Kinetics of mineral matter transformation during coal combustion

The transformation of individual minerals was investigated based on TG and DTG analysis at temperature up to 1700 K in inert and oxidizing atmospheres. The decomposition of minerals in inert atmosphere and the reaction with gaseous atmosphere was described by first order reactions for which the kinetic data were found. The evaluated kinetic parameters were then tested on a complex mineral matter of coals. It has been demonstrated on example of two different compositions that the mass loss during the transformation of coal mineral matter during combustion can be modelled as a mixture of individual minerals.     

Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion

NO-char reaction and char combustion in the presence and absence of mineral matter were studied in a quartz fixed bed reactor. Eight chars were prepared in a fluidized bed at 950 °C from four Chinese coals that were directly carbonized without pretreatment or were first deashed before carbonization. The decomposition of NO over these coal-derived chars was studied in Ar, CO/Ar and O₂/Ar atmospheres, respectively. The results show that NO is more easily reduced on chars from the raw coals than on their corresponding deashed coal chars. Mineral matter affects the enhancement both of CO and O₂ on the reduction of NO over coal chars. Alkali metal Na in mineral matter remarkably catalyzes NO-char reaction, while Fe promotes NO reduction with CO significantly. The effect of mineral matter on the emission of NO during char combustion was also investigated. The results show that the mineral constituents with catalytic activities for NO-char reaction result in the decrease of NO emission, whereas mineral constituents without catalytic activities lead to the increase of NO emission. Correlation between the effects of mineral matter on NO-char reaction and NO emission during char combustion was also discussed.     

Effect of inorganic matter on reactivity and kinetics of coal pyrolysis

Two Chinese coals, Shenfu subbituminous coal and Huolingele lignite, were used to investigate the effect of mineral matter in coal on reactivity and kinetic characteristics of coal pyrolysis. The experiments were carried out by using thermogravimetry to check the pyrolysis behavior of raw coal, HCl/HF demineralized coal and demineralized coal with inorganic matter (CaO, K₂CO₃ and Al₂O₃) added, respectively. The results showed that inherent mineral in coal had no evident effect on the reactivity and kinetics of coal pyrolysis. CaO, K₂CO₃ and Al₂O₃ all had a catalytic effect on the reactivity of coal pyrolysis, their effects were closely related to temperature region and coal types. The pyrolysis process of all the samples studied can be described by three independent first order kinetic model. Addition of inorganic matter the activation energy decreased and the characteristic temperature of coal changed.     

Factors governing reactivity in low temperature coal gasification. Part II. An attempt to correlate conversions with inorganic and mineral constituents

Links between extents of coal gasification and the amounts and compositions of mineral components in coals have been investigated. The influence of demineralisation and impregnation with various inorganic components on the pyrolysis and CO₂-gasification behaviour of two coals have been examined at 0.1 and 1 MPa. The effect of mineral matter on pyrolysis and gasification behaviour has also been examined by correlating actual conversions of a calibration set of 23 coal samples with the mineral matter-related bands of their FT-IR spectra. Whilst mineral matter contents clearly affect conversions during gasification, results from this work show that it is difficult to find systematic patterns, regarding the effect of specific inorganic components in different coals. The prediction of catalytic activity from amounts and compositions of particular inorganic components appears unlikely to be feasible. These findings confirm the difficulty of relating information on original structural features of coals to weight loss during gasification.     

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.     

