Wenqing Xu

Mercury removal from coal combustion flue gas by modified fly ash

Fly ash is a potential alternative to activated carbon for mercury adsorption. The effects of physicochemical properties on the mercury adsorption performance of three fly ash samples were investigated. X-ray fluorescence spectroscopy, X-ray photoelectron spectroscopy, and other methods were used to characterize the samples. Results indicate that mercury adsorption on fly ash is primarily physisorption and chemisorption. High specific surface areas and small pore diameters are beneficial to efficient mercury removal. Incompletely burned carbon is also an important factor for the improvement of mercury removal efficiency, in particular. The C-M bond, which is formed by the reaction of C and Ti, Si and other elements, may improve mercury oxidation. The samples modified with CuBr2, CuCl2 and FeCl3 showed excellent performance for Hg removal, because the chlorine in metal chlorides acts as an oxidant that promotes the conversion of elemental mercury (Hg0) into its oxidized form (Hg2+). Cu2+ and Fe3+ can also promote Hg0 oxidation as catalysts. HCl and O2 promote the adsorption of Hg by modified fly ash, whereas SO2 inhibits the Hg adsorption because of competitive adsorption for active sites. Fly ash samples modified with CuBr2, CuCl2 and FeCl3 are therefore promising materials for controlling mercury emissions.

Mechanism of Hg(0) oxidation in the presence of HCl over a commercial V2O5-WO3/TiO2 SCR catalyst.

Experiments were conducted in a fixed-bed reactor containing a commercial V2O5/WO3/TiO2 catalyst to investigate mercury oxidation in the presence of HCl and O2. Mercury oxidation was improved significantly in the presence of HCl and O2, and the Hg(0) oxidation efficiencies decreased slowly as the temperature increased from 200 to 400°C. Upon pretreatment with HCl and O2 at 350°C, the catalyst demonstrated higher catalytic activity for Hg(0) oxidation. Notably, the effect of pretreatment with HCl alone was not obvious. For the catalyst treated with HCl and O2, better performance was observed with lower reaction temperatures. The results showed that both HCl and Hg(0) were first adsorbed onto the catalyst and then reacted with O2 following its adsorption, which indicates that the oxidation of Hg(0) over the commercial catalyst followed the Langmuir-Hinshelwood mechanism. Several characterization techniques, including Hg(0) temperature-programmed desorption (Hg-TPD) and X-ray photoelectron spectroscopy (XPS), were employed in this work. Hg-TPD profiles showed that weakly adsorbed mercury species were converted to strongly bound species in the presence of HCl and O2. XPS patterns indicated that new chemisorbed oxygen species were formed by the adsorption of HCl, which consequently facilitated the oxidation of mercury.

Hg0 removal from flue gas over different zeolites modified by FeCl3.

The elemental mercury removal abilities of three different zeolites (NaA, NaX, HZSM-5) impregnated with iron(III) chloride were studied on a lab-scale fixed-bed reactor. X-ray diffraction, nitrogen adsorption porosimetry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and temperature programmed desorption (TPD) analyses were used to investigate the physicochemical properties. Results indicated that the pore structure and active chloride species on the surface of the samples are the key factors for physisorption and oxidation of Hg0, respectively. Relatively high surface area and micropore volume are beneficial to efficient mercury adsorption. The active Cl species generated on the surface of the samples were effective oxidants able to convert elemental mercury (Hg0) into oxidized mercury (Hg2+). The crystallization of NaCl due to the ion exchange effect during the impregnation of NaA and NaX reduced the number of active Cl species on the surface, and restricted the physisorption of Hg0. Therefore, the Hg0 removal efficiencies of the samples were inhibited. The TPD analysis revealed that the species of mercury on the surface of FeCl3-HZSM-5 was mainly in the form of mercuric chloride (HgCl2), while on FeCl3-NaX and FeCl3-NaA it was mainly mercuric oxide (HgO).

