Materials (partial list)
Corning, NY Johnson Matthey, Catalytic Systems Div.
Wayne, PA W.R. Grace & Co., TEC Systems Div. DePere, WI.
Baltimore, MD UOP
Des Plaines, IL Engelhard Corporation Iselin, NJ
Norton Chemical Process Products Corp. Akron, OH
Supports, metals/compounds, supported metals, other
Oxides, supported oxides, other Zeolites, adsorbents Noble metals, supported metals/oxides NOx reduction catalysts
Notes: This list is not comprehensive. The authors have attempted to provide current information but recognize that data may have changed. Inclusion of a company in this list is not an endorsement of that company's products and should not be construed as such.
is temperature sensitive. With the use of a suitable catalyst, the NOx reduction can be carried out at 300-400°C, a temperature normally available in a flue gas system. This process is called Selective Catalytic Reduction (SCR). Figure 5.20.21 is a schematic diagram for the SCR process. In this process, at least 1% O2 should be present in the flue gas, thus it is suitable for boiler and furnace applications. Several simple catalytic and noncatalytic systems are in commercial operation at several sites (see Table 5.20.14).
Under ideal reaction conditions, one mole of NH3 is required to convert one mole of NO; however, in practice, one mole of NH3 reduces about 0.8-0.97 moles of NOx. The excess NH3 is required because of the side reaction with O2 and incomplete mixing of the ammonia with the flue gas. Much of the ammonia slip ends up in the fly ash, and the odor can become a problem when the ash is sent to a landfill or sold to cement plants.
The SCR processes are simple, requiring only a proper catalyst and an ammonia injection system. The catalysts currently in use are TiO2-based which can be mixed with vanadium or molybdenum and tungsten oxides. These catalysts eliminate the formation of ammonium bisulfate which can plug the downstream equipment, a problem in earlier SCR systems.
A new Shell de-NOx catalyst comprises vanadium and titanium, in high oxidation states, impregnated onto silica with a high surface area (300 m2/g). The data for a commercial and semicommercial operation indicate consistent performance for periods up to one year (Groeneveld et al. 1988). The catalyst is deactivated by high concentrations of SO2, but the poisoning is reversible with heating. A version of this process is installed to control dust-containing flue gas. This process employs a parallel passage system in which the flue gas permeates through the catalyst separating into passages for feed and purified streams.
The Shell technology is being developed for other applications. In particular, a lateral flow reactor is being developed which demonstrates a low pressure drop. The design of this reactor should allow for convenient installation and maintenance (Groeneveld et al. 1988).
Flue gases, especially from coal-burning boiler units or power generation, contain both NOx and SOx, with fly ash and metal-containing particulates. Processes have been developed that convert the NOx to nitrogen, which is vented, and the SOx to sulfuric acid, which is removed by scrubbing. Figure 5.20.22 is a schematic of the SNOX process (Haldor Tops0e A/S, Denmark). This process recovers up to 95% of the sulfur in the SOx as sulfuric acid and reduces 95% of the NOx to free nitrogen. All fly ash and metals are essentially captured.
Table 5.20.15 is a recent list of catalyst suppliers, and Table 5.20.14 lists the manufacturers of environmental technologies (Environmental Processes '93 1993).
—Karl T. Chuang Alan R Sanger
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