Mineralogy of combustion wastes from coal-fired power stations

FUEL PROCESSING TECHNOLOGY Fuel Processing Technology 47 ( 1996) 261-280

Mineralogy of combustion wastes from coal-fired power stations Stanislav V. Vassilev *, Christina G. Vassileva Central Labomtory

ofMinerulogy and Crystallography, 92 Rukovski Str., Bulgarian Academy of Sciences, So$u 1000, Bulgaria

Received 1 August 1995; accepted 20 December 1995

Abstract A combination of methods, including separation procedures, light microscopy, SEM, TEM, XRD and DTA-TGA methods, were used to characterize the phase-mineralogical and chemical composition, microstructural and some genetic phase peculiarities of solid waste products from coal burning. Fly ashes, bottom ashes and lagooned ashes from eleven Bulgarian thermoelectric

power stations were studied. These products comprise inorganic and organic constituents. The inorganic part consists mainly of non-crystalline (amorphous) components and lesser amounts of crystalline components represented by various major, minor and accessory mineral phases. The organic constituent contains unburnt coal components represented by slightly changed, semicoked and coked coal particles. The origin of solid phases could be: primary - minerals and phases contained in coal and having undergone no phase transition (silicates, oxides, volcanic glass, coal particles); secondary phases formed during burning (magnetite, hematite, metakaolinite, mullite, anhydrite, lime, periclase, Ca-Mg silicates, glass, semicoke, coke); or tertiary minerals and phases formed during the transport and storage of fly ashes and bottom ashes (sulphates, carbonates and oxyhydroxides). Keywords: Fly ash; Mineral composition

1. Introduction

Different aspects related to the utilization and environmental influence of solid waste products from coal burning and gasification require a detailed knowledge of their phase-mineralogical and chemical composition. For this reason, intensive investigations

* Corresponding author. 0378-3820/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. P/I SO378-3820(96)01016-8

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have been carried out on fly ashes at thermoelectric power stations UPS) worldwide using different methods and approaches. The mineral composition [l- 111, morphological and microstructural characteristics of particle material [12- 171, as well as some genetic phase peculiarities [1,2,4,6-8,13,18,19] have been characterized to a certain extent. However, systematic studies on the mineralogy simultaneously of coals and their various by-products are restricted. Such parallel investigations are important in the elucidation of mineral and element behaviours and phase formations during coal combustion, and in the evaluation of possible utilizations and environmental effects of the combustion products. This paper provides a systematic characterization of phase-mineralogical, microstructural and genetic peculiarities of waste products related to coal burning at eleven Bulgarian TPS. The present work is a summary of the studies reported earlier [20,21] and it also includes additional data.

2. Material and methods Three types of samples from solid waste products, generated from dry-ash and slag discharge boilers in TPS, were collected and examined: bottom ash (BA) and slag from under the combustion chambers; fly ash (FA) from the hoppers of electrostatic and mechanical precipitators and collectors; and lagooned ash-slag (LAS) from the depositories near the TPS. The samples from BA, slag, FA and LAS were composed of a large number of single samples (30-70) and were taken at different time intervals (up to eight years). The composited samples studied are from eleven large Bulgarian TPS with pulverized coal-fired systems (Maritza-East 1, 2 and 3; Maritza-3; Kremikovtzi; Republica; Bobov Dol; Russe; Svishtov; Vama and Devnya) using Bulgarian lignitic and subbituminous coals, as well as Ukrainian bituminous and anthracitic coals. In addition, original coal, low-temperature ash (LTA) and high-temperature ash (HTA) samples from fossil fuel used in the aforesaid TPS were also studied. Sieving, heavy liquid and magnetic separations, as well as hand picking under a binocular stereomicroscope were applied to concentrate the minerals and phases present. Polarizing microscopes were used for optical observation under reflected and transmitted light. Polished sections were prepared by mounting samples in epoxy and duracryl pellets. For each sample and fraction separated, particles below 63 p,m in size were placed in a glycerine immersion medium and observed under transmitted light. Various samples were also observed in reflected light using a high-temperature microscope fitted with a heating stage which provided a maximum temperature of 1500°C in air atmosphere. Powder X-ray diffraction (XRD) patterns were recorded using a diffractometer with Co and Cu Ka radiation. Scanning (SEM) and transmission electron microscopic (TEM) studies were carried out with electron microscopes equipped with an EDAX analyzer. Samples for SEM examination and element determination were prepared from polished blocks and powder pellets, broken sample fragments and grain mounts. Samples for TEM examination were prepared from alcohol suspensions of powder. Mineral and phase identifications by TEM were based on electron diffraction patterns

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and elemental composition. Differential thermal (DTA) and thermogravimetric (TGA) studies were conducted in air atmosphere up to 1500°C. Semi-quantitative and quantitative determinations of the mineral and phase proportions in samples were performed using separation procedures, light microscopy (point counting method), XRD (semi-quantitative analysis) and SEM (micromorphometric particle analysis).

3. Results and discussion 3.1. Fly ash The fly ash is a finely dispersed heterogeneous material represented by particles generally below 100 Frn in size, while BA, LAS and especially slag are coarser grained agglomerates and fragments (Table 1; Plate I(a) to (cl>. The solid waste products are composed of inorganic, organic and fluid constituents (Tables 2 and 3). Common morphologies in FA at four Bulgarian TPS have been described and illustrated earlier [20]. Additional photomicrographs which show the occurrence and morphology of various phases and minerals in FA are included in the present study (Plates 1-3). FA is the most abundant residue of coal burning and detailed studies were performed predominantly on this by-product. 3.1 .I. Inorganic constituent: non-crystalline (amorphous) components The non-crystalline components in FA range commonly from 50 to 90 vol.%, while their contents in BA, slag and LAS are normally higher.

