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.     

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.     

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.     

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     

景德镇电厂粉煤灰的理化性质分析与表征

粉煤灰是烧煤发电后产生的一种工业废渣。本文采用Mastersizer2000型激光粒度分析仪、DSC/TG、XRD与SEM等测试手段,对景德镇电厂粉煤灰的理化性质进行了分析和表征。结果表明:景德镇电厂粉煤灰是由平均粒径大小为31.5μm的细微颗粒组成,粒度主要分布在1～100μm;其化学成分主要为SiO2、A12O3与Fe2O3,三者总含量高达84.4%,其次是MgO、K2O、CaO、Na2O;其物相成分主要有石英、莫来石、赤铁矿与玻璃相。     

Hydration of fly ash cement

It is necessary to establish the material design system for the utilization of large amounts of fly ash as blended cement instead of disposing of it as a waste. Cement blended with fly ash is also required as a countermeasure to reduce the amount of CO₂ generation. In this study, the influences of the glass content and the basicity of glass phase on the hydration of fly ash cement were clarified and hydration over a long curing time was characterized. Two kinds of fly ash with different glass content, one with 38.2% and another with 76.6%, were used. The hydration ratio of fly ash was increased by increasing the glass content in fly ash in the specimens cured for 270 days. When the glass content of fly ash is low, the basicity of glass phase tends to decrease. Reactivity of fly ash is controlled by the basicity of the glass phase in fly ash during a period from 28 to 270 days. However, at an age of 360 days, the reaction ratios of fly ash show almost identical values with different glass contents. Fly ash also affected the hydration of cement clinker minerals in fly ash cement. While the hydration of alite was accelerated, that of belite was retarded at a late stage.     

The effect of particle shape on the pressure drop across the dust cake

In order to observe the effect of particle shape of poly-dispersed dusts on filter performance, the pressure drop across the dust cakes of fly ashes from a conventional power plant (PC), fluidized bed combustion (FBC), and paint incinerator (FI) was measured over a metal filter element in the accurate conditions. A fluidized bed column was used to prepare the dust feed stream of uniform particle distribution. The fine particles of FI ash have a tendency to be agglomerated at low transport velocity. The aggregates were broken at high velocity of more than 21 cm/sec. FBC ash composed of jagged type particles and containing high concentration of unburned-carbon showed higher pressure drop than that of PC ash composed mostly of spherical particles. FI ash composed of aggregates of very fine carbon particles presented the highest pressure drop among the fly ashes tested. The shape factors of PC, FBC, and FI ash were estimated as 0.91, 0.76, and 0.65, respectively, by the Ergun equation. The results implied that the irregular particle tends to form a higher pressure drop and to be more compressible than spherical one. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

循环流化床锅炉脱硫灰和普通粉煤灰的特性研究

分析研究了循环流化床锅炉脱硫灰和普通粉煤灰的化学组成、矿物相组成和热行为等。研究结果表明,脱硫灰烧失量大,CaO含量高,其主要晶相为石英、方解石和硬石膏,还有少量莫来石,玻璃相含量较少。普通粉煤灰的主要晶相为莫来石和石英相。扫描电镜观察,普通粉煤灰主要以球形玻璃微珠为主,球形微珠表面光滑。脱硫灰则以不规则形状的颗粒为主,几乎无球形颗粒。两种粉煤灰的红外光谱分析和差热分析也表现出明显差异,主要归因于脱硫灰中大量存在的硬石膏和方解石。     

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.     

Classification of carbon in Canadian fly ashes and their implications in the capture of mercury

Fly ashes produced from Canadian power plants using pulverized coal and fluidized bed combustors were examined for their carbon content to determine their ability to capture mercury. The feed coal used in these power plants were lignite, subbituminous, high and medium volatile bituminous, their blends, and also blends of coal with petroleum coke (Petcoke). The carbon and mercury content of the coals and fly ashes were determined using the ASTM standard method and by the cold vapour atomic absorption spectrometry method. The carbon content of the fly ash was concentrated by strong acid digestion using HCl and HF. The quantitative and qualitative analyses of the carbon concentrate were made by using a reflected light microscope. The results show that the carbon content of fly ash appears to be partially related to depositional environment during coalification and to the rank of the coal. The Hg captured by the fly ash depends on the rank and blend of the feed coals and the type of carbon in the fly ash. The isotropic vitrinitic char is mostly responsible for the capture of most Hg in fly ash. The inadvertent increase in carbon content due to the blending of coal with petroleum coke did not increase the amount Hg captured by the fly ash. The fly ash collected by the hot side electrostatic precipitator has a low Hg content and no relation between the Hg and carbon content of the ash was observed. These results indicate that the quantity of carbon in the fly ash alone does not determine the amount Hg captured. The types of carbon present (isotropic and anisotropic vitrinitic, isotropic inertinitic and anisotropic Petcoke), the halogen content, the types of fly ash control devices, and the temperatures of the fly ash control devices all play major roles in the capture of Hg.     

