst this is slightly affected by thechemistry, the factor that most determines the catalytic efficiency of the filter is itsgeometry. It has also been observed [60] that the coating technique, except from theeffect on the pressure drop during soot loading, can determine the degree of contactbetween soot and the catalytic layer, thus affecting the soot�s oxidation activity(Figure 18.16).
Another issue observed [60] was the presence of complex multisite kineticsderiving from more than one different mode of contact of soot with the catalyst.
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Figure 18.14 Pressure drop development during soot loading of standard filters coated via anaerosol technique at different amounts of catalyst.
Color Fig.: 18.14
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Figure 18.15 Soot oxidation rate on monoliths coated with different catalysts.
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Figure 18.16 Effect of the coating technique on the soot oxidation rate.
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A detail of the soot oxidation rate plotted as an Arrhenius curve shows regions wherechanges in the slope of the curve are obvious (Figure 18.17), and could be associatedwith different modes of contact.
18.2.4.3.2 Combination of Direct Soot Oxidation Catalyst with Pt The combination ofdirect soot oxidation catalysts with Pt would broaden the operating window of theCDPF. By utilizing Pt for the conversion of NO to NO2, the indirect oxidation of sootcould be achieved at temperatures lower than those of direct soot oxidation. Underconditions where the NO concentration is poor in the exhaust, the direct sootoxidation catalyst could, in turn, be utilized to make up for the absence of NOx.Pt is also considered to enhance the oxygen transfer and, therefore, to enhance theperformance of the metal oxide used as a direct soot oxidation catalyst. An inves-tigation of the Pt deposition technique on CDPFs, combined with a direct sootoxidation, has been conducted [54] (Figure 18.18).
18.2.4.3.3 Conversion-Dependent Phenomena Microstructural phenomena thatoccur during catalytic soot oxidation were investigated [53], employing fuel-bornecatalysts (FBCs) deriving from fuels with different concentrations of additives. Thesestudies involved the oxidation of soot under isothermal conditions, with the oxidationrates being obtained as a function of soot conversion. These data allow the study ofconversion-dependent phenomena, which are of some importance when it is
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Figure 18.17 Effect of the presence of different kinds of soot-to-catalyst contact on the behavior ofsoot oxidation on soot loaded monoliths coated with one-fold gm�2 and four-fold gm�2.
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considered that soot is not a uniform entity, but rather exhibits a sub-primary particlenanostructure [14, 18, 19, 61, 62]. The conversion of soot could be visualized as asuccessive motion along a radius of a primary particle, which uncovers its internalparts (possibly with different reactivities). In these experiments it was noted that,depending on the concentration of the FBC and the temperature, different intra-particle phenomena set in (e.g., the uncovering of catalyst sites, catalyst particlemobility inside the soot particle creating channels or pits, similar to the situation foroxide catalyst particles on carbon [63–65]) that determine the profile of the sootoxidation rate (Figures 18.19 and 18.20).
The soot oxidation behavior of catalytic filters coated using different techniqueswas also investigated under isothermal conditions [53], visualizing the geometricrelationship between soot and the catalytic layer through the difference in the profilesof the soot oxidation rate as a function of conversion (Figure 18.21).
Figure 18.18 Comparison of direct soot oxidation-Pt filters coated via different methods withrespect to: (a) NO to NO2 conversion; and (b) indirect soot oxidation.
Color Figs.: 18.18 and 18.19
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Figure 18.19 Soot oxidation rate at 530 �C for all fuel-borne catalyst concentrations.
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18.2.5Simulation Approaches
Diesel particulate filter design, system integration and control, based on a traditionaldesign of experiments approach,may become very time-consuming and costly, due tothe high number of tests required. This provides a privileged window of opportunityfor the application of simulation. Recent reviews of advances in DPF simulationtechnology are available in Refs [42, 66]. While the interested reader should consultthese references (and their cited literature) for more detailed information on theunderlying assumptions regarding the treatment of the various physico-chemicalphenomena (including soot particle transport, deposition, and oxidation), an overviewof the mode of use of different simulation models in practice is available in Ref. [10].
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Figure 18.20 Soot oxidation rate at 620 �C for all fuel-borne catalyst concentrations.
Color Figs.: 18.20 and 18.21
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Advanced Coating (Material A)
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Conventional Coating (Material A)
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Figure 18.21 Comparison of soot oxidation rate at 620 �C, onmaterial A and B, coatedwith the twomethods (conventional and advanced).
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Examples of the Microflow Simulation approach include a computer-recon-structed porous DPF (Figure 18.22) and the flow field and, subsequently, sootdeposition inside the filter porous wall (Figure 18.23) via the reconstructed porousmaterial and employing Lattice Boltzmann-based methods [42, 67, 68].
DPF models coupled with the simulation of other emission control devices are,therefore, the appropriate tools for system optimization and control. An example of acoupled simulation of a DOC and a DPF in series is provided in Ref. [10].
