Cellular Ceramics / 5

416

5.2

Gas (Particulate) Filtration

Debora Fino and Guido Saracco

5.2.1

Introduction

Filtration through porous media can take place according to two main mechanisms: superficial or “cake” filtration and interstitial or “deep” filtration. In superficial filtration, a dust cake is formed over the porous filter during the process, eventually becoming the real filter media. Quite high efficiency of particle separation can be reached by the latter means, often exceeding 95 %. Conversely, deep filters lock dust inside their porous matrix by separation mechanisms such as inertial impaction, interception, and Brownian diffusion [1]. A lower filtration efficiency is generally reached (50–60 %) at equal pressure drop.

In both cases, cellular ceramics have met significant interest in the R&D community in the last decade for gas filtration applications. Figure 1 shows two examples of how cellular ceramics can serve as superficial or deep filtration devices.

Wall-flow monoliths (Fig. 1, left) having a honeycomb structure with adjacent channels blocked at opposite ends are typical surface filters: the filtered particles accumulate over the walls of the channels through which the dirty gas stream is fed. Foam filters (Fig. 1, right) trap particles inside their cellular structure (typically over their struts), as their pore size is generally much larger (> 200 mm) as opposed to their wall-flow counterparts (ca. 10 mm) [3].

A characteristic of wall-flow filters is their need for periodic cleaning or “regeneration”, since the accumulation of trapped particulate causes an unacceptable increase in backpressure [4]. Conversely, foam filters are often referred to as “nonblockable” traps, since particulate blow off takes place after a certain mass holdup is reached (dependent on the dust and the nature and geometry of the filter), and thus prevents the pressure drop from rising beyond a given value. However, this is generally not acceptable owing to the very low filtration efficiency it entails [5].

This chapter address the application opportunities of the above-mentioned media in the field of gas filtration. Special attention is paid to their coupling with catalysts [6–9] to constitute multifunctional reactors [10] (i.e., catalytic filters) capable of carrying out, in addition to filtration, a catalytic reaction for the abatement of gaseous pollutants or particulate. Ceramic foams, used commercially for the filtration of molten metals (Chapter 5.1), are attracting increasing attention as catalyst supports

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

5.2 Gas (Particulate) Filtration 417

Fig. 1 Particulate traps for hot gas filtration. Left: wall-flow honeycomb monolith (superficial filtration); Right: ceramic foam trap (deep filtration). (Reprinted from [2], Copyright 2000, with permission from Elsevier Science Ltd.)

due to their high thermal stability, high porosity, and increased tortuosity relative to honeycombs. This application requires the development of complicated catalyst deposition routes inside the porous matrix [11].

5.2.2

Properties of (Catalytic) Cellular Filters

The potential techno-economical advantages of catalytic filters are those typical of multifunctional reactors: 1) Fewer process units (substitution of at least two process stages with a single reactor in which all operations of interest are carried out simultaneously); 2) Cost reduction (a likely, though not inevitable consequence of point 1) should be a decrease in investment costs); 3) Energy saving (catalytic filters may allow a more efficient management of energy and heat recovery in the tail-end treatment of flue gases from large boilers [12]); 4) Space saving (perhaps the most obvious advantage of coupling two operations into a single unit).

However, to take full advantage of these opportunities catalytic filters should have: 1) Good thermochemical and thermomechanical stability; 2) High particulateseparation efficiency: particulate should not markedly penetrate the filter structure since this would lead to pore obstruction and/or to catalyst deactivation; 3) High catalytic activity to attain nearly complete catalytic abatement at superficial velocities that are as high as possible (e.g., those employed industrially for dust filtration are 10–80 m3 m–2 h–1 (standard temperature and pressure, STP) or even higher); 4) Low pressure drop (a certain increase in head compared with the virgin filters must be expected owing to the presence of the catalyst); 5) Sufficiently low cost to compete with conventional, well-established technologies. Requirement 3), in particular, should not be underestimated. For any degree of catalytic activity and filter thickness, a superficial velocity will indeed exist for which nearly complete catalytic abatement of pollutants can be achieved along with particulate filtration. However, it is

418 Part 5 Applications

mandatory to render this combination feasible at the superficial velocities typically adopted in industrial filtration. Otherwise, if the superficial velocity must be kept low to guarantee a sufficient residence time in the catalytic filter matrix for reaction purposes, the number of filter units would have to be larger than required for simple filtration when a given flow rate is to be treated. The investment costs will then increase and thus hamper the economic potential of the catalytic filters.

This chapter discusses how, and to what extent, the above properties can be exploited and the limitations overcome. Several flue gases (e.g., from coal-fired boilers, incinerators, diesel engines) are characterized by high loads of both particulate (e.g., fly ashes, soot) and gaseous pollutants (NOx, SO2, CO, volatile organic compounds, etc.), which must be removed for environmental protection purposes. In this context, several possible applications of catalytic filters based on cellular materials can be envisaged, some of which have already been successfully tested not only on the laboratory scale but also on the industrial scale.

