Christina G. Vassileva

Occurrence, abundance and origin of minerals in coals and coal ashes

The mineralogy of coal and coal ash samples from a wide variety of deposits worldwide has been studied by X-ray diffractometry, light microscopy, SEM, TEM, and DTA-TGA methods. The common major minerals identified in the crystalline matter of coals are quartz, kaolinite, illite, calcite, pyrite, plagioclase, K-feldspar and gypsum, and occasionally dolomite, ankerite, siderite, Fe oxyhydroxides and sulphates. A number of minor and especially accessory minerals are also present. The modes of occurrence and some genetic peculiarities of the minerals found are described and summarized. Minerals and phases of probable detrital origin include mainly silicates, volcanic glass, oxyhydroxides and phosphates. Authigenic minerals of syngenetic origin may be sulphides, clay minerals, carbonates and rarely sulphates and phosphates. Epigenetic minerals, formed by the infiltration of low-temperature hydrothermal solutions, may include sulphides, carbonates, sulphates, clay minerals, quartz, chlorides, and probably alkaline-earth hydroxides and zeolites. The alteration products of detrital and authigenic minerals may be Fe and Al oxyhydroxides, sulphates, kaolinite, illite, chlorite, muscovite, zeolites and calcite. The behaviour of these minerals and phases during low- and high-temperature ashing is also discussed.

Mineralogy of combustion wastes from coal-fired power stations

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).

Geochemistry of coals, coal ashes and combustion wastes from coal-fired power stations

Abstract Contents, concentration trends, and modes of occurrence of 67 elements in coals, coal ashes, and combustion wastes at eleven Bulgarian thermoelectric power stations (TPS) were studied. A number of trace elements in coal and coal ash have concentrations greater than their respective worldwide average contents (Clarke values). The highest values in coal ash are displayed by elements such as Rb, Cs, Ba, Cu, Sb, Bi, U, and Ag. Trace elements are concentrated mainly in the heavy accessory minerals and organic matter in coal. In decreasing order of significance, the trace elements in coal may occur as: element-organic compounds; impurities in the mineral matter; major components in the mineral matter; major and impurity components in the inorganic amorphous matter; and elements in the fluid constituent. A number of trace elements in the waste products, similar to coal ashes, exceed known Clarke contents. Trace elements are mainly enriched in non-magnetic, heavy and fine-grained fractions of fly ash. They are commonly present as impurities in the glass phases, and are included in the crystalline components. Their accessory crystalline phases, element-organic compounds, liquid and gas forms, are of subordinate importance. Some elements from the chalcophile (Cu, Zn, Ga, Ge, Pb, As, Sb), lithophile (Be, Ba, Ce, Hf, Sr, La, Zr, Mo, U) and siderophile (Sc, Cr, V) groups may release into the atmosphere during coal burning. For others, the combustion process appears to be a powerful factor causing their relative enrichment in the fly ash and rarely in the bottom ash and slag. Considerable amounts of Hf, some chalcophile elements (Zn, As, Tl, Pb) and Ag from stack emissions have probably entered the soil near TPS. Trace elements can also occur in water soluble forms in the fly ash (Li, Sr, Mo, Cs, V, Cr, Mn, As, Bi, B, F, Cl, Br, I) and probably contaminate the surface and subsoil waters. Some trace elements (Sr, Ba, Yb, Sc, Cd, Tl, Pb, Bi) may accumulate in the vegetation near TPS.

Behaviour of elements and minerals during preparation and combustion of the Pernik coal, Bulgaria

The chemical and mineral composition, including major (Al, Ca, Fe, K, Mg, S, Si, Ti), minor (Na, P) and trace (Br, Cl, Co, Cr, Cu, Li, Mn, Ni, Pb, Rb, Sr, Zn) elements and different minerals, of the Pernik subbituminous coals and their preparation and combustion solid waste products were studied. Feed coals, upgraded coals (high-grade and low-grade coals) and their waste products, namely coal slimes and host rocks generated from the Pernik coal preparation plant, as well as combustion waste products such as bottom ashes, fly ashes and lagooned ashes resulted from the Republica coal-fired thermoelectric power station were characterized. The occurrence and behaviour (partitioning, volatilization, condensation, capture and retention) of the above-mentioned elements and various minerals during coal preparation and combustion are described. The results indicate some technological problems and possible environmental pollution of the air, water, soil and vegetation with certain elements in the areas surrounding both thermoelectric power station and coal preparation plant.

