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.     

Characteristics and composition of fly ash from Canadian coal-fired power plants

Fly ashes were collected from the electrostatic precipitator (ESPs) and/or the baghouse of seven coal-fired power plants. The fly ashes were sampled from power plants that use pulverized subbituminous and bituminous feed coals. Fly ash from bituminous coals and limestone feed coals from fluidized-bed power plant were also sampled. The fly ashes were examined for their mineralogies and elemental compositions. The fly ashes from pulverized low sulfur coals are ferrocalsialic, those from high sulfur coals are ferrosialic and the fly ashes from the fluidized bed coals are ferrocalcic. The concentrations of As, Cd, Hg, Mo, Ni, and Pb in fly ash are related to the S content of the coal. Generally, those feed coals with a high S content contain higher concentrations of these elements. The concentrations of these elements are also greater for baghouse fly ash compared to ESP fly ash for the same station. The S content of fly ash from high S coal is 0.1% for pulverized ESP fly ash and 7% for baghouse fly ash from the fluidized bed, indicating that most of the S is captured by fly ash in the fluidized bed. The baghouse fly ash from the fluidized bed has the highest content of Cd, Hg, Mo, Pb, and Se, indicating that CaO, for the most part, captures them. Arsenic is captured by calcium-bearing minerals and hematite, and forms a stable complex of calcium or a transition metal of iron hydroxy arsenate hydrate [(M²⁺)₂Fe₃(AsO₄)₃(OH)₄·10H₂O] in the fly ash. Most elements in fly ash have enrichment indices of greater than 0.7 indicating that they are more enriched in the fly ash than in the feed coal, except for Hg in all ESP ashes. Mercury is an exception; it is more enriched in baghouse fly ash compared to ESP. Fly ash collected from a station equipped with hot side ESP has a lower concentration of Hg compared to stations equipped with cold side ESP using feed coals of similar rank and mercury content. Fly ash particles from fluidized bed coal are angular and subangular with cores of quartz and calcite. The quartz core is encased in layer(s) of calcium-rich aluminosilicates, and/or calcium/iron oxides. The calcite core is usually encased in an anhydrite shell.     

Comparison of calculated mercury emissions from three Alberta power plants over a 33 week period - Influence of geological environment

Feed coals and fly ashes from two coal-fired power plants burning Alberta subbituminous coal were analyzed for C, Cl, Hg, and S and calorific values (for feed coal only), every week for a period of 33 weeks. The feed coals used in this study were deposited in brackish water and are compared to the coals deposited in a freshwater environment. The Hg and char (unburnt carbon) content of the fly ash was monitored to determine the variation of Hg and its possible relationship to the char types in the fly ash. The feed coals have Hg content of 0.026-0.089 mg/kg and their fly ash contains 0.02-0.243 mg/kg of Hg. The C content of the fly ashes ranges from 0.15% to 0.51%. The carbon/char was separated from the fly ash using HF and HCl. Reactive vitrinitic (formed from woody part of plants) and less reactive inertinitic (natural char) chars were separated by density separations of various specific gravities using ZnBr₂.The char is mostly reactive vitrinitic (67-80 vol.%). Both stations have similar range of C content for their respective fly ashes. However, station 2 shows a much wider range of Hg in fly ash compared to station 1. In general, the fly ash from coal deposited under brackish water environment (stations 1 and 2) appears to have same or higher Hg content for lower C content compared to the fly ash from coal deposited under fresh water environment.The calculated emitted Hg for the period of 33 weeks for station 1 is estimated to be 64-90% of the total input of Hg with an average of 74%. The calculated emitted Hg shows a more complex pattern for station 2 and falls into two groups; with group (a) showing higher enrichment index for both Hg and S. The calculated emitted Hg for this group is 43-74% with an average of 57%, indicative of more Hg being captured by fly ash, possibly due to interaction between Hg and S. In the second group (b) the emitted Hg is calculated to be 74-95% with an average of 85%. The relative enrichment of both Hg and S in group (b) is low compared to group (a), indicative of possible slight paleo-weathering of the feed coal.The present study indicates that geological parameters such as paleo weathering and also depositional environment of the feed coal may influence the Hg content of fly ash.     

