Study On Desulfurization Wastewater Treatment Technology Of Coal-fired Power Plant

Emission reduction,reuse and zero emission of wastewater are important mission of power plants,especially after the implementation of the Action Plan for Prevention and Treament of Water Pollution.At present,more than 90%of coal-fired power plants in China use limestone-gypsum wet flue gas desulfurization technology(FGD).The composition of desulfurization wastewater in FGD is complex and there are many kinds of pollutants.Desulfurization wastewater is one of the most difficult to treat in power plants.With the improvement of environmental protection standards,zero emission treatment of desulfurization wastewater is an urgent problem to be solved at present and also a hot research topic in the field of zero emission at home and abroad.In this paper,the research objective was to optimize the treatment process of desulfurization wastewater from a coal-fired power plant.The water quality of desulfurization wastewater from five typical coal-fired power plants(A,B,C,D,E)was analyzed.The factors affecting the removal of calcium and magnesium ions from desulfurization wastewater by lime-Na2CO3 were studied by single factor optimization experiment.The factors of influencing settling characteristics of magnesium hydroxide were investigated.Based on the lime-Na2CO3,the effects of lime-CO2-Na2CO3 on removing calcium and magnesium ions in desulfurization wastewater were studied.The phase equilibrium of pseudo-ternary system(NaCl-Na2SO4-H2O)in desulfurization wastewater at 323.15-373.15K was studied.The desulfurization wastewaters of three different[NaCl]/[Na2SO4]ratios were selected for evaporation crystallization at 363.15K.The economic performance and risk analysis of desulfurization wastewater treatment process from coal-fired power plants were analyzed.The results are as follows:(1)Water quality analysis showed that:The pH of desulfurization wastewater from five coal-fired power plant was at 5.5～6.9.The content of COD was between 150～1000 mg/L.The salt content was 10000～30000 mg/L.The concentration of Mg2+was 600～5500 mg/L,The concentration of Ca2+was 700～2500 mg/L.Scaling tendency of calcium carbonate,calcium sulfate and magnesium carbonate were serious.There were also a large amount of Na+,Cl-,SO42-and NO3-.And the content of desulfurization wastewater of each power plant was quite different.(2)Under the determined experimental conditions,when the removal rate of calcium and magnesium ions in the desulfurization wastewater reached more than 99%,the dosage of lime was 7.67 g/L and Na2CO3 was 8.60 g/L.When the dosage of PAM flocculant was 300 mg/L,the settling rate of magnesium hydroxide reached the maximum.Compared with the magnesium hydroxide precipitation generated without adding crystal seeds and flocculant,the sedimentation rate can be increased by 13.9%.Adopting CO2 partially to replace Na2CO3,when the removal rates of Ca2+were both 99%,the dosage of Na2CO3 can reduce by 2.15 g/L comparing with the traditional lime-Na2CO3.The dosage of Na2CO3 can reduce by 25%per liter of wastewater treatment.(3)In this work,after pretreatment with lime-CO2-Na2CO3,the phase equilibrium of pseudo-ternary system(NaCl-Na2SO4-H2O)in desulfurization wastewater was studied.With the temperature increasing from(323.15 to 373.15)K,the crystalline region of Na2SO4 became bigger while the crystalline region of NaCl and the co-saturated region of NaCl and Na2SO4 became smaller.At 323.15K and 373.15K,the solubility of NaCl in pure water and desulfurization wastewater was basically same.The solubility of Na2SO4 in desulfurization wastewater was greater than that in pure water,especially at low temperature.The ratio of[NaCl]/[Na2SO4]and the temperature in the desulfurization wastewater had a great influence on the product types of evaporation crystallization.At 363.15K,"when[NaCl]/[Na2SO4]=1 in the desulfurization wastewater,Na2SO4 crystals were obtained by separating salt.When[NaCl]/[Na2SO4]=5 in the desulfurization wastewater,NaCl and Na2SO4 precipitated out crystallization successively,which was difficult to achieve salt separation.When[NaCl]/[Na2SO4]=10 in the desulfurization wastewater,NaCl crystals were obtained by separating salt.(4)The economic performance of desulfurization wastewater treatment process was analyzed using the cash flow technology and Monte-Carlo Simulation.It was concluded that the power generation subsidy should be 0.0024 CNY/kWh to make the project economically feasible.On this basis,Monte-Carlo Simulation showed that the probability of the internal rate of return of the project greater than 4%was 98.1%,and the probability of greater than 8%was 61.1%.The probability of the project reaching the lowest yield rate of drainage project was relatively high,and the financial risk of the project was relatively low.