Table 1 Sieve analysis of bottom ash (BA), coarse-grained fly ash (CGFA), fine-grained (LA), slag (S) and lagooned ash-slag (LAS) from 11Bulgarian TF’S (wt.%) Waste product

fly ash (FGFA), lagooned

Fraction (km) 1000-500

500-250

2.50-100

100-63

< 63

A. From dry-ash discharge boilers BA II-32 O-1 CGFA FGFA 0 LA l-8

4-35 O-10 o-3 4-13

21-42 3-40 O-10 16-37

16-43 23-38 l-24 28-47

1-6 7-19 2-12 5-15

1-4 7-54 55-97 6-32

B. From slag discharge boilers S 77-88 CGFA 0 FGFA 0 LAS 8-37

IO-16 0 0 2-15

2-5 l-2 1-3 3-7

o-3 8-10 2-6 8-19

o-2 13-16 4-9 7-24

o-1 72-78 83-92 20-54

> 1000

ash

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3.1.1.1.

Spherical and spheroidal particles. Spheres and spheroids are a result of softening, partial and complete melting, and vitrification of coal minerals such as clay minerals, chlorite, mica, feldspars, quartz and other mainly fluxing minerals and phases which have a lower melting point (MP). The quantities of spheres and spheroids vary within the limit of lo-80 vol.% in FA, as their participation increases to the finer granulometric fraction (< 63 pm). Dense and vesicular spheres, cenospheres and plerospheres are the best represented, while dermaspheres and ferrospheres are less common. The above-mentioned terms are often used in literature and some spherical particles have been illustrated and described elsewhere [7,12,14,22]. The size of spheres is dominantly in the range l-50 p,m. They exhibit different colours, however, grey, white, green, black and mixed spheres are ordinary. The dense and vesicular spheres originate respectively from fully and partly degasificated melts of various minerals. The cenospheres are thin-walled hollow spheres which are occasionally fragmented. They are larger (lo-250 pm) than dense and vesicular spheres. Their gaps are filled with gas and condensed gas formed during burning as a result of decomposition of organics, carbonates, sulphides, sulphates, hydrosilicates, and evaporation of pore water in coal. The locked liquid subsequently crystallizes in films. The presence of cenospheres could be explained by the appropriate and relative higher viscosity of the melt envelope and the possibility of retaining the gas bubble for a longer period during hardening. The quantity of cenospheres is mainly up to 15-20 vol.%. The plerospheres are cenospheres which pack (capsulate) other smaller pre-existing aggregates, particles and agglomerates. The dermaspheres are plerospheres which have crystal nuclei of

Table 2 Morphogenetic

scheme of fly ash, bottom ash, slag and lagooned

ash-slag description

1. Inorganic constituent -produced from inorganic and organic matter of coal 1.1.Non-crystalline (amorphous) components 1.1.1. Spherical and spheroidal particles (dense, vesicular, cenospheres, plerospheres, dermaspheres, ferrospheres, etc.) 1.1.2. Angular and irregular particles (dense, hollow, vesicular, tibres, agglomerates, etc.) 1.2.Crystalline (mineral) components 1.2.1. Crystals and aggregates 1.2.2. Grains and clusters 1.2.3. Ferrospheres and ferrispheres 1.2.4. Skeleton spheres and spheroids 1.2.5. Agglomerates, etc. 2. Organic constituent -produced from organic matter of coal 2.1. Coal components -slightly changed coal particles (dense, angular, irregular, rounded, scaly, agglomerates, etc.) 2.2. Semicoked components -transitional between coal and coked components 2.3. Coked components -spherical, spheroidal, angular and irregular particles (dense, hollow, vesicular, scaly, skeleton, agglomerates, etc.) 3. Fluid constituent a -produced from inorganic and organic matter of coal (gas-liquid inclusions in particles) a Particles composed

of all constituents

also occur.

Plate I. (a) SEM image of Bobov Dol bottom ash. Secondary electrons; (b) SEM image of Bobov Dal fly ash. Secondary electrons; (c) SEM image of Bobov Do1 lagooned ash. Secondary electrons; (d) TEM image of a semifused quartz grain on the surface of a glass aluminosilicate sphere in Bobov Do1 fly ash; (e) TEM image of a prismatic mullite crystal in Vama fly ash; (f) TEM image of a wollastonite grain in Maritza-East fly ash; (g) TEM image of a Ca-Mn silicate crystal in Vama fly ash; (h) TEM image of a muscovite flake in Bobov Do1 fly ash; 6) TEM image of needle forsterite crystals in Bobov Do1 fly ash; (j) TEM image of a prismatic amphibole fragment in Maritza-3 fly ash; (k) TEM image of chloritoid flakes in Varna fly ash; (1) TEM image of a talc flake in Maritza-East fly ash.

