A reexamination of the RapreNO sub x process

The recently proposed Rapre NO{sub x} process involves the injection of HNCO into combustion effluents in order to reduce their NO content via what was initially assumed to be a homogeneous gas-phase reaction. Although the process is thus similar to the Thermal DeNo{sub x} process it had the apparent advantage of operating at temperatures as low as 450{degrees}C, as contrasted with the 700{degrees}C minimum temperature of Thermal DeNO{sub x}. Further, the Thermal DeNO{sub x} process had the disadvantage that downstream of the reaction zone unreacted ammonia could react with SO{sub 3} to form NH{sub 4}HSO{sub 4} and cause fouling problems, a difficulty that using HNCO would seem to avoid. In this reexamination of the RapreNO{sub x} process the authors show that the process involves three different modes of NO reduction. The first is catalytic; NO reductions at temperatures significantly below 700{degrees}C are found not to occur in the absence of catalytic surfaces. While noncatalytic NO reductions were found to occur at 700{degrees}C in the presence of wet CO, NH{sub 3} will also reduce NO at these conditions. Modeling calculations indicate that for this mode of reaction HNCO reduces NO via a complex chain reaction mechanism very similar to that involved in the Thermal DeNO{sub x} process.