5.2.3

Applications

5.2.3.1

Diesel Particulate Abatement

The high efficiency of diesel engines, their low operating costs, high durability, and reliability have provided them with a leading role in the heavy-duty vehicle market. This wide occurrence entails careful evaluation of the related environmental effects. Among the emitted pollutants, diesel particulate raises serious health concerns due to its carcinogenity [13], owing to the presence of polyaromatic hydrocarbons (PAH) and nitro-PAH in its so called soluble organic fraction (SOF), as well as to its size falling in the lung-damaging range (10–200 nm). In the field of particulate emission control, attention has mainly been paid so far to improvements in engine design [14], modification of fuel formulation, and use of alternative nonfossil fuels such as natural gas, alcohols, or esters [15], as well as the use of filtering and nonfiltering aftertreatment devices. The EURO IV regulations imposing particulate emissions lower than 0.025 g km–1 for passenger cars will force car manufacturers to adopt new solutions from 2005 onwards. These innovative aftertreatment systems will likely be based on catalytic filters.

Some commercial systems using wall-flow filters have already been launched. The PSA (Peugeot-Citr.en Societ d’Automobiles) system (Fig. 2) is based on the following key components [16]:

This system is currently running on more than 500 000 cars with no apparent problems. However, it has some drawbacks [16]:

Fig. 2 Sketch of the PSA diesel particulate removal system. (Reprinted from [43], Copyright 2003, with permission from Elsevier Science Ltd.)

420 Part 5 Applications

Fig. 3 Pressure drop and optimization of regeneration period for the PSA system (courtesy of Peugeot Citroen Societes des Automobiles, La Garenne Culombe, France).

Another patented and commercially available system is the continuously regenerating trap (CRT) system by Johnson Matthey. It exploits the pronounced oxidative activity of NO2 towards carbonaceous particulate [19] and consists of a wall-flow trap with an upstream flow-through diesel oxidation catalyst, known as a preoxidizer (Fig. 4).

The preoxidizer converts about 90 % of the hydocarbons (HC) and CO present in the exhaust gas and promotes the reduction of at least 3 % of the nitrogen oxides. The most interesting feature of the CRT system, however, is its ability to promote continuous trap regeneration provided its operating temperature is kept in the range

Fig. 4 The Johnson Matthey CRT system (adapted from

www.jmcsd.com).

5.2 Gas (Particulate) Filtration 421

200–450 C. Above 200 C preoxidizer activity is sufficient to burn out the HC and CO as well as to convert NO to NO2, which can rapidly react with diesel particulate, leading to its combustion and to formation of NO. Above 450 C, thermodynamics start to disfavor NO2 formation. The use of continuous regeneration means that extreme temperature gradients within the trap are avoided, which prolongs the trap life expectancy. A satisfactory performance of over 600 000 km has been reported [20].

The major drawback of the CRT system lies in the sensitivity of the preoxidizer to the presence of sulfur compounds [20], which has hampered significant introduction in the market. However, low-enough fuel sulfur contents (50 ppm) will be made mandatory all over Europe in 2005, and this may lead to extensive introduction of this technology. However, another weak point of the CRT system is its dependence on the presence of NOx, as it is uncertain whether future diesel engines will produce sufficiently high NOx-to-soot ratios to allow satisfactory operation.

One of the most attractive ways of catalyzing the combustion of the soot accumulated on the filter is the use of a catalytic coating on the filter itself, that is, a catalytically coated trap (CCT). On the basis of their chemical composition, the supported catalysts for the combustion of diesel particulate can be divided into two classes: noble metals and base metal oxides, frequently combined with alkali metal compounds. Moreover, perovskite-type oxides have been recently studied [21] for the simultaneous removal of nitrogen oxides and diesel particulate.

The most common precious metals used are Pt, Pd, and Rh [22]. The noble metal is intended to initiate oxidation of the easily combustible hydrocarbons adsorbed on the soot surface. This provides a local heat release which initiates oxidation of the relatively less reactive carbonaceous particulate. Moreover, Pt activates oxygen effectively and this enhances the rate of soot oxidation and minimizes formation of partially oxidized compounds, such as CO. Precious metals are often added in very small quantities to catalysts based on transition metals (e.g., copper, vanadium) to improve their low-temperature activity compared to those of the transition metals alone [23].

The oxides and/or salts of several base metals (e.g., vanadium, copper, molybdenum, manganese, cobalt, chromium, iron), known to catalyze graphite oxidation [24], have been used as soot-combustion catalysts. In general, based on the catalytic oxidation of graphite, only those oxides which are capable of being reduced by carbon to the metallic or lower oxidation state are likely to be active catalysts in soot oxidation. For instance, studies by McKee [24] on the copper-catalyzed oxidation of graphite showed that copper(ii) salts form copper(ii) oxide, which interacts with graphite and is reduced completely to metallic copper, which finally be reoxidized. The net result of this oxidation–reduction cycle is transfer of oxygen atoms from the gas phase to the graphite. The reduction of copper oxides to copper metal by carbon is favored by the comparatively low free energy of formation of these oxides, whereas the oxides of aluminum and zinc are stable in the presence of carbon and are therefore catalytically inactive.

