Aggregation Experiments on Fine Fly Ash Particles in a Gradient Magnetic Field

Aggregation experiments were conducted on two kinds of fly ash particles in the size range of 0.023–9.314 μm in a gradient magnetic field produced by permanent magnetic rings. The two types of fly ash particles were obtained from Dongsheng and Datong coal combustion. The effect of particle size, total particle mass concentration, particle residence time in the magnetic field and gas velocity were examined. Experimental results showed that the removal efficiencies in a gradient magnetic field are much higher than those in a uniform magnetic field. The total and single‐sized particle removal efficiencies can be improved by increasing the total particle mass concentrations and the particle residence time in the magnetic field or reducing the gas velocity. Mid‐sized particle removal efficiencies are higher than those of the larger and smaller ones. With the increase in total particle removal efficiencies, the particle size corresponding to the maximum values of single‐sized particle removal efficiencies and the particle number median diameters both decrease. Both the single‐sized and total removal efficiencies for the particles from the Dongsheng coal combustion are higher than those from the Datong coal combustion.     

Significance of surface moisture removal on triboelectrostatic beneficiation of fly ash

The gas transport, triboelectrostatic beneficiation of coal combustion fly ash into carbon-rich and ash-rich products was studied relative to the effect of ash surface moisture. Increasing the humidity to which the ashes from American and Italian coal-fired utilities were exposed under process and ambient conditions affected carbon and ash separability. The effect of humidity and particle surface moisture became more important as particle size decreased: particles greater than 75 μm in diameter were nearly unaffected whereas particles smaller than 45 μm experienced up to a four-fold change in their separability upon changing their surface moisture contents. Although particle size influences the moisture adsorption, which in turn affects tribocharging, the decrease in adhesive forces between carbon and ash from otherwise intractable clusters during drying also may be a factor influencing triboelectrostatic beneficiation performance.     

Formation of fine particles from residual oil combustion: Reducing nuclei through the addition of inorganic sorbent

The potential use of sorbents to manage emissions of ultrafine metal nuclei from residual oil combustion was investigated by using an 82-kW-rated laboratory-scale refractory-lined combustor. Without sorbent addition, baseline measurements of the fly ash particle size distribution (PSD) and chemical composition indicate that most of the metals contained in the residual oil form ultrafine particles (∼0.1 μm diameter). These results are consistent with particle formation via mechanisms of ash vaporization and subsequent particle nucleation and growth. Equilibrium calculations predict metal vaporization at flame temperatures and were used to define regions above the dew point for the major metal constituents (iron [Fe], nickel [Ni], vanadium [V], and zinc [Zn]) where vapor-phase metal and solidphase sorbents could interact. The addition of dispersed kaolinite powder resulted in an approximate 35% reduction in the ultrafine nuclei as determined by changes to the PSDs as well as the size-dependent chemical composition.     

Aggregation experiments on fine fly ash particles in uniform magnetic field

Aggregation experiments on three fly ash samples in the size range of 0.023-9.314 μm were conducted in a uniform magnetic field. The fly ash particles were produced from combustion of three different bituminous coals. The coals were originated Dongshen, Datong and Xuzhou of China, respectively. A fluidized bed aerosol generator was used to disperse the fly ash particles to generate a constant aerosol. The aerosol particles aggregated when passing through the magnetic field. The variation of particle number concentration caused by particle aggregation was measured in real time by an Electrical Low Pressure Impactor (ELPI). The effects of several parameters, such as particle size, magnetic flux density, particle residence time in the magnetic field, total particle mass concentration and average gas velocity, on particle aggregation were examined. Experimental results indicated that removal efficiencies are the highest for particles with sizes in the middle of the size ranges tested. Increasing magnetic flux density, total particle mass concentration, particle residence time in the magnetic field or by reducing average gas velocity can increase removal efficiencies of single-sized and total fly ash particles. When fly ash particle magnetization reached saturation state, further increase of the magnetic flux density will have no effect on particle aggregation. The single-sized and total particle removal efficiencies of the three fly ashes are different under the same operating conditions. The removal efficiency is the highest for fly ash generated from Dongshen coal, followed by fly ash from Datong coal, and then fly ash from Xuzhou coal. Particle number median diameters decreases with the increase in the total particle removal efficiencies. The model prediction of particle aggregation under high total particle mass concentrations conditions indicated that the single-sized and total particle removal efficiencies will increase greatly with the increase in total particle mass concentration. The model predicted total removal efficiencies of the three fly ash particles are 53%, 43% and 14%, for Dongshen, Datong and Xuzhou coals respectively when total particle mass concentration is 40 g/m³.     

