Relation between Coal Mineral Matter and Deposit Mineralogy in

May 28, 2013 - Many of the currently accepted mechanisms for deposit attachment and ..... K is indicated, and both alkalis, Na and K, are taken into a...
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Relation between Coal Mineral Matter and Deposit Mineralogy in Pulverized Fuel Furnaces Robert A. Creelman,*,† Colin R. Ward,‡ Glenn Schumacher,§ and Lindsay Juniper∥ †

A & B Mylec Propriety Limited, 109−111 Bolsolver Street, Rockhampton, Queensland 4700, Australia School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales 2052, Australia § NRG Gladstone Operating Services, Post Office Box 5046, Gladstone, Queensland 4680, Australia ∥ Lindsay Juniper Proprietary Limited, Post Office Box 238, Sherwood, Queensland 4075, Australia ‡

ABSTRACT: Factors affecting ash deposition in pulverized fuel (pf)-fired furnaces include the nature of the coal mineral matter, the metallurgy of the furnace tubing, and operational considerations. The minerals in the coal exhibit responses that range from decomposition or vitrification through structural breakdown to retention of their original structure because they are stable at the temperatures involved. Furnace deposits consist of minerals and glasses derived from the coal mineral matter. Elements can move out of decomposed minerals into glasses if the source materials allow for higher diffusion rates. Upon cooling, crystallization occurs appropriate to the glass composition. This paper reviews the techniques used for mineralogical evaluation, the mechanisms of deposit formation, and interactions of the coal minerals that may cause deposits to develop. Many of the currently accepted mechanisms for deposit attachment and accumulation appear to be without any mineralogical/chemical foundation and require reconsideration in light of the reactivity of the different mineral matter components.

1. INTRODUCTION The pulverized fuel (pf) furnace is a complex machine, of which the effective operation requires considerable inputs from the operators; the best results therefore arise from a furnace being run by experienced crews. This is because there is still much empiricism in understanding and, hence, maximizing the pf furnace performance. Research into the problem of deposition in the pf furnace has been ongoing, with a number of review papers and site-specific studies appearing in the literature in recent years.1−5 Deposition in the furnace arises from a complex combination of coal properties, plant design, and operational demands. Most pf furnaces are designed and tuned for a specific coal type, and only limited coal variability can be tolerated. For this reason, the introduction of a new coal will require test burn programs. Consideration of coal properties alone is insufficient for a holistic understanding of deposition; detailed mineral analyses are required to understand the variability inherent in coal deposits. There is also a prime need to call upon the experience of the operators in deposit control as well as to develop a soundly based scientific understanding of depositional mechanisms. Only when there is an integrated approach are the best results possible. The behavior of coal in pf combustion has traditionally been assessed by reference to the chemistry of the coal and empiricisms as to the chemical behavior expected of the elements in combustion processes. This is, however, limited in effectiveness, especially when chemical analyses are used to predict outcomes based on element ratios and other empirical indices.6 There is an increasing realization that chemical analyses do not provide a complete characterization of coal and its combustion products; chemical data need to be supplemented by mineralogical data, which indicate how the various elements in the materials are linked together. © 2013 American Chemical Society

Determining the mineralogy of coals, coal ashes, and furnace deposits has been difficult in the past, especially on a quantitative basis. This particularly applies when fine particle sizes and amorphous components are involved. The advent of technologies that produce quantitative mineralogical data,7−9 however, has provided a basis for better understanding the problems that continue to plague the operators of pf furnaces: deposition, erosion, and corrosion. The systematic study of rocks and minerals in the geological sciences has provided a substantial body of knowledge,10,11 which, with only minor exceptions, is equally applicable to ash formation and deposition. Integration of X-ray diffraction (XRD), which provides definitive identification of mineral phases, and examination of sections under the optical or scanning electron microscope (SEM), which provides data on mineral textures and relationships, are useful in both geological and combustion studies. These techniques allow for linkage to the great body of knowledge that has developed on geological materials, primarily based on observation of mineralogical and textural features in the materials and interpretation of those observations in light of features produced by equivalent modern-day processes. The insights available to researchers from the application of mineralogical studies are removing the need for some of the empiricisms that have traditionally been used in combustion engineering. It is not that all empiricisms are redundant; there is still a strong need in engineering for this approach, but the Special Issue: Impacts of Fuel Quality on Power Production and the Environment Received: April 10, 2013 Revised: May 22, 2013 Published: May 28, 2013 5714

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Figure 1. (A) Schematic cross-section through a typical horizontally fired pf furnace, showing the various components and arrangements. (B) Typical design of a pf furnace, showing the dimensions of the gas path from component to component. The distances presented are suited to a highrank sub-bituminous to bituminous coal feed. Lower rank coals require higher residence times, resulting in larger furnaces.

mineralogical sciences are filling many gaps and placing the traditional cause and effect observations on a more substantial footing. Slagging and fouling in coal-fired boilers present great challenges to power plant operators. A recent International Energy Agency (IEA) publication5 indicates that slagging incidents cost the global utility industry several billion dollars each year. Slagging and fouling events result from a combination of coal properties, plant design, and plant operation. Much of the research into slagging and fouling has focused on coal properties in isolation. However, to actually reduce slagging or lessen its effects in operating plants, it is necessary to combine the coal properties with an understanding of what actually happens to the different materials when the coal is burned in particular furnaces and also as the combustion products move through the gas path.

above and the reduction in heat transfer as the deposits coat the tube surfaces that allow for heat transfer to the water (Figure 2).

