Coal Combustion Flue Gas Research Articles

31 塩素化合物共存下での石炭燃焼場における排ガス中の水銀挙動(水銀挙動・定量)

31 塩素化合物共存下での石炭燃焼場における排ガス中の水銀挙動(水銀挙動・定量)

Efficient removal of HCHO from simulated coal combustion flue gas using CuO-CeO2 supported on cylindrical activated coke

To achieve novel sorbent-catalysts with low cost and high activity for removing HCHO in flue gas, we prepared and investigated CuO-CeO2 supported on activated coke granules (CuCe/AC) for decomposition of gaseous formaldehyde (HCHO) from simulated coal combustion flue gas. The results indicated that Cu3Ce8/AC exhibited remarkable conversion performance and good stability for HCHO removal. The presence of NO and SO2 exhibited a slight inhibitive effect on HCHO removal in the presence of 6% O2. The physicochemical properties of CuCe/AC were also studied by combination of N2 adsorption/desorption, SEM, XRD, H2-TPR, XPS and FT-IR. The excellent conversion property of Cu3Ce8/AC was related to the high surface area, large pore volume, high dispersion of copper species, good adsorption capacity and great redox ability of metal oxides. Besides, the synergy between Cu and Ce species improved the mobility of activated oxygen species and the content of chemisorption oxygen, which facilitated the conversion of HCHO greatly. FT-IR results suggested that adsorbed HCHO on the samples surface was oxidized into intermediates, which could be further decomposed into H2O and CO2 by the surface active oxygen of Cu3Ce8/AC. Along with the advantages of Cu3Ce8/AC, it would be a promising material for the removal of HCHO in real industrial application.

Influences of Copper(II) Chloride Impregnation on Activated Carbon for Low-Concentration Elemental Mercury Adsorption from Simulated Coal Combustion Flue Gas

In this study, the Hg0 adsorption equilibrium and kinetics of a coconut-shell-based activated carbon impregnated with CuCl2 were examined with respect to their resulting physical and chemical properties. Integrating the results from N2 adsorption isotherm at 77 K, scanning electron microscopy, elemental analysis, X-ray photoelectron spectroscopy, and Hg0 adsorption experiments under N2 and simulated coal-combustion flue gases conditions, it was found that HCl pretreatment could enhance Hg0 adsorption of crude activated carbon; the Hg0 adsorption capacities of crude and HCl-pretreated activated carbon under N2 condition were 95.8 and 225.4 µg g–1, respectively. Additionally, CuCl2 impregnation further increased the adsorption capacity of crude. The Hg0 adsorption capacity of crude activated carbon with 8% CuCl2 impregnation was 631.1 µg g–1. However, the equilibrium Hg0 adsorption capacity decreased when Cu loading exceeded 8 wt%, suggesting that adequate forms of surface Cu, O and Cl interacting with flue gas components and Hg0, as well as the presence of pores with specific size ranges allowing rapid transport of the Hg molecules into the interior of the activated carbon and as energy sinker govern the overall chemisorption process. Pseudo-second-order kinetic model could best describe the adsorption behaviors of tested samples under both test conditions, indicating that Hg0 adsorbed on the activated carbon surface could be explained by bimolecular reaction mechanisms.

Mercury Adsorption Characteristics as Dependent upon the Physical Properties of Activated Carbon

Activated carbon is a widely used sorbent for the removal of mercury from coal combustion flue gas. Many studies have been conducted to understand the physical properties of activated carbon and the flue gas conditions that are important for increasing its mercury adsorption capability. The Brunauer–Emmett–Teller (BET) surface area and pore volume of activated carbon and the injection of acidic gases have been reported to influence the performance of activated carbon in mercury adsorption. However, the mercury adsorption mechanism of activated carbon is not understood in terms of the physical properties of activated carbon and the composition of the flue gas. Therefore, two representative raw materials of activated carbon, wood and coal, were used in this study. Activated carbon samples with varying physical properties were prepared by applying different carbonization and activation conditions. The prepared activated carbons were tested for mercury adsorption in three different simulated gas compositions:...

