Abstract: Coal is the most significant source of energy generation in China, but the high content of arsenic/selenium/lead in coal has made coal-fired power plants become one of the main anthropogenic emission sources of them. To solve the serious problem on arsenic/selenium/lead pollution from coal-fired power plants, this paper firstly introduces the discharge and harmfulness of arsenic/selenium/lead released from coal-fired power plants, and summarizes the relevant domestic and foreign regulations on heavy metals emission control, then points out that it is necessary to control their emission from coal-fired power plants in China. Secondly, the migration and transformation behaviors of arsenic/selenium/lead during coal combustion are illustrated from the perspectives of occurrence form, speciation transformation and mass distribution, focusing on their speciation characteristics and size distribution in particulate matters. Finally, the control technologies pre-, in- and post-combustion are reviewed, and the research progress on their removal by adsorbents and air pollution control devices (APCDs) is described in detail. Meanwhile, the potential for strengthening the removal by electrostatic precipitators equipped with low temperature economizer and agglomeration technologies is discussed. In conclusion, it is aimed to provide reference and guidance for realization of ultra-low emission of arsenic/selenium/lead in coal-fired power plants.
Abstract: Based on chemical thermodynamics equilibrium analysis, species distribution of As, Se and Pb in coal-fired flue gas was calculated and analyzed, and effect of S, Cl on them was studied. The results show that As exists in the form of As2O5, As4O6 and AsO in oxidizing atmosphere, Se mainly exists in the form of SeO2, and Pb is mainly solid PbSO4 below 1000 K and gaseous PbO above 1200 K. In reducing atmosphere, As is solid As2S2 at lower temperature; As2, AsS and AsN coexist at 900-1400 K, and all of As is changed into gas AsO above 2000 K. Se exists mainly as gaseous H2Se below 1100 K, SeS and Se2 are generated at 1100 K, gaseous Se and a small amount of gaseous SeO at 1800 K. Pb mainly exists as PbS and gaseous Pb above 1800 K at middle and low temperature. The ratio of AsS(g), PbS(g) and SeS(g) increases in the reducing atmosphere, but the distribution of As, Se and Pb is not affected in the oxidizing atmosphere. Cl has little effect on As and Se in both oxidizing and reducing atmosphere, but has a great influence on species distribution of Pb.
Abstract: The speciations of Arsenic (As) in coal will inevitably convert during the combustion process. The As speciations in coal and its by-products are closely related to human health and environmental safety which is urgent to be identified. However, there is a lack of pretreatment procedure and analysis method on the As species in coal-related products in power plants. In this study, the As species in coal, fly ash (FA), and gypsum were successfully determined by high performance liquid chromatography coupled with hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS). The instrument parameters, extract reagents, and pretreatment methods (i.e. ultrasound and microwave-assisted) were optimized. The whole separation time of inorganic As was shorten to 7 min after optimization, with the detection limit of 1.8 and 4.6 ng/g for As(Ⅲ) and As(Ⅴ), respectively. The efficient As extract reagent was the mixture of 1.0 mol/L H3PO4 and 0.1 mol/L ascorbic acid solution. Microwave-assisted (2000 W, 80 ℃, 40 min) and ultrasound-assisted (40 kHz, 20 ℃, 40 min) schemes were the optimal extraction methods for coal/FA and gypsum samples, respectively. Under the proposed microwave and ultrasound extraction procedure, the recovery of As(Ⅲ) and As(Ⅴ) could reach to 95.8%/104.5% and 90.6%/89.7%, respectively. The dominant occurrence of As species in coal was As(Ⅴ) with a small percentage of As(Ⅲ), while As(Ⅴ) was the only occurrence form observed in FA and gypsum. It is indicated that revealing the transformation of As(Ⅲ) to As(Ⅴ) is the key for gaseous As capture. The As species distribution investigation provides a scientific insight to the controlling of As emission from power plant.
