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The behaviour of inertinite macerals under pulverised fuel (pf) combustion conditions
Org. Geochem. Vol. 20, No. 6, pp. 779-788, 1993 Printed in Great Britain. All rights reserved
0146-6380/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
The behaviour of inertinite macerais under pulverised fuel (pf) combustion conditions C. G. THOMAS,t M. E. GOSNELL, 1 E. GAWRONSKI,1 D. PHONG-ANANT2andM. SHIBAOKA 1 tCSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde, NSW 2113 and 2Australian Combustion Technology Centre, ACIRL Ltd., PO Box 242 Booval, Qld. 4304, Australia Almtract--The usefulness of the current system of classification of the inertinite group of macerals for understanding the pulverised fuel (pf) combustion process is discussed and questioned. Results to date on the combustibility of inertinite macerals are indecisive, especially as inertinite is mostly regarded as a single entity. Simulated pf combustion experiments with a laser microreactor revealed that the inertinite macerals yielded a wide diversity of char morphologies. With one-to-one correlations between maceral and char, it was possible to determine which maceral was fusible or infusible (commonly called reactive and inert respectively). The microreactor is being developed to measure the burning parameters of individual maceral particles. For example, the data will show which macerals are slow burning (and by how much) and whether fusibility has any relevance to the speed of char burning. Key words--pf-combustion, fusibility, macerals, reactivity, char types, inertinite, classification
INTRODUCTION
Over the past 40 years the macerals in coal have been identified and classified in a progressive manner to enable the exchange of knowledge between scientists and technologists working with coal. This has been done largely by geologists and petrologists, with the result that the classification is by appearance, by deemed origin and in one instance alone, by its behaviour when heated. Because the macerals are heterogeneous at the microscopic level, the parameters chosen have been those which could be seen with the microscope: shape (or morphology) together with reflectance, colour and relief. The one exception has been the grouping together of several macerals into the Inertinite Maceral Group. The word inertinite was chosen (Stach, 1952) to combine those macerals that were "more or less inert or non-reactive in the carbonisation process" (Stach, 1982). At the time carbonisation was the only use for coal that had attracted attention to the relationship between coal structure and its processing behaviour. The choice of word was poor at the time and worse in hindsight. It labelled inertinite macerals with an inert image, when in fact, all that was meant was that under the slow heating conditions of coke-making these macerals did not fuse or show plasticity. Time has shown that even the identification of the infusible macerals was done with too little knowledge. Recently (Diessel, 1992) has discussed the question of the actual inertness of the inertinite macerals in the carbonisation process, and has shown for Gondawana coking coals (Diessel and Bailey, 1989) that 50% of the inertinite is fusible under the carbonisation process conditions. 779
The largest and most rapidly expanding use for coal is in the generation of electricity by pulverised fuel (pf) combustion processes. The concept that a large part of the organic matter of many commercial coals is inert, despite the fact that these can all be burnt, is very misleading. Many of these coals contain 70% inertinite, with this label implying that they are in some way less desirable. The problem is that the determination of the perceived inertness or relative inertness of the macerals in the inertinite group of macerals has never been quantified in any satisfactory manner. The pf combustion process has become the dominant method because the rapid rate of burning of the finely ground ( ~ 7 0 % < 75 pm) coal particles enabled compact boiler designs, whilst attaining high energy release per unit volume. This led to reduced capital costs for boilers and hence the rapid adoption of this process for generating electricity. These particles of organic matter burn away to ash in approx. 2-3 s, hardly a description of inert behaviour for constituents that can comprise 70% of the coal being consumed. To achieve complete carbon burnout the dimensions of the boiler and the rate of flow of the coal particles through the boiler must be matched to the time needed for burning the coal (Pohl et al., 1992). It is well known that coal rank has a strong correlation with burnout time (Jones et al., 1985a; Morrison, 1986; Shibaoka et aL, 1985; Oka et al., 1987) and that rank and volatile matter are closely correlated. Also as coal and char particle size increase, so the burnout time increases (Shibaoka, 1985; Smith, 1982; Walsh and Xie, 1992). Since rank and particle size alone can increase the burnout time by at least an order of magnitude, it is essential to match the boiler design to the coals intended to be
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C.G. THOMASet aL
used, to optimise the cost of the installation (IEA Coal Ind. Advisory Board, 1985). To design a boiler to burn all coals would be quite uneconomic. However, even when the factors discussed above have been used in the design, it sometimes turns out that the burnout of the coal is not as good as it should have been. Considering other coal properties that have been reported to affect burnout the most important is the maceral composition of the coal (Prado et al., 1987; Lockwood, 1987; Walsh and Xie, 1992). Although the behaviour of macerals has been discussed in the combustion literature in the past 15 years e.g. (Nandi et al., 1977; Sanyal, 1983; Bustin et al., 1983; Jones et al., 1985b; Falcon and Snyman, 1986; Hough and Sanyal, 1987; Skorupska et al., 1987; Crelling et al., 1988; Furimsky et al., 1990), they are almost completely ignored by utilities, and the maceral composition of a coal is not listed in coal specifications. This has occurred because coal was relatively cheap and a little unburnt carbon could be tolerated. As the coals used became more familiar to the user, adequate performance was eventually achieved. There were, and are, notable exceptions where a plant was tied to a particular coal which was unsuitable for the power station. Two things have changed. With the advent of the second coal age in the 1970s, coals are being moved around the world in increasingly large quantities. Utilities, often on the other side of the world are buying coals of which they have no direct experience. Secondly, these coals are being sourced from countries like Australia and South Africa whose Gondwana coals are substantially different in many ways to the Northern Hemisphere carboniferous coals (Taylor et aL, 1989; Hough and Sanyal, 1987; Jones et aL, 1985a). Yet, it is users in the Northern Hemisphere who are using these Southern Hemisphere coals, but naturally enough, interpreting them through Northern Hemisphere eyes. This paper examines the behaviour of the inertinite macerals in the pf combustion process and discusses some further work that needs to be done. SOME EARLIER WORK ON INERTINITE
There has been very limited research on inertinite combustion, and in most cases the workers have regarded inertinite as one entity (Nandi et al., 1977; Shibaoka and Ramsden, 1978; Sanyal, 1983; Lowenthal et al., 1986; Falcon and Snyman, 1986; Oka et aL, 1987; Hough and Sanyal, 1987; Crelling et al., 1988). There is ample evidence that only listing the three maceral groups namely, Vitrinite, Liptinite and Inertinite, can be quite misleading (Carpenter, 1988). The Inertinite Group of Macerals which comprise semifusinite, micrinite, macrinite, inertodetrinite, sclerotinite and fusinite are lumped together and labelled 'inertinite'. Even the UN-ECE (Economic Commission for Europe, 1988) in the classification system currently under discussion treats
