| More recently, semi-dry flue gas desulfurization techniques using finely divided lime slurry as sorbents for SO2 have drawn increasing interest. These semi-dry FGD techniques are projected to have lower capital and operating costs than the conventional wet FGD techniques. Further more, the waste product is dry powder rather than wet sludge. This offers potential advantages in the areas of waste handling and disposal. While there is an obvious economic benefit for these systems in terms of lowering of capital cost requirements, the higher operating costs incurred due to poor sorbent utilization may make them uneconomical for application to medium or high-sulfur coal application. With these factors in mind, current technologies of semi-dry flue gas desulfurization are reviewed while focus on the removal efficiency and utilization of calcium-based sorbents. The technologies include spray drying, Circulating Fluidized Bed, Gas Suspend Absorption and New Integrated Desulfurization. Since particulate collection device also have some effect on sulfur removal, these subjects are also reviewed. The preceding literature review points two major points that hinder the adoption of semi-dry scrubbing technologies: poor sorbent utilization and low SC>2 removals. Anything that can be done to overcome these limitations has the potential to improve the economic feasibility of semi-dry scrubbing processes. On the bases of great number of international and national references, one novel semi-dry flue gas desulfurization, which is new in China and suitable for the current situation in China, is presented in this paper. A bench scale test facility was designed and built. The process features simple design, operational reliability, low maintenance costs and removal of heavy metal.At the same time, its key components-atomizer has been researched in detail. The distribution characteristic of droplet size in space at different deepness of lime slurry and the quality rate of gas and liquid has been measured by the advanced FAM laser particle size-measuring instrument and analyzed to conclude the regularity of atomization.An experimental study has been performed on the semi-dry FGD. The influence of operation parameters on SOz removal has been investigated. The approach to saturation temperature and Ca/S mole ratio influence SCh removal strongly. The optimum operating parameters have been got. The optimum approach to saturation temperature is about 10癈, the optimum CA/S ratio 1.5, the optimum recycle ratio 5, and the optimum humidified water is about 30 percent of total water evaporated.The effects of additives were studied directly in the semi-dry FGD process. Hygroscopic additives had significant effects on improving lime utilization and SC>2 removal; Buffer additives had little or no effect. The mechanisms responsible for improvements in performance with the use of additives were investigated and discussed. Experiments showed that over 90% removal of SCh could be achieved in this semi-dry FGD process with the appropriate use of additives and operating conditions.An integral mathematical model was put forward to predict semi-dry FGD tower and fabric filter. The model of FGD tower was based on film theory and treated the atomized slurry droplet as asphere with the fluid phase uniformly distributed around the discrete individual sorbent particle. Both the gas phase mass transfer and liquid phase mass transfer coefficients were included in the tower model along with a relationship to predict the resistance to lime dissolution. This approach was said to allow predictions of the mass transfer coefficients and to model the enhancement due to increasing solid concentration as evaporation proceeds. It was investigated that there are different control mechanisms in different steps of the process. Efficiency predictions using this model have been compared with the test data taken from the pilot semi-dry/fabric filter facility. It can be seen that the calculated results from the model agreed well with the experimental results. |
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