Fine Fly Ash Particles Research Articles

Development of magnesium oxychloride cement with enhanced water resistance by adding silica fume and hybrid fly ash-silica fume

In this paper, a water-resistant magnesium oxychloride cement (MOC), an environment friendly cementitious material, was developed by incorporating silica fume and the hybrid of silica fume and fly ash. The effects of these admixtures on the water repellence of MOC were investigated via comprehensive macroscopic physical and mechanical tests and micro-scale studies. A formula design of MOC blended with 15% silica fume and 15% fly ash retained the compressive strength as high as 100% and 95% after 28 days' and 56 days’ immersion in water, respectively. Micro-scale studies, including XRD, FTIR, SEM, MIP, TG/DSC and XRM, were conducted to discover the mechanism of the enhanced water resistance of the new MOC. Besides, a clear and precise distribution of the hydration products in well-cured MOC specimens was demonstrated with the X-ray mapping (XRM) technique. It was found that adding silica fume or the hybrid of silica fume and fly ash optimised the porous structure of MOC and thereby increased the density and compressive strength leading to the enhanced water resistance. The filling effect of fine silica fume and fly ash particles, as well as the formation of Mg–Cl–Si–H gel, was found to be the main reason for the densified microstructure of MOC. The fractional contribution of phase 5, i.e. the main phase of MOC, decreased with the combined silica fume and fly ash, and this helped to relieve the internal stress and improve the stability of MOC, leading to enhanced water repellence.

Probability of Formation of Wear Debris during Initial Running-In Period of Sliding Wear of Al-Si (LM13)-10 wt.% Fly Ash Composites

In this paper, the effect of sliding distance on dry sliding wear behavior of LM 13 alloy reinforced with 10 wt.% fine fly ash particles was studied with special reference to the probability of formation of wear debris during the initial running-in period. The present test was conducted under varying sliding distance (1000, 2000, 3000, 4000 and 5000 m) at variable load 1, 2 and 3 kg and constant speed of 640 rpm. The worn-out surfaces were investigated under the scanning electron microscope (SEM). It was found that cracking of wear surface and formation of debris do not show any particular trend with sliding distance. The probability of formation of wear debris was calculated experimentally and also compared with Rowe’s theory of adhesion and fractal geometry model of wear prediction. SEM observation of wear surface suggested that the wear of material is controlled mainly by nucleation and propagation of cracks. The results clearly show that the mechanism of wear effected by the fracturing and work hardening of material.

Insight of particulate arsenic removal from coal-fired power plants

Arsenic is easily volatilized as vapors during coal combustion and predominantly transferred into particulate forms. The removal of particulate arsenic together with fine fly ash particles is essential to control arsenic emission during coal combustion. In this study, fly ashes and different size fraction samples (PM2.5, PM10, and PM20) were collected or prepared from four Chinese coal-fired power plants, with objectives of studying the arsenic distribution and speciation in fly ash, and understanding the retention relationship between the removal of ash particles and particulate arsenic. The results demonstrated that the concentration of arsenic of the first two ESP (electrostatic precipitator) hoppers was at similar level, then followed by a substantial increase in the rear hoppers. And the particle size distribution of fine fly ash particles from various ESP systems presented essentially similar trend. Furthermore, particulate arsenic was mainly present as arsenates probably through the interactions between arsenic vapors and accessory Ca/Fe/Al-minerals. Arsenic associated with calcium compounds tended to be enriched in coarse particles, while arsenic bound with Fe/Al-compounds was enriched in fine particles. Meanwhile, arsenic associated with calcium sulfate compounds was easier to be captured by ESP compared to calcium silicate-bound arsenic. The interactions between arsenic vapors and various accessory minerals not only affected the size distribution characteristics of particulate arsenic, but also had a great effect on its removal efficiency of ESP. Specifically, based on the ash resistivity calculation, arsenic-bearing fine particles with certain amount of Fe had suitable conductivity and favored the capture of particulate arsenic by ESP.

