Activated carbon for mercury control: Implications for fly ash management

As more utilities begin to use activated carbon injection (ACI) for mercury control, the potential for the presence of elevated concentrations of mercury, other air toxic elements, and activated carbon to impact fly ash management needs to be evaluated. Several Energy & Environmental Research Center (EERC) projects have allowed the collection of comparative baseline fly ash samples and associated fly ash–activated carbon (AC) samples from full-scale demonstrations of ACI for mercury emission control. These samples were evaluated for mercury and air toxic element content and mobility and for performance criteria to facilitate a better understanding of the impact of these components to specific utilization applications, including use as a mineral admixture in concrete. These data are compared with published data from samples collected at similar large-scale mercury emission control tests.     

The influence of carbon particle type in fly ashes on mercury adsorption

Recent research has shown that certain fly ash materials produced in coal combustion for power generation have an affinity for the mercury compounds present in flue gases. However, the exact nature of Hg-fly ash interactions is still unknown and the different variables that influence mercury adsorption need to be identified. In this work the microscopic components of fly ashes derived from the combustion of different types of feed blends of different coal rank and mercury adsorption were investigated. The aim of this research was to establish relationships between Hg retention and the type of unburned carbons present in various fly ashes. The fly ashes and fly ash fractions studied were used as sorbent beds for high mercury concentrations, conditions in which mercury retention is highly favored. From the results obtained it was confirmed that the role of the unburned carbon components in mercury capture may depend, among other factors, on the type of unburned carbon. Fly ashes capture different species of mercury depending on their nature and the type of anisotropic particles.     

Role of activated carbon structural properties and surface chemistry in mercury adsorption

In this work the performance of activated carbons prepared from raw and demineralised lignite for gas-phase Hg° removal was evaluated. A two-stage activation procedure was used for the production of the activated samples. In order to study the effect of mineral matter on pore structure development and surface functionality of the activated carbons, a demineralisation procedure involving a three-stage acid treatment of coals, was used, prior to activation. Hg° adsorption tests were realized in laboratory-scale unit consisted of a fixed-bed reactor charged with the tested activated samples. The examined adsorbent properties that may affect removal capacity were the pore structure, the surface chemistry and the presence of sulphur on the surface of activated carbons. The obtained results revealed that activated carbons produced from demineralised lignite posses a high-developed micropore structure with increased total pore volume and BET surface area. These samples exhibit enhanced Hg° adsorptive capacity. In all cases, mercury removal efficiency increased by sulphur addition. Finally, the starting material properties and activation conditions affect the concentration and the type of the oxygen groups on activated carbon surface, that have been determined with TPD-MS experiments.     

Composition and microstructure of alkali activated fly ash binder: Effect of the activator

The alkali activation of fly ashes is a chemical process by which the glassy component of these powdered materials is transformed into very well-compacted cement. In the present work the relationship between the mineralogical and microstructural characteristics of alkaline activated fly ash mortars (activated with NaOH, Na₂CO₃ and waterglass solutions) and its mechanical properties has been established. The results of the investigation show that in all cases (whatever the activator used) the main reaction product formed is an alkaline aluminosilicate gel, with low-ordered crystalline structure. This product is responsible for the excellent mechanical-cementitious properties of the activated fly ash. However the microstructure as well as the Si/Al and Na/Al ratios of the aluminosilicate gel change as a function of the activator type used in the system. As a secondary reaction product some zeolites are formed. The nature and composition of these zeolites also depend on the type of activator used.     

Catalytic effects of inorganic compounds on the development of surface areas of fly ash carbon during steam activation

This study was to investigate the catalytic effect of inorganic compositions present in fly ash carbons, high-unburned-carbon content fly ashes from coal-fired power plants, on the development of surface area during steam activation. Through this work, the relationships between the concentrations of alkali metals (Na and K) and a mixture of metals, Na-K-Ca and Na-K-Fe, in fly ash carbon and the surface areas of the produced activated carbon were studied.Six fly ash carbons were selected as feedstocks for activated carbon. SEM/EDS studies showed that there are two groups of inorganic particles present in fly ash carbons, in terms of the way they are associated with carbon particles: inorganic particles that exist as free single particles, and inorganic particles that are combined with the carbon particles. A series of froth flotation tests was conducted to separate the individual inorganic particles from the fly ash carbons. The concentration of the inorganic particles was analyzed using ICP-AES before and after froth flotation separation. Studies showed that the Na-K, Na-K-Ca, and Na-K-Fe which are combined with carbon particles had catalytic effects on the development of the surface areas of activated carbons. The higher the concentration of these catalytic particles, the more significant effects they had on the development of the surface areas with increasing carbon burn-off levels.     

