Study On Homogeneous Reaction Kinetics Of Arsenic In Combustion Process

Coal combustion,sludge and garbage incineration will produce a large number of heavy metals,which will cause harm to environment and human body when discharged into atmosphere.Arsenic is extremely polluting and toxic.Combustion process of toxic trace elements pollution problem such as arsenic has caused wide public concern,while so far the reaction mechanism of arsenic and transformation path is unclear.The reaction kinetics parameters of arsenic are serious lack.Essentially reveal of arsenic in the process of cmbustion reaction mechanism is of great significance to put forward the control strategy of arsenic,therefore kinetics of reaction mechanism of arsenic are studied in this thesis.The thermodynamic data of reaction components of arsenic,oxygen,chlorine and water vapor in flue gas during high temperature combustion is calculated firstly.The frequency analysis of the optimized ground state geometry of AsO,AsCl3 and AsH3 are calculated by quantum chemistry.The frequency analysis reveals the thermodynamic parameters within the temperature range of 298K-2000K,and the thermodynamic parameters is compared with data which is queried from National institute of standards and technology(NIST)database to lay a fondation for the next calculation,which prove it is feasible to study the reaction mechanism of arsenic by quantum chemical method.Microscopic mechanism of 26 homogeneous reactions between arsenic and O,Cl,H2O has been researched.The geometry optimization,energy calculation and frequency analysis of reactant,transition stste,intermediate and product is calculated by quantum chemistry density functional theory B3LYP method.The property of transition state is validated by vibration frequencies analysis,then the energy barrier of these reaction at 298K is worked out.On this basis,the pre-exponential factor and reaction rate constants within 500-2000K temperature range are calculated by classical transition state theory,and the dynamic parameters within the temperature range are obtained by nonlinear fitting.The reaction rate constants of arsenic and chlorine are compared,and the results showed the oxidation effect of Cl2 on arsenic compounds is better than that of HCL.By comparing three reaction channels of AsOOH and H atoms,it is found that H and O atoms directly connect to form OH with the smallest energy barrier and the reaction is the most likely to occur.H reacts with H atom of AsOOH to form H2 has the largest and most diffcult barrier to occur.The reaction rate constant of arsenic oxide or arsenic hydroxide with H is compared,and the results show the reaction rate of H-O is faster than that of H seize OH.The kinetic model of homogeneous reaction on As/O/H/Cl system is established.The effects of temperture and species concentration on reaction process is analysed.Thermodynamic equilibrium simulation shows arsenic mainly exists in the form of As4O6 below 1000K,and AsO and AsO2 above 1000K.Dynamic calculation results show that in As/O system,As2O3 is the main existing form of As when the temperature is below 1300K and AsO is above 1300K.In the As/O/Cl/H system,As exists mainly in the form of AsCl3 above 1200K and AsOOH below 1200K.H2O can promote the conversion of arsenic oxide to AsOOH,and the influence of H2O increases significantly with the increase of temperature.When the concentration of Cl2 exceeds 5ppm,the chlorination effect of arsenic increases sharply.