Cadmium isotope fractionation during coal combustion: Insights from two U.S. coal-fired power plants

Coal combustion, one of the principal energy sources of electricity in the United States, produces over 100 million tons of coal combustion products (CCPs) per year in the U.S. The reuse and disposal of CCPs has the potential to release toxic trace elements, including cadmium (Cd), into the environment. In this study, we investigated CCPs, including bottom ash (BA), economizer fly ash (EFA), and fly ash (FA), as well as feed coal (FC) and pulverized coal (PC) collected from two U.S. coal-fired power plants in New Mexico and Ohio with different coal supplies. The New Mexico plant uses high volatile C bituminous, low-sulfur coals mined from the San Juan Basin (Cretaceous Fruitland Formation) and the Ohio plant uses high volatile A bituminous, high-sulfur central Appalachian Basin coals (Upper Pennsylvanian Monongahela Formation). Mineralogical and elemental analysis showed that these CCP samples consist of ∼70% amorphous Al-Si-rich glasses and ∼30% mineral phases of quartz (SiO2) and mullite (Ai6Si2O13). The Cd isotope compositions (δ114Cd, normalized to NIST Cd standard 3108) of FA and EFA samples (ranging from −0.51 to +0.47‰) are distinctively heavier than those of BA samples (−0.75 to −0.52‰) in both power plants. We interpret this Cd isotope difference as a result of Cd condensation from the gas phase during flue gas cooling, instead of evaporation of Cd phase during coal combustion. Cd condensation is the main process to generate the isotopically heavy Cd signatures that preferentially partition on the fine FA particles. We also investigated Cd isotope compositions in different leachate products from a series of batch-leaching experiments with these CCPs, using diluted acetic acid, hydroxyl ammonium chloride, hydrogen peroxide followed by ammonium acetate, and 5% nitric acid, as a possible means to identify CCP-released Cd in the environment. Unusually and significantly heavier Cd isotope compositions were observed in each leachate of FA samples (+1.10 to +7.09‰), which fall far outside from the range of Cd isotope ratios observed in natural soils and rocks, but less so for the EFA samples (−0.43 to +1.18‰). Such an observation is consistent with the interpretation that isotopically heavy Cd preferentially partitions on the fine FA particles after coal combustion and is readily to be released during these leaching experiments. This study demonstrates that high-temperature coal combustion can lead to a very large degree of fractionation of Cd isotopes that can be used as a unique tracer for identifying anthropogenic metal inputs in the environment. The major Cd isotope fractionation process occurs as the Cd gas phase condenses on fine FA particles during the flue gas cooling stage after coal combustion.

Alkali metal transformation and ash deposition performance of high alkali content Zhundong coal and its gasification fly ash under circulating fluidized bed combustion

Alkali metal transformation and ash deposition during combustion of Zhundong (ZD) coal and its gasification fly ash (ZDf) were studied in a 0.4 t/d circulating fluidized bed. The Na in ZD coal was present primarily as Na2SO4. NaAlSi3O8 and KAlSi3O8 were the main compounds of Na and K in ZDf. Variation in the bed temperature had a significant effect on the transformation of Na, but it did not greatly affect the conversion of K. During combustion of the ZD coal, Na was mainly in flue gas, the Na in fly ash was mainly in the water soluble form, but in bottom ash it was mainly in the insoluble form. During combustion of the ZDf, Na was found mainly in the fly ash and flue gas, insoluble Na and K accounted for above 80.0% of Na and K in ashes. The release of Na during the ZDf combustion was less pronounced than that during the ZD coal combustion. The ash deposition during the ZD coal combustion occurred primarily as a result of the agglomeration and bonding of the ash particles that were rich in Na2SO4, and the deposition propensity was high. However, the ash deposition during the ZDf combustion was primarily caused by the accumulation of fine fly ash particles with a low Na content, the deposition propensity was low.

