Hybrid SNCR-SCR technologies for NOX control: Modeling and experiment

The hybrid process of homogeneous gas-phase selective noncatalytic reduction (SNCR) followed by, selective catalytic reduction (SCR) of nitric oxide (NO) was investigated through experimentation and modeling. Measurements, using NO-doped flue gas from a gas-fired 29 kW test combustor, provided data that allowed a fundamentally based catalyst monolith model, adapted from the literature, to be calibrated. The SCR model, which included pore and gas film diffusion (molecular and Knudsen) and Langmuir-Hinshelwood surface reaction kinetics, used published values for the catalyst preexponential factor and activation energy, and experimental measurements to determine the previously unavailable ammonia (NH3) adsorption coefficient on the catalyst surface. Then, with no further adjustment, a combined SNCR-SCR model was developed using literature values for the reaction rate coefficients of 112 elementary homogeneous reactions. This combined model agreed well with combined SNCR-SCR data. The combined model was then extrapolated to practical coal combustion conditions, and the economic benefits of a combined SNCR-SCR process were explored. With a desired total NO reduction of 95% and NH3 slip in the exhaust constrained not to exceed 5 ppm, adding SNCR led to only, modest decreases in the required SCR catalyst volume. Estimated system capital and annualized costs were not economically viable unless the SNCR portion by itself reduced more than 60% of the NO.