The effect of oxygen-to-fuel stoichiometry on coal ash fine-fragmentation mode formation mechanisms

Ash particles smaller than 2.5 μm in diameter generated during pulverized coal combustion are difficult to capture and may pose greater harm to the environment and human health than the discharge of larger particles. Recent research efforts on coal ash formation have revealed a middle fine-fragment mode centered around 2 μm. Formation of this middle or fine-fragment mode (FFM) is less well understood compared to larger coarse and smaller ultrafine ash. This study is part of an overall effort aimed at determining the key factors that impact the formation of FFM. This work examined the effects of oxygen-to-fuel stoichiometry (OFS).Pulverized Illinois #6 bituminous coal was combusted and the ash generated was size segregated in a Dekati low pressure inertial impactor. The mass of each fraction was measured and the ash was analyzed using scanning electron microscopy (SEM) and X-ray microanalysis. The FFM ash types were classified based on the SEM images to evaluate the significant fine-fragment ash formation mechanisms and determine any possible link between stoichiometry and formation mechanism.From the particle size distributions (PSDs), the coarse mode appears unaffected by the change in OFS, however, the OFS 1.05 lowered the fraction of ultrafine ash in relation to the higher OFS settings, and appears to increase the portion of the FFM. An intermediate minimum was found in the FFM at 1.3 μm for the 1.20 and 1.35 OFS tests but was not observed in the 1.05 OFS. SEM analysis also suggests that OFS may contribute to changing formation mechanisms.     

Investigation of sulphation behavior of two fly ash samples produced from combustion of different fuels in a 165 MWe CFB boiler

Emission of sulphur dioxide (SO₂) from combustion of fossil fuel is an important environmental issue. Circulating fluidized bed combustion (CFBC) technology can use limestone sorbent to achieve in situ SO₂ emissions control. This paper presents the chemical and physical analysis results of two fly ash samples derived from a 165 MWe CFBC boiler burning two different fuels with addition of limestone, as they pertain to sulphation behavior. One of the samples in this study was produced from combustion of a bituminous coal with high iron content, the other from firing of blended coal and petroleum coke fuel. The physical examination was conducted by scanning electron microscope (SEM) coupled with an energy dispersive X-ray (EDX) system for analysis of the surface structure or morphology of the sample, as well as the calcium and sulphur distribution. Some large particles derived from high-iron-content fuel were covered by dense iron shells; however, in general such a dense rim was found to not significantly impede the overall desulphurization performance in FBC in terms of the limestone utilization. The large particles (~ 100 μm in diameter) in both samples typically consisted of a CaSO₄ shell and an almost pure CaO core; however, numerous small particles of diameters of 10-20 μm consisted predominantly of CaO without sulphate shells. In particular, the emphasis of this investigation has been focused on the remaining capacity of the fly ash for reaction with sulphur dioxide and to clarify the effects of iron, both samples have been doped with additional iron content, and their sulphation behavior examined, and while both experienced a small reduction in sulphation capacity, the fly ash with the initial low iron content experienced the lowest reduction of sulphation capacity after doping, which is not supportive of the idea that iron has an important effect on sorbent capacity.     