Abstract: Alkali/alkaline earth metals(AAEMs) are widely present in various solid fuels. During the combustion process, the AAEMs undergo complex physical and chemical reactions with heavy metals and other minerals in the fuel, thereby affecting the migration and transformation of heavy metals. This paper mainly introduced the influence of AAEMs on the migration and transformation of As, Se, Pb and Cr, including the influence of alkali metals and alkaline earth metals on the migration and transformation of heavy metals, and the influence of particle agglomeration and coherence on heavy metal emissions. AAEMs can inhibit the volatilization of heavy metals. The combination of alkali metals and Cl elements reduces the production of PbCl2. The presence of alkali metals is beneficial to improve the adsorption efficiency of kaolin for Pb. AAEMs can form stable compounds with As and Se. However, at the same time, it should be noted that in the partial combination products of AAEMs and Cr, Cr exists in a hexavalent state and has high toxicity. AAEMs play a role in promoting and inhibiting the occurrence of agglomeration, respectively. An appropriate content of alkali metals is beneficial to reduce the release of heavy metals. By summarizing the influence of AAEMs on the migration and transformation of heavy metals, it is hoped to provide ideas for reducing the harm of heavy metals.
Abstract: The effect of water vapor on lead adsorption by kaolinite at high temperatures was studied using a drop tube furnace. The lead was in the forms of PbO and PbCl2. Firstly, effect of 0-20% water vapor was studied on adsorption of PbO (1100-1300 ℃) and PbCl2 (800-1300 ℃) by kaolinite. Then, mechanism of high-temperature adsorption of kaolin was revealed according to the analysis of XRD, SEM and residual hydroxyl group fraction. The results showed that water vapor reduced the loss of hydroxyl groups on kaolinite surface at high temperatures, hindering PbO adsorption and promoting PbCl2 adsorption. At the same time, due to production of inert mullite and collapse of pore structure of kaolinite at high temperatures, the optimal adsorption temperatures of PbO and PbCl2 were 1200 and 1000 ℃, respectively.
Abstract: The characteristics of Se capture by CaO, CaCO3, and MgO which are main components of Ca-/Mg-based mineral sorbents at 500-800 ℃ and that by calcite and dolomite were investigated, and the CaO obtained from calcine minerals were also used to capture Se. The results showed that capacity of CaO for Se capture was the highest, and the maximum value at 800 ℃ was 368 mg/g. Capacity of CaCO3 on Se adsorption at 700 ℃ was the largest and thermostability of the used CaCO3 was better. Se adsorption of Mg-based sorbents at medium temperature was obvious. The trend for Se adsorption capacity of calcite with increasing temperature was similar to that of CaCO3. Effect of calcite on Se capture was better than that of CaCO3, which was attributed to the higher specific surface area and pore volume of calcite. The ability of F-sor obtained from calcined calcite for Se capture was better than that of C-sor from calcined CaCO3 as well as that of CaO, which was likely due to higher specific surface area and pore volume of F-sor. Moreover, the used F-sor showed better thermostability at higher temperature, and the maximum adsorption capacity of F-sor was 403 mg/g.
Abstract: The As2O3 or PbO adsorption characteristics using typical mineral oxides as the sorbents were studied in a two-stage fixed-bed reactor under a simulated flue gas, and the density of atomic states, adsorption sites, and adsorption energy for the adsorption reaction were calculated by density functional theory (DFT). The results demonstrate that CaO has a large As2O3 adsorption capacity, with an arsenic adsorption capacity of 5.25 mg/g at 900 ℃, followed by Fe2O3, MgO, and Al2O3; and the adsorbed arsenic exists in the form of As3+ and As5+ arsenates. Kaolin and fly ash have large PbO adsorption capacities, with the maximum lead adsorption capacities of 6.69 and 2.75 mg/g, respectively, followed by SiO2 and Al2O3, and the adsorption capacity for lead with the 50%SiO2/50%Al2O3 mixture is higher than that with their single oxide. The oxygen atoms on the surface of the sorbents are the active sites for As2O3 and the unsaturated Si and Al atoms exposed on the surface of the sorbents are the active sites for PbO. In addition, the adsorption temperature and flue gas atmosphere have significant effects on the adsorption capacity and adsorption products of the sorbents.
