Experimental And Pilot-scale Studies On SO₂ Removal With Ammonia-based WFGD

From the perspective of energy resource condition, China's energy structure is coal-dominated. There are many kinds of pollutions produced by coal combustion. Because SO2 and NOx have serious influences on the environment, they have been the important goals of air pollution control in the world. Ammonia-based wet desulfurization process has drawn an increasing attention recently due to its advantages such as high desulfurization efficiency, easy utilization of by-products. Aimed at the increasing demand of Ammonia-based wet desulfurization process, the studies carried out systemically on the related mechanisms, and obtained the complete process applied to industrial production through a pilot experiment.In order to investigate the characteristics of the reaction between ammonium sulfite, the main desulfurizing solution, and the flue-gas-contained sulfur dioxide during the process of ammonia-based wet flue gas desulfurization in a power plant, the gas-liquid absorption reaction between sulfur dioxide and an ammonium sulfite solution was studied in a stirred tank reactor and a bench scale scrubber. Under the experimental condition of a stirred tank, the results indicate that the absorption of sulfur dioxide is controlled by both the gas-and liquid-films when the ammonium sulfite concentration is lower than 0.05 mol/L, and mainly by the gas-film at higher concentrations. In the latter case, the reaction rates are found to be zero-order with respect to the concentration of ammonium sulfite. The absorption rates of sulfur dioxide increase as the concentration of sulfur dioxide in inlet gas and the temperature increase. The reaction rate is of 0.6th-order with respect to the concentration of sulfur dioxide. The absorption reactions of NO2 into aqueous solution of (NH4)2SO3 were also investigated in a stirred tank reactor with a plane gas-liquid interface. It was also found that NO2 absorption rates was enhanced by the increasing concentration of (NH4)2SO3 but nearly remained constant if the concentration is greater than 0.1 mol/L. The absorption rates also increased with the increasing of the (NH4)2SO3 concentration, reaction temperature and the initial inlet concentration of NO2, but decreased as the oxygen concentration increased. The reaction rate is of first-order with respect to the concentration of sulfur dioxide, and of zero-order to the (NH4)2SO3 concentration in the solution when the concentration is greater than 0.1 mol/L.The process parameters were studied on the influence in desulfurization efficiency under the cold-state experimental condition of the scrubber. The experimental results indicate that the desulfurization efficiency is mainly controlled by pH value, and increase with the increasing pH value, flue gas velocity, and liquid-gas ratio. The increasing concentration of inlet SO2 decreases the desulfurization efficiency. Based on the two-film theory of gas-liquid absorption, the mathematic model of SO2 removal was built up with the bentch-scale scrubber as the physiacal model. The SO2 removal process in ammonia-based WFGD was simulated with this model. It is showed that the caculated results agree well with the experimental results.In the wet process, the presence of iron would catalyze the oxidation of sulfite. Iron leaching from fly ash has been investigated under simulated ammonia-based wet flue gas desulfurization conditions to determine the effects of reaction temperature, initial pH value, liquid-to-solid ratio and vibration frequency. The experimental results indicated that the ferric ion concentration in solution increased with the increase of temperature and vibration frequency, but increased with the decrease of pH value and liquid-to-solid ratio. The kinetics of iron leaching can be expressed as diffusion combined with a surface chemical reaction model. The reaction path can be described by the equation:1-(1-a)1/3=kt. The apparent activation energy was estimated to be about 20.01 kJ/mol. Such a value of the activation energy indicates that the leaching of iron under experimental condition is controlled by both chemical reaction and diffusion.Based on the results of the mechanism tests, a novel process of ammonia-bisulfite WFGD was developed. The pilot-scale experimental system was designed for 3000 Nm3/h of flue gas flow. The experimental results indicate that pH value is the dominant factor influencing desulphurization efficiency. The desulphurization efficiency increases with increasing pH value and liquor gas ratio, while decreases with increasing solution density and flue gas velocity. The oxidation of ammonium sulfite decreases with increasing pH value, salt concentration in solution. The increasing concentration of oxygen content in flue gas, reaction temperature and iron concentration in solution can also aggravate the oxidation of sulfite. Based on the operation results of the pilot-scale system, some process problems were solved. Therefore, the process flow and the parameters were determined. The industrial process was designed, and its Economic analysis was made.