DRIFTS investigation and DFT calculation of the adsorption of CO on Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2.

Molecular structures and vibrational spectra of the CO species adsorbed on the Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2 have been investigated by means of density functional theory (DFT) calculation and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The geometrical structures and vibrational frequencies were obtained at the MPW1PW91/SDD level. Theoretical calculation shows that the calculated IR spectra were in good agreement with the experimental results. The calculated results clarify the assignment of the adsorbed CO species on the surface of Pt/TiO2, Pt/CeO2 and FeOx/Pt/CeO2.

Role of NO in Hg-0 oxidation over a commercial selective catalytic reduction catalyst V2O5-WO3/TiO2

Experiments were conducted in a fixed-bed reactor that contained a commercial catalyst, V2O5-WO3/TiO2, to investigate mercury oxidation in the presence of NO and O2. Mercury oxidation was improved by NO, and the efficiency was increased by simultaneously adding NO and O2. With NO and O2 pretreatment at 350°C, the catalyst exhibited higher catalytic activity for Hg(0) oxidation, whereas NO pretreatment did not exert a noticeable effect. Decreasing the reaction temperature boosted the performance of the catalyst treated with NO and O2. Although NO promoted Hg(0) oxidation at the very beginning, excessive NO counteracted this effect. The results show that NO plays different roles in Hg(0) oxidation; NO in the gaseous phase may directly react with the adsorbed Hg(0), but excessive NO hinders Hg(0) adsorption. The adsorbed NO was converted into active nitrogen species (e.g., NO2) with oxygen, which facilitated the adsorption and oxidation of Hg(0). Hg(0) was oxidized by NO mainly by the Eley-Rideal mechanism. The Hg(0) temperature-programmed desorption experiment showed that weakly adsorbed mercury species were converted to strongly bound ones in the presence of NO and O2.

Effect of the properties of MnOx/activated carbon and flue gas components on Hg0 removal at low temperature

Manganese oxide loaded on activated carbon (Mn/AC) was synthesized using an impregnation method, and its capacity for Hg0 removal in simulated flue gas was investigated at 120 °C. The Hg0 removal performance was significantly enhanced by manganese oxide. The effects of the Mn loading on Hg0 removal were evaluated. X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) were employed to characterize the samples. In addition, the effects of individual flue gas components, including SO2, NO and HCl, on the Hg0 removal performance over the Mn/AC sorbent were investigated. The results indicated that 10% Mn/AC was the optimal sorbent under the simulated flue gas conditions. The XRD and BET analyses indicated that the Mn3O4 crystal particles and surface area were the primary factors that contributed to Hg0 removal. The competitive adsorption and formation of Mn(SO4)x were the primary reasons that SO2 inhibits Hg0 removal. In a pure N2 atmosphere, a low concentration of NO decreased the Hg0 removal due to consumption of active oxygen species. However, a high concentration of NO promoted Hg0 removal due to the formation of N-containing active species that were generated on the sample surface. The Mn/AC sample remained highly active toward Hg0 removal in the presence of HCl due to reaction with active chlorine species.

Removal of gas-phase Hg0 by Mn/montmorillonite K 10

Mn/montmorillonite K 10 (Mn/MK10) prepared by impregnation method was studied to remove Hg0 in simulated coal-fired flue gas. The samples were characterized by Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Effects of manganese loading value, reaction temperature and flue gas components on Hg0 removal efficiency were investigated. The results indicated that 4% Mn/MK10 was the optimal sample with outstanding Hg0 removal efficiency over the temperature range of 100–400 °C. The characteristic analysis demonstrated that amorphous MnO2 and active oxygen were crucial for Hg0 removal. Besides, NO had a promoting effect due to the formation of Hg(NO3)2. The addition of only 5 ppm HCl led to excellent Hg0 removal performance as HCl enhanced Hg0 conversion to HgClx. The inhibition effect of SO2 could be counteracted in the presence of NO and/or HCl. The Hg0 removal capacity showed a relative decrease when H2O (g) was added to simulated flue gas. Moreover, Hg0 removal performance was maintained at 80–99% (NO/SO2 = 0.26–1.71) in simulated flue gas without HCl, which appeared to be promising in industrial application.

