Mode of occurrence of arsenic in feed coal and its derivative fly ash, Black Warrior Basin, Alabama

An arsenic-rich (As = 55 ppm) bituminous feed coal from the Black Warrior Basin, Alabama and its derivative fly ash (As = 230 ppm) were selected for detailed investigation of arsenic residence and chemical forms. Analytical techniques included microbeam analysis, selective extraction, and As K-edge X-ray absorption fine-structure (XAFS) spectroscopy. Most As in the coal is contained in a generation of As-bearing pyrite (FeS₂) that formed in response to epigenetic introduction of hydrothermal fluids. XAFS results indicate that approximately 50% of the As in the coal sample occurs as the oxidized As(V) species, possibly the result of incipient oxidation of coal and pyrite prior to our analysis. Combustion of pyrite and host coal produced fly ash in which 95% of As is present as As(V). Selective extraction of the fly ash with a carbonate buffer solution (pH = 10) removed 49% of the As. A different extraction with an HCl-NH₂OH mixture, which targets amorphous and poorly crystalline iron oxides, dissolved 79% of the As. XAFS spectroscopy of this highly acidic (pH = 3.0) fly ash indicated that As is associated with some combination of iron oxide, oxyhydroxide, or sulfate. In contrast, a highly alkaline (pH = 12.7) fly ash from Turkey shows most As associated with a phase similar to calcium orthoarsenate (Ca₃(AsO₄)₂). The combined XAFS results indicate that fly ash acidity, which is determined by coal composition and combustion conditions, may serve to predict arsenic speciation in fly ash.     

Sulphur dioxide removal using South African limestone/siliceous materials

This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical composition and structural properties of the sorbent with regard to SO₂ removal in dry-type flue gas desulfurization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55-75 °C) and hydration period (4-10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe₂O₃ and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO₂ absorption capacity in terms of mol of SO₂ per mol of sorbent (0.1403-0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous structure with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents.     

Synthesis and characteristics of fly ash and bottom ash based geopolymers–A comparative study

The research was carried out to develop geopolymers mortars and concrete from fly ash and bottom ash and compare the characteristics deriving from either of these products. The mortars were produced by mixing the ashes with sodium silicate and sodium hydroxide as activator solution. After curing and drying, the bulk density, apparent density and porosity, of geopolymer samples were evaluated. The microstructure, phase composition and thermal behavior of geopolymer samples were characterized by scanning electron microscopy, XRD and TGA-DTA analysis respectively. FTIR analysis revealed higher degree of reaction in bottom ash based geopolymer. Mechanical characterization shows, geopolymer processed from fly ash having a compressive strength 61.4 MPa and Young's modulus of 2.9 GPa, whereas bottom ash geopolymer shows a compressive strength up to 55.2 MPa and Young's modulus of 2.8 GPa. The mechanical characterization depicts that bottom ash geopolymers are almost equally viable as fly ash geopolymer. Thermal conductivity analysis reveals that fly ash geopolymer shows lower thermal conductivity of 0.58 W/mK compared to bottom ash geopolymer 0.85 W/mK.     

Production of glass from coal fly ash

The coal fly ash from a Chinese thermal power plant was vitrified after the addition of ∼10 wt% Na₂O. The glass products have suitable viscosity at 1200 °C and displayed a good chemical durability. The heavy metals of Pb, Zn, Cr and Mn were successfully immobilized into the glass as determined by the toxicity characteristic leaching procedure method. Results indicate an interesting potential for the coal fly ash recycling to produce useful materials.     

Synthesis and characterization of novel solid base catalyst from fly ash

A new type of solid base catalyst was synthesized by chemical and thermal activation of fly ash, collected from Thermal Super Power Station situated in Kota, Rajasthan, India. The chemical activation was carried out by 50 wt.% NaOH followed by thermal activation at 450 °C. The modified physiochemical property of solid base fly ash (SBFA) was determined by X-ray diffraction, FT-IR spectroscopy, Scanning Electron Microscopy, N₂ adsorption-desorption studies and Flame Atomic Absorption Spectrophotometry. The results reveal that the catalyst is nano-crystalline in nature with crystallite size 11 nm and particle size in the range 840 nm to 6.95 μm. The surface basicity and therefore, catalytic activity in SBFA was originated by increased hydroxyl content as compared to fly ash, suggesting that the catalyst possess higher surface active sites. The basicity of the catalyst was measured by liquid phase, solvent free, single step condensation of benzaldehyde with cyclohexanone giving higher conversion (>70%) and selectivity (>80%) of desired product α,α′-dibenzylidenecyclohexanone. This excellent conversion shows that the catalyst has sufficient basic sites both on the surface and in the bulk, responsible for the catalytic activity. Furthermore, this catalyst may replace conventional environmentally hazardous homogeneous liquid bases making an ecofriendly; solvent free, solid base catalyzed process. The application of fly ash to synthesize a solid base catalyst finds a noble way to utilize this abundant waste material.     

Effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample

The effect of porous structure and surface functionality on the mercury capacity of a fly ash carbon and its activated sample has been investigated. The samples were tested for mercury adsorption using a fixed‐bed with a simulated flue gas. The activated fly ash carbon sample has lower mercury capacity than its precursor fly ash carbon (0.23 vs. 1.85 mg/g), although its surface area is around 15 times larger, 863 vs. 53 m²/g. It was found that oxygen functionality and the presence of halogen species on the surface of fly ash carbons may promote mercury adsorption, while the surface area does not seem to have a significant effect on their mercury capacity.     

Properties and potential applications of zeolitic materials produced from fly ash using simple method of synthesis

Very low energy-consuming procedure is proposed for synthesis of zeolitic materials from fly ash. Three different zeolite materials (X, P and S), rich in zeolite phases, Na-X (FAU), NaP1 (GIS) and sodalite (SOD) were produced from F-class fly ash, using NaOH and NaCl solutions under atmospheric pressure at temperature below 110 °C.Obtained zeolitic products were analysed for their composition and physicochemical properties then compared to the raw fly ash and commercial adsorbents. The zeolitization results in a significant increase of CEC (from 5.5 up to 239 meq 100 g^− 1), and the high ability to adsorb heavy metal ions (over 40 mg g^− 1) and retain complex and organic molecules (EGME), mostly evident for material X. Adsorptive purification of waste and working lubricating oils using zeolitic products allow to provide their commercial applications in petroleum industry. Leachability of toxic elements after standard post-reaction washing is environmentally safe.     

A study of fly ash-lime granule unfired brick

In this paper, the properties of fly ash-lime granule unfired bricks are studied. Granules were prepared from mixtures of fly ash and lime at fly ash to hydrated lime ratios of 100:0 (Ca/Si = 0.2), 95:5 (Ca/Si = 0.35) and 90:10 (Ca/Si = 0.5). After a period of moist curing, the microstructure and mineralogy of the granules were studied. Microstructure examination reveals that new phases in the form of needle-like particles are formed at the surface of granule. The granules were used to make unfired bricks using hydrothermal treatment at temperature of 130 ± 5 °C and pressure of 0.14 MPa. The microstructures, mineralogical compositions, mechanical properties and environmental impact of bricks were determined.The results reveal that the strengths of unfired bricks are dependent on the fineness of fly ash. The strength is higher with an increase in fly ash fineness. The strengths of the fly ash-lime granule unfired brick are excellent at 47.0-62.5 MPa. The high strength is due to the formation of new products consisting mainly of hibschite and Al-substituted 11 Å tobermorite. The main advantage of utilization of granule is the ability to increase the pozzolanic reaction of fly ash through moisture retained in the granule. In addition, the heavy elements, in particular Cd, Ni, Pb and Zn are efficiently retained in the fly ash-lime granule unfired brick.     

Chemical modification of coal fly ash for the removal of phosphate from aqueous solution

This study investigated the chemical modifications of coal fly ash treated with HCl and NaOH. Sorption behavior of phosphate from water solution on treated fly ash was examined. Results showed that the HCl-treated fly ash (TFA-HCl) had a greater specific surface area (SSA) than the NaOH-treated fly ash (TFA-NaOH) and untreated fly ash (FA). The XRF, XRD patterns, and SEM images revealed the decreased CaO content in the TFA-HCl and observed the presence of NaP1 and sodalite zeolites in the TFA-NaOH. The P sorption capacity of all studied fly ashes increased with increasing initial P concentration and mechanisms of P sorption were influenced by the equilibrium pH. Maximum phosphate immobilization capacity obtained from Langmuir model was in the following manner, TFA-NaOH > FA > TFA-HCl (57.14, 23.20, and 6.90 mg P g⁻¹, respectively). The decreased CaO content and acidic pH in the TFA-HCl were responsible for the lowest capacity of phosphate immobilization, because of unfavorable condition for calcium phosphate precipitation. In contrast, due to alkaline condition and relatively high calcium content, the precipitation of calcium phosphate was a key mechanism for phosphate removal in the FA and TFA-NaOH. The TFA-NaOH had a greatest phosphate immobilization, due to high CaO content and an increased SSA after the conversion of fly ash to zeolite. Both Langmuir and Freundlich models were good fitted for the TFA-NaOH, while was only Langmuir model for the FA and Freundlich model for the TFA-HCl. Results suggested that treating fly ash with alkaline solution was a promising way to enhance phosphate immobilization.     

