Mechanism Study Of Mercury Removal By Nanosulfur Particles Prepared By Nonthermal Plasma

Mercury is one of the toxic heavy metal elements with high volatility,almost all mercury in coal is released during combustion process.Adsorbent injection technology is considered as an effective strategy to remove mercury from flue gas.Excellent adsorbents should be characterized by high mercury removal efficiency,high active substance utilization rate and high product stability.Based on this,it is proposed to use nonthermal plasma to induce H₂S decomposition reaction to generate mercury removal active material-sulfur nanoparticles in the gas phase.This study provides a theoretical foundation for the preparation of high purity sulfur nanoparticles and the development of new technologies in the field of mercury removal from coal flue gas.Firstly,the influence and internal mechanism of different experimental parameters on the production of sulfur nanoparticles were investigated.It was found that the initial H₂S concentration and gas residence time were the key parameters that affected the size and number concentration of sulfur nanoparticles,however,the plasma discharge voltage has ignorable effect on particle size,but on number concentration.An increase in the gas concentration（2.5%to 20%）leads to an increase in the average size of sulfur nanoparticles（5.12 nm to 91.90 nm）and an increase in the particle number concentration.With the increase of residence time（3.6 s to 18.5 s）,the size of sulfur nanoparticles（3.70 nm to 20.62 nm）and particle number concentration increased.The increase of reduced electric field intensity would promote the ionization reaction of Ar molecules and the excitation,dissociation and ionization reactions of H₂S,and then promote the decomposition of H₂S.The increase of H₂S concentration would inhibit the decomposition and ionization reaction of H₂S,and then reduce H₂S conversion rate.Secondly,the mercury removal performance and reaction mechanism of sulfur nanoparticles were investigated.Analysis of solid elemental sulfur samples by high performance liquid chromatography-mass spectrometry（HPLC-MS）showed that the main component was S₈（>95%）and contained small amounts of S₆ and S₇.Compared with the micron-size elemental sulfur（mercury removal efficiency<10%）,the sulfur nanoparticles prepared by nonthermal plasma have excellent mercury removal efficiency（initial mercury removal efficiency is 88.2%）,which is due to the higher content of S₆（1.53%）and S₇（1.29%）in the sulfur nanoparticles.S₆ and S₇ were confirmed to be mercury removal active components in the elemental sulfur.Quantum chemistry calculation results showed that S₆and S₇ allotropes exhibited higher thermodynamic driving force and faster chemical reaction rate than S₈.The optimal adsorption temperature of nanosulfur particles was 110 ℃,which was suitable for coal-fired power plants equipped with low-low temperature electrostatic precipitator.Then,to broaden the mercury removal temperature window of nanosulfur adsorbent,nanosulfur modified activated carbon adsorbent was developed.There are not only solid nano sulfur particles deposited on the surface of activated carbon carrier,but also gaseous S_n（n=2～8）molecules in the discharge region during plasma modification process.Quantum chemistry calculation found all S_n molecules would undergo dissociation adsorption on activated carbon surface（adsorption energy of-440～-800 k J/mol）.Gaseous S₄ and S₅molecules adsorbed on the surface of activated carbon would serve as new active sites for mercury removal,and the oxidation process of Hg⁰ on S₄+AC and S₅+AC surface is chemical adsorption（adsorption energy of-100.3 k J/mol and-300.5 k J/mol,respectively）.Adsorption experiments showed that the optimum reaction temperature of nano-sulfur-modified activated carbon was 140 ℃,which was suitable for coal-fired power plants without low-low temperature electrostatic precipitator.Finally,from the practical application,the stability of the contaminated elements in the nano-sulfur adsorbents was investigated in the environment,and compared with the commercially used halogen（X=Cl,Br,I）modified adsorbents.Leaching experiments showed that Hg S formed by mercury removal products of nanometer sulfur adsorbent was stable,and the concentration of mercury in the leachate was lower than the detection limit of the instrument.Quantum chemical calculation was used to reveal the microscopic nature of the stability of pollutant elements.It was found that there was a correlation between the leaching characteristics of mercury and the ionic percentage of mercury forming chemical bonds on the surface of the adsorbent.