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Table 3 Minerals and phases identified, their content and probable origin (P, primary; S, secondary; T, tertiary) in fly ashes, bottom ashes, slags and lagooned ashes-slags from 11Bulgarian TPS; M, major phase ( > 1 wt.%); m, minor phase ( = l-0.1 wt.%); a, accessory phase ( < 0.1 wt.%) Mineral, Phase

Content

Origin a P

Silicates Quartz Cristobalite Kaolinite-metakaolinite Mullite Andalusite Plagioclase K feldspar Olivine Pyroxene Amphibole Zircon Chlorite Chloritoid Mica Vermiculite Talc Rankinite Wollastonite Larnite Melilite Monticellite Ca-Mn silicate

M a-M m-M m-M a m-M m-M a-m a-m a a a-m a a-m a a a-m a-M a-m a-M a-M a

Oxides and Hydroxides Magnetite Maghemite Hematite Limonite Magnesioferrite Ilmenite Spine1 Ferrian spine1 Chromite Chalcophanite Pyrolusite Cuprite Tenorite Zincite Rutile Anatase Brockite Lime Portlandite Periclase Brucite

m-M a-m M a-m a-m a a-m a a a a a a a a a a a-M a-M a-m a-m

s

X

X

X X X X

X X X X

X X

T

X

x

Ti

X

X

X

X X X X X

x

X X X X X X X X X X X X X

Yc

X

X

;

X X X

; X X X

X X X X X X

X

;

X

X

X X

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Table 3 (continued) Corundum y-At,% Al hydroxides W-Nb-Pb oxide

Sulphates Gypsum Auhydrite Fe sulphates Mg sulphates

a-m a a-m a

Na-K sulphates Barite

m-M m-M a-m a-m a-m a-m

Carbonates Calcite Manganocalcite Dolomite Cerussite Witherite

a-m a a-m a a

X

X

X

X

X X X

X X

Phosphates Apatite Svanbergite Vivianite Goyazite Ningyoite Sulphides and Sulphosalts Pyrrhotite Pb-Sb sulphosalt Chalcocite

a a a

others Glass Organic matter Fe carbide Graphite Scheehte

M M a a a

a The minerals and phases presented in waste. products could be primary (contained in coal, having undergone no phase transition), secondary (formed during the combustion process) and tertiary (formed during the transport and storage of the products).

mullite, hematite and other minerals, covered with glass aluminosilicate envelopes. Bulk chemical analyses show that the SiO,, Al,O, and K,O contents are higher, while the Fe,O,, MgO, CaO, Na,O and SO, contents are lower in the cenosphere-plerosphere separated fraction than in the respective FA at Bobov Do1 TPS. It seems that hollow spheres are more characteristic of FA obtained from coals enriched in finely-dispersed illite and quartz. The ferrospheres are Fe-enriched spheres, either amorphous or containing crystalline components. They are occasionally broken and their semi-spherical fragments demonstrate that some of them were hollow or covering other spheres and particles as crusts.

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Technology 47 (1996) 261-280

Table 4 Average chemical composition of various fly ash particles from 4 Bulgarian based on 53 electron microprobe analyses (wt.%) C

Oxides

Aa

SiO, TiO, Al,& Fe,O, MgO CaO Na,O

59.7 0.7 27.6 5.8 2.5 0.1

54.2 0.3 29.6 6.2 0.3 2.8

2.9

1.8 5.1

K2O

SD, Total

99.9

B

100.3

51.7 0.4 27.1 6.5 3.5 6.4 1.7 1.3 1.8 loo.4

D

E

51.5 0.6 25.1 9.0 3.7 8.3 0.7 1.5 0.2

51.2 0.6 28.2 9.1 6.2 0.8 2.8 1.8

100.6

100.7

F

G 46.6 0.5 26.6 9.3 2.3 8.5 1.2 1.7 3.8

loo.5

TF’S (in decreasing H

38.1 20.1 39.6 0.1 1.6

100.1

27.7 1.9 17.2 34.9 5.3 10.7 1.4 0.4 1.4 100.9

I

order of SiO,)

J 14.8

K

2.8

14.0 0.5 7.8 69.5 3.0 2.1 1.6 0.4 1.6

100.0

100.5

10.8 60.9 3.7 7.0

10.9 8.5 74.9 3.2 2.4

99.9

a A, cenospheres; B, kaolinite-metakaolinite particles; C, angular glass particles; D, glass spheres; E, plerospheres; F, glass spheroids; G, Fe-richangularglass particles; H, glass ferrospheres; I, ferrospheres and fenispheres of dendritic Fe oxide crystallization; J, ferrospheres and ferrispheres of skeleton Fe oxide crystallization; K, Fe oxide crusts.

The spheroids do not significantly differ from the spheres, however, they look more porous and vesicular. Their size is dominantly in the range lo-80 km. The spheroids hold two, three or more sector-distributed gas bubbles. Some of the finest solid spheres and spheroids are formed as a result of plastic cenospheres bursting and breaking up into many dense drops. The drops commonly adhere to the surfaces of coarser grained particles, which have already cooled, or mix with other particles still in a liquid or plastic state. 3.1.1.2. Angular and irregular particles. The angular and irregular glass particles are also formed from softening and melting of the coal inorganic matter or its vitrification. These particles are either dense or flecked with pores and gas bubbles. Their surfaces are occasionally collomorphous (clustered) due to particle grouping in liquid and plastic states and mutual germination of spheres, spheroids, debris and other particles. Their size is normally in the range 60-500 pm (in BA and slag up to several cm>. Glass fibres with a diameter of a few micrometers and a length up to 500 pm have also been recognized. The chemical composition of some aluminosilicate glass phases is shown in Table 4. Their chemical peculiarity is due to differences mainly in the clay mineral composition, some dissolved fluxing minerals and absorbed volatile elements in the fused material. Some aluminosilicate glass particles may also have primary origin because particles of such type have been found in coal [20,23]. 3.1.2. Inorganic constituent: crystalline (mineral) components The mineral phases (Table 3) in FA were identified by light microscopy, XRD, SEM

and TEM studies. The minor crystalline phases are generally determined after applying separation procedures, while the accessory minerals and phases were identified com-

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monly by SEM and TEM studies and, in rare cases, by light microscopy separation procedures.

