Cellular Ceramics / 5

426 Part 5 Applications

The use of catalytic foam filters has also been recently proposed and tested for the treatment of flue gases from stationary sources such as large boilers based on combustion of fuel, wood, peat, and coal on ferry boats, incinerators, or simply large diesel engines for power generation [43]. A specific filter baghouse reactor was conceived as shown in Fig. 8, on the grounds of the fact that foams can be produced in a tubular shape. A quite active Cs4V2O7 catalyst was deposited into zirconia-tough- ened alumina foam traps (pore size: 50 ppi, equivalent to about 400 mm, thickness 17 mm), whose abatement performance was evaluated in a specific pilot plant based on a diesel engine. Good abatement efficiency (about 50 %), coupled with low pressure drop across the trap (< 104 Pa), were obtained for superficial velocities (2 m s–1) and temperatures (about 400 C) of industrial interest. In this context, important cat- alyst-related issues are:

Fig. 8 Schematic view of a catalytic-trap baghouse and of the trap microstructure.

. Catalyst loading must be optimized to balance the need for high activity (filtration and catalytic conversion must be achieved in the limited space and time constraints imposed by the system) and the increase in pressure drop induced by the presence of the catalyst in the porous matrix. A catalyst deposition route based on filter impregnation with an aqueous suspension of the catalyst powders, followed by microwave drying and calcination, was adopted to achieve an optimal catalyst loading (14 wt %).

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. Catalyst thermochemical stability: the highly active pyrovanadate (Cs4V2O7) catalyst was found to be somewhat sensitive to water vapor. Some catalyst deactivation was indeed noticed in the long term at the front side of the filter. This is clearly visible in Fig. 9, where the orange color of the front side of the regenerated foam is attributed to the localized formation of the less active metavanadate CsVO3. Nevertheless, the activity remained high enough to guarantee the above-mentioned efficiency, but more stable catalyst compositions are probably needed to achieve long-term viability of this technology.

Fig. 9 View of a foam containing Cs4V2O7 catalyst after hydrothermal ageing. Left part: after regeneration by carbon combustion in oven at 450 C; Right part: before regeneration. Inlet side in front of view. (Reprinted from [43], Copyright 2002, with permission from Elsevier Science Ltd.)

In addition, Fig. 10 clearly shows the superior performance of such a catalytic trap in comparison to a noncatalytic one. The presence of the catalyst allows the rapid oxidation of a significant fraction of the trapped soot and thereby reduces its holdup in the foam and consequently the pressure drop.

Figure 10 allows some conclusions to be drawn on modeling issues, described later in more detail. After measurement of key kinetic parameters for the catalytic combustion of carbon particulate (i.e., activation energy, reaction order in oxygen) and a deep characterization of the permeation properties of the filter, a mathematical model was validated by using experimental data obtained with catalytic and noncatalytic traps [1]. The agreement of the model with the experimental data was in both cases good, which is particularly promising for design purposes.

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Table 1 Commercially available inorganic filters for high-temperature applications.

filters are generally made of a thin front layer with reduced pore size to achieve surface dust filtration and an inner structure with large pores to reduce pressure drop, provide sufficient mechanical strength, and to host the catalyst.

A new catalytic filter concept employing cellular materials has, however, been conceived recently with the perspective of applying it to the treatment of flue gases from waste incineration. In this context [51], Goretex fabric filter bags are widely adopted to remove both fly ashes and carbon particles suitably dispersed in the flue gases to remove dioxins and heavy metal traces. To provide the filters with the additional function of NOx and VOC (for example dioxins) removal by catalytic redox processes, a catalytically active ceramic foam candle can be inserted in each filter bag, according to the scheme reported in Fig. 11 [52]. As the Goretex filter bags generally work at 200–210 C, an innovative catalyst, active in such a temperature range for the selective catalytic reduction (SCR) of NOx with ammonia and simultaneous combustion of VOCs, must be developed and applied over the foam structure to obtain the above-mentioned multifunctional operation.

Currently adopted SCR units use V2O5–TiO2 catalysts which operate at 320–400 C in honeycomb catalytic converters fed with space velocities of 20 000 h–1 [53]. A specific catalyst has thus been developed at Politecnico di Torino, based on a mechanical mixture of TiO2 supported V2O5–WO3 [54] and CeO2-supported MnOx catalysts recently proposed by other authors [55].

