Sulfuric acid formation over ammonium sulfate loaded V2O5–WO3/TiO2 catalysts by DeNOx reaction with NOx

Operating the SCR DeNOx reactor at temperatures below 200°C results in a considerable saving in operating costs. Plant experience shows that on the catalysts in these second generation DeNOx plants, even for flue gases with SO2 concentration below 10mg/m3, over 1–2 years operating time sizeable quantities of ammonium sulfates accumulate. Ammonium sulfates deposited on V2O5–WO3/TiO2 catalysts react with NOx to nitrogen and sulfuric acid. Second-order rate constants of this reaction for temperatures of 170°C have been derived. It could be shown that the sulfuric acid formed on the catalyst is displaced by water vapour and desorbs resulting in gas phase concentrations of up to 6.5mgacid/m3 flue gas. Plant equipment downstream of the ammonium sulfate containing low temperature DeNOx catalysts has to be protected against the corrosive action of the sulfuric acid in the flue gases leaving the DeNOx reactor.