Important issues for the DPF performance at this level are the development oftransversal soot loading non-uniformities, induced by inlet cone flow and temper-ature maldistributions, and/or by heat losses to the environment from the DPFexternal surface. In addition, incomplete regenerations of the DPF must be de-scribed. The continuum multichannel approach [50–52] represents a computationallytractable and accurate tool to address the previously mentioned issues. The devel-opment of highly integrated simulators of multifunctional exhaust emission controlsystems requires the interfacing of multichannel models of DPFs, as well as of otherhoneycomb-type converters, to standard CFD solvers. Recent advances in thisarea have included the rigorous integration into the continuum multichannelframework of segmented filter designs, computationally efficient discretizations of
Figure 18.22 Computer reconstruction of various porous filters.
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non-axisymmetric filter geometries (e.g., oval and trapezoidal shapes) and intelligentcoupling to general-purpose CFD solvers to account for spatial distributions of inletconditions brought about by specific upstream exhaust piping layouts, as well as totemporal variations due to engine operation. An example of a 3-D simulation of aDPFis shown in Figure 18.24.
Color Figs.: 18.23 and 18.24
Figure 18.23 Visualizationof: (a) soot deposition; and (b) velocity through the filter wall, at differentsurface mass loads in an extruded ceramic (granular) filter wall (width of frame is 100mm [68]).
Figure 18.24 Example of temperature field evolution in a 285mm� 305mm in DPF duringregeneration.
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A previously developed 3-D simulation framework for porous materials [68–70]was applied to the case of NO–NO2 turnover in a granular silicon carbide CDPF.The detailed geometry of the CDPF wall was digitally reconstructed, and micro-simulation methods were used to obtain detailed descriptions of the concentrationand transport of the NO and NO2 species in both the reacting environment of thesoot cake, and in the catalyst-coated pores of the CDPF wall. From these results,NO–NO2 turnover was quantified and a comparison made with the turnoverpredicted by an approximate zero-dimensional analytical model. A good agreementwas found between the two models, thereby justifying use of the approximate butfast analytical model in large-scale simulations of CDPFs. Details of the simulationframework, the approximate analytical solution, and the simulation of NO–NO2
turnover in a simplified 2-D wall pore and in a reconstructed filter wall are availablein Ref. [69] (Figure 18.25).
The use of approximate but fast analytical approaches to describe the coupledtransport and reaction phenomena in filter walls [42, 71] opens new possibilities forembedding multifunctionally catalyzed DPFs in large-scale, multidimensional si-mulators at a fraction of the time required for conventional, brute-force computa-tional approaches. In addition, it becomes possible to consider the deployment ofefficient on-board monitoring and control algorithms, implementable in computa-tionally limited engine control units (ECUs).
18.2.6Effect of Ash Accumulation
As noted in Section 18.2.3.3, the use of additives in both the fuel and lubricants, aswell as deterioration of the engine, causes the formation of ash particles that arecollected on theDPF. These particles are incombustible, and as they are accumulatedon the DPF they cause an irreversible increase of the pressure drop.
Color Fig.: 18.25
Figure 18.25 NO2 concentration through the reconstructed CDPF wall with catalyst and at 300 �C.
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18.2.6.1 Rapid Ash Aging MethodThe evaluation of the effects of ash aging on filter performance is a time- and cost-consuming task that has slowed down the process of creating innovative filterstructures and designs. Consequently, a development of a methodology to producefilter samples that have been aged by accumulating ash, produced by the controlledpyrolysis of oil–fuelmixtures, would accelerate the evaluation offilterswith respect toash aging [72]. The �artificial� ash particles obtained in this way can be compared tothose from �real� engine operation [72], and they bear both morphological (size) andcompositional similarities to ash particles collected fromengine-agedDPFs. The ash-loaded samples are then evaluated with regards to their soot-loading behavior,filtration efficiency, and regeneration performance. It was observed that, for theuncoated filters, an ash mass load up to a certain level led to a decreased overallpressure dropduring soot loading,whereas in the case of coatedfilters the ash depositled to either an increased or decreased overall pressure drop, depending on the type ofcatalytic coating used (Figure 18.26). Ash particle deposition may also influence thefilter regeneration performance (Figure 18.27). Finally, the ash particle accumulationcaused substantial improvement in the filtration efficiency of the tested samples.
18.2.6.2 Ash Aging SimulationTo predict the behavior of the ash-aged filter with respect to pressure drop, the DPFpressure drop model has been extended to account for the presence of ashes in thechannels of the DPF [10, 27, 42]. Ash deposit growth dynamics was described with amechanistic model that exhibits different ash-deposition profiles, namely depositionalong the filter channel walls, and deposition at the end of the filter channel. The
Catalyst P
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Figure 18.26 Effect of the ash loading on a coated (designated as Catalyst P based on Pt-groupmetals) filter sample [72].
