The behaviour of bed material during fluidized bed gasification: The effects of mineral matter interactions

The behaviour of bed material during fluidized bed gasification: The effects of mineral matter interactions

The behaviour of bed material during f~uidized bed gasification: The effects mineral matter interactions James Williamson, Shaun S. West and Margar...

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The behaviour of bed material during f~uidized bed gasification: The effects mineral matter interactions James Williamson,

Shaun

S. West

and Margaret

of

K. Laughlin*

department of ~ateriais, ~mperiai Coffege, London, UK * British Coal Research Establishment, Stoke Orchard, Gloucester, (Received 12 May 7993)

UK

In the British Coal Topping cycle coal is partially gasified in a pressurized fluid&d bed gasifier to give a char and ash. A study of eight UK coals has shown that under these conditions each coal produced an ash which has a much finer size distribution than the parent coal. Changes in ash composition with size fraction have been related to the mineral matter in each coal. Each ash showed a decrease in SiO, and Al,O, with decreasing size and an increase in Fe,O, content. Fine pyrite particles have been observed to coat the surfaces of many of the larger clay particles. In instances where agglomeration between clay particles had occurred bonds of either Fe-S-O or an iron aluminosilicate glass were observed. Part washing one of the coals preferentially removed quartz, kaolinite and illite, thus enriching the remaining mineral matter in pyrite. The run-of-mine coal, although with a higher ash content, would be expected to show a lower propensity towards the formation of agglomerates. (Keywords: agglomeration;

bed material; Ruidized bed gasification)

Many advanced coal fired generation systems are under active development throughout the world. These systems offer the potential of higher conversion efficiencies combined with lower capital costs than conventional pulverized fuel (pf) fired units equipped with flue gas desulfurization, while at the same time meeting evertightening environmental constraints for processes with lower NO, and SO, emissions. In the UK, the British Coal Corporation is developing a combined cycle coal fired power generation system known as the British Coal Topping Cycle’. This system employs a pressurized fluidized bed gasifier to partially gasify a coal, with the combustion of the residual coal char in a circulating fluidized bed combustor. The air-blown gasifier produces a low calorific value gas which, after cleaning, is burnt in a gas turbine. Steam for a steam turbine is raised using a heat recovery system from the char combustor and the gas turbine exhaust. In the Topping Cycle, 7&80 wt% of the coal is gasified in the air-blown spouted bed, which operates at a temperature of just under 1000°C. A sorbent, limestone, is injected with the coal to retain the sulfur which would otherwise be released with the gases. Fluidized bed gasifiers are more tolerant to variations in coal size and quality than fixed bed and entrained phase gasifiers. Nevertheless, the optimum bed temperature is only arrived at after a careful consideration of the many process variables, including the coal and ash characteristics. This paper is based on a study which was undertaken to establish the flexibility of the process and 0016-2361(94~07/1039-07 P‘ 1994 Butterworth-~einema~n

Ltd

the operating window for a range of coals which may be used in the Topping Cycle process. The major minerals found in UK coals include clays, quartz, pyrite, limestone, dolomite and ankerite. With the exception of quartz, these minerals undergo rapid decomposition at the fluidized bed temperature, with the loss of H,O, CO, and sulfur. Agglomeration of ash particles can occur when two or more particles of bed material collide, adhere and a bond forms between them. At least one of the particles must have a molten or partially molten surface ‘s3 for this to occur. A molten surface can be formed by fusion of the mineral particles or as a result of the chemical interaction of two particles in which a liquid phase forms as part of a eutectic reaction; such a process is commonly referred to as a fluxing reaction. Once an initial bond has formed, diffusion and viscous flow may increase the contact area between the particles and this leads to a strengthening of the bond. Subsequent interaction with other particles in the bed can increase the size of the ash agglomerate, and this can eventually lead to a loss fluidization of the bed. In this paper the mineralogical characteristics of eight UK coals are reported. Some of these coals have been used as feed material for the British Coal experimental gasifier. One of the coals, Kiveton Park, was available as both a ‘partially washed’ coal and a ‘run-of-mine’ coal, with the washing process preferentially removing some of the clays. Bed material and sinters taken from a series of controlled trials have been examined to assess the nature of the interactions between mineral particles and aggregates.