高氯准东煤中典型矿物元素对颗粒物生成的影响

在高温沉降炉上,进行了高氯高碱准东煤、低氯低碱准东煤和由低氯准东煤处理制得的调质准东煤在含不同浓度HCl的模拟空气中的燃烧实验。利用DLPI进行了颗粒物的分级收集,对其质量粒径分布和矿物元素分布特性进行了讨论分析。结果表明,高氯高碱准东煤的超细模态颗粒物生成量和峰值粒径均明显大于其他两种煤,其主要成分为Na和Cl。Ca的迁移特性与其形态密切相关,原煤中的Ca主要以无机矿物形式存在,主要迁移进入粗颗粒物而对超细颗粒物生成贡献较小。气氛中加入不同浓度HCl气体后,低钠煤和调质煤产生细颗粒物中Cl含量均升高,但生成量无显著变化。表明HCl并未显著促进矿物的气化,但会促进NaCl形式矿物蒸气的形成,进而促进成核形成超细颗粒物。     

Effect of early stages of coal oxidation on its reaction with elemental sulphur

The present research shows how mild oxidation of coal mostly affects the evolution of H₂S produced in the reaction of coal with elemental sulphur. Coal samples oxidized at 30, 50, 80 and 150°C were reacted with sulphur in a temperature-programmed reactor. The H₂S produced in the reaction is very sensitive to the initial stage of the oxidation of coal. The strongest reduction in the amount of H₂S evolved was observed in the samples oxidized at 30°C. This temperature is lower than the one found in most coal storage places. The reaction with elemental sulphur could be used to monitor the initial stages of coal oxidation, which otherwise would be difficult to follow by conventional analytical methods.     

Application of percolation model to particulate matter formation in pressurized coal combustion

Coal is an important energy resource for meeting the future demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN^® model [Niksa, S., Combust. Flame, 100, (1995) 384-394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration.     

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.     

Thermodynamic study of the behaviour of minor coal elements and their affinities to sulphur during coal combustion

The behaviours of ten minor coal elements (Al, Ca, Fe, K, Mg, Mn, Na, P, Si and Ti) during coal combustion in the temperature range 400–2000 K, under both oxidising and reducing conditions, have been studied in detail by a thermodynamic equilibrium analysis.

The partitioning of these elements is calculated both in single minor element–coal–chlorine systems and in minor elements co-existing systems. Their vaporisation tendency is found in the order: (Si, Al)<(Fe, Ti)<(Ca, Mn)<(K, Na, P, Mg). Si, Ti, Al and P are present mostly as oxides and K and Na as chlorides, whatever the combustion conditions. Al, Ca, Fe, K, Mn and Na sulphates are dominant at low temperatures under oxidising conditions, whereas under reducing conditions most of them are sulphides and/or chlorides. Moreover, the interactions between these elements affect their major speciation: some species containing two elements among those studied are dominant in the minor elements co-existing systems. The affinities of minor coal elements to sulphur have been studied versus both temperature (400 or 800 K) and sulphur content (0.0062–6.20 wt.% in the coal), in order to find out their influence on the flue gas desulfurization. Two coal samples with different ash contents were considered, and it was found that the ash composition affects greatly the minor elements partitioning.     

Effect of mineral matter on product yield in supercritical water extraction of lignite at different temperatures

The effect of demineralization on conversion of Soma Lignite in supercritical water extraction was studied using a batch autoclave operated at 400, 450 and 500 °C under nitrogen atmosphere. The experiments were carried out to investigate the effect of mineral matter and temperature on gaseous, liquid, residue yield and composition of gaseous products. According to the results, main product in gaseous state is CO₂. Temperature is key factor affecting product distribution when compared the effect of minerals in lignite. As temperature was increased, yield of gas and solid residue increased, while yields of liquid decreased for raw and demineralized lignite samples. The removal of mineral matter caused to decrease the conversion for all lignite samples and to increase the carbon content of solid residue in supercritical water extraction.     

