新疆高钙煤混烧对灰中含钙矿物熔融特性影响

选用一种高钙和一种高硅铝新疆煤,在沉降炉中进行不同比例的混煤和单煤燃烧实验。采用计算机控制扫描电镜(CCSEM)分别对燃烧后总灰矿物成分和粒径分布进行分析。基于CCSEM分析获取单颗粒灰成分数据,采用热力学平衡方法对灰中矿物液相比例进行计算,分析混煤燃烧对灰中含钙矿物熔融特性影响。结果表明,煤中有机结合态Ca极易与煤中其他矿物元素发生交互反应,交互反应后含钙矿物种类取决于煤中内在矿种类。混煤燃烧会促进灰中含钙硅铝酸盐向含钙复杂硅铝酸盐转化,同时促进含钙矿物的熔融。在低温条件下,混烧煤灰中熔融含钙矿物粒径分布受碱金属粒径分布影响;但是高温条件下,混烧促进熔融含钙矿物向大粒径煤灰迁移。     

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.     

粒度对煤灰熔融特性的影响及作用机理初探

利用计算机控制扫描电镜(CCSEM)和5E-AFⅡ型智能灰熔点测试仪分别研究了A和B两种典型煤样的矿物组成及粒径分布和煤灰熔融温度。结果表明,煤灰熔融温度随粒径增大呈直线上升的趋势,当粒径大于100μm时,煤灰流动温度大于1 450℃。A、B煤中高岭石、石英、硅铝酸钾、蒙脱石矿物均以中小颗粒的形式存在,方解石分别以小颗粒、粗大颗粒的形式存在,铁氧化物则反之,且内在、外在矿物颗粒分布存在非均一性,这些是导致煤灰熔融特性产生重大变化的根本原因。     

O2/CO2燃烧对神华煤Ca和Fe交互反应影响

选用高Ca、高Fe含量的神华煤,在沉降炉系统中进行空气以及O₂/CO₂燃烧实验。利用X射线荧光探针(XRF)、X射线衍射仪(XRD)、先进计算机控制扫描电镜(CCSEM)分别对总灰元素、晶相组成以及主要矿物元素的共生特性进行深入表征。结果表明氧/燃料燃烧促进了Fe、Ca与其他矿物元素的交互反应,总灰中Fe在Fe-rich类矿物分布减少,而在铁铝硅酸盐(Fe-alsil),尤其是在Fe与Ca发生交互反应的Fe+Ca类矿物的分布增加,同时Ca也存在类似的分布规律。对总灰中Fe+Ca类矿物深入分析发现,氧/燃料燃烧条件下总灰中Fe+Ca类矿物结渣倾向更严重(Fe₂O₃/CaO摩尔比为0.5～3)。     

Effects of coal blending on the reduction of PM10 during high-temperature combustion 2. A coalescence-fragmentation model

A coalescence-fragmentation model has been developed to predict the behaviors of coal mineral particles during the combustion of pulverized bituminous coals or coal blends. Based on the computer-controlled scanning electron microscope (CCSEM) characterization of coal minerals, the particle size distributions (PSDs) and mineral species of ash particles can be simulated. In particular, the interactions among excluded minerals (mainly referring to the excluded Ca-bearing-species and Fe-bearing-species) and included minerals are accounted for in this model. The PSDs and the mineral species of ash particles are derived from the coalescence and fragmentation of coal mineral particles. Based on this proposed model, both of the predicted PSDs and the mineral species of ash particles are in good agreement with their corresponding experimentally measured values. And the comparisons further demonstrate that the combined effects of coalescence of included minerals and fragmentation of excluded minerals have direct impacts on the ash-forming process. In addition, for the coals rich in excluded Ca- and/or Fe-bearing-species, the interactions among included minerals and excluded minerals are another important mechanism governing ash formation for high-rank coals.     

不同炉型潞安煤灰理化及酸/碱溶解特性

对潞安煤经循环流化床、煤粉炉和气化炉处理后得到的煤灰进行了化学组成、矿物组成、特征基团、粒径分布、比表面积及微观形貌等理化性质的对比研究,并考察了其在酸浸和碱浸过程中Al^3+,Si^4+,Fe^3+,Ca^2+,K^+和Ti^4+等离子的溶解特性。结果表明:不同炉型潞安煤灰中铝、钙、硫等元素的含量有明显的差异;矿物组成包括晶相的鳞石英、方石英、莫来石、硬石膏及非晶相的偏高岭石、假莫来石、无定型二氧化硅等;相比较而言,气化炉灰中铝硅酸盐矿物结构更加不稳定。循环流化床灰颗粒呈具有一定孔洞结构的不规则状,而煤粉炉灰和气化炉灰均为光滑球形颗粒,循环流化床灰的粒径>煤粉炉灰的粒径>气化炉灰的粒径,循环流化床灰的比表面积>气化炉灰的比表面积>煤粉炉灰的比表面积。在HCl溶液中,Al^3+,Fe^3+,Ca^2+,K^+,Ti^4+的溶出率均较高;在NaOH溶液中,仅Si^4+和K^+的溶出率较高。不同炉型潞安煤灰中各元素的溶出率具有较大差异,主要与其矿物组成、结构稳定性、粒径和比表面积等相关。     

