ERTH 3117 COAL GEOLOGY AND PETROLOGY SECTION Lecture 13:Coal Rank, Grade and Type
COAL CLASSIFICATION Coal Type A classification of coal distinguished on the basis of the constituent plant materials; megascopic classification is a “lithotype”. Microscopic classifications use “microlithotypes” and “macerals”. Coal Grade A classification of coal based on degree of purity i.e. quantity of ash left after burning; dependent upon amount of mineral matter Coal Rank The classification of coals according to their degree of metamorphism or coalification (maturation) in the natural series from lignite to anthracite. THESE ARE INDEPENDENT PROPERTIES but can vary together spatially
FIRST CUT CLASSIFICATION IS RANK – IT DETERMINES UTILISATION Image courtesty of Australian Coal Association website
COAL RANK SERIES-SIMPLIFIED PROCESS Cartoon courtesy of Kentucky Geological Survey
NOT SO SIMPLIFIED MATURATION PROCESSFROM LEVINE (1993)COALIFICATION APPROX ASTM RANK PREDOMINANT PROCESSES PHYSICO-CHEMICAL CHANGESSTAGEPeatification Peat Maceration, humification, Formation of humic substances, increased gelification, fermentation, aromaticity concentration of resistent substances (lipids, minerals)Dehydration Lignite to sub- Dehydration, compaction, loss of o- Decreased moisture contents and O/C ration, bituminous bearing groups, expulsion of - increased heating value, cleat growth COOH, CO2 and H2OBituminisation Upper sub-bituminous Generation and entrapment of Increased Rvo, inc. fluorescence, increased A through high volatile hydrocarbons depolymerisation of extract yields, dec. in density and sorbate A bituminous matrix, increased hydrogen bonding accessibility, increased strengthDebituminisation Uppermost high volatile Cracking, expulsion of low Still increasing Rvo, Decreased fluorescence, A through low volatile molecular weight hydrocarbons, dec. molecular weight of extract, dec. H/C bituminous ESP. methane ratio, decreased strength, cleat growthGraphitisation Semi-anthracite to Coalescence and ordering of pre- Dec in H/C ration, stronger XRD peaks, inc. anthracite to meta- graphitic aromatic lamella, loss of sorbate accessibility, Rvo anisotropy, anthracite hydrogen and loss of nitrogen strength, ring condensation and cleat healing
COAL RANK PARAMETERS 100 75 86 90 91 92 95 65 71 80 50 60 52 61 40 33.5 35.6 31 36.0 36.4 36.0 35.2 30 23.0 22 14.7 14 10 11.7 8 %C ARB O N 7.00 % VM (daf) 5 percent ENERG Y (MJ/k g) 3 2.83 % IN S ITU MO IS T 1.97 2 1.58 Rvo 1.03 1 1 1 1 Rvm ax 0.63 0.42 0.20 0.1 ANTHRACITE ANTRHACITE PEAT BITUMINOUS BITUMINOUS BITUMINOUS BITUMINOUS BROWN WOOD HIGH VOL. COAL LOW VOL. MID VOL. SEMI- SUBSome parameters are more sensitive than others to thermal maturationThese rank parameters, along with type and grade, assist in predicting utilisation behaviour
GRADE Amount of impurities (i.e. mineral matter) in the coal Commonly analysed as “ash yield” Ash is the unburnable part of coal. It is most often sand and clay blown into the swamp or brought in by river or tides. Most commercial coals range from 3% to 9% ash. Why do we want to analyse for it? Mineral matter affects the coal processing and handling. Hard minerals increase the wear and tear on equipment during handling and crushing. The quantity of ash and its composition is important to determine the method of its removal, either as a dry ash or a slag during combustion. Other minerals and/or trace elements will affect the quality of the coke and resulting steel.