Fractionation of mercury stable isotopes during coal combustion and seawater flue gas desulfurization

In the current study, fractionation of mercury isotopes during coal combustion and seawater flue gas desulfurization (SFGD) in a coal-fired power plant using a SFGD system was investigated. Fourteen samples were collected from the power plant. The samples were pretreated with a combustion-trapping method and were analyzed with a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Compared with the raw coal, the bottom ash was enriched with lighter mercury isotopes with δ202Hg values ranging from −0.45 to −0.03‰. The fly ash was enriched with lighter mercury isotopes with δ202Hg values ranging from −1.49 to −0.73‰ for Chinese coal and from −1.47 to −0.62‰ for Indonesian coal. The δ202Hg of fresh seawater and desulfurized seawater was found to be −1.32 and −0.32‰ respectively. These δ202Hg values indicated that the desulfurized seawater was enriched with heavier mercury isotopes. Based upon the calculated results obtained from the mass balance equation, it was suggested that the stack emissions were enriched with lighter mercury isotopes. Mass independent fractionation was observed in most of the samples with a Δ199Hg/Δ201Hg ratio of approximately 0.96. The results help in improving the understanding of mercury isotope fractionation during coal combustion and SFGD, and are also useful in tracing the mercury emissions from coal fired power plants.

Process integration of a Calcium-looping process with a natural gas combined cycle power plant for CO2 capture and its improvement by exhaust gas recirculation

In this study, it was sought to find an efficient way to integrate a Ca-looping process with a Natural Gas Combined Cycle (NGCC) power plant for its post-combustion CO2 capture. Compared to its application to coal combustion flue gas, Ca-looping would incur augmented energy penalty when integrated with a NGCC of which the flue gas contains only 4.0mol% CO2. The reduced CO2 concentration in the feed requires the carbonator to operate at a lower temperature and more solids to circulate between carbonator and calciner for keeping up the carbon capture efficiency at 90%. However, this study demonstrated that such negative effects could be alleviated greatly by implementing 40% exhaust gas recirculation to the NGCC with the CO2 concentration in the flue gas increasing up to 6.8%. Accordingly, the resulting net power efficiency increased notably 1.6% points in comparison to its equivalent non-EGR NGCC case and it was only 0.9% points less than amine capture case. This study exhibited that exhaust gas recirculation would be crucial in decarbonising a NGCC power plant by Ca-looping.

Modeling of mercuric chloride removal in ductwork and fabric filter by raw activated carbon injection

Based on previous experimental adsorption kinetic and equilibrium data for mercuric chloride (HgCl2) adsorption onto raw activated carbon (AC), a comprehensive model was formulated for its removal in the ductwork and fabric filter by raw AC injection in coal combustion flue gas. Various factors that can impact the performance and sorbent utilization were investigated, including inlet HgCl2 concentration, sorbent loading, particle size, external and internal mass-transfer considerations, residence time, filtration time, injection mode, and pressure drop. Despite these considerations, in-flight removal of HgCl2 in the ductwork was found to be almost negligible. For the removal in fabric filter, the effect of particle size was found to be significant, particularly for continuous sorbent injection. On the other hand, a discontinuous injection mode delivering the same amount of sorbent in 10% of a cleaning cycle resulted in higher removal performance and sorbent utilization. However, at the end of the cleaning cycle, most of the sorbent capacity was not used (<0.2%). The pressure drop across the filter cake built by fly ash and sorbent was found to be manageable within typical operating limits.

A 59-year sedimentary record of metal pollution in the sediment core from the Huaihe River, Huainan, Anhui, China.

