Petrology of subbituminous feed coal as a guide to the capture of mercury by fly ash—influence of depositional environment

The mercury and maceral content of subbituminous feed coals burned at six Canadian power plants was measured and compared to the mercury and carbon content of corresponding fly ashes. The fly ashes were collected from electrostatic precipitator (ESP) pollution controls used at the power plants. Four feed coals from the Upper Cretaceous to Tertiary age Scollard Formation were deposited in freshwater depositional environment and contain 35–44% inertinite. Two feed coals from the Upper Cretaceous age Horseshoe Canyon Formation were deposited in a brackish water environment and contain 14% inertinite.The data show that abundant inertinite equates to more unburned carbon in ESP fly ash, and to enhanced capture of mercury in cold-side ESP fly ash. This indicates that the inertinite content of a coal seam may be indicative of the percentage of unburned carbon and the percentage of mercury captured by a cold-side ESP.     

Impact of co-combustion of petroleum coke and coal on fly ash quality: Case study of a Western Kentucky power plant

Petroleum coke has been used as a supplement or replacement for coal in pulverized-fuel combustion. At a 444-MW western Kentucky power station, the combustion of nearly 60% petroleum coke with moderate- to high-sulfur Illinois Basin coal produces fly ash with nearly 50% uncombusted petroleum coke and large amounts of V and Ni when compared to fly ash from strictly pulverized coal burns. Partitioning of the V and Ni, known from other studies to be concentrated in petroleum coke, was noted. However, the distribution of V and Ni does not directly correspond to the amount of uncombusted petroleum coke in the fly ash. Vanadium and Ni are preferentially associated with the finer, higher surface area fly ash fractions captured at lower flue gas temperatures. The presence of uncombusted petroleum coke in the fly ash doubles the amount of ash to be disposed, makes the fly ash unmarketable because of the high C content, and would lead to higher than typical (compared to other fly ashes in the region) concentrations of V and Ni in the fly ash even if the petroleum coke C could be beneficiated from the fly ash. Further studies of co-combustion ashes are necessary in order to understand their behavior in disposal.     

Parameters influencing the variation in mercury emissions from an Alberta power plant burning high inertinite coal over thirty-eight weeks period

Feed coals and fly ashes from a coal-fired power station burning Alberta subbituminous coal were examined for a period of thirty-eight weeks to determine the variation in emitted mercury. Feed coal samples were analyzed for proximate, calorific value and Hg content, while fly ash samples were examined for C and Hg contents. The maceral content of the feed coal was also determined. The emitted mercury was calculated and compared to mercury emitted from the stack according to a mass-balance calculation from a previous study for the same station.Mercury contents ranged from 0.029 to 0.066 mg/kg for feed coal, and from 0.069 to 0.112 mg/kg for fly ash. The carbon/char in fly ash was separated into reactive (vitrinitic/bimacerate) and less reactive (inertinitic) chars using ZnBr₂ at specific gravities of 1.7, 2.0, and 2.25 to 2.4. The result shows that there is a positive correlation between the carbon and mercury content of the fly ash. The reactive char particles in the fly ash may be responsible for the capture mercury in fly ash. The percentage of estimated captured mercury by fly ash increases with increasing carbon content (%) in fly ash. The percentage of emitted mercury for the period of 38 weeks is estimated to be within the range of 49% to 76% of the total input of mercury.     

Coal combustion by-product quality at two stoker boilers: Coal source vs. fly ash collection system design

Fly ashes from two stoker boilers burning Pennsylvanian Eastern Kentucky high volatile A bituminous coal blends were examined for their petrology and chemistry. The source coals have similar trace element contents. One of the ash collection systems was retrofitted with a baghouse (fabric filter) system, collecting a finer fly ash at a cooler flue gas temperature than the plant that has not been reconfigured. The baghouse ash has a markedly higher trace element content than the coarser fly ash from the other plant. The enhanced trace element content is most notable in the As concentration, reaching nearly 9000 ppm (ash basis) for one of the collection units. Differences in the ash chemistry are not due to any substantial differences in the coal source, even though the coal sources were from different counties and from different coal beds, but rather to the improved pollution control system in the steam plant with the higher trace element contents.     

