Characterization of coals for combustion using petrographic analysis: a review

Characterization of coals for combustion using petrographic analysis: a review Michael

Cloke and Edward

Lester

Coal Technology Research Group, Department Nottingham, Nottingham NG7 2RD, UK (Received 25 February 7993)

Petrographic analysis can give useful information relations between maceral types and reactivity are not all vitrinites are reactive. This is a consequence between coals from different parts of the world. associations of the different macerals, determined

of Chemical

Engineering,

University

of

about the combustion of pulverized coal. However, the not simple; for example, not all inertinites are inert, and of the formal definition of macerals and the variations The reflectance values of the maceral groups and the as microlithotypes, also need to be taken into account.

Mineral matter also has an effect on the type of char formed, and the situation is further complicated by the different char characterization schemes in the literature. The main conclusion from this review is that any predictive system needs to take all these factors into account, especially reflectance. (Keywords: coal; combustion; petrographic analysis)

Pulverized fuel combustion is used widely in power plants for the generation of electricity. Selection and testing of coals is vital to the efficient operation of these plants, and many different tests are carried out for this purpose. When coals are to be obtained from many different sources, especially from different parts of the world, this can become difficult. The aim of this review is to identify how petrographic analysis can be used in the assessment of coals and the diagnosis of combustion problems. Most workers identify two distinct stages in the combustion of pulverized fuel’. The first, pyrolysis, is over quickly, with times of 3&100ms reported*-‘; the second, char combustion, may require N 1 s6. Pyrolysis can be viewed as comprising two separate stages4: rapid release of volatiles, followed by their combustion. Char combustion can also be subdivided, into initial and residual stages7. The performance of a particular coal in a furnace therefore depends on how it reacts in the stages of pyrolysis and char combustion. Is either stage more important? One study8 indicated that early burnout is proportional to devolatilization rate, although char burnout is often thought of as more important because it takes longer’. However, changes occurring during pyrolysis determine the morphology of the char, and the char types present affect the overall combustion efficiency’. Frequently, coals that appear to be acceptable, and even desirable, on the basis of proximate and ultimate analyses can have peculiar burnout properties. For example, an American high-volatile coal gave an ash with a carbon content of 14 wt% lo, and a Fujian anthracite as much as 30 wt% carbon”, although both coals, according to standard tests, should have burned well. Reactivity tests on what appear to be similar coals have also shown wide differences1’,13. In all these instances the variations in reactivity were attributed to differences in the petrographic analyses of the coals. Such differences

0016-2361/94/03/0315X)6 c 1994 Butterworth-Heinemann

Ltd.

lie not only in the presence or absence of particular maceral groups but also in the way in which the macerals are associated in the different microlithotypes identifiable by petrographic analysis6. Poor combustion has both environmental and financial penalties. It has been suggested7 that > 5 wt% carbon in ash is not economically acceptable, and that industrial combustion problems could be foreseen by the use of petrographic analysis14. This review examines the extent to which this is true. The overall reactivities of macerals during combustion are considered, as well as the effects of maceral composition in the pyrolysis and char burnout stages. The effect of minerals on char formation is also mentioned. MACERAL

REACTIVITY

AND COMBUSTION

The three main maceral groups are vitrinites, liptinites and inertinites. It might be expected that the inertinite would be less reactive during combustion than the other fractions. However, although this may be true for a single coal, not all inertinites are inert and not all vitrinites are reactive. Gondwana coals are known to have high inertinite contents”, but they can be burnt satisfactorily. Australian coals may often burn more efficiently than vitrinite-rich coals from the northern hemispherel‘j. This is indicated by the low reflectance of the inertinite macerals in Australian coals compared with European coals17. Clearly, inertinites from different areas must be regarded as different entities with different degrees of reactivity. Pseudovitrinite, according to the ICCP”, is classified as a vitrinite, but it would be classed as inert Oxidized vitrinite has been during combustionlo913. recognized as behaving differently from most other types of vitrinite19-*l. Vitrinite combustion can thus vary from ‘good’ to ‘very poor’ depending on rank and speciation**,