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Fluidized bed gasification: J. Williamson et al. The pressurized gasijer test facility

the coal, but leaves the mineral matter unchanged. From a knowledge of the mineral phases present in the coal, and the chemical composition of a given size fraction of the ash, it is possible to calculate the percentage of each mineral present in that size fraction. The calculation, using a process known as ‘normative analysis’, has traditionally been a sequential process determining first one mineral and then another from the bulk chemical analysis. Nowadays, the calculation can be performed for all minerals simultaneously using a computer-based spreadsheet package, employing matrix algebra to solve the simultaneous equations. Coals, bed material and sinters from the gasifier were examined optically and with a scanning electron microscope (SEM) after setting the samples in a cold setting resin and polishing to a 0.25 pm diamond finish. Reflected light microscopy, using plane and crossed polarized light, was used to establish the distribution of iron-bearing phases. Elemental and image analysis was performed using a Jeol 6400 scanning microscope fitted with a low element energy dispersive spectroscopy (EDX) detector and a Tracer Voyager image analysis system.

The gasifier consists of a 10.8 m high vessel lined with aluminosilicate refractories. The internal diameter of 0.31 m expands to 0.45 m approximately 6 m above the base flange. Air, preheated to 300°C is injected through a central spout in the gasifier base along with steam. The gas jet promotes internal recirculation of solids within the gasifier which helps to break up any agglomerates which might form. The gasifier operates at pressures up to 2 MPa and temperatures of lOOO”C, with fluidizing velocities of 1.2 m s-l and bed heights of 6 m. In operation, the bed has a carbon content of approximately 50% by weight. Char and ash are removed continuously from the bed to prevent a build-up of mineral matter in the reactor. EXPERIMENTAL All the coals were supplied for the gasifier with a particle size of less than 3 mm. The size distribution of the particles in each coal was determined by mechanically sieving a sample of each coal until a constant weight was obtained on each sieve. Stainless steel sieves with mesh sizes of 3150, 1000, 500, 250, 100, 71, 50 and 38 pm were used. A sample of high temperature ash was prepared from each coal by heating the coal in a porcelain crucible to 950°C for 8 h, using an electric muffle furnace. The size distribution of the particles in the ash was then determined in the same way as for the coals. The chemical composition of the ash from each size fraction was determined by induction coupled plasma analysis (ICP). The mineral matter in the bulk coal was identified using X-ray powder diffraction analysis of a low temperature ash prepared in an oxygen plasma furnace. Low temperature ashing oxidizes the organic fraction of

Table 1

Chemical

composition

of the coal ash (wt%) and the mineral

RESULTS The chemical composition of the ash from each coal is shown in Table 1. One of the coals, Kiveton Park, was available as both a part washed coal and a run-of-mine coal. Washing the coal reduced the ash content from 22 to 19 wt%, by removing some of the clays and shales. The ash concentration of the other coals ranged from 12 to 55 wt% of the coal, although typically an ash content of 15-20 wt% was found. X-ray powder diffraction showed that the main minerals present in each of the coals was a mixture of kaolinite and illite, with generally lesser quantities of