Speciation of mercury in fly ashes by temperature programmed decomposition

Mercury (Hg) is a toxic trace element which is emitted mostly in gas phase during coal combustion, although some Hg compounds may be retained in the fly ashes depending on the characteristics of the ashes and process conditions. To improve the retention of Hg in the fly ashes a good knowledge of the capture mechanism and Hg species present in the fly ashes is essential. The temperature programmed decomposition technique was chosen to identify the Hg species present in fly ashes obtained from two Pulverized Coal Combustion (PCC) plants and a Fluidized Bed Combustion (FBC) plant. The fly ashes were then used as Hg sorbents in a simulated flue gas of coal combustion and gasification. The Hg compounds found in the fly ash from the FBC plant after elemental mercury retention were mainly HgCl₂ and HgSO₄. The Hg species present in the two fly ashes from the two PCC plants were HgCl₂ and Hg⁰. The Hg species formed in the coal gasification atmosphere was HgS for all three fly ashes. The only Hg compound identified in the fly ashes after the retention of mercury chloride was HgCl₂.     

Mercury and selenium retention in fly ashes: Influence of unburned particle content

Mercury and selenium are present as trace elements in coal and may be emitted to the environment in gas phase during coal conversion processes or be partially retained on the fly ashes. The present work explores the possibility that selenium may contribute to mercury capture in fly ashes in two different situations: firstly the power station itself, in order to evaluate the influence of typical working conditions, and secondly in a fixed bed of fly ashes enriched with Se, in order to study the capture of mercury in more severe conditions. It was found that the presence of selenium in fly ashes may improve their capacity to capture mercury. However, in the four fly ashes of different origin studied, selenium is not the most important component for mercury retention. In fact, the presence of selenium in fly ash samples enriched in unburned carbon does not have any significant effect on mercury retention.     

Thermal stability of mercury captured by ash

The thermal stability of mercury captured by ash was studied by sampling ash throughout the collection train of two Kentucky power plants. Sampling occurred over multiple years and involved both fresh and archived samples. During one ash collection episode, sampling was from the combustion of a single pulverized coal feed. The other collections involved ash from blended feeds. Ash was collected from economizer, mechanical and electrostatic precipitator hoppers. Feed coals, rejects and bottom ash were also sampled. Fractions of all the samples were heated in a thermal analyzer to maximum temperatures increased sequentially from 100 to 500 °C in 100 °C increments. The mercury content of the spent material was then determined by analysis of the solids for Hg. From this data the thermal decomposition temperature of the captured mercury was determined. The total mercury captured by each sample, thermal stability of the mercury in relation to collection site, and correlations between mercury capture and chemical composition of the sample were also determined. The data showed that mercury was released between 300–400 °C for all ash samples. The thermal release of Hg between 300–400 °C was studied in greater detail by following the Hg release in several samples at 25 °C intervals from 300–400 °C. The concentration of mercury captured in the ESPs hoppers was greater than in the ash collected from the economizer or mechanical separators.     

循环流化床中烟气飞灰汞迁移规律

在小型热态循环流化床试验台上进行褐煤、烟煤、无烟煤燃烧试验,研究3种典型煤的烟气气态汞和飞灰颗粒汞迁移规律。试验结果表明：褐煤、烟煤、无烟煤在燃烧过程中,炉膛温度、空截面风速、给煤量以及煤颗粒大小变化时,汞元素在烟气和飞灰之间的迁移规律相似;降低炉膛密相区温度和增大炉膛空截面风速可促进烟气气态总汞Hg^T（g）迁移到飞灰颗粒汞Hg（p）中,同时也促进烟气气态零价汞Hg⁰（g）向烟气气态二价汞Hg²⁺（g）和Hg（p）转化;增加给煤量,烟气气态总汞Hg^T（g）和烟气气态零价汞Hg⁰（g）减少,飞灰颗粒汞Hg（p）含量增加,并且影响Hg⁰（g）的转化;选择合适的煤颗粒粒度可以促进Hg⁰（g）的转化以及Hg^T（g）向Hg（p）迁移。随燃烧工况的变化,3种煤Hg^T（g）、Hg（p）和Hg⁰（g）含量变化趋势相似,但含量相差较大,Hg⁰（g）占Hg^T（g）的比例y值也不同,其中无烟煤的y值高于烟煤和褐煤的y值。     

