Combustion char morphology related to combustion temperature and coal petrography

Fuel Vol. 75, No. 9, pp. 1071-1082, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96 $15.00+0.00

PIESOOM-2361(96)00074-9

ELSEVIER

Combustion combustion petrography

Per Rosenberg,

char morphology related temperature and coal

Henrik

I. Petersen

to

and Erik Thomsen

Geological Survey of Denmark and Greenland (GEM), NV, Denmark (Received 6 February 1996; revised 17 March 1996)

Thoravej 8, DK-2400

Copenhagen

The morphology of chars sampled from various laboratory-scale reactors operating at temperatures from 800 to > 14OO”C,together with chars collected directly in the flame zone in a full-scale pulverized fuel combustion experiment, was examined. A coal and coal blend dominated by vitrinite-rich microlithotypes together with four coals dominated by inertinite-rich microlithotypes were used to produce the combustion chars. Char samples produced at temperatures above -1300°C have a morphotype composition very similar to the composition of the full-scale char samples, whereas the morphotype compositions of those produced at N 1150°Cor lower are significantly different. Correlation between coal petrography and char morphology and determination of char reactivity should thus be attempted only using chars produced at temperatures comparable with those for the intended use of the coal. A clear distinction between the high-temperature char samples (burnout 50-60 wt% daf) emerges which is related mainly to the parent coal petrography and probably secondarily to the rank. Vitrite, clarite and vitrinertitev may be correlated with the porous tenuisphere and crassisphere morphotypes, whereas inertite, durite, vitrinertite I, duroclarite and clarodurite may be correlated with the crassinetwork-mixed-network-mixed morphotype group. Copyright 0 1996 Elsevier Science Ltd. (Keywords: combustion char; morphology;coal petrography)

The burnout behaviour of coal during pulverized fuel (p.f.) combustion in power plants is known to be influenced by the reactivity of the chars produced in the early stage of the combustion process’. It is generally believed that the variations in char reactivity observed in both laboratory-scale and full-scale p.f. combustion experiments are related to the different char morphotypes. Hence combustion chars produced in laboratory-scale reactors are widely used when studying char reactivity and burnout behaviour. The criterion for identifying a char sample is normally based on the content of volatiles, and scant attention has been given to the influence of the experimental conditions, in particular temperature, on the resulting char morphology. Furthermore, the relation between char morphology and the petrographic composition of the parent coal has been studied, to predict the burnout behaviour of a feed coal with a particular petrographic composition1-6. Some studies have dealt with the morphology of chars derived from pyrolysis of maceral concentrates’,7’8. However, a more realistic approach is to focus on the pulverized raw coal, as this reflects the fuel actually used in p.f. combustion.

The present study deals with six feed coals of different petrographic composition and a suite of char samples derived from the coals in both laboratory-scale reactors and full-scale experiments. During this project it became

obvious that chars produced from different reactors lacked consistency of morphological characteristics. Hence, the objectives of this study were (1) to detect systematic variations in morphotype composition between char samples produced under realistic temperature conditions and those produced at lower temperatures or lower heating rates, and (2) to demonstrate some general relations between the morphology of high-temperature chars and parent coal petrography. EXPERIMENTAL Origin and petrography of the feed coals

Six coal samples were used in this study, including one coal blend: a Colombian coal (Co), two Australian coals (Au1 and Au 2), two South African coals (SAl and SA2) and a Polish coal blend (PO). The Colombian sample is a Tertiary coal from the north-eastern part of the country. It is from a 900 m thick coal-bearing formation which contains up to 55 coal seams with thicknesses ranging up to 26 m (ref. 9). The Australian and South African Permian coals were deposited in the ancient southern Gondwana continent that comprised South America, South Africa, Antarctica, Australia and India. The precursor peats of these coals were formed in a cold to cool-temperate

Fuel 1996 Volume 75 Number 9

1071

Combustion

char morphology:

P. Rosenberg

et al.

Table 2 Reactors production

Table 1 Petrographic and proximate analyses of the coals Co

Coal Vitrite (vol.%) Inertite (vol.%) Clarite (vol.%) Durite (vol.%) Vitrinertite (vol.%) Duroclarite (vol.%) Clarodurite (vol.%) Vitrinertoliptite (vol.%) Carbominerite (vol.%) Vitrinite (vol.%) Liptinite (vol.%) Inertinite (vol.%) Minerals (vol.%) % (%) Moisture (wt%) Ash (wt%) Volatile matter (wt%) Heating value (MJkg-‘)

21

Au1

6 23 2: 0 0 8 19 30 25 9 23 1 0 0 3 1 75 27 3 176 69 2 1 0.59 0.62 11.6 17.8 9.6 7.4 32.6 23.5 25.2 23.2

