Study On Evaporation Characteristics Of Desulphurization Wastewater Under Low-temperature Flue Gas Conditions

The application of wet desulphurizing process accounts for 93%in thermal power plant.In order to prevent the corrosion of the equipment inside the desulfurizer and keep the quality of the gypsum manufactured by it,the desulfurization waste water in the desulfurizer need to be discharged regularly.The desulfurization waste water has the characteristics of poor water quality,high heavy metals and high salt content.The chloride with higher concentration in desulfurization waste water can't be treated by traditional "trigeminy box" treating method,which causes damage to the environment.Therefore,it is necessary to carry out zero emission treatment on desulfurization waste water under the condition that the environmental protection policy is becoming stricter.At present,although the zero emission discharge processes of desulfurization waste water can achieve the goal of zero discharge,they all have the disadvantages of large initial investment,high operating cost,large floor space and easy scaling.If the concentrated pretreatment can be carried out on desulfurization wastewater,the amount of treated wastewater in the subsequent zero emissions of desulfurization wastewater will be decreased effectively,the operation cost will be reduced and the stable operation will be guaranteed.Much smoke heat is wasted in the desulphurizing process for the temperature of smoke will drop rapidly from 120 ? to around 50 ? before desulfurization in coal-fired power plant.This paper intended to set up a falling film evaporator in the inlet flue in front the desulfurization tower to concentrate and evaporate the desulfurization wastewater with 120 ? smoke.Which means it can utilize waste heat of smoke and effectively concentrate the desulfurization waste water.In this regard,the differential governing equation has been established to simulate the falling film evaporation process of desulfurization wastewater.It also studied the heat and mass transfer characteristics in the gas-liquid two-phase falling film countercurrent evaporation process in this process,and analyzed the effect of different physical properties of smoke and solution on the evaporation rate.And obtained the conclusion that the temperature of the smoke,the Reynolds number of the smoke and the Reynolds number of the solution promote the evaporation under certain conditions,and the humidity of the smoke and the concentration of the solution have inhibition on the evaporation.We also analyzed the characteristics of heat transfer of smoke in the process are analyzed and proposed that preheating the solution before evaporation can effectively improve the flue gas evaporation rate.The falling film evaporation test bench that can work stably in high smoke and flux environment has been designed and set up according to principle of similar modeling to guarantee that the solution liquid film will not rupture under the high smoke Reynolds number.Meanwhile,the paper also designed the two evaporation methods of windward falling film and leeward falling film to explore the influence of different factors on the evaporation effect of falling film.The analysis of the experimental and simulation results showed that the influence of the Reynolds number of the smoke,the temperature and humidity of the smoke in the inlet on the evaporation ability of the low-temperature smoke is consistent with the trend of the simulation results.Increasing the temperature and flow rate of the smoke in the inlet and reducing the humidity of the smoke in the inlet can promote evaporation.When the temperature of the smoke in the inlet increases,the effect of enhancing heat transfer is obvious.When the humidity of the smoke in the inlet increases,the mass transfer process occupies the dominant position of evaporation.The increasing of Raleigh number of solution can promote the evaporation of the solution,but when the falling film enters the flow state of the turbulent flow,the liquid film will rupture and scatter.Meanwhile,the effect of falling film evaporation on the windward side is better than that on the leeward,which indicates that it directly touches the mass transfer in the falling film evaporation under low temperature gas phase and plays a major role.There are three processes(heating process,stable evaporation process and accelerated evaporation process)in the evaporation process of solution.If the solution is preheated in advance,the evaporation efficiency can be effectively improved.When the gas phase is in a low temperature condition and the solution does not reach the state of crystallization,the chloride ions hardly evaporate.We obtained through the experiment that the evaporation rate of gas-liquid two-phase countercurrent falling film evaporation in per unit time is 2.86 kg/(m2·h)to 9.98 kg/(m2·h)in low temperature smoke,and the mass transfer coefficient was between 0.063m/s?0.166m/s in each working condition.