2. DEPOSIT FORMATION IN PF COMBUSTION SYSTEMS Boiler operators use the terms slagging and fouling to designate the various deposit types in the pf furnace.2 Slagging is generally defined as deposition that occurs in the boiler itself, and fouling is generally defined as deposits that accumulate beyond the superheaters into the convection zone (Figure 1). Within the flame zone of the furnace, there are the burners, the furnace nose, and the water wall tubes, horizontal for supercritical furnaces and vertical for subcritical furnaces. Above the flame zone, there are the radiant tube banks. Deposits can accumulate at all of these sites and eventually may be released to fall into the furnace. Slag can form around burners and build up deposits known as “eyebrows”. A special problem associated with eyebrows is the re-melt of sintered material to form a running slag that is dense and heavy. Accumulation around the burners interrupts the proper function of the swirl of the burners. Molten material can also accumulate in other parts of the furnace. At the nose, the gas streams enter the convection zone and descend through a series of reheater tubes, eventually passing through to the economizer. Deposits can form in all of these locations, dependent upon the coal type and operational parameters. Slag fall from the burners and the walls can occur in the boiler, and sometimes tonnes of material may be dropped down onto the furnace throat and onto the ash hopper. If the shedding is continuous, this is acceptable, but detachment of large pieces can damage the ash hopper or block the entrance to the ash hopper. There are two main effects of ash deposition in the furnace; the shedding of large blocks as described

Figure 2. Start of deposition on the webbing between the water wall tubes. It has been the practice to compartmentalize the various areas in the pf furnace and postulate the behavior of the deposits based on those temperatures. This practice presumes that the minerals from the coal are a “system”; that is, they are homogeneous, and that eutectics and other thermodynamic parameters can be applied. The truth is that the minerals are a very heterogeneous system, and each mineral group has its own behavior in the elevated thermal environment of the pf furnace. Furnace aerodynamics plays a critical part in the process. The metal temperature also plays a part, with preferential deposition on higher temperature surfaces, as shown in Figure 2, particularly evident in supercritical furnaces. The horizontal shelf formed by the lower tube may further support the deposit and allow it to grow. While it is possible to design a boiler to burn any given coal or a limited range of coals, it is not economically feasible to design the furnace to burn all available coals. A design is tailored to meet the specific combustion characteristics of a coal type or class. Generally, these are low- or medium-volatile bituminous coals. High-volatile 5715

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Figure 3. (A) BSE image of a polished section through a slag sample obtained using conventional SEM technology. (B) Mineral distribution map obtained from the same section using QemSCAN. Each image is approximately 800 μm across. The mineral key indicates the colors used to represent the different types of mineral and glass components. bituminous coals are the next category (lower in rank), and then lignites that may have low, medium, or high slagging propensities. The size of the furnace systematically increases as the rank of the coal to be burnt decreases.6,12

minerals, although chloride and sulfate ions may also occur in the pore waters, especially of lower rank coal deposits. Some of the elements forming the post-depositional minerals may have been derived from the organic matter as the coal increased in rank,14 but others may have been sourced from outside the coal-bearing sequence. Quartz, clays, and other minerals may also be introduced to coal products by incorporation of noncoal strata during mining;9 some of this material, along with possibly mineral-rich particles from the seam itself, may be removed by subsequent coal preparation processes or in pulverizing mill rejects before the coal is actually used. 3.2. Techniques for Mineralogical Analysis. Two complementary techniques can be used to obtain mineralogical data from coal, slag, and ash materials. The first of these is XRD, which has traditionally been used to provide definitive identification of crystalline phases but has more recently been extended, through pattern-matching software, such as Siroquant,16 to provide quantitative data on the abundance of both crystalline and non-crystalline phases in a wide range of coals and coal-derived materials.7−9 The second is the use of integrated SEM and image analysis techniques, such as QemSCAN (initially developed as QEM*SEM)17−19 and TESCAN integrated mineral analyzer (TIMA), which represent extensions of older CCSEM procedures20 and can provide a variety of sophisticated mineralogical data on coals and coal combustion products. 3.2.1. XRD. Although quantitative XRD analysis can be applied to untreated coal samples,7 it is more common, especially for coals with low ash percentages, to isolate the mineral matter from the coal by low-temperature oxygen plasma ashing21 and analyze the low-temperature ash (LTA) residue by XRD techniques.22 It is also possible to calculate the composition of the coal ash expected from the quantitative mineral assemblage indicated by XRD analysis and compare

