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.     

57Fe Mössbauer spectroscopic studies of fly ash from coal-fired power plants and bottom ash from lignite-natural gas combustion

A series of four fly ashes, representing a variety of geological origins, and a bottom ash sample derived from the combustion of lignite-natural gas mixtures have been studied by ⁵⁷Fe Mössbauer spectroscopy. The ashes are separated into magnetic and non-magnetic fractions to facilitate a study of the chemical state of the iron contained in the ash. The bottom ash contains no magnetic fraction whereas the magnetic fractions of the fly ashes range from 1.1 to 7.3%. The magnetic fractions contained iron in the form of magnetite, Fe₃O₄. Iron in the non-magnetic fly ash fractions occur as Fe⁺¹ and Fe⁺² mullites, and Fe⁺³ and Fe⁺² silicates. Only Fe⁺³ silicates are found in the bottom 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.     

Effect of size fraction and glass structure of siliceous fly ashes on fly ash cement hydration

Paper presents effect of size fraction and glass structure of fly ashes on cement hydration. Fly ashes below 16 μm and 16–32 μm, both from the 1st and 3rd section of electro-filter, were applied. Hydration heat, content of Ca(OH)₂ and unreacted C₃S were studied and compressive strength and microstructure were analysed. Results show that finer ashes have higher depolymerization degree of SiO₄ units in glass what increases pozzolanic reactivity. Incorporation of fly ashes below 16 μm from the 3rd section gives cement class 52.5 N. At 180 day, Ca(OH)₂ content decreases by 67% and C₃S hydration degree increases by 50% relative to control sample.     

Activation of the fly ash pozzolanic reaction by hydrothermal conditions

The effect of hydrothermal treatment on the pozzolanic reaction of two kinds of Spanish fly ashes from coal combustion (ASTM class F) is discussed. Characterization of the compounds formed as a result of hydrothermal treatment and the changes provoked in the starting fly ashes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared (IR) spectroscopy. α-C₂SH, CSH gel, different solid solutions of katoites (the cubic crystallographic variety of hydrogarnets series (C₃ASH₄)) and a mixed oxide (CaFe₂O₄) were formed depending on the kind of fly ash. The hydrated compounds are precursors of a new kind of low-energy cement called fly ash belite cement (FABC); besides, they have potential properties to intercalate toxic ions and therefore can be used as immobilization systems of these ions.     

The roles of lime and iron oxide on the formation of ash and deposits in PF combustion

This paper presents the results of a study to assess the slagging propensities of a suite of UK, Spanish and South African coals, ranging from lignites to anthracites. Laboratory deposits were collected on ceramic deposition probes at gas temperatures of ∼1250°C, using an entrained flow reactor that simulates the time-temperature conditions experienced by pulverised coal particles in a large utility boiler. The degree of sintering and consolidation of the deposits would not have been predicted from bulk ash chemistry, indicating the importance of mineral matter distributions in the pulverised coal. Deposits with similar base to acid ratios and Fe₂O₃ contents displayed a range of slagging propensities on CCSEM analysis, consistent with the visual ranking. CCSEM analysis of the fly ashes collected from the combustion gases revealed a similar chemical composition to the coal ash and ash collected at the base of the EFR. CaO was observed to have readily assimilated into the aluminosilicate fly ash particles. On deposition, the CaO distribution largely remained unchanged. Fe₂O₃ was redistributed on forming a deposit possibly aided by CaO already dissolved in the aluminosilicates. The study provides an insight into the observations made by boiler operators burning coals with high CaO and Fe₂O₃ ashes.     

The significance of fly ash in wet-dry scrubbing of SO2

A bench scale flue gas desulphurization spray dry scrubbing unit was employed to study the effect of fly ash on the removal of SO₂. The equipment consisted of a spray dryer with and ultrasonic nozzle for atomization and a pulse jet baghouse. The flue gas rate was 1500 l_N/h (dry gas). Four fly ashes, originating from different countries were investigated. The alkalinity and reactivity of the fly ashes were determined in a pH-stat equipment. Pure fly ash removed SO₂ in both the spray dryer and in the baghouse. An increase of humidity divided the fly ashes into two groups. The high calcium fly ash gave a considerably higher SO₂ removal than the medium and low calcium fly ashes which showed similar SO₂ removals. Fly ash did not enhance the removal of SO₂ when added to a lime slurry because lime suppresses the dissolution of the alkali in the fly ashes. The pressure drop build-up in the fabric filter showed a strong dependence on material properties.     