Au2

SAl

SA2

7 17 2 2 33 14 23 0 2 37 7 54 2 0.63 10.2 15.2 27.8 24.3

7 44 2 16 17 3 7 1 3 20 3 75 2 0.80 8.4 14.3 22.2 25.8

7 30 14 38 8 2 4 21 12 11 7 13 9 4 0 0 15 5 20 54 6 5 72 28 2 13 0.71 0.74/1.03 2.7 11.4 13.7 11.8 23.5 27.5 28.6 25.9

Fuel 1996 Volume 75 Number 9

conditions

for char

PO

climate which together with the tectonic setting, in particular slowly subsiding intracratonic and inner foreland basins, favoured oxidation of the organic matter and resulted in inertinite-rich coals10-16. The Polish coal is a blend of two coals of unknown origin. The petrographic composition and rank of the coals were determined in accordance with the standards outlined by Stach et a1.17. Each analysis was based on 500 point-counts. The results are listed in Table 1 together with proximate analyses. Total reflectance distributions (TRD) were obtained with a digital automatic image analysis system (PIA), which measured a total of 270 fields in each sample (Figure 1). The Colombian coal has a mean random reflectance R, of 0.59% indicating a high-volatile bituminous C rank. It has a high vitrinite content and is dominated by vitrinite-rich microlithotypes (Table 1). This is also reflected in the TRD curve, showing a well-defined peak in the vitrinite reflectance range (Figure 1). The transition to the inertinite macerals is distinct, without any pronounced proportion of components with intermediate reflectances. The Au 1 and Au2 coals are both of high-volatile bituminous C rank (Table 1). They are characterized by a high proportion of inertinite exhibiting a wide range of reflectances transitional between vitrinite and inertinite. These semi-inert macerals are shown on the TRD curve as a prominent ‘shoulder’ close to the vitrinite reflectance range (Figure 1). The semi-inert fraction may be regarded more reactive than the high-reflecting inertinite, due to a higher hydrogen content (higher H/C ratio)“. The high content of inertinite macerals is reflected in the dominance of inertinite-rich microlithotypes such as inertite, vitrinertite and clarodurite (Table 1). Reflectance measurements indicate a high-volatile bituminous B rank for the South African SA 2 coal and a high-volatile bituminous A rank for the SA 1 coal. Both samples are dominated by an extremely high proportion of the inertinite maceral group and of inertinite-rich microlithotypes (Table 1). The TRD curves are characterized by three peaks (Figure 1). The broad peak in the high-reflectance range represents the inertinite fraction and illustrates that a major part of the inertinite macerals is semi-inert and has reflectances transitional between vitrinite and inertinite. The middle

1072

and the basic operating

Reactor type

Experimental conditions

Full-scale, Fynsvierket block 07

Temperature -1500°C Particle size -70 pm 410MW tangentially fired p.f. boiler Burmeister & Wain 4AF-LN, low-NO, burner Full load, 3 vol.% oxygen in flue gas

1.5 M W tunnel reactor

Temperature ~1300-1400°C 4 vol. % oxygen in flue gas Particle size 97% < 90 pm IFRF block swirl burner Tunnel p.f. combustor

Laminar flow reactor

Temperature: see Table 3 Heating rate lo4 - 10’K s-’ Oxygen 6 and 12 vol. % Particle size 106-125 pm

Entrained flow reactor

Temperature 1330°C Heating rate 10’ K s-t 3 vol.% oxygen Particle size: 75-135 pm

Drop tube furnace

Temperatures 800, 1000 and 1150°C Atmospheric air Particle size 106-125pm

Mtie

Temperature 900°C Heating rate 1000K mint Atmospheric air Particle size 106-125 hrn

furnace

peak represents the vitrinite population, whereas the leftmost peak is situated within the reflectance range of liptinites, mineral matter and low-reflecting, commonly fluorescing, vitrinite (collodetrinite). The above coals are single coals, while the Polish coal probably is a blend of a high-volatile bituminous B coal (mean random reflectance R, = 0.74%) and a highvolatile bituminous A coal (mean random reflectance R, = 1.03%) (Table 1). The blend is dominated by vitrinite but contains a significant proportion of inertinite, which is also shown by the microlithotype composition. The TRD curve shows three peaks, which may be partly due to the blend of two coals of different rank. Char production

The char samples investigated in this study were sampled both from full-scale experiments at the Fynsvaerket p.f. power plantlg and from a suite of laboratory reactors (Table 2). Chars were produced at heating rates, temperatures and oxygen concentrations close to full-scale conditions in a 1.5 MW tunnel reactor at Research Centre Riso2’, in a laminar flow reactor (LFR)21-23 at the Sandia National Laboratories and in an entrained flow reactor (EFR) at Research Centre Ridered, and under conditions very far from realistic p.f. combustion in the drop tube furnace (DTF) at Studsvik Energy AB2’. At the low extreme were the muffle furnace (MF) experiments at Research Centre Riso26, where chars were produced at very low heating rates and with a final temperature not exceeding 900°C. Altogether 63 char samples were produced and analysed (Table 3). In most of the experiments it was possible to sample chars from different burnout levels. During the EFR, 1.5 MW and MF experiments, however, only one sample could be taken at -5Owt%

Combustion 20

char morphology:

P. Rosenberg

et al.