3. MINERALOGICAL EVALUATION OF COAL AND COAL COMBUSTION PRODUCTS 3.1. Mineral Matter in Coal. The material classed as “mineral matter” in coal includes not only crystalline mineral particles but also a range of non-mineral inorganic elements, associated in some way with the organic matter.13 Non-mineral inorganics, such as Ca, Al, Mg, and Fe, are usually abundant in lower rank coals but are lost from the coal because of changes in the organic components with rank advance.14 The mineral assemblages and modes of mineral occurrence reflect the depositional and post-depositional history of the coal. The nonmineral inorganics in lower rank coals may also reflect seam paleohydrology, resulting in differential concentration of ions in the pore waters of the coal bed.15 Clay minerals, predominately but not exclusively kaolinite, dominate the mineral matter of most coal beds. Together with quartz, which may form discrete fragments or particles, the clays may occur as thin layers within the coal, washed or blown into the original peat deposit. However, clay minerals and quartz may also occur in the microscopic cell lumens and pore spaces of the macerals (coalified plant particles) making up the organic matter of the coal, precipitated from solution in the pore waters of the peat bed. Other minerals, such as siderite or pyrite, may occur as nodules within the coal, also precipitated from solution in the waters of the original peat deposit.13 Post-depositional mineralization caused by circulating fluids may fill pore spaces and cleat fractures in the coal with minerals, such as calcite, dolomite, and pyrite. Oxidation of pyrite with exposure to the atmosphere may produce a range of sulfate 5716

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finely powdered coal feedstocks or as intimately mixed aggregates and intergrowths in the pf feed, with or without associated organic matter.19 Although conventional SEM studies can provide a basis for understanding the texture of the material, the integration with image analysis available through QemSCAN and similar systems allows for better understanding of the size, form, and relationships among the various components, including the nature and extent of any contacts between them. 3.2.3. Sequence of Mineralogical Evaluations. The sequence for evaluation of deposit formation ideally involves assessment of both the coal mineralogy and the ash and slag mineralogy, to establish the mineralogical changes that have taken place during coal utilization. Chemical analysis of the various materials is also useful, to cross-check the mineralogical results and also to relate the findings to previous studies and plant experience. Especially in the preliminary stages of an investigation, an electron microprobe analyzer (EMPA) may be of value in determining the detailed composition of particular phases in the coal and ash samples, including both mineral and glass components,9,19 as well as the organic matter in some cases.25 No single procedure can necessarily provide the complete solution, and integration of the various techniques is usually required.

that to the actual chemistry of ash from the same coal sample, determined by conventional ash analysis.7 This provides a useful way of checking the consistency of the quantitative XRD results against independent ash analysis data and increases user confidence in the XRD interpretations. In some cases, differences may be observed between the chemistry interpreted from the mineral percentages and the chemistry measured by direct analysis. This may be because some of the elements in question occur in the coal in a nonmineral form. This is especially common in lower rank coals, where significant concentrations of Fe and Ca may occur as inorganic elements either in solution in the pore water or associated in some way with the organic matter.14 Some of these elements may interact with organic sulfur during lowtemperature ashing to form crystalline sulfate phases, such as bassanite (CaSO4·1/2H2O),7 but others remain non-crystalline and are not detected by XRD analysis. Elements in this form, however, are likely to be more reactive than the same elements in crystalline phases, and thus, their presence should not be overlooked. In addition to quantifying the crystalline phases, it is also possible to quantify the amorphous component in ashes and slags derived from coal combustion.8,23 Calculations using such results also enable the chemical composition of the crystalline (mineral) and amorphous (glassy) components of the materials to be separately evaluated.24 3.2.2. Integrated SEM and Image Analysis. In integrated SEM and image analysis systems, such as QemSCAN, the electron beam of the SEM is scanned across the surface of a polished coal, ash, or slag sample to collect both a backscattered electron (BSE) image and a series of energy-dispersive X-ray (EDX) spectra from individual closely spaced points throughout the sample. As each EDX spectrum is collected, it is compared against a database derived from known spectra, the species identification protocol (SIP), and the mineral at that point in the sample is identified on the basis of chemistry and atomic weight. Mineralogical identification is usually made in 5−10 ms, and more than 700 000 individual EDX spectra can be quantified per hour. Samples can thus be characterized by analyzing hundreds or thousands of points automatically in a short time frame. As an example of the type of information available from this technique, Figure 3A represents a BSE image obtained by conventional SEM examination, and Figure 3B represents a phase distribution map produced using QemSCAN for a polished section through a slag sample. The mineral key accompanying the images shows the different phases that can be separately mapped using the SIP developed for this particular study. The technique in this instance allows for a range of different glass phases to be mapped separately based on the silica/alumina ratios, indicated in Figure 3 by various shades from green to blue. Silica/alumina glasses that are Febearing and in one case K-bearing are also indicated by different colors in the mineral map. For the sample in Figure 3B, the orange areas represent discrete Fe-bearing phases, such as hercynite or wuestite, the green areas represent the Si−Al−high Ca−Fe phase, and the blue spots are mullite. These integrated systems can be used in several different ways, including volumetric assessment of mineral components by point-counting and two-dimensional mapping of mineral distribution in polished sections of coal or coal utilization products.24 They can also be used to assess the extent to which particular minerals occur as “liberated” individual particles in

