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.     

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.     

Trace metal emissions from the Estonian oil shale fired power plant

Emission levels of selected trace metals from the Estonian oil shale fired power plant were studied. The plant is the largest single power plant in Estonia with an electricity production capacity of 1170 MW_e (1995). Trace metals were sampled from the flue gases by a manual method incorporating a two-fraction particle sampling and subsequent absorption of the gaseous fraction. The analyses were principally performed with ICP-MS techniques. The trace metal contents of Estonian oil shale were found to be in the same order of magnitude as of coal on average. The high total particle concentrations in the flue gases of the studied oil shale plant contribute, however, to clearly higher total trace metal emission levels compared to modern coal fired power plants. Although the old electrostatic precipitators in the plant have been partly replaced by state-of-the-art electrostatic precipitators, the majority of the boilers are currently equipped with the old precipitators. The results of the study show remarkably high concentrations of toxic heavy metals in the flue gases (e.g., Pb, Zn, Mn and As: >200 μg/m³ each) and clear accumulation of Pb, Cd, Zn, Tl and As on the fly ash. Additionally, significant portions of some heavy metals (e.g., Hg, Cd, As and Pb) were found in the absorption liquids of the sampling line, indicating the presence of either vaporous metal species or metals condensed on very small particles in the flue gases. The experimental results were interpreted by theoretical modeling using Global Equilibrium Analysis. The modeling could reasonably well explain the experimental results, especially the enrichment of certain trace metals in the fly ash as a result of volatilization/condensation phenomenon.     

Oil shale fueled FBC power plant – Ash deposits and fouling problems

A 41 MWth oil shale fired demonstration power plant was built in 1989 by PAMA in Mishor Rotem, Negev, Israel. The raw material for the plant is the local “oil shale”, which is in fact organic-rich marl. Since then, and until today, the unit is operated at high reliability and availability. At first, heavy soft fouling occurred due to the Circulating Fluidized Bed Combustion (CFBC) mode of operation, which caused a considerable reduction in the heat transfer coefficient of the heat exchangers. By going over to the Fluidized Bed Combustion (FBC) mode of operation the soft fouling phenomenon stopped at once, the heat transfer coefficient improved, and the power plant could be operated at its designed values. After five months of operation at the FBC mode the boiler had to be shut down because Hard Deposits (HD) blocked physically the passes in the boiler. These deposits could be removed only with the help of mechanical devices. During the first two years the boiler had to be stopped, at least, three times a year for deposit cleaning purposes.Research conducted at the plant and in the laboratories of the Geological Survey of Israel enabled us to understand the mechanism of formation of these deposits. The results showed that the HD are formed in two stages: (1) Deposition of very fine ash particles on the pipes of the boiler, as a result of the impact of larger particles on the pipes. The fine particles adhere to the pipes and to each other, and step by step build the deposit. The growth of the deposit on the pipe surface is always perpendicular to the particles flow direction. (2) The deposits harden due to chemical reactions. The joint experiments at the plant and at the laboratories of the Geological Survey showed:

(A) The rate of deposition depends mainly on the lime concentration in the fly ash.

(B) The lime concentration in the fly ash is a function of the clays concentration in the oil shale.

(C) The increase and hardening of the deposit with time is due to solid–gas reactions within the deposit. At first recarbonation occurs, reaction between CaO in the deposit and CO₂ (produced by the combustion) in the flue gas to form CaCO₃ (bonded deposits), and then sulfatization; the reactions of the sulfatization are:

(1) SO₂ in the flue gas with CaO and CaCO₃ in the deposit, leading to the formation of anhydrite CaSO₄; and

(2) SO₂ in the flue gas with the amorphous silicates in the deposit forming hydroxylellestadite Ca₁₀(SiO₄)₃(SO₄)₃(OH)₂.

These minerals are the hard deposits.The conclusions following these findings for the combustion of oil shales with a significant Ca-carbonate content are:

(A) The FBC is the preferred mode of combustion.

(B) The rate of deposition in the boiler depends mainly on the lime (free CaO) concentration in the Fly ASh (FAS).

(C) The ratio Ca-carbonates to silicates (Al, Fe, etc.), in the oil shale feed, determines the concentration of lime in the FAS.

