Mercury In Coal-fired Flue Gas Research Articles

Removal of Mercury from Coal-Fired Flue Gas and Its Sulfur Tolerance Characteristics by Mn, Ce Modified γ-Al2O3 Catalyst

Mercury pollution in the atmospheric environment is a matter of international concern. Mercury in coal-fired flue gas is the first human mercury emission source and has become the focus of national mercury pollution control. The catalytic performance of zerovalent mercury (Hg0) in coal-fired flue gas was studied by using manganese-cerium-aluminum oxide as catalyst. The effects of metal loading ratio, reaction temperature, calcination temperature, and O2 and SO2 concentration on the efficiency of Hg0 catalytic removal were investigated, and the Mn-Ce/γ-Al2O3 catalysts before and after the reaction were characterized by BET, SEM, XRD, and XPS to analyze the physicochemical properties of the samples. The results show that the mercury removal efficiency of the composite catalyst with Mn, Ce, and Al as the active component is higher than that of the single metal catalyst. The catalytic activity of Mn0.1Ce0.02Al is the best, the optimum reaction temperature is 150°C, the optimum calcination temperature is 400°C, and the O2 concentration in the conventional flue gas condition satisfies the effective oxidation of Hg0; SO2 in the flue gas can seriously inhibit the oxidation of Hg0.

Preparation of magnetic Co-Fe modified porous carbon from agricultural wastes by microwave and steam activation for mercury removal

In this article, a magnetic cobalt-iron modified porous carbon derived from agricultural wastes by microwave and steam activation was developed to remove elemental mercury in coal-fired flue gas. The effects of operating parameters on Hg0 capture were discussed. Reaction mechanism and regeneration performance were also studied. Results show that the activation of microwave and steam significantly improves the pore structure of the porous carbon. The ultrasound-assisted impregnation promotes the dispersion of cobalt oxides and iron oxides on the samples. The Co0.4Fe12/RSWU(500) sorbent exhibits highest Hg0 removal efficiency at 130 °C. The characterization analysis shows that cobalt oxides and iron oxides are the main active components for Hg0 removal. The XPS analysis suggests that the chemisorption oxygen and the lattice oxygen (derived from Co3+/Co2+ and Fe3+/Fe2+) participate in the Hg0 capture process. Moreover, the cobalt-iron mixed oxide modified porous carbon has a good regeneration performance, which is conductive to reduce the costs in the future application.

Exploring adsorption behavior and oxidation mechanism of mercury on monolayer Ti2CO2 (MXenes) from first principles

Developing low-cost, high adsorption capacity, and recyclable mercury sorbents is an important step for control of mercury in coal-fired flue gas. However, few sorbents meet all the requirements. Herein, we explore the adsorption behavior and oxidation mechanisms of mercury on 2D Ti2CO2 (MXenes) monolayer and defective Ti2CO2 monolayer with an Ov (Ov-Ti2CO2) through first-principles computations based on density functional theory (DFT). Our calculation results show that Hg0 adsorption on Ti2CO2 and Ov-Ti2CO2 monolayers are mainly physisorption and chemisorption mechanisms, respectively. Moreover, the presence of a single oxygen vacancy on Ti2CO2 monolayer can improve the interaction between the mercury and Ov-Ti2CO2 monolayer, thereby enhancing the adsorption energy of mercury by 66 kJ/mol. Hg0 has a strong interaction with Ti atom on Ov-Ti2CO2 monolayer by the atomic orbital hybridization and overlap. The adsorption of HgO molecule on Ti2CO2 and Ov-Ti2CO2 monolayers are chemisorption processes. Furthermore, electron density difference analysis indicates that the remarkable charge accumulation across the interface is closely related to the strong interaction between HgO molecule and Ti2CO2 or Ov-Ti2CO2 monolayers. The three-step reaction processes (Hg0 → Hg(ads) → HgO(ads) → HgO) are taken place on Ti2CO2 and Ov-Ti2CO2 monolayers, resulting in the formation of gaseous HgO molecule. The energy barrier for the Hg0 oxidation reaction step on Ov-Ti2CO2 monolayer (54.85 kJ/mol) is about half of that on Ti2CO2 monolayer (104.73 kJ/mol). The desorption steps of the resultant HgO molecule from Ti2CO2 and Ov-Ti2CO2 monolayers are the rate-determining steps, which need external energies of about 119.3 and 183.2 kJ/mol, respectively.