These results can be extended to soot combustion: Van Doorn et al. [25] found that very stable oxides, such as those of aluminum and silicium, exhibit no activity

422 Part 5 Applications

at all, whereas Ahlstr.m and Odenbrand [23] explained the high activity of V2O5 in terms of the capability of vanadium of existing in several oxidation states with small energy differences.

The combination of two of the above active metals [26], such as, for example, the use of copper vanadates, is also very frequent in the literature on the catalysis of soot combustion: in this case, copper cations can change their oxidation state easily between +II and +I, and vanadium can readily change its oxidation state between +V and +IV.

Alkali metal compounds, alone or added to transition metal compounds [27], proved to be active in the catalysis of soot combustion. The catalytic effect of alkali metals in carbon oxidation was explained in the early 1950s [28] by the fact that alkali metal atoms on the carbon surface act as sites for chemisorption of oxygen, which weakens the C–C surface bonds and promotes the desorption of gaseous oxidation products at low temperatures. Furthermore, the gasification activity of alkali metal compounds was found to be dependent on the nature of the alkali metal [29] and on the anion with which the alkali metal ion is associated. In the use of alkali metals in combination with transition-metal compounds, a synergistic effect can be observed [30]. Mross [31] reports on the addition of alkali metals to oxidic compounds: in this case, the electron-donating effect of the alkali metal ions increases the reactivity of the oxygen in the M=O bond (where M is a transition metal ion in the catalyst) and therefore catalyst doping with small quantities of alkali metal increases the reaction rate in the partial oxidation of hydrocarbons. Another possible explanation of the synergistic effect of alkali metal compounds is the capability of the alkali metal to lower the melting point of the catalytically active species and the viscosity of the resulting liquids [31]. Saracco et al. [32] showed that the addition of potassium chloride to several copper and potassium vanadates promotes the formation of eutectics with a melting point lower than those of pure vanadates. According to these authors, the role of the alkali metal compound (in this case potassium chloride) is that of bringing into the liquid phase the active components (e.g. vanadates), which, once the carbon surface has been wetted, catalyze its oxidation through redox processes.

The contact between soot and catalyst under operating conditions seems to be the most critical parameter controlling the reaction rate; this contact mainly depends on the outer surface of the catalyst layer and on the dispersion of the soot on the catalyst surface itself.

This problem was first underlined by Inui and Otowa [33], who compared the effect of support structure on soot conversion. They deposited the same amount of a copper-based catalyst on three different supports (a honeycomb monolith, a ceramic foam, and alumina pellets), uniformly loaded with the same quantity of a benzenegenerated soot. The ceramic foam showed better performance than the honeycomb or the pellets. This was explained in terms of better contact between soot and catalyst, since ceramic foams expose a larger catalyst surface compared with the other two counterparts. Inui and Otowa obtained further confirmation of the influence of contact on soot combustibility from tests employing two different methods for establishing this contact: the “dry method” (placing the deposited catalyst in an air stream containing the soot) and the “wet method” (dipping the catalyst into soot-containing benzene, followed by benzene evaporation). They observed that the soot loaded on

5.2 Gas (Particulate) Filtration 425

Fig. 7 Trap loading and regeneration cycles for a catalytic (LaCrO3) and a noncatalytic wall-flow trap (Cordierite by Corning). (Reprinted from [45], Copyright 2003, with permission from Korean Institute of Chemical Engineers.)

and active measures must be adopted. Runs were performed on an engine bench provided with a fuel post-injection system and a preoxidizer like that employed in the PSA system. After a prolonged trap-loading period (final soot content: 10 g L–1), regeneration was induced by the rapid temperature rise caused by fuel post-injection/combustion. Figure 7 shows how the presence of the catalyst over the trap enables much faster regeneration compared to a noncatalytic trap and has the benefit of savings in post-injected fuel and a lower fuel penalty for the particulate-abatement system.

This feature, together with the absence of fuel additives and the related storage and dosing systems, the high filtration efficiency of the wall-flow monoliths, and the good catalyst stability, makes this technology quite attractive for car manufacturers, who are believed to be adopting this technology soon.

In the context of diesel particulate removal from mobile sources two pioneering studies are noteworthy. Ciambelli et al. are currently developing a catalytic foam trap whose regeneration is induced by microwaves [46]. To optimize energy consumption and the related fuel penalty, the foam material must not absorb microwaves (the authors used alumina foams), whereas the catalyst composition must be tuned to maximize microwave absorption and redox properties. In this way, heat would be released exactly where needed (i.e., at the catalyst–soot locations) and thereby maximize overall system efficiency. Conversely, Setiabudi et al. [42] developed an aftertreatment device combining a catalytic foam trap ahead of a wall-flow catalytic filter. The foam was activated with a Pt catalyst whose role was mainly that of enabling NO to NO2 oxidation to promote the CRT effect. Besides this, the foam can act as a sort of particulate agglomerator by favoring contact among soot particles, to the benefit of their filterability. The combination of both filters allowed good particulate collection and a faster regeneration. The combination of two filters may increase the pressure drop somewhat.