The roles of lime and iron oxide on the formation of ash and deposits in PF combustion

This paper presents the results of a study to assess the slagging propensities of a suite of UK, Spanish and South African coals, ranging from lignites to anthracites. Laboratory deposits were collected on ceramic deposition probes at gas temperatures of ∼1250°C, using an entrained flow reactor that simulates the time-temperature conditions experienced by pulverised coal particles in a large utility boiler. The degree of sintering and consolidation of the deposits would not have been predicted from bulk ash chemistry, indicating the importance of mineral matter distributions in the pulverised coal. Deposits with similar base to acid ratios and Fe₂O₃ contents displayed a range of slagging propensities on CCSEM analysis, consistent with the visual ranking. CCSEM analysis of the fly ashes collected from the combustion gases revealed a similar chemical composition to the coal ash and ash collected at the base of the EFR. CaO was observed to have readily assimilated into the aluminosilicate fly ash particles. On deposition, the CaO distribution largely remained unchanged. Fe₂O₃ was redistributed on forming a deposit possibly aided by CaO already dissolved in the aluminosilicates. The study provides an insight into the observations made by boiler operators burning coals with high CaO and Fe₂O₃ ashes.     

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.     

水煤浆气化炉内飞灰的形成机理

基于实验室规模的多喷嘴对置式水煤浆气化炉,利用SEM、马尔文激光粒度仪和XRD表征气化炉内飞灰的粒径分布和组成,并分析了气化炉内飞灰的形成机理。结果表明,喷嘴平面处飞灰与气化炉出口处飞灰的粒径分布及化学组成存在显著差异,不同气化阶段飞灰的形成机理也不同。气化燃烧阶段飞灰的形成机理为部分固定碳燃烧和外在矿物转化,而在焦炭气化反应阶段,飞灰的形成机理为焦炭破碎和内在矿物释放及转化。     

准东煤燃烧中矿物质转化行为的CCSEM研究

在沉降炉中进行了准东煤的燃烧实验,利用计算机控制扫描电镜技术(computer controlled scanning electron microscopy,CCSEM)研究了煤中矿物质的转化行为。研究表明煤中主要矿物为方解石、高岭石、含铁类物质以及未分类矿物,燃烧后灰中石英、铁的氧化物、白云石的含量急剧增加,未分类矿物和方解石的含量下降。同时对3种重要致渣元素Na、Fe、Ca在燃烧前后的矿物转化行为及颗粒粒径分布进行了详细研究。     

Coal and coal ash characteristics to understand mineral transformations and slag formation

A knowledge of the composition and structure of minerals in coal is necessary in order to understand the mineral transformations and agglomerate or slag formation during combustion or gasification. Coal ash fusibility characteristics are difficult to determine precisely, partly because the ash contains many components with different chemical behaviours, and may vary from coal source to coal source.The first objective of this study was to determine if the most relevant characteristics of coal were representative of the typical coal from the South African Highveld region. Secondly, a detailed understanding of the coal and coal ash are needed in order to explain slag formation and mineral transformations.Based on standard coal properties, such as the ash content, volatile content, carbon content and maceral composition, it can be concluded that the coal sample used for this study was representative and comparable with the coal from the Highveld region.From the results obtained and the analysis done on the coal samples, it was observed that the mineral grains showed a wide range of types that ranged from pure coal to pure minerals. The types of mineral particles within the coal range from large irregular minerals to small irregular minerals on the edge of coal particles. Kaolinite and quartz can occur as fine inclusions in carbon rich particles or associated with mudstone, siltstone or sandstone, together with kaolinite infillings. The main minerals present in the coal feed are kaolinite, quartz, dolomite, calcite, muscovite, pyrite and microline. An abundance of calcium-rich particles, which are probably calcite and dolomite, were observed. These minerals are present throughout the coal structure and are not specific to one type of mineral grain or structure. An increase in Si and Al abundance in three different prepared coal fractions with increasing particle size distribution was observed the high density fractions are mainly situated in the coarser particles.After combustion or gasification, the major source of glass is derived from included minerals in carbon rich particles. It is clear that focus on the modification of the unclassified/amorphous phase, to increase viscosity (decrease slag formation or have a higher concentration of crystalline phases) at a certain temperature, or in general terms the ash fusion temperature of the coal, is important. Altering the ash chemistry involves the addition of a material to the coal to increase the viscosity.     