7

18

CoIal

1

6-

Au1 [b]

16 14

5-

..y 12 0

m

? a

10 8

0

2

1 %

3

5

4

3

0

Reflectance

4

5

4

5

% Reflectance 5

I

Au2[c] 4

1

0.

2

1 %

3

% Reflectance

Reflectance

4

SA2[ej

$ * P

5

4

1

62-

a

4-

2-

kll

0 0

2

1 %

3

4

5

0

1

2 %

Reflectance

3

Reflectance

Figure 1 Total reflectance distributions for the six feed coals. See text for further explanation

(daf) burnout, a level that ensures that the chars had reached a state of complete devolatilization. The burnout values were calculated on the dry, ash-free basis using the ash tracer method. Char morphology class$cation

Polished blocks suited for reflected light microscopy

were prepared by embedding the char samples in epoxy resin. Char classification was undertaken by point-counting 300 char particles in oil immersion for each sample. Particles < 25pm and fragments were not counted. Slightly modified point-counting was used, i.e. if the crosshairs fell in void spaces of the particle the entire particle was included in the classification. The

Fuel 1996 Volume 75 Number 9

1073

Combustion

Table 3 Code

char morphology:

P. Rosenberg

et al.

Char sample set Reactor type

Burnout (wt% daf)

Sample position

Temperature (“C)

CoA

Muffle furnace

33

_

COB

Drop tube

12

290 mm (215 ms)

cot

Drop tube

32

490 mm (363 ms)

800

COD

Drop tube

53

610mm (452ms)

800

CoE

Drop tube

64

740 mm (548 ms)

800

CoF

Drop tube

32

290mm (215ms)

1000

COG

Drop tube

66

490 mm (363 ms)

1000

CoH

Drop tube

27

140mm (104ms)

1150

co1

Drop tube

54

290 mm (215 ms)

CoJ

Laminar flow reactor

61

64 mm (47 ms)

1682 K (6% Or)

CoK

Laminar flow reactor

66

191 mm (95 ms)

1593 K (6% Or)

COL

Laminar flow reactor

69

254mm (117ms)

1536K (6% 02)

CoM

Laminar flow reactor

59

64 mm (47 ms)

1692K (12% 0,)

CON

Laminar flow reactor

70

129 mm (72 ms)

1645K (12% Ox)

coo

Full scale

25

Flame zone

COP

Full scale

63

Flame zone

-1500

CoQ

Full scale

79

-1500

CoR

1.5 MW tunnel reactor

50

Flame zone _

cos

Entrained flow reactor

52

AulA

Muffle furnace

24

250 ms _

AulB

Drop tube

28

290 mm (215 ms)

AulC

Drop tube

68

490 mm (363 ms)

800

AulD

Drop tube

76

740 mm (548 ms)

800

AulE

Drop tube

10

140mm (104ms)

1000

AulF

Drop tube

55

290mm (215ms)

1000

AulG

Drop tube

89

490 mm (363 ms)

1000

AulH

Drop tube

86

290 mm (215 ms)

Au11

Laminar flow reactor

42

64 mm (47 ms)

1682K (6% Oz)

AulJ

Laminar flow reactor

49

129mm (72 ms)

1642K (6% 02)

AulK

Laminar flow reactor

52

191 mm (95ms)

1593K (6% 02)

AulL

Laminar Sow reactor

56

254mm (117ms)

1536K (6% Oz)

900 800

1150

-1500

-1300-1400 1330 900 800

1150

AulM

Laminar flow reactor

46

64 mm (47 ms)

1692K (12% 02)

AulN

Laminar flow reactor

58

129mm (72ms)

1645K (12% 02)

Au10

Laminar flow reactor

66

191 mm (95 ms)

1591K (12% 02)

AulP

Laminar flow reactor

72

1532K (12% 02)

Au2A

Muffle furnace

28

254mm (117ms) _

Au2B

Drop tube

46

290mm (215ms)

800

Au2C

Drop tube

62

490mm (363 ms)

800

Au2D

Drop tube

75

610mm (452ms)

800

Au2E

Drop tube

51

140mm (104ms)

1000

900

Au2F

Drop tube

76

290mm (215ms)

1000

Au2G

Drop tube

91

490 mm (363 ms)

1000

Au2H

Drop tube

98

740 mm (548 ms)

1000

Au21

Drop tube

87

290mm (215ms)

1150

Au2J

Drop tube

97

390mm (289 ms)

Au2K

Laminar flow reactor

49

64 mm (47 ms)

1682K (6% 02)