Characterization of sintered coal fly ashes

Çan, Çatala?z?, Seyitömer and Af?in-Elbistan thermal power plant fly ashes were used to investigate the sintering behavior of fly ashes. For this purpose, coal fly ash samples were sintered to form ceramic materials without the addition of any inorganic additives or organic binders. In sample preparation, 1.5 g of fly ash was mixed in a mortar with water. Fly ash samples were uniaxially pressed at 40 MPa to achieve a reasonable strength. The powder compacts were sintered in air. X-ray diffraction analysis revealed that quartz (SiO₂), mullite (Al₆Si₂O₁₃), anorthite (CaAl₂Si₂O₈), gehlenite (Ca₂Al₂SiO₇) and wollastonite (CaSiO₃) phases occurred in the sintered samples. Scanning electron microscopy investigations were conducted on the sintered coal fly ash samples to investigate the microstructural evolution of the samples. Different crystalline structures were observed in the sintered samples. The sintered samples were obtained having high density, low water adsorption and porosity values. Higher Al₂O₃ + SiO₂ contents caused to better properties in the sintered materials.     

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.     

Determining suitability of a fly ash for silica extraction and zeolite synthesis

Zeolitic material is obtained from fly ash both by direct conversion of the ash or from SiO₂ extracts obtained from fly ash. This study focuses on determining the suitability of a fly ash for SiO₂ extraction and for zeolite synthesis by direct conversion. The SiO₂ extraction experiments from different fly ashes show that the main parameters governing the SiO₂ extraction are: (a) a high bulk SiO₂ content (>52%, for obtaining an extraction yield of 100 g SiO₂ kg^?1) in the starting fly ash, (b) a high proportion (>55%) of the bulk SiO₂ present in the degradable glass matrix and the highly soluble opaline fraction, and (c) a high bulk SiO₂/Al₂O₃ ratio (>2.0) of the fly ash. The results from experiments of zeolite synthesis by direct conversion demonstrate that the most important criteria for the selection of a fly ash for this process are: (a) a high content of Al₂O₃ and SiO₂(>65%) (b) a high glass content (>63%) and (c) relatively low SiO₂/Al₂O₃ ratio (<2.0). Multivariate analysis confirms the importance of the above‐mentioned variables and shows some additional variables that have influence on ash behaviour under alkaline conditions. It quantifies the use of those variables for determining the suitability of ashes for SiO₂ extraction and zeolite synthesis and is able to distinguish between the two. Copyright © 2004 Society of Chemical Industry     

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.     

Steam reactivation of 16 bed and fly ashes from industrial-scale coal-fired fluidized bed combustors

The hydration behaviour of sixteen ashes, obtained from different commercial-scale fluidized bed combustors, has been investigated. Hydration is important for both ash disposal and reactivation of excess lime present in the ashes for further use in flue gas desulphurization. The techniques used were instrumental and conventional chemical analysis, thermogravimetry and X-ray diffraction. The ashes comprised both fly ash and bottom ash, with particle size less than 2 mm. The ashes were heat treated in air to oxidize free carbon and then hydrated with pressurized steam at about 170 °C, alone and with addition of pure CaO.It has been shown that steam hydration is effective in quantitatively converting CaO to Ca(OH)₂, but in most cases the free lime content (i.e. CaO+Ca(OH)₂), expressed as CaO, decreases and added CaO enters into pozzolanic reactions with coal ash components, in part or even completely. Both the chemical evidence and X-ray phase analyses indicate that hydrated silicates and silicoaluminates are formed. The hydrated ashes are all able to take up additional SO₂ and it appears that the presence of amounts of Ca(OH)₂ detectable by phase analysis is not necessary for such capture.     

Investigation on the porosity development by CO2 activation in heavy oil fly ashes

The porosity evolution in heavy oil fly ashes subjected to activation with CO₂ has been examined. The work examined four different heavy oil fly ashes that, after preliminary acid leaching, have been pyrolyzed at 900 °C and then activated with CO₂ at the same temperature for different times.A different evolution of porosity was observed according to the different reactivity of the samples during activation. The activated samples have been characterised as regards the surface area and the pore volume. The scanning electron microscope-energy dispersive spectrometer microanalysis has been used to interpret the experimental results.     