4. REACTIONS OF MINERAL MATTER DURING COMBUSTION The combustion of the organic matter in the coal particles heats the inorganic portions of the coal to temperatures that approach the flame temperature in the pf furnace. This can be between 1300 and 1600 °C for bituminous coals but is less for lower rank coal combustion. At these elevated temperatures, most of the minerals in coal and also the non-mineral inorganic elements may go through a number of different chemical reactions and crystallographic transformations. For any particular mineral during combustion, there may be one of three outcomes: (i) The mineral may remain as a solid particle and be virtually unaffected by the heating process; it may change crystallographic phase (e.g., α-quartz to β-quartz; kaolinite to metakaolin) but otherwise retains its original chemistry and physical form. (ii) The mineral may change its structure in a fundamental way that allows it to release or accept other elements through diffusion processes. This includes elements released from an organic association. Organically associated elements, such as K, Ca, Na, and sometimes Al, are released from the macerals or pore water during burning of lower rank coals. The released elements may bond, for example, with metakaolin to form new mineral species.26 (iii) The mineral may decompose into subcomponents, one of which may escape as a gas. The residue left after this process has a different composition to the original particle and may also be more reactive because of its new structural form. The volatile component may escape with the flue gases (e.g., CO2 and SO2) and take no further part in the process, or it may become available for reaction with other minerals or mineral residues or with the furnace walls elsewhere in the combustion system. The extent to which such reactions occur, both within organic-rich coal particles and in liberated mineral assemblages, depends in part upon the size of the individual particles or mineral aggregates involved; however, in pulverized fuel, most particles are small enough to have potential for some reaction in the combustion process. It also depends upon the residence 5717

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Figure 4. Plot showing abundance of different phases during dynamic high-temperature XRD analysis of mineral matter isolated from a coal sample.26

techniques using heating-stage facilities.26 The crystal structure of kaolinite, for example, undergoes dehydroxlation at around 450 °C to form an amorphous material referred to as metakaolin.28 Metakaolin in the mixture tested then reacts with Ca, released from decomposition of calcite and/or dolomite, to form anorthite at around 1000 °C, reducing the proportion of amorphous material as the reaction takes place. A comparison to thermomechanical analysis (TMA) data also shown in Figure 4 indicates that this is essentially a solid-state reaction, analogous to the reactions that take place during geological metamorphism of rock materials. The anorthitebearing mixture begins to melt at around 1200 °C, generating a TMA response along with a reduction in the proportion of crystalline anorthite and a second increase in the amorphous component. 4.1. Behavior of Mineralogical Systems. It is not possible to review all of the information on which an understanding of mineralogical systems is based, but it is useful to review the more important concepts that may be used to interpret what is observed in association with coal combustion. Slag, for example, inherently forms from material that has been at least partially molten, with an analogy in geology to the igneous rocks that form from molten masses or magmas.10,11 Magmas that reach the surface are lavas, and therefore, lavas or volcanic rocks are the best analogy to slags. Similar to lava, as slag cools it will crystallize, and the formation of crystals depletes the remaining “melt” of the components making up the crystals that have formed. There reaches a point where a second mineral phase will form; thus, the melt will continue to produce crystals until the system is fully crystalline or the system is quenched, and the material will consist of two crystal types and glass. The use of ternary-phase diagrams to elucidate crystallization from glasses is useful but complex. The ternary diagrams used