phases

Part washed

Ash (wt%) Ash composition SiO,

present

in the parent

coal

coals

Run-of-mine

coal

Cotgrave

Daw Mill

Frickley

Harworth

Kiveton

Maltby

Thorsby

Bilsthorpe

Kiveton

12

18

13

14

19

16

15

55

22

47.6

49.3

49.8

48.9

52.5

50.9

50.7

60.0

56.0

Al,03

28.2

25.9

26.6

24.8

22.4

25.1

25.8

26.6

22.2

Fc,O, CaO

15.0

8.3

13.4

16.7

14.6

12.8

11.1

6.8

12.5

2.6

7.0

2.0

1.7

1.8

2.3

2.1

1.1

2.0

MgO

1.8

1.8

1.4

1.3

1.4

1.8

1.4

1.6

1.5

Na,O

1.8

0.7

1.6

0.9

1.0

0.8

2.3

0.9

2.8

K,O TiO,

3.0

2.0

2.5

3.0

2.7

3.1

1.1

0.2

1.0

0.9

_

1.0

3.5 1.0

_

_

MnO

0.2

1.0

0.0

0.0

_

0.0

0.0

_

_

P,O,

0.3

0.3

0.4

0.3

_

0.5

0.3

_

SO,

1.4

5.4

1.2

1.4

1.6

1.9

_

Mineral phases: Major phases

Trace phases

Fuel 1994

3.0

Kaolinite

Ankerite

Kaolinite

Kaolinite

Kaolinite

Kaolinite

Illite

Quartz

Kaolinite

Illite

Kaolinite

Illite

Illite

Illite

Illite

Pyrite

Kaolinite

Illite

Pyrite

Quartz

Pyrite

Pyrite

Pyrite

Pyrite

Quartz

Illite

Quartz

Quartz

Illite

Quartz

Quartz

Quartz

Quartz

Kaolinite

Pyrite

Calcite

Pyrite

Gypsum

Gypsum

Siderite

Gypsum

Magnetite

Gypsum

Ankerite

Hematite

Anhydrite

Rutile

1040

1.0

4.0

Volume

Rutile

73 Number

7

Gypsum

Pyrite

Ffuidized bed gasification:

-

Coal

- - -. -. - Ash

i 100

1000

10000

Size (pm) Figure 1 Size distribution temperature ash

Table 2

Size fraction

for

analysis

the

Kiveton

of Kiveton

Park

coal

and

high

Park coal ash Size fraction

38-50

[email protected]

1.4

5.8

4.6

6.2

<38 Ash (wt%) Ash composition SiO,

et al.

The SiO, contents show a general decrease as the particle size decreases, while the Al,O, content remains relatively constant. This suggests that more quartz is present in the coal in the coarse size fractions. Of major significance was the change in Fe,O, content of the ashes, with 9 wt% or less in size fractions greater than iOO/.~rn,but over 20 wt% for size fractions below 100 pm. Thus, it may be concluded that the level of pyrite is much enhanced in the finer size fractions. The only other oxide found to show significant changes was that of K,O, which fell from over 5 wt% for the coarsest ash to approximately 2.5 wt% for the finest ashes. Since the K,O is to be found principally in the illitic clays, this suggests a decrease in illite at the expense of the other minerals as the particle size decreased. From a knowledge of the minerals known to be present in the low temperature ash from each coal, and the composition of each size fraction of ash, the proportions of minerals required to give the ash composition following combustion or gasification was calculated by the process of normative analysis. The results for each coal are shown in Table 3. With one or two exceptions, the general pattern is for a decrease in the amount of quartz, illite and kaolinite as the size fraction of ash (and therefore coal) decreases, with a corresponding increase in the amounts of pyrite, ankerite and calcite. Bed material, and where available sinters or agglomerates from the fluidized bed, were characterized for ash content, the chemical composition of the ash and the crystalline phases present in the material. Results for the Kiveton Park coal are shown in Table 4. While the sinter material contains no carbon char, the bed material had an ash content of 60 wt%. Optical and SEM examination of bed material and sintered samples gave a valuable insight into the nature of the interactions between mineral particles in the fluidized bed. Since many of the trials with the gasifier produced no agglomerates or sinters, the following examples must be considered as extremes produced by hot spots or due to temperature excursions. Figure 2 is an example of a sintered ash, the micrograph being taken in the back scattered imaging mode at a low magnification. Thus, the higher the intensity of a given phase, the higher the mean atomic number. The sinter is

quartz, pyrite, calcite and gypsum. Traces of magnetite and hematite were found in some coals, but the usual form of Fe was as pyrite. The Fe,O, content of the coal ashes was found to lie between 6.8 and 16.7 wt%. The FeZOX contents of the ash are of importance, as pyrite plays a major role in forming agglomerates. Similarly, CaO may have a significant fluxing action on aluminosilicates, but for these coal ashes the CaO concentrations were, with one exception, less than 2.5 wt%. The size distribution of each coal and its respective ash have been compared. The result, shown in Figure 1, for the Kiveton Park coal, is typical for the suite of coals studied. Only 25% of the coal had particles smaller than 1000 pm, with a few per cent less than 100 pm. In contrast, the ash produced from this coal showed 50% less than 1000 pm and 30% less than 100 pm. Thus, on combustion many of the coal particles produce a residue of finer ash particles. The composition and amount of each of the ash fractions for the Kiveton Park coal is shown in Table 2.

