Factors influencing the transformation of minerals during pulverized coal combustion

The transformation of minerals and dispersed inorganic constitutents during pulverized coal combustion has been examined by burning utility sized coals (70% less than 200 mesh) in a laboratory-scale combustor. Experiments were conducted with four U.S. coals possessing different mineralogies. Size and composition of the initial minerals and the resulting ash were measured by a variety of techniques, including computer controlled SEM, low temperature ashing, deposition on a cascade impactor, and optical (Malvern) particle size analysis. Results for a Kentucky # 11 coal with a large amount of fine, included silicate minerals suggest that coalescence amongst illite, kaolinite, and quartz minerals was the dominant process, with occasional iron incorporation into the resulting glass. Pyrite was found to fragment to a limited extent. Illinois # 6 bituminous coal, possessing a similar mineralogy, yielded similar results. For a Beulah lignite coal containing large pyrite grains, mineral fragmentation was inferred from the data, increasing with increasing oxygen level. A high ash content San Miguel Texas lignite containing zeolite minerals demonstrated little mineral interaction during combustion. Differences in results obtained for the different coals highlighted the importance of understanding individual mineral transformations in predicting the formation and behavior of ash.     

Distribution of mineral matter in pulverized coal

Mineral transformations, and therefore fly ash evolution, during pulverized coal combustion, depend on the amount, composition and spatial distribution of the inorganic matter within individual pulverized coal particles. Thus, it is necessary to have information on the mineral composition of individual particles, as well as that of the raw pulverized coal. A model is proposed based on the assumption that mineral inclusions of size and composition determined using a CCSEM are distributed randomly in the coal. From this distribution it is possible to generate distributions of mineral content for any particle size and density fraction of coal. The model has been checked by comparing computed results with data on the compositional variations of narrowly and density classified fractions of an Upper Freeport bituminous coal. The results of individual coal particle compositions are used to generate information on the variability of the composition of the fly ash generated during combustion.     

Modelling ash deposition in pulverized coal-fired applications

A predictive scheme based on CCSEM flyash data and computational fluid dynamics (CFD) was developed to study the slagging propensity of coals. The model was applied to predict the deposition potential of three UK coals; Bentinck, Daw Mill and Silverdale, in a pilot scale single burner ash deposition test facility. The project is part of a collaborative research programme sponsored by the UK Department of Trade and Industry and involved various industrial organizations and universities. The objective is to understand the fundamental aspects of slagging in pulverized coal-fired combustion systems. The present model predicts the relative slagging propensity of the three coals successfully, and the predicted deposition patterns are also consistent with the observations. The results from the model indicate a preferential deposition of iron during the initial stage of ash deposition. The average compositions of the deposits become closer to that of the bulk ash when the accumulation of ash deposits is taken into account.     

Developing coal-combustion technologies

Improved and new methods for direct coal utilization are reviewed, including the following: direct burning of pulverized coals, clean-fuel-combined-cycle systems, atmospheric pressure fluidized-bed combustion, advanced and pressurized fluidized-bed combustion, slagging combustors, coal-oil mixtures, coal-water mixtures, other coal slurries, etc. The emphasis is on research needs identified by the DOE/Coal Combustion and Applications Working Group (CCAWG) and relating to such processes as coal cleaning, slagging and fouling, environmental control systems, on-line diagnostics, and modeling.     

Developing coal-combustion technologies

Improved and new methods for direct coal utilization are reviewed, including the following: direct burning of pulverized coals, clean-fuel-combined-cycle systems, atmospheric pressure fluidized-bed combustion, advanced and pressurized fluidized-bed combustion, slagging combustors, coal-oil mixtures, coal-water mixtures, other coal slurries, etc. The emphasis is on research needs identified by the DOE/Coal Combustion and Applications Working Group (CCAWG) and relating to such processes as coal cleaning, slagging and fouling, environmental control systems, on-line diagnostics, and modeling.     