Contents, modes of occurrence and origin of chlorine and bromine in coal

Contents, modes of occurrence and origin of Cl and Br in individual coal samples from 34 deposits worldwide (Bulgaria, Australia, the United States, Japan, Canada, South Africa, China, and Ukraine) were studied. Some relationships of Cl and Br contents with chemical and mineral composition, rank, age, ash yield and geographic location of coals are described. Despite of similar chemical and geochemical properties of Cl and Br some distinct differences in the association, behaviour and occurrence of both elements in coal were found and discussed. Chlorine and Br in coal may occur, in decreasing order of significance, as organic compounds, as impurity components in the crystalline and amorphous inorganic constituents, in the fluid constituents and as discrete minerals. Both elements could have an organic occurrence and association with water-soluble (mainly ionic) and water-insoluble (ionic and covalent) organic combinations. Chlorine was identified as an impurity in minerals and inorganic phases such as clay minerals, mica, feldspars, polyhalite, gypsum, siderite, volcanic glass, phosphates and other carbonates and sulphates. A characteristic Br association with illite and to a lesser extent with mica, kaolinite and Fe-bearing minerals was also found. Water molecules, hydroxyl groups and exchangeable cations in various minerals play a leading role for the inorganic occurrence and distribution of both elements. Discrete Cl minerals such as sylvite, halite, chlorapatite and probably carnallite were also identified. Significant amounts of Cl ions may associate with solutions in the inherent moisture of the coal mesoporosity. Limited proportions of both elements may also occur in gas–liquid inclusions of different solid phases. The favourable conditions for Cl and Br enrichments in coal are also discussed.

Some relationships between coal rank and chemical and mineral composition

Some relationships between coal rank (Cdaf) and the mineral and chemical composition of coals and their high-temperature ashes worldwide were studied. Low-rank coals (Cdaf <75 wt%) are relatively rich in moisture, volatile matter, ash, H, N, O and S, and their ashes are abundant in MgO, CaO and SO3. Coal ashes derived from higher-rank coals demonstrate increased contents of SiO2, Al2O3, Fe2O3, K2O, Na2O and TiO2. It was found that the occurrence, abundance and origin of mineral matter in coal depends on the coal rank to a certain extent. Coals enriched in illite, mica, chlorite, spinel, dolomite, siderite and hexahydrite, and partly in quartz, kaolinite and Fe oxyhydroxides, are of higher rank, while coals with increased contents of montmorillonite, feldspars, zeolite, Al oxyhydroxides, calcite, pyrite, gypsum and Fe, Al and Ba sulfates are of low rank. Higher-rank coals are abundant in ash-forming elements associated with probable detrital minerals, while low-rank coals show enrichment in ash-forming elements associated with probable authigenic minerals and organics. Various processes related to some changes in inorganic matter during coalification are discussed. The metamorphic progression and leaching behaviour of different minerals, phases and elements from low- to high-rank coals have resulted in essential changes in the mineral and chemical composition of coals.

Mineralogy and geochemistry of Bobov Dol coals, Bulgaria

Abstract The mineralogy, elemental composition, and modes of occurrence of 49 elements in nine composite samples of Bobov Dol high-ash coals were studied by optical microscopy, scanning and transmission electron microscopy, X-ray diffractometry and chemical analysis. The major minerals were quartz, kaolinite, illite, plagioclase and K-feldspar and the minor minerals and phases were pyrite, marcasite, siderite, calcite, dolomite, gypsum, muscovite, montmorillonite and volcanic glass. The accessory minerals include a wide variety of minerals, such as galena, pyrrhotite, magnetite, hematite, goethite, chromite, rutile, anatase, corundum, gibbsite, biotite, chlorite, zircon, enstatite, garnet, jarosite, alunite, barite, polyhalite, aragonite, ankerite, witherite, apatite, halite and sylvite. The modes of occurrence and some genetic peculiarities of the above-mentioned minerals are described. Thirty-three elements occurred in concentrations higher than the respective Clarke values; especially S, Rb, Nb, Hf, Zn, Cu, Pb, Mn, Ti and U. The concentration trends and modes of occurrence of the trace elements are also discussed. In decreasing order of significance, the trace elements probably occur as element-organic compounds, as impurities in the mineral constituents, as major components in the minerals, as major and impurity components in the inorganic amorphous constituents, and in the fluid constituents. The Bobov Dol coals have undergone complex syngenetic, diagenetic and epigenetic mineralization processes associated with hydrothermal and volcanic activities. These processes were also accompanied by intensive tectonic movements and, possibly, by a later change from continental to marine sedimentation in the area after burial of the coal.