Desulfurization of a Turkish lignite at various gas atmospheres by pyrolysis. Effect of mineral matter

An investigation was made of the removal of pyritic and organic sulfur by pyrolysis at ambient pressure of a Turkish lignite under nitrogen and carbon dioxide atmospheres and the effect of mineral matter on the sulfur removal in pyrolysis of HCl and HCl/HF-treated coal under carbon dioxide atmosphere. Results obtained indicated that both pyritic and organic sulfur removal increased with increasing pyrolysis temperature. The pyrolysis in carbon dioxide atmosphere had more effect on the organic sulfur removal at high temperatures. As a consequence of treatment of coal with HCl, pyritic sulfur removal increased but organic sulfur removal decreased. This implies that the removal of carbonates from coal negatively affects the organic sulfur removal. The observed decrease in organic sulfur removal may be related to the decrease in pyrolytic conversion. It was observed that HCl/HF treatment has an increased effect on the pyritic removal and organic sulfur removal during pyrolysis. The increase in organic sulfur removal after HF-treatment therefore might be due to the removal of clay minerals in the raw coal structure. In addition, it may be said that the presence of silicate minerals in the coal matrix can be induced that the easily removable organic sulfur compounds are converted to thermally stable and non-removable organic sulfur compounds (thiophenic or condensed thiophenic compounds) at these temperatures. Increase in the pyritic sulfur removal of HCl-treated and HCl/HF-treated coal samples may be attributed to the fact that increase of mass and/or heat transport in comparison with untreated coal as a result of elimination of mineral matter.     

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.     

Spontaneous combustion of carbonaceous stockpiles. Part II. Factors affecting the rate of the low-temperature oxidation reaction

In part I of this paper, the rate of the low-temperature oxidation reaction was found to be critical in determining the relative tendency of a carbonaceous material to self heat and hence undergo spontaneous combustion. A static isothermal apparatus was designed to directly measure rates of reaction at constant oxygen concentrations and at low temperatures. The results show the relative importance of extrinsic factors such as particle size, ambient humidity, oxygen concentration and concentration of reaction product. A distributed-rate model shows that the apparent fractional order of reaction with respect to oxygen can be explained by allowing for a distribution of first-order rate constants. The results are consistent with the porous structure of coal and with microscopic examination of oxidised coals.Sufficient samples were tested and analysed to perform statistical modelling, which relates the intrinsic properties measured by proximate, ultimate and petrographic analyses to the rate of oxidation of a coal. It was shown that the two factors that had the most statistical significance in determining the propensity for self-heating were the volatile content and the inherent moisture. It is suggested that the inherent moisture is related to the total surface area of the sample, and that the volatile matter component represents the reactive component. Measurement of these two parameters gives a reasonable prediction of a sub-bituminous coal's rate of reaction with oxygen at ambient temperature, and quickly identifies ‘at risk’ samples for further testing.A separate set of experiments was conducted to examine the comparative importance of the sorption of moisture. It was shown that the rate of vapour phase adsorption and desorption of moisture is slow compared to the oxidation reaction.     

煤粉密度对燃煤过程中颗粒物形成特性的影响

通过浮选实验先将典型烟煤分成高、中、低3个密度段,然后对3种不同密度原煤在沉降炉内进行热解和燃烧实验,研究原煤密度对颗粒物形成机理和特性的影响。实验采用低压撞击器（LPI）把颗粒物按不同粒径大小从0.03～10.0 μm共分13级,分别采集燃烧后的可吸入颗粒物。实验结果显示：低密度原煤对颗粒物形成的贡献最大,中密度次之,高密度最小,低密度原煤所含矿物质粒度最小,形成的焦的膨胀率、总孔体积和BET表面积最大,高密度原煤所含矿物质粒度最大,形成的焦的膨胀率、总孔体积和BET表面积最小,中密度原煤介于两者之间,3种密度原煤燃烧后形成的PM10颗粒物元素构成的相同点是：对于亚微米颗粒物,元素S＋碱金属元素＋其他元素>难熔元素,对于超微米颗粒物,难熔元素占80%以上,远远大于其他三类元素。     

Gasification of waste biomass chars by carbon dioxide via thermogravimetry. Part I: Effect of mineral matter