Comparative reactivity of treated FBC- and PCC-Fly ash for SO2 removal

Two types of fly ash from fluidized bed (FBC) and pulverized coal combustors (PCC) were treated with calcium hydroxide (Ca(OH)₂) to produce reactive SO₂ sorbents. Treatment was performed using 28.6 mass% Ca(OH)₂/fly ash mixtures slurried at 350 K for 8 h. Sulfation experiments were carried out in a thermogravimetric analyzer (TGA) at SO₂ concentration ranging from 0.11 to 0.67 vol% and 10(53–1153 K temperature range. At conditions close to those prevailing in an atmospheric FBC (1123 K, 3000 ppmv ami 20 vol% excess air), about 92% and complete conversion of CaO to CaSO₄ within 1 h reaction could be achieved with treated PCC and FBC fly ashes, respectively. Based on pore structural measurements for both sorbents, treatment enhanced the specific surface area (by about 8 times) as well as pore volume (by about 5 times). The shape of the N₂-adsorption/desorption isotherms, specific pore area, and pore volume distribution curves remained unchanged. Study of the intrinsic kinetics of the reaction between treated fly ash and S02 indicated a first order reaction with respect to SO₂ concentration up to 0.31 vol% SO₂ (3.36 × 10^?8 mol/cm³). Activation energies of 82.3 and 89.0 kJ/mol were calculated for treated PCC and FBC fly ashes, respsctively.     

Electron microscopy and phase analysis of fly ash from pressurized fluidized bed combustion

The characterization of the typical fly ashes from pressurized fluidized bed combustion system (PFBC) in Japan and Europe was carried out by electron microscopy and phase analysis using energy-dispersive X-ray spectroscopy (EDX). The purity of limestone as in-bed sulfur removal sorbent influences the desulfurization reaction. The high-purity limestone yielded both hydroxyl ellestadite and anhydrite in Japanese PFBC ashes, while dolomite-rich limestone yielded anhydrite in European PFBC ashes. When the high-purity limestone was used, hydroxyl ellestadite particles were observed as the independent particles or the rim around limestone particles. The Al₂O₃ content in the glassy phase was inversely proportional to the CaO content in the glassy phase, suggesting that the glassy phases were formed from metakaoline and calcite as end members. Since hydroxyl ellestadite, glassy phase and metakaoline are reactive under hydrothermal conditions, PFBC ashes are expected to be used as raw materials for autoclaved products.     

Activation of fly ash cementitious systems in the presence of quicklime: Part I. Compressive strength and pozzolanic reaction rate

In the first part of this study, the effect of industrially produced quicklime on the strength development and pozzolanic reaction rates of different fly ash/cement (FC) systems was investigated. Two high calcium fly ashes, diversified on their active silica and calcium oxide contents, and one with moderate calcium content were used. Strength development, hydration evolution, and pozzolanic reaction rates of the quicklime-fly ash-cement (QFC) systems were monitored and presented. Moreover, new efficiency factors were calculated for the activated systems in an attempt to optimize the quicklime addition in each case, whilst correlations were attempted between the nonevaporable water contents (W_n) and the cementitious efficiency factor (k values) of the activated systems. The addition of quicklime increased both the early and later strengths of the high-calcium fly ash specimens. For the two high-lime ashes tested, a 3% addition of quicklime was found to be the optimum dosage both for short and longer curing periods. It is possible that such an offer of lime fully employed the available silica from the ashes to form additional cementitious compounds, principally pozzolanic C-S-H. In the case of lower-lime fly ash, a small quantity of added lime (5%) was found to be effective only during the initial stages of the hardening process. When quicklime additions increased, no accelerating effect was detected, as a result of the diminished proportion of soluble silica in the pore solution. Identification of the generated hydration products, porosity of the activated mixtures and examination of their microstructure will be presented in Part II of the study.     

Adsorption of Cu from water using raw and modified coal fly ashes

In this study, we found that both raw and modified coal fly ashes effectively adsorb Cu²⁺ from wastewater. The adsorption capacities followed the order CFA> CFA-600> CFA-NaOH. The adsorption isotherms for Cu²⁺ on the raw and modified coal fly ashes fit the Langmuir, Freundlich, and DKR isotherms quite well. These adsorptions were endothermic in nature; the values of E (between 1.3 and 9.6 kJ mol⁻¹) were consistent with an ion-exchange adsorption mechanism. The adsorptions of Cu²⁺ onto CFA, CFA-600, and CFA-NaOH followed pseudo-second-order kinetics.     