(D) The rate of deposition in the boiler depends also on the geometry of the boiler and on the particles aerodynamic conditions in it.

Following these conclusions, the plant was able to reduce the shutdowns to twice a year. Furthermore, based on the understanding of the deposit formation mechanism, it will be possible to minimize shutdowns, for deposit cleaning, to only once a year in future similar oil shale fuelled power plants.     

Mössbauer studies of iron in Lurgi gasification ashes and power plant fly and bottom ash

Iron Mössbauer spectroscopy and X-ray diffraction methods were applied to the study of a selection of ashes produced in a Lurgi gasification plant and fly ash from a pulverized coal combustion. The ashes contained hematite, magnetite, and goethite. Sixty percent or more of the iron in these ashes was in the oxide form, with the remainder present in mullite and other silicate phases. Iron was divalent in the latter, and present as both Fe⁺² and Fe⁺³ in mullite. Ratios of Fe⁺² and Fe⁺³ varied from 0.3 to 0.7. By comparison, a water-quenched molten bottom ash was free of iron oxides and contained only amorphous silicate phases with virtually all iron in the divalent state.     

Use of mineral coal ashes in insulating refractory brick

Coal ash from Candiota thermoelectric power plant in southern Brazil is a waste generated in great amount. Current estimates indicate that coal ash production is likely to reach 2.000.000 tons per year. Of this total, only 10% are commercialized, so the price of this material is very low. In this work, it was investigated the use of coal ashes in a composition of refractory insulating brick. In coal combustion process, fly ashes and bottom ashes are generated. Bottom ashes are formed by partially fused ashes that precipitate to the bottom of the boiler, due to their coarser particle size. Therefore, it was analyzed fly ash addition in a refractory body and the addition of bottom ashes, partially replacing chamote (calcinated clay). The results showed that the effect of the ash is small in the fired materials. In terms of technological characterization, the bricks formulates with coal ash were considered suitable to commercial brick standards. Density and thermal conductivity of the refractory bricks with coal ashes had revealed to be similar the commercial products. Then, the use of ash is economical attractive and technical feasible. Translated from Novye Ogneupory, No. 6, pp. 60–63, June 2008.     

Ash deposition behavior of upgraded brown coal in pulverized coal combustion boiler

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The purpose of this study is to evaluate the ash deposition for Upgraded Brown Coal (UBC) and bituminous coal in a 145 MW practical coal combustion boiler. The UBC stands for Upgraded Brown Coal. The melting temperature of UBC ash is relatively lower than that of bituminous coal ashes. Combustion tests were conducted on blended coal consisting 20 wt.% of UBC and 80 wt.% of bituminous coal. Before actual ash deposition tests, the molten slag fractions in those coal ashes were estimated by means of chemical equilibrium calculations. The calculation results showed the molten slag fraction for UBC ash reached approximately 90% at 1523 K. However, that for blended coal ash decreased to 50%. These calculation results mean that blending UBC with bituminous coal played a role in decreasing the molten slag fraction. This phenomenon occurred because the coal blending led to the formation of alumino-silicates compounds as a solid phase. Next, ash deposition tests were conducted using a practical pulverized coal combustion boiler. A water-cooled stainless-steel tube was inserted in locations at both 1523 K and 1273 K in the boiler to measure the amount of ash deposits. The results showed that the mass of ash deposition for blended coal did not greatly increase, compared with that for bituminous coal alone. Therefore, appropriately blending UBC with bituminous coal enabled the use of UBC without any ash deposition problems in practical boilers.     

Physico-chemical characterisation of Indian biomass ashes

India stands fourth in biomass utilisation for various purposes like domestic, commercial and industrial applications. While extensive studies have been made for coal ash characterisation and utilisation, studies on characterisation of biomass ash and its utilisation has not been addressed. In this paper, biomass ash from five sources i.e. rice husk, bagasse, groundnut shell, cashewnut shell, and arecanut shell have been characterised. Chemical composition analysis, particle size analysis, thermal analysis, and microstructure analysis were carried out. Results show that in all ashes silica is the major compound with particle size ranging from 15 to 30 μm and having irregular shape. Ash powders originating from cashewnut shell, arecanut shell and groundnut shell also have compounds of calcium, magnesium and potassium. Bagasse and cashewnut shell ashes have high LOI due to presence of unburnt carbon, P₂O₅ and other volatiles.     