Leaching characteristics of fly ash

The disposal of fly ash as a byproduct of thermic power stations, results in significant environmental problems. The leaching of coal fly ash during disposal is of concern for possible contamination, especially for the aquatic environment when ash is in contact with water. The aim of this study was to investigate the leaching behaviour of fly ashes currently disposed in Kemerkoy Power Plant (Turkey) fly-ash-holding pond. The studies were conducted with fly ashes from the electrostatic precipitators (fresh fly ash) and from the fly ash pond (pre-leached fly ash). The fly ashes has alkaline in nature and pH ranges between 11.9 to 12.2. The pre-leached fly ash exhibited lower EC values (7,400 µS) than the fresh fly ash (10,300 µS). In contrast to Fe and Pb, the elements such as Cr, Cd, Cu and Co did not leach from the fly ash. The Ca and Mn concentrations decreased with increasing temperature whereas, Na and K concentrations increased. The results showed that the most important effects of fly ash leaching were pH, Na, Ca, K, Fe, Mg, Mn and Pb.     

Characterization of activated carbon obtained from Saudi Arabian fly ash

This study investigated the potential application of heavy oil burning fly ash as a precursor to prepare activated carbon. The raw fly ash obtained from a power plant is cleaned by nitric acid/hydrochloric acid and activated at 550–800 °C with hold times of 30 and 60 min to obtain fly ash activated carbon. The phosphoric acid is used as a chemical agent to improve the surface characteristics of the cleaned fly ash. The effects of process variables such as amount of chemical reagents, activation time and temperature were investigated according to two-levels full factorial design. The resultant activated carbons were characterized in terms of Brunauer, Emmett and Teller surface area and total and pore volume. The maximum specific surface area was found of 148.30 m²/g at 800 °C temperatures with 60 min holding time. The test showed that the surface area and pore volumes of the material are also significantly enhanced by the activation process.     

Mercury content of the Springfield coal, Indiana and Kentucky

With pending regulation of mercury emissions in United States power plants, its control at every step of the combustion process is important. An understanding of the amount of mercury in coal at the mine is the first step in this process. The Springfield coal (Middle Pennsylvanian) is one of the most important coal resources in the Illinois Basin. In Indiana and western Kentucky, Hg contents range from 0.02 to 0.55 ppm. The variation within small areas is comparable to the variation on a basin basis. Considerable variation also exists within the coal column, ranging from 0.04 to 0.224 ppm at one Kentucky site. Larger variations likely exist, since that site does not represent the highest whole-seam Hg nor was the collection of samples done with optimization of trace element variations in mind. Estimates of Hg capture by currently installed pollution control equipment range from 9–53% capture by cold-side electrostatic precipitators (ESP) and 47–81% Hg capture for ESP + flue-gas desulfurization (FGD). The high Cl content of many Illinois basin coals and the installation of Selective Catalytic Reduction of NO_x enhances the oxidation of Hg species, improving the ability of ESPs and FGDs to capture Hg.     

The occurrence of gold in fly ash derived from high-Ge coal

We present the first data on the mode of occurrence of Au in fly ashes from the Wulantuga and Lincang power plants in China, which burn high-Ge coal. Gold occurs as fine-grained drop-like particles with a size of n*0.01–0.2 μm on the surface of the glass globules. These features of the Au particles are proof for Au condensation from the gas phase and deposition on the surface of fly ash in the cooler zone of the electrostatic precipitator and baghouse filter.     

Leachability of trace elements from two stabilized low lime Indian fly ashes

Fly ash is a waste by-product obtained from the burning of coal by thermal power plants for generating electricity. When bulk quantities are involved, in order to arrest the fugitive dust, it is stored wet rather than dry. Fly ash contains trace concentrations of heavy metals and other substances in sufficient quantities to be able to leach out over a period of time. In this study an attempt was made to study the leachabilities of a few selected trace metals: Cd, Cu, Cr, Mn, Pb and Zn from two different types of class F fly ashes. Emphasis is also laid on developing an alternative in order to arrest the relative leachabilities of heavy metals after amending them with suitable additives. A standard laboratory leaching test for combustion residues has been employed to study the leachabilities of these trace elements as a function of liquid to solid ratio and pH. The leachability tests were conducted on powdered fly ash samples before and after amending them suitably with the matrices lime and gypsum; they were compacted to their respective proctor densities and cured for periods of 28 and 180 days. A marked reduction in the relative leachabilities of the trace elements was observed to be present at the end of 28 days. These relative leachability values further reduced marginally when tests were performed at the end of 180 days.     