269

and XRD after

3.1.2.1. Silicates. Silicate phases are mainly primary minerals and secondary products, and rarely tertiary phases of various detrital, syngenetic and epigenetic minerals in coal (Table 3). Quartz is the most widespread mineral. It is represented by angular to rounded (during softening r 1300°C) and fragmented grains. Generally, grain sizes are within the limit 5-70 km. Quartz may be of both primary (MP = 1713 “C) and secondary origin. The secondary quartz has undergone polymorphous transformations and has been formed from silica liberated during the phase transitions of clay minerals, mica and feldspars, which have experienced thermal effects at temperatures above 900°C. Some quartz (mainly skeleton type) also originate from the crystallization of melts. However, the dominant quartz proportions were fused, semifused or dissolved in melts (Plate l(d)). This silica forms glass or reacts with other components. The active melts produced from fluxing minerals may dissolve intensively the relict and newly formed refractory minerals below their melting temperature [24]. Cristobalite is primary and secondary in origin. The secondary cristobalite is a result of probable crystallization of organically bound and amorphous silica, recrystallization of opal, chalcedony, clay and other silicate minerals in coal. Some secondary cristobalite may also be formed from the crystallization of melts. Mullite (rarely andalusite) is secondary, generated mainly due to the decomposition and transformation of clay minerals, and to a lesser degree mica, feldspars and other aluminosilicates, at temperatures commonly above 1000°C. Some small mullite (MP = 1810°C) proportion may have a primary origin, especially for coals associated with volcanic rocks. However, this mineral is mainly a result of melts’ crystallization. Mullite is observed as individual prismatic (Plate l(e)) and needle (sword-shaped) crystals of 0.2- 15 pm in length or forming spheroidal skeletons where the lumens are filled in with glassy matter. It was seen occasionally as radial fan-shaped aggregates or building the nuclei in dermaspheres. Coals with high kaolinite concentrations yield FA enriched in mullite. XRD data also indicate significant amounts of mullite in a separated hollowsphere fraction. Feldspars are represented by angular to rounded grains of 20-30 km in size. The grains are sericitized, semifused, with pores on the surface, and occasionally they are resorbed by liquid drops. The origin of the feldspar-s could be primary (MP = 11181553 “C) and secondary. The latter feldspars are crystallized mainly from a silicate melt. Some feldspars, especially basic plagioclases, may also be formed from solid-phase reactions between aluminosilicates and liberated Ca, K and Na oxides during burning. Potassium feldspar could also have a secondary genesis as a result of polymorphous conversion to sanidine at temperatures above 900°C. Kaolinite-metakaolinite is a term representing the thermally changed clay matter. These phases are commonly new (secondary) formations and “transitional” to the amorphous components. They are mostly a result of kaolinite (MP = 1650-1810°C) semidestruction, since illite and montmorillonite clay minerals (MP = IOOO- 1300 “C) melt more easily and pass dominantly into the glass phases. These semiamorphous

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particles are of different dimensions, often represented by pseudohexagonal flakes of 0.1-7 pm in size forming aggregates. According to their chemical composition (Table 4), it is evident that they show a similarity to the composition of aluminosilicate glass components, with some exceptions. Coals with high concentrations of coarse-grained kaolinite yield FA enriched in refractory kaolinite-metakaolinite aggregates. Ca and Mg silicates (rankinite, wollastonite, lamite, melilite, monticellite and Ca-Mn silicate) are secondary phases (Plate l(f), (g)). They originate from solid-phase reactions between liberated alkaline-earth oxides and silica and aluminosilicates, or they are crystallized from silicate melts. Micaceous minerals are mainly of the muscovite type (Plate l(h)). These minerals are dominantly primary (MP = lOOO-1300°C) in origin. Olivine is observed as whiskers and needle crystals (Plate l(i)) normally in a glass matrix, and is probably a secondary phase due to its crystallization from a silicate melt enriched in Fe and Mg. Some olivine may also be of primary origin (MP = 1205-1890°C). Pyroxene minerals (MP = 13921557°C) probably are generated similarly to olivine. Zircon (MP = 2552°C) is a refractory mineral and its origin seems to be dominantly primary. Other minerals such as chlorite (MP = 1200 “C), amphibole (MP = 1030- 1140 “C>and chloritoid (Plate l(i) and (k)) may also be primary. Vermiculite most likely originates from biotite alteration, while talc (Plate l(1)) may be a result of reaction between dolomite and quartz [25]. 3.1.2.2. Oxides and hydroxides. Oxyhydroxide phases are dominantly primary minerals and secondary products, and more rarely tertiary minerals of detrital and authigenic minerals, and organic matter in coal (Table 3). Magnetite, hematite, maghemite, limonite and magnesioferrite can be primary minerals (MP = 1567-1592°C). However, they originate mainly from the: oxidation of pyrite, marcasite, siderite, ankerite, jarosite and magnetite; reduction of Fe oxide by carbon and hydrogen; Fe hydroxides dehydroxylation; and crystal growth in melts produced from Fe-bearing minerals in coal. Some morphological features and an enrichment in chalcophilic and siderophilic trace elements could be indicators of the Fe oxyhydroxides nature. Magnetite is present as angular to semirounded single grains of l-10 pm in size, as well as dendrites and octahedral crystals (commonly with a skeleton habit) in the glass. Rounded octahedral crystals enriched in Ti and Cr were also found. Magnetite grains are occasionally intensively martitezed. Hematite occurs as flakes (specularite), crusts, platy and lamellar crystals, commonly forming “iron roses”. In rare cases, it is observed as cubic and cubic-octahedral relict crystals inheriting the initial pyrite crystalline form. Some of the hematite grains show marks of mushketovitezation. Normally, the coarse-grained FA and BA are more abundant in magnetite than fine-grained FA which is enriched in hematite. This observation demonstrates a relatively higher oxidizing condition during formation of the finer FA. The crystalline ferrospheres are mainly of lo-40 pm in size and may be both hollow and dense ones. They are built up of a glassy aluminosilicate matrix that includes dendritic, lamellar and skeleton (“pine-tree” type) magnetite crystal forms and, to a lesser degree, hematite and magnesioferrite crystals. Among the ferrospheres, skeleton