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．     

CCSEM analysis of ash from combustion of coal added with limestone

Combustion of three Chinese coals, mixed with limestone physically, was carried out in drop tube furnace. The drop tube furnace consisted of two parts, the top side has a length of about 1.0 m and kept at 1573 K in all the runs, while the bottom-side has a length of 0.5 m and kept at 1173 K. SO₂ removal efficiency of about 80 and 73% were obtained in the combustion of Yanzhou with high and low sulfur, respectively. In contrast, for Datong coal, the De-S efficiency was only about 50% at the molar Ca/S ratio of 2.0; increasing Ca/S ratio to 3.0 had little effect on De-S efficiency. The combustion ashes were analyzed by several techniques including XRD, SEM-EDX and CCSEM (computer-controlled SEM). A novel calcium-based phase definition, based on CCSEM data was developed to investigate the modes of occurrence of added limestone in the ashes. Additionally, the mixture of limestone with kaolinite was injected into the furnace to study their transformation behavior under simulated coal combustion conditions. The governing mechanisms for limestone capturing sulfur and its reaction with the inherited minerals were correspondingly revealed. It was found that under the given coal combustion conditions, the calcium distribution in the ash varied with coal type and residence time. Briefly, more calcium was used for desulfurization or fixed into mineral; as time progressed, the inherited aluminosilicate, small sized excluded particles in the coal matrix, facilitated its reaction with limestone; it also reacted quickly compared to sulfation of limestone in coal combustion. This in turn hampered the efficient utilization of limestone in coal combustion.     

煤灰熔融黏温特性及对气流床气化的适应性

以21个中国典型煤样为研究对象,根据煤灰中CaO和Fe₂O₃含量,将之分为低钙低铁类、中钙中铁类、中钙高铁类、高钙低铁类、高钙高铁类等类别。利用高温黏度计测量煤灰熔渣黏温特性,并利用计算软件FactSage对煤灰熔融状态进行热力学平衡计算,研究了液相熔渣及固体矿物质结晶与熔渣黏度的关系,分析整理了煤灰最初硅铝比(SiO₂/Al₂O₃)、固体结晶物以及液相熔渣组成3个因素对煤灰熔融特性和熔渣黏温特性的影响,为根据煤灰组分分析来预测不同煤的熔渣黏温特性及对气流床气化的适应性提供了一个简单而实用的判断方法。对气流床气化液态排渣的适应性从高到低依次为:中钙高铁类、高钙低铁类、中钙中铁类、低钙低铁类和高钙高铁类。     

Transformation of mineral and emission of particulate matters during co-combustion of coal with sewage sludge

Co-combustion of coal with sewage sludge was carried out in laboratory-scaled drop tube furnace to understand the interaction between different fuels. The combustion conditions were selected as follows: the raw material feeding rate was 0.2-0.3 g/min, temperature was 1200 °C, the atmosphere of 10% O₂ and N₂ being balance was used to guarantee an air ratio of 1.5, and the residence time varied from 0.6 to 2.4 s. The coal/sewage sludge is kept at 50:50 (wt% to wt%), four fuel pairs were selected with respect to the mineral association within individual fuel. The results showed the obvious interaction between coal and sewage sludge during their co-combustion. For the carbon conversion, the devolatilization of mixing fuel occurred quickly; the combustion of both char and evolved volatile progressed almost completely. As a result, the unburnt carbon was almost zero in the fly ash. In addition, the evolution of both mineral and PM varied with the association of minerals in raw fuels. For both coal and sewage sludge rich in included minerals, they combusted separately in the furnace, less interaction occurred accordingly. Conversely, for both them rich in excluded minerals, the minerals reacted with each other to form much agglomeration, and therefore, the particle size of the fly ash was increased, while the amount of PM was decreased, which changed as the coarse fly ash particles. Finally, for the case of coal rich in excluded mineral and sludge rich in included mineral, their co-combustion led to the interaction of their minerals. As a result, more the fine particles were formed, which in part changed into PM. For the vaporized trace elements, they were adsorbed by the melt CaPO₄/Al-Si in the ash and accordingly, their contents in the particulate matter were reduced whereas their particle size distribution shifted to the large value.     

