Theoretical Study On As₂O₃ Formation And Transformation With As And AsO As Precursors In Coal-fired Flue Gas

Arsenic is a metal-like pollutant in coal-fired flue gas.It is carcinogenic and highly toxic.Emission into the atmosphere is extremely harmful to the environment and human health.The Ministry of Ecology and Environment has successively released policies such as the"Arsenic Pollution Prevention Technology",which set clear requirements for the pollution prevention and management of arsenic-containing coal use,coal-fired power plants and coal-fired industrial boilers.Due to the limitation of experimental conditions and the low arsenic content during the coal combustion process,the reaction is rapid.It is difficult to complete an accurate investigation through experimental methods.The related theoretical research on arsenic in coal-fired flue gas can be a pollutant for coal-fired boiler Control technology provides some theoretical support.Therefore,this paper applies density functional theory and transition state theory to study the mechanism of As₂O₃ formation and conversion with As and AsO as precursors in coal-fired flue gas,and uses thermodynamic and kinetic methods to conduct in-depth analysis of the reaction process.The results are as follows:（1）Under O₂ conditions,the maximum reaction energy barriers for the formation of As₂O₃ from As and AsO and homogeneous phases are 7.8 kcal/mol and 37.6 kcal/mol,respectively.It is easier to convert As and O₂ to As₂O₃ in the flue gas.Under SO₂conditions,the maximum reaction energy barriers for the formation of As₂O₃ from As and AsO and homogeneous phases are 24.9 kcal/mol and 41.3 kcal/mol,respectively.It is also easier to convert As and SO₂ to As₂O₃ in the flue gas.When the reaction energy barriers are 13.9kcal/mol and 38.6kcal/mol under the combined action of SO₂ and O₂,the conversion from As to As₂O₃ in the flue gas is easier.The reaction energy barriers are both lower than SO₂ alone and higher than O₂ alone.SO₂ acts as a negative catalyst in this process,which increases the reaction energy barrier and makes the reaction more difficult to occur.（2）Under O₂,O₃,and SO₂ conditions,the maximum reaction energy barriers of As₂O₃→As₂O₅ are 33.3kcal/mol,22.2kcal/mol,and 36.3kcal/mol,respectively.The ease of conversion to As₂O₅,SO₂>O₂>O₃,is the easiest to perform when reacting with O₃,which can occur when a small energy barrier is overcome.（3）In the range of 500-1900K,the forward and reverse reaction rate constants of each reaction increase with the increase of temperature,but the different reaction processes are affected to varying degrees by temperature.The equilibrium constant also varies with temperature depending on the endothermic and exothermic reactions.The degree of angle analysis performed shows that the conversion rate to As₂O₃ when O₂ acts alone is higher than that when SO₂,SO₂ and O₂ synergize.The presence of SO₂ not only increases the difficulty of the reaction but also reduces the conversion rate of the reaction.（4）During the reaction of As₂O₃ conversion,the value of the positive reaction rate constant curve at each temperature is O₂>O₃>SO₂,and the value of the reverse reaction rate constant is SO₂>O₂>O₃.Analysis of the equilibrium constant shows that the conversion degree of As₂O₃ is not much different when reacting with O₂and SO₂,and the conversion rates are low.When reacting with O₃,the equilibrium constant is less than10^-5,and the reaction is difficult to proceed in the forward direction.It is difficult to convert As₂O₅ in a large amount under the studied path.