time, because some particles may move through the system before the relevant reactions are able to occur. Another factor is the mode of occurrence of the minerals and inorganic elements in the pulverized coal and the opportunity that different phases may have to react with each other during the combustion process. There are fundamental differences between the reactions that occur in entrained inorganic particles and coal particles, which eventually form fly ash, and the reactions that occur within deposits on furnace walls. Fly ash is the product of mostly fine ash particles, and these have a very low ideal contact; that is, it is possible to a limited extent to relate one coal mineral to one ash particle. However, with slags and similar deposits, the degree of ideal contact is much higher and most of what is seen is the result of reactions that take place within the deposit. The chemistry of a fly ash in bulk differs greatly from that of a deposit, with the fly ash being the more refractory product.27 Within this framework, the individual components of the coal mineral matter can be classified into thermally active and thermally inactive phases. This is an important distinction and is fundamental in understanding how the mineral matter in the coal will behave during the combustion process. Whether the minerals occur as separate particles or aggregates liberated from the organic matter (excluded minerals) or as composite particles incorporated within a matrix of organic matter (included minerals) at the particle size of the coal feed is, perhaps, less significant than the nature of the minerals themselves and whether they have the inherent capacity to participate in reactions under the conditions involved. As an example of reactions that may occur, Figure 4 illustrates the relative abundance of different phases when the mineral matter (low-temperature oxygen plasma ash) of a coal sample is steadily heated, with contemporaneous evaluation of the material at different temperatures by quantitative XRD 5718

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in geological studies are made more complex by the need to reference pressure (P) as well as temperature (T). There are complex and dynamic inter-relationships between the chemistry of various melts and crystals that form in those melts. The thermodynamics of rock systems is complex and adapted to specific rock and mineral groups, but in a simplified form, it can help to elucidate the formation of slags.11 It is not the purpose of this paper to expand on this topic but, instead, to illustrate the use of ternary diagrams for understanding slagging dynamics based on a specific example. The FeO−SiO2−Al2O3 ternary-phase diagram,29 shown in Figure 5, can be used to explain the textures seen in the slag

Figure 6. BSE image of a slag in the polished section, showing the crystallization of mullite from an iron- and potassium-bearing glass. The bright edges around the dark crystals are iron- and potassiumenriched glass that is progressively expelled from the melt as the crystals form. The image is approximately 100 μm wide.

Figure 5. FeO−SiO2−Al2O3 ternary diagram, showing the effect of plotting ash analyses that have been modified by removal of the inactive oxide contributions to the raw ash. The plot shows only the reactive components, and these define the iron line or trend. Two important levers are included: the low-Fe lever that results in the two crystal mullite−cristobalite system and the high-Fe mullite−iron cordierite system.

deposits shown in Figures 6 and 7. There are a number of notable features in Figure 5: (1) The square points represent raw ash data. (2) The triangles are the raw ash data modified using the mineralogy to remove the inactive mineral species, as described below. (3) There are three lines in Figure 5: (a) the linear set defined by the triangles that is the Fe line or trend (note that two lines fall out because of a combination of mineral composition variation and analytical error; this is rare but does occur and is included for the purposes of illustration), (b) the mullite−cristobalite lever, and (c) the mullite−iron cordierite lever. Figure 6 provides an example of a similar sequence in a section through a power station slag. The majority of the area covered by the section is made up of glass, which is light-gray and homogeneous in the BSE image from the SEM. Small crystals of mullite (Al6Si2O13) occur within that glass, appearing as either small equidimensional “blocks” or elongate fibrous crystals, depending upon their orientation within the section. The bright edges around the dark crystals are Fe- and Kenriched glass that was progressively expelled from the melt as the crystals formed. This is an example of a diffusion front. A relatively large fragment of quartz occurs in the center of the image of a similar section in Figure 7, and a gas cavity or

Figure 7. BSE image of a slag in the polished section, showing replacement fronts where the original melt (glass 1) has been replaced by a more iron-rich melt (glass 2) plus cristobalite, while the original mullite remains. The image is approximately 300 μm wide.

vugh, black in color, occurs in the lower right of that image. In general, the quartz appears to have been thermally inactive and has preserved its original outline. However, both the quartz and (especially) the vugh are surrounded by a zone with more densely packed crystals of both mullite and cristobalite (SiO2), set in an almost white glassy matrix. Interpretation of the image suggests that mullite was the first phase to crystallize from the original molten material (glass 1). This continued until the alumina in the remaining melt was depleted, after which cristobalite began to crystallize, nucleating on the surfaces of the vugh or the quartz grain. Within the cristobalite-bearing zones, the glass (glass 2) is lighter in color, indicating a higher proportion of Fe. As crystallization proceeded, crystal formation removed Al2O3 and silica from the melt, depleting the remaining liquid of these components but enriching the melt in Fe, relative to the other components. In this particular 5719

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Figure 8. Montage of QemSCAN images representing a cross-section through a furnace deposit of around 10 mm thick, ranging from the furnace side on the left (A) to the tube side on the right (B). Colors represent different phases, as shown in Figure 3.