Classification of carbon in Canadian fly ashes and their implications in the capture of mercury

Fly ashes produced from Canadian power plants using pulverized coal and fluidized bed combustors were examined for their carbon content to determine their ability to capture mercury. The feed coal used in these power plants were lignite, subbituminous, high and medium volatile bituminous, their blends, and also blends of coal with petroleum coke (Petcoke). The carbon and mercury content of the coals and fly ashes were determined using the ASTM standard method and by the cold vapour atomic absorption spectrometry method. The carbon content of the fly ash was concentrated by strong acid digestion using HCl and HF. The quantitative and qualitative analyses of the carbon concentrate were made by using a reflected light microscope. The results show that the carbon content of fly ash appears to be partially related to depositional environment during coalification and to the rank of the coal. The Hg captured by the fly ash depends on the rank and blend of the feed coals and the type of carbon in the fly ash. The isotropic vitrinitic char is mostly responsible for the capture of most Hg in fly ash. The inadvertent increase in carbon content due to the blending of coal with petroleum coke did not increase the amount Hg captured by the fly ash. The fly ash collected by the hot side electrostatic precipitator has a low Hg content and no relation between the Hg and carbon content of the ash was observed. These results indicate that the quantity of carbon in the fly ash alone does not determine the amount Hg captured. The types of carbon present (isotropic and anisotropic vitrinitic, isotropic inertinitic and anisotropic Petcoke), the halogen content, the types of fly ash control devices, and the temperatures of the fly ash control devices all play major roles in the capture of Hg.     

Influence of iron species present in fly ashes on mercury retention and oxidation

The aim of this study was to assess the effect of iron species present in fly ashes produced from coal combustion on mercury retention and oxidation. To achieve this objective the work was divided into two parts. In the first part the relationship between the mercury and iron content in fly ashes of different origin and characteristics was evaluated. In the second, a series of fractions enriched in iron oxides were separated from the fly ashes to determine the effect of increasing iron content on mercury retention and oxidation. Special attention was paid to the influence of iron on mercury behavior in enriched carbon particles in fly ashes. From the results obtained it can be inferred that, in the range of fly ashes studied, iron species do not affect the retention of mercury and do not play any role in heterogeneous mercury oxidation.     

Removal of mercury in aqueous solution by fluidized bed plant fly ash

Coal combustion in power plant produces fly ash. Fly ash may be used in water treatment to remove mercury (Hg²⁺) from water or to immobilize mercury mobile forms in silts and soils. Experiments were carried out on two kinds of fly ashes produced by two circulating fluidized bed plants with different chemical composition: silico-aluminous fly ashes and sulfo-calcic fly ashes. For the two kinds of fly ashes, adsorption equilibrium were reached in 3 days. Furthermore, removal of mercury was increased with increasing pH. Sulfo-calcic fly ashes allow us to remove mercury more efficiently and more steady. The chemical analysis of fly ash surface was carried out by electron spectroscopy. The results show that mercury is bound to ash surface thanks to several chemical reactions between mercury and various oxides (silicon, aluminium and calcium silicate) of the surface of the ashes.     

Adsorption of surfactants on unburned carbon in fly ash and development of a standardized foam index test

The “foam index” test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. Many foam index test procedures are in common use, and it is difficult to compare results between laboratories. The present paper explores the possibility of standardizing the test for use with coal fly ash pozzolans. It does so by establishing that a series of commercial air-entraining admixtures (AEAs) and pure anionic surfactants all behave in a well-correlated manner, when tested using the same protocol, on a suite of 29 fly ash samples (both class F and class C) obtained from utilities throughout the United States. A pure, reagent-grade surfactant can be chosen as the basis for a standardized test; it appears as though dodecyl benzene sulfonate (DBS) is a good candidate material for such testing. The present results also confirm that it is the carbon in coal fly ash that is the main sink for AEA adsorption in concrete mixtures containing significant amounts of ash. The solution chemistry in the testing mixture is important, and use of cement in the test mix is strongly recommended. Careful attention also needs to be paid to the relative amounts of different components in the mixture to be tested.     