430 Part 5 Applications

Fig. 11 Schematic view of a multifunctional filter for the simultaneous abatement of fly ashes, nitrogen oxides, and dioxins. (Reprinted from [52], Copyright 2004, with permission from Elsevier Science Ltd.)

Such catalysts were lined inside a support structure [zirconia-toughened alumina (ZTA) foam disk; 50 ppi (equivalent to a pore size of 4 0 10–4 m); thickness: 10 mm; manufacturer: Saint Gobain] by special operating procedures (impregnation in ultrasonic bath followed by microwave drying and final calcination at 400 C). This procedure was tailored to achieve a catalyst distribution as even as possible throughout the disk body. Moreover, it enabled good adhesion to the porous walls of the structured support, and a rough surface of the catalyst layer with only a moderate increase in pressure drop across the porous medium. Subsequent impregnation steps, with intermediate drying stages, were carried out to reach a catalyst loading of 18 wt %.

The obtained catalytic foam was then tested on a synthetic gas mixture representing real incinerator flue gases (1000 ppm NO, 1/1 NO/NH3 molar ratio, 10 vol % O2,

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200 ppm benzene; balance He) to check its pollutant-abatement performance at superficial velocities of industrial interest. On the basis of the work in Ref. [56], the selection of benzene as a representative for PAHs is conservative, since its catalytic combustion is initiated at higher temperatures than those of the much more harmful PAHs and chlorinated aromatics mentioned above.

Figure 12 shows how the prepared filter can achieve NO conversion as high as about 90 % in the temperature range 200–250 C. Reduced conversion at higher temperatures is due to occurrence of the undesired oxidation of NH3 to NO.

Fig. 12 Experimental and calculated data concerning the NO conversion performance of catalytic foams (catalyst: MnOx.- CeO2+V2O5-WO3.TiO2) at different superficial feed velocities. (Reprinted from [52], Copyright 2004, with permission from Elsevier Science Ltd.)

A similar conversion efficiency was also obtained for benzene (80 % at ca. 210 C). A model developed by Saracco and Specchia [49] was employed to fit the experimental data after suitable evaluation of the reaction kinetics of the catalyst and the permeation properties of the catalytic foam. Figure 12 shows how good agreement between experimental data and model calculations could only be reached by assum-

ing that the catalyst loading of the filter was only half of the true value.

After careful structural analysis of the catalytic foams under the scanning electron microscope (SEM), a likely explanation for the fact that the observed abatement activities were lower than expected was found in the tendency of the deposited catalyst to occasionally clog the pores of the foam (see Fig. 13). Pore clogging would lead to portions of the porous matrix in which the catalyst is present but cannot be adequately reached by the reactants, and its activity thus remains unexploited. From this viewpoint and on the grounds of the earlier described model predictions, it can be assumed that only half of the catalyst present can actually exert its activity. Hence, additional developmental work must be spent to further improve the catalyst deposition route.

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5.2.4

Modeling

The modeling of mass transfer and reaction in catalytic filters and traps can be compared, in a first approximation, with the twin problem concerning honeycomb catalysts. The pores of the filters are counterparts of the channels of the monolith, whereas the catalyst layer deposited on the pore walls of the filter are related to the walls separating the honeycomb channels, which are in general exclusively made of catalytic material or catalyst-lined ceramic structures [57, 58].

Owing to the comparatively small size of the pores (100–500 mm, as opposed to a size of a few millimeters on average for the honeycomb channels) and the thinness of the catalyst layer (a few micrometers, compared some tenths of a millimeter for the catalytic wall of the honeycomb channels), both internal and external masstransfer limitations to pollutant conversion in catalytic filters can frequently be neglected. Hence, the achievable conversion per unit catalyst mass is maximized, which is favorable owing to the need to convert the noxious gases within short space and time constraints. The above-mentioned advantage is, however, compensated, at least in part, by the higher pressure drop entailed by the filter. However, this last feature is a rather obvious consequence of the combination of particulate filtration and catalytic reaction.