Utilization of fly ash coming from a CFBC boiler co-firing coal and petroleum coke in Portland cement

Fly ash coming from a circulating fluidized bed combustion (CFBC) boiler co-firing coal and petroleum coke (CFBC fly ash) is very different from coal ash from traditional pulverized fuel firing due to many differences in their combustion processes, and thus they have different effects on the properties of Portland cement. The influences of CFBC fly ash on the strength, setting time, volume stability, water requirement for normal consistency, and hydration products of Portland cement were investigated. The results showed that CFBC fly ash had a little effect on the strength of the Portland cement when its content was below 20%, but the strength decreased significantly if the ash content was over 20%. The water requirement for normal consistency of cement increased from 1.8% to 3.2% (absolute increment value) with an addition of 10% CFBC fly ash; and the free lime (f-CaO) content of CFBC fly ash affected the value of increasing. The setting time decreased with an increase of CFBC fly ash content. The volume stability of the cement was qualified even when the content of SO₃ and f-CaO reached 4.48% and 3.0% in cement, respectively. The main hydration productions of cement with CFBC fly ash were C-S-H (hydrated calcium silicate), AFt (ettringite), and portlandite.     

粒径对福建无烟煤在CFB锅炉中燃尽的影响

建立了福建无烟煤细颗粒燃烧模型,计算了其在容量35 t/h循环流化床锅炉炉膛内的燃尽时间和一次通过炉膛的停留时间,分析了不同粒径煤颗粒在不同燃烧温度和不同烟气流速时在CFB锅炉内的燃尽时间和停留时间的变化差异. 实验研究了福建无烟煤粒径对飞灰碳含量的影响及燃尽的影响. 结果表明,细煤颗粒的燃尽时间与停留时间均随粒径增大而增长,但燃尽时间增幅更明显,颗粒一次通过炉膛完全燃尽的临界粒径约为0.15 mm;粒径越大的颗粒其停留时间和燃尽时间对烟气流速和燃烧温度变化越敏感;无烟煤入炉粒径明显影响CFB锅炉飞灰含碳量,选用粒度为3~8 mm的偏粗颗粒为宜.     

High Temperature Interactions Between Residual Oil Ash and Dispersed Kaolinite Powders

The potential use of sorbents to manage ultrafine ash aerosol emissions from residual oil combustion was investigated using a downfired 82 kW laboratory-scale refractory-lined combustor. The major constituents were vanadium (V), nickel (Ni), iron (Fe), and zinc (Zn). The overall ash content of residual oil is very low, resulting in total ash vaporization at 1725 K with appreciable vaporization occurring at temperatures as low as 1400 K. Therefore, the possibility of interactions between ash vapor and sorbent substrates exists. Kaolinite powder was injected at various locations in the combustor. Ash scavenging was determined from particle size distributions (PSDs) measured by a Scanning Mobility Particle Sizer. Impactor samples and X-ray fluorescence (XRF) analyses supported these data. Injection of kaolinite sorbent was able to capture up to 60% of all the ash in the residual fuel oil. However, captures of ～ 30% were more common when sorbent injection occurred downstream of the combustion zone, rather than with the combustion air into the main flame. Without sorbent addition, baseline measurements of the fly ash PSD and chemical composition indicate that under the practical combustion conditions examined here, essentially all of the metals contained in the residual oil form ultrafine particles (～0.1 μ m diameter). Theoretical calculations showed that coagulation between the oil ash nuclei and the kaolinite sorbent could account for, at most, 17% of the metal capture which was always less than that measured. The data suggest that kaolinite powders reactively capture a portion of the vapor phase metals. Mechanisms and rates still remain to be quantified.     

The fate of sodium during pulverized coal combustion

The distribution of sodium in the different sizes of fly ash produced during coal combustion provides useful insights into the vaporization and condensation mechanisms for sodium. For the residual fly ash, the departure of the concentration of sodium from an inverse square dependence on particle size can be used to infer the degree of sodium vaporization. For low-rank coals most of the sodium vaporizes at combustion temperatures <1900 K; at higher temperatures the release of sodium decreases as increasing amounts react with silica. The fraction of sodium condensing on the submicrometre fume is influenced both by the amount of total ash vaporized and by the Kelvin effect.     

