Effects of SO2 and H2O on the ash deposition and NO emission during oxy-fuel combustion of high-alkali coal

Abstract

The oxy-fuel combustion is promising to realize the large-scale CO2 capture and effective reduction of nitric oxides. The recirculation of flue gas in oxy-fuel combustion can lead to high concentrations of SO2 and H2O, which probably affects the ash deposition of high-alkali coal because SO2 and H2O are important for the transformation of alkali and alkaline earth metals (AAEMs). In the present work, the effects of SO2 and H2O in recycled flue gas on the ash deposition and NO emission during the oxy-fuel combustion of high-alkali coal were experimentally evaluated in a vertical two-stage furnace system. The conversion ratio of fuel-nitrogen to NO (CNO) and ash deposition efficiency were measured, and then the ash deposits were subjected to a couple of tests for further analyses. The experimental results show that SO2 helps to reduce the NO emission and its impact on ash deposition depends heavily on its concentration. In the case with modest SO2 addition, the sulfations of sodium and calcium are enhanced to some extent. Consequently, the amount of small sticky particles (NaCl and sodium aluminosilicates) that attached to large particles decreases, and the large particles mainly made of calcium aluminosilicates become smaller and less sticky. The newly generated Na2SO4 and CaSO4 are insufficient to overcome the inhibiting effects, thus the ash deposition efficiency decreases. Under the condition of excessive SO2 addition, the sulfations are further promoted. The small particles rich in Na2SO4 and CaSO4 are substantially more, which can not only serve as the initial ash layer on the heating surface, but also strengthen the stickiness of large particles by attaching to their surfaces. Therefore, the ash deposition efficiency increases·H2O can reduce the NO emission, while it aggravates the ash deposition because H2O enhances the sulfation.