The significance of the amorphous or “glass” phases is that more rapid diffusion rates are possible than with crystalline structures, allowing for more rapid elemental exchange in much the same way as the elemental exchanges that take place in liquids. The most abundant amorphous or “glass” phase in coal combustion residues is a disordered mixture of Al2O3 and SiO2, derived from the breakdown of the clay minerals (e.g., metakaolin derived from kaolinite). This forms an aluminosilicate base with which other elements may interact (e.g., Ca in the formation of anorthite; Figure 4). For other elements to diffuse into (or react with) such an aluminosilicate “glass”, the incoming elements must be generated by an active source that also has liquid-equivalent diffusion rates. The incoming elements may be in one of two classes: fluxing elements or refractory elements. Fluxing elements are elements, such as Na, Ca, and K, that drop the solidus temperatures below those of the relatively refractory aluminosilicate (Al2O3·2SiO2) system derived from kaolinite. Refractory elements do not change or, in some cases, may even increase the solidus temperatures and, thus, do not affect “melting” points. Magnesium (as MgO) is perhaps the best (or only) example of a refractory element that may be added to such a system. Although solid-state interactions may also be involved, similar to metamorphism in geological terms,10 crystallization in a furnace deposit usually proceeds in a similar way to the processes that occur in magma. Some generalizations can be made about crystallizing slags: (a) The first crystal phase to form from a “pure” aluminosilicate melt is mullite, followed by cristobalite. Cristobalite appears when the alumina is exhausted. Any Fe in the system forms Fe spinels or enriches the glass in Fe. (b) Deposits in a furnace are not in equilibrium; this is also the case in some volcanic rocks.12 This means that FACT-Sage calculations34 cannot readily be applied. If such techniques are to be applied, they should be applied only to the reactive part of the total ash or mineral matter. The disequilibrium and variable compositions of the glasses in slags also pose a difficulty for any thermodynamic modeling. (c) Dependent upon the flux that enters the melt, the deposits can be classified in a way that is akin to magma types. The classification is (i) Fe melts: The first crystal that appears from these melts is mullite, followed by iron cordierite. Iron can crystallize in a number of Fe−Al silicates with different structures; the characteristic minerals in the Ferich system are iron cordierite and mullite. Such melts are best described using the FeO−SiO2−Al2O3 ternary diagram, as presented in Figure 6. (ii) Ca-rich melts: These are complex

case, K became enriched in the melt as well. Such a sequence of minerals, in this case glass−mullite−cristobalite, is termed a “paragenetic sequence”. The residual melt, enriched in Fe and K, then cooled quickly to form glass 2, which can be regarded as a “quenched product”, after which no further crystallization was able to take place. 4.2. Thermally Inactive Minerals. Quartz is the most common mineral found in coal that is thermally inactive under pf conditions. In gasifiers, where the residence time is longer, there is evidence that quartz may be reactive to some extent and a phase change from quartz to cristobalite may take place.30 This reaction is nevertheless relatively slow31 and would not have time to take place in suspended ash particles under pf furnace conditions. Another example occurs in the combustion of high alumina coals, which, although not common, may be characterized by the presence of Al2O3 minerals, such as boehmite, occurring as discrete pelletiodal bodies within the coal, derived from bauxitic weathering of the basin hinterland and transported into the original peat deposit.32 Upon heating these Al oxide minerals, a phase change takes place from α Al2O3 to χ Al2O3;33 however, both phases remain as solids, and as a result, Al2O3 in these particles is not available for any further reactions in the pf combustion system. The entry or exit of a component into or out of a mineral phase in the pf furnace is dependent upon diffusion rates. In solid particles, such as these, the diffusion rates are so slow in the pf furnace that they can be considered to be zero. 4.3. Thermally Active Minerals. A thermally active mineral is one that either decomposes or goes through mineral transformations that increase disorder in the crystallographic structure. Diffusion rates through disordered structures are considerably higher than through crystalline solids, providing potential for greater reactivity in the combustion environment. Clay minerals go through structural changes, for example by means of the reaction:28 kaolinite → metakaolin → mullite + cristobalite

The first product, metakaolin, is amorphous and essentially equivalent to a “glass”. This is an example of a type ii reaction (above). Carbonate, sulfide, and sulfate minerals decompose to liberate volatile components (CO2, SO2, etc), and represent examples of type iii reactions. The volatiles may then migrate to other parts of the system and react with residues from type ii or other type iii reactions to form new mineral phases. 5720