华能南京电厂粉煤灰中微珠特征及形成机理探讨

本文利用现代分析技术，对华能南京电厂粉煤灰中玻璃微珠的一些物理性质和微结构特征，如形貌、粒径分布、颗粒类型、矿物相种类和含量、漂珠的丰度、微珠的密度和壳壁厚度以及热稳定性等，进行了详细研究，并对空心微珠和成因机制进行了讨论。     

Leaching characteristics of selected Korean fly ashes and its implications for the groundwater composition near the ash disposal mound

Currently only 20% of the fly ash produced in Korea is utilised for industry, and the remainder is disposed as waste in landfill sites. Both anthracite and sub-bituminous coals are burnt in Korea. Fly ash and coal samples were collected from five different coal-fired power stations in Korea and analysed for their chemistry and mineralogy. Batch leaching tests were also carried out to investigate the leaching behaviour of selected fly ashes. The fly ash leachate chemistry was compared with the groundwater taken directly from the monitoring well installed in one of the power stations. The anthracite coals contain illite, pyrophyllite and kaolinite whereas kaolinite is the representative clay mineral for the sub-bituminous coals. Anthracite coals were higher in Si, Al and K than the sub-bituminous coals, reflecting higher mineral matter contents in the anthracite coals. Mullite and quartz are the main mineral phases for two different types of the fly ashes, with some iron oxides. The chemical compositions of the anthracite and sub-bituminous fly ashes are comparable with each other, except for extraordinary high concentrations of Cr for one anthracite fly ash. Most of the trace elements in the ash were enriched in the finer fraction, indicating surface associations. Although, some elements including Na, K, Ca and Cu were released rapidly in the initial stage of leaching, measurable amounts of metals were still detectable in the fly ash leachate treated several times with distilled water. Such leaching behaviour indicates slow and long-term leaching of elements associated with the glass fractions of the ash particle. This was confirmed by leaching of weathered fly ash, which had been disposed of for several years. Comparison of the ash leachate, treated with 0.1N-HCl, fly ash slurry in the ash pond and the groundwater indicate the influence of the ash leachate from the ash disposal mound on the groundwater composition.     

Characterization of the coal fly ash for the purpose of improvement of industrial on-line measurement of unburned carbon content

The influence of selected properties of fly ash on the measurements of an on-line analyser was described. Fly ash studied in the research originated from the pulverized coal fired boiler. The samples were taken using an inspection method within a period of 3 months. Systematic observation of the properties of the ash allowed monitoring of the work of the industrial analyzer during a relatively long period of the power plant work. Samples of coal fly ash were examined for their chemical and physical properties. Morphology was analysed by scanning electron microscopy. Unburned carbon content in fly ash was determined by using loss-on-ignition (LOI) and thermogravimetry analysis (TGA). The particle size distribution of fly ash was examined. Correlation between laboratory and on-line industrial measurements of the unburned carbon content of ash was discussed.     

粉煤灰回收氧化铝工艺研究进展

粉煤灰是火力发电行业煤炭燃烧工业的副产品,是中国固体废弃物污染的主要来源。粉煤灰富含氧化铝,所以又是潜在的铝土矿的替代品。从粉煤灰中提取氧化铝不仅可以减少粉煤灰对环境的污染,而且能缓解中国铝土资源短缺的问题。对粉煤灰性质、危害、粉煤灰利用现状以及中国铝土资源现状做了分析,论证了粉煤灰提取氧化铝的必要性;详细介绍了石灰石烧结法、硫酸铵烧结法和一步酸溶法的工艺路线、建设生产情况及各工艺的优缺点,为粉煤灰提取氧化铝的发展提出建议,并呼吁加强粉煤灰提取铝工艺方法的研究力度。     

