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

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

Variation in fly ash properties with milling and acid leaching

Coal-burning power plants that consume pulverized solid fuels produce large amounts of fly ash as a residue. Fly ashes have been used in construction, agriculture, metal recovery and water pollution control. This paper considers the variation in properties of fly ashes to be used in the field of construction.The fly ashes produced in two coal-burning power plants in Asturias (Spain) were physically and chemically characterized in order to determine their pozzolanic activity and reactivity. Fly ashes are used as hydraulically active additives as they are a finely divided inorganic material. They were examined to determine whether they could be used as special preservatives in cements and concretes. To improve their properties, they are mechanically activated by wet milling as well as chemically activated by leaching with 30% by weight sulfuric acid at different temperatures and times.     

The leachability of heavy metals in hardened fly ash cement and cement-solidified fly ash

The effect of mix proportion, leachant pH, curing age, carbonation and specimen making method etc. on the leaching of heavy metals and Cr(VI) in fly ash cement mortars and cement-solidified fly ashes has been investigated. In addition, a method for reducing the leaching of Cr(VI) from cement-solidified fly ashes is proposed. The results mainly indicate that: (1) either Portland cement or fly ash contains a certain amount of heavy and toxic metals, and the leaching of them from hardened fly ash incorporated specimens exists and is increased with fly ash addition and water to cement ratio; (2) the leachability of some heavy metals is greatly dependent on leachant pH; (3) when carbonation of cement mortars occurs the leaching of chromium ions is increased; (4) the amount of heavy metals leached from cement-solidified fly ashes depends more on the kind of fly ash than their contents in fly ash; and (5) with ground granulated blast furnace slag addition, the leaching of Cr(VI) from solidified fly ashes is decreased.     

Ge extraction from gasification fly ash

Water-soluble germanium species (GeS₂, GeS and hexagonal-GeO₂) are generated during coal gasification and retained in fly ash. This fact together with the high market value of this element and the relatively high contents in the fly ashes of the Puertollano Integrated Gasification in Combined Cycle (IGCC) plant directed our research towards the development of an extraction process for this element. Major objectives of this research was to find a low cost and environmentally suitable process. Several water based extraction tests were carried out using different Puertollano IGCC fly ash samples, under different temperatures, water/fly ash ratios, and extraction times. High Ge extraction yields (up to 84%) were obtained at room temperature (25 °C) but also high proportions of other trace elements (impurities) were simultaneously extracted. Increasing the extraction temperature to 50, 90 and 150 °C, Ge extraction yields were kept at similar levels, while reducing the content of impurities, the water/fly ash ratio and extraction time. The experimental data point out the influence of chloride, calcium and sulphide dissolutions on the Ge extraction.     

The Silicate/non-silicate distribution of metals in fly ash and its effect on solubility

In a study at DOE's National Energy Technology Laboratory, 32 Class F fly ash samples from pulverized coal (PC) power plants were dissolved in concentrated nitric acid and in hydrofluoric acid to estimate the distribution of metals in non-silicate and silicate matrices. Nineteen cations occurred to some extent in both phases. Using a column leaching method, the release of the metals was determined with four leachant solutions; the pH of the leachants ranged between 1.2 and 12. Although the amount of an element extracted from the fly ash was low, solubility in alkaline leachants was correlated with elements in non-silicate compounds. Solubility in acid solutions was correlated more strongly with silicate phase concentration.     

Chemical, leaching and toxicity characteristics of CFB combustion residues

Within the last years, attention has been focused on the development of clean coal technologies, based on the pulverized coal fired once-through boiler technology and the circulating fluidised bed combustion (CFBC) systems. The environmental problems provoked from those wastes due to their toxic trace element contents necessitate their detailed characterisation. Within the scope of this concept, two different coal types were used in a series of experimental trials. Fly and bottom ash samples were collected and characterised in terms of morphology, mineralogy, leaching and toxicity behaviour. According to the results, toxic trace elements are preferentially concentrated in the fly ash particles since they presented the smaller particles size. However, the chemical analysis of the ash leachates showed that are acceptable for safe disposal, since none of them exceeds the maximum EPA limits. Additionally, the Microtox toxicity test proved that fly ash leachates, which presented the higher heavy metals concentrations, caused the higher toxic effects.     

以粉煤灰为原料制备的硫酸铝溶液萃取除铁研究

以粉煤灰为原料制备的硫酸铝中含有大量的铁,产品质量达不到标准。实验用P204做萃取剂,用磺化煤油做稀释剂。研究了萃取温度、萃取级数和振荡时间对硫酸铝中含铁量的影响。并进行了反萃实验。在最佳条件下,产品中铁含量可达无铁硫酸铝要求。     

Leaching characteristics of fly ash from fluidized bed combustion thermal power plant: Case study: Çan (Çanakkale-Turkey)