the inertinite group of macerals as one entity-inertinite. Yet this collection of macerals comprises a diverse group which differ greatly chemically, physically, petrographically and in their response and behaviour when pyrolysed and combusted. It also means that the "averaged" behaviour of the inertinite in one coal, e.g. (Vleeskens and Nandi, 1986; Phong-anant et al., 1989) projects an image of "inertinite" which may be only partially valid for other coals whose "inertinite" comprises different mixtures of inertinite macerals. To continue this grouping is positively unhelpful and misleading. This is especially so for those industrialists responsible for the use of more than 70% of the coal in the OECD (IEA Coal Ind. Advisory Board, 1985). At the present time they are taught a classification system of nearly half a century's standing, which has little relevance for their major coal use--pf combustion. From the limited research data that are available, an unclear picture emerges e.g. with semifusinite showing greater combustion reactivity than vitrinite (Tsai and Scaroni, 1984) but inertinite showing the reverse with vitrinite (Shibaoka and Ramsden, 1978). Other pf combustion research has been conducted at 500°C (Crelling et al., 1988) with the authors indicating that the results were representative of a process at 1500-1700°C. The problems besetting these researchers and others have been the microheterogeneity of the macerals in many coals, the difficulties of making concentrates of individual macerals, and the problems involved in conducting the research under realistically simulated pf combustion conditions at temperatures ~ 1600°C (Morrison, 1986). Because the char morphology has such a significant effect on the apparent reaction rate, (Jones et al., 1985a; Oka et aL, 1987; Skorupska et al., 1987; Smith, 1982; Bend et al., 1992), and since char morphological development, (especially for many of the inertinite macerals) it so greatly affected by the rate of heating and the pyrolysis temperature, (Thomas et al., 1991a) it is imperative that research be conducted at true pf temperatures, heating rates, oxygen partial pressure and particle size. There is a further problem that the Combustion Petrologist has to face. In a coal, semifusinite is often the most abundant inertinite maceral (volume %). Yet this maceral itself has an acknowledged wide diversity, ranging in reflectance and morphology between fusinite and telinite (Stach, 1982). Understandably, when various samples of semifusinite are pyrolysed and the resulting chars examined a wide diversity of char types ensue (Thomas et al., 1991a; Bend et al., 1992). Some researchers have attempted to partially allow for this by dividing semifusinite into two parts-fusible and infusible e.g. (Nandi et al., 1977; Bustin et al., 1983; Falcon and Snyman, 1986; Jones et al., 1985a; Shibaoka, 1985; Skorupska et al., 1987). The division of semifusinite into fusible and infusible is regarded as significant by some (Jones et aL, 1985a;
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Inertinite maceral behaviour under pf combustion conditions Falcon and Snyman, 1986; Skorupska et al., 1987; Tait et al., 1989) but as a dubious exercise by others (Furimsky et al., 1990), a division which is not based on sound experimental data in the view of the present authors. Just to add to the confusion, many definitions of fusible and infusible macerals exist. These have been listed (Thomas et al., 1992a). Unfortunately, because this idea has been borrowed from coke makers (Schapiro et al., 1961; Gray et al., 1978), the fusible part is equated with 'reactive' and the infusible with 'non-reactive or inert'--labels quite appropriate for the carbonisation process. However, in the combustion context this leads to misunderstanding, because 'reactive' now has the meaning--"this coal will react faster with oxygen". Put another way, the message is--"reactive coal has a greater combustion reactivity", and thus "fusible semifusinite burns faster than infusible semifusinite". The division into the two parts was made because people believed that this was the case, but has anyone presented any data to validate such an hypothesis? This contentious inference has since been broadened to include other fusible and infusible macerals. It is said that fusible macerals burn faster than infusible macerals e.g. (Carpenter, 1988; Falcon and Snyman, 1986). There is no data to support such a concept--another myth has been born. There was just enough data to make it plausible, namely, that fusinite (infusible) had been observed to burn more slowly than vitrinite (fusible) at 1000°C (Shibakoa, 1969). However, this was an early result, obtained under conditions substantially different from the pf process, in terms of the particle heating rate and the combustion temperature. The extension from this to the concept that all fusible macerals burn faster than all infusible macerals has no justification. RESEARCH AT CSIRO (1987-1992)
Australia has many coals with high inertinite contents. Against the background just discussed and the fact that Australian inertinite-rich coals had experienced difficulty in the coking coal market 10 years ago, it was clear that a better understanding of the behaviour of the Gondwanan coals in the pf combustion process was necessary. In 1987 there was a widespread belief that 30% or less of the inertinite was fusible (reactive), e.g. (Falcon and Snyman, 1986; Furimsky et al., 1990). However char analyses of drop tube furnace residues indicated much higher levels of fusible inertinite. Hence the ability to measure and ascertain the validity of this oft quoted 'magic' number (Schapiro et al., 1961) was the first target, especially as coals with high levels of fusible macerals were considered more desirable e.g. (Nandi et al., 1977). Maceral heterogeneity is visible at the optical microscope level, thus a specific microscopic method was necessary. This has been described elsewhere (Thomas et al., 1992a; Thomas et al., 1991a; Thomas
et al., 1991b). Monomacerai particles are obtained from selected areas on a polished surface of a coal sample by quarrying them with a needle under a binocular microscope. The surrounding maceral area is subsequently characterised using an oil immersion objective to determine the reflectance and maceral type. One particle at a time is mounted on the tip of a needle and heated for ~ 4 0 ms by a CO2 laser beam from two sides in a laser microreactor (Fig. 1). The system simulates the heating conditions encountered in pf combustion, namely: heating rate ~ 106 °C. s -L, pyrolysis/combustion temperature ~ 1600°C, atmosphere--air, particle size ~ 100 #m. The particle temperature is monitored with a two colour micropyrometer, giving a temperature profile for each particle processed. The technique has the advantage that a characterised maceral particle is correlated one to one with the resulting char. After heating, the particle is removed from the needle, mounted in epoxy resin and polished using the method of Brunckhorst (1992) and its structure examined under a microscope. A whole coal reflectogram, where frequency is plotted against reflectance, shows how the inertinite macerals are distributed over a wide reflectance range, from vitrinite up to and including the high reflectance values of fusinite. To obtain samples of macerals representative of the whole coal, particles are dug from maceral quarries which have different reflectances, and which together cover the whole reflectance range. DISCUSSION OF CHAR TYPES FORMED FROM MONOMACERALS OF VITRINITE AND INERTINITE
Figure 2 shows examples of chars that have been produced in a study of six coals from both Australia and the Northern Hemisphere (Thomas et al., 1991a, 1992b). Captions for Fig. 2 are given in Table 1, with abbreviations in Table 2. Figures 2.1 and 2.2 show that vitrinite forms very thin walled
~LASER
[~ iEMP°C DLE
Fig. 1. Schematic diagram of laser microreactor.
