NOx Reduction Using Pilot-Scale SCR Under Oxyfuel Conditions

Abstract

This paper presents a comprehensive experimental study of selective catalytic reduction (SCR) of nitrogen oxides formed in the oxyfuel combustion of lignite coal in a 500 kWth pilot-scale bubbling fluidized bed (BFB) boiler using the V2O5 over TiO2 honeycomb catalyst and the ammonia as the reducing agent. The study also compares the performance of the SCR between oxyfuel and air combustion modes. The SCR unit is placed in the tail-end position of the flue gas processing scheme after the cyclone particle separator. Experiments were realized both in air- and oxyfuel combustion regime, evaluating the impact of relevant process conditions on the NOx reduction. The dependence of the NOx reduction efficiency on the catalyst temperature and on the normalized stoichiometric ratio (NSR) of the reducing agent molar flow to the molar flow of NOx were studied. The efficiency of the NOx reduction was evaluated based on the NOx concentration measured simultaneously before and after the SCR unit. In the off-gas, the concentration of unreacted reducing agent (ammonia slip) was measured as well. For evaluation of real NSR, ammonia and flue gas flow rates were measured during each experiment. The catalyst temperature was regulated within the experiments in the range of 260 to 300 °C and the NSR was chosen in the range of 0 to 1.2. From this experimental setup, the correlations of the NOx reduction efficiency related to the different NSR and catalyst temperature were evaluated. The results show the fundamental information about the differences between air and oxyfuel conditions in a real combustion environment. In general, the NOx conversion efficiency was significantly higher during air combustion. In order to get similar results of maximum NOx conversion as reached during air combustion, it was necessary to increase NSR up to 1.8 and more, however at the cost of significantly increased ammonia slip. Although the SCR performance is lower in oxyfuel combustion, it was possible to decrease NOX under 100 ppmv with NSR about 1 and without any significant ammonia slip, which is important prior to further CO2 treatment.