It is known that the concentration of elements of fly ash varies due to the used-coal and the used-lime qualities varying in different periods. In the Çan Thermal Power Plant (CTPP) located at northwestern Turkey, Çan (Çanakkale) basin coals, which are classified as lignite to sub-bituminous C coal with high total sulphur (0.4-12.22%) and a broad range of ash contents (3.2-44.6%) are mainly used. Performed studies reveal that some toxic elements exit in the coal, including As, U and V. Also, while the As, Cu, Co and Hg contents in coal increases, the sulphur contents in coal also increase. Additionally, trace elements that have inorganic compounds in coal are mobilized into air during the combustion process. This poses a big risk for human health and keeping the environment when Çan Basins low quality lignite is burned, it's the fly ash that contains several toxic elements which can leach out and contaminate the water resources.In this study, toxicity tests were conducted on the fly ash samples that were obtained from the fluidized bed combustion of Çan Thermal Power Plant. The results showed that water temperature, pH and the quality of the limestone used were the most important factors affecting the leaching properties. Concentration of some toxic elements found in the fly ash, such as; As, Cd, Cr, Pb, Se and Zn were analyzed. Concentration richness of some heavy metals were attributed to the increase of water temperature, especially when pH is lower than 5. At pH = 5 value, there is no clear explanation of each heavy metal presence in the fly ash from fluidized bed combustion thermal power plant.     

Heavy metal characterization of CFB-derived coal fly ash

The present study investigates the heavy metal content of coal fly ash (FA) samples coming from three different sampling points (secondary cyclone, cooler and filter) of a pilot plant combustion facility. The combustion experiments were carried out in a 0.1 MW Circulated Fluidized Bed (CFB) boiler using South African coal, with the addition of limestone for sulfur capture. FA was tested for the presence of selected heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES); batch leaching tests were conducted as well. The samples were also characterized in terms of their microstructure, chemical and mineralogical composition, total surface area and particle size distribution. Most of the studied metals (Cd, Cr, Cu, Ni, Mn, Zn) showed enrichment in the fine, filter FA particles, while Pb was mostly concentrated in the cooler sample. Regarding leaching characteristics of the examined samples, Cr was found to occur in considerable amounts. Although the use of CFB technology for the combustion of solid fuels steadily gets bigger worldwide, only a very limited number of studies have environmentally assessed CFB-coal FAs to date. Thus, the current study aims to contribute toward building a more integrated knowledge on the environmental impact of this abundant power production by-product.     

Comparison of chars in slag and fly ash as formed in pf boilers from B?dzin Power Station (Poland)

The amount of unburned organic matter in solid residues from coal combustion process is one of the indicators of the process and may influence their usage as by products. During the examination of slag and fly ash from B?dzin Power Station (Poland) the following forms of unburned coal were distinguished: crassispheres, tenuispheres, isotropic and anisotropic networks, tenuinetworks, honeycombs, inertinite and detritus. Slag characterises higher crassispheres and networks contents while high detritus contents are typical of fly ash. Spheres content in both slag and fly ash is always lower than vitrinite content in the feed coal. Fusinite content in the feed coal is lower than inertinite content in the solid residues. Other inertinite macerals probably participated in the formation of honeycombs.     

Characterization and treatment of wastewater produced during the hydro-metallurgical extraction of germanium from fly ash

The present work dealt with the characterization and treatment of liquid wastes produced during the hydrometallurgical recovery of germanium (Ge) from fly ash. The main metals causing reasons for environmental concern were arsenic (As), nickel (Ni) and antimony (Sb). Precipitation using calcium hydroxide (hydrated lime) was the most efficient method for treating wastewaters with high levels of As, Sb and Ni (up to 99.9% removal). However, relatively large quantities of sludge were produced due to the large doses employed. Reverse osmosis (RO) also proved to be an efficient method achieving heavy metal removals of over 90%. Large quantities of concentrate were produced which required subsequent treatment by lime precipitation. A combined treatment of RO and lime precipitation of the resultant concentrate stream yielded a 97.5% removal of metals. At the same time, a stream with a concentrated level of catechol was obtained that could be reused in the extraction of Ge.     

Fly ash effects: I. The morphological effect of fly ash

The morphological effect is an important part of fly ash effects. The paper analyzes emphatically this effect and points out that it is composed of the filling role, surface role and lubricating role. For different fly ash, these roles are different. They must be considered synthetically when the morphological effect is analyzed. Analyzing result shows that the filling role is relative to the particle size, the surface role is relative to the specific surface area and the water affinity and the lubricating role is relative to the shape of particle. The morphological effect of fly ash is the synthetical embodiment of these roles.     

酸浸法提取粉煤灰中氧化铝溶出规律的研究

酸浸法是粉煤灰提铝的主要方法之一。介绍了一种提取粉煤灰中氧化铝的方法,用不同浓度的硫酸作溶剂,用氟化钠作助溶剂,通过一定配比加入粉煤灰中反应。实验结果表明,当温度为沸腾温度、氟化钠添加量为0、硫酸浓度为12 mol/L、溶出时间为150 min时,氧化铝的溶出率可达80.19%。该工艺反应在低温常压下进行,避免了高温烧结工艺,节约了能源,降低了成本,具有较高的实用价值。     

Triboelectrostatic separation of fly ash and charge reversal

Triboelectrostatic separation has been investigated as a method for separating unburned carbon from coal combustion fly ash. It was found that when a fly ash is exposed to moisture before it undergoes separation, the charging properties of the components of the fly ash change significantly. The mineral and carbon components of the fly ash appear to charge oppositely to how they were charged before exposure to moisture. A correlation was found between the degree of charge reversal and the relative amounts of leachable ions, especially calcium and sodium ions, present on the surface of the ash.     