Ash approximates Mineral Matter Content,but does not encapsulate it Mineral matter includes other components lost upon combustion such as •CO2, •SO2 and •H20 of hydration •Salts (e.g. Cl) •Carbonates •Sulphides Often the volatiles from these minerals are the ones that foul or corrode boilers in power plants or that are emitted in the gases as pollutants From Ward, 1984 There are also different reporting standards
Coal Quality Those chemical and physical properties of a coal that influence its potential use Thomas, 2002Chemical properties:•Grade (ash yield and/or mineral matter content and composition)•Rank (degree of coalification or thermal maturity) ANALYTICAL TESTS•Type (composition described either by lithotype or maceral content)Physical properties:•Density•Abrasion index•Hardness or grindability•Particle size distribution•Flotation behaviour•Degree of oxidationPerformance properties:•Calorific value/specific energy•Ash fusion temperatures•Caking tests (free swelling index, Roga index)•Coking tests (Gray-King coke type, Fischer assay, Gieseler plastometer,dilatometer)
HOW TO CHARACTERISE A COAL SEAM? Coal Seam a stratum or bed of coal; upper contact with rock called “roof”, lower contact called “floor” Bench (of coal) a mineable section of coal or a unit of a coal seam that can be traced laterally for some distance; it is usually bounded by mappable rock partings or a significant change in lithotype; generally used as a basis for sampling. Synonomous with the term ply Parting a rock band or thin bed within a coal seam; often rock partings become thick and create a divergence of the coal beds known as a split. Band A significant layer within a seam or ply; if non-coal often referred to as “clay band” or “dirt band” or “tuff band”. Colloquial term “penny band” denotes thickness. Also used to describe the organic units within coal lithotypes. Coal Type a classification of coal distinguished on the basis of Ward Chapter 5 the constituent plant materials; megascopic classification is a “lithotype”.
DISTRIBUTION Geometry (thickness and areal extent) of the coal deposit is controlled by the depositional environment; i.e. available space between active water courses or in ponded depressions Aerial view of peat bog in Russia
MIRE EVOLUTION •Evolutionary sequence of mire development and peat accumulation manifested in the stratigraphy of coal types (megascopic and microscopic) •Lateral variability will occur due to variations in the substrate topography which is often “swamped” by mire development •Paludification (to make a “lake”) •Terrestrialisation (to make “land”) McCabe, 1984
Internal stratigraphy or layering within a coal deposit is controlled by plant succession, flooding from adjacent water courses (or volcanic ash falls) and degree of decay Primary peat woodland Black water brookSuccessionIf water table is raised or lowered, Oligotrophic bog lakethen these different vegetationzones will overlay one another
A Example of the Goonyella Middle Seam •Coal deposit varies from >10m to <5m •Thins to radially to the north, west and south •Organic and inorganic composition will change with respect to geometry A’
LATERAL CORRELATION OF LITHOTYPE PROFILESIN THE GOONYELLA MIDDLE SEAMA RELATIVE DISTANCE IS 50 km A’ rider Dull Main seam Dull banded Ply 2 datum Ply 310m Interbanded Ply 4 Bright banded Leader splits Lithotype profile from core leaderCan the composition of the plies change laterally?
LITHOTYPE PROFILES 0 100 •AT FACE OR IN CORE T M G B S H LE S EM IN •SEPARATE COAL 200 SEAM INTO MAPPABLE LA T O Gl4 300 “PLIES” OR “BENCHES” 400 •WHY? PLY 1 •SEAM H R E 500C A G CORRELATION PLY 2 600 •SELECTIVE MINING PLY3 700 •QUALITY CONTROL 800 •GEOTECHNICAL 900 PROPERTIES PLY 4 Band width rule 1000 5mm Australian system RAMP 27 LD CORE 3 mm US system 10 mm European system
