Selective catalytic reduction of nitrogen oxide with ammonia in a novel reactor called the floating gas-solid fluidized bed

摘要

The floating gas-solid fluidized bed (FGSFB) is a new type of gas-solid contacting device described earlier by Kwant et al. (Fluidization VII, Proc. 7th Engng Foud. Conf. on Fluidization, Brisbane, May, 1992). It is a tapered column provided with several coarse grids, in which catalyst particles are fluidized by a gas at a velocity decreasing with height from a value larger than the single particle terminal falling velocity U1 at the bottom to a value still higher than the minimum fluidization velocity at the top. This paper reports on the results of experiments concerning the application of the FGSFB as a reactor for the removal of NOx from flue gas. The selective catalytic reduction of NO with ammonia has been investigated in a column of 2 m height and cross-sectional area varying from 0.10 × 0.10 m2 at the bottom to 0.25 × 0.25 m2 at the top (apex angle: 4.7°); 1.7 mm diameter porous silica spheres containing V2O5/TiO2 were used as a catalyst material and fluidized by a simulated flue gas (at 523, 573, 623 or 673 K) from a natural gas burner. This gas contained approximately 500 vppm NO and was fed at flow rates varying from 200 to 600 m3/h. The results have been compared with those obtained in fixed beds operated at the same conditions. Almost the same NO conversion levels were found at equal space velocities; it was therefore concluded that the contacting efficiency corresponds to that in an ideally contacted (high Reynolds numbers) plug flow system. This means that there is no gas bypassing the solids in the FGSFB. The NO conversion behaviour of the FGSFB reactor was also simulated with a plug flow reactor model using simplified kinetics and an empirical correlation for the axial particle distribution. The S-shaped form of calculated curves, representing the NO conversion degree as a function of the axial position for different values of the reaction rate constant, are characteristic for a FGSFB: the slope increase in the bottom part of the reactor is caused by an increasing conversion rate due to a positive particle concentration gradient in the upwards direction. The decreasing slope beyond the inflection points corresponds of course to the depletion of reactants which becomes dominant upon approaching complete conversion.