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.     

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.     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

Colour measurement as a proxy method for estimation of changes in phase and chemical composition of fly ash formed by combustion of coal

Influence of technology on colour changes of fly ashes was studied in relationships with their chemical and phase composition. Dry bottom boilers at the Detmarovice Power Plant (the Czech Republic) were selected for this study. Combustion tests were performed using mixture of coal and mineral oil residues at the minimum and maximum output of the power plant. Fly ashes for chemical analysis, phase analysis and colour measurements were sampled from the four sections of electrostatic fly ash precipitator. Colour parameters indicate relationships with concentrations of elements which are preferentially bound in silicate matrix. The maximum output of power plant increases the concentration of glass which has decisive influence on values of colour parameters. The changes of colour parameters can indicate the conditions of the technological process. Relationships between colour and constituents of the fly ash are expressed by CIE Lab colour parameters.     

Ash fusibility and compositional data of solid recovered fuels

Several approaches are established to analyse the fouling and slagging propensities of coal ashes, but the same cannot be said of solid recovered fuel (SRF) ashes. This work has been conducted by using some fouling and slagging indicators, which are commonly applicable to coal ashes, on SRF ashes to ascertain their applicability.In this work, laboratory prepared ashes derived from municipal solid waste (MSW), sewage sludge, demolition wood, shredded rubber tyres, and plastic/paper fluff are analysed for their fusibility leading to fouling and slagging using three approaches; the ash fusibility temperatures (AFT), ternary phase diagrams, and fouling/slagging indices. The results from each approach are examined to determine the inclination of the ashes toward fouling and slagging. A subsequent inter-comparison of the methods was conducted to validate the methods which are in agreement and are applicable to SRF ashes. The study showed that ternary equilibrium phase diagram SiO₂-CaO-Al₂O₃, various fouling and slagging indices, and AFT can be used to complement each other to predict ash fusion properties, fouling and slagging propensities of SRF ashes.     

Modeling calcium dissolution from oil shale ash: Part 1. Ca dissolution during ash washing in a batch reactor

Batch dissolution experiments were carried out to investigate Ca leachability from oil shale ashes formed in boilers operating with different combustion technologies. The main characteristics of Ca dissolution equilibrium and dynamics, including Ca internal mass transfer through effective diffusion coefficients inside the ash particle were evaluated. Based on the collected data, models allowing simulation of the Ca dissolution process from oil shale ashes during ash washing in a batch reactor were developed. The models are a set of differential equations that describe the changes in Ca content in the solid and liquid phase of the ash-water suspension.     

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.     

Modeling calcium dissolution from oil shale ash: Part 2.: Continuous washing of the ash layer

In the present work a possible approach to the utilization of oil shale ash containing free lime in precipitated calcium carbonate (PCC) production is elucidated. This paper investigates the Ca (calcium) dissolution process during continuous washing of pulverized firing (PF) and fluidized bed combustion (FBC) oil shale ash layers in a packed-bed leaching column. The main characteristics of the Ca dissolution process from ash are established. The effect of water flow rate is investigated by conducting leaching experiments of oil shale ashes formed in boilers operating with different combustion technologies. The values of the overall and liquid phase mass transfer coefficients are evaluated based on experiments using the developed ash layer washing model. The model is a set of partial differential equations that describe the changes in Ca content in the stagnant layer of ash and in the water flowing through the ash layer. An example in which the model is applied to environmental assessment and estimation of Ca leaching from industrial oil shale ash fields is provided.     

煤灰基本特征及其微量元素的分布规律

对兖州矿区煤灰的化学成分、微量元素分布及灰的岩石学特征等方面进行了研究，并对灰产率与微量元素含量分布之间的关系进行了研究，同时对影响煤灰化学成分的因素方面进行了初步探讨。通过对兖州矿煤灰的采样分析可知,研究区煤灰由结晶物质、玻璃状物质和末燃尽有机质组成，其化学成分主要为SiO2,Al2O3,Fe2O3和CaO及少量的O3,P2O5,Na2O和TiO2,煤燃烧过程中微量元素发生了再分布，多数微量元素在煤灰中富集。同时它们在飞灰中富集的浓度明显高于底灰，即随着煤灰粒度的变小，它们在其中富集的浓度越高，其含量与煤灰的粒度成反比。微量元素Th,V,Zn,Cu,Pb与灰产率之间成正相关关系，而Cl与灰产率成负相关关系。     