由脱灰煤制备富勒烯

以 3种高变质程度的中国煤为原料 ,用电弧等离子体蒸发法制备富勒烯 .富勒烯产品的表征采用质谱、紫外光谱和红外光谱等技术 .采用碱法脱灰技术对 3种煤进行脱灰处理 ,考察了脱灰前后煤样的灰分和相应炭棒中碳含量对富勒烯产率的影响 .结果表明 ,由实验所用煤种均可制得富勒烯 .原料煤本身的化学结构和组成对富勒烯的产率有明显影响 ,表现为富勒烯的产率随炭棒中碳含量的增加而增大 .煤中的矿物质对富勒烯的形成有很强的抑制作用 ,表现为富勒烯的产率随原料煤中灰分的增加而降低 ,这一变化趋势通过对介休焦煤进行不同程度的脱灰处理而得到进一步证实.     

Carbonization of coal blends: mesophase formation and coke properties

Laboratory investigations of strength of cokes from blends of coals incorporating pitch were supported by 7 kg trials. The stronger cokes showed a greater interaction between coal and pitch to produce an interface component of anisotropic mozaics which is relatively resistant to crack propagation. The process whereby coal is transformed into coke includes the formation of a fluid zone in which develop nematic liquid crystals and anisotropic carbon which is an essential component of metallurgical coke. Strength, thermal and oxidation resistance of coke can be discussed in terms of the size and shape of the anisotropic carbon which constitutes the optical texture of pore-wall material of coke. Coals of different rank form cokes with different optical textures. Blending procedures of non-caking, caking and coking coals involve the interactions of components of the blend to form mesophase and optical texture. Petroleum pitches used as additives are effective in modifying the carbonization process because of an ability to participate in hydrogen transfer reactions.     

Petrology and minor element chemistry of combustion by-products from the co-combustion of coal,tire-derived fuel,and petroleum coke at a western Kentucky cyclone-fired unit

A western Kentucky power plant conducted a series of test burns with coal+tire-derived fuel (tdf) and coal+tire-derived fuel+petroleum coke blends. Collections of fuel, fly ash, and bottom ash/slag were made from the cyclone-fired unit under four fuel combinations: coal, coal+ca. 1% tdf, coal+ca. 3% tdf, and coal+ca. 3% tdf+petroleum coke. Fly ash carbons derived from the three fuel types can be distinguished, allowing an assessment of the impact of co-combustion on fly ash quality. While certain aspects of the ash chemistry are distinctive, Zn increasing in tdf-derived fly ash and Ni and V increasing in petroleum coke-derived fly ash, changes in the coal source between sampling dates complicate the assessment of the chemistry.     

The effect of coal blend fluidity on the properties of coke

The effect of maximum fluidity of coal blends on coke quality was investigated using high fluidity coals and pitch to increase the fluidity of the blends. The results show that high fluidity of the blends could improve the growth of anisotropic carbon in coke. It also improves the coke M₃₀, but this is restricted when using high fluidity coals as additives. There is no significant improvement of the M₁₀ by increasing the fluidity. The reactivity, however, could be reduced to lower than the expected value by increasing the coal blend fluidity.     

Pitch and petroleum coke additions to coke oven charges

Several pitch materials and a petroleum coke were added to coke oven charges in an attempt to make good metallurgical coke from Canadian coal of poor coking quality. Coal and petroleum pitches were added to a low fluid western Canadian coal of medium volatile bituminous rank, and the blends coked in a technical-scale moveable wall test oven having a 230-kg charge capacity. Pitches improved coke tumble test indices, the principal coke quality parameter related to blast furnace performance. Varying levels of petroleum coke were added to an eastern Canadian coal of high volatile bituminous rank, and the blends, some partially briquetted, were carbonized in a test oven. Tumble indices of coke from the partially briquetted charges approached an acceptable level. These investigations confirm that petroleum products as well as coal derivative can play a useful part in the production of a metallurgical strength coke from poor or non-coking coals.     

