Release and migration of Hg during desulfurization wastewater evaporation process: Whole process evaluation by experimental and theoretical study.

Hot flue gas evaporation technology is an effective strategy for zero liquid discharge of desulfurization wastewater. However, there is a potential risk that heavy metals such as Hg may be released from the wastewater during evaporation, disrupting the original balance of the power plant or even exceeding the Hg emission standard. Wastewater evaporation and Hg release behavior were obtained using a single droplet drying system. At an evaporation temperature of 300 °C, approximately 18.5% of Hg was released in the constant wet-bulb temperature period, and the remaining was released in the following evaporation periods. Furthermore, a fixed-bed experiment, in combination with density functional theory calculations, was used to investigate the possible migration mechanisms of released Hg. The results revealed that high HCl concentration, introduced fly ash, and precipitated evaporation products play a crucial role in the fate of Hg, and 85.3% of Hg finally turned into less harmful particulate-bound Hg. This study provides a new and effective strategy for evaluating the migration process of pollutants in wastewater treatment. Moreover, it will serve as an essential reference for advanced wastewater treatment and heavy metals control technologies in the future.

Experimental study on the treatment of desulfurization wastewater from coal-fired power plant by spray evaporation.

In this study, a pilot-scale evaporation tower system was built to treat the desulfurization wastewater by spray evaporation. The distribution characteristics of Cl- in the wastewater evaporation process were investigated. Besides, the morphology and physicochemical property of solid evaporation products from desulfurization wastewater were analyzed. In addition, the emission characteristics of fine particulates were evaluated. The results indicated that the increase of salt content in desulfurization wastewater increased the mass concentrations of Cl- in three phases, but the proportions of it remained almost unchanged, which were about 10%, 55%, and 35% in the gas phase, outlet solid phase, and bottom solid phase respectively. The increase of flue gas temperature can improve the content of Cl- in the gas phase, while the increase of wastewater pH inhibited the formation of gaseous HCl. The solid evaporation products from desulfurization wastewater had a prismatic crystal structure, which mainly included the sulfate and chloride salts, and the main elements including O, Na, Mg, S, Cl, K, and Ca. Besides, the peak values of particle size distribution in the bottom solid phase and outlet solid phase were 7.67 and 0.32 µm, respectively. For the particulate matters in flue gas, the spray evaporation of desulfurization wastewater can reduce the particle concentration, promote particle agglomeration, reduce the number concentration of fine particles, and improve the removal effect of PM10. When the inlet particle concentration was 7.62 g/m3, it can reduce the particle concentration at the tower outlet to 4.59 g/m3 and reduce the number and mass concentrations of PM10 after ESP by about 43.8% and 36.8%.

Removal characteristics of organic pollutants by the adsorbent injection coupled with bag filtering system.

Activated carbon (AC) injection coupled with bag filtering (ACI+BF) is a promising technology for the organic pollutant treatment in the flue gas of coal-fired power plants. The removal characteristics of six volatile organic compounds (VOCs) and adsorption pathways were investigated in a self-designed ACI+BF system. The results suggested that o-xylene had the highest removal efficiency and that was the lowest for benzene, which was influenced by their boiling points and saturated vapor pressures. The physicochemical properties of AC changed slightly after VOCs adsorption in the ACI+BF system. The VOCs removal process was dominated by physical adsorption even if the adsorption temperature was higher. With the increasing of adsorption temperature and VOCs concentration, the removal efficiency reduced; while that increased with increasing the AC feeding rate and residence time. The VOCs removal by the ACI+BF system could be divided into two processes, including the adsorption in pipeline and adsorption in the bag filter. Bag filter had an important contribution to the total removal efficiency. Increasing the length of the pipeline and reducing the dust cleaning frequency of the filter bag were useful in enhancing the organic pollutants removal efficiency.

Agglomeration and removal characteristics of fine particles from coal combustion under different turbulent flow fields.

Turbulent agglomeration is a promising pretreatment technology for improving the removal of fine particles in industrial flue gas, which can improve the particle removal effect of dust removal equipment safely and economically. However, due to the complexity of turbulence mechanisms, the relationship between turbulent flow fields and the agglomeration of fine particles is not known with precision, resulting a weak promotion effect for particle removal with this pretreatment technology. In this work, three kinds of turbulent agglomerators were constructed to investigate the agglomeration and removal characteristics of fine particles under different turbulent flow fields. The results demonstrated that the turbulent agglomerator with small-scale and three-dimensional vortexes in the flow field had the best effect in improving the agglomeration and removal of fine particles. Two kinds of agglomeration modes in turbulent agglomeration were proposed, one being agglomeration between fine particles in the vortex area, and the other the capture of fine particles by coarse particles. Furthermore, the motion trajectory, relative velocity and residence time of fine particles of different sizes in different flow fields were calculated by numerical simulation to investigate the interaction mechanism of particle agglomeration and turbulent flow fields. The results showed that a flow field with small-scale and three-dimensional vortexes can reduce the Stokes number (StK) and the relative velocity of particles of different sizes, and extend their residence time in a turbulent flow field, so as to obtain a better agglomeration effect for fine particles.