Experimental Research On Mercury Removal Over Magnetic Biochar In Oxyfuel Combustion Flue Gas

Combustion of coal and other fossil fuels has caused severe atmospheric mercury pollution,posing a serious threat to human health.Especially in the oxyfuel combustion system,the concentration of gaseous mercury is higher,which may adversely impact the stable operation of combustion equipment.Thus,it is necessary to develop effective mercury adsorption for oxyfuel combustion atmosphere.Magnetic biochar has a wide range of raw material sources,low preparation and use costs,and can be sprayed in the flue gas to remove mercury after recovery and reuse.However,in the oxyfuel combustion flue gas,the concentrations of acid gases,CO₂ and H₂O are very different from them in air combustion flue gas,which may cause impacts on mercury removal over magnetic biochar.In this paper,the effects of oxyfuel combustion flue gas components on the adsorption and oxidation efficiencies over magnetic biochar are studied.In this study,magnetic biochar was first prepared as sample,and the influence of CO₂and H₂O in the oxyfuel combustion flue gas on mercury adsorption and oxidation efficiencies was studied.The reaction mechanisms were explained by characterization and mercury desorption experiments.The experiments found that the C-Cl group on the magnetic biochar is the main active site for mercury removal.The mercury adsorbed after the mercury removal reaction mainly exists in the form of Hg Cl₂.High concentration CO₂can promote the formation C=O groups on the magnetic biochar,which can further promote the formation of organic bound mercury.The high concentration H₂O has a significant inhibitory effect on the adsorption process of mercury on the magnetic biochar.Then,the effects of enriched SO₃ in oxyfuel combustion flue gas on mercury removal were investigated.The experiments show that low concentration SO₃ can cause significant adverse effects on the adsorption of mercury,but with the increase of SO₃ concentration,the oxidation efficiency of mercury was significantly improved,which partially offsets the decrease of mercury adsorption efficiency caused by SO₃.In addition,SO₃ will also increase the content of chemically adsorbed oxygen on the sample,thereby promoting the formation of Hg O.When SO₃ and H₂O exist at the same time,inert substances such as sulfates will be formed on the magnetic biochar,resulting in a reduction of mercury removal efficiency.The high concentration SO₂ and NO in oxyfuel combustion flue gas are adverse to mercury adsorption.However,in the higher reaction temperature,SO₂ can promote mercury oxidation,thereby creating favorable conditions for mercury removal in wet desulfurization device.The reason is that more oxygen-containing functional groups are generated on the magnetic biochar and participate in the oxidation reaction of mercury.The addition of HCl can always significantly improve the adsorption efficiency of mercury.Spent magnetic biochar can be easily activated by heat treatment under 450℃for cyclic mercury removal.