| Mercury is a chemical element that is harmful to human health,ecosystems,and the environment.Due to persistent and bioaccumulative of mercury,it is more harmful to humans who are at higher levels of the biological chain.The accumulation of mercury in the human body can easily cause chronic diseases such as damage to the central nervous system of the human body,brain and liver.Therefore,effective control of mercury in the environment should be attentioned.Mercury emissions mainly come from human industrial activities.Among all anthropogenic mercury sources,mercury released from coal combustion in coal-fired power plants is one of the main sources.Therefore,effective control of mercury emissions from coal-fired power plants will greatly reduce mercury pollution in the environment.There are three types of mercury in coal-fired power plant flue gas,elemental mercury,oxidized mercury,and particulate mercury.Both oxidized mercury and particulate mercury can be removed in electrostatic precipitator and desulfurization tower.Elemental mercury is the most difficult to remove from the flue gas because of its poor solubility and high volatility.Therefore,elemental mercury has become the research focus in the process of mercury removal.At present,the mature technology for mercury removal is activated carbon injection.Due to the high cost of activated carbon,and its chemical and physical modification processes are cumbersome and time-consuming,the cheap and easily available biochar is used to prepare adsorbent for mercury removal by fast and efficient plasma modification method in this article.The plasma modification mechanism and the mercury removal mechanism of modified biochar have also been explored.Tobacco,rice,wheat,corn,millet,and black bean straw were pyrolyzed at 60? to obtain six kinds of original biochar.Cl2 or H2S plasma were used to modify six biochar.The mercury removal performance and physical and chemical characteristics of biochar before and after modification were tested.The mercury removal experiment results show that the mercury removal efficiency of Cl2 and H2S plasma modified biochar has been greatly improved.The mercury removal efficiency of the most effective modified biochar has increased by 69.1%?76.0%.It shows that the plasma modification method can effectively improves the mercury removal ability of biochar.Through infrared spectroscopy and other characterizations,it was found that chlorine-containing functional groups or sulfur-containing functional groups were generated on the surface of biochar modified by Cl2 or H2S plasma,respectively.After the modified biochar was subjected to mercury adsorption,the C-Cl and C-S functional groups were consumed.Combined with the results of mercury temperature program desorption,it can be suggested that the plasma can transfer the chemical elements in the modified gas which is helpful to remove mercury to the surface of the biochar,and combine with the biochar to form functional groups.Then the generated functional groups can combine with Hg0 to form HgCl2 or HgS.Cl2 plasma modified biochar showed good mercury removal performance at 30 and 150?.For H2S plasma modified biochar,it exhibited better mercury removal efficiency only at 30?.The mercury removal efficiency decreased significantly at 150?.Experiments and characterizations shown that the sulfur-containing functional groups formed on the surface of biochar are prone to fracture at high temperatures,thereby reducing their ability to remove mercury.During the oxygen-free plasma modification process,the number of oxygen-containing functional groups of biochar has increased,and the generated oxygen-containing functional groups can oxidize Hg0 to HgO.After investigation,the source of the oxygen-containing functional group is mainly the oxygen from the air adsorbed by the biochar during the storage process.Then the adsorbed oxygen combined with biochar to form oxygen-containing funtional groups under plasma environment.In addition,the plasma can cause the etching of the biochar surface,which creating a damage to its surface.During the plasma modification process,the changes in discharge voltage,discharge time,modification gas concentration,and modification gas flow rate all have effect on the mercury removal ability of biochar.Through experiments and characterizations,it was found that with the enhanced of discharge voltage from 2 to 8 kV,the mercury removal efficiency of plasma-modified biochar increased by 56.2%.The increase in efficiency is mainly due to the better plasma generation at higher voltages and the more mercury-removing functional groups formed.When the discharge time raised from 0 to 5 min,the mercury removal efficiency of biochar increases accordingly.When the discharge time is too long(10?30 min),the mercury removal efficiency decreases instead.Due to the long-term plasma modification,the surface of biochar has serious etching damage,thereby reducing the effect of mercury removal.As the concentration of modified gas improved,the mercury removal capacity of modified biochar was also increased.The mercury removal capacity of biochar reached saturation when the gas concentration reached 1%.The fast modification flow rate caused the active molecules generated under the plasma to be quickly taken out of the reactor,thereby reducing the modification effect and abating the mercury removal performance of biochar.During the mercury removal process,the experimental atmosphere(SO2,NO,NH3)has different influences on the mercury removal performance of Cl2 plasma modified biochar.Experiments shown that NH3 can improve the mercury removal efficiency of modified biochar,and its improvement effect reached the highest when the NH3 concentration was 1500 ppm,but shown a downward trend at 2000 ppm.The promotional effect of NH3 on the mercury removal of modified biochar is caused by converting Cl-to C-Cl to form more adsorption active sites.SO2 exhibited an inhibitory effect on the mercury removal process of modified biochar.Through X-ray photoelectron spectroscopy and other characterizations,it was found that SO2 easily combined with biochar to generate sulfate compounds on the surface of biochar,thereby blocking the active site.NO exhibited the ability to promote the oxidation ability of modified biochar for Hg0.The results shown that after the introduction of NO,the proportion of HgO generated on biochar was greatly increased.The simulation results of the plasma generation process shown that Cl2 and H2S mainly produced Cl atoms and S2 molecules in the plasma environment.For the mercury removal process simulation,the two adjacent C-Cl on the Cl2 plasma modified biochar showed the best adsorption energy for Hg0,which was-114.3 kJ/mol;on the H2S plasma modified biochar,only the C-S2 configuration showed better adsorption energy,which was-99.6 kJ/mol.The simulation data of plasma generation and mercury removal process of modified biochar are in good agreement.Therefore,the carbon-sulfur functional group on the surface of biochar modified by H2S plasma is in the form of C-S2 as the main mercury removal functional group.The modification mechanism and the mercury removal mechanism are further promoted. |
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