Fire resistance of fired clay bricks-fly ash composite cement pastes

This work aims to study the effect of substitution of fly ash for homra on the hydration properties of composite cement pastes. The composite cements are composed of constant proportion of OPC (80%) with variable amounts of fly ash and homra. The addition of fly ash accelerates the initial and final sitting time, whereas the free lime and combined water contents decrease with fly ash content. The fly ash acts as nucleation sites which may accelerate the rate of formation of hydration products which fill some of the pores of the cement pastes. The fire resistance of composite cement pastes was evaluated after firing at 250, 450, 600, 800 °C with rate of firing 5 °C/min with soaking time for 2 h. The physico-mechanical properties such as bulk density and compressive strength were determined at each firing temperature. Moreover, the phase composition, free lime and microstructure for some selected samples were investigated. It can be concluded that the pozzolanic cement with 20 wt% fly ash can be used as fire resisting cement.     

Heavy metal concentrations in bottom ash and fly ash fractions from a large-sized (246 MW) fluidized bed boiler with respect to their Finnish forest fertilizer limit values

The total and size fractionated concentrations of As, Cd, Cr, Cu, Ni, Pb and Zn in bottom ash and two fly ash fractions from a large-sized (246 MW) fluidized bed boiler were compared to Finnish statutory limit values for forest fertilizers, which came into force in March 2007. Fly ashes were sampled from the different fields (i.e. electrodes) of the electrostatic precipitator (ESP) unit treating the stack gases. The bottom ash and the fly ash from the first ESP field are suitable for use a forest fertilizer. Due to the elevated As concentration (40 mg/kg; d.w.), which exceeded its Finnish limit value of 30 mg/kg (d.w.), the fly ash from the second ESP field is not suitable as a forest fertilizer alone. The results of ash sieving indicated that an As concentration of 40 mg/kg (d.w.) for particle size less than 0.125 mm for fly ash 2 from the second ESP electrode field exceeded the As limit value of 30 mg/kg (d.w.). In addition, a Pb concentration of 170 mg/kg (d.w.) for fly ash 1 from the first ESP electrode field for particle size 0.5-2.0 mm exceeded the Pb limit value of 150 mg/kg (d.w.). These two specific fractions are therefore not suitable for used as a forest fertilizer alone.     

A new technique to reduce the radioactivity of fly ash utilized in the construction industry

High volume utilization of industrial wastes and by products is the solution for high disposal costs. Acceptable radioactivity levels in addition to other environmental factors is a key factor for safe utilization of wastes and byproducts of coal burning power plants. In general the radioactivity levels of most fly ashes are similar to natural materials. For higher radioactivity fly ash the radioactivity values must be reduced to acceptable limits. This can be done by mixing the fly ash with less radioactive natural materials. In this study a new technique involving the use of snow as an additive to the compaction water of fly ash is presented. Fly ash at optimum water content, and fly ash with additional 10% by weight snow are compacted, hermetically sealed to allow for equilibrium of ²²⁶Ra and ²³²Th with their decay products and cured for 28 days at the curing room. Radioisotope activity analysis are conducted with a gamma analyst integrated gamma spectrometer. The activities of ²³⁵U, ²²⁶Ra, ²³⁸U, and ²³²Th of the fly ash and snow-added fly ash samples compacted at optimum moisture content are determined. The control samples revealed radioactivity values above UNIPEDE maximum allowable limits. Addition of snow caused a decrease of 31-42% in the radioisotope activity levels to that of control samples in Bq kg⁻¹.The decrease in radioactivity is linked to increased void ratio after melting of ice, increased densification of matrix around the pores due to higher level of cementitious mineral formation. The decrease in the radioisotope activity levels will allow utilization of fly ash in highway embankment construction where large surface area exposure and large volume usage makes it more critical for human health. Another advantage of the developed technology is the reduction of transportation costs by more than ten per cent by using less material for construction.     

Evaluation of the radiological safety aspects of utilization of Turkish coal combustion fly ash in concrete production

The aim of this study is to evaluate radiological safety aspects of the utilization of fly ash in concrete manufacturing in the construction industry. The specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K in one hundred 55 concrete mixture samples incorporating 10, 20 and 30 wt.% of fly ash collected from the 11 coal-fired thermal power plants were measured by means of gamma-ray spectrometry with HPGe detector. The results of the measurement were used to evaluate the radiological safety aspects of utilization of the fly ash as cement replacement in concrete by assessing the radium equivalent activity, the gamma index, the absorbed gamma dose rate and the corresponding annual effective dose due to the external exposure in indoor. The results of evaluation show that all concrete mixture samples are within the recommended safety limits except for concrete mixture samples incorporating 30 wt.% fly ash of Kangal coal-fired thermal power plant.     