Abstract: Blended coal combustion technology was extensively used in coal-fired power plants in China. In order to investigate the in-situ reaction between trace elements and minerals in fly ash during blended coal combustion, a bituminous (HLH), anthracite (ZW) and the blended coal of these two parent coals were combusted in a drop tube furnace at 1150 ℃. The ash gathered at high temperature segment (HTA) and low temperature segment (LTA) of the furnace were analyzed, respectively. The results indicated that the retention rates of arsenic in HTA were lower than that in LTA, which suggested that arsenic would be re-absorbed by ash during cooling down of flue gas. For HTA the retention rates of arsenic in ash of ZW, Z3H1, Z1H1, Z1H3, HLH were 60.31%, 26.85%, 13.29%, 20.23% and 36.11%, respectively. The arsenic was more difficult to be captured by HTA of blended coal than that of parent coal. As for selenium, the retention rates in HTA of five coal samples were 24.68%, 23.60%, 20.58%, 15.19% and 38.13%, which had the same retention law as arsenic. The results of X-ray diffraction (XRD) demonstrated that the mineral morphology was changed obviously during blended coal combustion. Unlike parent coal, mullite appeared in HTA of blended coal, and peak of mullite was enhanced with proportion of ZW increased in blended coal. It was consistent with the trend of retention of As and Se in HTA. It illustrated that change of mineral species and in-situ reaction between minerals and trace elements significantly affected emission of arsenic and selenium during blended coal combustion.
Abstract: The interaction mechanism between the unburned carbon in fly ash and the arsenic pollutants in flue gas such as As, AsO, AsO2 and As2O3 was studied based on the density functional theory. The results show that the elemental arsenic is preferentially adsorbed at the carbon bridge site, with an adsorption energy in the range (-5.95)-(-5.88) eV; the AsO molecule preferentially combines with the unburned carbon in a way that the arsenic and oxygen atoms are bound with the surface carbon atoms respectively, forming a most stable configuration with an adsorption energy of -7.87 eV. When AsO2 is dissociated on the unburned carbon surface and form an AsO molecule and a surface reactive oxygen species, the system is the most stable, possessing an adsorption energy of -10.65 eV. While once the two oxygen atoms in a trigonal bipyramid As2O3 molecule first collide with the unburned carbon surface, it will be dissociated to small molecules of AsO and AsO2, forming a covalent bond with surface carbon. The adsorption energy is significantly reduced to -10.64 eV, compared with the undissociated case. The unburned carbon in fly ash is easy to bind with AsO or AsO2 small molecules, which locally tends to form a special five-member ring structure. Compared with As, AsO and AsO2, the most toxic trivalent arsenic As2O3 is chemically stable and not easy to adsorb. Catalytic pyrolysis of As2O3 into small molecules of AsO and AsO2 is expected to be a feasible measure to control the arsenic pollution in coal-fired power plants flue gas.
Abstract: The emmision of particulate matters and heavy metals such as As, Se and Pb from coal combustion into the atmosphere would cause a serious environmental and human health hazard. Therefore, a multiple agglomeration based on the principle of turbulent coalescence and wall surface adsorption was developed to investigate the agglomeration effects on the removal of particulate matter and particulate heavy metals. Firstly, a numerical simulation method was adopted to comprehensively study the pressure loss, the velocity uniformity and the particle agglomeration effect, and a folded blade was selected for the multiple agglomeration device. Subsequently, a pilot study at a coal-fired plant on the particle agglomeration at different flue gas velocities was carried out. It is found that the agglomeration rate of PM1 in the multiple agglomeration device is up to 32.84%. As the gas velocity is increased from 11.1 to 17.6 m/s, the agglomeration rate of PM2.5 shows a certain decline, indicating that an increase in gas velocity would lead to a shorter residence time of particles and thus a decrease in agglomeration rate of particles. By comparing the concentration changes of As, Se and Pb in the particles before and after agglomeration, it is found that the agglomeration process can enhance the adsorption to gaseous heavy metals and also aggregate the nano-particles rich in heavy metals, thus resulting in an increase in the concentration of heavy metals in PM1. The decrease of the absolute concentrations of As, Se and Pb in PM1 after coalescence shows a cooperative removal effect in the multiple agglomeration device on the particulate matter and particulate heavy metals.