A series of carbon dioxide gasification tests of waste biomass chars were performed in a thermogravimetric analysis system, at non-isothermal heating conditions. The effects of the inorganic constituents of the fuels on thermal conversion characteristics were examined. Reaction rates were determined by developing a power law model.The bulk of char gasification process occurred between 800 and 950 °C. Maximum reaction rate and conversion were exhibited by waste paper char, due to its higher surface area.Inherent alkaline and alkaline earth carbonates and sulphates acted as catalysts, by increasing the reactivity of the fuels in carbon dioxide and causing their degradation to start at lower temperatures (60-75 °C).The kinetic model fitted the experimental results accurately. Activation energy values and reaction order ranged from 180 to 370 kJ/mol and 0.4 to 0.6, respectively, among the chars, indicating a chemically controlled process.     

Spontaneous combustion of carbonaceous stockpiles. Part I: the relative importance of various intrinsic coal properties and properties of the reaction system

The likelihood of a coal stockpile or carbonaceous waste dump undergoing spontaneous combustion is strongly influenced by factors that relate to both the intrinsic properties of the material itself, as well as the properties of the reaction system. An understanding of these material properties is a pre-requisite for the simulation of the large-scale reaction system with the aim of preventing or containing this phenomenon.The first part of this paper investigates a range of coal properties that were considered to be the most critical in terms of heat ‘generation’ in a bulk coal medium. Certain properties, such as the heat capacity, the heat of reaction with oxygen and the activation energy for this reaction, were found to vary to a relatively minor degree for coals of varying rank and geological origin. However, rates of reaction with oxygen were found to vary by orders of magnitude for different coals. The reaction kinetics is consistent with a diffusion-limited shrinking-core model. A study of the coal properties that most strongly influence these rates of reaction is the subject of part two of this paper.     

煤中矿物质对神府煤快速液化的影响

利用共振搅拌反应器研究了煤中矿物质对神府煤高温快速液化的影响。原料煤经酸洗去除煤中大部分矿物质,脱灰率可达84．83％。脱灰后的煤在490℃进行加氢液化,结果表明,煤中矿物质的降低对神府煤高温快速液化有较大的影响,尤其影响了煤的初始高活性,煤转化率及液化产物的产率都降低,且煤中矿物质的降低会使矿物质自催化作用减弱,影响了神府煤液化产物质量。     

Role of mineral matter in the chemical reactivity of coal

Previous work showed that the solvent extraction yield of coal increased as a result of chemical reactions such as formylation, reductive acylation, acylation, amidomethylation, alkylation, reduction and depolymerization. In the work described in this Paper, dmmf coal obtained after demineralization with mixed HF-HCI acids was used for solvolytic extraction studies after depolymerization, alkylation, acylation, amidomethylation, reductive acylation and reduction reactions. In comparison with the original coal, mineral matter free coal showed less increase in extractability as a result of these reactions indicating that mineral matter present in the original coal was acting as a promoter for these reactions.     

Leaching of ashes and chars for examining transformations of trace elements during coal combustion and pyrolysis

One sub-bituminous coal and two bituminous coals were subjected to the combustion and pyrolysis by slow heating to a temperature ranging 550-1150 °C. Leaching of raw coals, ashes and chars with dilute HCl and HNO₃ was carried out, and leachate concentrations of major and trace elements were determined. Such a comparative leaching method was validated for characterizing the modes of occurrence of trace elements in coal and their transformations upon heating. Leaching results suggested that Be, V, Co, Cr and Ni were partially associated with organic matter, and As was partially associated with pyrite. During the ashing at 550-750 °C, the organically associated trace elements in coal formed some acid-soluble species. After the ashing at 1150 °C, Be, Co, Cr and Ni, together with Mn, Zn, and Pb, were immobilized in ash against leaching, whereas As was not immobilized. After pyrolysis, the organically associated trace elements in chars remained insoluble in both acids, and some HNO₃-soluble As in coal turned to a HNO₃-insoluble species.