Experimental Study On Efficient Removal Of SO₂ And Synthesis Control Of SO₃ Based On Wet Flue Gas Desulfurization System

Coal is the primary energy in China,mainly for power generation and heating.Emission of dust,SOx and NOx due to coal combustion is the largest source of air pollution.In order to reduce air pollution and improve air quality,China has published more stringent emission standards for coal-fired power plant,and the emission concentration of SV2 cannot exceed 35mg/Nm3.Meanwhile,the problems,such as visible plumes and secondary aerosol formation,caused by the SO₃ have drawn increasing attention to the control of emissions.There are many methods available for controlling the emission of SO2 from coal-fired boilers.Because of its high efficiency and reliability,limestone-gypsum wet flue gas desulfurization（WFGD）technology is the most commonly used technology for controlling the emission of SO2 in the world.Enhancing the SO2 removal efficiency and controlling SO₃ emission is an improtant way to achieve efficient emission reduction of SOx.Previous research has shown that the key to increase the SOx removal efficiency is improving the mass transfer condition in the spray scrubber.Based on such theoretical framework,this paper analyzes the characteristics of slurry pH under different desulfurization operating conditions.Then,the dual-pH control method was proposed and the investigation of the enhancement of SO2 removal in single spray scrubber using dual-pH was carried out.The method of improving gas-liquid contacting by installing sieve tray was discussed.In addition,the removal efficiencies of SO₃ in WFGD scrubber was investigated.Firstly,a dual-pH wet flue gas desulfurization system was built and the correlations of the pH differences between two slurry ponds as well as SO2 removal efficiency with flue gas volume,inlet SO2 concentration and spray density were investigated.Results show that pH differences depend mainly on the retention time of slurry in the ponds,which has an immediate impact on the dissolution of limestone.The SO2 removal efficiency can be significantly improved by introducing the dual-pH value zone into the WFGD system,and the improve performance is related to the method of pH control.Secondly,an experiment was performed to study the fluid dynamic characteristics of vapor-liquid two-phase flow on the sieve tray.Water-air system was used at atmospheric pressure and ambient temperature.The Influence of the superficial velocities,liquid rates,open area,holesizes,thickness of the tray and hole arrangement on the dry pressure drop,wet pressure drop and liquid hold-up was discussed.The results show that both dry and wet pressure drop as well as liquid hold-up increase with superficial velocities increase or open area decrease.And increasing the liquid rates will lead to the occurrence of flooding at lower superficial velocities.Then,the desulfurization performance test was conducted.And the comparison between the spray scrubber with sieve tray and the one without sieve tray was made by changing the major operating parameters such as inlet concentration of SO2,L/G,superficial velocities and pH value.It was found that the SO2 removal efficiency of the scrubber with sieve tray was about 2%higher than the one without sieve tray,when under the normal working condition.Finally,the WFGD experimental system was built.Based on the self-developed SO₃ tester and SO₃ generator,the particle feature of SO₃ aerosol in the wet spray scrubber was analyzed,and the effects of desulfurization operational conditions SO₃ removal efficiency were also investigated.The results show that SO₃ is converted to SO₃ aerosol as the flue gas entering into the scrubber,leading to a high number concentration of submicron particle（～107/cm3）.With the increase of the gas-liquid ratio and the inlet concentration of SO₃,or the decrease of inlet flue gas temperature and slurry temperature,the SO₃ removal efficiency increases.Moreover,the SO₃ removal efficiencies in the scrubber with sieve tray and the one without tray were compared.It shows that the tray could enhance the SO₃ removal performance in spray scrubber.