Two-stage NOx removal using high temperature urea SNCR and low temperature secondary additive injection

Abstract

In order to control Nitric Oxide (NO) in flue gases emanating from coal fired power plants, one possible way of using selective non-catalytic reduction (SNCR), at lower molar ratios and minimum ammonia slip, may be the combination of SNCR processes with a back-end NO oxidation process. At low molar ratios, residual NO that escapes from the high temperature urea SNCR (900°C to 1200°C) process, may be oxidized to NO2 by compounds containing (−OH) group injected at low temperature secondary stage and this NO2 can then be removed by scrubbing. In present study, low temperature (500°C to 800°C) gas phase oxidation of NO using compounds containing (OH) group such as methanol is initially investigated. Subsequently, experiments were performed by combining this low temperature gas phase oxidation at the back end of high temperature urea SNCR in a two-stage experimental setup. The results of initial (−OH) additive injections in flue gases revealed that the additive investigated has an optimum temperature window for efficient NO to NO2 conversion which ranges from 600 to 750°C. The optimum reaction time within this temperature window was found to be between 0.4 to 0.6 seconds. Investigations with two-stage experimental setup have shown that significant improvement in primary stage NOx reduction efficiency can be achieved by low temperature injection of additives in the second stage. A 50% to 150% increase in the overall removal was achieved using methanol solution in second stage with urea in primary stage. Through a selection of primary and secondary additives, with due considerations to their optimum temperature ranges, the efficiency could be increased to 97%. Low temperature NO removal, increase in efficiency and the low optimum amount of secondary additives (M=1.0) reflected the attractive features of two-stage NO removal technique.