An improved basis for characterizing the suitability of fly ash as a cement replacement agent

Fly ash is a critical material for partial replacement of ordinary portland cement (OPC) in the binder fraction of a concrete mixture. However, significant compositional variability currently limits fly ash use. For example, the performance of OPC‐fly ash blends cannot be estimated a priori using current characterization standards (eg, ASTM C618). In this study, fly ashes spanning a wide compositional range are characterized in terms of glassy and crystalline phases using a combination of X‐ray fluorescence (XRF), X‐ray diffraction (XRD), and scanning electron microscopy with X‐ray energy‐dispersive spectroscopy (SEM‐EDS) techniques. The compositional data are distilled to a unitless parameter, the network ratio (N_r), which represents the network behavior of atoms that form alkali/alkaline earth‐aluminosilicate glasses that make up fly ashes. N_r is correlated with known composition‐dependent features, including the glass transition temperature and amorphous XRD peak (“hump”) position. Analysis of heat release data and compressive strengths are used to evaluate the impact of fly ash compositions on reaction kinetics and on the engineering properties of cement‐fly ash blends. It is shown that fly ashes hosting glasses with a high network ratio (ie, having a less stable glass structure) are more reactive than others.     

Ash transformation during co-firing coal and straw

Co-firing straw with coal in pulverized fuel boilers can cause problems related to fly ash utilization, deposit formation, corrosion and SCR catalyst deactivation due to the high contents of Cl and K in the ash. To investigate the interaction between coal and straw ash and the effect of coal quality on fly ash and deposit properties, straw was co-fired with three kinds of coal in an entrained flow reactor. The compositions of the produced ashes were compared to the available literature data to find suitable scaling parameters that can be used to predict the composition of ash from straw and coal co-firing. Reasonable agreement in fly ash compositions regarding total K and fraction of water soluble K was obtained between co-firing in an entrained flow reactor and full-scale plants. Capture of potassium and subsequent release of HCl can be achieved by sulphation with SO₂ and more importantly, by reaction with Al and Si in the fly ash. About 70–80% K in the fly ash appears as alumina silicates while the remainder K is mainly present as sulphate. Lignite/straw co-firing produces fly ash with relatively high Cl content. This is probably because of the high content of calcium and magnesium in lignite reacts with silica so it is not available for reaction with potassium chloride. Reduction of Cl and increase of S in the deposits compared to the fly ashes could be attributed to sulphation of the deposits.     

Chemical fractionation for the characterisation of fly ashes from co-combustion of biofuels using different methods for alkali reduction

Chemical fractionation, SEM-EDX and XRD was used for characterisation of fly ashes from different co-combustion tests in a 12 MW circulating fluidized bed boiler. The fuels combusted were wood pellets as base fuel and straw pellets as co-fuel in order to reach a fuel blend with high alkali and chlorine concentrations. This fuel blend causes severe problems with both agglomeration of bed material if silica sand is used and with deposits in the convection section of the boiler. Counter measures to handle this situation and avoiding expensive shut downs, tests with alternative bed materials and additives were performed. Three different bed materials were used; silica sand, Olivine sand and blast furnace slag (BFS) and different additives were introduced to the furnace of the boiler; Kaolin, Zeolites and Sulphur with silica sand as bed material. The results of the study are that BFS gives the lowest alkali load in the convection pass compared with Silica and Olivine sand. In addition less alkali and chlorine was found in the fly ashes in the BFS case. The Olivine sand however gave a higher alkali load in the convection section and the chemical fractionation showed that the main part of the alkali in the fly ashes was soluble, thus found as KCl which was confirmed by the SEM-EDX and XRD.The comparison of the different additives gave that addition of Kaolin and Zeolites containing aluminium-silicates captured 80% of the alkali in the fly ash as insoluble alkali-aluminium-silikates and reduced the KCl load on the convection section. Addition of sulphur reduced the KCl load in the flue gas even more but the K₂SO₄ concentration was increased and KCl was found in the fly ashes anyhow. The chemical fractionation showed that 65% of the alkali in the fly ashes of the Sulphur case was soluble.