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.     

Mineral matter-organic matter association characterisation by QEMSCAN and applications in coal utilisation

The association of mineral matter with organic matter is extremely important for coal utilization process such as pf coal combustion. With the development of advanced analytical instruments such as QEMSCAN, it is now possible to measure directly the mineral matter-organic matter association on a particle-by-particle basis. The mineral matter and mineral-organic associations of a suite of fourteen CCSD coal bank coals (as pf) have been determined by QEMSCAN. An interface program was developed to make QEMSCAN data compatible with the CCSEM-based ash formation model developed previously in CCSD. Size and chemistry of flyash was predicted by a partial coalescence sub-model for included mineral grains, and a fragmentation sub-model for excluded mineral grains, respectively. The size and chemistry of predicted flyash was estimated on a particle-by-particle basis, and was used to rank the ash effect on heat transfer reduction for all the CCSD coals using the CCSEM-based model, in which coal property, furnace geometry and operational conditions have been taken into account. Other applications and further developments of the technique are also outlined.     

燃煤过程矿物质形态对亚微米颗粒释放的影响

为研究燃煤过程中亚微米颗粒的形成机理,选取了三种烟煤在沉降炉内燃烧,用X射线荧光光谱和透射电子显微镜能量色散谱仪联用对煤和亚微米灰中的矿物质进行分析,研究内生态和游离态的矿物质的转化过程.结果表明,与碳结构的结合方式决定了矿物质向亚微米颗粒的转化.游离态矿物质很少转化成亚微米灰颗粒,但其组成却影响着Si和Al的转化.     

The relationship between excluded mineral matter and the abrasion index of a coal

Predictions of the wear rates of components in grinding mills at pulverised coal-fired power stations are currently made using empirical relationships based on the ash content of the coals. However, modern coal characterisation techniques now allow the mineral inclusions in a coal that are responsible for the abrasive nature of the coal to be accurately characterised. Hence, there is scope to make improved predictions of wear based on a detailed knowledge of the mineral matter in a particular coal. It is first necessary, however, to understand the nature of the minerals and properties of the minerals in a coal that would contribute to abrasive wear. In this study known quantities of quartz, pyrite and slate have been added to a washed coal and the Abrasion Indices of the coal/mineral mixtures have been measured. The results show how the size, shape and hardness of excluded mineral matter contribute to the abrasive properties of a coal.     

Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral properties

Four pulverized coals were subjected to combustion in a laboratory-scale drop tube furnace to investigate the emission of suspended particulate matter smaller than 10 μm (PM₁₀) and to study the correlation of PM₁₀ emission with mineral properties of the coals. Combustion conditions of 1200 °C, 2.4 s and 20% atmospheric oxygen content were used and all the carbon was consumed under given conditions. The properties of PM₁₀ were studied including its concentration, particle size distribution and elemental composition. Two typical sizes were also subjected to Computer controlled scanning electron microscopy (CCSEM) analysis for determination of chemical species within them. To investigate the influence of coal mineral properties, the metallic elements in the raw coals were divided into three parts: organically bound, included inorganic particles and excluded ones. The results indicated that during coal combustion, about 0.5-2.5 wt% of inherent minerals changed into the suspended PM₁₀. With an increase in the coal ash content, the concentration of PM₁₀ increased proportionally. The resulting PM₁₀ had a bimodal size distribution with two peaks around 2.5 and 0.06 μm, respectively. SiO₂ and Al₂O₃ dominated the large mode around 2.5 μm, which is formed by the direct transformation of inherent minerals. On the other hand, SO₃ and P₂O₅ were prevalent in the small mode around 0.06 μm, which is formed by vaporization of these two elements. For other metals found in PM₁₀, the refractory metals were enriched in the large mode, with concentrations proportional to their content in the excluded minerals in the raw coal. Volatile metals were however enriched in the small mode since, they react with gaseous SO₂ and P₂O₅ to form sulfates and phosphates in the solid phase. The study showed that experimental observations agree with thermodynamic equilibrium considerations.     

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．     

Mechanisms of fine particulates formation with alkali metal compounds during coal combustion

Two types of coals with different sodium (Na) and potassium (K) concentrations were burned in an electrically heated drop tube furnace, to study the formation of particulates in association to alkali metal compounds from the coals. The particulates formed from these coals during combustion were separated by a low pressure impactor (LPI). The particulates collected in each stage of the LPI were analyzed, using an atomic adsorption spectrometer (AAS). The results obtained show that ash particles have bimodal particle size distribution for both coal types. This tendency could be due to the difference of the inherent minerals in raw coals. It was further observed that Na and K were enriched in the fine particulates for both types of coal. There was a dependence of the Na enrichment on the fine particles on concentration of excluded mineral in the raw coals. This work, therefore, showed that the appearance of alkali metal in the ash particles related to the nature of Na and K concentrations in raw coals.     