Structure in cokes from coals of different rank

A range of bituminous coals has been carbonized to 1273 K. Polished surfaces of the solid products, carbons or cokes, are examined for optical texture by optical microscopy. Fracture surfaces of the carbons are examined by scanning electron microscopy (SEM). The carbon from the lowest rank coal (NCB Code No. 702) is isotropic and fracture surfaces are featureless. Carbons from coals of ranks 602, 502 are optically isotropic but fracture surfaces are granular (size 0.1–0.2 μm), indicating small growth units of mesophase. In the carbon/coke from a 401 coal, the anisotropic optical texture and grain size are both ≈0.5–10 μm diameter. Coke from a coking coal (301a, 301b) has a layered structure extending in units of at least 20 μm diameter with sub-structures ~ 1.5 μm within the layers, indicating perhaps that the bedding anisotropy of these coals is not totally lost in the fluid phase of carbonization. The carbons from the higher rank coals have the bedding anisotropy of the parent coal. The combined techniques of optical microscopy and SEM (both before and after etching of the fracture surfaces of coke in chromic acid solution) reveal useful detail of structure in carbons/cokes and of the mechanism of carbonization of coking coals.     

Coal blend combustion: fusibility ranking from mineral matter composition☆

Although coal blends are increasingly utilized at power plants, ash slagging propensity is a non-additive property of the pure coals and hence difficult to predict. Coal ash tendency to slag is related to its bulk chemistry and ash fusion temperatures, and the present study aims to compare the results obtained from thermodynamic simulation with characterization of samples obtained as outcomes of plant-based coal-blend combustion trials at three utilities located in the Centre and North of Chile. Pulverized coal and plant residues samples from five families of binary blends tested in an experimental program were characterized for chemistry, mineralogy and maceral composition. The slagging was evaluated by determination of fusion curves using the MTDATA software and NPLOX3 database for the main coal ash oxides. The ranking obtained was approximately the same as obtained from carbon in the fly ashes and from plant residues observations. The thermodynamic modeling was a valid option to predict the fusibility during the combustion of blends.     

Combustion reactivity of different oil-fired fly ashes as received and leached

The combustion of heavy fuel oil for power generation leads to a great production of fly ashes, usually disposed of in controlled expensive landfills. As possible alternatives to their conventional disposal, hydrometallurgical processes are used for the recovery of valuable metals such as vanadium and nickel and the carbonaceous residues, so obtained, could be burned to recover energy.In this study, the combustion of oil-fired fly ashes, as received and leached, was investigated by thermo-analysis in atmospheres at different oxygen concentration (6, 12 and 21 vol%). Two commercial coals were used as reference samples. The ignition temperature and the thermo-kinetic analysis were used to compare the reactivity of the different samples.Besides, in order to value the feasibility of co-combustion of leached fly ashes with coals in pre-existent coal boilers, blends of leached fly ash with coal were prepared and their combustion behaviour was investigated.     

Trace element partitioning during the firing of washed and untreated power station coals

The effect of coal washing on trace element content and combustion behaviour of four world-traded coals has been studied at rig scale. The inputs and process outputs from a 1 MW combustion test facility, including coal, bottom ash, suspended fly ash, retained ash and flue gas, have been analysed for a standard suite of 17 trace elements. The results suggest that although coal cleaning significantly reduces the total ash content of the coal, the concentrations of individual trace elements are not reduced proportionately. Combustion of the washed coals resulted in increased concentrations of trace elements in the fly ash, although total fly ash loadings were reduced. Cleaning appeared to have little effect on concentrations of gaseous trace elements in the flue gas. The partitioning of the more volatile trace elements such as mercury and selenium between the vapour and solid phase was influenced by the amount of excess oxygen in the furnace, presumably affecting carbon-in-ash levels.The results suggest that the coal cleaning undertaken for these experiments did not significantly reduce the emissions to atmosphere of trace elements. The ultimate emissions will be determined by the efficiency of the dust capture systems.     