MEGASCOPIC CLASSIFICATION COAL LITHOTYPES (not including stone partings) first coined by Marie Stopes 1919- “On the Four Visible Ingredients of Banded Bituminous* Coal” *there’s another one for brown coal DULL BRIGHT SA DULL DULL w/MINOR BANDED DULL BANDED BANDED BRIGHT BRIGHT <1% bright 1-10% bright 10-40% bright 40-60% bright 60-90% bright >90% brightICCP DURAIN DURAIN DURO-CLARAIN CLARAIN VITRO CLARAIN VITRAINASTM DURAIN DURAIN DULL CLARAIN CLARAIN BRIGHT CLARAIN VITRAIN Others: fibrous coal=fusain=charcoal=mother coal shaley coal=bone coaly shale=carbonaceous mudstone cannel coal
COAL TYPE AND FRACTURE/CLEAT Cleat: the network of micro fractures coals develop when subjected to changes in stress or uplift. Bright banded coal Dull coal highly cleated poorly cleated thin to thick vitrain minor thin vitrain WHICH WILL BE MORE FRIABLE? WHICH WILL BE MORE PERMEABLE? (assuming the cleats are not mineralised)
COAL STRENGTH varies with RANK AND TYPE All samples at 0.2 MPa Confining Strength 35 33 30 Peak Strength MPa 25 25 20 Riverside Rank 1.2 Rank 0.53 15 15 Rank 0.7 Rank 0.8 10 Rank 1.2 Rank 1.33 5 Poly. (Riverside Rank 1.2) 0 2 BRT 3 BB IB 4 5 DB D6 Brightness
COAL BREAKAGE AND GRINDABILITY 90 90 80 80 mass % passing T10 at 0.075kWh/t Hardgrove Grindability Index 70 70 bright banded coals 60 60INCREASING FINES 50 50 all data 40 40 30 30 dull coals rock 20 dull to dull w/minor bright 20 dull banded to interbanded 10 bright banded 10 HGI ALL COAL T YPES 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Rank (Rvo) Data from Esterle et al, 2000
COAL TYPES CAN ALSO HAVE DIFFERENT GRADES STONE DULL+DM DB-BB 0 9% 32% 59% Raw ash=24.3% 100 %<2mm after 20 drops=26% B STHO STEM ING M AVERAGE Gl4 (27% of seam) 200 STONE DULL+DM DB-BB LE 13% 33% 54% Gl4 Raw ash=30.2% LA %<2mm after 20 drops=26.5 300 400 PLY 1 AVERAGE P123 (51% of seam) STONE DULL+DM DB-BB C AR E 500 H G 11% 42% 47% Raw ash=27.6% %<2mm after 20 drops=22.5 PLY 2 600 PLY3 700 800 AVERAGE P4 (22% of seam) STONE DULL+DM DB-BB 2% 2% 95% 900 Raw ash=9.5% PLY 4 %<2mm after 20 drops=32.2 1000 More partings, more dirt, higher ash RAMP 27 LD CORE
SO, IF YOU SHIFT THEMINING SECTIONWHATCHARACTERISTICSWILL OFTENCHANGE?IF I’M LOOKING FORPERMEABILITY TODRAIN OR PRODUCTGAS, WHAT WOULD ITARGET?
PLIES ARE THE BASIS FOR COAL SAMPLING •Core sample (good) •Face sample (good) •Grab sample (bad) If the coal seam is thick, or has a lot of vertical variability, it will have to be subdivided usually on the basis of “plies” or “benches” Ply=designated unit of sampling, often determined by stone partings or variation in lithotypes
COAL TYPE-SCALES OF CHARACTERISATION MEGASCOPIC MACROSCOPIC MICROSCOPIC telovitrinite BRIGHT BANDED id dv sf DULL 50 mm 250 microns What do you notice in the texture between the different types? Scales?
MICROSCOPIC CLASSIFICATION-COAL MACERALS ENCYCLOPÆDIA BRITANNICA maceral definition microscopic organic component of coal consisting of an irregular mixture of different chemical compounds. Macerals are analogous to minerals in inorganic rocks, but they differ from minerals in that they have no fixed chemical composition and lack a definite crystalline structure. Macerals change progressively both chemically and physically as the rank of coal advances. (Rank constitutes position in the lignite-to-anthracite series and is primarily based on increasing carbon content and increasing fuel value.) Macerals for “black coal”, i.e. bituminous coal, are classified into three major groups: vitrinite, inertinite, and exinite Vitrinite (Huminite is the term used for brown coals and lignites) is derived from woody plant tissue and includes the macerals collinite and telinite. Most coals have a high percentage of vitrinites. Inertinite group comprises fusinite, micrinite, sclerotinite, and semi-fusinite [and inertodetrinite, macrinite], which are all rich in carbon [due to primary oxidation from mouldering or charring]. Liptinite (Exinite) macerals, characterized by a high hydrogen content, include alginite, cutinite, resinite, and sporinite [liptodetrinite, suberinite, exudatinite, bituminite, fluorinite….].