Trace elements in coal: Associations with coal and minerals and their behavior during coal utilization - A review

Trace elements such as mercury, arsenic and selenium present in coal are known to be of concern for public health. Coal-fired power plants have resulted in emission of several tons of TEs in environment. These elements mostly evaporate during combustion and condense either homogeneously as sub-micron ash or heterogeneously onto already existing fine ash. The coal-mineral and mineral-mineral associations play an important role in the formation of fine particles and in subsequent condensation of trace elements in various phases. Any retention of these elements in fly ash particles is strongly influenced by their association with other minerals in individual coal and mineral grains. Clean coal technology development is, therefore, a priority area for research and needs continuous improvements in increased efficiency and decreased pollutant emission. The paper will include trace elements in different coals from around the world. It will consider different modes of occurrences present in coals, the ash formation and evaporation of trace elements and emissions. The typical emissions from typical power stations will be presented. The paper will also review different approaches adopted in estimating the deportment of these elements. The paper at the end would discuss control strategies for reducing emissions and future directions.     

Behaviour of coal mineral matter in sintering and slagging of ash during the gasification process

The mineral matter in typical feed coals used in South African gasification processes and the ash derived from gasifying such coals have been investigated using a variety of mineralogical, chemical and electron microscope techniques. The mineral matter in the feed coals consists mainly of kaolinite, with minor proportions of quartz, illite, dolomite, calcite and pyrite plus traces of rutile and phosphate minerals. The calcite and dolomite occur in veins within the vitrinite macerals, and are concentrated in the floats fraction after density separation. Some Ca and Ti also appear to be present as inorganic elements associated with the organic matter.Electron microscope studies show that the gasification ash is typically made up of partly altered fragments of non-coal rock, bonded together by a slag-like material containing anorthite and mullite crystals and iron oxide particles, with interstitial vesicular glass of calcic to iron-rich composition. Ash formation and characteristics thus appear to be controlled by reactions at the particle scale, allowing the different types of particles within the feed coal to interact with each other in a manner controlled mainly by the modes of mineral occurrence. Integration of such techniques provides an improved basis for evaluating ash-forming processes, based on quantitative phase identification, bulk and particle chemistry, and the geometric forms in which the different phases occur.     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

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.     

Analysis of sub-micron mineral matter in coal via scanning transmission electron microscopy

The fine-scale mineral matter in three US coals has been analysed via scanning transmission electron microscopy (STEM). The samples observed were a North Dakota lignite, a Kentucky bituminous, and a Pennsylvania semi-anthracite. Specific mineral types, differing among the three coals examined, appear to predominate at this fine size scale (particles ? 200 nm in diameter). Fe-rich and Ba-rich minerals in the lignite, a Ti-rich mineral in the bituminous and Ca-rich and Ti-rich minerals in the semianthracite were the predominant species found. The inherent mineral content in the observed organic background also differed from coal to coal. The distributions of mineral species in the size range ? 200 nm reported herein do not reflect the distributions in the larger size ranges obtained by more macroscopic techniques.     

Removal of mineral matter from some Australian coals by Ca(OH)2/HCl leaching

Four Australian coals, Blair Athol, Ebenezer, Newlands and Warkworth coals, were demineralized by Ca(OH)₂ digestion at 300°C, followed by washing with dilute HCl. Ash contents decreased from 8.8–15.4% to 1% for Blair Athol, Newlands and Warkworth coals, and from 14.9% to 2.8% for Ebenezer coal. The CaO/ash ratio is an important factor affecting coal demineralization; the maximum ash removals were achieved at CaO/ash ratios in the range 0.6–1 g/g for all the coals. The major minerals in the original coals were kaolinite and quartz, with some montmorillonite and carbonates. Kaolinite and quartz hydrothermally reacted with Ca(OH)₂ to form calcium-bearing hydrated silicates and aluminosilicates, such as tobermorite and hibschites, that could be dissolved in acid. With increasing CaO/ash ratio in excess of the optimum value, the removal of quartz significantly decreased; there also remains some calcium in treated coal, depending on digestion conditions and coal type. A lower ash removal from Ebenezer is due to a lower quartz removal and more remaining calcium.     

