Study On The Ptimum Oxidation Degree Of NO_x In Simultaneous Desulfurization And Denitrification By Urea

With the increasingly stringent emission regulations of SO₂ and NO_x in the marine engine filed,more and more attention has been paid to the research of desulfurization and denitrification simultaneously.In this paper,the current research status and development trend of simultaneous desulfurization and denitrification are introduced and the comprehensive evaluation is carried out.Urea has the following advantages:it is a stable compound,and safer to use and store on ships;the final products are N₂,CO₂ and?NH₄?₂SO₄,and?NH₄?₂SO₄ can be recycled;the secondary pollution can be avoided.But at present,the research and application of urea method in the ship field is less,and the denitrification efficiency is limited.In order to improve the denitrification efficiency by urea,many researchers used the technology of low temperature plasma to oxidize NO_x and then removed it with urea solution.Nowadays the conclusions on the problem of"the value of NO_x oxidation degree corresponding to the highest denitrification efficiency"in the present study of urea method is completely different.The optimum oxidation degree may be changed with the change of boundary conditions.Therefore,it is necessary to test the optimum oxidation degree of NO_x for the highest denitrification efficiency under the different boundary conditions,so that the oxidation technology can used more efficiently.The main work in this paper is as follows:In order to simulate the actual working conditions of ship's exhaust,a detailed design and calculation of the test bench for the simultaneous desulfurization and denitrification using urea was carried out.The results are as follows:the design of exhaust flow is 2?5 L/min,the tower diameter is 25 mm,the height of the tower is1 200 mm,the flooding velocity is 0.30 m/s,the actual gas velocity is 0.07?0.17 m/s,and the residence time of flue gas in the packing layer is 6?15 s.The optimum test conditions for simultaneous desulfurization and denitrification by urea are determined:?6×6 pall ring packing is choosed,packing height is 100cm,the spray density is 40 m³/?m²·h?,urea concentration is 5%?10%,exhaust flow is2 L/min,and the reaction temperature is 10?20?.Under the optimum test conditions,when the initial volume fraction of NO_x is 874×10^-6,SO₂ is 987×10^-6,O₂ is 15%,and the initial NO_x oxidation degree is 10%,the average removal efficiency of SO₂can reach more than 98%,and the average removal efficiency of NO_x is 56%.The denitration test was carried out further,and the influence rule of NO_x oxidation degree on denitrification efficiency was clarified.And the optimum NO_x oxidation degree under different boundary conditions such as exhaust gas flow,NO_x volume fraction and reaction temperature was determined.The results showed that,under this experimental condition,the average denitrification efficiency is 67%when the corresponding NO_x oxidation degree is 50%;changes of the exhaust gas flow,the volume fraction of NO_x,and the reaction temperatures has no effect on the rule of“the denitration efficiency increases with the increase of the NO_x oxidation degree”.Under the different boundary conditions,when the denitrification efficiency is up to about 90%,the corresponding NO_x oxidation degree is 90%.To further improve the efficiency of urea denitrification,realize high DeNO_x efficiency when NO_x oxidation degree is lower,test using NaOH solution is carried out and the results are compared to the urea.The results show that when the NaOH solution is used for denitrification,the maximum denitrification efficiency can reach90%and the corresponding NO_x oxidation degree is about 50%.The reason is that the NaOH solution can quickly neutralize the HNO₂ which is formed after the NO_x entered the liquid phase,thus greatly improving the efficiency of denitrification.In conclusion,the key problems of desulfurization and denitrification at the same time by using urea is not only to increase the NO_x oxidation dgree in the gas phase,but also to improve the reaction rate between urea and the NO_x dissolving into the liquid phase by appropriate reaction ways.