我国发电用煤的碱性成分在煤灰固硫中的作用

在煤燃烧过程中，煤灰自身有一定的固硫能力。本文根据我国发电用煤的煤质分析数据，将煤灰固硫率与煤中硫和碱性成分含量相关联，研究了实验室制灰和煤粉燃烧时煤中碱性成分的固硫作用，表明：实验室制灰条件下，钙在煤灰固硫中起主导作用，固硫率随煤中Ｃａ／Ｓ摩尔比的增加呈指数趋势增加，而其它成分（Ｎａ，Ｋ，Ｍｇ等）的作用不大；因钙的固硫作用类似于ＣａＯ吸收剂，实验室制灰时的固硫特性可应用于流化床燃烧；与此相比，煤粉炉中因温度高、停留时间短，钙的固硫作用显著降低，而其它碱性成分的贡献明显增大。     

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.     

Emission of trace toxic metals during pulverized fuel combustion of Czech coals

A study of the trace elements emission (As, Se, Cd, Co, Cr, Cu, Zn, Hg, Tl, Pb, Ni, Sn, Sb, V, Mn and Fe) from pulverized coal combustion has been made at six heating and power stations situated in the Czech Republic. The amount of chlorine in coal has considerable influence on volatilization of some elements such as Zn, Cu, Pb, Hg and Tl, which is explained by the formation of thermodynamically stable compounds of these elements with chlorine. Generally, the affinities for Cl follows the order Tl > Cu > Zn > Pb > Co > Mn > Sn > Hg. The experimental data indicates enrichment of some of the trace toxic elements in the emissions (Cu, Zn, As, Se, Cd, Sn, Sb, Hg and Pb) and good agreement was obtained by thermodynamic equilibrium calculations with a few exceptions. In the case of Fe, Mn, Co, Cr and Sn calculated values are overestimated in the bottom ash and there are zero predicted amounts of these elements in the fly ash. In comparison, the results from experiments show up to 80% of these elements retained in fly ash. This implies that there exist additional steps leading to the enrichment by Fe, Mn, Co, Cr and Sn of small particles. Such mechanisms could include the ejection during devolatilization of small inorganic particles from the coal of bottom ash particles, or disintegration of the char containing these metals to small particles of fly ash. On the other hand, there are slightly overestimated or similar values of relative enrichment factors for As, V, Cu, Cd, Sb, Tl and Pb in the fly ashes and zero predicted values for bottom ashes. Our experimental results show about 5% or less of these elements are retained in bottom ashes, so they probably remain in the bottom ash inside unburned parts of coal. Copyright © 2003 John Wiley & Sons, Ltd.     

Mineral transformation during combustion of coal blends

Mineral behaviour for two individual coals (I, J) and their two‐component coal blends and 800°C ash blends heating were studied. Ash samples were heated progressively from 800°C to IT (initial deformation temperature) at 100°C intervals under different conditions. Coal samples were heated from room temperature to the corresponding temperature. Mineral transformation at each temperature was determined by X‐ray diffraction and SEM measurements. The results show that Si, Al, Fe and Ca compounds have a great form variation during heating. Their forms at different temperatures depend on the chemical composition of the ash, the blending ratio and the atmosphere. For different coal ashes, the main mineral matters at 800°C were quartz, anhydrite, hematite, calcite and feldspar. As the temperature increased, oxidation, thermal decomposition, transformation and reaction occurred between the components. Comparing a 40% I+60% J ash blend with individual ashes, fayalite was formed at 1100°C for the blend; the reaction product existed in a glassy phase at 1300°C. For a coal blend having the same ash ratio as the ash blend, FeO reacted with amorphous SiO₂ or Al₂O₃ to form fayalite and hercynite at 1000°C. As the temperature increased to 1100°C, fayalite and hercynite increased obviously. At 1200°C, some iron inclusion compounds melted to become glassy phase matter. Compared with the ash blend, iron species undergo a different change during coal blend heating: fayalite and hercynite formed earlier, iron compounds melted to form a glassy phase at lower temperature. This may be caused by early combustion of the more reactive coal (J coal) in the blend inducing local variation in oxygen concentration gradients around the less reactive coal and consequently affecting the reaction atmosphere and Fe mineral behaviour and interaction. That is to say, for coal blends, the mineral transformation was affected by both the mineral species interaction and the combustion behaviour. The calculations were performed to examine the fate of mineral matter under different combustion conditions using a thermodynamic chemical equilibrium calculation program. Calculations from coal blends were comparable with experiments from ash blends, this is because the calculation program only considers the interaction among the mineral species but does not consider the combustion reaction. It indicates that combustion and the relative volatiles also affected the mineral behaviour and slagging during coal blend combustion. Meanwhile, the mineral species evaporations were measured at high temperature: the main evaporated species were Na, K pure species and compounds, Fe, FeO, SiO and SiO₂. The evaporation of Fe has an important effect on initial deposition. Calculations were comparable with the experiments. Copyright © 1999 John Wiley & Sons, Ltd.     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