“Simple” 3 component system in reflected light (generally oil immersion) •Vitrinite-will react when heated up (gray stuff) Liptinite Vitrinite •Inertinite-won’t react when heated up (white stuff) •Liptinite-will react, but you may not always want it to Inertinite hole
All of these analyses attempt to quantify the chemistry of thecoal that will impact or on its behaviour during utilisation For example during coking Ward, 1984 The vitrinite will react; increasing rank and vitrinite content willPhotomicrograph showing vitrinite-semifusinite transition before (left) and after increase reactivity of the coal(right) coking; Ro values given alongmargins.photo courtesy of Taylor et al, 1998 during coking
Australian Standard 2856-1986/1995bituminous coals MACERAL MACERAL SUBGROUP MACERAL ORIGINS GROUP*brown coal only VITRINITE TELOVITRINITE Textinite* Well preserved cell wall Criteria for recognition Occurs as bands or lenses Texto-ulminite* Partially gelified cell wall •colour Eu-ulminite* Telocollinite Completely gelified cell wall Gelified cell wall and filling •reflectance DETROVITRINITE Attrinite* Sparsely packed matrix of cell •morphology fragments Occurs in matrix •size Densinite* Finely packed matrix of cell fragments •polished relief Desmocollinite Gelified humic matrix •fluorescence GELOVITRINITE Corpocollinite Gelified cell filling in tissue (CT) in matrix (CM) Occurs in matrix Porigelinite* Vesicular humic gel in matrix Simple steps Poricorpocollinite Vesicular humic gelified cell filling •Is it grey, black or Eugelinite Humic gel LIPTINITE LIPTINITE Sporinite, cutinite, Spores, waxes, resin, cuticle, white? resinite, liptodetrinite suberin, algae, expulsed lipid, •Is it Structured (telo), Occurs in matrix alginite, suberinite, etc fluorinite, unstructured attrital exsudatinite, bituminite (detro) or gelified INERTINITE TELO-INERTINITE Semifusinite Partially oxidised tissue of low and moderate reflectance (gelo)? Occurs as lenses (mouldering, char?) •<20um grey? <30um Fusinite Oxidised tissue (char) Sclerotinite/Funginite Fungal spore/test/stalk white? DETRO-INERTINITE Inertodetrinite Oxidised cell wall fragment or cell filling of moderate to high Occurs in matrix reflectance Reflected light, oil Micrinite Fine grained oxidised material (<5microns) immersion lenses GELO-INERTINITE Macrinite Oxidised gel Occurs in matrix
SUMMARY • COAL TYPE IS CONTROLLED BY INGREDIENTS • It affects coal chemistry, texture, hardness, washability, and utilisation properties • INGREDIENTS ARE ORGANIC AND MINERAL • INGREDIENTS CHANGE AS A FUNCTION OF: – Depositional environment at time of peat formation • Swamp, marsh, bog; near the coast, far up river – Plant succession within the peat deposit (follows deposit geometry) – Palaeoclimate* – Botanical evolution* • INGREDIENTS RECORDED IN THE COAL LITHOTYPE PROFILE
Recommended ReadingStopes, M.C., 1919: On the four visible ingredients in banded bituminouscoals. Proc. Royal Soc. 90B, 470-87.Moore, T.A. and Shearer, J.C., 2003. Peat/coal type and depositionalenvironment—are they related? International Journal of Coal Geology 56,233– 252.Holdgate, G.R.; Kershaw, A.P.; Sluiter, I.R.K, 1995. Sequencestratigraphic analysis and the origins of Tertiary brown coal lithotypes,Latrobe Valley, Gippsland Basin, Australia. International Journal of CoalGeology Volume: 28, Issue: 2-4, pp. 249-275.Volkov, V. N., 2003. Phenomenon of the Formation of Very Thick CoalBeds. Lithology and Mineral Resources, Vol. 38, No. 3, 2003, pp. 223–232. Translated from Litologiya i Poleznye Iskopaemye, No. 3, 2003, pp.267–278.