Catalytic Afterburning

Afterburning processes enable the removal of pollutants such as hydrocarbons and volatile organic compounds (VOCs) by treatment under thermal or catalytical conditions. Combinations of both techniques are also known. VOCs are emissions from various sources (e.g. solvents, reaction products etc. from the paint industry, enaml-ing operations, plywood manufacture, printing industry). They are mostly oxidized catalytically in the presence of Pt, Pd, Fe, Mn, Cu or Cr catalysts. The temperatures in catalytic afterburning processes are much lower than for thermal processes, so avoiding higher NOx levels. The catalysts involved are ceramic or metal honeycombs with washcoats based on cordierite, mullite or perovskites such as LaCoOs or Sr-doped LaCoOs. Conventional catalysts contain Ba-stabilized alumina plus Pt or Pd. 

Both thermal and catalytical exhaust gas purification systems operate at pollutant concentrations 1.5-3 g/Nm autothermally. Since the efficiency of internal heat recovery is 80-90%, no additional energy is required for heating the exhaust gas. Thus both processes are environmentally sound and economical in operation. In Table 10-1 are some working temperatures compared for both processes. Limitations for catalytical processing are the catalyst sensitivity towards poison and overheating. 

Pollutant Temperature Catalytical processing PC) Temperature Thermal processing PC) 

Conversion at maximum process temperature (which may be fixed before) 

Area I kinetic region Area It diffusion region 

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The increasing use of sihconized coatings for weather durabiUty caused severe masking problems for the all-metal, filter mesh-like catalyst elements available in the 1970s. Interest in catalytic afterburners increased when dispersed-phase precious metal—alumin a-on-ceramic honeycomb catalysts offered economically attractive results. 

Catalytic afterburners are currently used primarily in industry for the control of solvents and organic vapor emissions from industrial ovens. They are used as emission control devices for gasoline-powered automobiles (see Chapter 31). 

The main advantage of the catalytic afterburner is that the destruction of the pollutant gases can be accomplished at a temperature range of about 315°-485°C, which results in considerable savings in fuel costs. However, the installed costs of the catalytic systems are higher than those of the direct-flame afterburners because of the expense of the catalyst and associated systems, so the overall annual costs tend to balance out. 

Thermal incinerators (gas-fired afterburners or catalytic afterburners) None... 

A catalytic afterburner, in which the surface action of catalysts allows incineration to take place at a temperature lower than a direct flame, reducing the auxiliary heat required, or... 

By careful choice of emulsifiers it is possible to provide microemulsions based on diesel oil which exhibit an ignition performance suitable for high-speed diesel engines. In contrast to emulsion fuels examined previously (23), these microemulsions show a pronounced net benefit in the NO and smoke emissions. The amounts of unbumed hydrocarbon and CO in the exhausts do increase but may be reduced by a catalytic afterburner. 

An airtight stove with a catalytic afterburner and a smoke chamber such as a double-drum stove or a design that provides for improved burning in a secondary combustion chamber. 

Economics of catalytic-afterburning plants for emission control of volatile organic compounds are strongly influenced by the lifetime of the employed catalyst,... 

Plans have been submitted for a catalytic afterburner. The installed afterburner is to incinerate a 3000-acfm contaminated gas stream discharged from a direct-fired paint baking oven at 350°F. The following summarizes the data taken from the plans ... 

The following additional information and rules of thumb may be required to review the plans for the catalytic afterburner ... 

Catalytic afterburner operating temperatures of approximately 950°F have been found sufficient to control emissions from most process ovens. 

The soot from diesel engines and wood smoke carry mutagenic polycyclic aromatic hydrocarbons and their nitro derivatives.45 Some typical ones are shown in 15.1, the first being a potent mutagen in the Ames Salmonella test. Catalytic afterburners (containing platinum) are now required... 

Catal54ic afterburning can solve various emission problems without generating secondary pollutants. There are numerous examples of off-gas purification in the chemical industry, the textile and furniture industry, and in printing works (see Section 10.3). Catalytic afterburning units (Fig. 8-2) are also successfully used for removing odors, e.g., in the foodstuffs industry. 

Catalytic afterburning (off-gas purification) Pt/Pd LaCeCoO (perovsldte) oxides of y W, Cu, Mn, Ife supported catalyst (honeycomb monolith or catalyst bed) or bulk catalyst 150-400 °C 200-700 "C... 

Fig. 8-2 Catalytic afterburning of the off-gases from a cyclohexanone plant (BASF, Antwerp)...

You can select a suitable catalyst for a catalytic afterburning process from monoliths or pellets. Which process parameters are mainly influenced by your choice ... 

Waste destruction by thermal decomposition makes good technical, environmental, and economic sense. Systems can be designed to meet almost any emissions standard oxygen could be used instead of air (even in excess enthalpy combustion systems) to achieve higher temperatures and reduce the amount of effluent gas a catalytic afterburner could be used to ensure complete combustion of trace compounds. 

Men et al. reported the operation of a small-scale bread-board methanol fuel processor composed of electrically heated reactors [15]. A methanol steam reformer, two-stage preferential oxidation reactors and a catalytic afterburner were switched in series. A fuel cell equipped with a reformate-tolerant membrane, which had a 20 W nominal power output, was connected to the fuel processor and operated for about 100 h. 

Shah and Besser presented results from their development work targeted at a 20 Wei methanol fuel processor-fuel cell system [66]. The layout of the system consisted of a methanol steam reformer, preferential oxidation, a catalytic afterburner and an evaporator. Vacuum packaging was the insulation strategy for the device, which is in line with other small-scale systems described above. A micro fixed-bed steam reformer coupled to a preferential oxidation reactor was then developed by the same group with a theoretical power output of 0.65 W. 

Figure 24.11 shows a microstructured coupled diesel steam reformer/catalytic afterburner developed by Kolb et al. [27], which was operated at temperatures exceeding 800 ° C. The reactor, which was coated with catalyst from Johnson-Matthey Fuel Cells, had separate inlets for anode off-gas and for air supply to the burner. Full conversion of the diesel fuel was achieved for a total operation time of 40 h with this reactor, which had a power equivalent of 2 kW thermal energy of the hydrogen produced. 

Wichert et al. reported about long-term experiments performed at a microstruc-tured coupled steam reformer/catalytic afterburner for LPG. The reactor was... 

The reformer unit was designed as a joint-part coupling the catalytic afterburner and the reformer layer in a sandwich-design, with the catalytic coated reformer substrate located between two burner plates (Figure 10). A commercial oxidation catalyst was used as burner catalyst, arranged as randomly packed bed above and below the reformer unit. 

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