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and certain inertinites can be more reactive than some vitrinites23p27. Thus, it is considered acceptable to assume that a certain fraction of inertinite will be reactive. For a maceral to be classed as reactive, it must exhibit thermoplasticity during pyrolysis and char formation; hence if ‘inerts’ are to be considered as reactive, some degree of structural alteration must be shown to have occurred. Lowreflectance inertinite is the most likely sub-maceral in which plasticity would be observed during combustion**, although medium-reflectance inertinite can also undergo the same degree of plastic deformation. Micrinite is within the inertinite group but can be considered to be reactive during combustion because it has a small surface area and is usually enclosed within vitrinite particles. It is also known to yield more volatile matter than other inertinite species, possibly owing to its connection with liptinite Hence as the vitrinite vesiculates, the fragmentsz9. enclosed micrinite will be either exposed and burnt or incorporated into the char walls and burnt along with the vitrinitic char14. The ICCP classification is specifically aimed at coal geologists, so its strict definitions of maceral groupings, e.g. semifusinite, pseudovitrinite, are not always useful for workers in coal utilization3’. It is therefore important to adopt a classification system relevant to the purpose at hand. For the prediction of burnout performance in combustion systems, coals must be divided into reactive and unreactive fractions. Predictions based on results using classical definitions may be misleading: for example, micrinite and semifusinite need not be considered completely inert, just as vitrinite is not always reactive. It may be that classical maceral classifications could be ignored, and the reflectance of the whole coal used to predict reactivity. Reflectance measurements have been used to differentiate between fusible, partly fusible and infusible particles, so it may be possible to characterize a coal realistically without the need to decide which sub-macerals should be put into a particular category. Inertinite fusibility limits (above which full fusion does not occur) have been quoted as a reflectance of 1.3% for subbituminous coals or 1.8% for higher-rank coals3i; an upper limit of 2.8% reflectance for fusible inertinite has also been reported 32, but the authors concluded that the use of inertinite reflectance alone is a very inaccurate method of assessing fusibility limits. Inertinite fusibility limits between 1.3 and 1.8% reflectance have also been reported, together with a dependence on vitrinite reflectance values between 0.5 and 1.2%33 or with R,,,=0.49%, where R,,, is the reflectance measured on telocollinite areas. Equations have also been developed34v35 to express the reflectance limit for inertinite fusion in terms of R,,,; examples are: For full and partial

fusion,

Rlim = 0.85 + 0.60R,,, For partial

analysis: M. Cloke and E. Lester

One cannot therefore successfully predict the way in which a coal may burn by considering only the three basic maceral groups. The question of reactivity is not a clear-cut issue: there exists some kind of scale between reactive and unreactive. It is therefore necessary to examine in great depth the effect of coal structure on the different stages of combustion. CHAR

MORPHOLOGY

One very important issue is the reactivity of the char formed initially during combustion, which is related to the char morphology. However, discussion of the behaviour of char during combustion is complicated by the number of char morphology classifications proposed. Most workers have created systems to suit their particular needs’0~24~25~35-39, and conclusions drawn from the use of one system may appear to contradict those from another. Table 1 shows the classifications used by several workers, grouped under similar char types labelled A-1. Not all the char types described by the various workers are included in the table, because in some cases reference is made to different sizes of char particle25,36, and in other cases qualitative distinctions using concepts such as isotropy and anisotropy3’, high and low reflectance’*, inclusion of minerals2s,3s, density of the network37 and other subdivisions within larger groups24. These different classes of char are also illustrated diagrammatically in Figure I. Table I and Figure 1 constitute an attempt to bring together the systems used and to explain the main char morphologies encountered. However, this is an area in which a standard classification would be most useful. In the descriptions which follow, the original classification given by the particular author is used. PYROLYSIS AND PETROGRAPHIC COMPOSITION During pyrolysis, volatiles are released and char particles are formed. The type of char formed can depend on the macerals present, the rank of the coal, the particle size and the temperature of char formation”. In this section the pyrolysis behaviour of individual macerals is discussed as well as that of microlithotypes (combinations of macerals present in the same particle).