Effects of ash fouling on heat transfer during combustion of cattle biomass in a small‐scale boiler burner facility under unsteady transition conditions

Combustion of cattle biomass (CB) as a supplementary fuel has been proposed for reducing emissions of NO_x, Hg, SO₂, and nonrenewable CO₂ in large coal‐fired power plants; however, its high ash content resulted in fouling and slagging problems when the CB was co‐fired with coals during small‐ and pilot‐scale tests. Ash depositions during combustion of the CB as a reburn fuel were investigated using a 30 kW_t (100 000 Btu h^?1) boiler burner facility with water‐cooled heat exchangers (HEXs) under unsteady transition conditions and short‐term operations. Two parameters were used to characterize the effects of the ash deposition: (1) Overall heat transfer coefficient (U) and (2) Burnt fraction (BF). A methodology was presented and empirically demonstrated for the effects of ash deposition on heat transfer under unsteady transition conditions. Experiments involving ash deposition during reburning the CB with coals were compared with experiments involving only ash‐less natural gas. It was found that the growth of the ash layer during reburning the CB and coals lowered the heat transfer rate to water in the HEXs. In low‐temperature regions, the thin layer of the ash deposition promoted radiation effects, while the thick layer of the ash deposition promoted the thermal resistance of the ash layer. A chemical analysis of the heavy ash indicated that the BF increased when a larger fraction of the CB was used in the reburn fuels, indicating better performance compared with coal combustion alone. However, the results of ash fusion temperature indicated the ash deposited during combustion of the CB and coals was more difficult to remove than the ash deposited during coal combustion alone. Copyright © 2010 John Wiley & Sons, Ltd.     

生物质与煤混燃灰熔融结渣特性的实验研究

通过对生物质燃料（锯末、玉米秸和麦秸）与煤混燃灰化学成分和熔融温度的测定,利用灰分的碱酸比B／A、硅比G、硅铝比S／A、积灰沾污特性指数Hw、磨损特性指数日。等判别指数对生物质纯燃、与煤混燃时的结渣、积灰和磨损特性进行了研究和分析。结果表明,生物质灰都具有结渣倾向,麦秸灰具有严重的积灰倾向,玉米秸灰和锯末灰有易积灰倾向。生物质灰的磨损倾向都较轻微。随着生物质与煤混燃比例的增加,结渣有加重趋势。灰中酸性氧化物和碱性氧化物的含量会直接影响灰的熔融温度。     

Investigation on ash deposition characteristics during Zhundong coal combustion

The reserves of Zhundong (ZD) coal in China are huge. However, the high content of Na and Ca induces serious slagging and fouling problems. In this study, the ZD coal was burned in a DTF (drop tube furnace), and the ashes collected at different gas temperature with non-cooling probe were analyzed to obtain the ash particle properties and their combination mode. The results showed that Na, Ca and Fe are the main elements leading to slagging when the gas temperature is about 1000 °C during ZD coal combustion, but their mechanisms are quite different. Some sodium silicates and aluminosilicates and calcium sulfate keep molten state in the ashes collected at 1000 °C. These molten ash particles may impact and adhere on the bare tube surface, and then solidified quickly. With the growth of slag thickness, the depositing surface temperature is increased. The molten ash particles might form a layer of molten film, which could capture the other high fusion temperature particles. The Fe₂O₃ sphere were captured by the formed molten slag and then they blended together to form a new molten slag with lower melting temperature.     