Toxicologic and Physicochemical Characterization of High-Temperature Combustion Emissions

Particles and combustion gases produced by two different high-temperature combustors, which burned pulverized coal and a No. 2 fuel oil-fly ash slurry, respectively, at adiabatic flame temperatures greater than 2400 K, were characterized. Effluent samples were taken at locations along the product gas stream and within the stack. Measurements of the particle size distributions, number concentrations, and gas species concentrations were made. The toxicity and mutagenicity of the effluent particles were determined. A large number of submicrometer particles were found in both cases of high-temperature combustion. The product emissions differed significantly in their particle size distribution and final chemical composition from those of conventional combustion systems having lower combustion temperatures.     

A characterisation of the surface properties of an ultra fine fly ash (UFFA) used in the polymer industry

Fly ash is the residue from the combustion of pulverised coal in thermal power stations. It consists of a variety of particle sizes of hollow silica-alumina spheres, which can be utilised as a custom made filler material in a number of industries, e.g. plastics, rubber, paint, refractories, etc. The 5 μm average particle size fraction is popular for its application in plastic products. In order to comply with the requirements for application in plastics, it is essential to understand and characterise the surface of the ultrafine glass spheres coming into contact and interacting with the organic phase of the final composite. This contribution describes the use of a number of techniques, such as EPMA (Electron probe micro-analysis), Raman spectrometry, zeta potential measurements and ICP-OES (Inductively coupled plasma optical emission spectroscopy) to investigate the surface composition and behaviour of the ultrafine fly ash particles.     

Particle size distribution profile of some Indian fly ash—a comparative study to assess their possible uses

Fly ash collected from three different thermal power plants in Eastern India were critically examined in respect to their particle size distribution. The objective of the investigation was to obtain proper distribution profiles of such ashes with the nature of combustors vis-à-vis their efficiency and to point out the possible areas of application/utilisation. For this purpose, particle size distribution profiles of the as received fly ash, the size fractionated ones as well as the magnetic and non-magnetic components, were determined. Distribution profiles reveal that fly ash obtained from Super Thermal Power Plants have better profiles than the fly ash obtained from captive power plants. It is observed that, in general, the finer fractions have a better profile in the sense that a considerable amount of the particles fall below 45 μm range; this is important in respect to utility in the construction industry. The magnetic particles are larger and rounder than the non-magnetic ones, and can probably be used for coal flotation. An important observation is that the fly ash collected from Stoker-fired thermal power plants possesses very poor distribution profile (AMD for as received fly ash ≥200 μm).     

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。     

A flat premixed flame reactor to study nano-ash formation during high temperature pulverized coal combustion and oxygen firing

This paper deals with the development of a laboratory reactor to study ultrafine (D < 100 nm) and nano (D < 30 nm) ash formation during pulverized coal combustion and oxy-firing. The reactor consists of an atmospheric pressure flat laminar premixed flame homogeneously doped with pulverized coal particles, monodisperse in size. It is accessible to diagnostics and sampling systems and it allows investigating the early stage of particle formation in a wide range of pulverized coal combustion operative conditions, in terms of gas composition and temperature. Coal combustion in an oxygen enriched gas mixture was investigated by performing on-line high resolution differential mobility analyses (DMA) and thermophoretic samplings for atomic force microscopy (AFM) image analyses. Ultrafine particle size distribution functions in a size range extending down to 1 nm have been measured. Three types of high volatile bituminous coals have been tested. Ultrafine particles, commonly neglected at the exhaust of pulverized coal combustors, form with huge number concentration and they represent a not negligible fraction of total ashes also in volume/mass. Nano-ashes are the most abundant in number and they also significantly contribute to ultrafine particle mass concentration. This not negligible contribution slight increases with the coal chlorine content while the shape of the nano-ash size distribution function is quite unaffected by the used coal type.     