Effect of HBr formation on mercury oxidation via CaBr2 addition to coal during combustion

Adding CaBr2 to coal to enhance elemental mercury (Hg0) oxidation during combustion has been an effective mercury control technology, but the added CaBr2 may increase levels of noxious Br2 or HBr gases in flue gas. Temperature-programmed decomposition (TPD) experiments were conducted to verify the effect of CaBr2 addition on Hg0 oxidation. The results indicated that the amount of Hg0 released initially decreased with increasing amounts of CaBr2 additive and then held steady. The optimal amount of additive was 200 μg g−1. CaBr2 addition effectively oxidized Hg0 released at relatively low temperatures only. The generation of HBr was confirmed by mass spectrometry. The formation of HBr occurred over a temperature range of 250 °C to 400 °C, and the HBr concentration first increased and then remained stable as levels of CaBr2 additive were increased in coal. The maximum concentration of HBr was 18 ppm and corresponded to 200 μg g−1 CaBr2. Further analysis indicated a strong, negative linear correlation between the amount of Hg0 released and the HBr concentration in flue gas. Based on these findings and previous studies, the possible mechanism of oxidation of Hg0 by CaBr2 was analyzed.

Support effect of Mn-based catalysts for gaseous elemental mercury oxidation and adsorption

Mn-Based catalysts with a Mn loading of 4 wt% were prepared using an impregnation method. Their Hg0 removal efficiencies were in the following order: 4% Mn/MK10 (montmorillonite K 10) > 4% Mn/SiO2 > 4% Mn/TiO2 > 4% Mn/Al2O3. Results from various characterization techniques, such as BET, XRD, STEM, H2-TPR, Hg0-TPD and XPS, suggested that the species, morphologies, and distributions of MnOx led to the different performances in Hg0 removal. The support effect on Hg0 adsorption and oxidation was studied. For 4% Mn/Al2O3 and 4% Mn/SiO2, physical adsorption dominates Hg0 removal at low temperature (150 °C), while chemical adsorption is more important at high temperature (350 °C). At both low and high temperatures, chemical adsorption and oxidation play leading roles in 4% Mn/TiO2 and 4% Mn/MK10, respectively. The effect of MnOx species and their morphologies on Hg0 oxidation was investigated. Amorphous MnO2 benefits Hg0 oxidation at both low and high temperatures, while amorphous Mn2O3 only facilitates Hg0 oxidation at high temperature. Hg0 oxidation on Mn-based catalysts follows a Mars–Maessen mechanism where the lattice oxygen of MnOx reacts with the absorbed Hg0.

CO 2 Emission in China’s Iron and Steel Industry

CO emissions have become a serious problem in China because of the country’s heavy reliance on fossil fuels as an energy source. The iron and steel industries, the energy consumptions of which are high compared to the rest of the world, are confronted with an increasing demand to reduce CO emissions. Data on CO emissions from iron and steel industries is a basic requirement for a certificate of CO reduction. By analyzing the production process and the influence factors of CO emissions during iron and steel production process, the scope of CO emissions were defined. Material Flow Analysis (MFA) was used to analyze carbon flow from iron and steel production process, and the CO emissions of a typical enterprise were calculated. The existing processing CO emissions reduction technologies were also analyzed, such as blast furnace top gas pressure recovery turbine (TRT), sintering waste heat power generation, converter low pressure saturated steam generation and so on. The technologies including blast furnace stock gas circulation technology, coke oven gas injection technology after reforming, carbon capture and storage technologies and so forth are considered to have a better prospect of application for CO emissions reduction.