Burning and physico-chemical characteristics of carbon in ash from a coal fired power plant

The paper addresses the relationship between the chemico-physical properties and the residual combustion reactivity of fly ashes from a full-scale front fired PF coal boiler. Ashes collected at different rows of electrostatic precipitators (EP) have been characterized for their particle size distribution, morphology, chemical composition and combustion reactivity. The combustion time of carbon in ash has been estimated for a wide range of temperatures using a thermobalance and a heated strip reactor.Results showed the existence of marked differences in the content of both carbon and inorganic elements according to the row of EP and the granulometric size of the samples. In contrast with this, the combustion reactivity of all ash samples was similar regardless of their collection point and particle size. Ash reactivity resulted to be approximately 100 times lower than that of the parent coal.The role of thermal annealing on the low reactivity of fly ashes and their propensity to undergo additional thermodeactivation upon further heat treatment has also been investigated. To this end coal and fly ashes have been heated under inert conditions up to 2000 °C and then characterised for their residual combustion reactivity. These tests showed that heat treatment does reduce the reactivity of coal but does not reduce any further the already low reactivity of fly ashes.     

Characterisation of the glass fraction of a selection of European coal fly ashes

Fly ash largely consists of the inorganic content of coal that remains after combustion. The crystalline phases present in fly ash may form upon cooling of a molten alumino‐silicate glass. This view is supported by the spherical shape of many fly ash particles, inferring that they have gone through a viscous fluid state. The amorphous content in fly ash is believed to dominate reactivity behaviour, under both alkaline and acid conditions, because glasses have a higher potential energy than the equivalent crystal structure and the variation of bond angles and distances in a glass makes the bond breakage easier. It is the degradation behaviour under alkaline conditions, and the subsequent release of silica from the glass phase, that is important in the use of fly ash for conversion to zeolites and for pozzolanic applications in cement. This research comprehensively studies the composition, quantity and stability of the glass phase in a series of nine fly ashes sourced from Spanish and Italian power plants. The quantitative elemental composition of the glass phase in each fly ash was determined. Samples of the ashes then underwent a series of tests to determine the internal structure of the ash particles. Heat treatment of most of the ashes results in mullite crystallising from the glass phase; this is the crystalline phase that is predicated to form by both the relevant phase diagrams and also by NMR spectroscopy. In the ashes, mullite is present as a spherical shell, tracing the outline of the particle but in some specific cases the mullite skeleton is made up of coarse crystals reach also the internal parts of the particles. The morphology and density of the mullite crystals in these shells varies greatly. This work has supported the view that some crystalline phases present in fly ashes, such as mullite, form upon cooling of the amorphous glass melt as opposed to direct conversion from existing mineral phases in the coal during the combustion process. Copyright © 2004 Society of Chemical Industry     

Surface tension measurements of coal ash slags under reducing conditions at elevated pressures

The aim of limiting the amount of CO₂ that is released together with other exhaust gases from power plants can be reached by technologies allowing for a systematic separation of this greenhouse gas. One such technology is the integrated gasification combined cycle power plant which makes use of a coal gasification step. For the gasification involving temperatures far higher than in typical pulverised combustion chambers, ash contained in the fuel is liquefied (slag) and must be removed from the cycle to guarantee safe operation of downstream equipment. To keep the efficiency of the power plant as high as possible, hot gas cleaning facilities are most desirable for this purpose. The design of these installations necessitates knowledge about thermophysical properties of coal ash slags, especially in reducing, pressurised atmospheres. In this work, the surface tension of 15 coal ash slags was measured in argon hydrogen gas of up to 10 bar absolute pressure according to the sessile drop method. Compared to experiments at 1 bar, surface tension values up to 42% lower were found on applying pressure. Additionally, shifts in the melting temperature interval of the ashes due to increased pressure were observed. The surface tension values obtained in pressurised atmospheres ranged from 270 to 490 mN/m with respect to temperature intervals where almost no data scattering occured.