煤燃烧过程中矿物质蒸发与煤灰的沉积特性

通过典型煤种的实验和模拟计算驿煤燃烧过程中主要矿物质的蒸发进行了详细的研究，探讨了煤灰组成、燃烧温度和燃烧气氛等因素对这些矿物质蒸发的影响，然后结合几种红典沉积判别指数和已知煤的结渣倾向，分析了煤中主要矿物质蒸发与煤灰沉积的关系，得出了与实际相符的新结论。     

670t/h锅炉结焦原因分析与燃烧器改造

煤粉锅炉燃烧时,结渣是锅炉一直存在的问题.锅炉设计煤种和燃烧方式存在着不适应,就容易导致炉膛结渣.锅炉燃烧煤种偏离设计煤种,而对燃烧的煤种特性分析不够深入,锅炉在燃烧时,由于燃烧器本身的调整不能适应煤种多变的情况,最终将导致炉膛结焦,甚至出现因结渣而引发锅炉停运.针对670 t/h锅炉因煤种和燃烧方式不相适应而导致炉膛结焦的原因进行分析,并对锅炉燃烧器进行重新布置,改善炉膛结焦.     

The evolution of mineral particle size distributions during early stages of coal combustion

Data are presented which illustrate the evolution of the particle size distribution (psd) of included mineral grains in coal particles during coal particle heatup and the early stages of combustion. The data indicate that pyrite fragments during its thermal decomposition to form pyrrhotite and oxides of iron. Two to three fragments are formed per original pyrite particle. The mineral psd for silica also significantly changes in the early residence time. The data indicate the production of many small particles, with diameters approximately 4–5 times smaller than the original particles. These results indicate that the original mineral grain size distribution in the coal is not a reliable indicator of the grain size distributions in the resultant char.     

Influence of phosphorus on ash fouling deposition of hydrothermal carbonization sewage sludge fuel via drop tube furnace combustion experiments

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.     

Studies of transformations of inorganic constituents in a Texas lignite during combustion

The mechanisms of coal ash formation were studied under closely controlled combustion conditions. Monticello lignite, from Titus County, Texas was combusted in a drop-tube furnace at 1500°C and fly ash was collected and aerodynamically size segregated into six stages. Short residence time chars were also produced in the drop-tube furnace. The coal was analyzed to determine both mineralogical and organically bound components using computer controlled scanning electron microscopy (CCSEM) and chemical fractionation techniques, respectively. Scanning electron microscopy/microprobe techniques were used to classify and determine the distribution of various ash particle types in each size fraction. Over 80% of the Monticello mineral content consisted of quartz and clay minerals and relatively large quantities of Ca, Mg, and possibly Fe were organically bound. Extensive reaction between the quartz and clay minerals and organically bound Ca resulted in amorphous and crystalline Ca-silicate and Ca-aluminosilicate phases in the fly ash. Finely subdivided discrete minerals or organically bound cations of Mg and Fe were concentrated in the finer fraction of the fly ash.     

Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse

Comparative combustion studies were performed on particles of pulverized coal samples from three different ranks: a high-volatile bituminous coal, a sub-bituminous coal, and two lignite coals. The study was augmented to include observations on burning pulverized woody biomass residues, in the form of sugarcane bagasse. Fuel particles, in the range of 75–90 μm, were injected in a bench-scale, transparent drop-tube furnace, electrically-heated to 1400 K, where they experienced high-heating rates, ignited and burned. The combustion of individual particles in air was observed with three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time–size histories. Based on combined observations from these techniques, in conjunction to morphological examinations of particles, a comprehensive understanding of the combustion behaviors of these fuels was developed. Observed differences among the coals have been striking. Upon pyrolysis, the bituminous coal chars experienced the phenomena of softening, melting, swelling and formation of large blowholes through which volatile matter escaped. Combustion of the volatile matter was sooty and very luminous with large co-tails forming in the wake of the particle trajectories. Only after the volatile matter flames extinguished, the char combustion commenced and was also very luminous. In contrast, upon pyrolysis, lignite coals became fragile and experienced extensive fragmentation, immediately followed by ignition of the char fragments (numbering in the order of 10–100, depending on the origin of the lignite coal) spread apart into a relatively large volume. As no separate volatile matter combustion period was evident, it is likely that volatiles burned on the surface of the chars. The combustion of the sub-bituminous coal was also different. Most particles experienced limited fragmentation, upon pyrolysis, to several char fragments, with or without the presence of brief and low-luminosity volatile flames; other particles did not fragment and directly proceeded to char combustion. Finally combustion of bagasse was once again very distinctive. Upon pyrolysis, long-lasting, low-luminosity, nearly-transparent spherical flames formed around slowly-settling devolatilizing particles. They were followed by bright, short-lived combustion of the chars. Both volatiles and chars experienced shrinking core mode of burning. For all fuels, flame and char temperature profiles were deduced from pyrometric data and burnout times were measured. Combustion rates were calculated from luminous carbon disappearance measurements, and were compared with predictions based on published kinetic expressions.     