Au2L Au2M

Laminar flow reactor Laminar flow reactor

54 58

129mm (72ms) 191mm (95ms)

1642 K (6% 0,) 1593K (6% 0,)

Au2N

Laminar flow reactor

61

254mm (117ms)

1536K (6% Or)

Au20 Au2P

Laminar flow reactor

49 62

64 mm (47 ms)

1692K (12% 02) 1645 K (12% 02)

AdQ

Laminar flow reactor

Au2R SAlA SAlB

Laminar flow reactor

66 71

191mm (95ms) 254mm (117ms)

Full scale Full scale

49 73

Flame zone

1532K (12% 0,) -1500

SAlC

Full scale

61

Flame zone Flame zone

-1500 -1500

1074

Fuel 1996 Volume 75 Number 9

Laminar flow reactor

129mm (72 ms)

1150

1591K (12% 02)

Combustion

Table 3

char morphology:

P. Rosenberg

et al.

Continued. Burnout (wt% daf)

Code

Reactor type

SAlD

Full scale

71

Flame zone

SA2A

1.5 MW tunnel reactor

43

_

Sample position

Temperature (“C) -1500 ,--1300- 1400

SA2B

Entrained flow reactor

52

250 ms

PoA

Full scale

63

Flame zone

-1500

PoB

Full scale

65

Flame zone

-1500

POC

Full scale

-1500

Full scale

61 _

Flame zone

POD

Flame zone

-1500

Table4 Char classification Description

Char type Tenuisphere

Spherical to angular, porosity > 80%, 75% of wall area < 5 pm

Crassisphere

Spherical to angular, porosity > 60%, 75% of wall area > 5 pm

Tenuinetwork

Internal network structure, porosity > 70%, 75% of wall area < 5 pm

Crassinetworkmixed networkmixed

Char with internal network structure with 75% of wall area > 5 pm or char with a fused and unfused part, porosity 40-70%

Inertoid

Dense, porosity 5-40%, 75% of wall area > 5 pm

Fusinoid-solid

Inherited cellular fusinite structure or solid particle with < 5% porosity

Mineroid

Particle with > 50% inorganic matter

Coal

Particle of unburnt coal

drawback of this technique is that the classification reflects the compositions based on numbers of the morphotypes and not on the mass percentage, which makes correlation with burnout difficult or even impossible. A counting method based on the mass contribution of each morphotype has been suggested by Vleeskens et al. 27, in which the particle is included only when the crosshairs cover solid matter. Identification of the char morphoty es followed the guidelines suggested by Bailey et al.P and at the ICCP annual meeting in Chania, Crete, 1993 (Table 4). Altogether seven different char morphotypes or groups and unburnt coal particles have been identified. However, the mineroid (> 50% mineral matter) and coal particle groups are deleted and the remaining six groups are normalized to 100%. As is evident from Table 4, the classification is based to a large extent on the porosity and wall thickness of the chars. It is generally not possible to distinguish all the variations visualized in the microscope. This is true in particular for the mixed group, where both the porosity and appearance of the particles vary significantly. RESULTS AND DISCUSSION Temperature influence on char morphology

Chars from the Colombian coal were produced in all the reactors used in this study. This gives an excellent opportunity for a direct comparison of the influence of different experimental conditions on the formation of the char morphotypes and further allows a comparison of the various laboratory-derived chars with full-scale

1330

p.f. combustion chars. A clear shift in composition between the low-temperature DTF chars and the hightemperature chars is observed with respect to the tenuisphere and crassisphere morphotypes and the mixed morphotype group (Figure 2). The three remaining morphotypes constitute a more random proportion of the samples. The low-temperature DTF chars are richer in the thin-walled morphotypes with high porosity, while the high-temperature chars are dominated by the thicker-walled crassisphere chars and the mixed morphotype group. However, the composition of the two DTF char samples produced at 1150°C is markedly closer to the composition of the hightemperature char samples with respect to the tenuisphere morphotype and the mixed morphotype group. This relation is also illustrated by photomicrographs of chars with comparable burnout levels produced in the various reactors (Figure 3). The two low-temperature DTF chars are dominated by almost perfectly shaped, highly fused, thin-walled spheres (Figures 3a, b), while the DTF chars produced at 1150°C (Figure 3c) are more complex in appearance, although still significantly different from the high-temperature chars (Figures 3&h). The hightemperature chars appear to be very similar. They are all dominated by thicker-walled morphotypes with small bubbles and they generally contain more vesicles. The presence of small bubbles in the wall has been suggested by Unsworth et al.28 to indicate pyrolysis after swelling has ceased. This phenomenon is apparently strongly temperature-dependent and it is suggested that the more intense development of devolatilization products at higher temperatures, together with a shorter period of plasticity, leads to the formation of bubbles. All chars derived from the Gondwana coals are compared as a group, since no single Gondwana coal was tested in all reactors. For simplicity, only chars with a burnout of -50-60 wt% (daf) together with all fullscale chars have been included in the graphs (Figure 4). Although the coals differ in both rank and origin, some general trends can be extracted from the graphs. Again the composition of the low-temperature DTF char samples is markedly different from that of the hightemperature samples, especially with respect to the tenuisphere and crassisphere morphotypes. Both the high- and the low-temperature char samples are rich in the mixed morphotype group, although the hightemperature char samples are significantly richer in this group. As with the Colombian char samples, only very few tenuinetworks are found and no clear trend in the distribution can be found. It is notable that the composition of the low-temperature DTF char samples produced at 1150°C approaches the composition of the higher-temperature char samples.

