Effect Of Steam On The Adsorption Of Sulfur Oxides On CaO Surface In CFB

The energy structure of China is characterized by rich coal,lean oil and low gas.Coal has dominated for decades.Combustion of fossil fuels such as coal will release pollutant such as SO₂ and cause atmospheric pollution.Circulating fluidized bed boiler has the characteristics of high combustion efficiency and wide adaptability to coal.It uses limestone for dry desulfurization during combustion,simple equipment,low cost,and is an advanced clean combustion technology.H₂O is one of the main components of coal-fired flue gas,and it can promote the desulfurization reaction.Therefore,the study of H₂O as affecting the adsorption of SO₂ on CaO provides a certain guiding significance for more efficient and clean use of coal resources.First,the adsorption of SO₂ on the surface of clean CaO was calculated first,and then the adsorption configuration of H2O on the surface of clean CaO was analyzed.The results show that the two most stable adsorption configurations of SO₂ molecules on the surface of CaO are chemisorption.There are two forms of H₂O on the surface of CaO:one is that H₂O still exists on the surface of CaO with the original molecular structure,and the other is that an O-H bond in H₂O is broken,in the form of an OH group and an H group Exist on the surface of CaO.All three configurations are chemisorption.Then,the configuration of SO2 adsorption on the CaO surface after H2O adsorption was calculated and compared with the adsorption of SO2 on the clean CaO surface.The results indicated that H₂O can have an effect on adsorption geometries for SO₂ and CaO（001）surface.When SO₂ adsorbs to the CaO surface near a water group of different forms（-H₂O,-H,-OH and-H&-OH）,the H group makes the adsorption energy 90 k J/mol higher with the sulfur p-orbital shifting downward and the other groups make little effect.When SO₂ adsorbs to the-OH surface and-H&-OH surface,bisulfite-like structures form with lower adsorption energies and trend to turn into more stable structures as intermediates.When SO₂ adsorbs to the-H₂O surface and form bisulfite-like structure,the H₂O group decomposes and form Ca（OH）₂ like-structure on the CaO surface.The new H groups mainly bond the bisulfite and make adsorption energy 45 k J/mol higher.Finally,the adsorption of SO₂ on the surface of CaO with different H₂O coverage was calculated.The results show that under different coverage conditions,the main adsorption of H₂O molecules is still O atoms on the surface of CaO.When the H₂O coverage is less than 1/2 ML,the H₂O molecules maintain a parallel geometric relationship on the CaO surface.When the H₂O coverage is 5/8 ML,the interaction between H₂O molecules is enhanced,resulting in a disordered molecule on the surface of CaO.On the surface of CaO with different H₂O coverage,SO₂ will break into bonds and form SO₃ similar structures.When the H₂O coverage is less than 1/2 ML,the SO₂ adsorption energy increases as the H₂O coverage increases.When the H₂O coverage continues to increase to 5/8 ML,the SO₂ adsorption energy decreases.The best H₂O coverage exists for the adsorption of SO₂ on the surface of CaO.