10

J. Williamson

71LlOO

(pm)

1W250 10.5

25&500 9.4

50ct1000

>lOOo

8.4

53.8

48.1

42.6

41.6

41.2

61.9

57.3

59.8

55.1

[email protected],

20.7

23.9

25.3

22.5

22.7

21.7

22.2

23.9

[email protected], CaO

22.8

25.1

24.3

26.1

5.8

8.9

6.9

9.1

1.3

1.7

1.9

2.2

0.6

2.6

2.2

2.2

MgO

1.2

1.2

1.2

1.4

1.8

1.6

2.3

1.5

Na,O

1.3

i.2

1.2

1.0

0.9

0.6

0.5

1.1

K,O TiO,

2.6

2.4

2.4

2.8

5.2

4.7

4.7

5.1

1.1

1.1

1.2

1.0

1.0

0.9

1.1

1.1

MnO

0.0

0.0

0.0

0.0

0.0

0.8

0.0

0.1

P,O,

0.7

0.7

1.0

0.9

0.0

0.7

0.0

0.8

SO,

0.0

0.0

0.0

0.1

0.0

0.2

0.2

0.0

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Fluidized bed gasification: J. Williamson et al.

Table 3

Proportions

of minerals

present

in each ash fraction

calculated

to give the chemical

Sieved ash fraction Mineral (wt%)

<38

Cotgrave Quartz Illite

38-50

50-71

71-100

lW250

composition

of the fraction

(pm) 2X-500

50&1000

>lOOO

Bulk

8

10

4

7

2

3

6

12

6

30

31

33

29

39

40

43

43

38

Kaolinite

36

35

38

39

36

43

46

41

41

Pyrite

25

24

22

11

18

11

4

2

14

1

0

2

14

4

4

1

1

2

Calcite Daw Mill Quartz

10

8

8

10

11

13

17

24

14

Illite

21

22

12

9

9

15

30

40

20

Kaolinite

38

40

44

45

43

42

35

25

41

9

8

9

8

6

6

6

5

6

16

22

27

29

31

23

12

5

20

5

8

8

8

10

9

10

16

11

28

30

32

36

32

44

46

44

43

4

13

0

1

Pyrite Ankerite Frickley Quartz Illite

26

21

24

26

Kaolinite

37

42

41

41

42

25

24

19

16

12

2

2

2

1

1

Pyrite

26

30

2

1

Gypsum Harworth Quartz

8

9

8

10

10

11

12

18

12

Illite

16

20

27

25

21

30

33

31

33

Kaolinite

30

33

33

34

37

39

42

41

38

Pyrite

46

38

31

30

24

18

13

10

18

1

1

1

1

1

1

0

0

I

Gypsum Kiveton Park Quartz Illite

15

7

5

6

25

21

22

16

18

30

30

31

34

46

52

55

52

34

Kaolinite

29

35

31

30

29

24

24

29

33

Pyrite

26

29

27

30

0

3

0

3

15

Maltby Quartz

13

11

10

8

13

9

18

21

13

Illite

30

27

29

37

43

38

36

42

38

Kaolinite

40

41

42

34

39

39

42

36

37

Pyrite

16

19

19

18

5

12

4

0

11

0

1

1

2

0

2

0

0

1

Gypsum Thorsby Quartz

10

9

8

8

9

9

10

18

10

Illite

44

53

56

63

59

62

67

65

69

Kaolinite

14

9

8

4

12

11

8

6

4

Pyrite

28

25

23

21

17

14

12

11

14

5

5

4

4

4

3

2

1

3

Gypsum Bilsthorpe Quartz

(run-of-mine) 26

22

22

19

24

20

21

21

19

Illite

40

41

40

38

42

4s

45

43

41

Kaolinite

28

29

30

33

32

32

31

32

39

Pyrite

6

8

I

10

2

1

2

4

2

Gypsum

0

0

0

1

0

2

0

2

1

Kiveton Park (run-of-mine) Quartz

25

14

16

12

19

21

28

20

Illite

40

43

51

45

50

46

43

41

Kaolinite

24

29

26

25

31

32

28

29

Pyrite

11

14

7

18

0

0

0

10

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Figure 2 Section through an ash agglomerate from pressurized fluidized clay-derived particles bonded by an iron-rich glassy phase. Back scattered