Fuel 1996 Volume 75 Number 9

1075

Combustion

char morphology:

P. Rosenberg

et al. 10

Tenuispheres [a] 1

Tenuinetwork [b]

9

8 7

0

6

,$?

0

5 4

Crassispheres [c]

20 18 16 14 -

lnertoid (f] ??

12 -

L$

0

lo6-O 6-

0 w

00

Figure 2 Chars derived from the Colombian coal. The individual morphotype distribution of each sample is shown. Notice the marked shift in composition between the low-temperature DTF chars and the high-temperature chars in [a], [c] and [d]

The chars derived from both Colombian coal and the Gondwana coals in the muffle furnace are significantly different from all other chars (Figures 2, 4). They are richer in the denser char types (mixed morphotype group and the fusinoid/solid morphotypes), and are more like coke than combustion chars. The present study shows that the char morphologies vary in a systematic way, depending on the temperature domain in which they are produced. If the various char morphotypes differ in reactivity and burnout characteristics, which is believed to be partly true, greater attention

1076

Fuel 1996 Volume 75 Number 9

should be given to the experimental conditions under which the chars are produced. The similarities in composition between high-temperature char samples formed in laboratory-scale and full-scale experiments indicate that it is possible in a laboratory-scale reactor to produce chars very similar to those found in the flame zone of a p.f. combustion boiler. The temperature and the heating rate should thus be very similar to the conditions in which the coals investigated are utilized. Preliminary results suggest a threshold temperature -1300°C for the production of chars that are comparable with full-scale

Combustion

char morphology:

P. Rosenberg

et al.

from the various

reactor

Fuel 1996 Volume 75 Number 9

1077

b)

d)

h)

Figure 3 Photomicrographs types, together with full-scale

of typical Colombian chars (see text)

coal-derived

char morphotypes,

taken at a burnout

at -50-60

wt%,

??

A

% A

I

8

I’

In

0

0

8 ! -I

OO

-

-

“A”,:“: Au20 SAlA SAlC f%i

-

-

-

Ailz AulJ

%i:g Au2E

AulC AulF

0

I

I

I

I

-ruuoPmm-lm(Do 00000000000

2%--

P2

1 mm’

>

0

%%i

I

%I

m

I

AuPL Au20 SAlA SAlC

0

I

“A”,:‘;’ Au20 SAl A SAIC

Atl%i AulJ

ii:: Au2E

ii:

AulA AuPA

I

I

%

0

0

O0

0

I

0

0

I

I

I

~~$~

“A”,:“: Au20 SAl A SAlC

Acl AulJ

%f? Au2E

i3

AulC AulF

I

o-

I

8

??

I

0

??

??

??

??

I

%

??

0

I

??

ruwPmm~co*o I I I I

a ??

I

I

I

??

I

??

I

I

Combustion

char morphology:

P. Rosenberg

et al.

Table 5 High-temperature char samples with -50-60 wt% burnout (daf) Char sample

Reactor type

Burnout (wt% daf)

Sample position

Temperature (“C)

CoJ

Laminar flow reactor

61

64 mm (47 ms)

1409 (6% 02)

CoM

Laminar flow reactor

64mm (47ms)

1419(12% 02)

COP

Full scale

59 63

Flame zone

-1500

CoR

1.5 MW tunnel reactor

50

cos

Entrained flow reactor

52

250ms

-1300-1400

AulJ

Laminar flow reactor

49

129mm (72 ms)

1369 (6% 0,) 1320 (6% Oz)

1330

AulK

Laminar flow reactor

52

191mm (95 ms)

AulL

Laminar flow reactor

56

254mm (117ms)

1263 (6% 02)

AulN

Laminar flow reactor

58

129mm (72ms)

1372 (12% 02)

Au2K

Laminar flow reactor

49

64 mm (47 ms)

1409 (6% 0,)

Au2L

Laminar flow reactor

54

129mm (72 ms)

1369 (6% 0,)

Au2M

Laminar flow reactor

58

191mm (95 ms)

1320 (6% 02)

Au2N

Laminar flow reactor

61

254mm (117ms)

1263 (6% 02)

