| Most of fuel consumed in China will be coal for several decades in future. The serious environmental pollution caused by coal combustion will be inevitable. As the largest source of anthropogenic mercury emissions, mercury emitted from coal-fired power plants has been identified as a hazardous pollutant to both human health and environment. At present, China is one of the largest source regions that release mercury into the atmosphere. Some measures and rules have been taken to control the mercury emissions from coal-fired power plants in some western developed countries, such as American, and Canada. However, the sifting of effective sorbents and researches of in situ full-scale demonstration for mercury control in coal-fired power plants are still weak in China. Therefore, it is necessary and important to find out the effective sorbents on mercury capture and to understand the mechanism of mercury removal.In this paper, several measures of mercury removal from coal-fired power plant were presented in detail. The solid sorbents researched extensively and their performances on mercury removal were emphatically introduced. The method of chitosan (CTS) modification and their applications on heavy metals removal were systematically summarized. The quantum chemistry theory studies on oxidation/adsorption of elemental mercury (Hg0) and the mechanism of CTS sorbents for heavy metals adsorption were deeply discussed. The possibility of modified chitosan sorbent on Hg0 capture from flue gases was analyzed.Five types of modified chitosan sorbents were synthesized using several advantages of chitosan. There are copper-templated CTS sorbent, silicon-modified CTS sorbent, iodine (or bromide) and acid modified CTS sorbent, iodine and acid modified bentonite (B)/CTS sorbent, and silver-modified B/CTS sorbent. The characterization of these sorbents was analyzed using N2 adsorption and desorption method, Fourier transform infra-red spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and Scanning electron microscope (SEM), et al. The results show that the surface areas of CTS, silicon-modified CTS, and B/CTS sorbent decrease after modification. The FTIR spectra demonstrate that the reaction of amide of CTS and Cu2+ occurs in the process of copper-templated CTS sorbent preparation. And the chemical reaction of iodine and sulfate ion with the amide of CTS occurs. TGA indicates that the thermal stability of CTS decreases after modifying by H2SO4, and potassium iodide, while it can be increased greatly when bentonite supported on the chitosan. XRD spectra show that the area under amorphous decreases after modification. This is caused by the fact that inter- and extra-molecular hydrogen bonds are destroyed due to the presence of H2SO4. Furthermore, the peak of I2 is observed in the iodine and H2SO4 modified CTS sorbent, which implies the occurrence of the interaction between H2SO4, KI and chitosan. XPS analysis reveals that nitrogen atom provides electron pair, and sulfur atom inclines to accept the electron. More active sites, such as S and Cl, can be easily obtained after silanization in silicon-modified CTS sorbent.Adsorption experiments of vapor-phase elemental mercury (Hg0) were studied using these sorbents in a laboratory-scale fixed-bed reactor with nitrogen as gas sources. VM3000 online mercury analyzer was applied to detect the inlet and outlet Hg0 concentrations. The results show that the parent CTS has no effect on Hg0 removal. The copper-templated CTS sorbents, which have memory function for Cu2+ vacancy, can adsorb the metal ion or vapor-phase metal which has similar radius as copper ion. These changes can improve the Hg0 and Hg2+ capture efficiency greatly. For the silicon-modified CTS sorbent, mercury removal efficiency improves with the presence of O2. For the iodine or bromide-modified CTS sorbents, mercury removal efficiency of CTS sorbents could be significantly promoted when appropriate amount of H2SO4 was added. Generally, the iodine and acid modified sorbents demonstrate higher mercury capture efficiency than that of bromine and acid modified sorbents. Silver-modified B/CTS sorbents exhibit somewhat poor mercury removal efficiency at higher reaction temperature, while they show higher mercury capture performance at room temperature. The results obtained under simulated flue gases show that presence of SO2 has negative effect on Hg0 removal. Somehow positive effect occurs when HC1 is added. For iodine and acid modified B/CTS and silver-modified B/CTS sorbents, both H2O and NO can enhance the adsorption of Hg0. Compared to iodine and acid modified B/CTS sorbents, silver-modified B/CTS sorbents show a more significant change caused by H2O and NO.According to the possible adsorption sites of iodine and acid modified CTS sorbent, the adsorption of Hg0 on modified CTS sorbent was investigated systematically by Density Functional Theory (DFT). The results show that the adsorption abilities in the sites 1#,2#, and 3# of parent CTS for Hg0 are very small. It is found that the amide of CTS exhibits higher adsorption abilities for H+ and HI, and their adsorption energies are 991 and 74kJ/mol, respectively. While for I2, the physical adsorption phenomenon is found. These sorbents modified by H+, HI, or I2 all have little effect on Hg0 removal. CTS reveals an excellent adsorption performance for H+ and I2 simultaneously and the adsorption energy is 1020 kJ/mol. The main contribution to the highest occupied molecular orbital (HOMO) of CTS-H+-I2 complex is from the p orbital of I atom near the amide of CTS. There is a significant contribution from the d orbital of Hg0 atom in CTS-H+-I2-Hg complex after Hg0 adsorption. The complex of CTS-H+-I2 displays a big binding energy of 127kJ/mol for Hg0 adsorption.Furthermore, theoretical explorations of single noble metal (Ag, Au, Pd, and Pt) on the three sites of CTS, respectively, and Hg0 adsorption on the single noble metal-CTS complexes were also conducted by DFT method. The results show that among the three sites of CTS, the site 2# of CTS exhibits the biggest adsorption energies for Ag and Au atoms, respectively. While for Pt and Pd atoms, the sites are 1# and 3#, respectively. It has been found that, in each site, the adsorption energies of noble metal on CTS are almost the same except Pt atom. Similarly, the adsorption energies of Hg0 on noble metal-CTS complexes are also the same. It has been found that, in each site, the adsorption energies of Hg0 on noble metal-CTS complexes occur in the following order:Pt> Pd> Au> Ag, which is also consistent with order of the adsorption energies of noble metal on CTS. It has been found that the adsorption energies of Hg0 on three M-T3 clusters occur in the following order:Au-T3> Cu-T3> Ag-T3. For the individual adsorption system, a clear correlation between adsorption energy and M-Hg distance (and Mulliken charges) is found. Hg0 adsorption energy is substantially increased upon NO being preadsorbed on M-T3 surface, however, a significant weakening of the Hg0 bonding to the sites takes place in the vicinity of a bound SO2 molecule. These phenomena are in agreement with the experimental results.
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