1 \

(Ref. 34)

and non-fusion,

Rlim= 1.31 +0.41R,,, For full and partial

i fusion,

1

Rlim= 0.74 + 0.67R,,, For partial

(Ref. 35)

and non-fusion,

Rlim= l.l8+0SlR,,,

316

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1

Diagrammatic

representation

of char types in Table 1

Characterization of coals for combustion using petrographic

analysis: M. Cloke and E. Lester

Table 1 Char classifications used by different workers, grouped by similar types’ E

F

G

H

I

dense

dense

dense

mixed

mixed

_

open solid

solid char

open solid

mixed

mixed

honeycombs

-

unfused char

unfused char

unfused char

tenuinetwork

mesosphere

inertoid

solid

fusinoid

mixed dense

mixed dense

thick walled lacy cenosphere cenosphere

solid

solid

solid

solid

solid

solid

cenosphere

cenosphere

honeycomb a&b

mesosphere

inertosphere

unfused a&b

unfused a&b

unfused a&b

unfused a&b

Ref. 24. Subdivisions made within the larger groups

cenosphere

cenosphere

network honeycomb

solid

solid

solid

solid

solid

solid

Ref. 37. Dense networks (small pores) described

thin-walled

thick-walled

network

honeycomb

solid disrupted

unfused disrupted skeleton

unfused

solid

solid

Char type

A

B

C

Ref. 25. Five sizes of each type described, and a mineralrich type

vesicular

vesicular

vesicular

Ref. 36. Five sizes of each type described

plastic

plastic

plastic

Ref. 27

cenosphere

cenosphere

Ref. 35. Isotropic and anisotropic distinctions made and mineroids described

tenuisphere

crassisphere

Ref. 39

thin walled cenosphere

Ref. 38. a is high-reflectance and b is low-reflectance

D

“Cenospheres are hollow spherical carbon particles, frequently produced by rapid devolatilization of coal particles; ‘tenui’ refers to walls of carbon < 5 pm thick, ‘crassi’ to walls > 5 pm thick

Vitrinite The type of char formed from vitrinite is influenced by coal rank. It has been known for some time that vitrinite produces spherical particles39. Vitrinite can yield tenuispheres 35, but tenuinetworks and crassispheres may result from higher-rank coalsp. Cenospheres arise predominantly from vitrinites4’, and vitrinites have been reported as never giving networks23,41; with biological and geographical factors being blamed for the apparent discrepancies. When heated, vitrinite decomposes and devolatilizes, leaving rounded vacuoies4’, although this is not entirely agreed upon’. As mentioned above, not all vitrinites are reactive, so there is a difference of opinion as to the combustion behaviour of vitrinite. Is vitrinite capable of forming large-pored spheres, or can it form various other char types? Pseudovitrinite is unlikely to become piastic in the same way as telocollinite, simply because of its different structure and chemical composition; it forms a more solid char with low porosity’O. The different ideas on the types of char formed by vitrinite may result from differences in experimental conditions43. The fragile tenuispheres produced by vesicated vitrinite may burn out or fracture as a result of thermai exposure. This may explain why fewer thin-walled spheres were found at higher temperatures in the work of Bailey et a1.44. The two factors with the most influence on the type of char formed by vitrinite are temperature and rank. The higher the temperature, the more thick-walled are the char particles formed. Networks appear to be prevalent in chars from lower-rank coals3*, and this may be related to coal aromaticity”? as coal rank increases, so does aromaticity4*, and this is therefore related to the proportion of cenospheres produced45. Of the three maceral types in a given coal inertinite has the highest C/H ratio, the lowest hydrogen content and volatile matter, and the highest degree of aromatic

