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In the proposed alternate arrangement, shown in Figures 3 and 4, the tempering air is injected fully or partially in the post-oxidation section of the exhaust gas. The temperature range of the oxidation catalyst is maintained at higher temperature than the reduction catalyst. In addition, a portion of the tempering air can be used as dilution air for maintaining concentration of the ammonia reductant below the lower explosion limit, or as a heating media to evaporate aqueous ammonia.
Considerations for the oxidation and reduction catalysts
The oxidation catalyst (CO oxidation) is capable of operating at 1,200°F (649°C).
However, the reduction catalyst (NOX reduction), depending on the active component’s chemical formulation and type of substrate, is typically utilized at about 850°F (454°C) and the operating temperature cannot exceed 1,100°F (593°C) for a prolonged period of time. The operating temperature of the reduction catalyst is restricted to maintain a stable operation and prevent sintering of the catalyst
In contrast to the reduction catalyst, the oxidation catalyst is a noble metal plated catalyst and its sintering effects are greatly reduced. Considering upper temperature limit, the injection of the tempering air is not required. The catalyst is capable of achieving more than 90 percent efficiency up to 1,200°F (649°C). However, below 600°F (315°C), the efficiency of the oxidation catalyst quickly deteriorates, and below 600°F (315°C) the catalyst is capable of oxidizing only about 70 percent of CO. The conversion of unburned hydrocarbons (UHC) also is highly dependent on the operating temperature and the type of unburned hydrocarbons present in the exhaust gas. The same minimum temperature limit exists for the VOCs. Similarly to the oxidation of CO and UHC, the VOC removal efficiency rate increases with the increase of the operating temperature of the exhaust gas. As a result, the optimal temperature window exists for efficient utilization of the oxidation catalyst, ranging from low efficiency utilization at 500°F (260°C) to high efficiency utilization starting at 700°F (371°C) to 1,200°F (649°C). The operating temperature window for the reduction catalyst ranges from 350°F (177°C) to 1,100°F (593°C).
Considering the emissions control catalysts’ operating temperature window, the oxidation catalyst is restricted by the lower temperature limit, however the reduction catalyst is restricted by its upper temperature limit that depends on its chemical formulation and ranges between 750°F (399°C) and 1,100°F (593°C). Exposing both catalysts to the same operating temperature, depending on the site specific requirements, might result in a loss of catalyst efficiency. Since both catalysts are active at higher temperatures, maintaining this operating temperature range for both catalysts throughout all operating loads is preferable, and overcooling of the exhaust gas with excessive amount of tempering air should be avoided. By injecting tempering air upstream of the oxidation catalyst, the operating temperature of the oxidation catalyst is decoupled from the operating temperature of the reduction catalyst. As a result, efficiencies of both oxidation and reduction catalysts are maximized.