Ash Vaporization in Circulating Fluidized Bed Coal Combustion

ABSTRACT

In this work, the vaporization of the ash forming constituents in circulating fluidized bed combustion (CFBC) in a full-scale 80 MW_th unit was studied. Ash vaporization in CFBC was studied by measuring the fly ash aerosols in a full-scale boiler upstream of the electrostatic precipitator (ESP) at the flue gas temperature of 125°C. The fuel was a Venezuelan bituminous coal, and a limestone sorbent was used during the measurements. The fly ash number size distributions showed two distinct modes in the submicrometer size range, at particle diameters 0.02 and 0.3 μm. The concentration of the ultrafine 0.02-μm mode showed a large variation with time and it decreased as the measurements advanced. The concentration of the 0.02-μm mode was two orders of magnitude lower than in the submicrometer mode observed earlier in the bubbling FBC and up to three orders of magnitude lower than in the pulverized coal combustion. Scanning electron micrographs showed few ultrafine particles. The intermediate mode at 0.3 μm consisted of particles irregular in shape, and hence in this mode the particles had not been formed via a gas to particle route. We propose that the 0.3-μm mode had been formed from the partial melting of the very fine mineral particles in the coal. The mass size distribution in the size range 0.01–70 μm was unimodal with maximum at 20 μm. Less than 1% of the fly ash particles was found in the submicrometer size range. Ninety percent of Mg in coal was organically bound, and it was found to react with quartz and aluminosilicate minerals inside the coal particle. No Mg was found to be released to the gas phase and Mg mass fraction size distribution was size independent. A fraction of halogens CI, Br and I were found to be in the gas phase after the combustion.     

Size Distribution and Sources of Trace Metals in Ultrafine/Fine/Coarse Airborne Particles in the Atmosphere of Shanghai

Airborne particulate matter (PM) samples in 13 different size-fractions from 0.0283 to 9.92 μm were collected in winter of 2007 at three sites in Shanghai, China. The PM exhibited a bimodal distribution with a major mode in the fine particle size range (D_p = 0.2–1 μm) and a minor mode in the coarse range (D_p = 1–10 μm), suggesting that fine particle pollution is dominant in the Shanghai atmosphere. Trace metals in PM exhibited the following distribution patterns: (1) unimodal distribution in the fine fraction (Pb, Cd, Se, Sn, Bi, and Zn), (2) unimodal distribution in the coarse fraction (Mg, Al, Fe, Ca, Ba, Sr, Ge, Zr, U, and rare earth elements), (3) bimodal distribution, with one mode in the fine fraction and one in the coarse fraction (Cu, Mn, K, Ga, V, Rb, and Cs), and (4) multimodal distribution (Na, Ti, Cr, Co, As, Ni, Mo, Ag, W, Pt, Au, S, and Cl) throughout the entire aerosol size spectrum. In addition to these size distributions, Aitken modes due to local origins were also evident for Se, Sn, Cu, V, Ti, Cr, Co, As, Ag, Mo, and Pt, whose respective mass in the ultrafine particles (<0.1 μm) was 10, 23, 13, 19, 23, 14, 67, 32, 79, 40, and 21%, with submicron mass median aerodynamic diameters (MMADs) in PM_0.02-9.92 (except Pt). In particular, the MMADs for Co and Ag were <0.1 μm, which increase potential health issues. The measured distributions are believed to result from a combination of processes including local anthropogenic and natural sources, such as traffic, coal combustion, and the steel industry.     

粉磨工艺对水泥厂煤粉中矿物分布规律的影响

采集了山东泰安中联水泥厂立式辊磨和管式球磨两种不同粉磨方式的出磨煤粉,通过浮沉、工业分析、粒度分析,研究了不同粉磨工艺对于煤粉粒度-灰分规律的影响。研究发现,两种不同粉磨工艺的出磨煤粉都呈现出灰分随密度级别的增大而递增,随着粒度的增加而递减的规律,各密度级别浮沉产率和粒度-灰分曲线相似,说明粉磨方式对煤粉中矿物规律没有影响。     