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Figure 18.27 Effect of ash particles on the soot conversion rate for uncoated and coated filters [72].
results showed good quantitative agreement with experimental data, and the modelcould be used to describe the dynamic ash transport and deposition phenomenainside the DPF [42].By considering two idealized modes of ash accumulation, namely: (i) ash only on
the wall (forming a layer in series with the porous wall and the soot cake); and (ii) ashonly at the end of the DPF channel (forming a plug that reduces the DPF length), itwas possible to derive [73] analytic approximations for the optimum cell density ofwall-flow filters that minimize the DPF pressure drop, under the combined con-straints of a prescribed filter volume, exhaust flow, temperature, as well as differentsoot and ash loadings inside the filter, thus facilitating the task of selection and thereliable deployment of DPFs over the vehicle life-cycle.
18.3Catalytic Technology for Gaseous Pollution Control
18.3.1Reduction of Gaseous Emissions from Mobile Sources
18.3.1.1 The Three-Way Catalyst (TWC)Themost frequent type of catalytic converter in automobiles is the so-called TWC forstoichiometric-operated gasoline engines, with a yearly worldwide production of over70millionunits. TWCsystemshave been applied to gasoline engines since the 1980s,and contain either Pt/Rh or Pd/Rh in themass ratio of approximately 5 : 1, with a totalloading of preciousmetals of about 1.7 g l�1. The TWCs simultaneously convert NOx,CO, andHC into N2, CO2, andH2O [4–6, 24]. The catalytic components are supported
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Figure 18.28 Scheme of a monolith-based automotive catalytic converter.
Color Fig.: 18.28
by a cordierite honeycomb monolith coated with high-surface-area alumina (seeFigure 18.28). This washcoat layer additionally contains thermal stabilizers (e.g.,La2O3), as well as the oxygen-storage component, ceria (CeO2). The ceria is able torelease oxygen under rich conditions, thus maintaining HC and CO abatement andavoiding the emission of H2S. The TWC process occurs exclusively within a narrowrangeofO2 content that is close to stoichiometric combustion conditions; that is,whenthe air coefficient l ranges from0.99 to 1.01 (Figures 18.29 and 18.30). To realize theseconditions, an oxygen sensor is usedwhichmeasures the air coefficient of the exhauststream, forcing the engine management system to regulate the air–fuel ratio.The TWC process involves a complex network of numerous elementary reac-
tions [46, 47], where the effectiveness of the catalyst is closely related to the specificactivity of the precious metals, as well as to their surface coverage. The transfer ofTWC technology to lean-burn engines – that is, lean-operated gasoline and dieselmotors – is problematic because of the insufficientNOx abatement. This is associatedwith the lower raw emissions of reducing agents, as well as the high content of O2,which enhances the oxidation of HCs and CO thus suppresses NOx reduction.Therefore, alternative concepts are required for the reduction of NOx under lean-burn conditions. For this purpose, SCR by NH3 and NSCs are considered in theautomotive industry, as discussed below.The catalytic CO oxidation is an essential reaction of TWCs and NSCs, and has
also been applied in diesel engines since the 1990s, using so-called DOC. Fur-thermore, the catalytic abatement of CO is also a state-of-the-art technology for gasturbine engines fed by natural gas. DOCs usually contain Pt as the activecomponent and reveal outstanding performance, although the highly expensive
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platinum can be substituted by the less active, but more inexpensive, palladium.The precious metal load of a DOC is approximately 3 g l�1. DOCs are also capable ofoxidizing gaseous HCs as well as HCs adsorbed onto soot particles (as discussedabove).
Figure 18.30 Dependenceof the post-TWCcatalyst emissionson the air value in gasoline-operatedautomobiles.
Figure 18.29 Typical engine raw emission of a gasoline operated engine.
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Over the past decade, a variety of computational tools has been developed tosimulate the transient behavior of the TWC under operating conditions [74–76],using sophisticatedmodels for chemistry andmass andheat transport. Thesemodelstoday allow for the simulation of driving cycles [77, 78] taking spatially nonuniforminlet conditions into account [79].
18.3.1.2 Selective Catalytic Reduction of NOx in Mobile Applications
18.3.1.2.1 Introduction Although, the SCR of NOx by NH3 has already been used innonmobile applications for over twenty years, it has also been introduced as thetechnology of choice for NOx removal in lean-burn engines, and in particular dieselengines. Indeed, the SCR process covers the relevant temperature range of dieselengines to provide effective NOx abatement and has, in the past few years, alsoadvanced to a state-of-the-art technology for heavy-duty vehicles. Unfortunately, inmobile applications the storage of NH3 is problematic, and consequently an aqueoussolution of urea (32.5 wt%) – referred to as �AdBlue� – is currently used. In this case,the urea solution is sprayed into the exhaust tailpipe, where ammonia is producedafter thermolysis andhydrolysis of the vaporizing urea–water droplets (Figure 18.31).Much of the current research is focused on optimizing the dosing system, and thedevelopment of vanadia-free catalysts [80], although the use of alternative reducingagents, such as HCs and hydrogen, has also been discussed.