C. G. THOMASet al.
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Fig. no. 2.1 2.2 2.3 2.4 2.5 2,6 2.7 2.8 2.9 2.10 2. l I 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24
Coal no. 4 1 4 3 5 1 5 2 2 6 6 3 3 4 3 6 3 2 6 4 5 2 5 3
Rank Rv (%) 1.14 0.45 1.14 0.70 0.81 0.45 0.81 0.76 0.76 0,99 0,99 0.70 0.70 1.14 0.70 0.99 0.70 0.76 0.99 1.14 0.81 0.76 0.81 0.70
Maceral VT VT VT VT SF SF SF SF SF SF SF SF SF SF SF SF SF SF SF F SF SF SF F
Table 1. Captions for Fig. 2 Maceral Pyrolysis refleCtanCe temp Maceral type (random) (°C) (see key) (%) 1420 Telocollinite 1.04 1500 Telocollinite 0.45 1300 Teiocollinite 1.12 1410 Telocollinite 0.70 1420 PG.FE.CS 1.18 1460 PG.CS 0.94 1730 PG.FE.CS 1.24 1820 PG.FE.CS 1.04 1430 WP.CS 1.38 1400 PG.CS 0.92 1625 1.35 PG.CS 1230 WP.CS 1.50 1580 WP.CS 1.50 1300 WP.TK.CS 1.65 1790 PG.FE.CS 1.55 1500 WP.FE.CS 1.45 1800 WP.FE.CS 1.85 1540 WP.CS 1.88 1620 WP.TK.CS 1.94 1550 BOGEN.CS 1.98 WP.TK.CS 1650 1.55 1320 WP.CS 2.15 WP.TK.CS 1.83 1550 1600 WP.TK.CS 2.60
cenospheres from both high and low rank thermal coals and that it also forms thick walled cenospheres with secondary vesiculation in the walls from both high and medium rank coals (Figs 2.3 and 2.4). The factors that influence thin and thick wall formation are not known, but probably include heating rate, oxygen partial pressure and probably, but most significantly, the maceral precursor. Figures 2.5 and 2.6 are examples of thin walled cenospheres formed from semifusinite (inertinite) macerals. As the reflectance of the semifusinite macerals increases, there is a tendency for the vesicles to be more equally sized, resulting in the formation of char types called network chars. These can have very thin walls as seen in Figs 2.7 and 2.8, indicating that at the time of devolatilisation the gas evolution coincided with the 'time window' where the coal/char was in a state of high fluidity. With increasing inertinitc reflectance there is a tendency for parts o f the walls to be thicker, indicating zones of lower fluidity (or plasticity) for these more viscous chars, Fig. 2.9. F o r some inertinite macerals that have reflectances similar to vitrinite a thick walled network char may form, Fig. 2.10. F o r inertinite with higher reflectances, the
Table 2. Abbreviations in Table 1 Maceral type Char type Vitrinite VT Fused FD Fusinite F Unfused UF Semifusinite SF Cenosphere CE TN Cell structure CS Thin Well preserved WP Thick TK NK Part gelification PG Network Fine FE Mixed MX Thick TK Transitional TRANS Solid S Open Solid OS
Char type (see key) FD.MONO.CE FD.TN.CE FD.TK.CE FD.TK.CE FD.TN.CE FD.TN.CE FD.TN.NK FD.TN.NK FD.MX.NK FD.TK.NK FD.MX.NK FD.MX.NK FD.MX.NK TRANS TRANS TRANS UF.OS UF.OS UF.OS UF.OS UF.OS UF.S. UF.S UF.S
Scale 50 #m = " "mm 10 25 19 24 16 20 16 25 23 25 25 17 19 25 15 24 39 23 31 16 24 40 27 24
char morphology shows a mixture of network and banded structures, Fig. 2.11, with the mix varying, both between and within different coals, Figs 2.12 and 2.13. The transition from macerals that fuse (showing fluidity or plasticity during their formation) to those that do not (infusible macerals) tends to occur over a narrow range o f reflectance values, with some particles showing mixed fused/unfused structures. Figure 2.14 shows a particle with a few vesicles in a structure which otherwise shows little signs of plasticity. The particles seen in Figs 2.15 and 2.16 show the same mixed characteristics, although with quite different char morphologies from that seen in Fig. 2.14. Observation with the transmission electron microscope (TEM) o f the darker areas in the walls of the particle in Fig. 2.16 show them to consist of very finely vesiculatecl material. A b o v e the transition zone the particles are essentially unfused (Figs 2.17-2.24). Examination of the maceral precursor o f the particle shown in Fig. 2.22 shows rounded pores, and there is difficulty in distinguishing these remnant structures from pyrolysis vesicles, when observing the char particle. F o r inertinite with reflectances in the upper range, the effect of heating is to crack the structure, Fig. 2.24. Sometimes these cracks open up, Fig. 2.23, and in some of the unfused particles, the original structure, which combines separate components or elements, can be seen to have exfoliated textures and formed a more open structure, with good access for the oxidising gases during the subsequent char combustion (Figs 2.17-2.21). The broad diversity of behavioural responses shown in these chars are poorly described if they are all regarded as inertinite chars, without further differentiation (Figs 2.5-2.24).
Inertinite maceral behaviour under pf combustion conditions These chars can be classified into two groups, fused and unfused. Since the experimental technique gives a one-to-one correlation between the char and its maceral precursor, the macerals can be classified into fusible and infusible. When a plot was made of the distribution of the fusible and infusible macerals with respect to the reflectance of these macerals, it
783
was found that a distinct pattern emerged for each coal studied (Thomas et al., 1991a, 1992b). From the evaluation of up to 100 char particles per coal it was possible to determine the reflectance value which separated fusible macerals from the infusible macerals. For the six coals the boundary lies in a band between reflectance values of 1.4-1.75%, with
Figs 2.1-2.8. Selected photomicrographs of vitrinite and inertinite chars after rapid pyrolysis in the laser microreactor. See Table 1 for descriptions.
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C.G. THOMASet
no clear trend shown with the limited results available (Fig. 3). DISCUSSION OF RESULTS
The division of the inertinite macerals into fusible and infusible, mirrors the results found in carbonisation studies (Diessel and Bailey, 1989). However,
al.
under the dynamic (that is very high heating rate) conditions of pf combustion, the boundary values are higher. This has the result that an additional onethird, approximately, of the inertinite macerals were observed to be fusible. For the Australian coals 75% on average was fusible compared with 50% in the carbonisation studies. However, for coals that have a larger part of their inertinite macerals with high
ii
Figs 2.9-2.16. Selected photomicrographs of inertinite chars after rapid pyrolysis in the laser microreactor. See Table 1 for descriptions.
Inertinite maceral behaviour under pf combustion conditions reflectances (i.e. above 1.8% e.g. Northern Hemisphere coals) it could be expected that high levels of unfused chars will be present even under pf combustion conditions. For the two carboniferous coals tested, the average value of fusible inertinite was only 51% of the total inertinite (Thomas et al., 1991c).
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SIGNIFICANCE OF FUSIBLE MACERALS ON
COAL COMBUSTIBILITY Microscopic analysis of partially burnt chars obtained from laminar-flow entrainment reactor experiments are shown in Fig. 4 (Young et al., 1989). During these combustion experiments the chars are taken from ports down the reactor tube, with each
Figs 2.17-2.24. Selected photomicrographs of inertinite chars after rapid pyrolysis in the laser microreactor. See Table 1 for descriptions.
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C.G. THOMASet al.
I "~
0,8
•~
0.4
+ -I-
I 0.2
0
I 0.4
I 0.6
p~÷
I 0.8
i 1.0
I 1.2
Coal Rank (FIv %)
Fig. 3. Mean reflectance boundary between fusible/infusible inertinite macerals vs Coal Rank (Rv, %) for six coals. sequential sample showing increased burnout. Thus for the lowest coal with 54% inertinite the carbon remaining (i.e. not burnt out) decreases from 56 to 36% (ports 2, 3 and 4). Analysis of this carbonaceous material (i.e. char) gives the proportions of the chars that have exhibited fusibility during their formation, and in Fig. 4 these are expressed as a percentage of the whole char population (fused and unfused). If the fused material would have burnt more quickly than the unfused, then the "fusible material in char sample (%)" would have decreased and the burnout plot for that coal would drop, from left to right (ports 1-4). For the seven inertinite rich coals shown, five show a slight increase and two a slight decrease during their respective burnouts. The reasons for the different behaviours of these coals is not known. At this stage the average rate of burnout of fusible and infusible chars can be regarded as very similar in the region where burnout increases from 30 to 80% (Fig. 4). In other work, the burnout was 98-99% for both vitrinite and inertinite concentrates (Phong-anant et al., 1989). These results gave further evidence that lOO
59
n E 70 CO
30
•
2O
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10
T H E MEASUREMENT OF T H E SPEED OF COMBUSTION OF INDIVIDUAL MACERALS--FUTURE WORK
The huge diversity of char types (Fig. 2) deriving from monomacerals provides a clue to why other researchers have found such a wide range of chemical reactivities (i.e. combustion rates) for coal particles reacted at the same temperatures (Smith, 1982; Prado et al., 1987; Sanyal et al., 1991). Some researchers (Walsh et al., 1992) are using guesstimates of the proportion of slow burning material in their coal in their attempts to model the combustion behaviour successfully, In order to determine the combustion rate of individual maceral particles the laser microreactor is being upgraded. This will enable kinetic measurements to be made on individual particles under pf combustion conditions, seen as important by many workers e.g. (Walsh and Xie, 1992). The research will assist in the understanding of the relationships between macerals and their char structures (and these structures as they change during burnout) with their combustion reactivity. A major improvement to the equipment is to use the particle temperature information to operate a feedback control system for the laser to keep the particle burning at a fixed temperature. It is also necessary to know the coal particle's size, volume and weight. By imaging a silhouette (Fig. 5) of the particle
39
;=
._c 50
•
the division of macerals into the fusible/infusible parts was not a meaningful or helpful action, in terms of understanding the pf combustion behaviour of coals. The present authors do not dispute that there are macerals which yield char which burn faster, and others which burn slower, but are inclined to the view that morphological factors, such as wall thickness and access of oxidising gases to internal surfaces, are the important parameters, as seen in Fig. 2. It is well established that burning rates of pulverised coal are controlled by the combined effects of pore diffusion and the chemical reaction on the carbon surface (Smith, 1982). What is less well understood, is how pore structure and hence the diffusion of gases through that structure vary during burnout.
