Experimental Study On NO And N₂O Removal In CFB Boiler

Although circulating fluidized bed (CFB) boiler is a coal clean combustion technology, the concentration of nitrogen oxides (NO and N₂O) in its exhaust flue gas is still too high to meet the stringent emission standard in the future. Based on the understanding of SNCR reaction mechanisms, the reaction path of ammonia (NH₃) over circulating ash was explored. Under CFB boiler condition, the reactions were categorized into four groups as the oxidation of NH₃, the reduction-oxidation reaction between NH₃ and NO, the decomposition of N₂O and the reduction of N₂O by NH₃. In this study, an experimental system with a premixed fix bed reactor was set up and a series of experiments were conducted on the four groups of reactions over circulating ashes and their major components. Corresponding theoretical analysis was performed and the catalytic effect of the circulating ash was assessed.It was found that the circulating ashes, with large specific surface area and rich in Fe₂O₃, Fe3O4 and CaO, behaved as a catalyst for both the reactions of NH₃ oxidization and the reactions of NO reduction with NH₃. Compared with quartz sand, the circulating ashes would result a higher de-NO efficiency at lower temperatures and a remarkable lower concentration of NH₃ slip in exhaust gas. Over the circulating ashes, the optimal temperature window for NO removal is 950K3/NO molar ratio m is about 1.0. Experiments also found that the presence of bed materials in the reactor would slightly accelerate the formation of N₂O.It was well known that N₂O concentration could be decreased by thermal decomposition under high temperature or through reducing reaction. However, the relation between the properties of circulating ash and decomposition reaction rate was unknown. In this study, the chemical kinetics parameters for N₂O heterogeneous catalytic decomposition were measured while homogenous decomposition was deducted. It was found that N₂O decomposition reaction rate was mainly determined by the composition of bed materials and their specific surface area. A model based on the specific surface-area-weighted kinetic data of major components was developed to predict the catalytic decomposition of N₂O over circulating ash. The predictions agreed with the experimental data with a minor discrepancy. It was also found that under the CFB boiler temperature, the injection of NH₃ would promote the reduction of N₂O. Furthermore, over the circulating ash, the reduction of N₂O with NH₃ was catalyzed even at a lower temperature and much less amount of NH₃ slip in exhaust gas. However, experiments showed that the presence of O2, CO2 and H2O vapor could remarkably reduce the efficiency for N₂O removal. Thus, with the consideration of the catalytic oxidation of NH₃ over circulating ash, the optimal temperature for N₂O removal by NH₃ injection with O2 presence were suggested to be 1025K2O removal under certain conditions. The cost-effective ultra-low nitrogen oxides emission without using extra catalyst for CFB boilers is feasible.