Study On Elemental Mercury Adsorption Mechanism Of Biochar Based On Density Functional Theory

The elemental mercury is removed by catalyzed oxidation and adsorbtion in the flue gas of coal fired power plants.However,the existing elemental mercury adsorbtion is not widely used due to its competitive adsorption,high cost and narrow temperature range.Therefore,the biochar obtained during biomass pyrolysis will be one of the most promising,green,cheap and efficient mercury adsorbent.Biochar is a mixture of high-quality heterogeneous and complex structure.The mechanism of elemental mercury adsorption by biochar is obtained by exploring the characteristics of biochar structure from the perspective of micro-molecular.It has great scientific significance and practical value to develop the technology.In this thesis,the walnut shell was selected to prepare biochar.Based on ultimate analysis and proximate analysis,the chemical composition of biochar were studied by FTIR and XRD.The characteristics of biochar molecular structure were analyzed and characterized by 13C-NMR.The molecular structure of biochar was constructed.It was found that the walnut shell biochar consisted of C,H,O and N.The aromatic carbon was the main component of the biochar molecular structure,while the aliphatic carbon acted as the unit of the aromatic structure.The molecular structure model of biochar was mainly composed of aromatic structure,and contained one methyl group,four hydroxyl groups and eight carbonyl groups.The molecular formula was C₅₅H₃₇NO₁₄ and M_r=935.The13C-NMR prediction calculated spectra of the constructed molecular model were in good agreement with the experimental spectra,and were consistent with the characterization results such as FTIR and elemental analysis.The total potential energy of the optimized three-dimensional structure under the UFF force field was the largest,which was 1855kJ·mol^-1.While the potential energy under the MM2 force field was the smallest,which was 529.4kJ·mol^-1.Meanwhile,the heat generation of the three optimized constellations was calculated by the semi-empirical PM6 method of quantum chemistry,and the optimized structure under the Dreiding force field was more stable.Based on the density functional theory,the suitable biochar fragment structure was selected to optimize the simulation,and the effect of oxygen-containing functional groups and metal ions on the mercury adsorption performance of biochar was calculated.The adsorption height,adsorption energy and Mulliken layout of biochar were calculated.The results indicated that the aldehyde group and carbonyl functional group promoted the physical adsorption and chemisorption of biochar.The hydroxyl functional group had no effect on the mercury adsorption capacity of biochar.The ether group inhibited the mercury adsorption capacity of biochar by the calculation of adsorption energy.Compared with unmodified carbon-based adsorbents,biochars loaded with Fe³⁺,Co³⁺,Ni²⁺,Cu²⁺and Zn²⁺could increase their adsorption capacity for mercury,and the adsorption process had both physical adsorption and chemical adsorption.The adsorption energy of the modified biochar was 224.9.1,236.9,78.3,184.5 and 91.1kJ·mol^-1,respectively,which were larger than the adsorption energy of unmodified biochar that was 19.1kJ·mol^-1,so chemical adsorption was dominant on the adsorption process.In addition,the carbon atom adsorption site adjacent to the metal ion was the main adsorption site.