18.3.1.2.2 Pre-Catalyst Processes When the urea– water solution (UWS) is sprayedinto the hot exhaust stream [81], the subsequent generation of NH3 proceeds in threesteps [82, 83]:
1) Evaporation of water from a fine spray of UWS droplets:
ðNH2Þ2COðaqÞ! ðNH2Þ2COðs or 1Þþ 6:9H2OðgÞ;2) Thermolysis of urea into ammonia and isocyanic acid:
ðNH2Þ2COðs or lÞ! ðNH3ÞðgÞþHNCOðgÞ
Figure 18.31 SCR-Denoxtronic-systems at Robert Bosch GmbH, using an urea solution asammonia source. Illustration courtesy of Robert Bosch GmbH.
Color Fig.: 18.31
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3) Hydrolysis of isocyanic acid:
HNCOðgÞþH2OðgÞ!NH3ðgÞþCO2ðgÞ:
As the evaporation and spatial distribution of the reducing agent upstream of thecatalyst are crucial factors for the conversion of NOx, the dosing systemmust ensurethe correct preparation of the reducing agent, under all operating conditions. Anoverview of the different exhaust gas and spray characteristics that occur in passengercars and trucks is provided in Table 18.3. Appropriate spray properties of the ureasolution will also avoid the deposition of urea on walls, which could lead to theformation of melamine complexes [84].
The chain of effects, from the injection point to catalyst entrance, which includesthe injection of theUWS, the evaporation and thermolysis and hydrolysis of theUWSdroplets, the impingements of droplets on the tailpipe wall, and the potentialformation of films, is shown schematically in Figure 18.32. The filmsmay evaporate,
Table 18.3 Exhaust gas properties and spray parameters for urea dosing system using UWS inautomotive applications [85].
Location Value
ExhaustExhaust velocity (m s�1) 5–100Exhaust temperature (K) 400–1000Wall temperature (K) 350–900
SpraySauter mean diameter (mm) 20–150Injection velocity (m s�1) 5–25Injection temperature (K) 300–350
Figure 18.32 Chain of effects from the injection point to catalyst entrance.
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but may also lead to the solid depositions mentioned above. Meanwhile, CFDsimulations can be used to comprehend the interaction of spray injection and theturbulent flow of the exhaust gas in the tailpipe. The numerically predicted thicknessof thewallfilm, and its composition, is shown in Figure 18.33. As urea andwater havedifferent boiling points, the urea concentration in the droplets will vary with time. Asthe evaporation time and, consequently, the droplet composition also depends on theinitial droplet size and the interaction with the hot exhaust gas stream, the impingingdroplets will vary not only in size but also in urea concentration. Meanwhile, CFD
Figure 18.33 CFD simulation of interactions of flow, spray, and wall describes wall film formation;wall film thickness (a) and urea fraction in film (b) [85].
Color Fig.: 18.33
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simulations have also been used for the rapid prototyping of SCR-systems. Inparticular, the position and angle of the injection nozzle, and the position of thecatalyst in relation to the tailpipe, must be selected so as to produce an optimumdistribution of the reducing agent at the front face of the catalyst.
18.3.1.2.3 Catalytic Conversion of NOx by NH3 [86] As the ammonia is formed, itbecomes available as a reducing agent for NOx, mainly NO andNO2, which is carriedout in a structured catalyst integrated into the tailpipe; this is very similar to the TWCapproach discussed above. At higher temperatures (>450 �C), however, the ammoniareacts with oxygen in an undesirable parallel reaction to produce N2, N2O, or NO. Incontrast, at temperatures below 200 �C, ammonia and NOx may form solid depositsof ammonium nitrate and nitrite.
NOx can be reduced continuously by NH3 on a SCR catalyst, resulting in theselective formation of nitrogen and water. At this point, it should be mentioned thatthe SCR procedure is the only technique converting NOx selectively into N2, evenunder strongly oxidizing conditions. Hence, SCR was considered the technology ofchoice when deNOx became an issue for lean-burn engines. In fact, the SCRprocess covers the relevant temperature range of diesel engines, providing effectiveNOx abatement, to a point where during the past few years it has advanced tobecome state-of-the-art deNOx technology for heavy-duty vehicles. Whilst theoperational range of the SCR procedure is limited at low temperatures (<200 �C)by the kinetics of the catalyst, above approximately 450 �C it is slightly restricted bythe oxidation of NH3. The stoichiometry of the main desired reactions can bedescribed as follows [87–92]:
4 NOþ 4 NH3 þO2 ! 4 N2 þ 6H2O ðstandard SCRÞ
6 NO2 þ 8 NH3 ! 7 N2 þ 12 H2O ðNO2 SCRÞ
NOþNO2 þ 2 NH3 ! 2 N2 þ 3 H2O ðfast SCRÞThe standard SCR reaction is most important if NOx originates from high-
temperature combustion processes, where very little NO2 is present. However, inexhaust streams containing larger amounts of NOx, the fast SCR reaction proceedingwith at least a 10-fold higher rate than the standard SCR reaction may become thepredominant reaction [93]. DOC systems integrated into the exhaust-gas after-treatment systems (see Figure 18.31) convert NO to NO2, thus changing theNO2/NOx ratio at the entrance of the SCR catalyst drastically and leading to majorincrease in SCR, particularly below 250 �C. However, when designing this pre-catalyst it must be remembered that molar ratios of NO2/NOx> 0.5 can lead toNH4NO3 deposits, thus deactivating the SCR catalyst.