MgO含量对高钠煤灰熔融特性的影响

为研究MgO含量对高钠煤灰熔融特性的影响,配制了不同MgO含量的高钠合成灰并对灰熔融温度进行了测试。利用FactSage 7.0提供的热力学数据库建立了SiO₂-Al₂O₃-Fe₂O₃-CaO-MgO-Na₂O多元体系,模拟不同MgO含量的高钠合成灰的熔融过程。使用X射线衍射（XRD）和扫描电子显微镜（SEM）对合成灰的矿物质组成及微观形貌进行了研究。结果表明,随着MgO含量的增加,灰熔融温度先降低后升高。当MgO质量分数由0增加到5%时,高温下灰中生成大量低熔点的透辉石,透辉石会与霞石等矿物质形成低温共熔体,导致灰熔融温度降低。进一步增加MgO含量,高温下灰中生成镁黄长石、镁橄榄石和镁硅钙石等高熔点矿物质,使灰熔融温度升高。二元相图和似三元相图的结果表明,全液相温度随MgO含量的变化趋势与灰熔融温度相同。对本研究中的煤种,当MgO质量分数为30%时,可以有效提高灰熔融温度并抑制熔融液渣的生成。     

几种典型固废与神华煤掺烧的结渣特性

以三种典型工业有机固废和神华煤掺烧在实验室气氛炉中燃烧形成的混合灰样为研究对象,通过表观形貌分析、熔融性温度分析、X射线衍射图谱分析（XRD）和扫描电镜耦合X射线能谱分析（SEM+EDS）研究了在空气氛围下固废与煤耦合燃料的结渣特性和结渣机理。结果表明,与原煤灰相比,活性炭和药渣显著降低了混合灰的熔点,促进结渣;树脂提高了混合灰的熔点,不易渣化熔融。添加活性炭和药渣后,混合灰中产生了较多的霞石、钠/钾长石类等低熔点物质,容易结合成含钠钾硅酸盐的低温共熔体,并抑制了莫来石的生成。药渣中含有大量的磷酸铝、磷酸铁钙等含磷矿物质,易与含钙矿物质和赤铁矿形成无定形玻璃相的低温共熔体,相同条件下,添加药渣的结渣现象比活性炭更严重。添加树脂后的混合灰中生成了大量的莫来石等高熔点物质,与氧化铝、石英共同建构灰的“骨架”,保持较好的抗渣化特性。总体来讲,神华煤中掺烧20%的该树脂是可行的,针对活性炭,混烧比例不宜高于10%,而药渣则以低于5%为宜。     

Influences of minerals transformation on the reactivity of high temperature char gasification

Two Chinese coals were used in this study and coal chars were prepared at different temperatures. High temperature gasification of coal chars with CO₂ was investigated in a bench scale fixed-bed reactor and the transformations of minerals from these two coals were also studied from 1100 to 1500 °C. Mineral matters produced at different temperature and ash generated after gasification were collected and analyzed by XRD and FTIR. It was found that the iron oxides were only catalytic mineral matters existing at high temperature. And gasification behaviors above ash melting temperature were different for different mineral composition, especially the content and form of iron oxide, which not only accelerates the gasification reaction, but also reduces the influence caused by melting minerals.     

An investigation on the heterogeneous nature of mineral matters in Assam (India) coal by CCSEM technique

This is a very first preliminary investigation on the distribution of heterogeneous nature of mineral matter in one of the industrially important Assam (India) pulverized coal using computer-controlled scanning electron microscopy (CCSEM). The results show that clay minerals, quartz, pyrite, and pyrrhotite form the bulk of the mineral matter. Minor minerals, such as calcite, dolomite, ankerite, barite, oxidized pyrrhotite, and gypsum, are also observed in the sample. The particle size distribution (PSD) of the included minerals is generally observed to be finer than that of the excluded ones in the coal. As a consequence, the coal rich in included minerals has more small mineral particles, which may affect its reactivity. Regarding the association of individual mineral species, the proportion of included to excluded is found to be higher in major cases. With regard to the modes of occurrence of major inorganic elements, it is found that Si mostly occurs as quartz and clay minerals, while Al mostly occurs as silicate minerals. Fe is primarily present as iron sulfides, iron oxide, and Fe-Al-silicate. S is partitioned into iron sulfides and gypsum. Most Ca occurs as carbonates and gypsum, with a minor fraction associated with clay minerals. Mg is mainly present as dolomite and clay minerals, with a very minor fraction present as ankerite. The majority of alkali elements are associated with aluminosilicates. P is mostly associated with kaolinite and/or present as more complex compounds containing Al, Si, and other elements as apatite is found to be absent in the coal studied. Ti is mainly present as rutile and kaolinite.     

Shell粉煤气化炉大渣块形成机理探讨

为了弄清实际生产过程中Shell粉煤气化炉大块渣的形成机理,文章运用XRD、SEM-EDX技术,对比分析气化炉稳定运行期间排出的灰渣和堵渣停车期间采集的大块渣的矿物组成、表观形貌及微区化学组成。结果表明:在Shell气化炉内大块渣形成期间,煤中含铁和含钙矿物存在明显的富集和析晶现象,在高温还原气氛下含钙矿物与石英形成大量低温共熔物—钙长石,钙长石在熔融状态下易吸附小块渣样,从而使渣块增长变大;含铁矿物质在气化炉里逐渐演变成(Fe-S-O)共熔体,这种物质具有很强的吸附粘结性,从而使渣块迅速增长变大。