Geology Science of coal

1. ERTH 3117
COAL GEOLOGY AND PETROLOGY SECTION
Lecture 13:
Coal Rank, Grade and Type

2. 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

3. FIRST CUT CLASSIFICATION IS RANK – IT DETERMINES UTILISATION
Image courtesty of Australian Coal Association website

4. COAL RANK SERIES-SIMPLIFIED PROCESS
Cartoon courtesy of Kentucky Geological Survey

5. NOT SO SIMPLIFIED MATURATION PROCESS
FROM LEVINE (1993)
COALIFICATION APPROX ASTM RANK PREDOMINANT PROCESSES PHYSICO-CHEMICAL CHANGES
STAGE
Peatification 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 H2O
Bituminisation 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 strength
Debituminisation 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 growth
Graphitisation 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

6. Teichmuller and Teichmuller, 1982

7. 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-
SUB
Some parameters are more sensitive than others to thermal maturation
These rank parameters, along with type and grade, assist in predicting utilisation behaviour

8. 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.

9. 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

10. Coal Quality
Those chemical and physical properties of a coal that
influence its potential use
Thomas, 2002
Chemical 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 oxidation
Performance 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)

11. 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”.

12. 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

13. 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

14. 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 brook
Succession
If water table is raised or lowered,
Oligotrophic bog lake
then these different vegetation
zones will overlay one another

15. 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’

16. LATERAL CORRELATION OF LITHOTYPE PROFILES
IN THE GOONYELLA MIDDLE SEAM
A RELATIVE DISTANCE IS 50 km
A’
rider
Dull
Main seam
Dull banded
Ply 2 datum
Ply 3
10m
Interbanded
Ply 4
Bright banded
Leader
splits
Lithotype profile from core leader
Can the composition of the plies change laterally?

17. 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
500
C 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

18. 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% bright
ICCP DURAIN DURAIN DURO-CLARAIN CLARAIN VITRO CLARAIN VITRAIN
ASTM DURAIN DURAIN DULL CLARAIN CLARAIN BRIGHT CLARAIN VITRAIN
Others: fibrous coal=fusain=charcoal=mother coal
shaley coal=bone
coaly shale=carbonaceous mudstone
cannel coal

19. 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)

20. 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

21. COAL BREAKAGE AND GRINDABILITY
90 90
80 80
mass % passing T10 at 0.075kWh/t
Hardgrove Grindability Index
70 70
bright banded coals
60 60
INCREASING 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

22. 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

23. SO, IF YOU SHIFT THE
MINING SECTION
WHAT
CHARACTERISTICS
WILL OFTEN
CHANGE?
IF I’M LOOKING FOR
PERMEABILITY TO
DRAIN OR PRODUCT
GAS, WHAT WOULD I
TARGET?

24. 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

25. 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?

26. 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….].

27. “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

28. All of these analyses attempt to quantify the chemistry of the
coal that will impact or on its behaviour during utilisation
For example during coking
Ward, 1984
The vitrinite will react; increasing
rank and vitrinite content will
Photomicrograph showing vitrinite-
semifusinite transition before (left) and after increase reactivity of the coal
(right) coking; Ro values given along
margins.photo courtesy of Taylor et al, 1998 during coking

29. Australian Standard 2856-1986/1995
bituminous 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

30. 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

31. Recommended Reading
Stopes, M.C., 1919: On the four visible ingredients in banded bituminous
coals. Proc. Royal Soc. 90B, 470-87.
Moore, T.A. and Shearer, J.C., 2003. Peat/coal type and depositional
environment—are they related? International Journal of Coal Geology 56,
233– 252.
Holdgate, G.R.; Kershaw, A.P.; Sluiter, I.R.K, 1995. Sequence
stratigraphic analysis and the origins of Tertiary brown coal lithotypes,
Latrobe Valley, Gippsland Basin, Australia. International Journal of Coal
Geology Volume: 28, Issue: 2-4, pp. 249-275.
Volkov, V. N., 2003. Phenomenon of the Formation of Very Thick Coal
Beds. Lithology and Mineral Resources, Vol. 38, No. 3, 2003, pp. 223–
232. Translated from Litologiya i Poleznye Iskopaemye, No. 3, 2003, pp.
267–278.