As the hydrolysis of urea may not be fully completed at the entrance of the SCRcatalyst, the front section of the SCR structure is composed of a urea hydrolysiscatalyst, such as alumina or titania. The subsequent part, which forms the �real�SCR catalyst, is commonly extended by an NH3 oxidation catalyst to avoid any slipof NH3.
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The most common SCR catalyst used today is a TiO2-supported WO3/V2O5; thisis normally used in the form of an homogeneous monolith, although in a fewapplications charcoal catalysts may also be used. The mechanism of the SCRreaction on a V2O5 catalyst was elucidated both experimentally [94–97] and byusing quantum-mechanical calculations [98]. The reaction follows an Eley–Rideal-type mechanism, where two different active sites are involved (Figure 18.34) thatare in such close proximity that they represent a Brønsted acid site (V5þ�OH) anda (V5þ¼O) redox site. In the first step, NH3 is adsorbed onto the Brønsted site toproduce NH4
þ ; this subsequently interacts with the neighboring redox site,leading to a reduction of the latter. The gaseous or weakly adsorbed NO thenreacts with the activated N species to form N2 and H2O. In the final stage, the(V4þ�OH) group is reoxidized into (V5þ¼O), again resulting in the production ofH2O.
18.3.1.2.4 Alternate Catalysts [99] Currently, amajor trend in automotive SCR is thesubstitution of V2O5 catalysts by harmless materials. In the meantime, catalyticconverters based on V2O5 have been prohibited in Japan and California, on the basisof the toxicity of the active component; similar discussions regarding this point arecurrently ongoing in the European Union. An additional problem is that V2O5
demonstrates a limitedhigh-temperature stability, whichmay cause difficultieswhencoupling SCR with particulate filter systems in passenger cars. At present, Fe-ZSM5zeolite is considered to be the most-favored type of catalyst as an efficient substitutefor the classical V2O5 patterns [80, 100–102].
18.3.1.2.5 Alternative Reducing Agents Alternative reducing agents have been in-vestigated extensively over the past two decades,mainly hydrocarbons (HC-SCR) andhydrogen (H2-SCR).
In the case ofHCs, additional fuelmay be injected in the raw exhaust or the exhaustline upstream of the SCR catalyst. Unfortunately, difficulties in achieving a highconversion and selectivity over the catalysts [103], the catalyst stability at hightemperature, and problems in meeting HC limits have so far prevented HC-SCRfrom becoming widespread in terms of its applications.
Figure 18.34 Mechanism of the SCR reaction on V2O5/TiO2 catalysts [98].
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H2-SCR, which is currently still undergoing fundamental development, is ofparticular interest for low-temperature exhaust gases [104]. The conversion ofNOx byH2 already operates at stoichiometric (TWC) and rich (NSC in regeneration phase)conditions. However, low-temperature H2-SCR has also been reported for stronglyoxidizing conditions using Pt catalysts. Although a very good performance isobtained below 100 �C, the narrow range in activity and the high selectivity towardsN2O remain challenging issues. The mechanism of the NO reaction by H2 on Pt/Al2O3, under lean conditions, implies the dissociative adsorption of NO on reducedPt sites [105]. The recombination of two N atoms leads to the evolution of N2, whilethe oxygen is retained on the Pt surface. The production of N2O is explained by thecombination of a surfaceN atomwithNOadsorbed onto neighboringPt sites. Finally,the effect of the hydrogen is to regenerate the active Pt sites. Apotential source for theonboard production of H2 is the processing of fuel by catalytic partial oxidation orsteam reforming [106–108]. Additionally, the temporary generation of H2might alsooccur by engine management; that is, after the injection of fuel.
18.3.1.3 NOx Storage CatalystsNOx storage reduction catalysts were originally developed for lean SI engines, and arecurrently being transferred for use in diesel-powered passenger cars. The NSCprocedure is based upon the periodic adsorption and reduction of NOx [4, 109], theprinciple of which is illustrated in Figure 18.35.
The catalysts consist of Pt, Pd, and Rh in the mass ratio of approximately 10 : 5 : 1,with a total precious metal load of approximately 4 g l�1. The NSC contains basicadsorbents such as Al2O3 (160 g l
�1), CeO2 (98 g l�1) and BaCO3 (29 g l
�1, denoted as
Figure 18.35 Reduction of NOx emissions of lean operated engines. Principles of a NOx storage/reduction catalyst (NSC).
