Application of SNCR/SCR Combined process for effective operation of SCR Process

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This paper have examined the optimum combination of SNCR and SCR by varying SNCR injection temperature and NSR ratio along with SCR space velocity. NOx reduction experiments using a SNCR/SCR combined process have been conducted in simple NO/NH<TEX>$_3$</TEX>/O<TEX>$_2$</TEX> gas mixtures. Total gas flow rate was kept constant 4 liter/min throughout the SNCR and SCR reactors, where initial NOx concentration was 500 ppm in the presence of 5% O<TEX>$_2$</TEX>. Commercial catalyst, sulfated V<TEX>$_2$</TEX>O<TEX>$\_$</TEX>5/-WO<TEX>$_3$</TEX>/TiO<TEX>$_2$</TEX>, was used for SCR NOx reduction. The residence time and space velocity were around 1.67 sec, 2,400 h<TEX>$\^$</TEX>-1/ and 6,000 h<TEX>$\^$</TEX>-1/ in the SNCR and SCR reactors, respectively. SNCR NOx reduction effectively occurred in a temperature window of 900-950<TEX>$^{\circ}C$</TEX>. About 88% NOx reduction was achieved with an optimum temperature of 950<TEX>$^{\circ}C$</TEX> and NSR=1.5. SCR NOx reduction using commercial V<TEX>$_2$</TEX>O<TEX>$\_$</TEX>5/-WO<TEX>$_3$</TEX>-SO<TEX>$_4$</TEX>/TiO<TEX>$_2$</TEX> catalyst occurred in a temperature window of 200-450<TEX>$^{\circ}C$</TEX> 80-98% NOxreduction was possible with SV=2400 h<TEX>$\^$</TEX>-1/ and a molar ratio of 1.0-2.0. A SNCR/SCR(SV=6000 h<TEX>$\^$</TEX>-1/) combined process has shown same NOx reduction compared with a stand-alone SCR(SV=2400 h<TEX>$\^$</TEX>-1/) unit process of 98% NOx reduction. The NH<TEX>$_3$</TEX>-based chemical could routinely achieve SNCR/SCR combined process total NOx reductions of 98% with less than 5 ppm NH<TEX>$_3$</TEX> slip at NSR ranging from about 1.5 to 2.0, SNCR temperature of 900<TEX>$^{\circ}C$</TEX>-950<TEX>$^{\circ}C$</TEX>, and SCR space velocity of 6000 h<TEX>$\^$</TEX>-1/. Particularly, more than 98% NOx reduction was possible using the combined process under the conditions of T<TEX>$\_$</TEX>SNCR/=950<TEX>$^{\circ}C$</TEX>, T<TEX>$\_$</TEX>SCR/=350<TEX>$^{\circ}C$</TEX>, 5% O<TEX>$_2$</TEX>, SV=6000 h<TEX>$\^$</TEX>-1/ and NH<TEX>$_3$</TEX>/NOx=1.5. A catalyst volume was about three times reduced by SNCR/SCR combined process compared with SCR process under the same controlled conditions.