Experimental And Mechanism Study Of Elemental Mercury Oxidation In Flue Gas Over Aluminum-based SCR Catalysts

The volatility, persistence, bioaccumulation and toxicity of mercury make it an important and urgent task to develop efficient ways to reduce mercury emission from coal-fired power plants. Currently, there are several technologies available to remove Hg0from coal-fired power plants, such as sorbent injection, catalytic oxidation, and photochemical oxidation. Among these methods, selective catalytic reduction (SCR) technology, which exhibits the co-benefit of promoting mercury oxidation in the process of NOx reduction, has been in the spotlight recently. Although vanadium based SCR catalysts have been successfully developed for industrial use, there still be some drawbacks where improvement can be made. Novel catalysts which can overcome these shortcomings would probably be adopted as the next generation SCR catalysts. Hg0oxidation studies over these SCR catalysts would be of great scientific and practical value.CuO-MnO2-Fe2O3/y-Al2O3(CMFA) SCR (selective catalytic reduction) catalyst prepared by improved impregnation method was first studied for gas-phase mercury oxidation in a simulated coal combustion flue gas at typical SCR temperatures. Brunauer-Emmet-Teller (BET), X-ray Diffractogram (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the catalyst. The CMFA catalyst yielded nearly80%oxidation efficiency using a simulated flue gas (O2, CO2, HC1, NO, SO2, H2O and balanced with N2). A3-day experiment indicated that CMFA had a long service life and would be promising in industrial applications. In the presence of O2, an enhancing effect of HC1, NO and SO2was observed. Without O2, NO still promoted Hg0oxidation due to the formation of multi-activity NOx species. Meanwhile, SO2had little effect on Hg0oxidation and HC1inhibited Hg0adsorption. Hg0oxidation over the CMFA catalyst might follow the Eley-Rideal mechanism, where gaseous or weakly bonded Hg0reacted with adsorbed active species on the catalyst surface to generate Hg2+. Although NH3would inhibit Hg0oxidation, once NH3was cut off, the efficiency fully recovered.To elucidate the mercury species exsited on the CMFA catalyst surface, a temperature programmed thermal decomposition (TPTD) method was applied in this study to analyze the characteristics of the mercury species formed on the CMFA catalysts. A series of pure mercury compounds mixed with fresh CMFA were first studied for qualitative calibration. Then, the TPTD method was employed to identify the Hg species on used CMFA catalysts pretreated under different operation conditions. The formation of HgO, HgCl2, Hg(NO3)2and HgSO4during the oxidation process was confirmed and the reaction pathways were proposed. The mercury species present were mainly HgCl2after CMFA catalysts were used to oxidize Hg0under simulated flue gas conditions. HgO and HgSO4were found to exist in very low concentrations. This has shown that the TPTD method is an efficient technique for mercury speciation on the catalyst surface.The kinetic results would provide insights into the understanding of Hg0oxidation mechanism. The reaction kinetics of Hg0oxidation on CMFA catalysts were studied on a fixed-bed reactor. The results showed that the influence of internal and external diffusion could be ignored when gas flow rate was2000ml/min with the sized of catalysts particle of0.1mm. The reaction was first order with respect to Hg0concentration, zeroth-order with respect to HCl and O2. The kinetic studies demonstrated that Hg0oxidation over CMFA catalyst follows the Eley-Rideal mechanism. Based on the kinetic results, Hg0oxidation efficiencies can be roughly estimated using the given amount of the catalysts and flue gas conditions.In order to investigate the activity of CMFA catalyst under real flue gas conditions, the performances for selective catalytic reduction (SCR) were explored in a200MW coal-fired power plant boiler tail flue. The catalytic efficiencies for Hg0oxidation were tested using Ontario Hydro Method (OHM) as well. Besides, traditional CuO/γ-Al2O3catalyst was used as comparision. NH3-TPD (NH3temperature programmed desorption) and H2-TPR (H2temperature programmed reduction) were used to characterize the catalysts. The presence of the BrΦnsted-acidic sites and Lewis-acidic sites were responsible for the superior activity. H2-TPR results indicated that the redox properties of the catalyst play an important role in the catalytic reaction. The optimum temperature range for the SCR of NO over fresh catalysts was300～400℃. In the optimum temperature range, the efficiencies of the two catalysts were over80%, with a maximum efficiency of nearly90%obtained at350℃. Reducing the catalysts amounts to half did not change the efficiency dramatically. The highest SCR activity was obtained with NH3/NO molar ratios ranging from1.0to1.2. Abrasion resistance test indicates that the mechanical strength of the catalysts can bear the industrial scale and long-term erosion of fly ash. The activities of the catalysts remained constant during a200h test. Meanwhile, the catalysts were able to maintain excellent NO reduction efficiency after three times recycling. CuO/γ-Al2O3and CMFA achieved40%and80%Hg0oxidation efficiency, respectively. The results provide a promising catalyst for NO and Hg0removal for coal-fired power plants.Low temperature SCR technology has been in the spotlight recently. Thus, MnOx-CeO2/y-Al2O3(MnCe) low temperature SCR catalysts prepared by sol-gel method were employed for low-temperature Hg0oxidation on a fixed-bed experimental setup. BET, XRD and XPS were used to characterize the catalysts. MnCe catalysts exhibited more than80%Hg0oxidation activity at250℃under the simulated flue gas (O2, CO2, NO, SO2, HC1, H2O and balanced with N2). Only a small decrease in mercury oxidation was observed in the presence of1200ppm SO2, which proved that the addition of Ce helped resist SO2poisoning. An enhancing effect of NO was observed due to the formation of multi-activity NOX species. The presence of HC1alone had excellent Hg0oxidation ability, while10ppm HC1plus5%O2further increased Hg0oxidation efficiency to100%. Hg0oxidation on the MnCe catalyst surface followed the Langmiur-Hinshelwood mechanism, where reactions took place between the adsorbed active species and adsorbed Hg0to form Hg2+. NH3competed with Hg0for active sites on the catalyst surface, hence inhibiting Hg0oxidation.