Plate 2. (a) TBM image of a corundum grain in Ma&a-East fly ash; (b) TBM image of prismatic rutile crystals in Bobov Do1 fly ash; (c) TEM image of a octahedral ilmenite crystals in Bobov Do1 fly ash; (d) TEM image of a prismatic ilmenite crystals in Vama fly ash; (e) IBM image of a chromite grain in Vama fly ash; (f) TBM image of prismatic anatase crystals in Maritza-East fly ash; (g) TEM image of a prismatic brockite crystal in Vama fly ash; (h) TEM image of a zincite grain in Bobov Dol fly ash; (i) TEM image of needle pyrolusite crystals in Maritza-East fly ash; (j) ‘IBM image of a octahedral cuprite crystal in Vama fly ash; (k) TBM image of a octahedral cuprite crystal in Vama fly ash; (I) TEM image of a prismatic tenorite crystal in Ma&a-3 fly ash; (m) ‘IBM image of a chalcophanite grain in Bobov Do1 fly ash.

Plate 3. (a) TEM image of a W-Nb-Pb oxide grain in Maritza-East fly ash; (b) SEM image of an initial Fe, Na, Mg, K and Ca sulphate crystallization onto the surface of a ferrosphere in Bobov Do1 fly ash. Secondary electrons; (c) SEM image of an advanced Fe, Na, Mg, K and Ca sulphate crystallization onto the surface of a ferrosphere in Bobov Do1 fly ash. Secondary electrons; (d) TEM image of a manganocalcite grain in Maritza-3 tly ash; (e) TEM image of a prismatic apatite fragment in Maritza-3 fly ash; (f) TEM image of a pseudocubic svanbergite crystal in Bobov Do1 fly ash; (g) TEM image of a pyrrhotite gram in Ma&a-East fly ash; (h) TEM image of a Pb-Sb sulphosalt grains in Kremikovtzi fly ash; (i) TEM image of a chalcocite grain in Bobov Do1 fly ash; (j> TEM image of scheelite crystals in Bobov Dol fly ash; (k) TEM image of a Fe carbide sphere adhered to a glass sphere in Maritza-East fly ash.

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spheres and spheroids enriched in hematite and limonite (ferrispheres) were also found. A “glass skin” formation, which is typical of dermaspheres, was also observed on some of these iron roses. The Fe-enriched spherulites could be considered as: (1) representing the limit of “split” growth [26] of melted drops; (2) spheroidal relics with a cosmic [27] and volcanic origin; and (3) relics of altered pyrite-marcasite framboids and clusters (roses), as well as relics of siderite concretions, lenses and spheroidal aggregates observed in coal [23]. The last origin is common and the morphological similarity in habit and size, and some typomorphic trace elements are a confirmation. Similar mineral pseudomorphs have been observed elsewhere [18]. The chemical composition of some Fe-rich particles is given in Table 4. Ca and Mg oxyhydroxide minerals are secondary and tertiary. They are a product of carbonates’ decomposition or originate from organically and sulphate combined Ca and Mg. The refractory lime (MP = 2570-2585 “C) and periclase (MP = 2800°C) commonly occur as unaltered cores, which are enclosed in portlandite and brucite coarse grains. Lime is also observed as short prismatic crystals or scaly aggregates, which are occasionally localized as unmelted clusters in the glass or on glass spheres. The portlandite growth is a pseudomorph of such lime clusters. Ca and Mg hydroxides are mainly formed during ash storage due to the lime and periclase hydration to more stable portlandite, brucite and later to carbonates. In addition, tertiary calcite cenospheres produced by lime hydration and subsequent carbonatization were also found in depositories [28]. Ca and Mg oxyhydroxides dominantly occur in products generated from low-sulfur and low-silica coals. In high-sulfur coals there is an intensive formation of sulphates, while in high-silica coals Ca-Mg silicates are usually formed. Spine1 is primary (MP = 2135°C) and secondary, when its formation is a result of probable solid-phase reaction between Mg, Al and Fe oxides, or recrystallization of clay and mica minerals. Corundum (Plate 2(a)) can be primary (MP = 205O”C), however, this mineral results mainly from the recrystallization of clay minerals, some crystallization of organically bound alumina, as well as Al hydroxides dehydroxylation. y-Al,O, is probably formed by the recrystallization of mica and clay minerals. Rutile (MP = 1827 “C), ilmenite (MP 2 1592 “C) and chromite (MP = 1450-2 180 “C) are refractory minerals and their origin seems to be dominantly primary (Plate 2(b)-(e)). Rutile may be secondary when it is a result of crystallization in the melt or the polymorphous transformation of anatase (Plate 2(f)) and brockite (Plate 2(g)). Rutile, ilmenite and chromite can also be secondary if they originate from the oxidation of organically combined Ti, Fe and Cr. Zincite (Plate 2(h)), pyrolusite (Plate 2(i)), cuprite (Plate 2(i) and (k)) and tenorite (Plate 2(l)) are probably secondary and their origin is connected with the oxidation of sphalerite, Mn and Cu sulphides, or with crystallization of organically linked Zn, Mn and Cu. Chalcophanite (Plate 2(m)) and W-Nb-Pb oxide (Plate 3(a)) probably also originate from the oxidation of Zn, Mn, W, Nb and Pb that are bound in organics. 3.1.2.3. Sulphates. Sulphate phases are rarely primary minerals and commonly secondary and tertiary products of authigenic (mainly epigenetic) minerals and organic matter in coal (Table 3). Gypsum and anhydrite are found as platy, needle or wedge-shaped crystals of l-10