150~um
54
particle
KEY 5 9 = Inertlnite (%) i
0 0 90
i
i
J
1
i
i
80
70
60
50
40
30
10
2
Carbon Remaining (% W/W, Dry Basis)
Fig. 4. Fusible material (%) in drop tube char samples vs carbon remaining (%).
of needle
Fig. 5. Silhouette of coal particle on tip of needle.
Inertinite maceral behaviour under pf combustion conditions
3.
4.
Segment
5.
Coal Particle Fig. 6. Average size, and volume calculation of particle, from multiple silhouettes. every 9 ° degrees (Fig. 6) and using an image analysis software package, both the size and the volume of the particle can be calculated. After pyrolysis, if vesiculation and swelling have occurred, the sizing can be repeated to determine the new size and volume (i.e. determine percentage swell). The weight loss as burning proceeds can be measured by weighing the particle on the needle, using a microbalance with a resolution of 10 ng. The period of combustion can be determined from the temperature trace obtained from a new four colour micropyrometer. A video record of the burning particle reveals if there have been any catastrophic events such as fragmentation or rupture. Calculation of the kinetic parameters from these measurements will enable the relative speed of combustion of different macerals and their corresponding chars to be compared. A combustibility profile for a particular coal can be drawn up which will reveal which are the faster burning macerals, and, more importantly, which are the slow burning ones. If the percentage of the slow burning macerals is significant then it is likely that this maceral aspect could give a burnout problem, i.e. high carbon-in-ash percentages. CONCLUSIONS 1. The present classification system for black coals is quite unsuited for the use of combustion petrologists. The grouping together of some diverse macerals into the inertinite maceral group is meaningless from a combustion viewpoint, because they differ chemically, physically, morphologically and widely in their reflectances, all of which influence the very different ways these macerals respond when subject to rapid pyrolysis and combustion conditions. 2. The CSIRO laser microreactor has been used successfully to determine which macerals fuse
6.
7.
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(and which do not) under properly simulated pf combustion conditions. Pyrolysis of monomacerals from the inertinite group of macerals has yielded a range of char types varying from 'vitrinite-like' cenospheres to solid lumps of fusinite. The grouping of these char types under one heading--inertinite char--is simplistic and very misleading. Any usefulness that may have existed in this maceral grouping has long past. Inertinite is not inert in the pf combustion process. It all burns. What has to be determined is the relative rate of combustion of the variety of chars from within both the vitrinite and inertinite groups of maeerals. Comparison of char morphologies from vitrinite and inertinite particles reveals that thick walled 'bulky' char particles can result from the pyrolysis of macerals from either group. These constitute the slower burning or 'harder to burn' char particles. A method is proposed which has the ability to measure the relative combustibility of pf sized maceral particles under true pf combustion conditions. This method will reveal whether fusible ("reactive") macerals burn faster than infusible ("inert") macerals, as well as finding out which are the slow burning chars and relating these to their maceral precursors. REFERENCES
Bend S. L., Edwards I. A. S. and Marsh H. (1992) The influence of rank upon char morpholol~y and combustion. Fuel 71, 493-501. Brunckhorst L. F. (1992) A method for mounting, grinding and polishing very small laser-hit coal grains for microscopy and image analysis. Research School of Earth Sciences, Australian National University, Canberra, Australia. Submitted to Fuel. Bustin R. M., Cameron A. R., Grieve D. A. and Kaldreuth W. D. (1983) In Coal Petrology; its Principles, Methods and Applications. Geol. Ass. of Canada, Short Course Notes, Vol. 3, p. 195. Carpenter A. M. (1988) Coal Classification IEACR/12, p. 61. IEA Coal Research, London. Crelling J. C., Skorupska N. M. and Marsh H. (1988) Reactivity of coal macerals and lithotypes. Fuel 67, 781-785. Diessel C. F. K. (1992) The nature of inertinite and its effect on hydrogenation, carbonization and combustion. Keynote address. Int. Cmmtte. Coal Petrology. In Abstracts: 9th Ann. Meeting, Soc. Org. Petrology, Pennstate Univ., July 1992. Diessel C. F. K. and Bailey J. G. (1989) The Application of Petrographic Techniques to Carbonisation and Combustion Research at the University of Newcastle. Mineralogy-Petrology Symp., Sydney NSW, The AusIMM Sydney Branch, Australia, 117-121. Economic Commission for Europe (1988). International codification system for medium and high rank coals. ECE/COAL/ll5, p. 6, Table 3, United Nations, New York. Falcon R. M. S. and Snyman C. P. (1986) In An Introduction to coal petrography: Atlas of Petrographic constituents in
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the bituminous coals o f Southern Africa. The Geological Society of South Africa, Review Paper No. 2, plate 7. Furimsky E., Palmer A. D., Kalkreuth W. D., Cameron A. R. and Kovacik G. (1990) Prediction of coal reactivity combustion and gasification by using petrographic data. Fuel Process Technol. 25, 135-151. Gray M. R. J., Goscinski J. S. and Shoenberger R. W. (1978) Selection of coals for coke making. Prec. Joint Conf. by Iron and Steel Soc. A I M E and Soc. Min. Eng. AIME, 3 October, Pittsburgh, PA, U.S.A. Hough D. C. and Sanyal A. (1987) The role of petrography in the classification and combustion of coal. Energy World 146, 7-10. IEA Coal Industry Advisory Board (1985) Coal Quality and Ash Characteristics. Paris, France, OECD/IEA, 63 p. Jones R. B., McCourt C. B., Morley C. and King K. (1985a) Maceral and rank influences on the morphology of coal char. Fuel 64, 1460-1467. Jones R. B., Morley C. and McCourt C. B. (1985b) Maceral effects on the morphology and combustion of coal char. In Prec. 1985 Int. Conf. on Coal Sei. Sydney, 28-31 October, pp. 669-672. Lockwood F. C. (1987) Transfer of fundamental results to the modelling of burners and boilers. In Fundamentals o f the Physical-Chemistry o f Pulverised Coal Combustion (Edited by Lahaye J. and Prado G.), pp. 395-423. Martinus Nijhoff Pub. Dordrecht. (Co-op with NATO). Lowenthal G., Wanzi W. and van Heck K. H. (1986) Kinetics of swelling and plasticity of coal during rapid pressurized pyrolysis and hydropyrolysis. Fuel 65, 346-353. Morrison G. F. (1986) In Understanding Pulverised Coal Combustion, p. 26. IEA Coal Research, London. Nandi B. N., Brown T. D. and Lee G. K. (1977) Inert coal macerals in combustion. Fuel 56, 125-130. Oka N., Murayama T., Matsuoka H., Yamada S., Yamada T., Shinozaki S., Shibaoka M. and Thomas C. G. (1987) The influence of rank and maceral composition on ignition and char burnout of pulverized coal. Fuel Process Teehnol. 15, 213-224. Phong-anant D., Salehi M. R., Thomas C. G., Baker J. W. and Conroy A. (1989) Burnout and reactivity of coal macerals. In Prec. Int. Conf. on Coal Science, Tokyo, pp. 253-256. Pohl J. H., Hurt R. H., Mitchell R. E. and Goldberg P. M. (1992) Interpretation, correlation and use of measured rate of coal combustion. Prec. 5th Australian Coal Science Conf., Melbourne, 30 November-2 December. The Australian Institute of Energy, pp. 217-224. Prado J., Froelich D. and Lahaye J. (1987) Heterogeneous combustion of residual coke particle. In Fundamentals o f the Physical-Chemistry o f Pulverised Coal Combustion (Edited by Lahaye J. and Prado G.), pp. 219-244. Martinus Nijhoff PUb. Dordrecht. (Co-op with NATO). Sanyal A. (1983) The role of coal macerals in combustion. J. Inst. E. 56(427), 92-95. Sanyal A., Livingston W. R., Marsh H., Skorupska N. M., Field D. J. and Edwards I. A. S. (1991). Petrography of lignitic and bituminous coals: application to assess coal combustibility for power generation. Prec. Effects o f Coal Quality on Power Plants-2nd Int. Conf. EPRI GS-7361. Proj. 2256-8. July 5-3 to 5-20. Sehapiro N., Gray R. J. and Eusner G. R. (1961) Recent developments in coal petrography. Blast Furnace Coke Oven and Raw Materials Committee, Prec. 20, 89-112. Shibaoka M. (1969) An investigation of the combustion processes of single coal particles. J. Inst. Fuel 59, 59-66. Shibaoka M. (1985) Microscopic investigation of unburnt char in fly ash. Fuel 64(2), 263-269. Shibaoka M. (1986) Carbon content of fly ash and size distribution of unburnt particles in fly ash. Fuel 65, 449-450. Shibaoka M. and Ramsden A. R. (1978) Microscopic investigation of the behaviour of inorganic material in