Pozzolanic fly ash as a hydraulic barrier in land fills

The liner plays an important role in controlling migration of contaminants present in the leachate in waste containment systems such as land fills and impoundments. Although questions have been raised about the performance of clay liners, they are increasingly used singly or as double liners in disposal sites. Though the clay liners possess many advantages such as low permeability and large attenuative capacity, they also possess high shrinkage potential and hence can crack under unsaturated conditions causing instability and increase in leakage rates. Further, the permeability of the clay linear can increase due to clay–pollutant interaction. This study examines the potential of pozzolanic fly ash as a hydraulic barrier in land fill. The behaviour of three different types of fly ashes, showing a range of physical properties and chemical composition from three different sources are reported in the study. Geotechnical properties, needed to evaluate the use of fly ashes as barriers, such as shrinkage, compaction, permeability, consolidation and strength characteristics are reported. The results show that fly ashes possess low shrinkage and hence do not crack. Compacted fly ashes undergo very little volume changes. They also show that pozzolanic fly ashes develop good strength properties with time. Pozzolanic fly ashes containing sufficient lime develop strength even without addition of lime. Non-pozzolanic fly ashes do not develop strength even on addition of lime. Fly ashes generally consist of silt size particles and consequently possess high permeability. However, pozzolanic fly ashes with lime exhibit low permeability on curing because of the formation of gelatinous compounds which block the pores. Thus, pozzolanic fly ashes appear to be promising for construction of liners to contain alkaline leachate.     

Environmental assessment of elements and polyaromatic hydrocarbons emitted from a Canadian coal-fired power plant

A detailed assessment of elements was carried out at a power plant rated at approximately 760 MW of electricity using western Canadian subbituminous coal. The concentrations of elements of environmental concern (As, Cd, Cr, Hg, Ni, and Pb) in milled coals, ashes, stack-emitted materials. Speciation of As, Cr, and Ni were determined. The polyaromatic hydrocarbons (PAHs) emitted from the stack were also measured. The rates of input of elemental input for As, Cd, Cr, Hg, Ni, and Pb were 28, 0.94, 230, 0.44, 44, and 88 kg/day, respectively; of which only 0.16, 0.01, 0.40, 0.27, 0.15, and 0.04 kg/day, respectively, were stack emitted. The total stack emission of toxic elements is 1.02 kg/day, with Cr being the highest contributor to this group with 0.4 kg/day. However, Hg at 0.27 kg/day has the highest percentage rate of emission at about 60%, while Cd has the lowest at about 1%. The electrostatic precipitator (ESP) removes a significant portion of the elements indicated by their relative enrichment (RE) ratios greater than 0.7. The results show that most of the elements in milled coal are low compared to world coals and other Canadian milled coals.Mercury is mostly ( 81%) emitted as gaseous elemental mercury (GEM), with 19% as reactive gaseous mercury (RGM). Particulate mercury is very low and averages about 0.1% of the total mercury at this station. Most of the arsenic in the milled coal is primarily associated with pyrite or as arsenate in its less toxic form of As⁺⁵ (> 95%). In both bottom and fly ashes, more than 95% of the total arsenic is present as As⁺⁵. Chromium in the milled coal and bottom ash is mostly non-toxic (Cr⁺³). The more toxic Cr⁺⁶ comprise less than 5% of the total Cr in the ESP fly ash and the stack-emitted ash. Nickel in milled coal and ashes is in the form of non-toxic Ni⁺², predominantly in coordination with oxygen.The emitted PAHs include acenapthene, fluorene, 2-methyl-fluorene, phenathrene, anthracene, fluoanthene, and pyrene; which are emitted from stack at the combined rate of 3.6 g/day. The concentrations of elements of environmental concern (As, Cd, Hg, Ni, and Pb) emitted to the atmosphere by the power plant in the zone of maximum impact at ground level are lower than those listed in the Health Guidelines of the USEPA and Canadian National Air Pollution Surveillance. This is based on stable meteorological conditions, down wind from the power plant for a maximum distance of 3 km.     

