1. Mineralogical Changes in South African Coals during Pulverised-fuel Combustion Tests
Ratale H. Matjie1, Zhongsheng Li2*, Colin R. Ward2, Johan Kosasi3, John R. Bunt1, Christien A. Strydom1
1: North-West University, Potchefstroom 2520, Republic of South Africa
2: University of New South Wales, Sydney, NSW 2052, Australia
3: ALS-ACIRL Pty Ltd, PO Box 242, Booval, Qld 4304 Australia
*Present address: CSIRO Energy Flagship, North Ryde, NSW 2113, Australia

2. Coalfields of South Africa
Witbank
Highveld
Permian coals occurring in the Karoo Basin
Main coal-bearing sequence is the Vryheid Formation of the Ecca Group
Coals are typically dull and inertinite-rich
Samples for present study are from the Highveld Coalfield, SE of Johannesburg

3. Project Objectives
Evaluate the partitioning of maceral and mineral- matter components in a run-of-mine South African coal with beneficiation
Export-quality coal
Coal for domestic power generation
Determine mineralogy of the furnace deposits and ashes produced by each coal during controlled pulverised-fuel combustion tests
Evaluate the mineralogical processes that occur during pf combustion based on the resulting observations, mineralogical and chemical data

4. ALS-ACIRL Combustion Test Facility
Down-fired furnace 2.9 m high and 0.7 m diameter
Oxidising and low-pressure environment
Normal firing rate 150 kW
Coal feed rate 25 kg/hour (depending on coal CV)
Particle residence time approximately 3 seconds

5. Coal Samples Tested Run-of-Mine Highveld Coal Dense Medium Cyclone
<38 mm
Quality Parameters
Low-ash
High-ash
Moisture
4.6
3.6
Vol Matter
29.7
19.9
Fixed Carbon
52.2
36.9
Ash
13.5
39.6
Sulphur
1.0
1.9
VM (daf)
36.3
35.0
CV (MJ/kg)
26.6
16.8
Petrology – mineral-free
Vitrinite 56 26
Liptinite 7 4
Inertinite 37 70
Rvrand
0.59
Dense Medium Cyclone
Low-ash Coal
<1.4 g.cm-3
13.5% ash
High-ash Coal
>1.4 <1.9 g.cm-3
39.6% ash
Pulverised to 70-76% passing 75 m
Pulverised to 70-76% passing 75 m
Similar to South African export coals
Similar to South African domestic power coals

6. Mineral Matter
LTA of low-ash coal has higher percentages of calcite and ankerite
These minerals typically occur as cleat infillings in vitrinite
Low-ash coal
High-ash coal
LTA
17.2
47.8
Quartz
7.3
21.0
Kaolinite
55.3
57.7
Illite
5.4
4.7
Mica  -
3.1
I/S
1.3
0.3
Calcite
5.0
1.1
Ankerite
12.3
Siderite
0.1
Pyrite
2.2
Apatite
0.6
Goyazite
1.2
Rutile
0.8
Bassanite
9.6
1.7
Low-ash coal has a higher vitrinite content
Bassanite is also more abundant in LTA of low-ash coal
Consistent with a higher percentage of non-mineral Ca, also in the vitrinite
LTA of high-ash coal has more quartz and mica
Consistent with greater abundance of stony particles in denser float-sink fraction
High-ash coal has higher inertinite content; more porosity for mineral impregnation

7. Ash Chemistry Ash of high-ash coal has higher SiO2 percentage
Consistent with higher quartz content of mineral matter
Higher K2O also reflects higher proportion of illite/mica in high-ash material
Ash of low-ash coal has higher CaO and MgO percentages
Consistent with higher percentages of calcite and ankerite in mineral matter
Also consistent with greater proportion of vitrinite containing non-mineral Ca
Ca-rich low-ash coal retains more S as SO3 in the laboratory-prepared ash
Low-ash coal
High-ash coal
Ash
13.5
39.6
SiO2
40.16
55.82
Al2O3
29.16
28.70
Fe2O3
3.55
3.33
TiO2
1.46
1.40
CaO
10.94
4.58
MgO
3.48
1.57
Na2O
0.69
0.48
K2O
0.68
1.10
P2O5
1.31
0.51
SO3
8.32
4.12
Total
99.75
101.61

8. Ash Fusion Temperatures
 Oxidising Atmosphere C
Low-ash Coal 
High-ash Coal 
Initial deformation
1360
1390
Spherical
1370
1500
Hemispherical
1380
1510
Flow
1400
1550
Low-ash coal has lower ash fusion temperatures
Consistent with higher CaO percentage
Ash from low-ash coal collapses more quickly (i.e. has lower viscosity) than ash from high-ash coal
May reflect higher proportion of (more rigid) quartz framework particles in high-ash material

9. Sampling Points and Operating Temperatures
Zone 1
Slagging Panel 1
Zone 2
Slagging Panel 2
Zone 3
Slagging Panel 3
Zone 4
Transition Section
Tunnel and
Fouling Probe Section
Sampling Points and Operating Temperatures
 Coal Feed
Furnace Coarse
Ash (FCA)
 Electrostatic Precipitator
and Dust Collector

