Dust, SO2 and NOX emission control technologies have reached a higher level for coal-fired flue gas, but the PM2.5, SO3 and Hg control technologies are relatively lagged. Separate processing units for control PM2.5, SO3 and Hg take up space and have high cost. The wet flue gas desulfurization (WFGD) system was used to remove SO2 for coal-fired boiler flue gas, the desulfurization wastewater was produced in the process of desulfurization. Evaporation treatment technology is a good application prospect in the numerous technologies. And for this, a novel technology was proposed to enhance air pollutant control devices (APCDs) removal of PM2.5, SO3 and Hg during desulfurization wastewater evaporation in the flue. Then, the paper gone on to study the macroscopic removal performance and micro removal mechanism, respectively. Based on the basis of desulphurization wastewater evaporation technology and combined with chemical and turbulent agglomeration technology, the electrostatic precipitator (ESP) removal PM2.5 was strengthened. Combining the technology of alkaline absorbent spray removing SO3, the removal efficiency of SO3 was enhanced in APCDs. By using chlorine ion, the oxidation and removal of Hg0 in desulfurization wastewater were promoted. At last, the removal mechanism of PM2.5, SO3 and Hg were studied by using the density functional theory (DFT). This technology realized desulfurization wastewater zero discharge and collaborative removal of PM2.5, SO3 and Hg, which achieved the aim of waste control by waste.
Based on heat and mass transfer equations of droplet, the mathematical model of droplet flue vaporization was established. The law of heat and mass transfer in the process of the droplet evaporation was obtained, and the variation of droplet diameter with time was also obtained. Aiming at the parameters of a typical 300 MW flue, the model was used to calculate the evaporation law of the droplets with different diameters and different temperature changes. The results showed that the evaporation time for the droplet with the diameter of 40 μm were 0.44 s and 0.15 s, respectively for 150 ℃ and 350 ℃. Based on the established the coal-fired thermal experimental platform, the characteristics of droplet evaporation in actual flue were studied by the visualization method of fluorescent tracer. The results indicated that the influence of temperatures and diameters on the droplet evaporation time were in good agreement with the calculated results in the model. The particle concentration had little effect on the droplet evaporation time.
Based on the coal-fired thermal system, the ESP removal of PM2.5 was investigated experimentally with different flue gas temperature and humidity caused by desulfurization wastewater evaporation. Also, the law of strengthen ESP removal of PM2.5 was discussed, and physical property change of fine particles in flue gas were analyzed in the wastewater evaporation process. In addition, the law of strengthen ESP removal of PM2.5 was discussed by coupled with chemical and turbulent agglomeration technology. The results showed that the ESP breakdown voltage raised with desulfurization wastewater evaporation, which was beneficial to charged particles and the improvement of dust efficiency. Desulphurization wastewater evaporation promoted particles agglomeration, besides, the fly ash resistivity was reduced. The PM2.5 concentration at ESP outlet decreased by about 15% under typical conditions, the removal efficiency of PM2.5 increased with increasing amount of evaporation. With the addition of agglomerants, the peak diameter of particle was increased in the flue gas, particles were bonded together by chemical agglomeration agent to form larger aggregates, and the number removal efficiencies of particles from 0.1 μm to 1.0 μm were increased by 20%-30%. The diameter of particle further increased by chemical coupled turbulence agglomeration technology, agglomeration efficiency was the highest by chemical coupled turbulence agglomeration technology, and agglomeration efficiency was the lowest by turbulence agglomeration technology, the increase of velocity and particle concentration were good to strengthen particles agglomeration. The ESP removal efficiency increased by 10% to 20% with turbulence agglomeration technology under typical conditions, and the removal efficiency increased by 20% to 30% with turbulence agglomeration, the removal efficiency increased by 40% to 50% with chemical coupled turbulence agglomeration technology.
The removal characteristics of SO3 and Hg by desulfurization wastewater evaporation were investigated experimentally based on the coal-fired thermal system. The SO3 binary nucleation mechanism was analyzed. The experimental results showed that SO3 formed sulfuric was fast by binary nucleation in actual coal-fired flue gas, the flue gas acid dew point temperature increased with the improvement of humidity and SO3 concentration. When the temperature dropped the acid dewpoint, H2SO4 condensed and adsorbed on the surface of fly, the adsorption capacity followed the empirical formula of Weber-Morris. Furthermore, SO3 condensation on the ?y ash was mainly controlled by internal diffusion. When added alkaline absorbents (NaOH, Na2CO3, NaHCO3) to desulfurization wastewater, the removal efficiency of SO3 followed by NaOH > Na2CO3 > NaHCO3, the removal efficiency of SO3 increased with increasing amount of evaporation and alkaline absorbents, moreover, smaller atomized droplet was beneficial to improve the SO3 removal efficiency. When the n(Na/S) was 2, the SO3 removal efficiency was greater than 70% for three kinds of alkaline absorbents. The Hg concentration at WFGD outlet decreased by desulfurization wastewater evaporation before SCR and ESP, the Hg0 oxidation ef?ciency was increased significantly when wastewater evaporated before SCR. The ESP removal efficiency of HgT increased by about 15% with wastewater evaporated before ESP, the WFGD removal efficiency of Hg2+ reached more than 80%, respectively.
Based on the above experimental results, the collaborative removal mechanism of PM2.5, SO3, Hg were studied by DFT, SiO2 was as dust particles in simulation calculation. Vibrational frequencies, free energy, natural bond orbital (NBO) were calculated to reveal the interaction between chemical agents and SiO2. the reasonable calculation model was established, which contained SO3, chemical agent, SiO2. And the interactions between SO3, H2SO4, SiO2, chemical agent were explored. Finally, the oxidation process and mechanism of Hg0 on V2O5 were analyzed by establishing the Hg0 and V2O5 model. In the results, the chemical agent molecule and SiO2 were integrated by hydrogen bonds (OH?O). The longer molecular chain molecules, the higher free energy between chemical agent molecule and SiO2, which was benefitted to particle agglomeration. In SO3, H2O, chemical agent, SiO2 coexisting system, SO3 combined with H2O formed H2SO4 first, and chemical agent and SiO2 showed the adsorption ability of SO3, however, adsorption ability of chemical agent for SO3 was better than SiO2. The oxygen in V2O5 showed oxidation ability to Hg0, more importantly, the chloride ion strengthened Hg0 oxidation on the V2O5, and generated the intermediate (HgCl), further generated HgCl2.