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Recent research determined that the conventional flue gas desulfurization (FGD) wastewater zero liquid discharge(ZLD)processes have many limitations, such as long process, high investment, high operating cost, and maintenance. In this experimental study, a FGD wastewater was evaporated using the exhausted heat from the flue gas in a 330 MW coal-fired power plant. The numerical simulation and engineering practice results were compared, and the key operating parameters were obtained, such as boiler load conditions, the ratio number of droplets captured by the flue wall to the total number of droplets, optimization of atomizing nozzle layout, the atomization droplet diameter, flue gas enthalpy value, accurate calculation on the change of flue gas, and fly ash characteristics after the injection of FGD wastewater. We found that under different boiler load conditions, the higher temperature and the faster the speed of the flue gas, the less time it takes for the complete evaporation of the wastewater droplets. Different amounts of FGD wastewater can be completely dried, the properties of fly ash do not change much, and no negative impacts were found on the downstream process. Moreover, it was found that when the optimum atomization cone angle of a single flue structure was 65°, the size of vortex inversely proportional related to the distance between sprayer and the wall of flue duct, that was conducive to the continuous diffusion of the local droplets in the nozzle region to other regions. This wastewater evaporation treatment method is a feasible technology for ZLD of FGD wastewater with characteristics of short process, lower investment, low operating cost, and less maintenance than conventional membrane method for wastewater reduction and evaporative crystallization system.
Coal-fired thermal power generation predominates in china, with 65.56% electricity capacity generated by coal-fired power in the year of 2016. In the process of coal combustion, sulfur compounds in coal will form sulfur containing flue gas, and the emission without proper treatment will cause an air pollution. Therefore, the flue gas desulfurization (FGD) of coal-fired power plants is a necessary technique which can control the source of air pollution. FGD technology is widely used due to an efficiency above 95%. High concentration of salt containing wastewater is produced during the desulfurization process. The typical FGD wastewater contains heavy metal ions, high turbidity, high hardness, high chlorides, high sulfate, and a low pH value, shown in Table 1. The conventional treatment method for this wastewater is by chemical precipitation. Heavy metal ions in wastewater are precipitated by chemicals addition into the wastewater, however, there are many drawbacks for this conventional process, such as high operating cost because of the large amount of chemical agent needed, high maintenance cost, difficulty in recycling, and harmful elements. With the increasing environmental protection restrictions, the zero liquid discharge (ZLD) for FGD wastewater has become the focus of pollution prevention and control methods in coal-fired power plants.
Normally, ZLD system for FGD wastewater include two-stage softening, pretreatment system for reverse osmosis(BWRO, SWRO or DTRO) system, Forward Osmosis(FO) membrane process and crystallizer processes. These papers provided some cases and application results for these kinds of treating methods, such as the OASYS FO membrane system which was applied in ChangXing power plant,China. These processes are used to obtain 40–60% wastewater recovery and remove the Ca 2+, Mg 2+, heavy metals, and SO 4 2− in the wastewater. These systems also need to supply large amount of additional energy and chemical agents for operation, large investment, and high operating cost. Additionally, the process of the whole system has high manpower and maintenance requirements. FGD wastewater evaporation by flue gas technology from the coal-fired power plant was introduced by some researchers. It was found that if the evaporation is incomplete, it may cause corrosion to flue duct and downstream devices because of acidic FGD wastewater. Quantitative relation between the water evaporation rate and various factors have not been determined yet, and dynamics model with high fitting degree is still rare. These problems constrained the theoretical research and commercial application for this technology. A thermogravimetric analyzer (TGA) with differential scanning calorimetry (DSC) fuction and scanning electron microscope (SEM) were used to evaporate to a droplet and observe the morphology of the crystals. It was found that the difference of the temperature and the concentration leads to a higher crystallization rate at higher temperature increasing rate. Evaporation rate per unit volume is relatively high at low volume of the droplet. FGD wastewater was sprayed into the flue duct in a coal-fired power plant, and the wastewater contacted with the high temperature (120℃–150℃) flue gas. When the FGD wastewater was evaporated, the residual solid and salt in the wastewater would be captured by the downstream dedusting system (ESP, Electrical Static Precipitator). Compared with the conventional FGD wastewater ZLD processes, no additional energy is needed in this system, because the exhausted heat of the flue gas is used as the energy source to evaporate the wastewater. Although there are many previous studies documenting leaching characterization, evaporation effectiveness and transient evaporation, there are few studies devoted to the mechanism, the degree of consistency between theory, and engineering application of this technology.
The key factor of flue gas spray evaporation technology is to evaporate the volume of FGD wastewater within a certain time, with no negative effect on the flue structure and downstream process. Therefore, the tiny droplets formed by FGD wastewater should be completely vaporized as soon as possible after being sprayed into the flue duct. Otherwise, if the droplets are not completely evaporated, the flue duct and downstream ESP will be corroded by the wastewater, because the FGD wastewater is acidic. In this paper, the evaporation processes of FGD wastewater droplets group were studied by numerical simulation software. Different amount of FGD wastewater were spray into the flue duct of a coal-fired power plant. The relation between simulation results by software and the engineering application results were studied. The influences of various parameters on the wastewater droplets evaporative characteristics, such as boiler load conditions, the ratio number of droplets captured by the flue wall to the total number of droplets, atomization cone angle, volume of wastewater droplet, temperature of flue gas, and fly ash characteristics after the injection of FGD wastewater were evaluated. In order to verify the numerical model and provide guidelines for future commercial applications, the experiment which is base on the numerical simulation results and the engineering measured data was performed. This paper presents the numerical simulation methods, and then compares the engineering results with these simulation results. The main goal is to achieve a guideline for the ZLD evaporation process of FGD wastewater.
In this study, 3 two-fluid (compressed air and wastewater) nozzles were uniformly arranged in the vertical section of flue duct in a 330 MW coal-fired power plant. The nozzles were set in the elevation of 13.8 m. The FGD wastewater was pretreated by the chemical precipitation, the effluents stored in a buffer tank and then sent to the wastewater dispenser by the spray feed pump. The water dispenser distributes the water to the three atomization nozzles. An air purifier removes oil and small
The evaporation effect of droplets is mainly determined by the following parameters: flue gas phase temperature and transport characteristics, liquid phase temperature and velocity of movement, heat and mass transfer efficiency of gas-liquid two phases. According to the boundary conditions given in Table 2, the load of 50%, 75% and 100% were analyzed, the three kinds of gas phase conditions with different flue gas temperature (120 ℃, 125 ℃, 128.9 ℃) and flue velocity (9.19 m·s−1、11.56 m·s−1
In this paper, from the perspective of theory and engineering practice, the ZLD technology of FGD in coal-fired power plants was studied. Several major conclusions can be summarized by this method.
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