Experimental Study On Simultaneous Desulfurization And Denitrification From Flue Gas By Wet Process Based On Α-FeOOH/H₂O₂ Preoxidation

Nowadays,coal is still an important primary energy source for industrial development in China,and coal-fired power plays a critical role in production and living,but the nitrogen oxides and sulfur oxides in the flue gas of coal combustion cause a series of environmental problems and endanger human health seriously.At present,the selective catalytic reduction method and the limestone-gypsum method are widely used in the industry to remove nitrogen oxides and sulfur oxides in flue gas,respectively.However,these technologies have several disadvantages,such as large occupation,high operating cost,difficulty in catalyst regeneration and complex operating system.Therefore,it is of great research significance and engineering value to research and develop a technology that combines efficiency,economy,and can simultaneously remove nitrogen oxides and sulfur oxides.Our group proposed a method for simultaneous desulfurization and denitrification by preoxidation coupled ammonium-based liquid phase absorption.The method utilizes the strong oxidizing free radicals after catalytic decomposition of H₂O₂ to pre-oxidize the insoluble NO into high-valent nitrogen oxides that are easily soluble in water,and then uses the liquid phase absorption device for tail gas treatment.In this study,α-Fe OOH was used as the catalyst,the preoxidation system and the liquid-phase absorption system were studied on the self-built experimental bench,and the deactivation and regeneration of the catalyst were studied.Firstly,α-Fe OOH was used as a catalyst to conduct flue gas preoxidation experiment.Compared with the catalyst-free preoxidation experiment,the preoxidation efficiency can greatly improve after adding 2g of catalyst.The optimal preoxidation efficiency is obtained to 80%at flue gas preheat temperature of140°C,vaporization temperature of 140°C,catalytic temperature of 160°C,H₂O₂ concentration of 10mol/L and H₂O₂ flow of 2.5 m L/h.The study finds that in the catalytic preoxidation experiment,with the increase of the vaporization temperature and the catalytic temperature,the preoxidation efficiency of NO first increases and then decreases.Increasing the concentration of H₂O₂ can promote the NO removal efficiency significantly;at low H₂O₂ flow rate,increasing H₂O₂ flow rate can promote the preoxidation efficiency,but when the H₂O₂ flow rate is excessive,the preoxidation efficiency decreases instead.Combined with economy and efficiency,subsequent preoxidation controls to a vaporization temperature of140°C,a catalytic temperature of 140°C,H₂O₂ concentration of 10 mol/L and H₂O₂ flow of 2.5 m L/h,the preoxidation efficiency reaches 76.4%and the catalytic efficiency is stable.The experimental results show that when the O₂ concentration is 1%-9%,the NO preoxidation efficiency remain unchanged basically.In the simultaneous SO₂ and NO removal process,a small amount of SO₂ gas can promote preoxidation,when the SO₂ concentration is 1000 ppm,the NO preoxidation efficiency can achieve 86%.The catalyst characterization results show that crystal type,functional group,pore structure and other parameters of theα-Fe OOH remain unchanged after the long-term catalytic reaction,so theα-Fe OOH has good stability.Secondly,the liquid-phase absorption denitration of ammonium-based,preoxidation coupled ammonium-based liquid phase absorption denitration and simultaneous desulfurization and denitrification were studied.Ammonium-based liquid phase absorption denitration experiments results show that in each experimental absorption solution,ammonium sulfite plays a key role in NO removal.Considering that urea solution is easy to react with SO₂ and the product is ammonium sulfite,so the ammonium sulfite/urea solution is most suitable for the removal of NO.As the concentration and the volume of the absorption solution increasing,the efficiency of liquid phase denitrification gradually increases,but the rate of growth is slowing until the denitration efficiency doesn’t change much or remains unchanged.When the flue gas is1.5L/min and the NO concentration is 500ppm,600m L of 5 wt%ammonium sulfite/urea absorption liquid can be used to achieve 56.6%denitration efficiency.The preoxidation coupled liquid phase absorption denitration experiments show that when the H₂O₂/NO molar ratio is 2.5,the total denitration efficiency reaches 87%,and the efficiency increases when the molar ratio increases.The NO removal efficiency is higher than 90%when the H₂O₂/NO molar ratio is bigger than or equal to 5.And in the preoxidation coupled liquid phase absorption simultaneous desulfurization and denitration experiment,the SO₂ gas concentration has almost no effect on the NO removal efficiency,and the SO₂ removal efficiency can be maintained at approximately 100%.Finally,Na OH was used as the catalyst deactivation factor to simulate the effect of different loading ratios on the preoxidation efficiency,and the deactivated catalyst was regenerated by water washing and0.05mol/L H₂SO₄ washing.The results show that low loading of Na OH causes a significant decrease in the catalytic efficiency.When loading ratios are 0.01,0.05,and 0.1,the catalyst still retains the temperature characteristics,but the catalyst with a loading ratio of 0.01 only retains 50%-70%of the catalytic activity in the temperature range of 120-200°C,and the catalytic activity decreases faster with the increase of the loading.When the ratio is 0.25-0.75,the catalyst completely loses the original temperature characteristics of the catalyst,only a little catalytic ability in the low temperature section of 80-120°C,when the loading molar ratio is 1,the catalyst is completely deactivated.The catalytic activity of the deactivated catalyst can improve to different degrees through water washing and acid washing regeneration.It is worth noting that the catalytic efficiency of the acid-washing regenerated catalyst is still higher than the raw one.The efficiency can reach 83.5%when a vaporization temperature of 140°C and a catalytic temperature of140°C and injecting 10mol/L H₂O₂ at 2.5m L/h.The XRD,FTIR,SEM,NH₃-TPD,BET results show that the physical cause of catalyst deactivation is the deposition of Na OH particles on the catalyst surface and pores,but the main reason is that the reaction of Na OH with the acid sites on the catalyst surface,which causes the decrease of the catalyst surface acidity,and deactivates the catalyst.The acid washing regeneration because of the introduction of new acid sites Fe₂（SO₄）₃ makes the catalyst more efficient.