coal during combustion. Ash Deposits and Corrosion due to Impurities in Combustion Gases (Edited by R. W. Bryers). Hemisphere Publishing Corp. Shibaoka M., Thomas C. G., Young B. C., Oka N., Matsuoka H., Tamaru K. and Murayama T. (1985) The influence of rank and maceral composition on combustion of pulverized coal. In Prec. 1985 Int. Conf. on Coal Science, Sydney, 28-31 October, pp. 665-668. Skorupska N. M., Sanyal A., Hesselman G. J., Crelling J. C., Edwards I. A. S. and Marsh H. (1987) The use of an entrained flow reactor to assess the reactivity of coals of high inertinite content. Prec. Int. Conf. on Coal Sci., Maastricht, The Netherlands, Elsevier, 827-831. Smith I. W. (1982) The combustion rates of coal chars: a review. Nineteenth Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh, 1045-1065. Stach E. (1952) Die Vitrit-Durit Mischungen in der petrographischen Kohlenanalyse. Brennstoff-Chem. 33, 368. Stach E. (1982) The macerals of coal. In Stach's Textbook o f Coal Petrology (Edited by Stach E., Mackowsky M. T. et aL, p. 127. Berlin, FRG, Gebriider Borntraeger. Tait K. M., Banks G. N. and Whaley H. (1989) Combustion characteristics of Canadian coals, Vol. 2. Canmet report 89-4E. Energy Research Laboratories, Ottawa, Canada. I1A OG1, pp. 146. Taylor G. H., Liu S. Y. and Diessel C. F. K. (1989) The cold-climate origin of inertinite-rich Gondwana coals. Int. J. Coal Geol. 11, 1-22. Thomas C. G., Phong-anant D., Shibaoka M., Gawronski E. and Gosnell M. E. (1991a) Final report "Study of combustion reactivity and burnout of coal macerals under pf conditions". NERDDP EG83/184. Dept. Primary Ind. and Energy. GPO Box 84, Canberra, ACT 2601, Australia. Thomas C. G., Shibaoka M., Gosnell M. E., Gawronski E. and Phong-anant D. (1991b) Coal combustion studied with a laser microreactor--a video. In 1991 Int. Conf. on Coal Science Prec. (Edited by International Energy Agency Coal Research Ltd), pp. 424-427. ButterworthHeinemann, Oxford. Thomas C. G., Shibaoka M., Phong-anant D., Gawronski E. and Gosnell M. E. (1991c) Determination of percentage reactives under pf combustion conditions, In 1991 Int. Conf. on Coal Science Prec. (Edited by International Energy Agency Coal Research Ltd), pp. 48-51. Butterworth-Heinemann, Oxford. Thomas C. G., Shibaoka M., Gawronski E., Gosnell M. E. and Phong-anant D. (1992a) Reactive inertinite in pf combustion. Part I: A laser microreactor--its use in coal research. Fuel. Thomas C. G., Shibaoka M., Gawronski E., Gosnell M. E. and Phong-anant D. (1992b) Reactive inertinite in pf combustion. Part II: Determination of reactive inertinite in pf combustion. Fuel. Tsai C.-Y. and Searoni A. W. (1984) Roles of various constituents in pulverised coal combustion. Meeting of Soc. o f Mining Engineers of AIME, Denver, Colorado, 24-26 October. Vleeskens J. M. and Nandi B. N. (1986) Burnout of coals. Comparative bench-scale experiments on pulverized fuel and fluidized bed combustion. Fuel 65(0, 797-802. Walsh P. M., Douglas R. E., Battista J. J. and Zawadski E. A. (1992) Effect of coal quality on unburned carbon loss. 3rd Int. Conf. on the Effects o f Coal Quality on Power Plants. EPRI, San Diego, CA. 25-27 August. Walsh P. M. and Xie J. (1992) Etficiency of pulverised coal combustion. Central States Sect., Spring Technm. Meeting. The Combustion Inst., Ohio. 26-28 April. Young B. C., Thomas C. G., Shibaoka M., Gawronski E. and Holcombe D. (1989) Relative effects of rank, maceral composition and char structure on the combustion reactivity of pulverized coal. In The Joint Int. Conf., The Combustion Institute, The University of Sydney, 24-27 September, pp. 118-120.
0146-6380/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
The behaviour of inertinite macerais under pulverised fuel (pf) combustion conditions C. G. THOMAS,t M. E. GOSNELL, 1 E. GAWRONSKI,1 D. PHONG-ANANT2andM. SHIBAOKA 1 tCSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde, NSW 2113 and 2Australian Combustion Technology Centre, ACIRL Ltd., PO Box 242 Booval, Qld. 4304, Australia Almtract--The usefulness of the current system of classification of the inertinite group of macerals for understanding the pulverised fuel (pf) combustion process is discussed and questioned. Results to date on the combustibility of inertinite macerals are indecisive, especially as inertinite is mostly regarded as a single entity. Simulated pf combustion experiments with a laser microreactor revealed that the inertinite macerals yielded a wide diversity of char morphologies. With one-to-one correlations between maceral and char, it was possible to determine which maceral was fusible or infusible (commonly called reactive and inert respectively). The microreactor is being developed to measure the burning parameters of individual maceral particles. For example, the data will show which macerals are slow burning (and by how much) and whether fusibility has any relevance to the speed of char burning. Key words--pf-combustion, fusibility, macerals, reactivity, char types, inertinite, classification
INTRODUCTION
Over the past 40 years the macerals in coal have been identified and classified in a progressive manner to enable the exchange of knowledge between scientists and technologists working with coal. This has been done largely by geologists and petrologists, with the result that the classification is by appearance, by deemed origin and in one instance alone, by its behaviour when heated. Because the macerals are heterogeneous at the microscopic level, the parameters chosen have been those which could be seen with the microscope: shape (or morphology) together with reflectance, colour and relief. The one exception has been the grouping together of several macerals into the Inertinite Maceral Group. The word inertinite was chosen (Stach, 1952) to combine those macerals that were "more or less inert or non-reactive in the carbonisation process" (Stach, 1982). At the time carbonisation was the only use for coal that had attracted attention to the relationship between coal structure and its processing behaviour. The choice of word was poor at the time and worse in hindsight. It labelled inertinite macerals with an inert image, when in fact, all that was meant was that under the slow heating conditions of coke-making these macerals did not fuse or show plasticity. Time has shown that even the identification of the infusible macerals was done with too little knowledge. Recently (Diessel, 1992) has discussed the question of the actual inertness of the inertinite macerals in the carbonisation process, and has shown for Gondawana coking coals (Diessel and Bailey, 1989) that 50% of the inertinite is fusible under the carbonisation process conditions. 779
The largest and most rapidly expanding use for coal is in the generation of electricity by pulverised fuel (pf) combustion processes. The concept that a large part of the organic matter of many commercial coals is inert, despite the fact that these can all be burnt, is very misleading. Many of these coals contain 70% inertinite, with this label implying that they are in some way less desirable. The problem is that the determination of the perceived inertness or relative inertness of the macerals in the inertinite group of macerals has never been quantified in any satisfactory manner. The pf combustion process has become the dominant method because the rapid rate of burning of the finely ground ( ~ 7 0 % < 75 pm) coal particles enabled compact boiler designs, whilst attaining high energy release per unit volume. This led to reduced capital costs for boilers and hence the rapid adoption of this process for generating electricity. These particles of organic matter burn away to ash in approx. 2-3 s, hardly a description of inert behaviour for constituents that can comprise 70% of the coal being consumed. To achieve complete carbon burnout the dimensions of the boiler and the rate of flow of the coal particles through the boiler must be matched to the time needed for burning the coal (Pohl et al., 1992). It is well known that coal rank has a strong correlation with burnout time (Jones et al., 1985a; Morrison, 1986; Shibaoka et aL, 1985; Oka et al., 1987) and that rank and volatile matter are closely correlated. Also as coal and char particle size increase, so the burnout time increases (Shibaoka, 1985; Smith, 1982; Walsh and Xie, 1992). Since rank and particle size alone can increase the burnout time by at least an order of magnitude, it is essential to match the boiler design to the coals intended to be