Effects of fly ash and bottom ash on the frictional behavior of composites

Immense quantities of coal combustion by-products are produced every year, and only a small fraction of them are currently utilized. Our recent work has focused on developing value-added products especially from fly ash, bottom ash, and flue gas desulfurization (FGD) scrubber sludge. We explored the potential utilization of fly ash, bottom ash, and sulfate-rich scrubber sludge as frictional modifiers and additives for automotive frictional composites. The surfaces of the frictional composites, fabricated from scrubber sludge and fly ash or bottom ash, were characterized with the help of scanning electron microscopy (SEM). The mechanical properties of by-products containing composites were evaluated using a dynamic mechanical analyzer (DMA). The frictional behavior of the composites was probed with the help of friction assessment and screening test (FAST). The frictional results suggested that fly ash or bottom ash had a profoundly different effect on the frictional coefficient (μ) and wear of the composite than those observed for scrubber sludge particles. It appeared that fly ash or bottom ash particles had abrasive characteristics and gave frictional composites a higher μ-value. The FAST test also revealed that the fluctuations in the μ-value were a minimum for composites that contained 20 vol% fly ash or bottom ash among the ash-derived composites. The composites that contained 30 vol% fly ash or bottom ash showed fade after approximately 60 min of continuous FAST test. We compared the frictional and wear performance of our composites with a commercial automotive brake, and it appeared that frictional composites could be formed which contained up to 20 vol% fly ash or bottom ash and 25 vol% scrubber sludge.     

Mineralogical and elemental composition of fly ash from pilot scale fluidised bed combustion of lignite, bituminous coal, wood chips and their blends

The chemical and mineralogical composition of fly ash samples collected from different parts of a laboratory and a pilot scale CFB facility has been investigated. The fabric filter and the second cyclone of the two facilities were chosen as sampling points. The fuels used were Greek lignite (from the Florina basin), Polish coal and wood chips. Characterization of the fly ash samples was conducted by means of X-ray fluorescence (XRF), inductive coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), particle size distribution (PSD) and X-ray diffraction (XRD). According to the chemical analyses the produced fly ashes are rich in CaO. Moreover, SiO₂ is the dominant oxide in fly ash with Al₂O₃ and Fe₂O₃ found in considerable quantities. Results obtained by XRD showed that the major mineral phase of fly ash is quartz, while other mineral phases that are occurred are maghemite, hematite, periclase, rutile, gehlenite and anhydrite. The ICP-OES analysis showed rather low levels of trace elements, especially for As and Cr, in many of the ashes included in this study compared to coal ash from fluidised bed combustion in general.     

An overview of the chemical composition of biomass

An extended overview of the chemical composition of biomass was conducted. The general considerations and some problems related to biomass and particularly the composition of this fuel are discussed. Reference peer-reviewed data for chemical composition of 86 varieties of biomass, including traditional and complete proximate, ultimate and ash analyses (21 characteristics), were used to describe the biomass system. It was shown that the chemical composition of biomass and especially ash components are highly variable due to the extremely high variations of moisture, ash yield, and different genetic types of inorganic matter in biomass. However, when the proximate and ultimate data are recalculated respectively on dry and dry ash-free basis, the characteristics show quite narrow ranges. In decreasing order of abundance, the elements in biomass are commonly C, O, H, N, Ca, K, Si, Mg, Al, S, Fe, P, Cl, Na, Mn, and Ti. It was identified that the chemical distinctions among the specified natural and anthropogenic biomass groups and sub-groups are significant and they are related to different biomass sources and origin, namely from plant and animal products or from mixtures of plant, animal, and manufacture materials. Respective chemical data for 38 solid fossil fuels were also applied as subsidiary information for clarifying the biomass composition and for comparisons. It was found that the chemical composition of natural biomass system is simpler than that of solid fossil fuels. However, the semi-biomass system is quite complicated as a result of incorporation of various non-biomass materials during biomass processing. It was identified that the biomass composition is significantly different from that of coal and the variations among biomass composition were also found to be greater than for coal. Natural biomass is: (1) highly enriched in Mn > K > P > Cl > Ca > (Mg, Na) > O > moisture > volatile matter; (2) slightly enriched in H; and (3) depleted in ash, Al, C, Fe, N, S, Si, and Ti in comparison with coal. The correlations and associations among 20 chemical characteristics are also studied to find some basic trends and important relationships occurring in the natural biomass system. As a result of that five strong and important associations, namely: (1) C-H; (2) N-S-Cl; (3) Si-Al-Fe-Na-Ti; (4) Ca-Mg-Mn; and (5) K-P-S-Cl; were identified and discussed. The potential applications of these associations for initial and preliminary classification, prediction and indicator purposes related to biomass were also introduced or suggested. However, future detailed data on the phase-mineral composition of biomass are required to explain actually such chemical trends and associations.     

