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Show INN 0 V A nONS OF CAT AL VTIC COMBUSTION Introduction Catalytic Destruction of VOC (Volatile Organic Compounds) contaminants in an air stream is a very economical solution to plant emission reduction. The process involves lower temperature operation than the corresponding thermal options and often allows heat recuperation without the requirement of exotic alloys in the heat exchangers. This feature further reduces the comparatively low fuel consumption of the catalytic systems. While these economic factors have made the catalytic systems attractive to the end user, the applications have been limited by the vent stream constituents that could be tolerated by the catalyst formulation. Catalysts used in VOC oxidation processes have had limited applicability because the catalyst could be deactivated by some contaminants, masked by some, and poisoned by others. The former limitations were so broad that only select applications were considered an appropriate fit. Aggressive research programs by catalyst suppliers have developed formulations to eliminate these limitations, or render them less harmful. This paper will review the system's construction, give better understanding of these limitations, and indicate the future trends of the technology. General Principles of Catalytic Combustion Catalytic Oxidation of VOC contaminants in a waste stream is a process typically performed with waste streams where the VOC concentration does not exceed 25% of the LEL (Lower Explosive Limit) and where the oxyge~ concentration of the flue gas is at or above 2.0%. The system is designed to raise the waste stream temperature to the design inlet temperature of the catalyst, typically 260-370·C (500-700·F), by a combination of fuel gas burning and heat recuperation. The catalyst outlet temperature is a function of the amount of heat available for release contained in the VOC, and the required Destruction Removal Efficiency (ORE) required for the application. This temperature is usually limited to a peak value of 663°C (1225°F). The exact amount of catalyst required is determined by the DRE requirement and the exact VOC constituents being oxidized. More difficult constituents such as CH4, etc. require greater volumes of catalyst for destruction than do most longer-chained hydrocarbons. Construction of the catalyst bed (substrate) takes several forms, such as pellets, random packing and structured packing (monolithic). The material may be metal (stainless steel) or ceramic. These shapes provide the base to which the catalyst is applied. Application is a two stage process which includes a wash coat of high surface area alumina and a second thin coat of the design formulation of catalytically active materials across this enhanced alumina surface. The catalyst then is a very thin layer of active materials finely dispersed across this extended surface. This maximizes the number of active sites available to the VOC constituents as the waste stream flow moves across the catalyst surface. 1 |