Effect of combustion temperature on the emission of trace elements under O2/CO2 atmosphere during coal combustion

The effect of combustion temperature on the emission of trace elementswas studied under O2/CO2 atmosphere during coal combustion in a laboratory scale fluidized bed combustor. The elemental composition of fine fly ash particles collected with a low pressure impactor(LPI)was quantified by X-Ray F1uorescence Spectrometer (XRF). The elemental composition of coal and bottom ash was quantified byinductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results indicate that the contents of Mn, Zn, Cd and Cr in the fly ash increase with the rise of combustion temperature. It is found that the enrichment of Zn and Cd is greater in the submicrometer particles than the supermicrometer particles, but Mn and Cr do not enrich in the submicrometer particles. Mn, Zn, Cd and Cr display one peak around 0.1 μm. The relative enrichment factor (Rij) of four elements is in the order of Zn, Cd, Mn and Cr. Zn and Cd are mostly retained in fly ashwhileMn and Cr are retained in both the fly ash and bottom ash.

Effect of particle size of fly ash on the properties of lightweight insulation materials

The paper reports the effects of fly ash particle size on the properties of the lightweight insulation materials. The microstructure and mechanical properties of lightweight insulation materials are separately investigated by SEM (scanning electron micrograph) and tabulate thermal conductivity apparatus methods. In addition, the effects of particle size distribution of fly ash on the properties of lightweight insulation materials is studied by grey incidence analysis. The results show that when the ball milling time increases, the particle size of fly ash decreases and the distribution becomes narrow. At the same time, the polymerization state of fly ash would be depolymerize from high poly structure to low polymeric structure. Moreover, the linear shrinkage, bulk density, compressive strength, acid resistance and thermal conductivity of lightweight insulation materials have also increased, but the apparent porosity decreases. And the studies highlight that when the content of fine fly ash particles increases, the nucleation and growth of mullite is accelerated and the sintering driving force is improved at high temperature, which have a positive effect on improving the physicochemical properties of materials. The particles ranging from 20 to 30μm has a great influence on the linear shrinkage, bulk density, compressive strength, acid resistance rate and thermal conductivity of lightweight insulation materials.

Desirability based multiobjective optimisation of abrasive wear and frictional behaviour of aluminium (Al/3.25Cu/8.5Si)/fly ash composites

Aluminium alloy (Al/3.25Cu/8.5Si) composites reinforced with fly ash particles of three different size ranges (53–75 μm, 75–103 μm and 103–125 μm) in 3, 6 and 9 wt-% were fabricated using liquid metallurgy technique. Pin on disc abrasive wear tests were carried against the disc surface fixed with SiC emery paper (120 grades). A mathematical model was developed to predict the abrasive wear and coefficient of friction of the composites. Analysis of variance technique was used to check the validity of the developed model. Composites reinforced with coarse fly ash particles exhibited better abrasive wear resistance than those reinforced with fine fly ash particles. Abrasive wear in composites with fine fly ash particles is a combination of adhesive wear and abrasive wear. Larger fly ash particles present in composites gets fractured into fine particles and entrapped between the composite pin and the disc, thereby decreasing the wear rate. Worn surfaces of the pins were then analysed using scanning electron microscopy to study the wear mechanisms of the composites. The abrasive wear was optimised using desirability based multiobjective optimisation technique.

Characterising long-term wet-stored fly ash following carbon and particle size separation

The paper describes a study carried out to characterise fly ash produced from hard-coal, following recovery from long-term pond and stockpile storage, and wet process separation. Three power station sites, representative of those in the United Kingdom, were considered and initial work examined the characteristics of recovered material from both types of storage area. Most of this had high loss-on-ignition (LOI) and was relatively coarse, with that in ponds generally having greater variability. Fly ash particles had reaction products on their surfaces and the presence of agglomerates was also noted. Methods including, pre-screening, column (hydraulic) separation, froth flotation and lamella hydraulic classifier were considered for processing. Three trials were carried out using various combinations, and it was possible to separate carbon and fly ash to give material groups ranging from carbon-rich to low LOI/high fineness, although fine fly ash particles (<5μm) were distributed throughout these. Relatively minor changes in chemical composition and mineralogy of the separated materials were noted, except for particle groups with high LOI (reflecting fly ash dilution). Similar effects were generally obtained with both pond and stockpile storage. Mass balance calculations were made to quantify material distributions for the three processing trials carried out. Examples are given of yields achieved for potential end-uses from these, which indicate that optimisation to target particular fly ash properties should be possible.

