Abstract: Mercury emitted from coal-fired power plants seriously harms human health and the ecological environment. In this paper, the effect of pollutant control devices on the removal of mercury from the flue gas in the ultra-low emission coal-fired power plants were first summarized; in particular, the impact of denitration, dedusting, and desulfuration devices after the ultra-low emission retrofits on the mercury removal was highlighted. Subsequently, the research progress on the non-carbon-based adsorbents used in the flue gas cleaning, including fly ash, natural minerals, noble metals, metal oxides, and metal sulfides, was then reviewed; the factors that may influence their performance in mercury adsorption were evaluated. Lastly, based on the current research progress, it is proposed that special attention should be paid in the future to the stability and leaching toxicity of adsorbed mercury as well as the regeneration and recycling of the spent adsorbents.
Abstract: Natural gas condensate, a by-product in natural gas exploitation and utilization, is an excellent raw material for naphtha production. However, the natural gas condensate always contains a trace amount of mercury, which may damage the human health and corrode the downstream processing units such as heat exchangers. It is highly demanded that the mercury in natural gas condensate can be identified specifically and analyzed accurately and quickly, which remains a big challenge in natural gas processing industry up to now. This paper reviewed the analytical methods for the determination of mercury species in natural gas condensate, by evaluating the advantages and disadvantages of various measures in terms of mercury extraction and detection in natural gas condensate. It was observed that the gas chromatography-inductively coupled plasma-mass spectrometer with high accuracy and high mercury recovery rate is the best technique at present to determine the mercury species in the natural gas condensate. The state of mercury species and content of mercury are two key factors to govern the selection and optimization of the adsorbent and the process for the efficient removal of mercury in natural gas condensate in different scales, which exacts a novel technique for the fast yet accurate characterization of various mercury species to meet the demands in the natural gas industry.
Abstract: In view of the poor resistance of manganese oxide octahedral molecular sieve (OMS-2) catalyst to sulfur in the oxidation of elemental mercury, CeO2 was used to modify the OMS-2 catalyst. The mechanism for the enhancement of the resistance of the OMS-2 catalyst to sulfur by modifying with CeO2 was investigated, with the help of thermodynamic analysis, fixed-bed reaction test and various characterization methods like nitrogen sorption, XRD ICP and XPS. The results indicate: The OMS-2 catalyst modified by Ce has a large surface area and void-rich structure, which can adsorb more Hg0 through chemical adsorption; More Mn defects are formed on the OMS-2 catalyst through modifying with Ce, leading to an increase in the electron mobility, the proportion of adsorbed oxygen (Oβ) species, and the density of catalytically active sites; The Ce-modified OMS-2 catalyst can quickly re-oxidize the reduced Hg0 or HgSO4 species (facilely formed on the pristine OMS-2 catalyst surface in the presence of SO2) to HgO, which can then improve the apparent Hg0 oxidation efficiency. The results should be helpful for the development of high-performance anti-thio catalysts for mercury oxidation.
Abstract: The MnOx/PG catalysts were prepared by supporting Mn oxides to palygorskite (PG) and used to remove Hg0 in simulated flue gas, which were analyzd and characterized by the specific surface area analysis (BET), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the co-effect of MnOx and PG significantly enhances the Hg0 removal performance; the MnOx/PG catalyst with 8% MnOx loading has the highest Hg0 removal activity. The Hg0 removal efficiency can maintain above 95% at 210 ℃ with a space velocity of 6000 h-1 and an inlet Hg0 concentration of 80 μg/m3 for 400 min. O2 can promote the Hg0 removal by MnOx/PG catalysts, while SO2 and H2O have inhibitory effects. In the presence of O2, the inhibitory effect of SO2 can be obviously weakened. The results of XRD, XPS and temperature programmed desorption experiments (TPD) indicate that the active components MnOx disperse well on the surface of PG. The removal process of Hg0 on the MnOx/PG catalyst includes the steps of adsorption, oxidation and reaction, and HgO and HgSO4 are generated and adsorbed on the catalyst.
Abstract: Simultaneous removal of SO2, NOx and Hg0 by using O3 and corn bio-charcoal/coconut shell activated carbon was compared in a fixed bed. The effects of temperature, adsorption time and ratio of O3/NO on the removal efficiency of NO, Hg0 and SO2 by corn/coconut shell charcoal were studied, and the corn/coconut shell charcoal was characterized and analyzed. The results indicated that the oxidation rate of NO and Hg0 increases with the increase of O3/NO ratio, while the oxidation rate of SO2 first increases and then decreases slightly. The increase of temperature inhibits the oxidation of NO but promotes the oxidation of Hg0 and SO2. At 140 ℃ and O3/NO ratio of 1.4, the oxidation rate of NO, Hg0 and SO2 is 99%, 78.6% and 3.5%, respectively. As O3/NO ratio increases from 0.4 to 1.4, the removal efficiency of corn charcoal for NOx increases from 4.6% to 93%, and coconut shell charcoal increases from 4.5% to 79%. Corn/coconut shell charcoal can reduce part of NO2 and increase NO concentration at the outlet. NOx adsorption performance on corn charcoal is relatively better, while those of Hg0 and SO2 on coconut charcoal are relatively stronger. Coconut shell charcoal has stronger physical adsorption capacity than corn charcoal. The relative content of oxygen-containing functional groups C-O and C=O on the surface of corn charcoal is higher than that of coconut shell charcoal, while its relative content of COOH and O=C-O is lower.
