| Condensable Particulate Matter(CPM)is a material that is vapor phase at stack conditions but which condenses and/or reacts upon cooling and dilution in the ambient air to form solid or liquid PM immediately after discharge from the stack.As an air pollutant that has received wide attention in recent years,CPM is featured with high emission concentration,small size of formed particles,and complex composition.Most of the current studies focus on the emission characteristics of CPM and control methods are synergistic removal by using the existing air pollutant control devices.This paper first investigated the condensation characteristics of CPM and showed that the condensation of CPM in EPA Method 202,which can achieve surface deposition and space condensation.Among them,the CPM tends to be in the surface deposition state,which indicates the feasibility of CPM removal by cooling condensation.Based on this finding,a flue gas spray experimental system was built and used to focus on the transformation and removal mechanism of CPM in coal-fired flue gas in two spray cooling processes,humidification spray and condensation spray,in which humidification spray corresponds to the flue gas humidification cooling process in wet desulfurization and condensation spray corresponds to the saturated wet flue gas condensation cooling process after wet desulfurization.In the investigation of the transformation and removal mechanism of CPM during the humidification spray process,the roles of the spray zone and the mist removal zone,realized by installing a mist eliminator,in the spray tower were explored separately.The measured data shows that the CPM removal efficiency of the spray zone alone can reach up to 62%,and the removal efficiency would increase to 73%after the installation of the mist removal zone.The transformation and removal mechanism of CPM in the humidification spray tower is included three aspects:First,part of the CPM condenses and converts into Filterable Particulate Matter(FPM);second,part of the CPM is dissolved in the wet surface of the spray droplet or mist eliminator.Third,some of the FPM converted from CPM is intercepted and removed from the spray droplet or mist eliminator surface.In addition to this,some inorganic components contained in the spray droplets will enter the gas phase with the evaporation of the droplets,resulting in an increase of the inorganic components in the discharged CPM.In the investigation of CPM transformation and removal mechanism in condensation spray process,the relationship between CPM removal efficiency and spray liquid temperature was explored,and it was found that the removal efficiency increased with the decrease of spray liquid temperature,and the removal efficiency was up to 72%.In addition,experimental data show that condensation spray can result in the almost complete removal of NH4+,Cl-,and SO42from the inorganic components of CPM,as well as alkanes,esters,and alcohols from the organic components.The main mechanism of CPM removal in the condensing spray tower is the dissolution of spray droplets,homogeneous and heterogeneous condensation of CPM gaseous precursors;in addition,when the saturated flue gas undergoes the condensation spray process,condensation of water vapor occurs on the surface of the spray droplets,generating a directional Stefan flow to enhance mass transfer,which promotes CPM precursor and FPM trapping by the spray droplets.In conclusion,in the application of two kinds of spray cooling operations for flue gas including humidification spray and condensation spray,both can achieve different efficiencies of CPM removal,and the mechanism of removal includes condensation,dissolution,and interception,etc.Based on this,further research can be conducted in the future to develop methods for efficient and synergistic CPM removal in the spray cooling process. |
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