Abstract: In order to investigate the influence of spraying the agglomeration adsorbent on the removal efficiency of fine particles and heavy metals, the sampling and tests on particles and heavy metals were carried out before and after the dedusters and the desulfurization towers in the Unit 1 of a power plant in Hubei Province with a capacity of 330 MW and equipped with a double-chamber four-electric-field electrostatic precipitator. The test results show that after spraying the agglomeration adsorbent in the flue, the proportion of the particulate heavy metals at the ESP inlet increases, the Se element increasing significantly in PM2.5 and PM10, while the heavy metal content in the gas phase decreases, indicating that the agglomeration adsorbent can improve the coagulation efficiency of particulate heavy metals, leading to the small particulate and heavy metals in gas phase being transferred to large particulates. In gypsum, the content of heavy metals is significantly reduced after agglomeration, indicating that heavy metals can join the desulfurized gypsum. Also, the heterogeneous agglomeration enhances the effect of ESP on the removal of heavy metals. At the point before final discharge to the chimney, there is a significant decrease in the content of heavy metals compared with the non-agglomerated condition, which indicates that the heavy metals discharged into the atmosphere after agglomeration are significantly reduced, and the heterogeneous agglomeration plays a key role in the control of heavy metals.
Abstract: This paper studied physicochemical and leaching characteristics of three hazardous trace elements As, Se and Pb in hetero-aggregation fly ash (HAFA) and coal fly ash (FA)samples which were collected from coal-fired power plant. The results show that the peak of particle size of HAFA is 138.04 and is 60.26 μm of FA; the fine particles agglomerate into large particles after agglomeration; the contents of heavy metals of As, Se and Pb in HAFA are all higher than those of in FA. The content of heavy metals in gypsum produced in the subsequent desulfurization process is decreased. The batch leaching experiments show that leaching concentrations of three metals in HAFA are suppressed by mild and alkaline condition for As, by acidic and alkaline condition for Se, and by alkaline condition for Pb. The column leaching experiments show that the leaching abilities of the trace elements in HAFA are inhibited in both acidic and aqueous solution.
Abstract: During coal and coal gangue combustion, many heavy metal pollutants are emitted and cause serious environmental problems. In this paper, the environmental effect values of As and Pb emission during coal gangue and coal combustion in the 330 MW pulverized coal boiler, 50 kW circulated fluidized bed boiler and laboratory were calculated by ReCiPe2016. The results show that when coal combustion in 330 MW pulverized coal boiler, the environment effect values of As for bottom slag, fly ash and flue gas are 3.28×10-6, 2.68×10-5 and 3.89×10-3 respectively; while the environment effect value of Pb for bottom slag, fly ash and flue gas are 8.57×10-6, 6.00×10-5 and 4.83×10-2, respectively. The environmental effects of As and Pb in bottom slag are lower than those in the fly ash; and the environmental effects of As and Pb on air are higher than those on soil. Moreover, when coal combustion in the 50 kW circulated fluidized boiler, the effect values of As and Pb in fly ash on environment are 3.26×10-5 and 1.28×10-4; and the effect values of As and Pb in bottom slag are 1.16×10-6 and 1.43×10-5 respectively. The results also show that when coal gangue combustion in the laboratory, the effect values of As and Pb emission increase with increasing of the temperature; and the proportions of total environmental effects of As and Pb on air are higher than those on soil. Besides that, this study also indicates that the effect of Pb emitted into environment is higher than that of As at the same conditions during coal combustion both in circulated fluidized boiler and pulverized coal boiler. The results may provide basic data for predicting the environmental effects of As and Pb during coal gangue combustion in circulating fluidized bed for life cycle impact assessment.
Supervisor：Chinese Academy of Sciences
Sponsors by：Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences Chinese Chemical Society
