The reduction of nitric oxide in simulated combustion effluents by hydrocarbon-oxygen mixtures

Abstract

A study has been made of the chemistry of a gas-phase process for the reduction of nitric oxide in a flowing system of simulated combustion effluents (CO and CO2 in N2, H2O being inert with respect to this reaction). The reductant consists of mixtures of hydrocarbons (e.g., isobutane or gasoline) and carefully controlled amounts of oxygen. The latter brings the rate and nature of the products of the otherwise slow reaction between hydrocarbon and NO close to the requirements of practicability. The reactions were carried out in a ceramic flow reactor at temperatures in the range of approximately 1200°K to 1700°K and with residence times of 50 to several hundred milliseconds. In addition to temperature, the most critical parameters are the molar ratios R1=[HC]/[NO] and R3=[O2]/[HC] where HC is any one of the hydrocarbons used (methane, ethane, isobutane, isobutylene and gasoline). When the fraction of the initial NO remaining, is plotted as a function of R3 (for a given R1), a characteristic minimum is formed, along with a maximum for HCN. Increasing R1 results in greater reduction of NO but tends to increase HCN production. For a ratio of R1=1 at an initial NO concentration of 1000 ppm, total fixed nitrogen (TFN) remaining after reduction (NO+HCN) may range from 10% to 45% of the original concentration at the minimum of the TFN-R3 curve, depending on temperature, residence time and stoichiometry. The HCN can be oxidized to residual NO. Various aspects of the basic chemical mechanism are examined and it is suggested that O2 provides effective reducing free radicals, e.g., CH and CH2, by hydrogen abstraction of the hydrocarbon. They can then reduce NO through reactions typified by the exothermic step, CH+NO=HCO+N.