An approximately 59-year (1955-2014) sedimentary record of metal elements (Cu, Pb, Zn, Ni, Co, Mn, and Fe) in a sediment core, collected from the Huaihe River, Huainan City, Anhui Province, China, was reconstructed by using 210Pb geochronology. Copper, Zn, Ni, Co, and Mn evaluated by enrichment factor (EF) indicated minor contamination due to water pollution accidents of the Huaihe River that occurred in 1990s and 2004. Lead presented the most severe pollution among the metals studied, especially during 1957-1974. The use of leaded petrol and atmospheric deposition of coal combustion flue gases could have contributed to Pb contamination. In spite of the general good quality (mean sediment pollution index (SPI) 35.69) of the sediment core evaluated by SPI based on the principal component analysis, worse sediment qualities in the upper section (<6cm, 2004) were still observed, suggesting intensive human activities causing the increasing concentrations of metals in recent decades.

Mercury co-beneficial capture in air pollution control devices of coal-fired power plants

Coal contains trace amount of mercury (Hg). Hg compounds in coal combustion flue gas are speciated into elemental (Hgo), oxidized (Hg2+), and particle-bound (Hgp). Air pollution control devices (APCDs) installed for removing NOx, particulate matter and SOx have a co-beneficial effect on Hg capture. In this paper Hg emission and speciation across APCDs in coal-fired power plants in S. Korea were measured; total emission and the co-beneficial control in existing APCDs were estimated for the year 2013. Hg concentration in flue gas at the outlet of the boiler and stack ranged from 7.5 to 9.9μg/Sm3 and 1.0 to 1.2μg/Sm3, respectively. APCDs (selective catalytic reactor (SCR)+electrostatic precipitator (ESP)+wet flue gas desulphurization (FGD)) were able to capture 80.8% of Hg as a co-beneficial control. SCR, ESP, and wet FGD independently and in a group promoted co-beneficial Hg control. Annually, 7.2t of Hg was captured as a co-beneficial control in existing APCDs of coal-fired power plants in South Korea. Even though, Hg co-beneficial controls in plants with advanced APCDs are significant, emphasis should be given for the direct control of Hg.

Multipollutant removal of Hg0/SO2/NO from simulated coal-combustion flue gases using metal oxide/mesoporous SiO2 composites

This study investigated the multipollutant control effectiveness for Hg0/SO2/NO using V, Mn, and Cu oxides/mesoporous SiO2 particulate composites. The presence of metal oxides decreased the specific surface area and pore volume and altered the surface morphology and sizes of the raw SiO2 particles. V4+/V5+, Mn4+, and Cu+/Cu2+ were the major valence states in the VOx, MnOx, and CuOx/SiO2 composites, respectively. Amorphous and highly dispersed metal oxides presenting in the SiO2 composites was confirmed by the X-ray diffraction characteristic peaks. Multipollutant control of Hg0 and NO using the metal oxide/SiO2 composites was achieved at both 150 and 350°C under the simulated coal-combustion flue gas condition. CuOx/SiO2 with >40% Hg0 removal at 350°C further implies that strong chemical bonds could form between Hg species and the surface groups of CuOx/SiO2. The metal oxide/SiO2 composites had up to 62% NO removal enhancement with injection of NH3 at 350°C, indicating the effectiveness of catalytic reduction of NO. The CuOx/SiO2 composite showed the greatest Hg0 and NO removal performance, which may stem from the presence of active CuOx in high content and its large surface area and mesoporosity.

Mechanism of mercury-iodine species binding on carbonaceous surface: Insight from density functional theory study

Iodated activated carbons were found to be effective for the mercury uptake from coal combustion flue gas. However, the detailed mechanism of mercury binding remains unclear. The adsorption mechanisms of mercury iodine on carbonaceous surface were studied by density functional calculations. A cluster model of five benzene rings was used to represent the active sites. The Mulliken bond population and total atomic charges analysis were performed to better understand the interaction of mercury-iodine species with surface. The results show that the binding of mercury-iodide species on surface include several adsorbing and desorbing processes. HgI is strongly adsorbed on the surface and the preferred adsorption mode is Hg-down mode. HgI is very stable existing on surface because the evolution of HgI is highly endothermic process. HgI2 adsorption is not stable, and some of the geometrical configurations of the intermediates show the dissociation of HgI2. The dominant interaction pathway is the dissociation of HgI2 with the binding of HgI and I on carbon surface.