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。     

Effects of Xinjiang high calcium coal co-firing on melting characteristics of Ca-bearing minerals in ash

In this study, a high-calcium coal, a high-silicon-aluminum Xinjiang coal and their blends were burnt in a drop tube furnace. The computer-controlled scanning electron microscope (CCSEM) was used to analyze the total ash mineral composition and particle size distribution after combustion. Based on CCSEM analysis, the composition data of single particle ash was obtained. The thermodynamic equilibrium method was used to calculate the liquid phase ratio of minerals in the ash, and the effect of coal blending on the melting characteristics of calcium-containing minerals in the ash was analyzed. The results show that the organically bound Ca easily interacts with other minerals in the coal. The mineral species of Ca-bearing minerals in the bulk ash mainly depend on the included minerals in coal. Co-firing will promote the conversion of calcium-containing aluminosilicate in the ash to calcium-containing complex aluminosilicate, and at the same time promote the melting of calcium-containing minerals. Under low temperature conditions, the particle size distribution of molten calcium-containing minerals in co-fired coal ash is affected by the particle size distribution of the alkali metal; however, under high temperature conditions, co-firing promotes the migration of molten calcium-containing minerals to large particle size ash.     

Transformation of alkali and alkaline earth metals in low rank coal during gasification

Transformation of alkali and alkaline earth metals (AAEM) in low rank coals during gasification was examined by combining computer-controlled scanning electron microscopy (CCSEM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Two sub-bituminous coals were pyrolyzed at 1500 °C using a drop tube furnace, and the resultant chars were then gasified in CO₂ atmosphere at the same temperature. Total amounts of AAEM species in the raw coals and the chars were determined by ICP-AES. Minerals in the raw coals and ash particles in the chars were analyzed by CCSEM.AAEM species were mainly present in the raw coals as dispersed species, organically associated cations or fine mineral particles (<1 μm), which cannot be quantified by CCSEM. It was found that the dispersed Ca species were first converted into fine ash particles upon the devolatilization and then most of the particles interacted with inherent clay minerals to form complex aluminosilicates. In the case of Na and K, the dispersed species mostly vaporized and the interaction with inherent minerals was not observed.     

Quantitative X-ray powder diffraction analysis of clay minerals in Australian coals using Rietveld methods

The mineralogy of clay-rich mineral matter isolated from a range of Australian bituminous coals has been evaluated in quantitative terms from X-ray powder diffraction (XRD) patterns using a Rietveld-based data processing technique. The chemical composition of coal ash derived from this mineral matter has been calculated and compared to the directly determined composition of the ash prepared from the same coal samples. Although there are some minor differences due in part to uncertainty regarding the actual composition of several minerals, the compositions indicated by the two methods show a relatively high correlation, suggesting that the Rietveld technique provides mineralogical analyses that are consistent with independently determined chemical data. Comparison of the normalised clay mineral percentages from the Rietveld analysis to quantitative interpretations based on a peak intensities in glycolated and heat-treated oriented aggregates of the respective clay fractions also shows a high correlation, confirming mutual consistency of the two different mineralogical analysis methods. Such quantitative mineralogical data are significant to a range of coal exploration, mining and utilization activities, including seam correlation, material handling and ash and slag formation in combustion processes.     

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.     

Submicron ash formation from coal combustion

In recent years, fine particles have been found to be the cause of various harmful effects on health, and many countries have imposed restrictions on emission of these particles. Fine ash particles are formed during coal combustion in power stations and, if not collected in the air pollution control devices, are emitted into the atmosphere. The fine ash particles can remain airborne for long periods and can result in deleterious health effects when inhaled and deposited in the lungs.Previous studies have shown that combustion of coals of different rank can result in differences in the amount and chemistry of the submicron ash particles. This study examines the variability occurring between the submicron ashes formed from coals of similar rank. Five Australian bituminous coals were burned in a laminar flow drop tube furnace in two different oxygen environments to determine the amount and composition of submicron ash formed. The experimental setup is described and the repeatability of the experiments is discussed. The variability in the submicron ash yield as a percentage of the total ash collected and the submicron ash composition are presented and discussed. This paper presents experimental results rather than a detailed discussion on its interpretation. However, the results indicate that the condensation of evaporated species is responsible for the formation of ash particles smaller than 0.3 μm.     

Glow discharge excited low temperature ashing: A new technique for separating mineral matter of coals

Low temperature ashing techniques are widely used to determine mineral matter content and in the analysis of the composition of the inorganic matter in coal and coal products. This Paper presents a new technique which makes it possible to ash coals in 4 (low-rank coals) to ≈8 (high-rank coals) h per gramme. The problem of the formation of LTA artefacts is described and differences between authigenic and artefactic minerals are indicated.