焦化除尘粉制备活性炭的研究

研究了以焦化除尘粉为原料,采用水蒸气为活化剂制备活性炭的可行性.利用正交实验探讨了活化温度、活化时间、水蒸气流量等对活性炭吸附性能的影响.结果表明:影响除尘粉基活性炭制备的主次因素为活化温度、活化时间、水蒸气流量;制备除尘粉基活性炭的最佳条件为活化温度850.C,活化时间90 main,水蒸气流量10.73 mL/min,制备除尘粉基活性炭的碘吸附值为490.5215 mg/g,产率为35.16％.     

Effect of fly ash on autogenous shrinkage

The correlation between autogenous shrinkage and degree of hydration of fly ash was determined with the selective dissolution method. Then, the relationship between the degree of hydration of fly ash and autogenous shrinkage was examined. The results showed that the degree of hydration of fly ash increased as its Blaine surface area increased. The degree of hydration of fly ash increased with time, and autogenous shrinkage increased corresponding to the increase in the degree of hydration of fly ash. Moreover, it was found that the total quantity of Al₂O₃ in cement-fly ash samples affected autogenous shrinkage at early ages, but the long-term influence was very small.     

Size distribution of unburned carbon in coal fly ash and its implications

A set of nine coal fly ashes, obtained from various US utilities, were fractionated by standard dry-sieving techniques. The carbon contents of the different size fractions were measured, and the nature of the carbon particles was microscopically examined. Significant differences were found in the distribution of carbon in class F and class C ashes. The ‘foam index’ test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. This test measures adsorption of air entraining admixtures (AEAs) by the carbon in the fly ash. Application of this test to the different ash fractions confirmed that the smallest particle size fractions of ash make the major contribution to AEA adsorption. The carbon from class F ash has a comparable capacity for AEA adsorption as carbon from class C ash, when compared on a surface area basis. What makes the class C carbons apparently ‘worse’ is the fact that they have much higher surface areas than class F carbons (and it is only by virtue of the low carbon mass in most class C ashes that problems with these ashes are not more common). The importance of accessibility of the surface is also clearly seen from these results.     

Significance of surface moisture removal on triboelectrostatic beneficiation of fly ash

The gas transport, triboelectrostatic beneficiation of coal combustion fly ash into carbon-rich and ash-rich products was studied relative to the effect of ash surface moisture. Increasing the humidity to which the ashes from American and Italian coal-fired utilities were exposed under process and ambient conditions affected carbon and ash separability. The effect of humidity and particle surface moisture became more important as particle size decreased: particles greater than 75 μm in diameter were nearly unaffected whereas particles smaller than 45 μm experienced up to a four-fold change in their separability upon changing their surface moisture contents. Although particle size influences the moisture adsorption, which in turn affects tribocharging, the decrease in adhesive forces between carbon and ash from otherwise intractable clusters during drying also may be a factor influencing triboelectrostatic beneficiation performance.     

The impact of fly ash resistivity and carbon content on electrostatic precipitator performance

Despite the wide use of electrostatic precipitators for particulate control in pulverised coal combustion, there are still some aspects surrounding the performance of these devices which are not fully understood, particularly with respect to high carbon ash. The introduction of low NO_x burners to meet more stringent emissions regulations has also resulted in an increase in the carbon present in fly ash. An investigation was carried to assess how the operating conditions of a pilot-scale single-stage electrostatic precipitator test facility (ESPTF) affected the collection efficiency of two high carbon fly ashes. Whilst our findings demonstrated the effectiveness of the ESPTF in collecting both fly ash and carbon particles, the performance of the high-resistivity fly ash was significantly better than expected, particularly at higher temperatures. It was concluded that ESP performance of any particular fly ash and the separation of the carbon fraction may not necessarily be predicted by its resistivity alone.     

Characterization of the coal fly ash for the purpose of improvement of industrial on-line measurement of unburned carbon content

The influence of selected properties of fly ash on the measurements of an on-line analyser was described. Fly ash studied in the research originated from the pulverized coal fired boiler. The samples were taken using an inspection method within a period of 3 months. Systematic observation of the properties of the ash allowed monitoring of the work of the industrial analyzer during a relatively long period of the power plant work. Samples of coal fly ash were examined for their chemical and physical properties. Morphology was analysed by scanning electron microscopy. Unburned carbon content in fly ash was determined by using loss-on-ignition (LOI) and thermogravimetry analysis (TGA). The particle size distribution of fly ash was examined. Correlation between laboratory and on-line industrial measurements of the unburned carbon content of ash was discussed.     