Catalytic oxidation removal of gaseous elemental mercury in flue gas over niobium‐loaded catalyst

Nb‐Co‐Ce/Al2O3 catalysts prepared by impregnation, sol‐gel method, and co‐precipitation were examined for elemental mercury (Hg0) oxidation in a simulated coal combustion flue gas. The catalysts were characterized by SEM‐EDS, BET, XRD, FTIR, and XPS techniques. The performances of different niobium‐loaded catalysts on Hg0 oxidation efficiency with regard to preparation method, assistant, reaction temperature, and typical individual flue gas were investigated. The results showed that the Hg0 oxidation efficiency performed at the highest value for niobium‐loaded catalysts at 523 K. SO2 was observed to have a negative effect on Hg0 oxidation. The niobium‐loaded catalysts prepared by the sol‐gel method and co‐precipitation exhibited higher Hg0 oxidation efficiency than that prepared by impregnation, due to higher specific surface area and more exposure of active sites. However, it also results in these catalysts being affected by SO2 more easily. An enhancing effect of O2 was observed and an addition of HCl would promote the Hg0 oxidation further. Results also indicated that synergistic effects between NbOx, CoOx, and CeOx could promote the sustainable capacity of Hg0 oxidation for Nb‐Co‐Ce/Al2O3 catalysts. Hg0 oxidation over Nb‐Co‐Ce/Al2O3 catalysts is thought to follow a Mars‐Maessen mechanism in which lattice oxygen derived from NbOx would react with absorbed Hg0.

Integrated removal of NO and mercury from coal combustion flue gas using manganese oxides supported on TiO2

A catalyst composed of manganese oxides supported on titania (MnOx/TiO2) synthesized by a sol–gel method was selected to remove nitric oxide and mercury jointly at a relatively low temperature in simulated flue gas from coal-fired power plants. The physico-chemical characteristics of catalysts were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses, etc. The effects of Mn loading, reaction temperature and individual flue gas components on denitration and Hg0 removal were examined. The results indicated that the optimal Mn/Ti molar ratio was 0.8 and the best working temperature was 240°C for NO conversion. O2 and a proper ratio of [NH3]/[NO] are essential for the denitration reaction. Both NO conversion and Hg0 removal efficiency could reach more than 80% when NO and Hg0 were removed simultaneously using Mn0.8Ti at 240°C. Hg0 removal efficiency slightly declined as the Mn content increased in the catalysts. The reaction temperature had no significant effect on Hg0 removal efficiency. O2 and HCl had a promotional effect on Hg0 removal. SO2 and NH3 were observed to weaken Hg0 removal because of competitive adsorption. NO first facilitated Hg0 removal and then had an inhibiting effect as NO concentration increased without O2, and it exhibited weak inhibition of Hg0 removal efficiency in the presence of O2. The oxidation of Hg0 on MnOx/TiO2 follows the Mars–Maessen and Langmuir–Hinshelwood mechanisms.

Preparation of Fly-Ash-Modified Bamboo-Shell Carbon Black and Its Mercury Removal Performance in Simulated Flue Gases

Toxic mercury emissions from various industrial sources, especially coal combustion, have become an increasing concern in recent years, and effective methods of removing these emissions from coal combustion flue gases are being sought. In this study, fly-ash-modified bamboo-shell carbon blacks were prepared, and the properties and mercury removal abilities of the prepared carbon blacks were evaluated. The fly ash acted as a support for the carbon structure during the carbon black preparation, and demonstrated the ability to fix a portion of the carbon contained in volatile compounds during carbonization, resulting in a higher carbon yield of the carbon black containing fly ash; it had a higher BET surface area and a higher mercury removal ability than carbon black without fly ash. Strong pore structure, in the range of 1000–10000 nm, appeared after the addition of fly ash, which became stronger with higher fly-ash content. Additionally, the mercury compounds generated through the catalytic effect of fly a...