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BaO equivalent) [110]. In the lean phase of the engine (general operation mode), theNOx of the exhaust is adsorbed onto the basic components of the NSC,mainly on thebarium carbonate, to form a nitrate.When the storage capacity is reached, the engineis operated at rich conditions (l� 0.9) for a few seconds, and this leads to an exhaustcontaining CO, HCs, and H2 as reducing agents for catalyst regeneration (back-transformation of the nitrate to the carbonate):
Storage (lean) phase:
. NO oxidation over noble metal:
NOþ 12O2 !NO2 j
. NOx storage on Ba sites:
BaCO3 þ 2NO2 þ 12O2>BaðNO3Þ2 þCO2
. Regeneration (rich) phase:
BaðNO3Þ2 þCO=H2=HC>BaCO3 þ 2NOþCO2=H2O
Obviously, this global scheme is an oversimplification, and many researchinvestigations have been devoted to elucidating the intrinsic kinetics [111–116].
The effect of the Ba component is to adsorb NOx at temperatures above 250 �C,whereas substantial storage is also provided by Al2O3 and CeO2 at lower tempera-tures [117]. However, below 250 �C the effectiveness ofNSC catalysts is limited by thekinetics of the NO2 production on the Pt component, while above 400 �C the thermalstability of the NOx surface species represents the limiting factor.
For the NO2 adsorption on the Ba sites, two parallel pathways of nitrate formationare suggested [118]. The first route involves the adsorption of NO on Ba to formnitrites, which are subsequently oxidized by gas phase O2 into nitrates. The secondroute includes the catalytic NO oxidation on Pt into NO2, followed by its immediateadsorption in the form of nitrates. It has also been suggested that barium peroxidespecies might serve as the crucial sites for nitrate formation [119].
As compared to SCR, the most important advantage of the NSC technique is thefact that no additional liquid tank (Urea-SCR), additional injection management/system (HC-SCR), or chemical reactor (H2-SCR) is needed. However, a substantialconstraint is the susceptibility to sulfur poisoning. In parallel to NO, SO2 is alsooxidized on Pt, followed by the adsorption of SO3 onto the basic substrate. The sulfatespecies produced lead to a drastic deactivation of the NOx storage sites. Although, thepoisoned sites of ceria and alumina can be regenerated thermally above 300 �C, thereleased SOx is readsorbed selectively onto the Ba species. The regeneration ofpoisoned Ba sites is inadequate, even under rich conditions and high temperatures,as the sulfate groups are partially converted into highly stable BaS [110].
In recent years, several groups have coupled detailed kinetic models and CFD toprovide a numerical simulation of NOx storage catalysts under varying conditionsand using different modeling approaches [116, 120–124]. These approaches have
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differed especially in the applied reaction mechanisms, and the treatment of masstransfer in the single channels and the washcoat. Furthermore, most studies haveconsidered NO oxidation only in the lean phase, and its reduction in the rich phase.The applied gas matrix is sometimes far from a realistic exhaust gas, because itcontains neither water nor CO2, both of which have amajor influence on the catalyticactivity of the noble metal and the morphology of barium. As the storage of nitrogenoxides is a slowprocess compared to the subsequent reduction, the observed behaviorwas explained by the different molar volumes of BaCO3 and Ba(NO3)2, the so-called�shrinking core� model [116, 120].
State-of-the art simulations have included detailed reaction mechanisms, radialmass transfer limitations in the catalytic channels and the washcoat, and have beencapable of describing the transient behavior of theNSCat varying inlet conditions. Asan example, Figure 18.36 shows the numerical predicted and experimentally mea-sured NO and NO2 profiles in a laboratory flat-bed reactor, using a commerciallymanufacturedPt/Ba/Al2O3modelNSCas function of axial position and storage time.The effect of NO2 formation in the first section of the catalyst, and its subsequentstorage, is clearly captured. The model applied a detailed reaction scheme for theprocesses on the noble metal Pt [125], and a lumped scheme coupled with theshrinking core model for the storage component Ba.
18.3.2Reduction of Gaseous Emissions From Stationary Sources
18.3.2.1 Catalytic Technologies for NOx Removal [1]Nitrogen oxides arise from the oxidation of nitrogen-containing compounds of thefuel (fuel NOx), the oxidation of atmospheric nitrogen from combustion with air(thermal NOx), and by the oxidation of intermediate combustion species (promptNOx). Often, a combination of combustionmodifications and catalytic gas cleaning isused, for example, low-NOx burners, and SCR. In addition, a selective noncatalyticreduction (SNCR) step can be applied by injecting ammonia into the furnace.Primary NOx formation can substantially improved by oxy-fuel combustion – thatis, combustion with pure oxygen or enriched air.