Contents, modes of occurrence and behaviour of chlorine and bromine in combustion wastes from coal-fired power stations

Abstract Contents, modes of occurrence and behaviour of Cl and Br in combustion wastes from eleven Bulgarian coal-fired thermoelectric power stations (TPS) were studied. Despite some differences, there are indicative close relationships between the contents, redistributions, modes of occurrence and behaviour of Cl and Br in fly ash (FA), bottom ash (BA) and stack emissions during coal combustion. Chlorine and Br occur in inorganic and organic combinations in FA and BA. Both elements are normally present, in decreasing order of significance, as: (1) impurities in the glass phases (dominantly for Br); (2) impurities in the various crystalline components (dominantly for Cl) such as surface-bound Ca–Na–K–Mg sulphates and phosphates; (3) organic compounds in char; (4) impurities in the fluid constituents; (5) probable discrete inorganic phases. Dominant proportions (up to 96%) of the initial Cl and Br in coal are normally emitted during combustion process in TPS. The main reasons for Cl and Br volatilisation, condensation, capture and retention during coal combustion are described. The results also indicate some possible environmental pollution of the air, water and soil with Cl and Br in the areas surrounding the large TPS.

Relations between ash yield and chemical and mineral composition of coals

Abstract Some relations between the ash yield (db) and the mineral and chemical composition of coals and their high-temperature ashes were studied. Low-ash coals ( 2 O and TiO 2 . High-ash coals (> 20 wt%) have increased contents of H, N, O, S and minerals such as quartz, illite, calcite, pyrite and gypsum, while their ashes are enriched in SiO 2 , Fe 2 O 3 and K 2 O. Medium-ash coals (10–20 wt%) have high concentrations of C and Al 2 O 3 (ash basis) and minerals such as kaolinite, mica and siderite, and show the lowest contents of plagioclases and K-feldspars. The low-ash coals are enriched in ash-forming elements associated with probable authigenic minerals and organics, whereas higher-ash coals demonstrate an enrichment in ash-forming elements associated with probable detrital minerals. Various processes related to changes in ash yield and inorganic matter during coalification are discussed. The material supply and leaching behaviour of different minerals, phases and elements during peat formation and conversion to coal as well as the various plant constituents and regional, depositional and paleoenvironmental conditions are responsible for the most essential changes in the mineral and chemical composition or ash yield of coals.

Comparative chemical and mineral characterization of some Bulgarian coals

Abstract The mineralogy and chemical composition of lignitic, subbituminous and bituminous coal and coal-ash samples from six Bulgarian deposits, namely Maritza West (MW), Sofia (S), Maritza East (ME), Bobov Dol (BD), Pernik (P) and Balkan (B), were studied and a model for comparative characterization of their composition was done. Major minerals identified in the crystalline matter of Bulgarian coals are commonly quartz, kaolinite, illite, calcite, pyrite, plagioclase, K-feldspar and gypsum and occasionally dolomite, siderite, Fe oxyhydroxides and Fe sulphates. Some genetic features of the inorganic matter in coal and coal ash are described and compared. The coals studied show high detrital (P, B, BD, ME) or high authigenic (S, MW) mineral abundance with sulphide–sulphate (P, BD, ME, MW) or mixed carbonate and sulphide–sulphate (B, S) authigenic mineral tendencies. The high-temperature ashes of these coals belong to sialic (P), sialoferricalcic (B, BD), sialoferric (ME) or ferricalsialic (MW, S) chemical types. They have high (B, P, BD) or low (ME, MW, S) silicate mineral abundance with sulphate (B, P, ME, MW, S) or oxyhydroxide (BD) mineral tendencies.