Colloidal Silicalite Coating for Improving Ionic Liquid Membrane Loading on Macroporous Ceramic Substrate for Gas Separation

A thin layer of colloidal silicalite was coated on a macroporous alumina substrate to improve the effectiveness in loading and supporting ionic liquid (IL) membrane on macroporous ceramic substrate. The [bmim][BF4] IL and CO2 gas separation were used as the model system in this research. The colloidal silicalite top layer enabled the formation of a pinhole-free IL membrane with significantly reduced load of IL as compared to the bare alumina substrate because the former had a smaller and more uniform inter-particle pore size than the latter. The supported IL membrane was extensively studied for CO2 separation in conditions relevant to coal combustion flue gases. The silicalite-supported IL membrane achieved a CO2/N2 permselectivity of ~24 with CO2 permeance of ~1.0×10-8 mol/m2·s·Pa in dry conditions at 26˚C and reached a CO2/N2 separation factor of ~18 with CO2 permeance of ~1.56×10-8 mol/m2·s·Pa for a feed mixture containing ~11% CO2 and ~9% water vapor at 50oC. This supported IL membrane exhibited excellent stability under a 5-bar transmembrane pressure at 103˚C and chemical resistance to H2O, SO2, and air (O2). Results of this study also indicated that, in order to fully realize the advantages of using the colloidal silicalite support for IL membranes, it is necessary to develop macroporous ceramic supports with optimized pore size distribution so that the IL film can be retained in the micron-thin silicalite layer without penetrating into the base substrate.

Supercritical CO2 Brayton cycles for coal-fired power plants

This paper investigates the supercritical CO2 cycles performance, from thermodynamic consideration and within realistic industrial modeling hypotheses, for coal power plant application. It proposes a design of such a power cycle for a first step implementation of the technology. Main findings are the following: even with the low temperature heat available in the coal combustion flue gas, a recompression cycle is mandatory for this application: the difference between this option and a standard Brayton cycle is more than 4.5%pt efficiency. Compared to no-reheated cycle, single reheat is an effective configuration with 1.5%pt efficiency increases. Another process improvement such as double reheat cycle, double recompression cycle and an advanced flue gas economizer configuration induce efficiency gain between 0.3 and 0.5%pt. Influence of the heat sink temperature stability has been quantified: 1.5%pt reduction for 5 °C increase with a minimal cycle pressure of 7 MPa; however, performance stability could be improved by adapting the main compressor inlet pressure. As a conclusion, CO2 supercritical coal-fired power plant theoretically offers interesting performances, of 47.8%-LHV efficiency, with existing materials at current operating conditions in a relatively near timeframe.

Catalytic Destruction of a Surrogate Organic Hazardous Air Pollutant as a Potential Co-benefit for Coal-Fired Selective Catalytic Reduction Systems

Catalytic destruction of benzene (C6H6), a surrogate for organic hazardous air pollutants (HAPs) produced from coal combustion, was investigated using a commercial selective catalytic reduction (SCR) catalyst for evaluating the potential co-benefit of the SCR technology for reducing organic HAP emissions. Bench-scale experiments were performed using simulated coal combustion flue gases under a broad range of SCR reaction conditions. C6H6 was added at 1 or 17 ppm into the flue gas mixtures with different concentrations of sulfur dioxide (SO2), nitrogen oxide (NO), hydrogen chloride (HCl), and ammonia (NH3) to simulate the combustion of bituminous and sub-bituminous coals. The destruction of the C6H6 across the catalyst was measured by a total hydrocarbon analyzer and a resonance-enhanced multiphoton ionization time-of-flight mass spectrometer (REMPI–TOFMS) for the experiments with high (17 ppm) and low (1 ppm) concentrations of C6H6, respectively. The operating parameters of the SCR process, including the ...