A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags

Gasification or combustion of coal and biomass is the most important form of power generation today. However, the use of coal/biomass at high temperatures has an inherent problem related to the ash generated. The formation of ash leads to a problematic phenomenon called slagging. Slagging is the accumulation of molten ash on the walls of the furnace, gasifier, or boiler and is detrimental as it reduces the heat transfer rate, and the combustion/gasification rate of unburnt carbon, causes mechanical failure, high-temperature corrosion and on occasions, superheater explosions. To improve the gasifier/combustor facility, it is very important to understand the key ash properties, slag characteristics, viscosity and critical viscosity temperature. This paper reviews the content, compositions, and melting characteristics of ashes in differently ranked coal and biomass, and discusses the formation mechanism, characteristics, and structure of slag. In particular, this paper focuses on low-rank coal and biomass that have been receiving increased attention recently. Besides, it reviews the available methodologies and formulae for slag viscosity measurement/prediction and summarizes the current limitations and potential applications. Moreover, it discusses the slagging behavior of different ranks of coal and biomass by examining the applicability of the current viscosity measurement methods to these fuels, and the viscosity prediction models and factors that affect the slag viscosity. This review shows that the existing viscosity models and slagging indices can only satisfactorily predict the viscosity and slagging propensity of high-rank coals but cannot predict the slagging propensity and slag viscosity of low-rank coal, and especially biomass ashes, even if they are limited to a particular composition only. Thus, there is a critical need for the development of an index, or a model or even a measurement method, which can predict/measure the slagging propensity and slag viscosity correctly for all low-rank coal and biomass ashes.     

混煤煤质及燃烧特性研究

针对混煤的煤质特性和燃烧特性开展实验研究,以指导燃煤电站科学合理的燃用混煤。研究结果表明,混煤的元素分析、工业分析及发热量满足质量加权平均,但混煤的可磨性和灰熔融特性不满足加权平均,低灰熔点煤中掺烧高灰熔点煤能显著提高混煤灰熔点,改善锅炉燃烧过程中的结渣问题,混煤灰熔点变化受到单煤灰成分的影响。热重实验分析表明,混煤的剧烈燃烧阶段与单煤存在明显差异,混煤的燃烧特性介于参与掺混的单煤之间,但不满足线性叠加,其燃烧过程存在着不同程度的交互作用。混煤的着火特性接近于易燃煤,而燃尽特性与难燃煤相近。除此以外,随着氧浓度的降低,混煤的燃烧特性明显变差,易燃煤对氧浓度的变化更加敏感。     

Comparative fixed/fluidized bed experiments for the thermal behaviour and environmental impact of olive kernel ash

Olive kernel can play an important role as a fuel for heat and power production in the island of Crete, substituting a large part of conventional fuels. However, combustion of this biofuel may create operational and environmental problems related to its inorganic constituents. Thus, the thermal behaviour of the ashes in terms of slagging and fouling propensities and their environmental impact upon disposal to local soils were investigated, through lab-scale fixed/fluidized bed combustion tests. Bottom and fly ashes were characterized by mineralogical, chemical, morphological and fusibility analyses, as well as standard leaching tests and the results under the different combustion configurations were compared.Olive kernel ash was rich in Ca, Si and P minerals and contained substantial amounts of alkali. Under the conditions of the combustion tests, no signs of bed agglomeration or ash deposition were noticed; however, when combustion takes place in a fixed bed this should be operated below 1100 °C, to avoid ash melting and the companion problems. Trace elements showed little preference for the fly ash. The elements Cr, Cu, Ni and Mn were enriched in fixed bed ash. Toxic metal ions were released in low quantities in the soil, below the legislative limit values, with the exception of Cr. The low leachability of trace elements from the fixed bed ash was attributed to the alkaline nature of the ashes, the mineralogy, the chemistry and the buffering capacity of the soil. The high extraction rates of Mn, Zn and Cr, from the fly ash, suggest that these elements were associated with carbonates, sulfides, sulfates or organic matter.     

The effect of air staged combustion on NOx emissions in dried lignite combustion

Experiments were carried out in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of BRXL lignite and its dried coals. The impact of moisture content, multiple air staging, pulverized coal fineness and burnout air position on NOx emissions under deep, middle and shallow air-staged combustion conditions. Moreover, the impact of blending coals on NOx emissions was investigated in this paper. The unburned carbon concentration in fly ash was also tested. Experimental results based on the combustion of BRXL lignite and its dried coals show that NOx emissions can be reduced drastically by air-staged combustion. NOx emissions reduce with the increase of the air that is staged and the distance between the burner and burnout air position. Dried coal of BRXL lignite emits a smaller amount of NOx than that of BRXL lignite. However, the dried degree of BRXL lignite is closely related to R₉₀ fineness. Dried coal with optimal moisture content yields least NOx emissions. When deep or middle staged combustion was adopted, the application of multi-staged combustion is conducive to NOx reduction. However, when shallow staged combustion was adopted, NOx emissions are higher in multi-staged combustion than that in single-staged combustion with M_S = 0.54. Thus, the existence of a certain concentration of O₂ in reduction zone would significantly reduce NOx emissions. The blending coals that dried coals of BRXL lignite were blended with bituminous coals emit a larger amount of NOx than that of the dried coal alone. NOx emissions decrease with the increase of the proportion of dried coal in the blending coal. Moreover, the unburned carbon concentration in fly ash of dried coal in staged combustion is lower than that of BRXL lignite in staged combustion. On the whole, the dried coal of BRXL lignite is conducive to NOx reduction in staged combustion.     

