Experimental Investigation On Improving The Removal Of Fine Particles By Heterogeneous Condensation

Large amounts of fine particles (PM2.5) emitted from coal combustion in coal-fired plantcan not be efficiently removed by conventional dust removal devices such as electrostaticprecipitators, cyclones and wet scrubbers and so on, and may pose a serious threat to thepublic health because they often consist of toxic components and can easily enter the humanrespiratory tract. The coal-fired plants are mostly equiped with wet flue gas desulfurization(WFGD) system, which makes the hot flue gas contact normal temperature scrubbing liquid,causing the gas phase temperature to decrease and the relative humidity of flue gas to increase.With a little water vapor added into the desulfurized flue gas in the condenser posed at theoutlet of the scrubber ,the removal efficiency of PM2.5 can be considerably improved by theenlargement of heterogeneous condensation of water vapor.The relationship of temperature and humidity characteristics of the desulfurized flue gasmeasured by temperature and humidity sensor and operating parameters during the WFGDprocess was investigated experimentally for the typical WFGD technology oflimestone-gypsum process. The results show that when liquid-to-gas ratio is more than9L/m3,the relative humidity of the desulfurized flue gas can amount to 80%,while the absolutehumidity 92g/m3,increasing 86g/m3 than that at the inlet of the scrubber, as is very beneficialto improve the removal efficiency of fine particles by adding a little vapor and when thetemperature of the scrubbing liquid changes from 34℃to 54℃, the absolute humidityincreases from 41g/m3 to 89g/m3 ,while, the relative humidity only from 81.4% to95.8%,mainly because the temperature increasing of the desulfurized flue gas caused by thatof the scrubbing liquid, so is possible and necessary to decrease the temperature by adding alittle colder water through the demister in order to improve the supersaturation.Thetemperature and humidity characteristics of the desulfurized flue gas and operating parametersduring the the WFGD process has great influent on the removal efficiency of fine particles,which was investigated experimentally for the typical WFGD technology oflimestone-gypsum process by adding a little water vapor into the condenser posed at the outletof the scrubber. The results show that the removal efficiency increases with increasing theliquid-to-gas ratio and the desulfurized flue gas humidity while decreasing the scrubbingliquid temperature and the concentration of fine particles .A mathematical model of ideal mixing between wet flue gas and water vapor was established ,by which the temperature andhumidity characteristics of the mixture with the different amount of vapor and the originalcharacteristics of the wet flue gas were numerically analyzed. The results show that increasingthe amount of vapor and the humility of the original flue gas and especially reducing thetemperature of the original flue gas ,can increasing the supersaturation of the mixture. As aresult , optimization operation conditions during the the WFGD process and adding littlewater vapor into the condenser can make the removal efficiency increased by 40%, by theenlargement of heterogeneous condensation of water vapor.In addition, the steam quality , the style of adding vapor and the condenser structure wasanalyzed by the basic principle of thermodynamics and field synergy principle so as toimprove the supersaturation of the mixure in the condenser and the removal efficiency of fineparticles. The results show that the mixture efficiency between vapor and flue gas can beimproved by adding dry saturated vapor through ejector ,and field synergy principle can beused into designing the condenser structure so as to reduce the amount of vapor condensingon the wall of the condenser and improve the efficiency of vapor applying.