10. Deposit Chemistry - Low-ash Coal
SP1
SP2
SP3
FCA FT
Tunnel
Mass (%)
0.13
0.04
10.9
4.4
15.2
SiO2
41.53 -
45.18
43.57
46.90
Al2O3
27.93
22.82
27.50
23.40
Fe2O3
5.61
7.72
5.01
7.35
TiO2
1.29
1.13
1.42
1.34
P2O5
1.21
0.78
1.09
0.79
CaO
11.75
8.22
11.17
9.14
MgO
3.54
2.46
3.62
3.26
Na2O
0.35
0.42
0.56
K2O
0.59
0.52
0.57
0.55
SO3
1.58
0.96
3.63
2.35
MnO
0.12
0.11
LOI
3.00
9.44
1.30
4.28
FP1
FP2
FP3
FPort
ESP DC
0.09
0.33
0.35
0.54
21.3
46.6
42.55
44.00
44.19
43.39
43.66
56.86
29.83
30.78
30.15
29.76
27.88
23.43
3.91
3.20
3.40
4.05
3.55
7.78
1.58
1.73
1.72
1.61
1.53
1.66
1.38
1.28
1.25
1.27
1.18
0.48
10.65
10.55
10.56
11.31
8.86
3.17
3.60
3.73
3.75
3.87
3.33
2.30
0.96
0.69
0.64
0.50
0.68
1.21
0.67
0.66
0.59
3.47
2.87
2.46
2.33
1.46
1.06
0.11
0.12
0.10
0.08 -
5.69
SP = Slagging Panel FP = Fouling Panel ESP = Electrostatic Precipitator
FCA = Furnace Coarse Ash FPort = Fouling Port DC = Dust Collector
FT = Furnace Transition
- = Not analysed (insufficient sample)

11. Deposit Chemistry – High-ash Coal
SP1
SP2
SP3
FCA FT
Tunnel
Mass (%)
0.24
0.03
48.2
7.1
9.7
SiO2
57.31
54.61
54.73
59.08
56.20
53.78
Al2O3
27.76
30.92
32.33
27.73
30.06
26.72
Fe2O3
4.09
3.82
3.07
4.25
3.51
4.64
TiO2
1.33
1.54
1.61
1.32
1.50
P2O5
0.46
0.59
0.62
0.44
0.53
0.48
CaO
4.56
3.62
3.38
4.36
5.12
5.35
MgO
1.55
1.30
1.53
1.86
1.92
Na2O
0.83
0.52
0.31
0.45
K2O
1.11
1.03
1.19
1.00
1.02
SO3
0.66
2.07
1.68
0.02
0.22
1.14
MnO
0.05
0.04
0.06
LOI
0.34 -
0.30
FP 1
FP2
FP3
ESP DC
0.09
0.13
0.18
19.0
15.0
54.89
54.49
55.20
55.12
50.35
31.69
32.47
32.28
32.61
30.20
3.20
2.82
2.92
4.37
3.82
1.59
1.68
1.65
0.59
0.61
0.58
0.60
0.86
4.56
4.45
4.68
5.13
1.69
1.70
1.73
2.16
0.43
0.30
0.33
0.67
1.04
0.98
0.96
1.01
0.85
1.45
1.57
1.50
0.93
1.55
0.04
0.05
0.07 -
0.27
0.28
0.36
1.30
Deposits from high-ash coal have higher SiO2 and lower CaO than deposits from low-ash coal
Deposit chemistry for each coal is very similar, regardless of sampling point

12. Deposit Mineralogy – Low-ash Coal
SP1
SP2
SP3
FCA FT
Tunnel
FP1
FP2
FP3
FPort
ESP DC
Quartz
5.9
6.6
6.9
12.3
11.0
13.2
8.4
8.2
8.8
8.5
9.1
19.7
Mullite
18.2
29.4
31.5
21.1
25.7
24.6
28.2
29.1
26.9
26.3
27.6
19.8
Cristobalite
0.4
7.1
1.0
1.5
Anhydrite
4.6
3.4
0.9
5.3
2.5
3.6
2.9
2.4
1.6
Gypsum
1.2
Anorthite
23.2
12.1
6.7
0.3
Gehlenite
3.1
Rutile
0.1
0.2
Hematite
1.8
1.1
Magnetite
Maghemite
0.7
0.5
0.6
Mg-ferrite
Lime
0.8
1.3
1.4
Periclase
1.7
Amorphous
47.2
57.8
56.8
44.8
42.7
54.1
55.4
54.8
56.9
57.7
54.9

