基于密度泛函理论的燃煤飞灰未燃尽碳与烟气砷作用机理

Interaction mechanism between unburned carbon in coal-fired fly ash and arsenic in flue gas based on the density functional theory

基于密度泛函理论研究了燃煤飞灰中未燃尽碳(unburned carbon,UBC)组分对气态单质砷As及其氧化物AsO、AsO2和As2O3的作用机理。结果表明,单质砷优先吸附于碳桥位,吸附能在(-5.95)-(-5.88)eV。AsO分子中的砷、氧原子分别与碳原子成键时,吸附构型最稳定,吸附能最低为-7.87 eV。当AsO2在未燃尽碳表面解离形成一个AsO和表面活性氧时,体系最稳定,吸附能为-10.65 eV。当三角双锥As2O3分子以两个氧原子首先碰撞未燃尽碳表面时,将解离形成AsO和AsO2小分子,并分别与表面碳成键,此时体系吸附能相较于未解离情形而言显著降低,达到-10.64 eV。飞灰未燃尽碳与AsO或AsO2小分子的结合较紧密,局部倾向于形成特殊的五元环结构。毒性最强的三价态砷As2O3,相较于As、AsO和AsO 2而言,化学性质稳定,不易发生吸附。将其催化裂解为AsO、AsO2小分子,有望成为可行的燃煤电厂烟气砷污染控制措施。

The interaction mechanism between the unburned carbon in fly ash and the arsenic pollutants in flue gas such as As,AsO,AsO2 and As 2O 3 was studied based on the density functional theory.The results show that the elemental arsenic is preferentially adsorbed at the carbon bridge site,with an adsorption energy in the range(-5.95)-(-5.88)eV;the AsO molecule preferentially combines with the unburned carbon in a way that the arsenic and oxygen atoms are bound with the surface carbon atoms respectively,forming a most stable configuration with an adsorption energy of-7.87 eV.When AsO2 is dissociated on the unburned carbon surface and form an AsO molecule and a surface reactive oxygen species,the system is the most stable,possessing an adsorption energy of-10.65 eV.While once the two oxygen atoms in a trigonal bipyramid As2O3 molecule first collide with the unburned carbon surface,it will be dissociated to small molecules of AsO and AsO2,forming a covalent bond with surface carbon.The adsorption energy is significantly reduced to-10.64 eV,compared with the undissociated case.The unburned carbon in fly ash is easy to bind with AsO or AsO2 small molecules,which locally tends to form a special five-member ring structure.Compared with As,AsO and AsO2,the most toxic trivalent arsenic As2O3 is chemically stable and not easy to adsorb.Catalytic pyrolysis of As2O3 into small molecules of AsO and AsO2 is expected to be a feasible measure to control the arsenic pollution in coal-fired power plants flue gas.