Elemental mercury vapor capture by powdered activated carbon in a fluidized bed reactor

A bubbling fluidized bed of inert material was used to increase the activated carbon residence time in the reaction zone and to improve its performance for mercury vapor capture. Elemental mercury capture experiments were conducted at 100 °C in a purposely designed 65 mm ID lab-scale pyrex reactor, that could be operated both in the fluidized bed and in the entrained bed configurations. Commercial powdered activated carbon was pneumatically injected in the reactor and mercury concentration at the outlet was monitored continuously. Experiments were carried out at different inert particle sizes, bed masses, fluidization velocities and carbon feed rates. Experimental results showed that the presence of a bubbling fluidized bed led to an increase of the mercury capture efficiency and, in turn, of the activated carbon utilization. This was explained by the enhanced activated carbon loading and gas-solid contact time that establishes in the reaction zone, because of the large surface area available for activated carbon adhesion/deposition in the fluidized bed. Transient mercury concentration profiles at the bed outlet during the runs were used to discriminate between the controlling phenomena in the process. Experimental data have been analyzed in the light of a phenomenological framework that takes into account the presence of both free and adhered carbon in the reactor as well as mercury saturation of the adsorbent.     

Design of an optical instrument to measure the carbon content of fly ash

The polarisation ratio resulting from the backscattering of linearly polarised incident light by fly ash particles is indicative of the carbon content. Determination of this parameter is useful for characterising the efficiency of coal burning furnaces. This paper describes the development of an instrument to make these measurements, which is designed to be attached furnace ducts. The performance of the instrument is tested in laboratory measurements on a range of ashes from different coals. It is concluded that if the mineral content of the ashes is not known then the carbon mass fraction can be determined to within ±1%. If the mineral content is known then the use of neural network analysis can reduce this to ±0.5%.     

Development of fly ash and iron ore tailing based porous geopolymer for removal of Cu(II) from wastewater

This work aims to verify the feasibility of utilizing iron ore tailing (IOT) in porous geopolymer and intends to broaden the application of porous geopolymer in heavy metal removal aspect. Porous geopolymer was prepared using fly ash as resource material, which was partially replaced by IOT at level of 30%, by weight, with H₂O₂ as foaming agent and removal efficiency, adsorption affecting factors, adsorption isotherms and thermodynamics of Cu²⁺ by the developed porous geopolymer were investigated.The experimental results uncover that the porous amorphous geopolymer was successful synthesized with total porosity of 74.6%. The transformation of fly ash and IOT into foaming geopolymer leads to the formation of porous structure encouraging Cu²⁺ sorption. Batch sorption tests were carried out and geopolymer dosage, Cu²⁺ initial concentration, pH, contact time and temperature were the main concern. Both Langmuir and Freundlich models could explain the adsorption of Cu²⁺ on the porous geopolymer due to the high fitting coefficients. The uptake capacity reaches the highest value of 113.41 mg/g at 40 °C with pH value of 6.0. The thermodynamic parameters ΔHº, ΔSº and ΔGº suggests the spontaneous nature of Cu²⁺ adsorption on porous geopolymer and the endothermic behavior of sorption process.     

Substitution of quartz and clay with fly ash in the production of architectural ceramics: A mechanistic study

Quartz and clay are substituted gradually by fly ash using a triaxial ceramic formulation under simulated industrial conditions and the effects of fly ash substitution on the macroscopic properties and microstructures of the sintered ceramics are evaluated systematically. With the substitution of 35 wt% (1250 °C), the ceramic sample exhibited optimal properties, including linear shrinkage of 15.61%, bulk density of 2.39 g cm^-3, water absorption of 0.62% and flexural strength of 41.70 MPa, due to the accelerated densification and fly ash-spurred needle-shaped mullite. The microstructure analysis shows that the sintered matrix consists of three types of particles, quartz-, clay- and feldspar-like particles showing sintering behavior with respect to filling the glassy matrix with preserved morphology, precipitating mullite crystals, and fusing with the surrounding glassy matrix, respectively. The strength of the fly ash - containing ceramics is analyzed by the dispersion-strengthening mechanism and porosity and the results indicate that the fly ash particles affect the mechanical strength due to Griffith flaws when the total porosity is less than 25% and pores at higher total porosity. This study provides a viable strategy to recycle industrial fly ash in the production of architectural ceramics.     

Effect of fly ash on the physico-chemical and mechanical properties of a porcelain composition

The ceramic industry is one of the largest consumers of natural raw materials but has also the capacity and potential to make significant contributions in solving environmental problems by consuming solid rejects of various industries.