Catalytic filters

The NO t constituent in the exhaust of machines firing natural gas is some 150 to 160 ppm, and for distillate fuels typically 260 ppm. In order to reduce these levels to the targets quoted above, catalytic filters can be used, but the systems currently available are expensive. As an alternative, certain manufacturers are developing low-NO burners but these limit the user to natural gas firing. 

The oxidation of soot is a slow process, owing to its refractory character. Therefore, the soot particles are trapped in a filter, thus increasing the reaction time. However, catalysis is still required. The design of a catalytic filter is a challenge mainly because solid/solid contact is too poor for efficient catalysis [23]. Several ideas have been put forward. 

For a= 1, soot in the catalytic layer is oxidized fast leaving the soot in the thermal layer unreacted. This has been observed with some early catalytic filters. As a decreases the soot from the top layer replaces more rapidly the soot oxidized in the catalytic layer increasing the global oxidation rate. The corresponding soot layer thickness evolution is shown in Fig. 22. For values of a close to 1 (e.g. 0.9) the catalytic layer is totally depleted from soot at some instances, followed by sudden penetration events from the soot of the thermal layer. These events are clearly shown in the thickness evolution for oc = 0.9 in... 

The two-layer model for soot oxidation with N02 in a catalytic filter can be written as follows ... 

In order to avoid the unfavorable process conditions, different flue-gas treatment processes for combustion plants based on catalytic filters were developed, which combine fly-ash removal with SCR of ISKh with NH3 [4—8], The advantages of these processes are space and treatment-cost savings, reduced internal and external mass transfer resistances compared to honeycomb SCR catalysts, heat recovery from offgases with good efficiency, and low corrosion problems due to the removal of both dust and NOx at high temperatures. 

Catalytic filters should possess the following properties 1) high thermal, chemical, and mechanical stability 2) high dust separation efficiency attained by cake filtration (no penetration of particulates into the filter structure) 3) low cost 4) high catalytic activity (operation at high superficial velocities) and 5) low pressure drop. 

More recent investigations have been conducted into NOx-assisted soot oxidation [55, 56]. NO2 was suggested as an oxidation agent to assist the combustion of particulate matter in the presence of oxygen in the exhaust gas [57]. NO2 can be produced by catalytic oxidation of NO prior to the catalytic filter. 

One of the potentially wide-spread applications under development is catalytic filters for air pollution control. This combines separation and catalytic oxidation into one unit operation. One possibility is the oxidation of volatile organic carbon (VOC) by employing a porous honeycomb monolithic ceramic membrane filter. Inside the pores are deposited an oxidation catalyst such as precious metals. The resulting VOC removal efficiency can exceed 99% [Bishop et al., 1994]. 

It should be possible to use compressed metal powders as, for example, a catalytic filter, but the present authors are not aware of any industrial examples of this. Raney metals powders, however, have been employed in some liquid-based processes in shallow beds through which reactants pass. Because Raney metals are fine grained, pressure drop can be a problem so it is more common to use them in an unstructured way in slurry reactors, as, for example, formerly in the oils and fats industry [3]. Raney metals can have high surface areas when freshly prepared, but this decreases quickly in use, particularly when exposed to elevated temperatures. Pressure drop considerations are less significant for beds of metal granules, but there is less effective use of metal than with fine powders. For granules, surface areas in the region of 30-35 cm g are typical for silver used in... 

The potential advantages of catalytic filters are those typical of multifunctional reactors (reduction of process units, space and energy savings, cost reduction, etc.). Later on some examples will elucidate these points (see Section IV). However, the following properties should be possessed by catalytic filters so that these opportunities actually can be exploited ... 

Several flue gases (e.g., from coal-fired boilers, incinerators, diesel engines) are characterized by high loads of both particulates (e.g., fly ashes, soot) and gaseous pollutants (NO, SO2, VOCs, CO, etc.), which need to be removed for environmental purposes. In this context, several possible applications of catalytic filters can be envisaged, some of which... 

Figure 6 Schematics of two alternative routes for treating the flue gases of a PFBC coal boiler, (a) Process based on conventional technologies (b) process employing catalytic filters. Legend for scheme (a) I. PFBC boiler 2. air preheater 3. fabric filter 4. air compressor 5. turbine 6. SO2 wet scrubber 7. economizer 8. postheater 9. DeNOx catalytic unit. Legend for scheme (b) 1. PFBC boiler 2. air preheater 3. SOi dry-scrubber 4. catalytic filter unit 5. turbine 6. air compressor 7. air preheater.

Even considering very high engineering and contingency costs for the catalytic filters, the overall treatment costs (running plus investment) of the flue gases from PFBC coal boilers can likely be reduced by 15-30%, by varying the above process parameters. 

Contrary to early expectations, the cost of the catalytic filter apparatus was higher than the sum of the fabric filters and SCR reactor costs, primarily because the cost of the baghouse (which has to work at more than 400X and is not produced routinely, at present, but only for tailored processes) is higher than that of the catalytic filters. 

On the basis of this last observation, it is easy to predict that if the catalytic filter technology can penetrate the market more deeply, investment cost will likely lower, due to higher productions, thus further improving the above economic advantages. Moreover, even better results eould be accomplished if higher lime utilization efficiency were achieved in the dry-scrubbing unit, a point addressed by several research programs all over the world [32-35]. 

Patents were filed by Babcock Wilcox [21-23] concerning the so-called SOx-NOx-Rox Box process, according to which, in line with Fig. 6b, contemporary SO2 and NOx removal (the former by adsorption on lime, the latter by catalytic reduction with ammonia) is accomplished by the use of catalytic filters, prepared as described in Section III. A schematic of the catalytic baghouse assembly is presented in Fig. 7. The results of the application of such technology to the treatment of a lab-scale atmospheric fluidized-bed coal boiler (capacity 0.5 MWe) were reported in Refs. 9 and 29. The achieved abatement efficiencies were 70-80% for SO2, 90% for NOx(NH3/NO ratio = 1 ammonia slippage = 10-15%), and 99% for particulate. Since March 1992 a 5-MWe demonstration project, funded by the U.S. Department of Energy and by the Ohio Coal Development Office,... 

Figure 7 Schematic of the catalytic filter baghouse by Babcock Wilcox for the SO.-NO. Rox Box process. (Reprinted from Ref. 9, with permission of the American Institute of Chemical Engineers. Copyright 1992, all rights reserved.)...

Figure 8 Potential application of catalytic filters in the IGCC cycle. Legend 1. gasifier 2. catalytic filter unit 3. combustion chamber 4. gas turbine-compressor setup 5. boiler for heat recovery from exhaust gases 6. condenser 7. steam turbine.

Besides the cited application opportunities of catalytic filters, surely many others can be proposed for solving specific problems of flue gas treatment. 

The modeling of mass transfer and reaction in catalytic filters can be compared, in a first approximation, with the twin problem concerning honeycomb catalysts. The pores of the filters will have as counterparts the channels of the monolith, whereas the catalyst layer deposited on the pore walls of the filter will be related to the wall separating the honeycomb channels, which in general are made exclusively of catalytic material. Considering, for example, the DeNOx reaction. Fig. 9 shows schematically the NO concentration profiles within the channels/pores and the catalyst wall/layer of the two reactor configurations. 

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