煤焦燃烧反应动力学参数与煤种的通用关系

根据作者以前提出的关于煤焦在空气中燃烧的表面反应动力学的新思想，即其表面反应活化能Ｅ与煤种无关，只取决于其温度，但其反应频率因子ｋ０，ｃｈ与煤种有密切关系，在此基础上，本文提出了一种确定煤焦燃烧反应动力学参数的新方法。在这一方法中，确定了Ｅ＝１８０ｋＪ／ｍｏｌ，它适用于在空气中燃烧的任何一种煤焦的反应。     

准东煤掺烧煤矸石安全性与经济性分析

高岭土与低钠煤可以有效抑制燃用准东煤时炉内的沾污结渣问题,但由于其价格升高,为降低电厂燃煤成本,因此提出将煤灰成分相似的煤矸石作为准东煤防结渣添加剂.分析煤矸石作为添加剂抑制炉内沾污结渣的可行性,根据入炉煤灰中折算Na2O含量低于3.0％,确定掺烧比例,并在某150 MW机组上进行实炉掺烧试验.通过掺烧试验分析可见,煤...     

Coal ash fusion temperatures—New characterization techniques,and implications for slagging and fouling

The well-documented shortcomings of the standard technique for estimating the fusion temperature of coal ash are its subjective nature and poor accuracy. Alternative measurements based on the shrinkage and electrical conductivity of heating samples are therefore examined with laboratory ash prepared at about 800°C in crucibles, as well as combustion ash sampled from power stations. Sensitive shrikage measurements indicate temperatures of rapid change which correspond to the formation of liquid phases that can be identified on ternary phase diagrams. The existence and extent of formation of these phases, as quantified by the magnitude of “peaks” in the test, provide alternative ash fusion temperatures. The peaks from laboratory ashes and corresponding combustion ashes derived from the same coals show clear differences which may be related to the evaporation of potassium during combustion and the reactions of the mineral residues to form combustion ash. A preliminary evaluation of data from nine power stations indicates that shrinkage measurements can provide an alternative approach to characterizing slagging.     

Combustion performance and slagging characteristics during co-combustion of Zhundong coal and sludge

Zhundong coal (ZDc) with a very large reserve is faced with severe problems of slagging and fouling during combustion in boilers because of the high-Na content. Sludge, the by-product of urban sewage treatment, is also faced with the problem in utilization. In this study, the co-combustion of ZDc and sludge was investigated in a laboratory-scale experimental apparatus before further studies in larger-scale setups. The experimental results confirm an interaction between ZDc and sludge during co-combustion, which was mainly caused by the Na catalytic action and improved the combustion performance of the co-fuels. The catalytic effect was particularly significant at low sludge mixing ratios. The reactions between Na-based compounds in ZDc and Si/Al/P-rich minerals in sludge, forming high-melting-point phosphates and aluminosilicates, not only increased Na retention in residual ash reducing the risk of fouling on tail-heating surfaces in boilers, but also raised the ash fusibility of the co-fuels avoiding low-temperature sintering. Even so, to prevent slagging, the high combustion temperature above 900 °C should be avoided during co-combustion because of the high Na retention in residual ash. Moreover, the high heavy metal retention in residual ash decreased the pollution caused by heavy metal volatilization during sludge combustion.     

