Research On Resource Utilization Of Flue Gas Desulfurization Ash By Semi-dry Method From Power Plant

Coal is the leading primary energy resource in China, and the vast derect burning of coal leads to environmental problems, such as serious pollution of smoke and SO2and acid rain. Flue gas desulfurization projects have been built by coal-fired power plants in succession to fit the national requirements of energy conservation and emission reduction. Semi-dry desulphurization technology is suitable for the current conditions of China and widely used for the advantages of occupying a smaller area, less investment, no wastewater produced, simple operation and maintenance compared with the wet method. However, the high calcium desulfurization ash (by-product) has become a new solid waste, obtained from semi-dry desulfurization technology, which is different in composition and properties from flyash. The abandoned desulphurization ash will lead to secondary pollution, take up a lot of precious resources of soil, and even affect the normal operation of desulfurization project, if it is not utilized reasonably in time, with the semi-dry desulphurization device being installed and operated continuously by plants all over the country. The treatment and resource utilization of flue gas desulfurization ash by semi-dry method has become an urgent problem.Based on the analysis in composition, content and morphology, this paper seeks the appropriate resource utilization methods for the high calcium desulfurization ash, produced by NID (Novel Integrated Desulphurization) semi-dry flue gas desulfurization technology, which is from a steel company power plant. This paper discusses the feasibility of preparation of light calcium carbonate by calcium from desulfurization ash through choosing appropriate leaching liquid and carbonation agent, studies the influence of relevant factors on leaching calcium and calcium carbonate synthesis separately, optimizes technological parameters, and determines the appropriate technological conditions. The feasibility of treating waste water with the desulfurization ash and the residue of the ash after calcium leaching is also studied.The effects were investigated of factors like concentrations of ammonium chloride solution, leaching time, extent of the excess of ammonium chloride and temperature on leaching rate for calcium and magnesium in the experiment of leaching desulfurization ash. Based on experiments of investigating single factors, respectively, the order of influence importance was determined through orthogonal test as temperature, the concentration of ammonium chloride solution, time and extent of the excess of ammonium chloride. The optimum condition was temperature20℃, concentration of ammonium chloride solution2.0mol·L-1, extent of the excess of ammonium chloride15%and leaching time40min. Under the best condition, the leaching rate of calcium reached82.14%while the leaching rate of magnesium was6.44%.With the leaching liquid of desulfurization ash obtained under the optimum condition of leaching calcium, Calcium carbonate were prepared through liquid-liquid reaction and gas-liquid reaction, with two carbonation agents of (NH4)2CO3and CO2respectively. For the liquid-liquid reaction, influence factors of temperature, concentration of (NH4)2CO3solution, CaCl2/(NH4)2CO3mole ratio and pH were investigated on the quality of CaCO3synthesised. The order of influence importance was determined through orthogonal test as temperature, pH, concentration of (NH4)2CO3solution, CaCl2/(NH4)2CO3mole ratio. The optimum condition was temperature20℃, initial concentration of (NH4)2CO3solution2.5mol·L-1, pH=10, and CaCl2/(NH4)2CO3mole ratio1:1. For the gas-liquid reaction, influence factors were investigated of temperature, stirspeed, flow gas velocity, volume ratio of air to CO2and pH on the quality of CaCO3synthesised. Fixing initial pH, the order of influence importance was decided through orthogonal test as air/CO2volume ratio, stirspeed, temperature and flow gas velocity. The optimum condition was temperature25℃, stirspeed500·min-1, flow gas velocity20mL·min-1and air/CO2volume ratio3:2. The content was analyzed through X-ray diffraction method and the morphology was investigated by scanning electron microscope (SEM) of the calcium carbonates prepared by two processes. Under the best condition, calcium carbonate particles obtained by liquid-liquid reaction were spherical, particle size about2μm, purity98.7%, the white degree97.1; calcium carbonate particles made by gas-liquid reaction were square, particle size about3μm, purity97.6%, the white degree96.8. The desulfurization ash residue after leaching calcium and dry desulfurization ash were used to deal with imitative wastewater of ammonium chloride and the desulfurization ash achieved better effect on denitrogenation. When concentration of imitative wastewater of ammonium chloride was10mg·L-1, under the condition of temperature20℃and stirspeed250r·min-1, the denitrogenation rate reached58.64%. The effects were investigated of the initial concentration of waste leachate, temperature, pH, dosage of desulfurization ash, and concentration of humic acid on the treatment of rubbish leachate by desulfurization ash. The order of importance was confirmed through orthogonal test as diluted times, dosage of desulfurization ash, pH and temperature. The optimum condition was initial leachate diluted200times, temperature40℃, pH=5, adding0.28g desulfurization ash to every25mL wastewater. Under the best condition, the removal rates of ammonia nitrogen, CODCr, chroma and turbidity were20.01%,88.31%,69.72%,78.33%respectively.