Mineralogy and geochemistry of Greek and Chinese coal fly ash

In this paper the mineralogy and geochemistry of Greek and Chinese coal fly ash are examined. Annual production of fly ash in China is around 160 Mt while in Greece lignite fly ash accounts around 10 Mt. Even though the mineralogical and chemical composition of the fly ashes coming from these two countries differs, there are common questions on the utilization of this material. The variation of the Greek fly ash’ chemical composition, from Ca-poor to Ca-rich fly ash, has resulted to applications such as dam construction, use in cement and possibly in concrete and road construction. The Chinese fly ash, which is rich in mullite, is broadly applied for brick making.     

Enhanced mechanical properties of wood ash and fly ash as supplementary cementitious materials

The incorporation of fly ash (FA) and wood ash (WA) in concrete as supplementary cementitious materials (SCM) was studied. The chemical composition of ordinary Portland cement, FA and WA was determined according to ASTM C-114. SEM and optical microscopy were used for the analysis of concrete. Setting time, compressive strength, water absorption and acid resistance of the concrete with different percentages of SCM ranging from 0 to 60% were evaluated. The results obtained showed that setting time and rate of water absorption increased with the increase in percentage of SCM. After 7 and 28 days, the compressive strength of concrete with 20% FA as SCM was higher than that with substitution with 20% WA. Resistance of concrete against sulphate attack increased with an increase in the percentage of FA. It was found that incorporating more than 20% WA resulted in a decrease in sulphate attack resistance.     

Hydraulic conductivity of compacted cement-stabilized fly ash

When combined with portland cement and compacted, fly ash is a high-strength material. In some instances, it may also be desirable to control the hydraulic conductivity (k) of the compacted mixture. Therefore, a study was performed to assess the effects of water content (w), cement content, curing time, and compaction effort on the hydraulic conductivity of compacted cement-stabilized fly ash. When compacting relatively dry mixtures (w < 20%), k is independent of compaction effort, and is on the order of 10⁻⁵ cm/s. When compacting between w of 20% and optimum water content (w_opt), compaction effort affects k, and, at a given w, k decreases by about an order of magnitude when increasing from standard to modified proctor effort. When wet of w_opt, k is on the order of 10⁻⁶ cm/s regardless of compaction effort or water content. With respect to curing time, extended curing time has relatively little effect on k within a 60-day time frame. Based on the results of this study, an approach to construction quality assurance testing can be applied to estimate k based on in situ measurement of dry density (ρ_d) and w.     

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.     

Performance characteristics of high-volume Class F fly ash concrete

More than 88 million tonnes of fly ash is generated in India each year. Most of the fly ash is of Class F type. The percentage utilization is around 10 to 15%. To increase its percentage utilization, an extensive investigation was carried out to use it in concrete. This article presents the results of an experimental investigation dealing with concrete incorporating high volumes of Class F fly ash. Portland cement was replaced with three percentages (40%, 45%, and 50%) of Class F fly ash. Tests were performed for fresh concrete properties: slump, air content, unit weight, and temperature. Compressive, splitting tensile, and flexural strengths, modulus of elasticity, and abrasion resistance were determined up to 365 days of testing.Test results indicated that the use of high volumes of Class F fly ash as a partial replacement of cement in concrete decreased its 28-day compressive, splitting tensile, and flexural strengths, modulus of elasticity, and abrasion resistance of the concrete. However, all these strength properties and abrasion resistance showed continuous and significant improvement at the ages of 91 and 365 days, which was most probably due to the pozzolanic reaction of fly ash. Based on the test results, it was concluded that Class F fly ash can be suitably used up to 50% level of cement replacement in concrete for use in precast elements and reinforced cement concrete construction.     

Thermal expansion of slag and fly ash from coal gasification in IGCC power plant

Integrated gasification in combined cycle (IGCC) is an electrical power generation system which is characterized to be a clean coal technology different than conventional process in combustible treatment. IGCC process gives rise to inorganic solid wastes in the form of vitreous slag and fly ashes with singular thermal properties. The gasification of the fuel takes place at high temperature and pressure in reducing atmosphere. Under that conditions, gases such as H₂, N₂ or CO, which are the main components of the gas mixture in the gasifier, show a high solubility in the melt and during the cooling remain enclosed in the vitreous slag. When these wastes are afterward thermal treated in oxidizing conditions, two phenomena occur. The development of a crystalline phase by devitrification of the glassy matrix and the releasing of the enclosed gas, which starts at temperatures nearly to the softening point. At higher temperatures the bubbles with increasing kinetic energy tend to ascent with difficult through the viscous liquid phase and promotes an expansive reaction, giving rise to a foam glass-ceramic product. This paper has been focused on the study of thermal expansion in slag and fly ash samples from the ELCOGAS IGCC power plant located in Puertollano (Spain).