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pm and over in length. Radial anhydrite aggregates are observed as well. They are crystallized mainly on the glass spheres and spheroids. The genesis of the gypsum is secondary and tertiary and that of anhydrite is mostly secondary. Anhydrite can occasionally be an original mineral due to its incomplete decomposition (I 9001220°C). However, it is commonly a newly formed phase in FA. Anhydrite originates from: (1) gypsum dehydration; (2) pore-water crystallization; (3) reactions between Ca oxide produced from the decomposition of carbonates, anhydrite and organic matter reacting with the sulfur oxide and sulphuric acid formed from organic matter and sulfur-bearing minerals during the combustion process. The Fe sulphates are primary (decomposition temperature = 480-810°C) or are formed from incomplete Fe sulphide oxidation (5 400-700°C). They could even be products of iron and sulfur combined in organics. The origin of barite (decomposition temperature = 1582 “C), Mg sulphate (decomposition temperature = 1122-l 124 “C) and alkaline sulphates (MP = 884-1069°C) is probably similar to that of anhydrite. Their occurrence may be primary or related to the transformations and reactions of organically bound Ba, S, alkaline and alkaline-earth elements, their proper minerals in coal and flue gas. The mechanism for the formation of alkaline and Ca sulphates at the surface of ash particles, as well as their enrichment towards the finest fly-ash fractions (< 20 pm) have been discussed elsewhere [29,30]. An interesting case is the formation of complex Fe, alkaline and alkaline-earth sulphates on ferrospheres and ferrispheres. These Fe oxyhydroxide spheres originate from pyrite framboids and they are coated with a glass film resulting from melted clay crusts associated with pyrite framboids [23]. It can be seen (Plate 3(b) and (cl> that the sulphate growth is a pseudomorph of the dendritic and skeleton Fe oxide crystallization, similar to the aforementioned portlandite formation. 3.1.2.4. Carbonates. Carbonate phases are mostly primary and tertiary minerals, and rarely secondary products of authigenic minerals and organic matter in coal (Table 3). Some of the calcite and dolomite could be primary because of the incomplete decomposition of coarse-grained particles at lower temperature ( < 700-950 “C). These carbonates are also secondary and mainly tertiary due to the carbonatization of Ca and Mg oxyhydroxides by flue gas in TPS and air and water in depositories. The origin of manganocalcite (Plate 3(d)), witherite and cerussite is probably similar to that of calcite and dolomite. 3.1.2.5. Phosphates. Phosphate phases are primary minerals and secondary products of detrital and authigenic minerals and organic matter in coal (Table 3). Apatite (MP = 1270-1660 “C) is a refractory mineral and its origin seems to be dominantly primary (Plate 3(e)). Minerals such as svanbergite (Plate 3(f)), vivianite, goyazite and ningyoite may be primary, however, some phosphate species probably also have secondary origin according to their layer attachment to the surface of glass spheres. Similar Ca phosphate occurrence was observed elsewhere [l 11. 3.1.2.6. Sulphides and sulphosalts. Sulphide and sulphosalt phases are primary minerals and secondary products of authigenic minerals in coal (Table 3). Pyrrhotite (MP = 1027- 1175 “C> seems to be a product of incomplete pyrite oxida-

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tion, solid-phase conversions of chalcopyrite, and hematite mushketovization (Plate 3(g)). Pb-Sb sulphosalt (Plate 3(h)) may be primary. Chalcocite (MP = 1130°C) could be related both to its availability in coal and to the chalcopyrite solid-phase conversion during burning (Plate 3(i)). 3.1.2.7. Others. Scheelite (MP = 1580°C) is a refractory mineral and its origin seems to be dominantly primary (Plate 3(i)). Iron carbide (Plate 3(k)) is likely secondary because of iron reduction by carbon during the combustion process. It could also be a product of the abrasion of the TPS’s metal surfaces. Graphite may be primary (MP = 3527 “C) and secondary in origin.