Fly ash—a potential source of soil amendment and a component of integrated plant nutrient supply system

In sub-tropical climate the high rainfall and high temperature is responsible for low soil productivity due to losses of bases and low organic matter content in soil. In acid lateritic soil low availability of P and high content of Al and Fe posses nutritional imbalance which is generally corrected by lime materials. Alkaline fly ash can be used in such problematic soil as an amended material and also it acts as source of plant nutrition for crop production. An attempt was made to develop an integrated plant nutrient supply system utilizing the fly ash along with other organic wastes like paper factory sludge, farm yard manure, crop residue and chemical fertilizers for rice-peanut cropping system. Direct and residual effects of fly ash were assessed based on crop yield, nutrient uptake and changes in soil characteristics. The application 10 t ha⁻¹ of fly ash in combination with organic sources and chemical fertilizer increased the grain yield and nutrient uptake of rice, and pod yield of peanut compared to chemical fertilizers alone. The heavy metal contents in plant and soil system was analyzed and found to remain below the permissible level. The results indicated that fly ash could be applied safely to tropical agro eco-systems for retaining productivity of acid lateritic soil.     

The effect of fly ash on fluid dynamics of CO2 scrubber in coal-fired power plant

Uncaptured fly ash and/or suspended solids from wet flue gas desulfurization (WFGD) scrubbing solutions are one of several factors that will influence the performance and robustness of carbon dioxide capture systems in coal-fired power plants which will be installed prior to the exhaust stack. In this study, a 100 mm ID packed column scrubber was tested with different concentrations of ash in various chemical solutions to evaluate the influence of solids on the fluid dynamics of the packing material. Data reported here are collected from three solutions including water, 30 wt% MEA (monoethanolamine), and 20 wt% potassium carbonate. The packing selected for this study was a 16 mm polypropylene pall rings. Compressed air was used to simulate flue gas at near ambient temperature and pressure.     

The effect of BaCO3 addition on the sintering behavior of lignite coal fly ash

The effect of BaCO₃ (witherite) addition on the sintering behavior of lignite coal fly ash taken from the Seyitömer power plant of Kütahya/Turkey was examined at temperatures of 1100, 1150 and 1200 °C in air atmosphere. Bloating of the fly ash samples sintered at 1150 °C was prevented, that is, the decomposition temperature of CaSO₄ in the fly ash is shifted to a higher temperature, and their physico-mechanical properties (porosity, water absorption, bulk density and bending strength) were improved with BaCO₃ addition. Positive effects of BaCO₃, however, were not seen on the fly ash samples sintered at 1100 °C. All the fly ash samples sintered at 1200 °C were bloated due to the gas evolving and also they melted. During the thermal treatment at 1150 °C a phase transformation from CaSO₄ (anhydrite) to BaSO₄ (Barite) occurred in the fly ash with BaCO₃ addition as seen from the X-ray diffraction (XRD) patterns and the bar shaped fly ash samples with BaCO₃ saved their structural integrity up to 1150 °C.     

NaOH-activated ground fly ash geopolymer cured at ambient temperature

NaOH-activated ground fly ash geopolymers, cured at room temperature, were studied in this paper. Ground fly ash (GFA), with a median particle size of 10.5 μm, was used as source material. NaOH concentrations of 4.5-16.5 M (M) were used as an alkali activator. Compressive strength tests and microstructure observations using SEM, EDX, XRD and FTIR were performed. Results indicated that GFA gave higher strength geopolymer paste compared to original fly ash. Ground fly ash could be used as a source material for making geopolymers cured at room temperature. An increase in NaOH concentration from 4.5 to 14.0 M increased the strength of GFA geopolymer pastes. Microstructure studies indicated that NaOH concentrations of 12.0-14.0 M created new crystalline products of sodium aluminosilicate. The compressive strengths at 28 days of 20.0-23.0 MPa were obtained with the NaOH concentrations of 9.5-14.0 M. Increasing the NaOH concentration beyond this point resulted in a decrease in the strength of the paste due to early precipitation of aluminosilicate products.