Experimental Research On SO₃ Generation And Control In Ultra-Low Emission Coal-Fired Power Plant

Coal combustion produces a large number of pollutants.After the transformation of ultra-low emission in coal-fired power plant,the emission of conventional pollutants such as dust,SO₂ and NO_x in the flue gas have been controlled effectively,but there are few research on emission of SO₃ acid mist,which is harmful and needs attention.With the introduction of selective catalytic reduction（SCR）technology,the concentration of SO₃ acid mist in flue gas increase greatly.When the temperature of flue gas decrease,SO₃ acid mist will form micro-sized aerosol particles,which are difficult to remove by existing desulfurization and dust removal devices in coal-fired power plants.Chemical agglomeration enhanced dust removal technology,as a technical scheme with low cost,no secondary pollution can remove many pollutants synergistically,which have attracted the attention of researchers gradually.In this paper,SO₃ generation and removal by fly ash coupling chemical agglomerator are studied,and SO₃ generation and control in 3×300 MW ultra-low emission coal-fired power plant are sampled and analyzed.The removal efficiency of SO₃ acid mist between wet electrostatic precipitation technology and chemical agglomeration enhanced dust removal technology coupling other devices is compared,in hopes of providing a new idea for SO₃ removal synergistically in coal-fired power plant.Firstly,micron-sized Fe₂O₃ catalyst was prepared and three commercial SCR catalysts were selected.The effects of temperature on the catalytic oxidizing SO₂ over four catalysts were investigated.The results show that at 700℃,micron-sized Fe₂O₃ have the highest oxidizing performance of SO₂,and the conversion of SO₂/SO₃ reach 9.5%.Temperature has different effects on the catalytic oxidizing SO₂ over three commercial SCR catalysts,which are affected by the composition of the catalysts.V₂O₅ is the active center,which determines the conversion of SO₂/SO₃ directly.WO₃ plays a role of auxiliary catalytic that improves the activity of V₂O₅.The conversion pathway of SO₂/SO₃ was analyzed by density functional theory.The results show that the barrier of SO₂/SO₃ conversion is 74.6 kJ/mol in the presence of Fe₂O₃,which is much smaller than that of SO₂/SO₃ conversion without catalyst,195.0kJ/mol.It indicates that the difficulty of SO₂/SO₃ conversion is decreased greatly.Secondly,three organic macromolecule compounds,were selected to prepare agglomeration solution,then the removal of SO₃ by chemical agglomeration was studied experimentally.The removal efficiency of SO₃ by chemical agglomeration coupling fly ash in different temperature,type and concentration of chemical agglomerate was investigated,and its mechanism was analyzed.The experimental results show that the removal efficiency of SO₃ by chemical agglomeration solution improved significantly;with the increase of temperature and concentration in the experimental conditions,the removal efficiency of SO₃by unit volume agglomeration solution increases,which is related to the structure of organic macromolecule compound molecule in aqueous solution and functional groups on organic polymer chains;fly ash contains a large number of pore structure and alkaline metal oxides,so chemical agglomeration solution coupling fly ash can further improve SO₃ removal efficiency.Finally,the analysis of SO₃ generation and control for 3×300 MW ultra-low emission coal-fired units is carried out.The results show that the SO₂/SO₃ conversion rate is 0.8%¹.4%in the furnace and 0.8%¹.3%in the SCR unit.It shows that SO₃ concentration increases greatly in flue gas with the introduction of SCR device.The removal efficiency of SO₃ acid mist by wet electrostatic precipitation coupling electrostatic precipitation and wet desulfurization technology is basically over 80%at medium and high load,with the highest removal efficiency of 87%,while chemical agglomeration enhanced dust removal coupling electrostatic precipitation and wet desulfurization technology has little influence on the removal of SO₃ acid mist,and the removal efficiency can reach 90%at most.Both of them can greatly improve the removal efficiency of SO₃ in coal-fired power plants.