A mathematical model of ash formation during pulverized coal combustion

A mathematical model of ash formation during high-rank pulverized coal combustion is reported in this paper. The model is based on the computer-controlled scanning electron microscope (CCSEM) characterization of minerals in pulverized coals. From the viewpoint of the association with coal carbon matrix, individual mineral grains present in coal particles can be classified as included or excluded minerals. Included minerals refer to those discrete mineral grains that are intimately surrounded by the carbon matrix. Excluded minerals are those liberated minerals not or at least associated with coal carbon matter. Included minerals and excluded minerals are treated separately in the model. Included minerals are assumed to randomly disperse between individual coal particles based on coal and mineral particle size distributions. A mechanism of partial-coalescence of included minerals within single coal particles is related to char particulate structures formed during devolatilization. Fragmentation of excluded minerals, which is important particularly for a coal with a significant fraction of excluded minerals, is simulated using a stochastic approach of Poisson distribution. A narrow-sized sample of an Australian bituminous coal was combusted in a drop-tube furnace under operating conditions similar to that in boilers. The particle size distribution and chemical composition of experimental ash were compared to those predicted with the model. The comparisons indicated that the model generally reflected the combined effect of coalescence of included minerals and fragmentation of excluded minerals, the two important mechanisms governing ash formation for high-rank coals.     

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.     

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.     

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.     

工业煤粉中矿物质的分布规律

采用浮沉实验和激光粒度法研究了工业煤粉中矿物质的分布规律,考察了原煤的变质程度、原煤的灰分高低以及煤粉制备工艺对分布结果的影响.结果表明,工业煤粉都呈现出灰分随着煤粉粒度增加而降低的趋势,并且随着变质程度和原煤灰分的提高,灰分随煤粉粒度增加降低的幅度增大,而煤粉制备工艺对该规律的影响不明显.     

Effect of Coal Type and Residence Time on the Submicron Aerosol Distribution from Pulverized Coal Combustion

Three types of pulverized coal were burned in a laboratory furnace under various combustion configurations. Pulverized samples of Utah bituminous, Beulah (North Dakota) lignite, and Texas lignite coals were burned at a rate of 2.5 kg/hr in a laboratory furnace. Aerosol size distributions were measured at various positions within the convection section, and temperature and gas compositions were measured throughout. The evolution of the submicron particle size distribution within the convection section for the three coals was similar, although the location of the initial particle mode at the convection section inlet varied with coal type. While staged combustion of Utah bituminous coal had a variable effect on the volume of submicron aerosol produced, staged combustion of the lignites caused a definite increase in the submicron aerosol volume. Vapor enhancement due to a localized reducing atmosphere, which would effect coals of higher ash volatility, is thought to explain this behavior.     

660MW燃煤锅炉细微颗粒物中次量与痕量元素的分布特性

应用承重撞击器(DGI)采样系统在南昌某电厂2^#锅炉电除尘器前进行颗粒物采集,并同时采集了原煤样和底灰样。对飞灰的质量粒径分布、底灰和飞灰中次量与痕量元素的分布特性进行了分析。结果表明PM_1.0和PM_2.5质量分别占PM₁₀质量的16.0%~17.4%和46.9%~50.6%;Na、Mg、P、S主要富集在亚微米颗粒物中,Al、Si、Ca、Ti、Fe、K主要富集在超微米颗粒物中;随着颗粒物粒径的减小,As、Cd、Cr、Pb的浓度逐渐增大,且在亚微米颗粒物中的增幅大于超微米颗粒物,Mn在各级颗粒物中浓度相近;As、Cd、Cr、Pb大量富集于亚微米颗粒物之上,Mn在各级颗粒物中富集特性无明显差异,且各痕量元素挥发特性存在以下规律:As>Cd>Cr>Pb>Mn;文中给出了无控制条件下痕量元素的排放因子,PM_1.0中各元素排放比例存在以下规律:As>Cd>Cr>Pb>Mn。     

Effects of Dilution Sampling on Fine Particle Emissions from Pulverized Coal Combustion