Table 5 Morphology

and size of uncommon

minerals and phases in fly ashes from Bulgarian

TPS

Morphology Grains and particles: 1. Angular -Pb-Sb sulphosalt, chromite, chalcophanite, zincite, anatase, amphibole, wollastonite, Fe carbide 2. Scaly -chloritoid, mica, vermiculite, talc, graphite 3. Rounded -pyrrhotite, Pb-Sb sulphosalt, chalcocite, spine1 ferrian, chalcophanite, zincite, anatase, y-AlzO,, wollastonite 4. Irregular -pyrrhotite, Pb-Sb sulphosalt, pyrolusite, W-Nb-Pb oxide, Fe carbide 5. Spheroids -wollastonite, Fe carbide Crystals: 1. Platy -magnesiofenite, rutile, vermiculite 2. Isometric -rutile 3. Pseudocubic -andalusite, Ca-Mn silicate, svanbergite 4. Prismatic -chalcocite, ilmenite, tenorite, anatase, brockite, W-Nb-Pb oxide, apatite, amphibole, zircon 5. Octahedral -cuprite, scheelite 6. Rhombohedral -manganocalcite 7. Bipyramidal -zircon 8. Needle -pyrolusite, olivine Aggregates: 1. Needle -pyrolusite, olivine 2. Polycrystalline -zincite, rutile Size 0.01-0.1

pm Pb-Sb sulphosalt,

0.1-l

cuprite, tenorite, rutile, anatase, andalusite,

manganocalcite,

apatite, graphite

pm pyrrhotite, Pb-Sb sulphosalt, magnesiofenite, ilmenite, spine1 ferrian, chromite, pyrolusite, cuprite, tenor&e, zinc&e, rutile, anatase, y-Al,O,, W-Nb-Pb oxide, olivine, amphibole, mica, talc, Ca-Mn silicate, manganocalcite, apatite, svanbergite, Fe carbide, graphite, scheelite

I-IO pm chalcocite, chalcophanitc, pyrolusitc, talc, wollastonite, Fe carbide

y-Al,O,,

olivine, zircon, chloritoid,

mica, vermiculite,

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The accessory minerals and phases (Table 3) are commonly represented by grain dimensions between 0.1-l ym (Table 5). Their phases occur on the surface of coarser particles, as inclusions in matrix phases and in particular as discrete crystals, aggregates, grains and clusters. This observation could explain the increased concentration of the majority of trace elements in the finer FA. The accessory phases show a considerable variety, however, their reliable identification is complicated due to some having extremely fine dimensions and very low (trace) quantities. Their origin is commonly hypothetical and difficult to explain because of the scarce information on coal accessory minerals, as well as on their behaviour under various physico-chemical effects, The results demonstrate that when there is detailed information on the modes of mineral occurrence in coal and their behaviour during heating, it is possible to make a reliable prediction of formation of the waste products during coal burning and gasification. The present studies also support the possibility that a number of trace elements in FA occur not only as impurities in the glass, crystalline and organic phases, but also as various proper mineral phases and discrete particles of Sr, Ba, Zr, Nb, W, Ti, Cr, Mn, Cu, Zn, Sb, Pb, U and probably of Y, MO, La, Ce, Hf, V, Ga, Ge, As, Sn, Ag, etc. Some modes of element occurrence can elucidate various problems related to: coal burning; waste product formation, storage and utilization; as well as to environmental protection. For example, the occurrence of some minerals and phases is related to specific physico-chemical conditions of formation and weathering, while their amount and morphologies are informative for the separation and extraction of different valuable components. In addition, an extended knowledge of the discrete minerals and phases, and their behaviour during heating could also explain the susceptibility of some elements: to accumulate in some products; to emit into the atmosphere by stack emissions in solid, liquid and gas states; as well as to migrate in and out of the waste depositories near TPS. These aspects will be discussed and summarized in a future report, 3.1.3. Organic constituent The organic constituent or the unburnt coal (char) components are represented by slightly changed, semicoked and coked particles. The last two types are produced from the partial and complete melting of the various organic constituents, while the first type is represented by particles which were exposed to temperatures not higher than 550°C. The slightly changed coal particles are typical for the coarse-grained fractions above 100 km. The porous coke spheres and spheroids are hollow and have thick faces, in contrast to the glass ones. Some of them have a fine-scaly texture and others are skeletons built up of complex interwoven plates and fibres. The organic constituent quantity in FA is within the range 0.1-25 wt.%. 3.2. Bottom ash and slag BA (from dry-ash discharge boilers) does not show great qualitative differences from FA with regard to the phase composition and particle morphology. The content of the amorphous and char components in BA is always higher than in FA. BA is occasionally enriched in clay minerals, quartz, mica, feldspars and other unmelted mineral aggregates

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which are coarser grained in coal and have experienced a considerable particle fractionation in boilers. On the other hand, BA shows diminished contents of S and Fe-bearing minerals in comparison with FA at some TPS. The slag (from slag discharge boilers) demonstrates significant phase and morphological differences in comparison with its respective FA. The slag is almost fully amorphous and it does not contain unburnt coal components, while FA is enriched in these components (normally lo-25 wt.%). The crystalline matter in the slag is represented by trace amounts of quartz, cristobalite, magnetite, plagioclase, mullite, hematite, gypsum and calcite. These minerals crystallize in the silicate melt, in pores or on the slag surface. BA and especially slag demonstrate increased concentrations of Fe*+ glass phases and mineral species due to the more reducing conditions during BA and slag formation. 3.3. Lagooned ash-slag LAS normally shows trends to an intermediate phase composition and particle morphology with BA, slag and FA. However, this product also has some specific peculiarities caused by transport, separation during discharge and storage, and additional changes related to LAS weathering in the depositories. For LAS the following is typical: a solution and redistribution of some sulphates, carbonates and chlorides; some increase of hematite and maghemite contents on account of magnetite oxidation; a hematite limonitization; an absence of anhydrite because of its hydration to gypsum; some Ca-Mg silicate hydration; a transformation of lime and periclase into portlandite, brucite, calcite, dolomite and gypsum, known as linking substances for the different LAS particles in depositories; and an encrusting of the particles with fine weathered material. During the flow of LAS pulp from transport pipes, there are also some changes in particle distributions along the sedimentation regions. 3.4. Genetic generalization