Interestingly, the potent greenhouse gas nitrous oxide (N2O) cannot be removed bythe normal SCRprocess. This emerges as a particular in nitric acid production plants,where the threat of environmental harm is inevitable, unless innovative end-of-pipeN2O removal technologies can be developed to effect the reduction of the N2Oproduced as waste. The current approach to this problem has focused on the use oftransition-metal, ion-exchanged zeolites for the decomposition of N2O.
SCR with ammonia is by far themost relevant technology for the catalytic removalofNOx from stationary sources, andhas been implemented since the 1980, notably todealwithNOxproducednot only by power plants but also by industrial boilers and gasturbines. It should be noted that SCRwith ammonia would utilize a similar approachfor both mobile and stationary applications, except that the size of the structuredcatalyst would be much larger in the latter case.
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The selective catalytic reduction of NOx was first carried out using Pt catalystsalthough, due to the high N2O selectivity of this catalyst, base metal catalysts havesubsequently been developed forNOx reduction. Vanadia supported on titania (in theanatase form) and promoted with tungsten or molybdenum oxide exhibits the bestcatalytic properties. Although BASF were the first to describe vanadia as an activecomponent for SCR, a TiO2-supported vanadia for the treatment of exhaust gaseswas
Figure 18.36 Axial profile of NO (a) and NO2 (b) concentration at varying storage time in the leanphase of Pt/Ba/Al2O3 catalyst at 350 �C [126, 127].
Color Fig.: 18.36
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also developed in Japan. Anatase is the preferred support for SCR catalysts for twomain reasons. First, it is only moderately sulfated under real exhaust gas conditions;in fact, its catalytic activity even increases after sulfation [128]. Second, vanadia can bespread in thin layers on the anatase support, which in turn leads to highly activestructures with large surface areas. Unfortunately, the amount of vanadia in atechnical catalyst is limited to only a few weight%, because it is also catalyticallyactive for SO2 oxidation.
The mechanism of the standard SCR reaction over vanadia-based catalysts isgenerally assumed to proceed via an Eley–Rideal mechanism that involves adsorbedammonia and gas-phase NO (as described above). As the rate of the SCR reactionunder industrially relevant conditions is quite high, external and intraparticlediffusion resistances play an important role, especially for the frequently usedhoneycomb monolith or plate-type catalyst geometry operating in a laminar flowregime. These geometries must be used to minimize the pressure drop over thecatalyst bed. Monolithic elements usually have channel sized of 3–7mm, a cross-section of 15� 15 cm, and lengths of 70–100 cm. Monoliths or packages of platecatalysts are assembled into standard modules which are then placed in the SCRreactors as layers. Notably, thesemodules can be easily replaced to introduce fresh orregenerated catalysts.
SCR reactors can be used in different configurations, depending on the fuel type,the flue gas composition, the NOx threshold, and other factors. The first possibility isa location directly after the boiler (a �high-dust� arrangement), where the flue gasusually has the optimal temperature for the catalytic reaction. However, dustdeposition and erosion, as well as catalyst deactivation, will be more pronouncedthan in other configurations. A second option, which is common in Japan, is to placethe SCR reactor after a high-temperature electrostatic precipitator for dust removal(�low-dust� arrangement). In this case, although damage of the catalyst by dust can beprevented, the deposition of ammonium sulfate (which in the high-dust configura-tion mainly occurs on the PM in the gas stream) may become more critical. It is forthis reason that especially low limits for ammonia slip must to be met. Finally, theSCR reactor may be located in the cold part after the flue gas desulfurization unit, inthe so-called �tail-end� arrangement. In this case, in order to achieve the requiredreaction temperature the exhaust gases must be reheated by means of a regenerativeheat exchanger and an additional burner. A major benefit here, however, is thatcatalysts with very high activities can be used, as no poisons will be present and SO2
oxidation need not be considered.New promising catalysts for the removal of NOx include iron-exchanged
zeolites, such as MFI and BEA. Although field tests in the flue gases of powerplants have shown a quite strong deactivation, notably by mercury [129], thesecatalysts appear to be especially suited for �clean� exhaust gases, such as in nitricacid plants. The main advantages of iron zeolite catalysts include a broadertemperature window for operation, and the ability also to reduce N2O emissions.Uhde GmbH has recently developed the EnviNOx� process for the simultaneousreduction of NOx and N2O, which uses iron zeolite catalysts provided by S€ud-Chemie AG.
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18.3.2.2 Technologies for Removal of Other EmissionsCatalytic technologies may play a minor role in the removal of other gaseousemissions from stationary sources; however, for the sake of brevity, the readershould consult comprehensive reviews produced by Gabrielsson and Pedersen [1]and by Spivey [2].
Catalytic combustion may be applied to remove VOCs [1], and also to reduce theformation of gaseous pollutants, in particular NOx [130, 131]. Further details on thesetopics are available in the reviews of Forzatti et al. [130] and Hayes andKolaczkowski [131].
