Study On Formation And Emission Control Of Aerosols During Ammonia-based Wet Flue Gas Desulfurization Processes

The technology of wet ammonia flue gas desulfurization(WFGD)has the advantages of high SO₂ removal efficiency and high absorbent utilization rate,whose byproducts can be used as resources.However,new aerosol generation occurs during the desulfurization processes,leading to excessive discharge of particulate pollutants from the stack,which aggravates particle pollution in the atmosphere.Therefore,to solve the aerosol emission problem,experimental research was conducted on the generation,the transformation,and the emission control of ammonia aerosols,which was of great significance for the innovation and development of the ammonia desulfurization technology.Experiments were conducted on the simulated ammonia WFGD experimental platform with the electrical low pressure impactor plus(ELPI⁺)being used as the primary instrument for aerosol measurement.The investigation was mainly on the aerosol emission characteristics after the ammonia WFGD system and the comparison of the aerosols formed by the flue gas entrainment and the heterogeneous reactions.The results showed that the aerosols formed in ammonia WFGD were mainly submicron whose dominant components were(NH₄)₂SO₃·H₂O and(NH₄)₂SO₄.The major source of the aerosol formation was the heterogeneous reactions between the NH₄ volatilized from the desulfurization solution,the SO₂ in the flue gas,and the water vapor,which were highly dependent on the desulfurization solution pH and the inlet flue gas temperature.The entrainment of desulfurization droplets by flue gas was also important in generating aerosols,which was influenced by the solution concentration and the flue gas superficial velocity in the scrubber.The contribution of SO₃ to the ammonia aerosol formation was explored experimentally on a coal-fired flue gas experimental system.Based on that,the heterogeneous reaction simulation experimental system was designed and built to study the SO₂/SO₃ related reactions and their aerosol-generating characteristics.The results showed that the SO₃ removal efficiency was relatively high in an ammonia WFGD system because SO₃ would react with NH₃ and water vapor to generate aerosols and thus be consumed.SO₃ has stronger abilities than SO₂ in producing aerosols,which might not be affected by the variations of reaction temperature and water content within the scope of WFGD parameters.However,it would increase with the rise of the NH₃/SO₃ molar ratio.In contract,SO₂related reactions would be inhibited in higher temperatures(>65?)and lower NH₃/SO₂ molar ratio,resulting in decreased aerosol production.Due to the essentiality of NH₃ to the aerosol formation during ammonia WFGD processes,experimental enquiries were conducted based on an ammonia WFGD system regarding the ammonia emission characteristics and the influencing patterns.According to the existing phase upon discharge,the ammonia emission was divided into the particulate ammonia and the gaseous ammonia emission.A train of classification sampling system was developed accordingly.The results showed that the ammonia WFGD system would produce tremendous ammonia emission along with the flue gas,of which the gaseous ammonia accounted for about 20%while the particulate ammonia made approximately 80%contribution.It would raise the ammonia emission to increase the flue gas temperature,the desulfurization solution temperature,the solution concentration,and the solution pH.The variation of the particulate ammonia emission was largely affected by the solution concentration and the flue gas temperature.whereas the gaseous ammonia emission was mostly affected by the solution pH,then by the flue gas temperature.The total ammonia emission after WFGD could be reduced by around 40%by adjusting the flue gas temperature,the solution concentration,and the solution pH.Combined with the aerosol generation and emission pattern in the ammonia-based WFGD system,experimental research was conducted to reduce aerosol emission through the desulfurization parameter optimization,the internal assembly installation,and the water spray purification.The results showed that lowering appropriately the inlet flue gas temperature,the solution temperature,and the solution pH could achieve aerosol emission decrease to different extents.Reducing the flue gas temperature to 60?and the solution pH to 4.5 would diminish the aerosol mass emission by about 47%.The installation of a sieve tray with the free area of 35%and the hole diameter of 10 mm in the desulfurization system,coupled with lowering appropriately the liquid-to-gas ratio(L/G),could achieve about 50%aerosol emission abatement(mass concentration).When the desulfurized flue gas was washed by sprayed branch water at the L/G of 4?10 L/m³,the ultimate aerosol emission could be reduced by around 60%in numerical concentration and about 45%in mass concentration,the size of which was concentrated in the range of<0.1?m and>1.0?m.According to the definition in Method 202 developed by the United States Environmental Protection Agency,the aerosols formed in and discharged from an ammonia-based WFGD system can be divided into the filterable particulate matter(FPM)and the condensable particulate matter(CPM).Experimental research was conducted based on a simulated ammonia-based WFGD system to investigate the CPM emission characteristics and how it was influenced by desulfurization parameter variations.The results showed that the CPM proportion in the total aerosol emission was about 70%in terms of mass concentration,in which the inorganic components accounted for more than 90%that were mainly SO₄^2-(70%)and NH₄⁺(20%).Most of the emitted CPM was submicron in size,of which about 70%was generated by the NH₃-H₂O-SO₂ heterogeneous reactions.The CPM emission was positively related to the flue gas temperature,the desulfurization solution temperature,the solution concentration,and the pH value.Around 50%CPM reduction could be achieved by reducing the solution pH to 4.5.