Mercury in coal and the impact of coal quality on mercury emissions from combustion systems

The proportion of Hg in coal feedstock that is emitted by stack gases of utility power stations is a complex function of coal chemistry and properties, combustion conditions, and the positioning and type of air pollution control devices employed. Mercury in bituminous coal is found primarily within Fe-sulfides, whereas lower rank coal tends to have a greater proportion of organic-bound Hg. Preparation of bituminous coal to reduce S generally reduces input Hg relative to in-ground concentrations, but the amount of this reduction varies according to the fraction of Hg in sulfides and the efficiency of sulfide removal. The mode of occurrence of Hg in coal does not directly affect the speciation of Hg in the combustion flue gas. However, other constituents in the coal, notably Cl and S, and the combustion characteristics of the coal, influence the species of Hg that are formed in the flue gas and enter air pollution control devices. The formation of gaseous oxidized Hg or particulate-bound Hg occurs post-combustion; these forms of Hg can be in part captured in the air pollution control devices that exist on coal-fired boilers, without modification. For a given coal type, the capture efficiency of Hg by pollution control systems varies according to type of device and the conditions of its deployment. For bituminous coal, on average, more than 60% of Hg in flue gas is captured by fabric filter (FF) and flue-gas desulfurization (FGD) systems. Key variables affecting performance for Hg control include Cl and S content of the coal, the positioning (hot side vs. cold side) of the system, and the amount of unburned C in coal ash. Knowledge of coal quality parameters and their effect on the performance of air pollution control devices allows optimization of Hg capture co-benefit.     

粉煤灰扰动样现场取样问题初步探讨

本文结合福州电厂粉煤灰室内试验结果分析，探讨粉煤灰扰动样现场取样及试验控制干密度的等问题，该电厂粉煤灰的沉积规律与其它电厂有所不同，灰料颗粒组细分布和距排灰口远近基本无关，取样也不能按以往经验进行。     

Assessing bioaccessible fractions of arsenic,chromium, lead,selenium and zinc in coal fly ashes

The utility of a physiologically based extraction test for evaluating the bioaccessibility of metals from fly ash in the human gastrointestinal system was evaluated in the present research. Calcium-rich and silica-rich fly ashes collected from eight power plants in India and United States of America were assessed for bioaccessibility for arsenic, chromium, lead, selenium and zinc. The results from the physiologically based extraction test were compared with those from a sequential extraction procedure that is often applied to solid wastes. Based on the physiologically based extraction test results, more than 40 % of the arsenic was found to be bioaccessible for all the ashes while selenium was very accessible for the calcium-rich ashes. Lead was found to be insignificantly bioaccessible in calcium-rich as well as silica-rich fly ashes. The mobilization of metals in the first three steps of the sequential extraction procedure was similar to the mobilization in the physiologically based extraction test for selenium for all ashes and for arsenic and chromium for most ashes, but the sequential extraction procedure mobilized more zinc than did the physiologically based extraction test. These results indicate that while sequential extraction procedures can provide good estimates of the bioaccessibility of many elements, extraction tests that more closely simulate physiological conditions can provide more accurate measures of bioaccessible concentrations of metals.     

Mineralogical and chemical composition of magnetic fly ash fraction

Magnetic fractions of coal fly ashes from three power plants were obtained by wet magnetic separation method. Quartz and mullite were the crystalline minerals dominating the nonmagnetic fractions. Magnetic fractions contained magnetite, hematite, and, to a lesser extent, quartz and mullite. Iron speciation by Mössbauer spectroscopy indicated the presence of Fe²⁺ and Fe³⁺ in aluminosilicate glass in magnetic fractions apart from magnetite and hematite. Chemical analyses revealed that magnetic fractions had about 2.5 times higher concentrations of Co and one to two times higher concentrations of Ni, Cu, Zn, Mo, and Cd. The dominant magnetic minerals were ferrimagnetic, and multi domain and stable single domain grains contributed mainly to the magnetic enhancement of fly ash samples.     

Mercury in Eastern Kentucky coals: Geologic aspects and possible reduction strategies

Mercury emissions from US coal-fired power plants will be regulated by the US Environmental Protection Agency (USEPA) before the end of the decade. Because of this, the control of Hg in coal is important. Control is fundamentally based on the knowledge of the amounts of Hg in mined, beneficiated, and as-fired coal. Eastern Kentucky coals, on a reserve district level, have Hg contents similar to the USA average for coal at mines. Individual coals show greater variation at the bench scale, with Hg enrichment common in the top bench, often associated with enhanced levels of pyritic sulfur. Some of the variation between parts of eastern Kentucky is also based on the position relative to major faults. The Pine Mountain thrust fault appears to be responsible for elemental enrichment, including Hg, in coals on the footwall side of the thrust.Eastern Kentucky coals shipped to power plants in 1999, the year the USEPA requested coal quality information on coal deliveries, indicate that coals shipped from the region have 0.09 ppm Hg, compared to 0.10 ppm for all delivered coals in the USA. On an equal energy basis, and given equal concentrations of Hg, the high volatile bituminous coals from eastern Kentucky would emit less Hg than lower rank coals from other USA regions.     