Abstract: The effect of NO on the performance of Cu-Mn spinel sorbent in the removal of Hg0 from flue gas was investigated by using experimental and density functional theory (DFT) calculation methods. The results indicate that NO shows slightly inhibitory effect on the Hg0 removal on the CuMn2O4 sorbent at high temperature (>250 ℃), probably due to the competitive adsorption between NO and Hg0 on the sorbent surface. At low temperature (< 250 ℃), NO has little influence on the Hg0 removal. The mercury species adsorbed on the CuMn2O4 sorbent exist mainly in the forms of Cu-Hg amalgam and Hg(NO3)2. NO can be oxidized into NO2 over the CuMn2O4 sorbent and NO2 then react with adsorbed mercury to form Hg(NO3)2. Cu and Mn atoms are identified as the active sites for the adsorption of NO and Hg0. There is a strong interaction between NO and CuMn2O4 surface, which is closely associated with the orbital hybridization of Cu, Mn, and N atoms.
Abstract: The spent fluid catalytic cracking (SFCC) catalysts were activated by an "internal instant vaporization (ⅡV)" method and used in the removal of Hg0 from a simulated flue gas in a fixed bed reactor; the effect of various operation parameters such as the SFCC activation conditions, adsorption temperature, and flue gas components on the Hg0 removal efficiency was investigated. The results indicate that the SFCC catalyst activated with methanol or ethanol performs adequately in terms of Hg0 removal, whilst the calcination temperature also has a great influence on the activation of the SFCC catalyst. O2 in the flue gas favors the Hg0 removal, whilst NO facilitates the oxidation of mercury and displays a positive effect on the mercury removal in the presence of O2, accompanying with the formation of N-containing active species on the activated SFCC catalyst surface. SO2 in the flue gas, depending on its concentration, may exert the effect of catalytic adsorption or competitive adsorption on the Hg0 removal. Approximately 100% Hg0 can be removed in the stream of 6% O2, 12% CO2 and 0.06% NO at 120 ℃ by using the activated SFCC catalyst with ethanol as an organic solvent and calcined at 120 ℃, suggesting that the spent FCC catalysts after activation can be a potential adsorbent for the removal of Hg0 from the coal-fired flue gas.
Abstract: Antimony is a trace element with potential toxicity. As a major source of atmospheric antimony pollution in China, the fate of antimony released during coal combustion has been attracting increasing concern. In this study, the contents and occurrence modes of antimony in coals were summarized, with subsequent discussions on the vaporization and transformation behavior of antimony in the coal combustion process. The partitioning of antimony in bottom ash, size-segregated fly ash particles as well as flue gas were also presented. Regarding the potential control methods for antimony emission, technologies facing pre-combustion, combustion and post-combustion stages were proposed respectively. It aims to provide a guideline for understanding the behavior of antimony migration and emission control during coal combustion.
Abstract: The high-silicon coal in Xuanwei area of Yunnan is selected to study the transformation behavior of minerals and the distribution and enrichment of heavy metals during the combustion process. The minerals in high-silicon coal are mainly composed of quartz, kaolinite, pyrite and anatase. The mullite in fly ash may come from the transformation of quartz and kaolinite in coal; the quartz in fly ash mainly comes from the original quartz component in coal or is formed by the conversion of SiO2-Al2O3 system. Analyzing the enrichment characteristics of several heavy metals in high-silicon coal and its fly ash, it can be found that Cr, Cu, and As are enriched in the high-silicon coal, and Mo is the heavy metal enriched in the electric fields of the ESP, while Se contents in high-silicon coal and fly ash in China are both lower than the world average level. The contents of radioactive elements of Th and U in the fine-particle high-silicon fly ash are higher than the average of world coal ash, and the enrichment factors in the fly ash in the four electric fields of the ESP are 1.51 and 1.59, respectively.
Abstract: Distribution of rare earth elements (REEs) in six speciations extracted from coal and coal gangue and their combustion products (slag and fly ash) generated by three different power plants in China were determined by sequential extraction procedure combined with inductively coupled plasma mass spectrometry method. The results show that the REEs mainly occurred as acid soluble and silicate & aluminosilicate fraction, e.g., approximately 42.54% and 45.62% in coal gangue, 32.85% and 57.13% in lignite, and 18.10% and 75.46% in bituminous coal, respectively. However, REEs in the combustion products were mainly presented in silicate & aluminosilicate fraction regardless of coal or coal gangue, reaching up to approximately 80% of the total REEs content. During combustion, around 36%, 23%, and 5% from the other five fractions (water soluble, ion-exchangeable, acid soluble, organic, and sulfide) were transformed to silicate & aluminosilicate fraction from coal gangue, lignite, and bituminous coal, respectively. In the case of coal or coal gangue, the amount of each REEs in the same extracted fraction was different, but the distribution trend of REEs from La to Lu in each fraction was followed in the same rule. In the case of slag and fly ash generated from coal or coal gangue, distribution of REEs from La to Lu in each fraction showed the different trend between fly ash and slag. This was due to the fly ash exposed in flue gas system was much longer than the time for slag formation.