循环流化床粉煤灰的组成形貌与性能研究

通过X-射线衍射分析、扫描电镜、需水量比和强度活性指数试验,研究了循环流化床粉煤灰的组成、形貌及性能。研究结果表明,循环流化床粉煤灰与煤粉燃烧锅炉粉煤灰相比,其玻璃相含量较低,颗粒形状多数不规则,表面粗糙,且烧失量一般也较高。循环流化床粉煤灰的需水量比较大,其强度活性指数也较低,部分循环流化床粉煤灰的强度活性指数还低于70.0%。使用粉煤灰需注意循环流化床粉煤灰与煤粉燃烧锅炉粉煤灰的差别。     

超低排放燃煤机组硒的迁移转化及飞灰对其富集特性

采用微波消解法和氢化物发生-原子荧光光谱法考察了9台超低排放在役机组硒迁移转化规律,探究了循环流化床（CFB）和煤粉炉（PC）机组飞灰特性差异对硒吸附能力的影响。燃烧后煤中硒几乎全部呈现挥发态,底渣中残留量极低。与浓度归一化和质量分布法相比较,相对富集系数法可以客观地评价燃煤副产物中硒的富集能力,两类机组中硒均主要富集于飞灰中。CFB较低炉膛温度和添加CaO可以降低入炉煤中硒释放比例并增强飞灰对硒的吸附能力,故其底渣和飞灰中硒的富集程度均高于PC,导致脱硫石膏中硒富集程度低于PC。飞灰对硒的吸附量随比表面积或孔容积增大而增大,但随粒径或孔径增大而减小。CFB飞灰中未燃尽碳含量高、形状不规则、表面粗糙且存在较多蜂窝状孔隙,导致其对硒的富集程度高于PC飞灰。     

煤粉炉和循环流化床锅炉飞灰特性对其汞吸附能力的影响

通过分析两台容量相近的循环流化床锅炉和煤粉锅炉飞灰样品的粒径分布、表面结构特性、未燃尽碳含量、反应性和汞含量，探究两种类型锅炉飞灰特性差异及其与飞灰汞吸附能力的关系。结果表明：循环流化床和煤粉锅炉尾端除尘设备排灰口飞灰汞的含量分别为1584.0 ng/g和503.7 ng/g，其原因与飞灰粒径、未燃尽碳含量和表面特性相关。对于循环流化床锅炉，飞灰中汞含量随其粒径和反应性温度的减小而增加，随未燃尽碳含量增加而增加，且与比表面积和吸附量呈正相关关系。对于煤粉锅炉，粒径为75～53 μm的飞灰对汞吸附能力较强，未燃尽碳含量明显小于循环流化床所产生飞灰的含量，飞灰比表面积随粒径变化不大，由此导致煤粉锅炉除尘设备排灰口所取样品对汞的吸附能力远低于循环流化床锅炉相对应位置飞灰对汞的吸附能力。     

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.     

Studies of sintering of coal ash relevant to pulverized coal utility boilers: 1. Examination of the Raask shrinkage—electrical resistance method

The Raask shrinkage and electrical resistance method for determining the onset of sintering of fly ash has been tested on a soda lime glass, coal ASTM ashes, fly ashes and pulverized synthetic mineral mixtures. Results with the glass confirmed those of Raask and showed that particle size distribution and bulk density affected the resistance values: the sinter point was indicated by a change in the characteristic temperature coefficient of resistance. The shrinkage and electrical sinter points were usually the same, but a high Na content lignite gave 850°C for shrinkage but 600°C for characteristic resistance change. Sinter points were 200°C less for fly ash than for ASTM ash. The fly ashes had a slight enrichment of alkalies and a decrease in Fe content compared to the total ash. Holding a compact at a temperature above the sinter point gave increased strength and shrinkage, but no decrease in electrical resistance, indicating that the decrease in resistance as temperature increased was due to the establishment of contact points between particles, with little effect of the continued growth of a contact neck. Addition of pulverized sodium silicate or iron silicate glasses to a synthetic ash also reduced the sinter points, as did alkali additions, whereas addition of pyrite did not. It was concluded that the method was a valuable tool, but sources of variability had to be determined and controlled.     

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.