Analysis of Parameters Influencing Wear and Frictional Behavior of Aluminum–Fly Ash Composites

Aluminum alloy (Al/3.25Cu/8.5Si) composites reinforced with fly ash particles of three different size ranges (53–75, 75–103, and 103–150 μm) in 3, 6, 9, and 12 weight percentages were fabricated using a stir-casting technique. Pin-on-disc wear tests were conducted with 20, 30, and 40 N loads and sliding speeds of 2, 3, and 4 m/s for a constant time period of 10 min. A mathematical model was developed to predict the wear volume and coefficient of friction of the composites. An analysis of variance (ANOVA) technique was applied to check the validity of the developed model. Student's t-test was utilized to determine the significance of factors. Composites reinforced with coarse fly ash particles exhibit superior wear resistance to those reinforced with fine fly ash particles. A scanning electron microscope (SEM) study of the worn surfaces of the pins was performed to confirm the results of the model.

PCDD/F Formation Catalyzed by the Metal Chlorides and Chlorinated Aromatic Compounds in Fly Ash

The entrained fine fly ash particles carried into the post-combustion region and collected by air pollution control devices were capable of such surface-mediated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) formation. The secondary formation of PCDD/Fs in the flue gas and ash from the municipal solid waste incinerator (MSWI) systems has attracted considerable public concern. The objective of this study was to examine reactions in fly ash from an municipal solid waste incinerator (MSWI) operated with metal chlorides and chlorinated aromatic precursors. When 10 wt% FeCl2, ZnCl2 and CuCl2 were added onto the fly ash medium, the PCDD/Fs concentration is 8.8, 25.5 and 369.5 ng/g, respectively. This indicates that the formation of PCDD/Fs was significantly increased by CuCl2. In the case of CaCl2 addition, the results showed that it acts in a minor role as a catalyst to promote the formation of PCDD/Fs. In addition, the results of adding HgCl2 has no transformation effect on the formation of PCDD/Fs in the residue and gas phases. Moreover, in an experiment using chlorinated aromatic precursors, the formation of PCDD/Fs was significantly increased by pentachlorophenol (PCP). This study suggests that the filter cake ash treatment should be studied extensively to control dioxin yields on the fly ash matrix.

Concentration and association of minor and trace elements in Mukah coal from Sarawak, Malaysia, with emphasis on the potentially hazardous trace elements

Abstract The ash yield and concentrations of twenty-four minor and trace elements, including twelve potentially hazardous trace elements were determined in Mukah coal from Sarawak, Malaysia. Comparisons made to the Clarke values show that Mukah coal is depleted in Ag, Ba, Be, Cd, Co, Mn, Ni, Se, U, and V. On the other hand, it is enriched in As, Cr, Cu, Pb, Sb, Th, and Zn. Among the trace elements studied, V and Ba are associated predominantly with the clay minerals. Manganese, Cr, Cu, Th, and Ni are mostly bound within the aluminosilicate, sulphide and/or carbonate minerals in varying proportions, though a portion of these elements are also organically bound. Arsenic, Pb and Sb are mostly organically bound, though some of these elements are also associated with the sulphide minerals. Zinc is associated with both the organic and inorganic contents of the coal. Among the potentially hazardous trace elements, Be, Cd, Co, Mn, Ni, Se, and U may be of little or no health and environmental concerns, whereas As, Cr, Pb, Sb and Th require further examination for their potential health and environmental concerns. Of particular concern are the elements As, Pb and Sb, which are mostly organically bound and hence cannot be removed by physical cleaning technologies. They escape during coal combustion, either released as vapours to the atmosphere or are adsorbed onto the fine fly ash particles.