Au20

Laminar flow reactor

49

64 mm (47 ms)

1419 (12% 0,)

SAlA

Full scale

49

Flame zone

-1500

SAlC

Full scale

61

Flame zone

-1500

SA2B

Entrained flow reactor

52

250ms

PoA

Full scale

63

Flame zone

-1500

POC

Full scale

61

Flame zone

-1500

Table 6 Average composition (%) of the char samples Char type Tenuisphere Tenuinetwork Crassisphere Crassinetworkmixed networkmixed Inertoid Fusinoid-solid Number of analyses

AvCo

AvAul

AvAu2 AvSAl

AvSA2 AvPo

9 3 54

4 4 17

1 2 15

9 4 17

2 0 6

24 6 31

30

73

74

64

89

29

4 0 5

2 2 4

6 2 5

5 1 2

3 0 1

6 4 2

(AvCo, AvPo, AvAul, AvAu2, AvSAl) were calculated from these samples (Table 6). The average compositions were used for correlation with the petrography of the parent coals. Classification of the char morphotypes reveals some clear differences between the samples, in particular between the chars derived from the Colombian coal (AvCo) and the Polish coal blend (AvPo) and the chars derived from the Australian and South African coals (AvAu 1, AvAu2, AvSA 1, AvSA2) (Table 6). The AvCo sample is dominated by the crassisphere morphotype, that is, high-porosity char with few prominent degassing pores (Figure Sa, b). Second is the group comprising crassinetwork and mixed morphotypes. Likewise, the highly porous tenuisphere morphotype is present in a significant proportion (Table 6, Figure 5~). The AvAul and AvAu2 samples derived from the Australian coals are dominated by a high proportion of the crassinetwork-mixed char group, followed by the crassisphere morphotype (Table 6, Figure 5a, b). The other char types are subordinate. The South African AvSAl and AvSA2 samples are likewise dominated by the crassinetwork and mixed char types (Figure 54, in particular the AvSA2 sample, where the content is very high (Table 6). The AvSAl sample is comparable compositions

1330

with the Australian samples in having a pronounced proportion of the crassisphere morphotype. In contrast, the AvSA2 sample contains only subordinate proportions of the highly porous tenuisphere, tenuinetwork and crassisphere char types. The Polish coal blend char, AvPo, is rather intermediate in composition, having a high proportion of the crassisphere and tenuisphere morphotypes and the crassinetwork-mixed group. A principal-components analysis was carried out using the computer program Sirius29. The 36 hightemperature char samples, containing six morphotypes, were loaded into the data matrix, allowing a twoprincipal-components model to be extracted. The model explains almost 98% of the total variance in the data set, thus losing only -2% of the information. The score plot of the data set shows a two-dimensional representation of the variance, explained by the multivariate model of the data set (Figure 6). The overall similarities among the char samples can thus be studied and clustered. The separation in the principal-component 1 (PCl) direction is prominent and separates the Australian and South African samples from the Colombian and Polish samples. In the principal-component 2 (PC2) direction, a weaker separation of the South African and Australian samples can be observed together with a weak separation between the Colombian and Polish samples. The Australian char samples were produced in only one reactor type and are thus very similar, in contrast to the Colombian char samples, which were produced from different reactors and consequently have higher variances. Nevertheless, a good separation between the four groups can be observed, indicating a good correspondence between the petrography of the feed coal and the resulting char morphology. The separation in the PC1 direction may be explained by the different petrographic compositions of the parent coals, whereas the separation in the PC2 direction may be caused by differences in rank, as already indicated by the

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b)

4

Figure 5

Examples of char morphotypes: (a) and (b) crassisphere chars (AvCo); (c) tenuisphere char (AvCo); (d) mixed network char (AvSA2) 30

N

r; if

*O 10

g i

0

z2 -10 %

-20 -30

I

-60

-40

-20

0

20

40

60

Principal Component 1 Figure 6 Score plot from the principal-components analysis of the high-temperature chars, showing the major variances in the data set projected on two principal components. See text for further explanation. ?? , Co-chars; 0, Aul-chars; 0, Au2-chars; A, SAl-chars; v, SAZ-chars; 0, PO-chars Table 7

petrographic analyses. However, the small number of feed coals does not allow a statistically valid conclusion to be made. One South African sample falls in the field of the Australian samples; this cannot be explained and may be incidental. Hence the most significant factor controlling the char composition appears, for the coals in this study, to be the petrographic composition of the parent coal. The Colombian coal and the Polish coal blend are characterized by a high proportion of the vitrinite-rich microlithotypes vitrite, clarite and vitrinertite V, and this seems to have favoured the formation of the crassisphere morphotype, and also tenuisphere chars. The proportion of the three microlithotypes in the Colombian coal amounts to 57vol.%‘, and the average content of crassispheres in the AvCo char sample is 54% (Table 7, Figure 7); if the tenuisphere chars are included, the value is 63% (Table 7). In the Polish coal blend the three microlithotypes amount to 44 vol.%, whereas the AvPo char sample contains 31% of crassispheres