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systems that produce the Ca feldspar (anorthite) and related Ca−Na aluminosilicates if Na is present. They are best described using the CaO−SiO2−Al2O3 ternary diagram. (iii) Alkali melts: After mullite, feldspars crystallize. Na-bearing systems produce albite and other minerals of the plagioclase series (albite−anorthite). K-rich systems will produce sanidine. They are best described using the Na2O−SiO2−Al2O3 and K2O−SiO2−Al2O3 ternary diagrams. 4.4. Relationships between Phases in Slags. Figure 8, derived from a number of separate QemSCAN images (phase distribution maps) that have been stitched together, represents a cross-section of about 10 mm in length through a furnace deposit. The sequence from left (A) to right (B) represents a transition from the glass-rich slag on the furnace side of the deposit to a crystalline material with abundant feldspar near the contact with the boiler tube, developed through progressive crystallization of the different mineral species over time. Figure 9 shows a selection of phase distribution maps through different sections of the slag deposit, chosen to illustrate the evolution of the different slag types. Figure 9A is from the glassy “sauce-like” material on the furnace side of the slag, showing early crystallization of mullite (blue) from an iron-rich glass (gray). The iron-rich matrix gives the outer surface of this material a vitreous red−brown color in hand specimen, because of the high iron content. Mullite was the first crystal to appear from the melt upon cooling, and if cooling was to proceed, cristobalite would be the second crystal phase to form. This particular slag, however, was quenched before cristobalite could appear. Figure 9B represents crystallization of the melt underneath the glassy material of Figure 8A. Mullite is still present (dark blue), but an An40 plagioclase (Na−Ca feldspar) is beginning to crystallize (pink), leaving a residual groundmass made up of an Al−Si−Fe−Ca glass (green). The glass is the residue that was left in the melt as the plagioclase species crystallized. Patches of gray material, which represent residual Fe-rich glass, are also present. Note the clustering of the mullite (deep blue color) crystals around the edges of a vugh (bubble) in the upper right of Figure 9B. This is a common texture seen in slags and represents draining of the melt, usually through a fracture, after mullite crystallization has begun. The significance of this image is that it accounts for part of the Na uptake into the slag; the An40 plagioclase is a solid solution with Na-bearing albite making up 60% of its total composition and Ca-bearing anorthite (An) making up the remaining 40%. The image in Figure 9C shows the further crystallization of the An40 plagioclase in clumps that are now surrounded by Fe− K glass (magenta color around the crystals). Note that the orange areas are Fe-rich hercynite/wuestite “particles” in the light-pink crystallized plagioclase and the green areas are expelled Al−Si glass. This image again illustrates the Na uptake into the slag, because the An40 plagioclase has the Na-bearing albite molecule making up 60% of its composition. Also significant are the magenta areas that are K-rich and are associated with high Fe. An almost symbiotic relationship between Fe and K is indicated, and both alkalis, Na and K, are taken into account. Large plagioclase crystals appear as laths in Figure 8D. This image can be directly compared to Figure 8C discussed above. The QemSCAN imaging shows the residual glass between the laths to be Fe−K glass, from which “ferrite” is crystallizing. The green Si−Al phase, which was expelled as the feldspar

Figure 9. Phase distribution maps obtained using QemSCAN for different parts of the furnace deposit shown in Figure 8: (A) early crystallization of mullite from an iron-rich glass, (B) crystallization under the glassy surface layer, (C) appearance of potassium−iron glass phases and further crystallization of An40 plagioclase, (D) crystallized slag showing plagioclase laths and Fe−K glass matrix, and (E) crystallized slag showing plagioclase laths and Fe−K glass matrix intermixed with Fe-rich glass. The scale bar in each case is approximately 20 μm. The color scheme representing different phases is shown in Figure 3.

crystallized, occurs along the margins of the feldspars in discontinuous lines. Figure 8E is similar to Figure 8D, but some of the glass matrix is iron-rich, as indicated by the gray color. The Fe−K glass is still present, and the expelled Al−Si glass is more clearly seen than in the previous image.

5. MECHANISMS OF DEPOSITION IN A PF FURNACE 5.1. Mechanisms of Slagging. Slagging in the furnace is a three-stage process: (i) adhesion of specific materials to the metal in the furnace, (ii) accumulation of a deposit on top of the adhesive layer, aided by progressive development of higher temperatures because of reduction in heat flow through the buildup and heat escape into the boiler tubes beneath (this can be referred to as the sinter stage), and (iii) post-depositional changes because the deposits sitting on the furnace walls and tubes are receiving energy from the flame zone. Adhesion onto a metal surface is conventionally understood to be the result of an initiation layer that, because of a 5721

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Figure 10. False-color QemSCAN image of the deposit on a Cr-bearing boiler tube. Included is the color coding for the species in the image. The corroded metal of the tube displays shapes typical of acid reactions. The iron oxide layer is Cr-impregnated and with included S-bearing species. It is the Fe oxide layer that is the actual initiation layer. These observations conform well to what Birks36 has postulated about NaCl effects on hot corrosion.

