Selective Catalytic R E D U C T I O N of N I T R O G E N Oxide with a M M O N I a in a Novel Reactor

-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 En#ng Found. 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 U, 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 NO t 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 x 0.10 m 2 at the bottom to 0.25 x 0.25 m 2 at the top (apex angle: 4.7°); 1.7 mm diameter porous silica spheres containing V2Os/TiO 2 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 ma/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. INTRODUCTION The floating gas-sol id fluidized bed (FGSFB) has already been in t roduced in an earlier publ icat ion (Kwant et al., 1992). Gas and fluidized spherical particles are contacted in a tapered column in which grid inserts prevent the format ion of large heterogeneities and of severe solids-circulation flows. The gas velocity at the ent rance of the column is higher than the single particle terminal falling velocity U, (typically 5 -10 times Ut), yielding a type of fast fluidized regime at the bo t tom of the bed with relatively few particles dis tr ibuted homogeneously over the lowest intergrid compartments_ Towards the top of the tapered column the superficial gas velocity decreases gradually, which may result in the .bubbling fluidization regime near the top of the bed or, ultimately, in a fixed bed. The F G S F B can be applied for convers ion of gas phase reactants over a solid catalyst. A good example *To whom correspondence should be addressed. of such a conversion process is the removal of ni t rogen oxides (NOx), which are present in flue gas from coal fired boilers used for electric power product ion, by the selective catalytic reduction (SCR) process. This opera t ion is carried out by mixing flue gas and a m m o n i a after which the mixture is contacted with a vanad ia / t i t an ia conta in ing catalyst. It was reported (Kwant, 1994) that for a highly selective V2Os/TiO 2 catalyst on a SiO2 carrier, the convers ion process proceeds according to the following overall reaction equation: 4 N O + 4NH3 + 02 ~ 4N2 + 6 H 2 0 . This reaction is fur ther referred to as the SCR reaction. The flue gas from a coal fired boi ler has a temperature between 573 and 673 K, and is typically loaded with 1 0 2 0 g / m 3 fine fly ash particles ( < 100/am). Var ia t ions (up to 4 0 ~ ) in the flue gas flow rate occur quite frequently (roughly four times a day) due to the usual f luctuations in electricity demand. Besides, the instal lat ion is turned off and