Table 4

Properties

of Kiveton

Park

bed material

Bed material Ash (wt%) Ash composition SiO, [email protected], Fe,& CaO MgQ Na,O K,Q TiO, MnO P,Q, SO,

Kiveton

Park coal, showing

large

and sinter Sinter

60

100

61.1 23.0 6.8 0.8 1.3 0.7 5.0 0.9 0.2 0.1 0.1

62.1 22.9 5.5 0.7 2.0 1.0 4.3 1.0 0.1 0.2 0.2

Glassy

phase

Not detected

Yes

Major

phases

Quartz Magnetite FeS

Quartz Magnetite Hematite FeS

Hematite

Mullite

Trace phases

bed gasifier trial with a part washed image, bar = 1 mm

highly porous, with large particles derived from the clays bonded in a variety of ways. Particles of a kaolinite origin show little sign of fusion and, characteristically, series parallel striations. An EDX analysis shows almost entirely Si and Al. Particles derived from illites frequently show signs of internal fusion and rounding of external edges. These particles also show internal rounded porosity and an EDX analysis gave variable amounts of K, Fe, Ca and Mg. These oxides therefore act as fluxes and lower the melting point and viscosity of the particle.

Figure 3 Section through illite-derived particles from the part washed Kiveton Park coal coated and bonded by an Fe-S-O phase. Back scattered image, bar = 100 nm

The bond between particles is an iron aluminosilicate which was clearly fluid at the temperature at which it was formed and on cooling this has solidified to a glass. Most of the clay particles are seen to have at least a thin layer of an iron aluminosilicate on the surface. Examination of surfaces at higher magnification frequently shows a thin layer with a very high iron composition; this may be an iron oxide or may also contain sulfur. An optical examination of bed material shows many of the shale particles have an iron-rich

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Figure 4 Illite (top) and kaolinite (bottom) derived particles bonded by an iron-rich aluminosilicate glassy phase which shows partial devitrification. Back scattered image, bar = 1 mm

coating which is birefringent in crossed polarized light, indicating the presence of Fe,O,. When bed material and sinters were examined at higher magnification, the bonding between the two particles was shown to be either of an Fe-S-O composition or an iron aluminosilicate. Figure 3 is an example of the Fe-S-O bond which holds two illite derived particles together. Both illite particles show evidence of internal fusion; the Fe-S-O phase which holds the particles together has completely coated both particles with a layer lCL50pm thick. Figure 4 shows a particle of illitic origin (top) bonded by an iron aluminosilicate phase to a kaolinite particle (bottom). The glassy bond shows evidence of crystallization, with the formation of fine dendrites of an iron spine1 (bottom left and top right). A relatively sharp interface between the kaolinite particle and the glass suggests little dissolution has occurred, while the illite particle which itself has partially fused shows some evidence of chemical attack from the aluminosilicate bond.

calcite to be present in essentially one size fraction (71-100 pm). An optical examination of this coal revealed that this was the cleat size for this mineral. When the composition of the Kiveton Park bed material and sinters is compared with the original bulk coal ash, a decrease in the concentration of Fe,O, is observed. This suggests that some of the finer pyrite particles were elutriated from the fluidized bed, and indeed the material collected from the primary cyclone was found to be rich in iron compounds. Nevertheless, pyrite retained in the bed has been observed to play a crucial role in the formation of agglomerates. The bonds between large shale particles were either of an Fe-S-O composition or an iron aluminosilicate. The evidence would all suggest that an initial bond may be formed between a decomposed pyrite particle and particles from either illites or kaolinite. The phase diagram for the system FeO-FeS5 shows that FeS has a melting point of approximately 117O”C, see Figure 5. However, even the smallest amount of oxidation of FeS to Fe0 will produce a eutectic liquid at 940°C. Therefore, once an FeS particle has stuck to the surface of a clay particle, as oxidation proceeds the amount of liquid present will rapidly increase. The complete wetting of the surface of clay particles with an Fe-S-O composition (Figure 3) supports this theory. To provide an Fe-S-O layer up to 50 pm in thickness would require a succession of pyrite particles to adhere, gradually building up this layer. An examination of the bed material from a number of runs shows that agglomerates can form with the minimum of bond formation. However, once an Fe-SO bond has formed, oxidation will continue with Fe0 reacting with fine clay particles on the surface of the clays. Illite derived particles, with variable amounts of CaO, MgO, K,O and Fe0 appear to react more rapidly with Fe0 than kaolinites, which were essentially pure aluminosilicates. Given time and a high enough temperature, significant