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.     

Combustion and De-SOx behavior of high-sulfur coals added with calcium acetate produced from biomass pyroligneous acid

This paper first aims to evaluate the possibility of synthesizing organic calcium through the use of pyroligneous acid and raw limestone. The factors affecting this reaction were investigated including the particle size of limestone, the type of pyroligneous acid and reaction conditions as well. Secondly, two low-rank coals were mixed with the calcium-enriched pyroligneous acid to evaluate the possibility of desulfurization in the combustion furnace. The soluble calcium within pyroligneous acid is in the form of calcium acetate having a low decomposition temperature around 400 °C, far lower than that of raw limestone being about 700 °C. Due to its solubility, calcium within pyroligneous acid was loaded readily on the coals, forming the ultrafine particles in the impregnated coal matrix. During coal combustion, the loaded calcium underwent quick decomposition prior to char combustion and subsequently, the formed ultrafine calcium oxide captured the evolved sulfur oxide. Around 85% desulfurization efficiency was achieved for coal combustion. S/Ca molar ratio around 1.0 was formed for the particles less than 1.0 μm, whereas it decreased gradually with the increasing particle size.     

熟料和粉煤灰的颗粒尺寸分布与水泥性能的灰色关联分析

将不同比表面积的熟料和粉煤灰分别混合制得不同比表面积的粉煤灰水泥,分别测定粉煤灰水泥胶砂强度和熟料、粉煤灰的颗粒尺寸分布.运用灰色系统原理分别就熟料和粉煤灰的颗粒尺寸分布对水泥强度的影响进行数学分析.研究表明:熟料颗粒粒径在0～30 μm为正关联,>30 μm为负关联,其中10～20 μm颗粒对水泥强度的影响最大;粉煤灰颗粒粒径在0～40 μm为正关联,且其活性较高,>40 μm颗粒为负关联,其活性没有得到充分发挥.     

A scanning electron microscopy study of the transformations of organically bound metals during lignite combustion

The transformation of organically bound alkaline metals on the surface of char particles during combustion of pulverized Montana lignite was studied in detail by scanning electron microscopy. The char particles were obtained by burning the lignite in a laboratory flow furnace in which the extent of burnout was controlled by collecting the char and quenching the combustion reaction at various residence times. The atomically dispersed alkaline earth metals in the raw coal form minute submicron grains of ash on the surface of the char after slight (10–20%) char burnout. As combustion of the char proceeds, these grains coalesce and ultimately form ash droplets ranging in size from ≈ 1 to 10 μm. Significant interaction between alkaline coal ash and the silicate minerals embedded in the char partition was observed. Most of the mass of the mineral matter in coal coalesces to form a few ash particles in the size range 10–20 μm. In addition, several thousand smaller particles, 1 to 8 μm in diameter, are produced by the shedding of ash particles during combustion.     

Fundamental ash deposition characteristics in pulverized coal reaction under high temperature conditions

Three types of coal with the different melting temperature and ash content were burned under the condition of high-temperature air pulverized coal reaction. A water-cooled tube was inserted into the furnace to make the ash adhere. Particle size and composition distributions of ash particles in both reacting coal particles and depositing layer were analyzed, using a Computer Controlled Scanning Electron Microscope, to study the deposition behaviors of ash particles. As a result, quantity of the ash deposition on the tube surface increases with a decrease of the melting temperature of coal ash. Index of fraction of the ash deposition depended on the coal type. For structure of the deposit layer, fine particles of size less than 3 μm mainly consisted of the initial layer for three types of coal, and the thickness was about 30 μm. Deposition of fine particulates of about 3 μm became a trigger of initial deposition at the stagnation point of tube even if irrespective of coal type is burned. The chemical compositions of ash particles in the reacting particles differed from those in the initial deposition layer. The deposition phenomenon relates to the particle size distribution of ash formed, the flow dynamics surrounding the probe, the chemical compositions in each ash particle and so forth.