Non-thermal plasma injection-CeO2-WO3/TiO2 catalytic method for high-efficiency oxidation of elemental mercury in coal-fired flue gas

The combined method of non-thermal plasma injection and 3%CeO2-WO3/TiO2 catalysis system was proposed to enhance Hg0 oxidation in flue gas. Compared with the plasma system, the combined system had excellent oxidation performance. Approximately 86.6% of Hg0 was oxidized and energy yield reached 32.9µgkJ−1 at 2.6JL−1 and 110°C . The combined system widened the temperature windows of CeO2-WO3/TiO2 catalyst (80–300°C), showed good resistance to H2O and SO2, and also overcame the inhibitory effect of NO on Hg0 oxidation in the plasma system·NH3 inhibited the Hg0 oxidation; however, the coexistence of NO with NH3 significantly mitigated the inhibition. In the combined system, the CeO2-WO3/TiO2 catalyst could catalytically decompose the ozone generated by gas discharge, and increased the production of high adsorbed active oxygen, which was favorable for Hg0 oxidation process. The Hg0 oxidation process followed the homogeneous reaction mechanism and the Mars-Maessen mechanism in NTP injection system and CeO2-WO3/TiO2 system, respectively.

Theoretical Study of Mercury Species Adsorption on MgO(001) Surface

Traditional thermal power generation releases mercury, which will cause important environmental issues. As one of the main components of fly ash produced by coal-fired power generation, MgO can adsorb mercury in HCl atmosphere. Aiming to investigate the interaction mechanism of MgO with mercury in coal-fired flue gas in HCl atmosphere, density functional theory method is used to study the adsorption of mercury species on the fly ash component. The MgO(001) surface is built to study the stable adsorption configurations of Hg0, HCl, HgCl and HgCl2 on all possible adsorption sites. Results show that Hg0, HgCl and HgCl2 are all chemisorbed on MgO(001) surface weakly, while HCl is dissociatively adsorbed. HgCl cannot exist stably on the surface, so it has no obvious effect on the adsorption and oxidation of Hg0.

The role of iodine monochloride for the oxidation of elemental mercury

The removal of Hg 0 by the homogenous gas-phase reaction and particle-induced reaction was investigated under various conditions. Iodine monochloride was found to be efficient for Hg 0 oxidation, with the apparent 2nd-order rate constant of about 10.5(±0.3) × 10 −17 cm 3 molecules −1 s −1 and 5.7(±0.3) × 10 −17 cm 3 molecules −1 s −1 at 273 K and 373 K, respectively. The pilot-scale tests showed that the removal of Hg 0 by ICl increased significantly in presence of flyash. It was predicted that over 90% of Hg 0 removal efficiency can be obtained with 0.2 ppmv ICl and 20 g/m 3 flyash in flue gas. Though the reaction between Hg 0 and ICl was by far faster than that of Hg 0/Cl 2, the major product was found to be HgCl 2 rather than HgI 2, which implicated that iodine might partly act as the accelerant in Hg 0 oxidation by facilitating the formation of certain intermediates. The results indicated that using ICl to oxidize elemental mercury in coal-fired flue gas can save the consumption of iodine, and it appeared to be a promising oxidant to enhance the removal of Hg 0.

Analysis of mercury species present during coal combustion by thermal desorption

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg 0) and oxidized (Hg 2+, primarily as HgCl 2). Because Hg 2+ is more condensable and far more water soluble than Hg 0, the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl 2 < HgS < HgO < HgSO 4. Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.

Geological sequestration of carbon dioxide: implications for the coal industry

The predominant forms of mercury in coal-fired flue gas are elemental and oxidized. During combustion, the mercury in the coal is transformed into three species that include particle-bound mercury, vapor-phase elemental mercury, and vapor-phase oxidized mercury, primarily in the form of HgCl2. For the best possible removal of mercury from flue gas, a high level of oxidation is helpful. Because HgCl2 is water soluble, it can be removed during the wet flue gas desalifurization process. Particle-bound (Hgp) is easily removed by dust control equipment, such as baghouse filters and electrostatic precipitators. Existing air pollution control devices (APCDs) are capable of removing mercury to some extent. Existing APCDs include electrostatic precipitators (ESPs), fabric filters (FF or baghouse), flue gas desulfurization (FGD), and selective catalytic reduction (SCR). ESPs are designed to reduce fly ash emissions by creating an ionized field that removes charged particles. They have low operating costs, but a limited capacity to remove mercury because it is not generally adsorbed in the fly ash at combustion temperatures. SCR technology is used to reduce emissions of nitrogen oxides by installation of a fixed catalyst bed with reductant injection to reduce NOx to N2. In conjunction with SCR, low NOx burners create a fuel-rich primary combustion zone. This reduces the amount of thermal and fuel NOx created during combustion.