DISCUSSION The Topping Cycle has been designed to take a wide variety of UK coals, thus the coals chosen for study have both variable ash contents and ash compositions. Many UK coals have a relatively high iron content and the iron is generally present in the coal as pyrite, FeS,. Under both oxidizing and reducing conditions pyrite readily decomposes at temperatures above 400°C to give pyrrhotite, FeS, and sulfur4. At relatively low partial pressures of oxygen the pyrrhotite will oxidize, going first to magnetite, Fe,O,, and eventually to hematite, Fe,O,. Both Fe2+ and Fe3+ ions will lower the liquidus temperature and viscosity of aluminosilicates, with Fe’+ being the most effective as a network modifying ion. When the chemical composition of each size fraction of ash from a given coal was examined, significant enrichment of the finer fractions in Fe,O, was observed. Thus, when calculating the proportions of each mineral which had produced a given size fraction of ash (Table 3), pyrite enrichment in the finer size fractions is frequently significant. One of the coals, Cotgrave, showed

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FeO,,,+Fe%, 800

Ill

Fe0

25

50

Mole % FeS Figure 5

The system FeO-FeS

75

FeS

Fluidized

amounts of a liquid-glassy phase can form. Viscous flow sintering, in which both surface tension and wetting play a role, aid the consolidation process. Thus iron, and in particular pyrite, is seen to play a major role in the formation of agglomerates. Since for all the coals, the pyrite is seen to be concentrated in the finer ash fraction, this will increase its reactivity. One of the coals, Kiveton Park, has been studied as both a ‘run-of-mine’ coal and a ‘part washed’ coal. Part washing reduced the ash content from 22 to 19 wt%, but increased the Fe,O, content from 12.5 to 14.6 wt% by the removal of some of the clays. Thus, in this instance part washing of the coal to remove some of the mineral matter would actually increase the agglomeration potential of the ash.

CONCLUSIONS Coals ashed under conditions similar to those in the pressurized fluidized bed gasifier gave ashes with finer size distributions than the parent coal. The composition of the ash changed with size, becoming depleted in SiO, and Al,O, and enriched with Fe,O, as the size fraction of the ash decreased. The ash composition may be related to the mineral matter in the coal. Pyrite has been shown to be concentrated in the finer size fractions of the coal. Fine iron particles were seen to have coated the surface of clay particles present in the bed material from the gasifier.

bed gasification:

J. Williamson

et al.

In instances where agglomeration between clay particles had occurred, the bonds were either of Fe-S-0 or an iron aluminosilicate. Both Fe-S-O and iron aluminosilicates may completely coat the surface of particles derived from kaolinite or illite. Part washing of one of the coals preferentially removed quartz and clays, thus enriching the remaining mineral matter in pyrite. Such treatment would therefore be expected to increase the agglomeration potential of the ash. ACKNOWLEDGEMENTS The authors are grateful to British Coal for permission to publish this paper. One of us (S. S. W.) gratefully acknowledges a research bursary provided by British Coal. REFERENCES Minchener, A. J., Cross, P. J. I., Smith, M. A., Gale, J. J. and Dawes, S. G. in ‘Advanced Clean Coal Technology for Power Generation’, Conference on Fluidized Bed Combustion Technology and the Environmental Challenge, Institute of Energy, Adam Hilger, Bristol, 1991, pp. 331-337 Kline, S. D., Mason, D. M., Carty, R. H. and Babu, S. P. in ‘Proceedings of the Utilization of High Sulphur Coals III’ (Eds R. Markusziewski and T. D. Whealock), Eisevier Science, New York, 1990, pp. 687-695 Laughlin, K. M. and Reed, G. R. in ‘Proceedings of the Sixth International Conference Coal Science, Newcastle’, ButterworthHeinemann, Oxford, 1991, pp. 396399 Williamson, J., Groves, S. P. and Sanyal, A. Fuel 1987, 66, 461 Asanti, P. and Kohlmeyer, F. J. Zeit anorg. Chem. 1951, 265, 94

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