Influence of the co-firing on the leaching of trace pollutants from coal fly ash

The (co)-firing of low-cost alternative fuels is expected to increase in the forthcoming years in the EU because of the economic and environmental benefits provided by this technology. This study deals with the impact of the different coal/waste fuel ratio of the feed blend on the mineralogy, the chemical composition and especially on the leaching properties of fly ash. Different blends of coal, petroleum coke, sewage sludge, wood pellets, coal tailings and other minor biomass fuels were tested in PCC (pulverised coal combustion) and FBC (fluidized bed combustion) power plants. The co-firing of the studied blends did not drastically modify the mineralogy, bulk composition or the overall leaching of the fly ash obtained. This suggests that the co-firing process using the alternative fuels studied does not entail significant limitations in the re-use or management strategies of fly ash.     

Carbonization and liquid-crystal (mesophase) development. Part 4. Carbonization of coal-tar pitches and coals of increasing rank

Coal-tar pitches, from coals of different rank and with various quinoline-insoluble contents, were carbonized under pressure (67 to 200 MN m⁻²) to maximum temperatures of 923 K. The resultant cokes were examined by optical and scanning electron microscopy in terms of size and shape of anisotropic structures within the coke. Natural quinoline-insolubles and carbon blacks both destroyed growth of the mesophase and development of anisotropy. Graphite particles (<10 μm) promoted growth and coalescence of the mesophase. Fourteen coals, of carbon content 77 to 91 wt%, VM 41 to 26%, were similarly carbonized under pressure. In the lower-rank coals no microscopically resolvable anisotropic mesophase was produced, but at a carbon content of 85% anisotropic units 1–2 μm in diameter were detected, increasing in size at a carbon content of 90% to 5 μm diameter. Results are discussed in terms of the origins of anisotropic mosaics observed in cokes, their variation in size with coal rank, and their significance in the carbonization of coal.     

Laboratory study of air-water-coal combustion product (fly ash and FGD solid) mercury exchange

Recent laboratory research has indicated that coal fly ash derived from subbituminous and bituminous type coals is a sink for atmospheric mercury (Hg), however lignite-based ash was found to emit Hg to the air. Solids collected from systems with components that enhance Hg removal (i.e. activated carbon injection (ACI), flue gas desulfurization (FGD), and selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR)) may have higher Hg concentrations and therefore a higher potential for Hg release. For this study we investigated the potential for Hg release to the air and water from coal combustion products (CCPs) collected from coal-fired units with FGD equipment, SCR and SNCR equipment, and sorbent injection for Hg removal. In the laboratory study, most dry samples acted as sinks for atmospheric Hg in the dark at 25 °C. When exposed to light or increased temperature (45 °C), deposition of Hg to the fly ash substrates in most cases continued but decreased. Wet FGD samples emitted Hg. However, they became a sink for atmospheric Hg or exhibited low Hg emission rates when dried. Mercury flux in the dark at 25 °C was correlated with fly ash carbon content (LOI). Most liquid extracts derived using the synthetic precipitation leaching procedure (SPLP EPA method 1312) had very low Hg concentrations (<13 ng/l).     

Study of coal ash deposition in an entrained flow reactor: Influence of coal type,blend composition and operating conditions

An entrained flow reactor has been used to characterize the slagging behaviour of twelve coals/blends, under different experimental conditions. Besides measuring the weight of the deposits, a detailed characterization of the coals, fly ashes and deposits has been performed using ASTM procedures and Computer-Controlled SEM. Two different indices are used to represent the deposition behaviour: capture efficiency (CE) is a measure of the intrinsic tendency of the particles to form a deposit, while the energy-based growth rate (GRE) also considers the ash and energy contents of the different fuels. The coals tested ranged from lignites to anthracites and showed widely different values for those slagging indices. The experimental results are interpreted making use of a detailed analysis of the physico-chemical properties of the coals and fly ashes. In particular, a consistent relationship is found between deposit growth rates and the aerodynamic diameter of the fly ash particles. Coal blending showed in all cases non-linear behaviours with respect to the parent coals. The experimental programme was also extended to evaluate the influence of variations in some experimental parameters, including exposure time, gas temperature, surface temperature and distance from injection. The results obtained are useful both to characterize the experimental method and to determine the influence of some parameters relevant to deposition phenomena.