18.4Outlook
The currents trends in the implementation of exhaust-gas after-treatment systems forcars and trucks are determined by an increasing complexity. The system oftenincludes several of the components described above. As an example, Figure 18.37illustrates the recently developed system that includes DOC, NSC, DPF, and SCRtechnology. Whilst the management of this �chemical plant� underneath a carrepresents a major challenge, the even lower legislative emission limits plannedfor the future may require even greater complexity of designs and operationstrategies.
Yet, on the other hand, integrated systems are becoming increasingly attractive,with Kolios et al. having recently proposed a heat-integrated reactor concept forcatalytic reforming and automotive exhaust purification [132].
Figure 18.37 Exhaust-gas treatment of the E320 BLUETEC. Illustration courtesy of Daimler AG.
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Clearly, increasing efforts will have to be made to substitute the expensive noblemetal catalysts, or at least reduce the amount needed. These include:
. An optimization of the technical device (geometry, flow conditions, washcoatstructure, zone coating) to reduce any mass transfer limitation of the overallreaction rate.
. A maximization of the dispersion of the catalyst particles.
. Finding alternative noble metal-free catalysts; in particular, some promisingresults have recently been reported with the use of nanostructured catalysts.
Acknowledgments
The studies of A.G. Konstandopoulos (A.G.K.) in the Diesel Emissions area havebeen supported in part by the European Commission framework programs throughthe industrial collaborative projects DIDTREAT, CERFIL, MULTISENS, ART-DEXA,PSICO-DEXA, SYLOC-DEXA, STYFF-DEXA, FLOWGRID, IMITEC, COMET,MAAPHRI, IPSY, PAGODE, TOP-EXPERT, ATLANTIS, the Hellenic General Sec-retariat for Research and Technology through collaborative projects EPET-II, PAVE,EPAN, and from a number of automotive industries and their suppliers, includingHonda, Centro Ricerche Fiat, Ibiden, The Dow Chemical Company, AMR, andJohnson Matthey. A.G.K. is very grateful to these sponsors, and also to colleagues atthe APT Laboratory for their hard work and support. Particular thanks go to MsSouzana Lorentzou, for her valuable assistance in putting together the presentchapter.
O. Deutschmann would like to thank W. Boll, D. Chatterjee, G. Eigenberger, R.J.Kee, J. Koop, S. Kureti, N.Mladenov, S. Tischer, V. Schmeisser, andM. Votsmeier forsupport of his research, and many fruitful discussions and collaborations. Thanksalso to Y. Dedecek for her help with editing the manuscript. O.D. gratefully acknowl-edges the financial support of the German Research Foundation (DFG), and ofmanyautomotive industries and suppliers.
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Abstract
In this chapter, stationary and automotive technologies for gaseous and particulatepollution abatement are discussed. Stationary pollution abatement technologiesinvolve the reduction of NOx emissions from the exhaust gases of power plants,industrial boilers, and gas turbines, and are based on selective catalytic reduction(SCR) using ammonia. In automotive pollution control, internal combustion enginesrepresent the major source for the emission of NOx, CO, and unburned hydro-carbons (HCs), whereas diesel engines contribute to the emission of soot and NOx.These gaseous pollutants can be minimized with primary measures, for example,modification of the combustion process, and with additional secondary measures(exhaust purification by catalysts). The most frequent type of catalytic converter inautomobiles is the three-way catalyst (TWC) in stoichiometric operated gasolineengines, which simultaneously converts NOx, CO, and HCs. NOx emissions fromlean-operated engines can be removed by urea-SCR and NOx storage reductioncatalysts (NSC). Catalytic CO oxidation is an essential reaction of the TWC and NSC,and is also applied in diesel engines using the diesel oxidation catalyst (DOC) that canoxidize gaseous HCs as well as HCs adsorbed onto soot particles. Diesel particulatefilters (DPFs) are used to remove soot from diesel exhaust by filtration of the particlesthrough the porouswalls of thefilter. TheDPFapplication requires regeneration; thatis, oxidation of the stored soot particles, which is accomplished by indirect (NO2-assisted) and direct (redox metal oxide-based) catalytic oxidation of the accumulatedsoot particles in the DPF.
Keywords: Soot; pollution control; catalysis; NOx; emissions; particle filters;three-way catalysts; selective catalytic reduction.
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Author QueryNote: As the volumes have been renumbered because of canceledchapters “please check all numbering very carefully with regard to can-celed chapters and renumbering.”
1. please insert the title of the thesis.
2. please provide thesis title.
3. please give the technical paper number.
4. please give the technical paper number.
5. refs 70 and 71 have the same SAE numbers - please check.
6. please give the technical paper number.
7. Ref 108 - Please update the year of publication, volume number andpage range.
8. Please check for correctness of missing date in Ref. 126.
9. Please check for correctness ofmissing footnotes (Table 18.1 a and b).