Abstract: The content and state and leaching characteristics of the two trace elements, viz., chromium (Cr) and arsenic (As), in the solid product of ultra-low emission units in the coal-fired power plants are investigated. The results show that the contents of chromium and arsenic in the fly ash are in general higher than those in the bottom slag. In the fly ash samples of units 1#, 2#, and 3#, the most abundant chromium and arsenic species appear in the exchangeable state and oxidizable state, respectively, whereas in the ash sample of unit 4#, the residues account for the most chromium and arsenic species. The leaching concentration of arsenic is lower than 0.01 mg/L as specified in the groundwater environmental standard (GB 14848—2017); however, the leaching concentration of chromium in the fly ash of 2# and 3# units is higher than the emission limit, to which close attention should be paid.
Abstract: The distribution and enrichment characteristics of arsenic in five circulating fluidized bed (CFB) units with capacity between 25 to 350 MW and five pulverized coal furnace (PC) units with capacity between 300 to 600 MW were investigated using microwave digestion and hydride generation-atomic fluorescence spectrometry. By comparing the conventional wet digestion method and three kinds of mixed-acid microwave digestion systems, the appropriate digestion method was determined to be HNO3-HCl-HF acid solution mixed in a volume ratio of 6:2:2 with microwave digestion. The majority of the arsenic in coal evaporates during combustion and captured by the fly ash, the arsenic content in the bottom slag is only 1.95-9.75 μg/g. Most arsenic in the flue gas is adsorbed by the fly ash, most of the adsorbed arsenic is successively captured by the dust collector and desulfurization system. The arsenic contents in the fly ash and gypsum is 8.68-17.63 μg/g and 1.71-4.0 μg/g, respectively. Combustion temperature is the key factor affecting the release of arsenic. PC has a higher furnace temperature than CFB and makes more arsenic volatilize from coal and less arsenic remain in bottom slag. Meanwhile, a higher combustion temperature in PC unit produces more glassiness as aluminosilicate in the fly ash, which can capture the arsenic from the flue gas. Therefore, the arsenic content in the fly ash from PC unit is 12.08-17.63 μg/g, which is significantly higher than that from CFB, 8.68-13.84 μg/g. Moreover, the furnace temperature increases with the boiler load, which makes the ratio of arsenic content in the fly ash to the feed coal show an increasing trend. The ash content of the coal used for CFB and PC units is 33.96%-59.63% and 15.05%-41.67%, which makes the relative enrichment factor of arsenic in CFB higher than that in PC. Furthermore, more fine particles escaped from the dust collector should be captured by the desulfurization system, resulting that the arsenic concentration in the desulfurization gypsum of CFB unit is higher than that in PC unit.
Abstract: Calcium oxide (CaO) has been widely used as an adsorbent in the purification of heavy metals in coal-fired flue gas. However, the adsorption efficiency is limited and a further modification is needed. The cerium (Ce) modification can redistribute the surface electrons and enhance the chemical activity of CaO. Therefore, the Ce-CaO (100) periodic model was established to study the adsorption mechanism of mercury, selenium, and lead pollutants in the coal-fired flue gas. The results show that, except for the physical adsorption of Hg0 on the Ce-CaO (100) surface, the other heavy metal pollutants are chemically adsorbed on the surface. The Ce-site and O-site are the main active adsorption sites of heavy metal pollutants. Intense charge transfer and strong interaction are observed between adsorption molecules and Ce-CaO (100). Moreover, the adsorption capacity of Ce-doped CaO (100) surface for heavy metal pollutants has been improved, especially the significantly increased capture capacity on Se0, SeO2 and HgCl2.
Abstract: Based on the principle of thermodynamic equilibrium, reactions between heavy metals As, Se and Pb in flue gas of coal burning and main minerals CaO, Al2O3, Fe2O3 and MgO in fly ash were studied. The results show that As reacts with CaO at 1600 K to form Ca3(AsO4)2, and its temperature range becomes narrower with increasing CaO concentration, indicating that CaO can inhibit volatilization of As in coal. As reacts with Al2O3 at 1700 K, reaction of As with Fe2O3 forms FeAsO4. As and MgO exist in the form of Mg3(AsO4)2(s) between 600 and 1500 K, and turns into As2O5(s) below 600 K. Se and CaO, MgO exist in the form of CaSeO3(s) and MgSeO3(s), respectively, below 600 K, but does not react with Al2O3 and Fe2O3. CaO and Pb react at 900-1100 K to form (CaO)2(PbO2)(s). Pb reacts with Al2O3, and solid (PbO)(Al2O3)6(s) is formed at 900-1200 K. Fe2O3 and MgO have no effect on species distribution of Pb.
Supervisor：Chinese Academy of Sciences
Sponsors by：Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences Chinese Chemical Society