Table 2

Microlithotype classification

Microlithotype

Maceral groups

Vitrite Liptite Inertite Clarite Durite Vitrinertite Trimacerite

>95% >95% >95% > 95% > 95% > 95% > 95%

Vitrinite Liptinite Inertinite Vitrinite + Liptinite Liptinite + Inertinie Vitrinite + Inertinite Vitrinite + Inertinite + Liptinite

bonding’5,46. These characteristics affect the types of char structure formed on pyrolysis. Inertinite is capable of forming almost all of the known types of char, from networks and tenuispheres6,37 to dense solids4’. Like vitrinite, inertinite behaves according to its rank, and its plasticity can be related to its reflectance24,2Q,48. Plasticity has been linked to aromaticity, and lower-rank inertinites swell during combustionz3. Laser heating of a single-maceral particle has revealed that low-reflectance inertinite vesiculates and forms porous char36; this has demonstrated for the first time the definite behaviour (in realistic combustion conditions) of one maceral type. Liptinite This maceral group has the highest hydrogen content and volatile matter, the lowest aromaticity and the lowest reflectance. Its volatile matter is roughly twice that of the associated vitriniter5, and as a result, liptinite has been linked with flame stability14. Coals of high liptinite content have shorter combustion times49. Liptinite can begin to burn at _ 300°C4’ and at this temperature it liquefies and vaporizes, forcing holes in the surface of the softened particle 48. Hence liptinite is significant only in the pyrolysis stage ofcombustion and does not contribute significantly to the char. However, even the behaviour of liptinite is not entirely agreed upon, since concentrated liptinite macerals have been found to show almost inert

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behaviour in reactivity tests”, although relevant to their behaviour in vitrinite5’.

using petrographic this

is not

Microlithotypes Macerals often exist as associations which, at the microscopic level, are known as microlithotypes. These may need to be taken into account when assessing combustion efficiency 4o. Table 2 lists the microlithotypesi and the macerals associated with each. Vitrinite-rich microlithotypes generally produce lowdensity chars, reducing the combustion time. Vitrites and clarites have been found to give tenuispheres and tenuinetworks3’. An increase in the porosity of the char is also a result of a higher vitrite content in coa15’. However, vitrite in larger inertinite-rich fragments does not burn so readily, since the vesiculation of vitrinite is restricted when it is compacted in inertinite bodies48, where vitrinite can exist as thin layers bound by inertinite macerals such as macrinite and fusinite2’. Other microlithotypes with lower vitrinite contents, such as clarodurites, vitrinertite-I and duroclarites, form thickerwalled chars or mixed porous chars52. Durites, which tend to produce inertoids, contain > 5% vitrinite53, mixed dense solids and fusinoids44. Consequently, particles consisting of reactive macerals are generally much more able to form porous chars with relatively large surface areas than are microlithotype particles consisting of bonded high-reflectance material, which generally form closed chars with high densities and relatively low porosity.

EFFECT OF MINERALS

ON CHAR

FORMATION

The main influences on char formation are associated with the organic part of the coal. However, the effects of mineral matter should not be underestimated, since mineral matter cannot be considered as inert in the combustion process54. The presence of certain types of minerals can affect the type of char formed55. Reactivity can also be influenced by mineral matter56. Vital properties such as flame stability and burnout efficiency can be affected by the presence or absence of certain minerals5’~‘*. Poor burnout can result from the high heat capacity of mineral matter, which delays the heating process. The addition of positive ions, such as K+, Na+ and Ca2+, to a coal has been shown to increase the proportion of cenospheric char formed58. The addition of Na,CO, increases the number of crassi-walled spheres59. CaO not only increases crassisphere formation but also lowers the reactivity of char surfaces6’. The larger the particle, the greater the amount of mineral matter present 61. Clarites are known to congregate in the larger size fractions of coa14, so it is possible for this potentially reactive microlithotype to form thicker-walled unreactive chars if it contains significant amounts of mineral matter. Even though it is evident that mineral matter adversely affects burnout, the total removal of mineral matter is impractical and is also undesirable as regards the requirements of modern burner@‘. However, it would be unacceptable to allow excessive amounts of mineral matter to be present as a result of inefficient cleaning. The evidence suggests that in any assessment of char formation and reactivity, mineral matter elTects need to be considered.