The formation of combustion wastes from coal burning has a direct relationship with the mineral and chemical composition of coal, and the technological process of fuel preparation and burning, as well as with waste removal and storage. The inorganic matter of coal consists of various minerals and phases. These minerals and phases may or may not undergo physico-chemical changes in the combustion chambers to give the basic phase composition of the solid waste products. Additionally, some minerals and phases are originated from the crystallization and vitrification of liberated components from organic matter. Major processes which drive these phase formations are mainly transformations in solid state; solid-phase reactions; formation of new solid, liquid and gas phases and reactions between them. The behaviour of each coal particle is individual and the particle falls under the influence of various physico-chemical and aerodynamic conditions during burning. The coal particles experience thermal effects from I 500~600°C to the maximum determined temperature in boilers ( = 1600-1700°C) for a variable residence time. That is why some coal and mineral particles undergo weak changes such as incomplete burning and transformation to semicoke, metakaolinite and glass, and

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partial decomposition of some sulphides, sulphates, hydroxides and carbonates. Other minerals experience various and complex physico-chemical transformations such as: decomposition (sulphates, carbonates, sulphides); oxidation (sulphides, sulphates, magnetite); reduction (hematite); dehydration and dehydroxylation (sulphates, clay minerals, Fe and Al hydroxides); hydration (anhydrite, lime, periclase, Fe sulphates); carbonatization and sulphatization (lime, periclase, alkaline and iron oxides); reversible and stable polymorphic transformations (quartz, feldspars, Ti oxides); melting and solution (quartz, clay minerals, mica, feldspars, oxides, chlorides, sulphates); and other combined conversions and reactions between pre-existing and new-formed compounds. When FA, BA and melts leave the high-temperature zone in boilers, they experience sharp phase transitions (in particular slag and BA) which lead to intensive glass formation and limited mineral crystallization. The mineral formations are a result of crystal growth in silicate melts (quartz, cristobalite, mullite, magnetite, olivine, pyroxene, feldspars), recrystallization (clay minerals, feldspars, mica, oxides), solid-phase reactions (Ca-Mg silicates, basic plagioclases), as well as some alterations and substitutions. In addition, an extensive volatilization of some components from the organic and inorganic matter occurs during burning and a subsequent deposition and crystallization of condensed vapor phases (sulphates, oxyhydroxides, phosphates, chlorides) onto different solid particles may also occur. Other minerals in these products are pre-existing and relict coal minerals (clay minerals, magnetite, hematite, feldspars, rutile, zircon, other refractory silicates and oxides) which have not undergone lattice-transformations. The morphological peculiarities of the new crystal phases demonstrate a fast crystal growth in unsteady conditions where the basic inorganic matter represents a “frozen” system. The fly ash shows qualitative composition of the crystalline matter similar to the high-temperature coal ash (450~815°C) prepared in the laboratory furnace under air atmosphere. The main distinctions are related to mineral abundances, particle morphologies and the presence of some Fe’+ minerals and higher-temperature phases such as mullite, andalusite, corundum, cristobalite, Ca-Mg silicates, spinels, sanidine and glass which are more characteristic of FA.

4. Conclusions Fly ash, bottom ash and lagooned ash comprise inorganic and organic constituents. The inorganic part consists mainly of non-crystalline (amorphous) components (glass spheres, spheroids, angular and irregular particles) and lesser amounts of crystalline components represented by various major (quartz, magnetite, hematite, mullite, feldspars, gypsum, anhydrite, kaolinite-metakaolinite, Ca-Mg silicates, lime, portlandite, cristohalite), minor (mica, calcite, olivine, spinel, maghemite, limonite, magnesioferrite, periclase, brucite, corundum, Al hydroxides, pyroxene, chlorite, Fe, Na-K and Mg sulphates, barite, dolomite) and accessory (r-utile, apatite, svanbergite, Fe carbide, chloritoid, zincite, pyrolusite, cuprite, zircon, chromite, etc.) mineral phases. The organic constituent contains unburnt coal components represented by slightly changed, semicoked and coked coal particles. The origin of solid phases could be: primary minerals and phases contained in coal and having undergone no phase transition (quartz,

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kaolinite, mica, feldspars, volcanic glass, rutile, zircon, coal particles); secondary phases formed during burning (magnetite, hematite, metakaolinite, mullite, anhydrite, lime, periclase, Ca-Mg silicates, glass, semicoke, coke); or tertiary - minerals and phases formed during the transport and storage of fly ashes and bottom ashes (gypsum, calcite, hematite, limonite, portlandite, brucite). The results demonstrate that when there is detailed information on the modes of mineral occurrence in coal and their behaviour during heating, it is possible to make a reliable prediction of the formation of the waste products during coal burning and gasification.

Acknowledgements The authors sincerely thank Mr. K. Bozhilov and Mr. M. Tarassov from the CLMC for their assistance with TEM and SEM examinations.

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