A fixed bed study of formation and reduction of nitric oxide over different sand materials at fluidised bed temperatures and concentrations

摘要

There are several homogeneous and heterogeneous reactions that are of importance for the formation and reduction of nitric oxide at fluidised bed conditions. This investigation concerns three heterogeneous processes, namely the reduction of nitric oxide with carbon monoxide, the formation of nitric oxide from ammonia and oxygen and the oxidation of carbon monoxide. The reactions were studied in a fixed-bed quartz reactor with quartz, silver sand, olivine sand as bed materials in the temperature range 600-950°C. The residence time within the bed was 50-380 ms and the gas concentrations 380-1000 ppm nitric oxide, 400-7600 ppm carbon monoxide, 1000 ppm NH{sub}3 and 1% oxygen. The gases were diluted in argon. The natural sands have more catalytic effect than the quartz of analytical grade. Olivine sand was the most active of the sands investigated. The crystalline structure of olivine sand changed under oxidising and reducing conditions. Olivine sand contains iron oxide, which gives a more catalytic surface compared to the natural quartz sand. Drying of the gases reduced the oxidation of carbon monoxide substantially on quartz and silver sand, but only a few percent on olivine sand, probably because of the high iron content of olivine sand. The activation energy and preexponential factor for the reduction of nitric oxide with carbon monoxide over olivine sand were determined to 76.5 kJ/mole and 1905 m{sup}3/kmole·s respectively.