A dilution sampler was used to examine the effects of dilution ratio and residence time on fine-particle emissions from a pilot-scale pulverized coal combustor. Measurements include the particle size distribution from 0.003 to 2.5 μm, PM_2.5 mass, and PM_2.5 composition (OC/EC, major ions, and elemental). Heated filter samples were also collected simultaneously at stack temperatures in order to compare the dilution sampler measurements with standard stack sampling methodologies. Measurements were made both before and after the bag house, the particle control device used on the coal combustor, and while firing three different coal types and one coal–biomass blend. The PM_2.5 mass emission rates measured using the dilution sampler agreed to within experimental uncertainty with those measured with the hot-filter sampler. Relative to the heated filter sample, dilution did increase the PM_2.5 mass fraction of selenium for all fuels tested, as well as ammonium and sulfate for selected fuels. However, the additional particulate mass created by gas-to-particle conversion of these species is within the uncertainty of the gravimetric analysis used to determine the overall mass emission rate. The enrichment of PM_2.5 selenium caused by dilution did not vary with dilution ratio and residence time. The enrichment of PM_2.5 sulfate and ammonium varied with fuel composition and dilution ratio but not residence time. For example, ammonium was only enriched in diluted acidic aerosol samples. A comparison of the PM_2.5 emission profiles for each of the fuels tested underscores how differences in PM_2.5 composition are related to the fuel ash composition. When sampling after the bag house, the particle size distribution and total particle number emission rate did not depend on residence time and dilution ratio because of the much lower particle number concentrations in diluted sample and the absence of nucleation. These results provide new insight into the effects of dilution sampling on measurements of fine particle emissions, providing important data for the ongoing effort of the EPA and ASTM to define a standardized dilution sampling methodology for characterizing emissions from stationary combustion sources.     

Fine particle and trace element emissions from an anthracite coal-fired power plant equipped with a bag-house in China

Fine particle and trace element emissions from energy production are associated with significant adverse human health effects. In this investigation, the fine particles and trace elements emitted from the combustion of pulverized anthracite coal at a 220 MW power plant were determined experimentally in the size range from 30 nm to 10 μm with 12 channels. The particulate size distributions and morphological characteristics before and after the bag-house were evaluated. The uncontrolled and controlled emission factors of particles are compared with the calculated values from the US Environment Protection Agency, AP-42. Size-classified relative enrichment factors of As, Hg, Se, Cd, Cr, Cu, Al, V, Zn, Mn, Fe were obtained. Relative distributions of trace elements between bottom ash, fly ash and flue gas are determined by mass balance method. The bag-house collection efficiencies of particles and trace elements in the particulate phase are obtained. Finally, the controlled and uncontrolled emission factors of elements of different particulate size fractions are obtained, which will provide useful information for PM_2.5 and PM₁₀ emission inventory development, toxic and hazardous pollutant emission estimates and emission standards established for metal-based pollutants from a pulverized coal-fired boiler.     

燃煤过程中颗粒物的形成机理研究进展

介绍了煤粉燃烧过程中颗粒物的形成机理，包括亚微米飞灰和残灰颗粒的主要形成途径．亚微米颗粒主要来自无机物的气化-凝结过程，在高温条件下无机矿物首先以氧化物、次氧化物或原子的形式气化，当温度降低时，无机蒸气通过均相成核、异相冷凝、凝并、团聚等过程形成细微颗粒．残灰由残留在焦炭颗粒中的矿物转化而成，焦炭破碎和表面灰的聚合是决定残灰最终粒径分布的主要过程，除此之外，对于含外来矿物较多的煤种，矿物破碎对残灰颗粒的形成也有十分重要的影响．最后对燃煤过程中颗粒物的形成机理研究提出了建议．     

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.     