13. Deposit Mineralogy – High-ash Coal
SP1
SP2
SP3
FCA FT
Tunnel
FP1
FP2
FP3
FPort
ESP DC
Quartz
16.7
16.2
14.5
15.6
17.7
20.2
16.9
12.4
15.3
15.4
11.9
Mullite
29.6
32.7
33.9
27.7
32.6
17.9
37.0
31.9
35.0
34.5
30.5
Cristobalite
0.1
2.5
0.3
0.2
Anhydrite
1.7
1.1
0.9
1.5
1.8
1.6
0.6
1.3
Gypsum
1.2
0.4
Anorthite
8.4
0.8
10.5
6.0
Rutile
0.7
0.5
Hematite
Magnetite
Maghemite
Lime
Periclase
Amorphous
44.4
47.1
48.2
43.8
41.6
57.8
43.6
53.6
47.2
47.3
48.0
55.0
Quartz more abundant in deposits from high-ash coal  essentially non-reactive mineral
Anhydrite, gypsum, lime and periclase more abundant in deposits from low-ash coals  residues of higher CaO, MgO
Slightly greater proportion of amorphous material (glass) in the deposits from the low-ash coal sample
Feldspar (anorthite) and cristobalite only in SP1, FCA, FT and Tunnel deposits from both coal samples

14. Ash Distribution Percentage of total ash in each sampling section
Low-ash coal produced a greater proportion of fine ash, captured in ESP and dust collector
High-ash coal produced a greater proportion of coarse ash, captured at bottom of furnace and in adjacent transition and tunnel sections
Very little ash was retained on the slagging or fouling panels for either coal sample
Sampling Sites
Low-ash Coal
High-ash Coal
Slagging Panels
0.21
0.30
Coarse Ash to Tunnel
30.5
65.0
Fouling Panels
1.31
0.40
ESP + Dust Collector
67.9
34.0
Consistent with a larger proportion of coarse mineral grains and stony particles in the high-ash coal sample and a higher proportion of fine mineral residues and neo-formed phases in the ash derived from the low-ash material

15. Slagging Panel Deposits – Low-ash Coal
SP-1
SP-3
Colour
Cream-grey
Cream
Base layer
1 mm
Powder
0.5 mm
Sinter layer
5 mm
Nodules
Firm, easily crushed
None
Top Slagging Panel
Bottom Slagging Panel

16. Slagging Panel Deposits – High-ash Coal
SP-1
SP-2
Colour
Grey
Cream
Base layer
1 mm
Powder
Sinter layer
20 mm
Nodules
Firm, easily crushed
3-10 mm
SP-3
Colour
Cream
Base layer
1 mm
Powder
Sinter layer
3 mm
Soft
Top Slagging Panel
Top Slagging Panel Port

17. Fouling Probe Deposits
Low-ash Coal
High-ash Coal
Deposit mass - Tube 1
9.3 g
43.7 g
Deposit mass - Tube 2
32.8 g
64.1 g
Deposit mass - Tube 3
34.5 g
84.3 g
Deposit colour
Cream
Deposit texture
Powder
Powder and sinter
Low-ash Coal
Mass retained on the slagging panels decreased progressively from SP1 to SP3
Mass held on the fouling probes increased progressively from FP1 to FP3
No major slagging or fouling problems indicated for either coal sample
High-ash Coal

18. Changes in Mineral Matter with Heating
LTA of Ca-rich Coal Sample
Metamorphism
Melting
Dehydroxylation
Kaolinite breaks down at 450C to form amorphous metakaolin
Calcite and dolomite break down to form oxides
CaO reacts with metakaolin from 900C to form anorthite
This is a solid-state reaction, before melting on TMA curve
Anorthite disappears with melting at 1200C and more amorphous material is formed
Dynamic high-temperature XRD data
French et al., Pittsburgh Coal Conference 2001

19. Processes of Feldspar Formation
Feldspar (anorthite) is found only on top slagging panel (SP-1) and in FCA, transition and tunnel samples for both coals
May have been formed in deposits on SP-1 (1350 C), which became detached and fell into the lower part of the furnace
Absence of fused deposits on panel suggests solid-state reaction rather than crystallisation from melt; this is also consistent with the ash fusion temperatures
Temperatures on SP-2 and SP-3 ( C) were too low for feldspar formation
Some feldspar may also have formed directly in hottest part of fireball
If so would have required good contact between phases plus a short reaction time

20. Sulphate and Oxide Phases
Anhydrite/gypsum found in ashes throughout the stream from both coal samples
Formed from interaction of CaO with SO2 in combustion gases
More abundant in low-ash coal materials, probably due to higher CaO percentage from carbonates and non-mineral inorganics
Lime (CaO) and periclase (MgO) found in ashes from low-ash sample
Probably reflects lesser availability of SO2 in gases due to the lower sulphur content of the low-ash coal product

21. Conclusions
Beneficiation of a single ROM coal may produce fractions with quite different maceral assemblages and mineral matter characteristics
Higher proportions of coarse ash may be produced from high-ash (domestic power) coals than from low-ash (export) coals
Feldspar (anorthite) is formed only at high temperature (1350 C), either in fireball or on topmost slagging panel
Absence of melting on panel suggests mainly solid-state reaction
Lime (and periclase) formed from low-ash coal, with high Ca and Mg but insufficient sulphur for sulphate development
Quantitative XRD analysis, in conjunction with coal petrology, can be used to better understand beneficiation and combustion processes