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C.G. THOMASet aL
used, to optimise the cost of the installation (IEA Coal Ind. Advisory Board, 1985). To design a boiler to burn all coals would be quite uneconomic. However, even when the factors discussed above have been used in the design, it sometimes turns out that the burnout of the coal is not as good as it should have been. Considering other coal properties that have been reported to affect burnout the most important is the maceral composition of the coal (Prado et al., 1987; Lockwood, 1987; Walsh and Xie, 1992). Although the behaviour of macerals has been discussed in the combustion literature in the past 15 years e.g. (Nandi et al., 1977; Sanyal, 1983; Bustin et al., 1983; Jones et al., 1985b; Falcon and Snyman, 1986; Hough and Sanyal, 1987; Skorupska et al., 1987; Crelling et al., 1988; Furimsky et al., 1990), they are almost completely ignored by utilities, and the maceral composition of a coal is not listed in coal specifications. This has occurred because coal was relatively cheap and a little unburnt carbon could be tolerated. As the coals used became more familiar to the user, adequate performance was eventually achieved. There were, and are, notable exceptions where a plant was tied to a particular coal which was unsuitable for the power station. Two things have changed. With the advent of the second coal age in the 1970s, coals are being moved around the world in increasingly large quantities. Utilities, often on the other side of the world are buying coals of which they have no direct experience. Secondly, these coals are being sourced from countries like Australia and South Africa whose Gondwana coals are substantially different in many ways to the Northern Hemisphere carboniferous coals (Taylor et aL, 1989; Hough and Sanyal, 1987; Jones et aL, 1985a). Yet, it is users in the Northern Hemisphere who are using these Southern Hemisphere coals, but naturally enough, interpreting them through Northern Hemisphere eyes. This paper examines the behaviour of the inertinite macerals in the pf combustion process and discusses some further work that needs to be done. SOME EARLIER WORK ON INERTINITE
There has been very limited research on inertinite combustion, and in most cases the workers have regarded inertinite as one entity (Nandi et al., 1977; Shibaoka and Ramsden, 1978; Sanyal, 1983; Lowenthal et al., 1986; Falcon and Snyman, 1986; Oka et aL, 1987; Hough and Sanyal, 1987; Crelling et al., 1988). There is ample evidence that only listing the three maceral groups namely, Vitrinite, Liptinite and Inertinite, can be quite misleading (Carpenter, 1988). The Inertinite Group of Macerals which comprise semifusinite, micrinite, macrinite, inertodetrinite, sclerotinite and fusinite are lumped together and labelled 'inertinite'. Even the UN-ECE (Economic Commission for Europe, 1988) in the classification system currently under discussion treats
the inertinite group of macerals as one entity-inertinite. Yet this collection of macerals comprises a diverse group which differ greatly chemically, physically, petrographically and in their response and behaviour when pyrolysed and combusted. It also means that the "averaged" behaviour of the inertinite in one coal, e.g. (Vleeskens and Nandi, 1986; Phong-anant et al., 1989) projects an image of "inertinite" which may be only partially valid for other coals whose "inertinite" comprises different mixtures of inertinite macerals. To continue this grouping is positively unhelpful and misleading. This is especially so for those industrialists responsible for the use of more than 70% of the coal in the OECD (IEA Coal Ind. Advisory Board, 1985). At the present time they are taught a classification system of nearly half a century's standing, which has little relevance for their major coal use--pf combustion. From the limited research data that are available, an unclear picture emerges e.g. with semifusinite showing greater combustion reactivity than vitrinite (Tsai and Scaroni, 1984) but inertinite showing the reverse with vitrinite (Shibaoka and Ramsden, 1978). Other pf combustion research has been conducted at 500°C (Crelling et al., 1988) with the authors indicating that the results were representative of a process at 1500-1700°C. The problems besetting these researchers and others have been the microheterogeneity of the macerals in many coals, the difficulties of making concentrates of individual macerals, and the problems involved in conducting the research under realistically simulated pf combustion conditions at temperatures ~ 1600°C (Morrison, 1986). Because the char morphology has such a significant effect on the apparent reaction rate, (Jones et al., 1985a; Oka et aL, 1987; Skorupska et al., 1987; Smith, 1982; Bend et al., 1992), and since char morphological development, (especially for many of the inertinite macerals) it so greatly affected by the rate of heating and the pyrolysis temperature, (Thomas et al., 1991a) it is imperative that research be conducted at true pf temperatures, heating rates, oxygen partial pressure and particle size. There is a further problem that the Combustion Petrologist has to face. In a coal, semifusinite is often the most abundant inertinite maceral (volume %). Yet this maceral itself has an acknowledged wide diversity, ranging in reflectance and morphology between fusinite and telinite (Stach, 1982). Understandably, when various samples of semifusinite are pyrolysed and the resulting chars examined a wide diversity of char types ensue (Thomas et al., 1991a; Bend et al., 1992). Some researchers have attempted to partially allow for this by dividing semifusinite into two parts-fusible and infusible e.g. (Nandi et al., 1977; Bustin et al., 1983; Falcon and Snyman, 1986; Jones et al., 1985a; Shibaoka, 1985; Skorupska et al., 1987). The division of semifusinite into fusible and infusible is regarded as significant by some (Jones et aL, 1985a;
781
Inertinite maceral behaviour under pf combustion conditions Falcon and Snyman, 1986; Skorupska et al., 1987; Tait et al., 1989) but as a dubious exercise by others (Furimsky et al., 1990), a division which is not based on sound experimental data in the view of the present authors. Just to add to the confusion, many definitions of fusible and infusible macerals exist. These have been listed (Thomas et al., 1992a). Unfortunately, because this idea has been borrowed from coke makers (Schapiro et al., 1961; Gray et al., 1978), the fusible part is equated with 'reactive' and the infusible with 'non-reactive or inert'--labels quite appropriate for the carbonisation process. However, in the combustion context this leads to misunderstanding, because 'reactive' now has the meaning--"this coal will react faster with oxygen". Put another way, the message is--"reactive coal has a greater combustion reactivity", and thus "fusible semifusinite burns faster than infusible semifusinite". The division into the two parts was made because people believed that this was the case, but has anyone presented any data to validate such an hypothesis? This contentious inference has since been broadened to include other fusible and infusible macerals. It is said that fusible macerals burn faster than infusible macerals e.g. (Carpenter, 1988; Falcon and Snyman, 1986). There is no data to support such a concept--another myth has been born. There was just enough data to make it plausible, namely, that fusinite (infusible) had been observed to burn more slowly than vitrinite (fusible) at 1000°C (Shibakoa, 1969). However, this was an early result, obtained under conditions substantially different from the pf process, in terms of the particle heating rate and the combustion temperature. The extension from this to the concept that all fusible macerals burn faster than all infusible macerals has no justification. RESEARCH AT CSIRO (1987-1992)