Impact of heavy metals in Indian fly ashes on its application as soil ameliorant

Coal contains various organic and inorganic substances including trace quantities of the heavy metals. Therefore, the combustion of coals releases some of the ultimately to the environment of some heavy metals elements in the form of their oxides and in a redistribution of these heavy metals in the surface soil and water bodies, particularly in the vicinity of coal-fired power plants. The fly ash and pond ash of different thermal power stations of India were mixed with soil at different doses separately to compare their impact on crop produce and soil. The present study deals with the presence of the heavy metals in the coal ash and its movement through the amendment of coal ash to the soil and crop produce.     

分级燃烧最佳一次风空气系数的实研究

在一维煤粉燃烧炉上进行了不同煤种、不同细度的分级燃烧试验。实验发现,分级燃烧对高挥发份 煤种以及同一煤种的细煤粉的Nox排放浓度的降低效果更显著,而且在分级燃烧条件下,同一煤种细煤粉的 飞灰含量较粗煤粉低。另外,还得到了不同煤种在分级燃烧条件下的最佳一次风空气系数。图9表2参3     

不同煤种下循环流化床灰渣特性的试验研究

在一台 0 .5MW的循环流化床燃烧炉上对 4种不同煤种分别进行燃烧试验 ,对燃烧产生的灰渣的分析结果表明了煤种特性如挥发分、灰分和含碳量等对循环流化床燃烧过程的灰渣形成及其排放特性有很大影响 ,并获得了煤中挥发分、灰分及含碳量对底渣粒径及其含碳量、飞灰粒径及其含碳量、飞灰份额及燃烧效率等影响特性 ,对循环流化床锅炉的设计和运行有一定的指导意义。     

Emissions of organic hazardous air pollutants during Chinese coal combustion

This paper provides a general investigation of the emissions of organic hazardous air pollutants (HAPs) during the combustion of several typical Chinese coals. First, the distribution of four types of HAP, i.e., aliphatics, cyclic hydrocarbons, monoaromatic compounds and PAHs, in the CH₂Cl₂ extracts of six Chinese coals were studied and the influences of the extractive times and coal varieties were also evaluated. Second, the partitioning of these HAPs in the flue gas during coal combustion in a small-scale reactor were investigated, depending on oven temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) and coal varieties. The behaviors of HAP in the combustion flue gas were compared with those in the CH₂Cl₂ extracts. Finally, combustion was conducted at given conditions in two laboratory-scale reactors: a fluidized bed and a fixed bed. Two coals (Shengmu bituminous coal and Xunhuan anthracite coal) and one coke were considered in this case. The HAP partitioning both in flue gases and in ashes were evaluated and compared between the two combustors.     

Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace

Zhundong coalfield is one super-large coalfield recently discovered in China. However, the utilization of Zhundong coal in power plants has caused serious ash-related issues mainly due to its high-sodium feature. The ash deposition problem on convection heat exchanger surfaces is still particularly difficult to resolve and its mechanism has yet to be fully understood. This study deals with the ash deposition and alkali metal migration behaviors on convection heat exchanger surfaces between 400 and 800 °C during combustion of Zhundong coal using a lab-scale drop tube reactor. Experimental results show that the sodium content in ash deposit of Zhundong coals increases obviously as the deposition temperature decreases from 800 to 600 °C, while it is almost unchanged below 600 °C. The contents of iron and calcium in ash deposits exhibit nonmonotonic variations as the deposit probe temperature varies between 400 and 800 °C. Quartz and calcium sulfate are main crystalline phases in ash deposit of Zhundong coals. Calcium is inclined to present as calcite and lime at low deposition temperature, while high temperature facilitates calcium sulfation. Sodium of crystalline phase is found as albite and sodium sulfate at low deposition temperature. Both condensation of gaseous alkali metals and formation of low-melting minerals were responsible for the ash deposition phenomenon on convection heat exchanger surfaces involved in combustion of Zhundong coal. The sodium content in ash deposit decreases considerably with the increasing combustion temperature while the case of iron variation is opposite due to its low-volatility. In addition, the Na content in ash deposits increases obviously with the access air ratio reduced from 1.2 to 1.05, but the local weakly reducing atmosphere leads to less iron within ash deposits. Clarification of sodium migration and evaluation of ash deposition behaviors during combustion of Zhundong coal is helpful for a better exploration of the functional mechanism of ash deposit and then large-scale utilization of high-sodium coals.     

