Study On Wet Denitrification Of Fe(Ⅱ)EDTA Composite Absorbent

The problem of air pollution caused by coal combustion is becoming more and more serious.The flue gas emission in the combustion process is increasing,and the nitrogen oxides in the flue gas have caused great harm to human health and ecological environment.95%of the NOx is NO,and the solubility of NO in water is very low and the reactivity is relatively low,so the removal of NO becomes a difficulty.In this paper,metal chelating agent Fe（Ⅱ）EDTA was used to remove NO,but oxygen in flue gas easily oxidized Fe（Ⅱ）EDTA to Fe（Ⅲ）EDTA,thus reducing the denitration efficiency.Therefore,regarding the shortcoming that Fe（Ⅱ）EDTA is easy to oxidize,urea dioxidide（TD）,sodium bisulfite（SD）and ascorbic acid（VC）were respectively used in combination with Fe（Ⅱ）EDTA to absorb NO,specifically including the following contents:First,sodium bisulfite（SD）combined with Fe（Ⅱ）EDTA was used to remove NO and SO₂ simultaneously in a self-made bubbling reaction system.The effects of O₂ volume concentration,SD concentration,initial pH value,absorption of coexisting gas and temperature on the removal of NO and SO₂ in mixed liquid were studied.The results showed that SD could significantly improve the NO removal rate of Fe（Ⅱ）EDTA,and the optimal Fe（Ⅱ）EDTA:SD ratio was 3 m M:0.026 M.With the increase of temperature,pH value,SO₂concentration or O₂ concentration,NO removal firstly increased and then decreased.In all experiments,the removal rate of SO₂ was between 92%and 99%,and there was no obvious fluctuation in the removal rate of SO₂ under different operating conditions.Finally,through the analysis of different ion concentration,the main absorption products were determined as sulfite,sulfate,ammonium,nitrite and nitrate.Secondly,urea dioxidide（TD）combined with Fe（Ⅱ）EDTA was used for wet denitration.The effects of O₂ volume concentration,TD concentration,initial pH value and absorption temperature on the denitration efficiency of the absorption system were investigated.The results showed that with the increase of O₂ volume concentration,TD concentration and temperature,the NO removal rate firstly increased and then decreased.When pH value increased from 6.5 to 10.5,NO removal rate increased monotonically to a certain extent.In addition,response surface methodology（RSM）and central composite design（CCD）were used to investigate the effects of the four variables on NO removal and their interactions.The calculation model shows that pH value has a major positive independent effect on NO removal.The interaction between any two factors was explained by 3D surface and plane graph,and the optimal absorption conditions were determined as follows:O₂volume content of 6.0%,TD concentration of 0.02 M,initial pH value of 10.5,and absorption temperature of 42℃.Under these conditions,the best NO removal rates were 95.8%and 94.3%,respectively.By comparing the experimental and simulated values,it can be concluded that the process parameters of response surface optimization are reliable and reasonable.Finally,ascorbic acid（VC）combined with Fe（Ⅱ）EDTA was used for wet denitration.First of all,the reduction of Fe（Ⅲ）EDTA by VC was investigated,and the Fe（Ⅲ）EDTA could be reduced efficiently by VC no matter under aerobic or anaerobic conditions.Then the effects of operating parameters such as Fe（Ⅱ）EDTA concentration,Vc concentration,initial pH value and temperature on NO removal in the bubbling device were investigated.The results showed that VC had a significant effect on the removal of NO by Fe（Ⅱ）EDTA.The absorption rate of NO increased with the increase of VC concentration or Fe（Ⅱ）EDTA concentration.Low temperature or low O₂ concentration is beneficial to the absorption of NO,and the absorption rate of NO increases first and then decreases with the increase of pH value.In addition,kinetic studies show that compared with conventional Fe（Ⅱ）EDTA method,Fe（Ⅱ）EDTA mixed with VC can more effectively reduce the total mass transfer resistance and increase NO flux,and the absorption process is controlled by liquid film.Finally,the study mechanism showed that NO was mainly converted into nitrite and nitrate.