Australia has many coals with high inertinite contents. Against the background just discussed and the fact that Australian inertinite-rich coals had experienced difficulty in the coking coal market 10 years ago, it was clear that a better understanding of the behaviour of the Gondwanan coals in the pf combustion process was necessary. In 1987 there was a widespread belief that 30% or less of the inertinite was fusible (reactive), e.g. (Falcon and Snyman, 1986; Furimsky et al., 1990). However char analyses of drop tube furnace residues indicated much higher levels of fusible inertinite. Hence the ability to measure and ascertain the validity of this oft quoted 'magic' number (Schapiro et al., 1961) was the first target, especially as coals with high levels of fusible macerals were considered more desirable e.g. (Nandi et al., 1977). Maceral heterogeneity is visible at the optical microscope level, thus a specific microscopic method was necessary. This has been described elsewhere (Thomas et al., 1992a; Thomas et al., 1991a; Thomas
et al., 1991b). Monomacerai particles are obtained from selected areas on a polished surface of a coal sample by quarrying them with a needle under a binocular microscope. The surrounding maceral area is subsequently characterised using an oil immersion objective to determine the reflectance and maceral type. One particle at a time is mounted on the tip of a needle and heated for ~ 4 0 ms by a CO2 laser beam from two sides in a laser microreactor (Fig. 1). The system simulates the heating conditions encountered in pf combustion, namely: heating rate ~ 106 °C. s -L, pyrolysis/combustion temperature ~ 1600°C, atmosphere--air, particle size ~ 100 #m. The particle temperature is monitored with a two colour micropyrometer, giving a temperature profile for each particle processed. The technique has the advantage that a characterised maceral particle is correlated one to one with the resulting char. After heating, the particle is removed from the needle, mounted in epoxy resin and polished using the method of Brunckhorst (1992) and its structure examined under a microscope. A whole coal reflectogram, where frequency is plotted against reflectance, shows how the inertinite macerals are distributed over a wide reflectance range, from vitrinite up to and including the high reflectance values of fusinite. To obtain samples of macerals representative of the whole coal, particles are dug from maceral quarries which have different reflectances, and which together cover the whole reflectance range. DISCUSSION OF CHAR TYPES FORMED FROM MONOMACERALS OF VITRINITE AND INERTINITE
Figure 2 shows examples of chars that have been produced in a study of six coals from both Australia and the Northern Hemisphere (Thomas et al., 1991a, 1992b). Captions for Fig. 2 are given in Table 1, with abbreviations in Table 2. Figures 2.1 and 2.2 show that vitrinite forms very thin walled
~LASER
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Fig. 1. Schematic diagram of laser microreactor.
C. G. THOMASet al.
782
Fig. no. 2.1 2.2 2.3 2.4 2.5 2,6 2.7 2.8 2.9 2.10 2. l I 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24
Coal no. 4 1 4 3 5 1 5 2 2 6 6 3 3 4 3 6 3 2 6 4 5 2 5 3
Rank Rv (%) 1.14 0.45 1.14 0.70 0.81 0.45 0.81 0.76 0.76 0,99 0,99 0.70 0.70 1.14 0.70 0.99 0.70 0.76 0.99 1.14 0.81 0.76 0.81 0.70
Maceral VT VT VT VT SF SF SF SF SF SF SF SF SF SF SF SF SF SF SF F SF SF SF F
Table 1. Captions for Fig. 2 Maceral Pyrolysis refleCtanCe temp Maceral type (random) (°C) (see key) (%) 1420 Telocollinite 1.04 1500 Telocollinite 0.45 1300 Teiocollinite 1.12 1410 Telocollinite 0.70 1420 PG.FE.CS 1.18 1460 PG.CS 0.94 1730 PG.FE.CS 1.24 1820 PG.FE.CS 1.04 1430 WP.CS 1.38 1400 PG.CS 0.92 1625 1.35 PG.CS 1230 WP.CS 1.50 1580 WP.CS 1.50 1300 WP.TK.CS 1.65 1790 PG.FE.CS 1.55 1500 WP.FE.CS 1.45 1800 WP.FE.CS 1.85 1540 WP.CS 1.88 1620 WP.TK.CS 1.94 1550 BOGEN.CS 1.98 WP.TK.CS 1650 1.55 1320 WP.CS 2.15 WP.TK.CS 1.83 1550 1600 WP.TK.CS 2.60
cenospheres from both high and low rank thermal coals and that it also forms thick walled cenospheres with secondary vesiculation in the walls from both high and medium rank coals (Figs 2.3 and 2.4). The factors that influence thin and thick wall formation are not known, but probably include heating rate, oxygen partial pressure and probably, but most significantly, the maceral precursor. Figures 2.5 and 2.6 are examples of thin walled cenospheres formed from semifusinite (inertinite) macerals. As the reflectance of the semifusinite macerals increases, there is a tendency for the vesicles to be more equally sized, resulting in the formation of char types called network chars. These can have very thin walls as seen in Figs 2.7 and 2.8, indicating that at the time of devolatilisation the gas evolution coincided with the 'time window' where the coal/char was in a state of high fluidity. With increasing inertinitc reflectance there is a tendency for parts o f the walls to be thicker, indicating zones of lower fluidity (or plasticity) for these more viscous chars, Fig. 2.9. F o r some inertinite macerals that have reflectances similar to vitrinite a thick walled network char may form, Fig. 2.10. F o r inertinite with higher reflectances, the
Table 2. Abbreviations in Table 1 Maceral type Char type Vitrinite VT Fused FD Fusinite F Unfused UF Semifusinite SF Cenosphere CE TN Cell structure CS Thin Well preserved WP Thick TK NK Part gelification PG Network Fine FE Mixed MX Thick TK Transitional TRANS Solid S Open Solid OS
Char type (see key) FD.MONO.CE FD.TN.CE FD.TK.CE FD.TK.CE FD.TN.CE FD.TN.CE FD.TN.NK FD.TN.NK FD.MX.NK FD.TK.NK FD.MX.NK FD.MX.NK FD.MX.NK TRANS TRANS TRANS UF.OS UF.OS UF.OS UF.OS UF.OS UF.S. UF.S UF.S
Scale 50 #m = " "mm 10 25 19 24 16 20 16 25 23 25 25 17 19 25 15 24 39 23 31 16 24 40 27 24
char morphology shows a mixture of network and banded structures, Fig. 2.11, with the mix varying, both between and within different coals, Figs 2.12 and 2.13. The transition from macerals that fuse (showing fluidity or plasticity during their formation) to those that do not (infusible macerals) tends to occur over a narrow range o f reflectance values, with some particles showing mixed fused/unfused structures. Figure 2.14 shows a particle with a few vesicles in a structure which otherwise shows little signs of plasticity. The particles seen in Figs 2.15 and 2.16 show the same mixed characteristics, although with quite different char morphologies from that seen in Fig. 2.14. Observation with the transmission electron microscope (TEM) o f the darker areas in the walls of the particle in Fig. 2.16 show them to consist of very finely vesiculatecl material. A b o v e the transition zone the particles are essentially unfused (Figs 2.17-2.24). Examination of the maceral precursor o f the particle shown in Fig. 2.22 shows rounded pores, and there is difficulty in distinguishing these remnant structures from pyrolysis vesicles, when observing the char particle. F o r inertinite with reflectances in the upper range, the effect of heating is to crack the structure, Fig. 2.24. Sometimes these cracks open up, Fig. 2.23, and in some of the unfused particles, the original structure, which combines separate components or elements, can be seen to have exfoliated textures and formed a more open structure, with good access for the oxidising gases during the subsequent char combustion (Figs 2.17-2.21). The broad diversity of behavioural responses shown in these chars are poorly described if they are all regarded as inertinite chars, without further differentiation (Figs 2.5-2.24).