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CHAR

BURNOUT

Many variables affect char burnout, including temperature, oxygen level, redisence time and char morphology (mainly structure, porosity, density and optical texture) 9-35,4o Like that of macerals, the behaviour of char is also not entirely certain. Some workers clearly state that vitrite chars are more reactive than inertite chars8,33,63. Vitrite char has been estimated to burn two to four times as fast as inertite char63. However, it has also been suggested that inertite can burn as fast as vitrite25,64, depending on the type of char formed. This does not apply to solid char formed from inertite, which is slowhas been found to ignite burning I7 . Some semifusinite before vitrinitez7, and burn faster24. The same conclusion was also drawn from a comparison of two coals of different origin, one with 59% inertinite and the other 8% inertinite. In combustion studies, both appeared to be equally reactive65. PREDICTION

METHODS

From the above, some general conclusions can be drawn about the relations between macerals and char types and between the latter and burnout, although care is needed in interpreting the data. Hence it may be possible to develop methods for predicting the burnout characteristics of each type of coal. Some of the major factors that might be included in such methods are discussed below. Rank The rank of a coal can be expressed by the reflectance of the vitrinite, which is related to C/H and C/O ratios and volatile matter l5 . At present, the use o frank appears to be the most accurate means of predicting combustion behaviour37,38,65m70. The reactivities of the macerals are related to rank67, so rank is related to burnout71. Char reactivity decreases as the rank of the parent coal increases63. Rank has been correlated with the change from predominantly thin-walled to thicker, crassi-walled chars45. Fusibility and fluidity have also been linked with rank72. Predictions are generally valid above a certain rank level’ 3,72: predictions for coals of R,,, < 0.45 are not particularly reliable. On the other hand, in one study rank was found not to affect char type25, but it did affect pyrolysis time and hence indirectly influenced burnout performance. As stated previously, some inertinites, which are of lower reflectance, are not truly inert. Thus, even when an assessment is based on petrographic analysis, rank is an important factor. Inertinite content The inertinite content of a coal has often been used to explain burnout characteristics8~‘2~13~20~72~73. Most authors have associated high inertinite contents with poor burnout, but some have been more specific in relating inertinite content to burnout, stating that the achievement of >90% burnout was dependent on the inertinite content*. Again, not all workers agree as to the nature of inertinite, and not all would agree that inertinite content does correlate with burnout performance24. Mixed prediction methods The percentages of reactive macerals (vitrinite and liptinite) in a coal have been used together with rank74. In other work, the reflectances of liptinite and vitrinite

Characterization

of coals for combustion

were found to correlate well with the heating value of the volatile matter arising from coa175. The calorific values of coals have been shown to be related to the vitrite content’r, and the porosities of the chars produced from the same coals were also shown to be linked to these two properties6. Reflectance, a measure of coal rank, is directly related to the grey scale measured using image analysis76. The advantage of the grey scale is that it can take account of both petrography and rank. Since these are such important factors in the assessment of a coal’s combustion performance, future prediction methods could well include grey-scale data coupled with other factors such as particle sizer6,17, presence of microlithotypes48,51, char porosity45 and calorific value6. CONCLUSIONS

12 13 14 15

16 17

18

19 20 21 22 23

1. Coal combustion

2.

3.

4. 5.

6.

is probably not best served by the strict petrographic definitions found in the ICCP handbook. Inertinite is not all inert, and vitrinite is not always reactive. Such deviations can often be dealt with in terms of reflectance values; for example, unreactive pseudovitrinite has a high reflectance. Reactive semifusinite in Gondwana coals is generally distinguished by its low reflectance relative to fusinite and other inertinite material. There is a need to define a char morphology classification which can be used by all workers in the field, instead of the range of systems hitherto proposed. Mineral matter affects the type of char formed. The addition of certain cations can increase char reactivity. In maceral associations (microlithotypes), the behaviour of the macerals may differ from that in their isolated state. Even when vitrinite is reactive, it may be adversely affected by association with inertinite. Microlithotype analysis may well prove to be more valuable than simple maceral analysis in prediction of combustion behaviour. Any prediction method needs to take into account the effects of both rank and petrography, whether or not an automated system is used.

using petrographic

24

25

26

27 28

29 30 31

32

33

34

REFERENCES 1

6

7 8

9

10 11

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