The influence of unburned carbon particles on electrostatic precipitator collection efficiency

Laboratory tests have shown that the chemical composition of fly ash (in that unburned coal) as well as its size distribution has significant influence on the dust cleaning process. Likewise the design of discharge electrodes has shown a strong influence on the dust cleaning. Tests of precipitation efficiency were carried out on a laboratory electrostatic precipitator (ESP) model using fly ash samples of diverse size distribution and unburned coal content collected from several grate boilers. Test results show explicit dependency of the ESP precipitation efficiency on physical and chemical characteristics of the fly ash, design of discharge electrodes and amount of electrical energy delivered to the ESP. Mercury concentration measurements show higher levels in the fly ash than in the fired coal indicating high sorption capacity of the fly ash. Prior observation suggests good mercury adsorption on fine fly ash particles in the presence of elemental coal. Hence the improvement of ESP collection efficiency of fine particles containing unburned coal may help decrease the emission of mercury.

Plerosphere and its role in reduction of emitted fine fly ash particles from pulverized coal-fired power plants

Fine particles (PM 2.5) emitted from the stacks of the coal-fired power plants are of environmental concern since they can easily enter the human respiratory track. The detailed study of the fly ash particles using scanning electron microscope/electron dispersive spectrometry (SEM/EDX) show that fine solid spherical particles (microspheres) are contained by the large cenosphere particles (>50 μm) during the combustion process. The resulting macro particles are known as “plerosphere”, which are typically impregnated by the fine microspheres. The coal-fired power plants’ particle control devices such as the electrostatic precipitators (ESP) and baghouse filters tend to capture the large plerospheres, more efficiently. Therefore, the result of this study suggests that the containment of the microspheres by plerospheres during the coal combustion process can effectively reduce the amount of fine particles and associated elements released into atmosphere.

Fine particle and trace element emissions from waste combustion — Comparison of fluidized bed and grate firing

Waste-to-energy applications are increasingly being used to simultaneously reduce the amount of waste and produce electricity and heat. As waste materials typically contain high concentrations of impurities that are transformed during combustion, the flue gases need to be cleaned efficiently in order to avoid harmful emissions to the environment. In this investigation, we determined experimentally fly ash particle characteristics – particle size, composition and concentration – as well as particle and trace element emissions during waste combustion in two full-scale plants using different combustion technologies but similar gas cleaning technology. The two combustion plants were a grate-fired boiler, and a circulating fluidized bed (CFB) boiler. Both boilers used selective non-catalytic reduction (SNCR) for NO x control together with novel integrated flue gas desulfurization (NID) for flue gas cleaning. The trace elements included in the investigation were As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Tl, and V. Fine fly ash particles were formed from the gas phase ash-forming species by nucleation and condensation. Concentration of particles smaller than 1.0 μm by diameter (PM1.0) at grate-fired plant was 1.0–1.4 g/Nm 3, approximately four times the PM1.0 concentration at CFB combustion, 0.25–0.31 g/Nm 3, as determined upstream of the flue gas cleaning system NID. The average total fly ash mass concentration was higher at CFB combustion than at grate firing with 4.6 g/m 3 and 1.4 g/m 3, respectively. Particle and trace element emissions were very low from both grate-fired and fluidized bed plants. Fabric filter particle collection efficiency was 99.99% by mass at both plants. All the measured emissions were clearly below the limit values set by European Waste Incineration Directive (WID).

Back pressure equal channel angular consolidation—Application in producing aluminium matrix composites with fine flyash particles

Abstract Back pressure equal channel angular consolidation (BP-ECAC) is an innovative process to synthesise bulk materials from particles. It enables particles from nano to micro scales to be consolidated into fully dense materials at much lower temperatures and under much simpler conditions. It is particularly useful in consolidating particles with non-equilibrium microstructures and in producing metal matrix composites (MMCs). As an example, BP-ECAC was used in this investigation to produce aluminium matrix composites with flyash particles. Compared to the conventional powder metallurgy (PM) and ingot metallurgy, BP-ECAC is more efficient and capable of incorporating very fine particles at higher volume fractions with more uniform particle distribution. In particular, it can be used to produce master composite billets for subsequent melting and casting.