Inertinite maceral behaviour under pf combustion conditions These chars can be classified into two groups, fused and unfused. Since the experimental technique gives a one-to-one correlation between the char and its maceral precursor, the macerals can be classified into fusible and infusible. When a plot was made of the distribution of the fusible and infusible macerals with respect to the reflectance of these macerals, it
783
was found that a distinct pattern emerged for each coal studied (Thomas et al., 1991a, 1992b). From the evaluation of up to 100 char particles per coal it was possible to determine the reflectance value which separated fusible macerals from the infusible macerals. For the six coals the boundary lies in a band between reflectance values of 1.4-1.75%, with
Figs 2.1-2.8. Selected photomicrographs of vitrinite and inertinite chars after rapid pyrolysis in the laser microreactor. See Table 1 for descriptions.
784
C.G. THOMASet
no clear trend shown with the limited results available (Fig. 3). DISCUSSION OF RESULTS
The division of the inertinite macerals into fusible and infusible, mirrors the results found in carbonisation studies (Diessel and Bailey, 1989). However,
al.
under the dynamic (that is very high heating rate) conditions of pf combustion, the boundary values are higher. This has the result that an additional onethird, approximately, of the inertinite macerals were observed to be fusible. For the Australian coals 75% on average was fusible compared with 50% in the carbonisation studies. However, for coals that have a larger part of their inertinite macerals with high
ii
Figs 2.9-2.16. Selected photomicrographs of inertinite chars after rapid pyrolysis in the laser microreactor. See Table 1 for descriptions.
Inertinite maceral behaviour under pf combustion conditions reflectances (i.e. above 1.8% e.g. Northern Hemisphere coals) it could be expected that high levels of unfused chars will be present even under pf combustion conditions. For the two carboniferous coals tested, the average value of fusible inertinite was only 51% of the total inertinite (Thomas et al., 1991c).
785
SIGNIFICANCE OF FUSIBLE MACERALS ON
COAL COMBUSTIBILITY Microscopic analysis of partially burnt chars obtained from laminar-flow entrainment reactor experiments are shown in Fig. 4 (Young et al., 1989). During these combustion experiments the chars are taken from ports down the reactor tube, with each
Figs 2.17-2.24. Selected photomicrographs of inertinite chars after rapid pyrolysis in the laser microreactor. See Table 1 for descriptions.
786
C.G. THOMASet al.
I "~
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I 1.2
Coal Rank (FIv %)
Fig. 3. Mean reflectance boundary between fusible/infusible inertinite macerals vs Coal Rank (Rv, %) for six coals. sequential sample showing increased burnout. Thus for the lowest coal with 54% inertinite the carbon remaining (i.e. not burnt out) decreases from 56 to 36% (ports 2, 3 and 4). Analysis of this carbonaceous material (i.e. char) gives the proportions of the chars that have exhibited fusibility during their formation, and in Fig. 4 these are expressed as a percentage of the whole char population (fused and unfused). If the fused material would have burnt more quickly than the unfused, then the "fusible material in char sample (%)" would have decreased and the burnout plot for that coal would drop, from left to right (ports 1-4). For the seven inertinite rich coals shown, five show a slight increase and two a slight decrease during their respective burnouts. The reasons for the different behaviours of these coals is not known. At this stage the average rate of burnout of fusible and infusible chars can be regarded as very similar in the region where burnout increases from 30 to 80% (Fig. 4). In other work, the burnout was 98-99% for both vitrinite and inertinite concentrates (Phong-anant et al., 1989). These results gave further evidence that lOO
59
n E 70 CO
30
•
2O
U.
10
T H E MEASUREMENT OF T H E SPEED OF COMBUSTION OF INDIVIDUAL MACERALS--FUTURE WORK
The huge diversity of char types (Fig. 2) deriving from monomacerals provides a clue to why other researchers have found such a wide range of chemical reactivities (i.e. combustion rates) for coal particles reacted at the same temperatures (Smith, 1982; Prado et al., 1987; Sanyal et al., 1991). Some researchers (Walsh et al., 1992) are using guesstimates of the proportion of slow burning material in their coal in their attempts to model the combustion behaviour successfully, In order to determine the combustion rate of individual maceral particles the laser microreactor is being upgraded. This will enable kinetic measurements to be made on individual particles under pf combustion conditions, seen as important by many workers e.g. (Walsh and Xie, 1992). The research will assist in the understanding of the relationships between macerals and their char structures (and these structures as they change during burnout) with their combustion reactivity. A major improvement to the equipment is to use the particle temperature information to operate a feedback control system for the laser to keep the particle burning at a fixed temperature. It is also necessary to know the coal particle's size, volume and weight. By imaging a silhouette (Fig. 5) of the particle
39
;=
._c 50
•
the division of macerals into the fusible/infusible parts was not a meaningful or helpful action, in terms of understanding the pf combustion behaviour of coals. The present authors do not dispute that there are macerals which yield char which burn faster, and others which burn slower, but are inclined to the view that morphological factors, such as wall thickness and access of oxidising gases to internal surfaces, are the important parameters, as seen in Fig. 2. It is well established that burning rates of pulverised coal are controlled by the combined effects of pore diffusion and the chemical reaction on the carbon surface (Smith, 1982). What is less well understood, is how pore structure and hence the diffusion of gases through that structure vary during burnout.
150~um
54
particle
KEY 5 9 = Inertlnite (%) i
0 0 90
i
i
J
1
i
i
80
70
60
50
40
30
10
2
Carbon Remaining (% W/W, Dry Basis)
Fig. 4. Fusible material (%) in drop tube char samples vs carbon remaining (%).
of needle
Fig. 5. Silhouette of coal particle on tip of needle.
Inertinite maceral behaviour under pf combustion conditions
3.
4.
Segment
5.
Coal Particle Fig. 6. Average size, and volume calculation of particle, from multiple silhouettes. every 9 ° degrees (Fig. 6) and using an image analysis software package, both the size and the volume of the particle can be calculated. After pyrolysis, if vesiculation and swelling have occurred, the sizing can be repeated to determine the new size and volume (i.e. determine percentage swell). The weight loss as burning proceeds can be measured by weighing the particle on the needle, using a microbalance with a resolution of 10 ng. The period of combustion can be determined from the temperature trace obtained from a new four colour micropyrometer. A video record of the burning particle reveals if there have been any catastrophic events such as fragmentation or rupture. Calculation of the kinetic parameters from these measurements will enable the relative speed of combustion of different macerals and their corresponding chars to be compared. A combustibility profile for a particular coal can be drawn up which will reveal which are the faster burning macerals, and, more importantly, which are the slow burning ones. If the percentage of the slow burning macerals is significant then it is likely that this maceral aspect could give a burnout problem, i.e. high carbon-in-ash percentages. CONCLUSIONS 1. The present classification system for black coals is quite unsuited for the use of combustion petrologists. The grouping together of some diverse macerals into the inertinite maceral group is meaningless from a combustion viewpoint, because they differ chemically, physically, morphologically and widely in their reflectances, all of which influence the very different ways these macerals respond when subject to rapid pyrolysis and combustion conditions. 2. The CSIRO laser microreactor has been used successfully to determine which macerals fuse
6.
7.
787
(and which do not) under properly simulated pf combustion conditions. Pyrolysis of monomacerals from the inertinite group of macerals has yielded a range of char types varying from 'vitrinite-like' cenospheres to solid lumps of fusinite. The grouping of these char types under one heading--inertinite char--is simplistic and very misleading. Any usefulness that may have existed in this maceral grouping has long past. Inertinite is not inert in the pf combustion process. It all burns. What has to be determined is the relative rate of combustion of the variety of chars from within both the vitrinite and inertinite groups of maeerals. Comparison of char morphologies from vitrinite and inertinite particles reveals that thick walled 'bulky' char particles can result from the pyrolysis of macerals from either group. These constitute the slower burning or 'harder to burn' char particles. A method is proposed which has the ability to measure the relative combustibility of pf sized maceral particles under true pf combustion conditions. This method will reveal whether fusible ("reactive") macerals burn faster than infusible ("inert") macerals, as well as finding out which are the slow burning chars and relating these to their maceral precursors. REFERENCES
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