A New Scheme for Synergetic Removal of NH3 and SO3 and Particulate Matter in the Flue Gas of Coal-Fired Boiler

Abstract

In China, coal-fired boilers generally utilize the selective catalytic reduction (SCR) denitration technology to achieve low NOx emission. The SCR catalyst will inevitably convert some of the SO2 in the flue gas into SO3. NH3 escaping from the SCR denitration system can react with SO3 generated in the flue gas to form (NH4)2SO4 and NH4HSO4. NH4HSO4 is highly hygroscopic, viscous and corrosive and easy to induce ash deposition on the heat transfer surfaces at the temperature below its dew point. Since the condensation temperature of NH4HSO4 is just in the operating temperature range of the rotary air preheater (RAPH), the RAPH suffered more serious sticky ash deposition problems seriously impacting the economical and safe operation of the boiler. In this study, a new scheme was proposed for synergetic removal of NH3 slipped from SCR, SO3 and particulate matters in flue gas. The flue gas outlet temperature of the RAPH is raised to the dew point temperature of NH4HSO4 by reducing the heat transfer area, and then NH3 and SO3 in the flue gas can be used as the flue gas conditioning agent to modify the adhesion property and specific resistance of the particles. Therefore, by using this new scheme, not only the particulate matters but SO3 and NH3 escaped from SCR in the flue gas can be removed in the electrostatic precipitator, and the ash deposition corrosion problem on the low temperature heating surface can be greatly alleviated. The key to this new system is the proper parameters design to achieve sufficient adsorption for NH3 and SO3 and the appropriate agglomeration of particulate matters. Therefore, the adsorption and agglomeration characteristics of ash particles were experimentally studied. The results showed that the agglomeration of ash particles can be significantly enhanced by the adsorption reaction, which can increase the mean size of particle matters. The molar ratio between the absorbed N and S contents always maintained in the range of 1.2–1.8. The adsorption of the SO3 and NH3 would interact with the ash particles leading to the morphology changes. Besides, the adsorption of NH3 and SO3 could strengthen the agglomeration of fine ash particles, and produces highly cohesive and relatively large particles, resulting in high collection efficiency due to the reduced rapping losses and re-entrainment. The experimental results initially proved that the proposed system would be a promising approach for the alleviation of the sticky ash deposition and for the synergetic removal of NH3 and SO3 and particulate matters. Further more detailed study would be made to examine the effects of the ratio of NH3 and SO3 in the gas, the adsorption temperature and the particle size on the adsorption rates and adsorption ratio of NH3 and SO3, and the specific resistance of the particles, and the particle agglomeration, in order to obtain proper operating parameters for the synergetic removal system.KeywordsParticulate matterAdsorptionAgglomerationSynergetic removalParameter design