Thermal Analysis And Optimization Of Desulfurization Wastewater Flue Gas Evaporation System

Most coal-fired power plants are equipped with a complete flue gas desulfurization system in order to reduce the SO2 pollution caused by coal combustion.The desulfurization method is mainly based on the mature technology and widely used limestone-gypsum wet flue gas desulfurization process,with various types.As one of the products of wet flue gas desulfurization,desulfurization wastewater contains a large amount of pollutants such as heavy metal salts and cannot be discharged directly.With the increasingly stringent national emission standards for pollutants from thermal power plants,achieving "zero discharge"of desulfurization wastewater has become one of the hot research hotspots at the moment.In the "zero discharge" process of desulfurization wastewater,the use of boiler flue gas heat to realize the evaporation and drying of wastewater is one of the processes with better overall performance.The "zero discharge" system of desulfurization waste water and flue gas evaporation and drying is different,and its impact on the thermal economy,equipment reliability and investment of the power plant is also different.Therefore,the dissertation constructs different "zero discharge"systems for the evaporation of desulfurization wastewater,and establishes the corresponding thermal calculation model to carry out simulation calculations in combination with specific projects.The results obtained can provide basis and reference for the technical and economic analysis of the "zero discharge"project of desulfurization wastewater,and have certain theoretical value and practical significance.Firstly,by establishing the energy balance model of wet flue gas desulfurization system,it is concluded that the moisture content of flue gas at the outlet considering the external energy entering into the desulfurization tower is 0.69%higher than that without considering the external energy entering into the desulfurization tower,and the external energy entering can be ignored in the calculation of desulfurization wastewater quantity.Combined with the water balance model of desulfurization system and C1balance model,the situation of different chlorine content in coal-fired power plant is deduced The theoretical calculation method of desulfurization wastewater discharge under different conditions.According to the specific engineering example,the desulfurization wastewater output of a 300MW Coal-fired Unit is obtained QW=3940.87kg/h,which can provide basic data for subsequent calculation of evaporation system.Secondly,the calculation model of saturated wet flue gas moisture content,air preheater outlet flue temperature calculation model,flue gas acid dew point calculation model,and boiler thermal efficiency change calculation model were established,and three different flue gas evaporation and desulfurization wastewater systems were constructed.The thermal calculation model redefines the calculation method for the loss of flue gas evaporative desulfurization wastewater at medium temperature.Combining engineering examples,the unit parameters under different evaporation conditions are calculated.The results show that a 300MW unit uses a low-temperature flue gas concentration system to extract about three times the flue gas volume of a medium-temperature flue gas evaporation system;use the medium-temperature flue gas evaporation system to preheat the air The flue gas temperature at the outlet of the boiler drops by about 4℃,the thermal efficiency of the boiler drops by 0.29%,and the standard coal consumption rate for power generation increases by 1g/(KW·h);the amount of waste water entering the desulfurization in the evaporator in the medium and low temperature series evaporation system increases by 10%,and the boiler thermal efficiency Decrease by 0.0025%,the standard coal consumption rate for power generation increased by 0.02g/(KW-h),and the temperature of the flue gas at the outlet of the air preheater decreased by 0.025℃.For the pre-concentration scheme of desulfurization wastewater,a steam turbine extraction multi-effect evaporative desulfurization wastewater system was constructed.A three-effect evaporative desulfurization wastewater model was built using Aspen plus software,and a 300MW power plant thermal system model was built using Ebsilon software to simulate and calculate the flow,concentration,and thermal economy changes of the power plant after extraction of the desulfurization wastewater under different extraction volumes.The results show that if the salt content of the concentrated liquid exceeds 50%,the required steam volume increases sharply;the salt content exceeds 80%,the economics of the multi-effect evaporation system drastically deteriorates;the heat consumption rate of the multi-effect evaporation desulfurization wastewater system for extraction of steam increases at most 3.5kJ/(KW-h),the standard coal consumption rate for power generation will increase by up to 0.2g/(KW-h).Finally,the paper compares the thermal economy index of the steam turbine extraction multi-effect evaporation system with the flue gas evaporation system.Compared with the medium temperature flue gas evaporation system,the steam extraction multi-effect evaporative desulfurization wastewater system has better thermal economic performance;compared with the flue gas evaporation system arranged in series,the thermal economic performance of the two is not much different,and it needs to be based on economic factors such as investment cost Make comparisons to choose the best solution.