Correlation between feed coal petrography and char morphology Samples

Microlithotypes/ Char morphotypes

SAl/AvSAl

SA2/AvSA2

Po/AvPo

Vitrite + clarite+ vitrinertite V (vol. %)

57

44

15

20

15

13

Crassisphere (%) Crassisphere + tenuisphere (%)

54 63

31 55

17 21

15 16

17 26

6 8

Inertite + durite + vitrinertite I + duroclarite + clarodurite (vol.%)

38

39

80

71

79

81

Crassinetwork-mixed mixed (%)

30

29

73

74

64

89

1080

Aul/AvAul

AuZ/AvAu 2

Co/AvCo

network-

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100 80

80

60

60 %

%

40

40

-

Figure 7

Crassispheres Vitrite+Clafite+Vitriinertite V

Histograms showing the correlation between coal microlithotypes and char morphotype groups

(Table 7, Figure 7). If the tenuisphere morphotype is included, the value increases to 55%, thus emphasizing the high proportion of tenuisphere chars produced by the coal blend. In the Australian and South African coals with low proportions of the vitrinite-rich microlithotypes, correspondingly low proportions of the crassisphere morphotype are produced during combustion. The two Australian feed coals, Au1 and Au 2, contain 15 and 20 vol. % respectively of vitrite, clarite and vitrinertiteV, and the two resulting chars, AvAul and AvAu2, contain 17 and 15% respectively of the crassisphere morphotype (Table 7, Figure 7). The South African SAI coal contains 15 vol. % of these microlithotypes, whereas 17% of the crassisphere morphotype is present in the AvSAl sample. The values for the SA2 and AvSA2 samples are 13 vol.% and 6% respectively (Table 7, Figure 7). Because of the generally low proportion of tenuisphere chars derived from the Australian and South African coals, the results are not significantly altered by including this morphotype (Table 7). These results general relationship between suggest a the vitrite + clarite + vitrinertite V microlithotypes and mainly the crassisphere morphotype, but also the tenuisphere morphotype. The relation is in particular pronounced for vitrinite-rich coals. This is in agreement with the results of Bailey et aL2; however, these authors also found a strong, rank-dependent correlation with tenuinetwork chars. None of the present coals produced significant proportions of the tenuinetwork morphotype, so the present study cannot confirm this correlation. The proportion of crassinetwork and mixed morphotypes is very high in the chars from the four inertiniterich coals from Australia and South Africa. The total sum of inertite, durite, vitrinertite1, duroclarite and clarodurite in the Aul, Au2, SAl and SA2 coals is 80,7 1, 79 and 8 1 vol.% respectively. The proportion of the crassinetwork-mixed morphotype group in the corresponding char samples, AvAul , AvAu2, AvSAl and AvSA2, is 73, 74, 64 and 89% respectively (Table 7, Figure 7). A weak correlation may be seen, but the SAl -AvSA 1 pair is an outlier. It may be suggested that a higher proportion of crassisphere plus tenuisphere

morphotypes was formed despite the inertinite-rich composition of the SAl coal. The Colombian coal shows a good correspondence between the microlithotypes and char types: 38vol.% and 30% (Table 7, Figure 7). The correlation between the Polish coal blend and the AvPo char sample is less satisfactory, being 39vol.% and 29% respectively (Table 7, Figure 7). No extraordinarily high content of the dense inertoid and fusinoid-solid morphotypes was observed in the Australian and South African chars despite the high inertinite content of the feed coals; this may be related to the semi-inert character of the inertinite. Bailey et al2 correlated durite and inertite with inertoid, mixed, solid and fusinoid morphotypes. The char samples derived from the present parent coals rich in the microlithotype inertite do not contain equivalent proportions of the inertoid, mixed, solid and fusinoid char types, and it has thus not been possible to demonstrate that correlation. The score plot suggests that rank influences the char morphology, and it must be noted that this factor may account for some of the discrepancies in the correlations between the petrographic composition of the feed coals and the resulting char morphology. It is probably necessary to consider the rank to strengthen the correlation. CONCLUSIONS Systematic variations have been found in the texture of chars produced in different temperature domains from the same Colombian raw coal. Similar variations have been found in chars produced from a suite of Gondwana coals with common characteristics (high inertinite content), although the coals differ in both rank and origin. The results suggest that experimental work on correlation between coal petrography and char morphology, and determination of char reactivity and burnout characteristics, can be accomplished only if the chars are produced under realistic temperature conditions similar to those in the intended use of the coal. Preliminary results suggest a threshold temperature of -1300°C for producing char morphologies that are comparable with those produced in a p.f. combustion boiler.