selective acidic attack of the alumina layer by the molten salt at unique sites situated primarily along the alumina grain boundaries. At the base of the pits formed, base metal oxides were growing, which eventually caused the mechanical breakup of the alumina scale, allowing the molten salt to gain access to and attack the metal surface. When water vapor was added to the atmosphere, its reaction rate apparently increased and the morphology of the product became more clearly crystalline, represented by stacks of hexagonal plates, with void structures down their length. These features indicate a strongly volatile species to be involved in the reaction mechanism involving water vapor.36 An ash particle containing Na (e.g., as a non-mineral inorganic element) and S may interact with SO2 in the gas stream to form sodium sulfate (Na 2 SO 4 ). Na 2 SO 4 is documented to thermally transform at 884 °C.2 Formation of metakaolin in the presence of such a Na2SO4 melt enables the diffusion of the two components producing albite (NaAlSi3O8). Albite has a melting temperature of 1100 °C, and thus, the process increases the potential for low ash fusion temperatures. Figure 10 is an image through an iron−chromium boiler tube sectioned to show a covering deposit on the corroded surface of a steel tube. The initiation layer is Fe oxide with Cr through the mass. The “scalloping” of the tube is due to corrosion and is evidence of corrosion mass loss. The “sinter” stage of the deposit is a complex of glasses and crystals, one of which is mullite. This Fe (Cr) oxide layer contains sulfur and the sinter Ca sulfate. This is consistent with the findings by Birks.36 If the deposits adhering to the surfaces of a pf furnace remain at higher temperatures for prolonged periods, they may

mechanism where alkalis that have volatilized from the combusting coal, forms a layer onto which incoming particles adhere and accumulate. Bryers2 claims Na to be the most important component in the initial layer of ash deposition. Na is capable of both evaporation−condensation diffusion and solid-state diffusion. This suggests that Na has the potential to have a significant influence on ash deposition. Although Na may occur in feldspar particles (e.g., albite) or as an exchangeable ion attached to some of the clay minerals, the most reactive forms of Na that may be found in coal are NaCl and Na2SO4. NaCl volatilizes at 750 °C to the gaseous form of NaOH.35 NaOH has the potential to react with extraneous silicates, inherent silicates, and char to form Na aluminosilicates.2,35 Detailed mineralogical investigations by the authors have seen no evidence to support these notions of adhesion, and the suggested chemistry is questioned. The metal−salt interaction has been a concern in the aerospace industries because of the potential of salts to damage turbine blades for not only aircraft but also marine turbines. Birks,36 conducting experiments with water, NaCl, and SO2 in combustion gases, found that, in all cases, NaCl converted to Na sulfate, with the volatilization of NaCl from the original salt particles being responsible for the development of a uniform coating of Na sulfate on the alumina substrate that resulted from oxidation of the Al alloys. The main role of NaCl in hot corrosion is that Cl provides the means of generating corrosive liquids.36−40 The alumina substrate is a scale growing on the alloy, and the liquid salt aggressively attacks the protective alumina layer. Birks36 found that this attack is initiated rapidly at 700 °C by 5722

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transform to a more crystallized mass that is hard and dense. It is rare to see total crystallization of a deposit, but total crystallization has been observed by the authors. The temperature that drives crystallization can be relatively low, because the constant heat energy input allows for solid-state diffusion. Even at temperatures as low as 300 °C, crystallization will proceed if the deposit is kept at that temperature for long periods (months). This suggests that the temperature of formation for a specific mineral is not a determinant for formation conditions; it is determined by the energetics associated with crystallization combined with both liquid- and solid-state diffusion. Tian et al.41 postulated that, in the alkali system, anorthite was a replacement product when the alkalis reacted with the first-forming mullite within alkali-rich glasses. Although many slags have been examined directly by electron-beam instruments, including QemSCAN, this replacement has in the authors’ experience never been seen; rather the feldspars that are the alkali-bearing mineral species crystallize directly from the glass. This demonstrates the value of direct observation and the general application of petrological systematics.

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6. CONCLUSION Deposition in a pf furnace is the result of a complex combination of coal properties, plant design, and operational demands. Consideration of coal properties alone is insufficient for a holistic understanding of deposition; there is a need to call upon the experience of the operators in its control as well as to have a soundly based scientific understanding of depositional mechanisms. When there is an integrated approach, best results are possible. The use of mineralogical tools and systematics, developed by petrologists over many decades, is of great assistance to the scientific understanding of slagging in the pf furnace. Petrological techniques use the direct observation of the minerals and glasses that constitute slags, and those observations add to the understanding of how slags form at a fundamental level, limiting the empiricisms used to explain the various phenomena. Petrological observation of pf furnace deposits, when combined with the body of literature from the aerospace industry,36 seriously questions our previous understanding of initiation layers on metal boiler surfaces, and revisions are necessary in this area. The types of deposits seen in a furnace can be grouped into at least three classes: Fe-dominated, Ca-dominated, and alkalibearing classes. This is analogous to rock classifications common in petrological studies. Just as petrologists say “to every rock its proper name”, those who study furnace deposits may in the future say “to every deposit its proper name”. The classification of slag types and knowledge of mineral assemblages in coals allow for much better prediction of slagging. At all times, the design of the furnace is the other prime consideration and must be assessed in conjunction with the mineralogy and petrology involved.

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5723

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