循环流化床锅炉的飞灰含碳量问题

循环流化床锅炉的飞灰含碳量问题近年来受到关注。对实际运行的多台燃烧各种燃料的220t／h锅炉的飞灰样品测定表明：飞灰的含碳量具有明显的不均匀性。分析了煤质、分离器及运行条件对飞灰含碳量的影响。结果表明：循环流化床锅炉燃烧过程中焦炭反应性逐渐下降;焦炭燃烧过程中发生的爆裂、磨损、失活等行为与煤种有关,对循环流化床锅炉飞灰碳燃尽有很大影响。气固混和不均匀是导致较高的飞灰含碳量的原因之一。图7表2参13     

Concentration and distribution of uranium in Chinese coals

Representative coal uranium concentrations are essential not only to better estimate the environmental and health effects associated with the uranium emissions from coal utilization but also to utilize the potentially valuable uranium in coal combustion by products. The average uranium concentrations of coals in China are estimated based on uranium analyses of 1535 coal samples and coal reserves tonnage. The results showed that the estimated average weighted uranium concentrations of Late Carboniferous and Early Permian (C₂–P₁), Late Permian (P₂), Late Triassic (T₃), Early and Middle Jurassic (J₁₋₂), Late Jurassic and Early Cretaceous (J₃–K₁), and Eogene and Neogene (E–N) coals are 2.91, 5.43, 3.67, 1.18, 1.84, and 3.92 μg/g, respectively. The overall average weighted uranium concentration of coals in China is 2.31 μg/g. The background values of uranium in coals of China were dominated by detrital materials of terrigenous origin, but the anomalous enrichments of uranium were attributed to synsedimentary volcanic ashes and epigenetic low-temperature hydrothermal fluids and magmatic hydrothermal inputs.     

Preliminary study of trace element emissions and control during coal combustion

Hazardous trace element emissions have caused serious harm to human health in China. Several typical high-toxic trace element coals were collected from different districts and were used to investigate the emission characteristics of toxic trace elements (As, Se, Cr, Hg) and to explore preliminary control methods. Coal combustion tests were conducted in several bench-scale furnaces including drop tube furnace (DTF), circulating fluidized bed (CFB) combustion furnace, and fixed-bed combustion furnace. Calcium oxide was used to control the emission of arsenic and selenium. The granular activated carbons (AC) and activated-carbon fibers (ACF) were used to remove mercury in the flue gas from coal combustion. The chemical composition and trace element contents of ash and particulate matter (PM) were determined by X-ray fluorescence (XRF) spectrometry and inductively coupled plasma-atomic emission spectrometry (ICP-AES), respectively. The speciation and concentration of mercury were investigated using the Ontario-Hydro method. X-ray diffraction spectrometry (XRD) was used to determine the mineral composition of production during combustion experiments. With the addition of a calcium-based sorbent, arsenic concentration in PM₁ sharply decreased from 0.25–0.11 mg/m³. In fixed-bed combustion of coal, the retention rates of selenium volatiles were between 11.6% and 50.7% using lime. In the circulating fluidized-bed combustion of coal, the content of selenium in ash from the chimney was reduced to one-fourth of its original value and that in leaching water from the chimney decreased by two orders of magnitude using lime. Calcium-based sorbent is an effective additive to control the emission of As and Se during coal combustion. The emission of chromium is influenced by the occurrence mode of Cr in coal. Chromium emission in PM_2.5 during coal combustion is 55.5 and 34.7 μg/m³ for Shenbei coal and mixed Pingdingshan coal, respectively. The adsorptive capacity of granular activated carbon for Hg⁰ is significantly enhanced through ZnCl₂-impregnation. The activated carbon fibers showed decent efficiency in mercury adsorption, on which surface oxygen complex showed positive effects on mercury adsorption.