Fabrication and characterisation of Al–7Si–0.35Mg/fly ash metal matrix composites processed by different stir casting routes

In the present investigation, the effect of three different stir casting routes on the structure and properties of fine fly ash particles (13 μm average particle size) reinforced Al–7Si–0.35Mg alloy composite is evaluated. Among liquid metal stir casting, compocasting (semi solid processing), modified compocasting and modified compocasting followed by squeeze casting routes evaluated, the latter has resulted in a well-dispersed and relatively agglomerate and porosity free fly ash particle dispersed composites. Interfacial reactions between the fly ash particle and the matrix leading to the formation of MgAl 2O 4 spinel and iron intermetallics are more in liquid metal stir cast composites than in compocast composites.

Fly ash–airborne particles from Ptolemais–Kozani area, northern Greece, as determined by ESEM-EDX

The aim of this study is to investigate the contribution of fly ash from lignite-fired power stations to the air pollution of the Western Macedonia region of northern Greece. Several filters that capture airborne particles were collected over a year's period (March 2003 to February 2004), from seven sampling sites spread throughout the area. In addition, fly ash was collected from two power plants in this area along with stack ash (fly ash that escaped the electrostatic precipitators and trapped at the stack's exit). Both fly ash and filters were analysed with the help of Environmental Scanning Electron Microscopy (ESEM), coupled with energy dispersive X-ray analysis (EDX) in order to determine the morphological, mineralogical and chemical characteristics of fly ash particles. Cenospheres, vesicular spheres, spheroids, dense spheres and chars were the main recognized fly ash particles. Similar airborne particles were also present in almost every filter collected in the area. Sampling sites situated nearby power plants had apparently more fly ash particles. Fine fly ash particles (< 5 μm) were transported to distant sites, while larger fly ash particles (≥ 10 μm) were mostly recorded in the vicinity of power stations. The analysis also revealed the presence of potential toxic elements in the fly ash, a fact with important environmental implications.

Aggregation mechanism of fine fly ash particles in uniform magnetic field

Aggregation of fly ash particles with size range of 0.023-9.314 μm in a uniform magnetic field was investigated for removing them. A binary collision-aggregation model evaluating particle aggregation coefficient was developed. Based on the model, particle removal efficiency was calculated by solving the General Dynamic Equation. The comparison was done between the calculated and experimental data. The modeling aggregation coefficient shows that the aggregation coefficient increases with particle size, and the bigger the size difference between two particles is, the more strongly the gravity difference promotes aggregation. For the mid-sized particles, their removal efficiencies are higher than those of the smaller and bigger ones. The effect of the magnetic flux density on total particle removal efficiency is similar to that on aggregation coefficient. Before particles are saturatedly magnetized, their total removal efficiency increases with an increase in the magnetic flux density. Higher removal efficiency can also be caused by prolonging the particle residence time in the magnetic field or increasing their mass concentration. The particle number median diameter decreases with an increase in the total removal efficiency. Calculation results are found to coincide essentially with the experimental data.

Properties of flash hydrated and agglomerated particles of CFB fly ashes

The fly ash (high carbon content and high unreacted CaO) recirculation in CFB is a typical method to improve the carbon burnout efficiency and the calcium utilization ratio. While the effectiveness of it is limited by the resident time and the reactivity of the re-injected fly ash particles. In the present research, an improved fly ash recirculation method is suggested in which the CFB fly ash is mixed with water or the mixtures of additives (such as waste water of paper mill, cement, sodium silicate, and carbide slag) and water in a blender. Then, this mixture is re-injected into the combustion chamber of CFB by a sludge pump. Because the temperature in CFB is higher, the fly ash was flash hydrated. At the same time, it was dehydrated and agglomerated. The size of agglomerates is bigger than that of original particle and their attrition rate is lower. Therefore the resident time of agglomerates is much longer than that of fine fly ash particles. The absorption of SO 2 is higher than that of original particles, too. This results in high carbon burnout efficiency. The hydrated lime also improves the calcium utilization.