A Two-Stage Virtual Impactor for In-Stack Sampling of PM2.5 and PM10 in Flue Gas of Stationary Sources

Two ISO standard methods for in-stack sampling and measurement of PM_(2.5) and PM_(10) mass concentrations in flue gas from stationary sources were published in 2009 (ISO 23210, conventional cascade impactors) and 2012 (ISO 13271, virtual impactors). The performances of these two methods in terms of PM_(2.5) separation efficiency and the accuracy of measured mass concentration were compared at the same sampling point and conditions both using laboratory-scale model flue gas with different dust concentrations and using real flue gas sampled from a test facility for pulverized coal combustion. The virtual impactor showed very satisfactory performance for PM_(2.5) mass concentration and separation efficiency within the investigated range of mass concentrations and ambient conditions. The conventional cascade impactor method overestimated PM_(2.5) mass concentration by more than 25% due to particle bounce and re-entrainment of coarse particles from the collection plates. During in-stack PM_(2.5) sampling from coal combustion flue gas with reactive components at high temperature, the use of greased plates with the conventional impactor caused overestimation of mass concentration, even when grease with high temperature endurance was used. The use of a quartz-fiber filters on the impaction plates reduced overestimation but particle bounce and re-entrainment still remained.

Effect of fly ash composition on the retention of mercury in coal-combustion flue gas

Fly ash from coal-fired power plant has been reported to be a promising sorbent for mercury capture. However, the nature of the Hg–fly ash interaction is still unknown. In this work, the thermal decomposition method is used to evaluate the effect of the fly ash composition on the retention of mercury in coal-combustion flue gas. Eight fly ash samples were collected from the electrostatic precipitators of eight full-scale pulverized coal-fired power plants. The high-temperature treatment is performed to gradually decompose the carbon and mercury species in the fly ashes, and each fly ash are divided into four groups. The content of the carbon and mercury present in all the examined samples was measured. The results show that the most mercury species are decomposed from the fly ash at 300°C. The quantity of carbon in the fly ash alone does not determine the amount of mercury retained at any temperature. The decomposed mercury content is related to the mercury content of the fly ash. The carbon particle with ultra-strong mercury capture capacity cannot be found in the fly ash. Furthermore, X-ray fluorescence spectrometry (XRF) is used to identify the elemental composition of inorganic components of all the native fly ashes. Multivariate linear regression method is used to assess the dominated components that determine the mercury retention capacity of these fly ashes. The results indicate that the carbon, silicon, iron, sulfur, and magnesium in fly ash all play a major role in mercury retention. Hence, both carbon and inorganic components influence the retention of mercury in the fly ash.

Removal of Gaseous Elemental Mercury by Cylindrical Activated Coke Loaded with CoOx-CeO2 from Simulated Coal Combustion Flue Gas

Co–Ce mixed oxides were loaded on commercial cylindrical activated coke granules (CoCe/AC) by an impregnation method to remove gaseous elemental mercury (Hg0) from simulated coal combustion flue gas at low temperature (110–230 °C). Effects of the Co/Ce molar ratio in Co–Ce mixed oxides, mixed oxides loading value, reaction temperature, and flue gas components (O2, NO, SO2, H2O) on Hg0 removal efficiency were investigated, respectively. Brunauer–Emmett–Teller analysis, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) analysis were employed to analyze the characteristics of the samples. Results showed that up to 92.5% of Hg0 removal efficiency could be obtained over Co4.5Ce6/AC at 170 °C. The remarkably high Hg0 removal ability of Co4.5Ce6/AC mainly depended on the synergetic effect between cobalt oxide and ceria. Additionally, different with the pure N2 condition, the existence of O2 and NO...