Characterisation of the properties of size fractions from ten world coals and their chars produced in a drop-tube furnace

A range of coals from different parts of the world was studied to determine if there were any common relationships that could be determined to gain a clearer understanding of the distribution of coal properties within different particle-sizes. The properties examined were proximate analysis, maceral analysis and %Unreactives from image analysis. Each fraction was also pyrolysed in a drop-tube furnace at 1300°C, 1 vol% oxygen and a residence time of 200 ms and the resulting chars analysed for morphology using image analysis. There were substantial variations between the particle-size distributions of the different coal samples even though they were ground to the same specification for trials on a combustion rig. Ash distributions showed in all cases that the smallest particle size (−38 μm) had either the highest ash level or was very close to it. However, the trends in ash level for increasing particle size showed variations between coals with some coals showing increases in ash towards the larger particles. Fusinite content did not necessarily concentrate in the smallest size fraction, however, liptinite content did increase with particle size. %Unreactives generally increases with particle size and is related to char morphology through an empirical parameter, the ACA [5]. In addition the ACA [5] parameter showed the effect of both particle size and %Unreactives on char morphology and clearly showed the significant influence of particle size on burnout. A parameter such as this could, therefore, be used in burn-out models and further correlated with %Unreactives and particle size.     

Comparison of Coal Ash Particle Size Distributions from Berner and Dekati Low Pressure Impactors

This article presents the differential mass size distributions of coal combustion particulate matter (PM) determined with the Berner low-pressure impactor (BLPI, Hauke Model 25-4/0.015) and a newer generation of low pressure impactor, the Dekati low-pressure impactor (DLPI, Dekati Ltd Model 6281). The collection characteristics of the BLPI and DLPI are compared and cutoff diameters are calculated. Samples were collected in the post-combustion zone of a 19 kW vertical downflow combustor from two coal types. Both BLPI and DLPI represent a tri-modal distribution and give statistically similar characterizations of the coal ash particle size distribution. Distributions generated from DLPI data have higher fractions of submicron particles compared to those generated from BLPI data. The DLPI's two additional stages may provide greater resolution in the submicron region than the BLPI.     

粒度级配对神华煤成浆特性影响的试验研究

为了研究粒度级配对神华煤成浆特性的影响,通过筛分和干法调浆,探讨了不同粒度分布煤粉的成浆性,结果表明:在添加剂用量为0.3%(干基/干煤)时,具有连续级配特征的原始煤粉可制备出质量分数为61%的煤浆;利用筛分法,分别去除原始煤粉中0.045 mm以下、0.045 mm^0.15 mm、0.15 mm^0.3 mm和0.045 mm^0.3 mm部分,得到4种具有不连续级配特征的样品,其所制煤浆流变性发生了较大变化,其中去除0.045 mm^0.3 mm部分的样品的成浆质量分数提高了3个百分点,在黏度符合要求的前提下流动性大幅提高;连续级配和非连续级配的煤浆均存在黏度与流动性不统一的现象,当级配中粗细颗粒粒径差较大且细颗粒含量达到一定值时,这种现象更加明显。采用粒度级配技术制浆,其细颗粒含量存在一个合理的区间。     

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。     

煤粉炉掺烧生活垃圾对灰渣特性的影响研究

利用小试规模煤粉炉,研究掺烧不同比例生活垃圾对燃煤灰渣特性的影响,主要包括飞灰元素组成、飞灰粒径分布、飞灰形貌、灰熔点和结渣特性等影响变化研究。结果表明,随着生活垃圾掺烧比例的增加,灰分中Ca、Fe、Cl和S元素含量增加,Al、Mg、K、Na、Ti和Si含量降低,飞灰球形颗粒分布减少,层状堆积结构增多;灰渣熔融特征温度呈平缓下降趋势,但变化范围小于2%,影响较小;掺烧量为25%时,飞灰表面发现少量褐色大颗粒。总之,生活垃圾掺烧对燃煤灰渣特性影响很小,该结果为实际煤粉炉开展掺烧生活垃圾试验提供了一定理论基础。     

焦化除尘灰的反应性研究

为了扩大高炉喷吹用煤煤源,降低高炉喷吹用煤成本,本试验探索用廉价的焦化除尘灰代替部分无烟煤用于高炉喷吹的可行性。在对高炉喷吹用煤进行工业分析和粒度分布研究的基础上,进一步研究了高炉喷吹用煤的反应性及其影响因素。初步研究表明:高炉喷吹用煤中配入适当比例的焦化除尘灰,对高炉喷吹用煤的反应性影响不大。在焦化除尘灰的配比小于15%的条件下用作高炉喷吹是可行的。