Development of Nitric Oxide Oxidation Catalysts for Fast SCR Reactions

Development of Nitric Oxide Oxidation Catalysts for the Fast SCR Reaction At the University of Kentucky Center for Applied Energy Research we propose to reduce SCR system costs in coal fired boiler applications by developing nitric oxide (NO) oxidation catalysts for use in conjunction with conventional SCR catalysts. Two main reactions contribute to NOx reduction with ammonia: NOx formed in combustion processes is typically composed of >90% NO and reaction (I) therefore dominates. In the case that equimolar amounts of NO and N02 are present, NOx reduction occurs according to equation (2). This is the so-called "fast" SCR reaction, for which the rate constant greatly exceeds that of reaction (I). This implies that the rate of NOx conversion can be accelerated by use of an oxidation catalyst upstream of the SCR unit to convert ca. 50% of the NO to N02. Studies with diesel engine exhaust have shown that by promoting the fast SCR reaction in this manner, required SCR catalyst volumes can be effectively halved. Given that NO oxidation catalysts typically operate at high space velocities (and hence low catalyst volumes are required), the use of such a catalyst presents the opportunity to significantly reduce overall catalyst volumes and hence capital costs. The improved NOx reduction activity which results ITomthe use of an oxidation pre-catalyst can also be used to reduce the SCR operating temperature, albeit that this temperature must be set above the dew-point for ammonium bi(sulfate) deposition. The specific objectives of the proposed research are two-fold, being firstly to identify a catalyst which is selective for the oxidation of NO to N02 under typical flue-gas conditions while possessing minimal activity for the oxidation of S02, and which shows adequate stability with respect to long term operation in a flue-gas environment. Secondly, the activity and manufacturing cost of the catalyst should be such that a 25% saving in total catalyst costs can be realized. To this end, candidate catalysts will be prepared and screened for NO oxidation activity under conditions representative of flue gas from coal-fired utility boilers. A key feature of the screening will be the use of both fresh catalysts and catalysts which have first been subjected to accelerated aging (as commonly practiced in the automotive catalyst industry). In this manner the screening experiments will provide insights into the intrinsic activity of the specific catalyst compositions, as well as the propensity for deactivation upon long term exposure to SOx. The most promising catalyst identified from the screening will be put through a program of optimization and subsequently subjected to a durability test lasing one month in order to gauge its long term performance.