Study On The Mechanism Of Desulfurization And Denitrification Of Phosphorus-containing Composite Pulp

Removal of SO₂ and NOx released from middle and small–scaled coal–burning boiler is one important strategy for air pollution control.It is the general trend to develop the desulfurization and denitrification technology with the characteristics of collaborative removal of SO₂ and NOx,low cost,high efficiency,recycling of sulfur and nitrogen resources,and no secondary pollution.Accordingly,based on the analysis of production features of phosphor–chemical enterprises,phosphorus sludge and phosphate rock slurry are being developed as absorbents to remove SO₂ and NOx simultaneously.Phosphorus sludge is the waste during the production of yellow phosphorus,but it still contains a lot of yellow phosphorus.Yellow phosphorus can react with O₂ to form O₃ and other oxidizing substances that can oxidize NO toward high valence nitrogen oxides with good water solubility.These high valence nitrogen oxides and SO₂ are dissolved into the phosphate rock slurry,and then rapidly convert into nitric acid and sulfuric acid,under the catalysis of metal ions dissolved from phosphate rock slurry.This special route not only minimizes the emission of NOx and SO₂,but also fabricates the nitric acid and sulfuric acid in situ to decompose phosphate rock for getting phosphoric acid.Therefore,it is a promising technique by employing phosphorus sludge and phosphate rock slurry to eliminate SO₂ and NOx simultaneously.In this dissertation,the mechanism of simultaneous desulfurization and denitrification using yellow phosphorus and phosphate rock slurry as absorbents was studied,which lays a theoretical foundation for the popularization and application of this technology.（1）The desulfurization reaction processes with phosphate rock slurry as absorbent and catalytic mechanism of metal ions towards SO₂ absorption were investigated.Phosphate rock slurry exhibits good absorption performance for SO₂,and the desulfurization efficiency can reach 100%.The structure of phosphate rock and the composition of absorption liquid in the process of desulfurization are analyzed.The experimental results indicate that the Ca Mg（CO₃）₂ and Ca₅（PO₄）₃F form phosphate rock are the main components for SO₂ absorption.H₂SO₄,formed from the conversion of SO₂,preferentially reacts with Ca Mg（CO₃）₂ until it is exhausted,and then reacts with Ca₅（PO₄）₃F.The accumulation of PO₄^3–and H⁺ions resulted from the dissolution of Ca₅（PO₄）₃F causes negative effect on SO₂ removal.Hence,the desulfurization efficiency decreases gradually.In the spent phosphate rock,Ca₅（PO₄）₃F is still the main component and Ca SO₄·2H₂O is formed.The trace metal ions Mn²⁺and Fe³⁺dissolved from phosphate rock can greatly reduce the activation energy of SO₃^·–radical formation,which made more reactive oxygen species,such as,SO₄^·–,SO₅^·–,HSO₅^–and OH·,generate to oxidize S（Ⅳ）.As a result,high S（Ⅵ）conversion efficiency and SO₂absorption efficiency are maintained.（2）The Ca₅（PO₄）₃F embedded with carbonate can be liberated from carbonate when phosphate mine tailings are employed as absorbent to remove SO₂,which provides a new way for recycling of phosphorus resources.Experimental results show that the content of Ca Mg（CO₃）₂ in phosphate mine tailings is much higher than that of phosphate rock.In the process of desulfurization,Ca Mg（CO₃）₂ reacts firstly with sulfuric acid to form Ca SO₄·2H₂O and H₂O,so as to ensure the higher p H value of absorption slurry and high desulfurization efficiency.Hence,phosphate mine tailings have better desulfurization performance than the raw ore.（3）It was investigated that NOx absorption mechanism in the mixed absorbent composed of yellow phosphorus and phosphate rock slurry.Experimental results show that the key to denitrification is to oxidize NO to high valence nitrogen oxides,by O·,OH·and O₃ active substances produced by the reaction of yellow phosphorus and oxygen.These high valence nitrogen oxides dissolve in water to form nitric acid,and then react with phosphate rock to produce nitrate,so as to realize denitrification.Ca Mg（CO₃）₂ and Ca₅（PO₄）₃F,the main components of phosphate rock,show excellent absorption properties for high valence nitrogen oxides and are detected in phosphate rock before and after denitrification reaction.（4）Synergistic promotion mechanism of SO₂ and NOx during yellow phosphorus and phosphate rock slurry adsorption process is revealed.The results show that 100%desulfurization and denitrification efficiency can be obtained at low temperature of 65^oC.Temperature,stirring speed,yellow phosphorus concentration and flue gas flow have a significant impact on NOx oxidation removal.Compared with single desulphurization and single denitrification,the removal efficiency and efficient removal time of SO₂ and NOx were significantly improved when yellow phosphorus and phosphate rock slurry are used as absorbent to desulfurization and denitrification simultaneously.The SO₃^·–can be obtained directly by the reaction of S（Ⅵ）and NO₂in the absorption of NOx and SO₂,which reduces the activation energy of S（Ⅳ）oxidation to SO₃^·–and increases the yield of SO₃^·–.Moreover,a large number of oxidizing substances,such as,SO₅^·–,SO₄^·–,OH·and HSO₅^–,are generated to oxidize S（Ⅵ）to S（Ⅵ）,resulting in efficient SO₂ removal.Meanwhile,the formed oxidized substances can promote the conversion of NO,NO₂ and NO₂^–towards HNO₃,leading to a high denitrification efficiency.