Technical Note Reactive silica and strength of fly ashes

The use of fly ash in geotechnical engineering depends greatly on its pozzolanic reactivity. Though many factors influence the reactivity of fly ash it is well recognized that reactive silica and lime content play a major role. A new, accurate and reliable method for the determination of reactive silica content of fly ash has been established. The reactive silica content, obtained as acid soluble silica in about 2 to 3 N hydrochloric acid, is found to correlate well with unconfined compressive strength of fly ashes. The reactive silica content of fly ash is also important in the stabilization of soils using fly ash. © Rapid Science Ltd. 1998     

Strontium isotopes as tracers of airborne fly ash from coal-fired power plants

Fly ash generated by coal-fired power plants is in part collected by filters in the emission stacks while a small portion is vented into the atmosphere. Since many of the coalfired power plants in the western United States are located in the desnrt, the ability to monitor fly ash emissions requires a chemical tracer that utilizes desert soil and plant interactions with the fly ash deposited in the desert environment. This investigation presents the results of a controlled greenhouse experiment in which a native desert plant, the brittlebush (Encelia farinosa), was grown on admixtures of desert soils and fly ash. The fly ash is strongly enriched in Sr and the brittlebush is a Sr accumulator. The data demonstrate that (1) the brittlebush isotopically equilibrates with desert soils whose fly ash components are as low as 0.25% by weight, (2) the fly ash Sr is apparently more available to the plant, than Sr derived from the soils, and (3) the difference between the⁸⁷Sr/⁸⁶Sr ratio of the fly ash (0.70807) and soils (0.71097 to 0.71117) warrants further investigations in the natural environment to determine the practicality of this method as a natural tracer of fly ash in the environment.     

Radioelemental study of Kolaghat,thermal power plant,West Bengal,India: possible environmental hazards.

Coal combustion in power plants in India produces large quantities of coal-related wastes, e.g. fly ash and bottom ash. Indian coals used in power stations are of high ash content, thus resulting in the generation of large amounts of fly ash (~100 million tons/year). Combustion of coal results in enhanced concentration of most radionuclides found in waste materials. In the present work, an attempt has been made to assess the radiological impact of the Kolaghat thermal power plant in West Bengal, India. The fly ashes and coal from the power plant were analysed for ²³⁸U, ²³²Th and ⁴⁰K by a NaI (Tl)-based gamma-ray spectrometer. The results show that ²²⁶Ra and ²³²Th range from 81.9-126 and 132-169 Bq/kg in fly ash and 25-50 and 39-55 Bq/kg in coal. These results are high compared to those of other thermal power plants of India. Hence, the Kolaghat fly ash has a significant amount of radioactivity which, if not properly disposed, will be a serious threat to the ambient environment.     

Leaching characteristics of a high-calcium fly ash as a function of pH: a potential source of selenium toxicity

Using a modified extraction procedure, the effect of pH on the leaching of selected elements from Ca-rich (Type C) power plant fly ash was studied. Continuous additions of acetic acid were used to maintain pH values of fly ash slurries at 4.0, 6.0 and 8.0 for 24 h and an additional set was leached at its natural pH (average 11.8) value. Analyses for Se, As, Ca, Cd, Cr, Fe, Na and Pb showed that the highest concentrations occur in the leachate at pH 4.0 and decline with increasing pH. Concentrations of Cr and Fe increased slightly between neutral and high pH. Arsenic, Cd, Cr, Pb and Se concentrations exceeded the Environmental Protection Agency's toxicity criteria at pH 4.0. Selenium was above its toxicity level at pH values near 7 but the other elements were below their respective toxicity levels near neutral pH. Because recent studies show adverse effects of Se on aquatic life at far lower concentrations than the current Environmental Protection Agency's standard, high-Ca, power plant fly ashes represent a potentially hazardous pollutant to surface and subsurface waters.