Catalytic Oxidation Of Gas-phase Elemental Mercury And Bivalent Mercury Aqueous Absorption-Reduction Behaviour Study

Mercury, due to its persistence, bio-accumulation and neurological toxicity, has received tremendous attentions. The coal-fired power plant was the major source of the gas-phase mercury, which was mainly in elemental form and could not be removed by the existing dust removal and desulfurization system. The widely industry application of the active carbon injection method(ACI) for mercury capture from coal-derived flue gases was limited by its rather high operating cost and the harmful effect on the fly ash quality. A co-effective and economic alternative was to transfer the elemental mercury to more soluble bivalent form, following removed in wet absorption process. Thus, the mercury removal process of the gas-phase oxidation and aqueous absorption combined system was systematically investigated in this dissertation.Firstly, several catalysts were prepared by doping different metals into the TiO2 for mercury oxidation tests by sol-gel method, which was optimized by our group. By the activity tests and loading content optimization, a cobalt doped catalyst having an oxidation efficiency of 100% within the temperature reaction window of 150-300℃was obtained. The chosen catalyst was then characterized by XRD, XPS, TEM, HRTEM and TPR, indicating that the main active phase of the catalyst was Co3O4. Furthermore, a modified Mars-Maessen mechanism was proposed by us through the analysis of the performance of HCl and O2 during the reaction, in which the performance of HCl and O2 as well as their effects on mercury oxidation were defined.Secondly, the removal of bivalent mercury by the wet Ca-based FGD system, the Dual-Alkali FGD system and the magnesia FGD system were evaluated in a simulated flue gas absorption device, respectively. Main gas and liquid conditions as well as some aqueous ions on mercury removal were experimental studied. The results showed that three FGD systems all showed high efficiency on bivalent mercury absorption, but the reduction amount of absorbed bivalent mercury was also very large. Within the pH value of 4.5-5.5, about 70% of the absorbed mercury was reduced to elemental form in the wet Ca-based FGD system, while that were around 30% in both the Dual-Alkali and magnesia FGD systems. The increase of sulfate and chloride ions in the absorbent greatly inhibited the mercury reduction. The magnesium ion had different effects on mercury reduction in the Ca-based FGD system and Dual-Alkali system. NO and O2 in the gas were beneficial to the mercury removal, while SO2 would somewhat lower the removal efficiency.Finally, the UV-vis spectra and some other technologies were adopted to investigate the mercury reduction behaviors in the presence of magnesium, sulfate and chloride ions, respectively. The enhancement of reduction in the Ca-based FGD system was contributed to the increased surface area of CaSO3·0.5H2O suspended solid particle, thereby increasing the active sites of mercury reduction. And the reduction was inhibited by the formation of MgSO30 ion pairs in the Dual-alkali system by the addition of magnesium. The inhibitions by the addition of sulfate and chloride were attributed to the formation of HgSO3SO42-, Cl2HgSO32- and ClHgSO3-, respectively. Cl2HgSO32- was recognized as the more stable one compared to ClHgSO3- in the presence of chloride. Aqueous HgSO3 decomposition was the key step of mercury reduction, since presences of sulfate, chloride and excess sulfite ions all limited its decomposition.