Modeling of a coal-fired slagging combustor: Development of a slag submodel

In a slagging combustor or furnace, the high combustion temperature makes the molten slag layer cover the wall and capture the particles. If these particles contain combustible matter, they will continue to burn on the running slag. As a result, the total amount of ash deposition will be much greater than that in dry-wall combustors and the total heat flux through the deposition surface will change greatly. Considering the limitations of existing simulation methods for slagging combustion, this paper introduces a new wall burning model and slag flow model from the analysis of particle deposition phenomena. Combined with a conventional combustion simulation program, the total computational frame is introduced. From comparisons of simulation results from several kinds of methods with experimental data, the conclusion is drawn that the conventional simulation methods are not very suitable for slagging combustion and the wall burning mechanism should be considered more thoroughly.     

Modeling the behavior of selenium in Pulverized-Coal Combustion systems

The behavior of Se during coal combustion is different from other trace metals because of the high degree of vaporization and high vapor pressures of the oxide (SeO₂) in coal flue gas. In a coal-fired boiler, these gaseous oxides are absorbed on the fly ash surface in the convective section by a chemical reaction. The composition of the fly ash (and of the parent coal) as well as the time-temperature history in the boiler therefore influences the formation of selenium compounds on the surface of the fly ash. A model was created for interactions between selenium and fly ash post-combustion. The reaction mechanism assumed that iron reacts with selenium at temperatures above 1200 °C and that calcium reacts with selenium at temperatures less than 800 °C. The model also included competing reactions of SO₂ with calcium and iron in the ash. Predicted selenium distributions in fly ash (concentration versus particle size) were compared against measurements from pilot-scale experiments for combustion of six coals, four bituminous and two low-rank coals. The model predicted the selenium distribution in the fly ash from the pilot-scale experiments reasonably well for six coals of different compositions.     

Fluidized-bed combustion: The Australian experience and its application to Asian and Pacific basin countries

Fluidized-bed combustion (FBC) is a promising method for the efficient combustion of coal. It is especially suited to those fuels having properties that present difficulties in conventional combustion systems such as high mineral matter content or low ash fusion temperature. Examples are given of pilot-plant trials to burn coal washery wastes and alkali-salt-bearing low-rank coals.     

影响煤焦比表面和孔隙特性因素的灰色关联分析

煤焦比表面积和孔隙特性直接决定着煤焦的反应性和煤焦的燃尽时间。影响煤焦比表面积和孔隙特性的因素很多,其关系也相当复杂。本文对实验测得的不同煤种煤焦的比表面积和平均孔径与各影响因素之间的关系进行了灰色关联分析。结果发现,水分、挥发分和矿物质对煤焦的比表面积影响较大,而镜质组含量、固定碳和灰分对煤焦的孔隙特性有较大影响。     

煤中FeS2和含铁粘土矿物在锅炉结渣过程中的作用

采用 57Fe Mossbauer谱学方法并辅以 X 射线衍射分析和灰成分分析等方法 ,对取自一台 2 2 0t h锅炉的平朔煤煤粉、飞灰和结渣沉积物中的含铁成分进行了分析、测定 ,以探讨煤中 Fe S2 和含铁粘土矿物在炉内的变化和在结渣形成过程中的作用。通过煤粉和飞灰中不同成分的含铁份额的比较 ,表明在炉内 Fe S2 最终转变成氧化物 ,而含铁粘土矿物最终转变成玻璃体。对内层沉积物成分的研究表明 ,Fe S2 的炉内产物在水冷壁上的选择性沉积是初始结渣形成的主要原因 ,而含铁硅酸盐矿物的产物对初始结渣的贡献极小 ,虽然平朔煤中上述两种成分的含铁份额相差无几。