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It has been possible to demonstrate a general correlation between the petrography of the feed coals and the morphotype composition of the corresponding chars derived from high-temperature reactors and a p.f. power plant. Petrography seems to be the main factor controlling the char morphotype composition, with rank perhaps of minor importance. The minor importance of rank may be a consequence of the comparatively narrow rank range of the coals studied. Vitrite, clarite and vitrinertitev may in particular be correlated with the porous crassisphere morphotype, and probably also the tenuisphere morphotype, whereas the inertite, durite, vitrinertite I, clarodurite and duroclarite microlithotypes may be correlated with the crassinetworkmixed-network-mixed morphotype group. One of the difficulties in establishing a more detailed correlation may lie in the crassinetwork-mixed-network-mixed group, which contains a wide range of morphologies and porosities.

8 9 10 11

12 13 14 15 16 17

18 19 20

ACKNOWLEDGEMENTS The authors are grateful to the Danish Energy Agency, Elsam I/S and Elkraft A.m.b.a. for financial support. L. H. Sorensen (Research Centre Rise) is thanked for preparing laboratory-scale chars, and K. Laursen (Geological Survey of Denmark and Greenland, GEUS) for sampling full-scale chars at the Fynsvaerket p.f. power plant, Denmark. Fynsvrerket is thanked for giving access to the full-scale experiments. D. Jutson (GEUS) is thanked for reading and improving the manuscript.

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Thomas, C. G., Gosnell, M. E., Gawronski, E., Phong-Arrant, D. and Shibaoka, M. Org. Geochem. 1993,24I, 779 Crelling, J. C., Skorupska, N. M. and Marsh, H. Fuel 1988,67, 781 Walker, S. ‘Major Coalfields of the World’, IEACR/Sl, IEA Coal Research, London, 1993 Hagelskamp, H. H. B. and Snyman, C. P. Fuel 1988,67, 307 Hobday, D. K. In ‘Coal and Coal-bearing Strata: Recent Advances’ (Ed. A. C. Scott), Geological Society of London, Special Publication 32, 1987, p. 219 Holland, M. J., Cadle, A. B., Pinheiri, R. and Falcon, R. M. S. Znt. J. Coal Geol. 1989, 11, 143 Hunt, J. W. Int. J. Coal Geol. 1989, 12, 589 Hunt. J. W. and Smvth. M. Znt. J. Coal Geol. 1989. 11.23 Martini, I. P. and Johnson, D. P. Znt. J. Coal Geol. ‘198j, 7,365 Taylor, G. H., Liu, S. Y. and Diessel, C. F. K. Znt. J. Coal Geol. 1989,11, 1 Stach, E., Mackowsky, M.-Th., Teichmiiller, M., Taylor, G. H., Chandra, D. and Teichmiiller, R. ‘Stach’s Textbook of Coal Petrology’, 3rd Edn, Gebrtlder Borntraeger, Berlin-Stuttgart, 1982 Diessel, C. F. K. Fuel 1983,62, 883 Laursen, K. and Larsen, 0. H. ‘Slagging and Fouling, Full Scale Testing at Fynsvrerket Unit 7’, 1995 Jensen, P. A., Rathman, O., Clausen, S., Andersen, S. A., Beth, N. and Kirkegaard, M. Final Report Em. No. 1323/8920, Rise-R-637, Research Centre Rise, Denmark, 1992 Hardesty, D. R., Pohl, J. H. and Stark, A. H. Sandia Report SAND78-8234, Sandia National Laboratories, 1978 Mitchell, R. E., Hurt, R. H., Baxter, L. L. and Hardesty, D. R. Sandia Report SAND92-8208, Sandia National Laboratories, 1992 Jensen, P. T. and Mitchell, R. A. Subreport, Energy Research Project No. 1323/87-16, 1993 Sorensen, L. H., Clausen, S., Asstrup, P., Jensen, P. A., Porsdal, H. and Olsen, A. ‘Fundamentals in Combustion, Pyrolysis and Reactivity Measurements with a New Atmospheric Entrained Flow Reactor, and Description of a Burning Particle in a Controlled Atmosphere’, Report, Research Centre Riser, Denmark, 1991 Jensen, P. T. and Thomsen, E. Status Report, Energy Research Project No. 1323/87-16, 1990 (in Danish) Sorensen, L. H. ‘Fuel Reactivity as a Function of Temperature, Pressure and Conversion’, Ph.D. Thesis, Research Centre Rise, Denmark, 1994 Vleeskens, J. M., Menendez, R. M., Roos, C. M. and Thomas, C. G. Fuel Process. Technol. 1993,36,91 Unsworth, J. F., Barratt, D. J. and Roberts, P. T. ‘Coal Quality and Combustion Performance’, Coal Science and Technology 19, Elsevier, Amsterdam, 1991 Kvalheim, 0. L. and Karstang, T. V. Chemom. Intell. Lab. Syst. 1987,2,235