The LaMnO3, CeMnO3, and PrMnO3 perovskite oxides were synthesized by the sol–gel method and employed as the catalyst of selective catalytic reduction (SCR) for NO removal and synergistic mercury removal from coal combustion flue gas. The experimental results indicated that CeMnO3 exhibited the best NO and Hg0 removal activity among the three catalysts. NO conversion over CeMnO3 could reach a maximum value of 89.2% in the atmosphere of 4% O2 + 400 ppm NO + 400 ppm NH3, and the optimal reaction temperature for the NO removal was 200–250 °C, demonstrating good low-temperature catalytic activity. Hg0 removal efficiency of CeMnO3 decreased with the rise of the reaction temperature. The individual flue gas components of O2, HCl, NO, and CO2 had promotions on the Hg0 removal over CeMnO3, while SO2, NH3, and H2O displayed inhibitory actions on the efficiency. The performance of CeMnO3 on simultaneous NO and Hg0 removal was the best at 200–250 °C in simulated coal-fired flue gas. Specifically, the NO conversion and Hg0 removal efficiency were, respectively, 73.7% and 80.9% at 200 °C, and both the efficiencies remained excellent stability during a long experimental period. A series of characterization analyses were also carried out to identify the physicochemical properties of the catalysts. The results demonstrated that the superior catalytic performance of CeMnO3 mainly derived from the abundance of chemisorbed oxygen and the great activity of Mn3+ besides Mn4+ in the catalyst.
La1–xAxMn1–yByO3 (A = Ca, Sr and Ce, B = Cu, Co and Fe, x = 0/0.2, y = 0/0.2) perovskite catalysts were employed for simultaneous NO and Hg0 removal. The perovskite structure is beneficial for low temperature catalysis. The substitution of A-site cations with cerium (Ce) cations significantly improved the catalytic activity of perovskite catalyst. 90% NO conversion and 98% Hg0 oxidation was attained using La0.8Ce0.2MnO3 catalyst at 200 °C. Hg0 oxidation posed negligible effect on NO reduction. Compared to the N2 plus 4% O2 atmosphere, Hg0 oxidation was significantly facilitated by selective catalytic reduction atmosphere. The enhancement in Hg0 oxidation was probably attributed to NO2 originated from NO. Furthermore, a possible reaction mechanism was proposed, in which surface oxygen, Mn4+ and Ce4+ contributed to NO and Hg0 removal. Such knowledge provides useful information for the development of effective and economical NO and Hg0 removal technology for coal-fired power plants.
Economic sustainability is one of the main factors restricting development of mercury pollution control technology in the flue gas. Renewable magnetospheres separated from fly ash can be used as adsorbent for mercury removal from coal-fired flue gas. Based on the actual working condition of the 1000 WM coal-fired unit, this paper analyzes the impact of magnetosphere and mercury recovery technology on the economic benefits of this technology. The results show that the recovery of magnetospheres and mercury can obtain higher economic returns. This not only can realize the deduction of operating costs, but also has certain economic investment potential. When the magnetic separation rate is greater than 6% and the selling price of the magnetosphere is greater than 720 CNY/t, the revenue of the magnetosphere recovery system can offset the operating cost of whole system. The internal rate of return of the system can reach 50% at most. When the selling price of mercury selenide is lower than 450 CNY/g, the single mercury recovery income is insufficient to offset the mercury removal cost. Within the selling price range of 450–500 CNY/g mercury selenide, the system can generate revenue, and the product qualification rate directly affects the profit and loss of the system. The coupling of magnetospheres and mercury recovery systems can make economic and technical parameters more moderate. When the selling price of magnetospheres is higher than 600 CNY/t, the magnetic separation rate is higher than 6%, and the selling price of mercury selenide is higher than 350 CNY/g, the internal return of the system will fluctuate within 13%∼120%, and the static investment recovery period will remain within 6 years. Overall, this study can provide economic guidance for mercury removal technology with renewable magnetospheres and provide data support for further commercialization of technology.
Efficient and economical technologies are essential to the control of SO2, the emission of which poses serious health concerns and environmental risks. Photocatalysis is an attractive method for reducing SO2 emissions. To reduce energy consumption for excess moisture evaporation, a dry photocatalytic oxidation (DPCO) system was used instead of a traditional gas–liquid process in this study. Considering that TiO2 is a widely applied photocatalyst for the purification of gaseous pollutants, this study investigated the photocatalytic removal of SO2 over different TiO2-based nanofibers. Results show that the reduction of SO2 was mainly due to oxidation. Under ultraviolet irradiation, the removal of SO2 was enhanced by the presence of NO2, which was formed by the oxidation of NO. More interestingly, the SO2 removal efficiency remains 100% over cerium-based titania nanofibers with an increase in gas humidity, indicating that this sample has excellent resistance to H2O. This is very beneficial for application in an actual flue gas atmosphere, where H2O is inevitable. In contrast, H2O played a bifacial effect in the photocatalytic removal of SO2 over copper-based titania nanofibers. Under low levels of H2O (<4%), competitive adsorption for active sites leads to the deactivation of photocatalytic activity, while addition of 8% H2O resulted in more SO2 dissolution. Nevertheless, the promoting effect was limited; competitive adsorption was the major factor. Accordingly, the main reaction products are H2SO4 and H2SO3. These indicate that combining photocatalysis technology with TiO2-based nanofibers is a promising strategy for oxidizing SO2 during a DPCO process.
Mercury, a toxic heavy metal, poses significant risks to human health. Coal-fired power plants are the largest anthropogenic sources of mercury emissions, making mercury removal from flue gas imperative. Among various mercury adsorbents, metal selenides show promising potential in mercury capture. Here, we reported the synthesis of a novel three-dimensional hierarchical flower-like material, SnSe2, and its debut application in flue gas mercury capture. Under the influence of low-coordinated selenium, the introduction of abundant selenium vacancy defects led to the exposure of additional active sites in the adsorbent. Additionally, the presence of Sn enhanced gas selectivity and promoted electron transfer processes, thereby augmenting Hg0 adsorption and oxidation performance. Benefiting from these advantages, SnSe2 exhibited superior mercury removal performance over a wide temperature range (30–180 °C), with a saturated mercury adsorption capacity of 2027.23 μg/g, surpassing that of commercial activated carbon. Furthermore, the presence of NO in flue gas improved mercury adsorption performance, while high concentrations of SO2 do not affect mercury removal efficiency, as elucidated by adsorption kinetics models. Moreover, the mercury adsorption mechanism was demonstrated through mercury temperature-programmed desorption (Hg-TPD) and density functional theory (DFT) calculations. Finally, toxicity characteristic leaching procedure (TCLP) experiments confirmed SnSe2 as an efficient and permanent adsorbent for mercury, offering insights into the application of novel mercury removal materials.
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