The most frequently used approach to modeling catalytic filters is of the pseudohomogeneous type. The porous matrix is considered as ideally homogeneous by lumping the prevalent constituting parameters (porosity, tortuosity, permeability, catalyst concentration, etc.) into average values per unit volume. This allows material, component, and energy balances throughout the porous structure to be described by means of a set of partial differential equations (see, e.g., Ref. [59] for a typical set of these equations).

One of the most accurate modeling studies on catalytic filters, performed by Saracco and Specchia [59, 60], involved a c-Al2O3-coated a-Al2O3 granular filter on which a model reaction (2-propanol dehydration, catalyzed by the c-Al2O3 itself) was performed. The major conclusion drawn by the authors was that a certain degree of catalyst bypass occurs in filters prepared with a single catalyst deposition step. In other words, they concluded that a single catalyst deposition step induces a uneven distribution of catalyst in the filter, which means that some pores have a higher catalyst loading than others. This forced them to adopt for these filters a pseudohomogeneous model based on a bimodal pore size distribution to account for the fact that the less catalytically active pores are also more permeable, which implies lower overall pollutants conversion throughout the filter. However, a second deposition cycle seemed to remediate the above uneven distribution, and thus enabled the use of a simpler model based on a monomodal pore size distribution.

However, on the basis of a similar modeling approach for NOx-abatement filters [49] (see also at the end of Section 5.2.3.2), it was also emphasized that high loadings of catalyst may result in a similar unevenness problem as long as pore plugging prevents some catalyst from being reached by the permeating gases.

434 Part 5 Applications

All the above modeling studies showed therefore the importance of having a proper knowledge of the pore texture and catalyst distribution within the catalytic filter, since these parameters can seriously affect its performance. This suggests, in line with Ref. [61], the need for proper characterization of the porous structure of catalytic filters (pore connectivity, pore size distribution, presence of dead-end pores, etc.), since each of these features may play a primary role in reactor performance. On the basis of such characterization, valuable information could be drawn for selecting or optimizing preparation routes. This is an even more stringent requirement for cellular structures for diesel particulate removal where, in addition to the catalytic function, filtration mechanisms must taken into account.

Several EU projects (GROWTH GRD2-2001-50038: “Simulation Tool for Dynamic Flow Analysis in Foam Filters” – STYFF-DEXA; GROWTH 99 GRD1-1999-10588: “System Level Optimization and Control Tools for Diesel Exhaust Aftertreatment” – SYLOC-DEXA; GROWTH 99 GRD1-1999-10451: “Advanced Regeneration Technologies for Diesel Exhaust Particulate Aftertreatment” – ART-DEXA) have been or are being carried out to develop and apply an experimentally validated simulation tool that allows accurate characterization of foam materials applications and to enable the systematic optimization of cellular structures with high separation efficiency for diesel exhaust aftertreatment.

In such efforts, pseudohomogeneous models [1, 5] have been developed that include, in addition to mass and heat balances, proper expressions for the filtration mechanisms of inertial impaction, interception, and Brownian diffusion, as well as an additional mechanism for the blow-off of trapped particulate in the flowing gases. The model could properly predict the experimental behavior of the trap, as already discussed (Fig. 10), with the use of a single fitting parameter related to the re-entrainment mechanism.

This performance could perhaps be further improved by a completely different approach based on the Lattice Boltzmann (LB) method which is more suitable to achieve a real understanding of the physical and chemical phenomena occurring inside the foam matrix and how they interact.

Pseudohomogeneous approaches, typical of conventional computational fluid dynamics (CFD) methods based on adaptive unstructured computational grids, become rather time-consuming due to the necessary high grid resolution near com- plex-shaped surfaces, as found for example inside a filter material. An efficient alternative for the analysis of this kind of flow is generally considered to be the LB method [62]. With this method boundary conditions on complex, rigid surfaces can be satisfied on a fixed equidistant grid (lattice) with second-order accuracy. Essential advantages lie in the simple underlying algorithm, which facilitates parallel implementation, intrinsic stability, and capability to deal with arbitrarily shaped geometrical boundaries. This method is therefore ideally suited for studying flow phenomena in detail. The underlying geometrical model for given materials can be generated by computer tomographic methods [3, 63, 64] or lower order correlation functions [65], as shown in Fig. 15. A typical computed velocity pattern inside a porous ceramic foam is shown in Fig. 16.