Photocatalytic Removal Of Elemental Mercury From Natural Gas

As clean and low-carbon energy,natural gas can reduce the emission of pollutants such as SO₂,CO₂,NOx and dust while reducing the consumption of coal and oil,and fundamentally improve the problem of environmental pollution.Mercury in natural gas mainly exists in the form of zero-valent mercury,which is not easy to settle,easy to migrate,and has highly biological toxicity.During the extraction,transportation and storage of natural gas,elemental mercury not only corrodes processing equipment and transportation pipelines,but also causes catalyst poisoning.Therefore,elemental mercury needs to be removed from natural gas at the source.There is an urgent need to develop an efficient,economical and widely applicable natural gas mercury remover.The operating temperature of natural gas is low,close to normal temperature.Photocatalytic mercury removal can be applied in low temperature environment with high catalytic efficiency.TiO₂ is widely used due to its good thermal stability,mechanical strength and high photocatalytic efficiency.After modification with transition metal oxides,the recombination of photogenerated electron-hole pairs can be reduced and the sulfur resistance can be improved.In this paper,a photocatalyst for mercury removal from natural gas was prepared by bimetallic modification of CeO_x or MCe（M=Cu,Fe,Mn）on TiO₂ by sol-gel method.Compared with powder reactors,honeycomb ceramics have higher light source utilization and are easy to industrialize and scale up,so catalyst precursors are coated on the surface of honeycomb ceramics.The mercury removal performance of photocatalysts was systematically studied on a photocatalytic reaction device.The effects of element doping amount,catalyst calcination temperature,reaction temperature,light intensity and natural gas composition on the mercury removal efficiency of the catalyst were experimentally studied.UV-Vis,XRD,N2 adsorption-desorption,XPS,Hg-TPD The catalyst was characterized by other characterization methods,and the reaction process on the surface of the catalyst was described with the analysis of the results.The results show that the optimal Ce loading and catalyst calcination temperature were 1mol%and 400℃,respectively.Under the conditions of reaction temperature of 60℃,UV light intensity of 3.0 m W/cm² and coated value of 0.5 g,the mercury removal efficiency of1Ce Ti-400 reached the best.O₂ can replenish the active oxygen consumed on the surface of the catalyst in time,and H2S will generate active elemental sulfur on the surface of the catalyst,so the presence of O₂ and H2S improves the mercury removal efficiency of the catalyst.CO₂ competes with Hg⁰ for active adsorption sites,which inhibits the mercury removal performance of the catalyst to a certain extent.In addition,H2S can alleviate the inhibitory effect of CO₂.When the atmosphere contains H2S,1Ce Ti-400 has a superior oxidation efficiency on Hg⁰.The characterization results of Hg-TPD and XPS showed that Hg S existed on the surface of the catalyst after the reaction.In continuous experiments,1Ce Ti-400 has good stability,and the mercury removal efficiency is stable at about 90%.Calcination at 400℃can effectively regenerate the catalyst,but after multiple regenerations,the mercury removal efficiency drops significantly and remains around 55%.The mercury removal efficiency of CeO_x-TiO₂ catalyst was inhibited when the H2S concentration was higher than 400 ppm,so the CeO_x-TiO₂ was further modified with Cu/Fe/Mn three metals to improve its H2S resistance.The experimental results show that the mercury removal performance of the catalyst after Cu modification was significantly improved.When the doping ratio of Cu/Ce was 1:2 and the reaction temperature was 25℃,the catalyst average mercury removal efficiency was as high as 90.40%.The addition of Cu improves the average mercury removal efficiency（2 h）of the catalyst by about 7%in the atmosphere with H2S concentration higher than 400 ppm.H2S in the atmosphere was favorable for mercury removal,but reduces the oxidation performance of the catalyst.What′s more,the addition of Cu improved the regeneration performance of the catalyst.After the Cu2Ce Ti-400 catalyst was regenerated for many times,the mercury removal efficiency remained at about 87%.