Chapter 3 The organic structure of lignites

79

Chapter 3

THE ORGANIC STRUCTURE OF LIGNITES This Chapter discusses the organic structural features of lignites, beginning with the lithologic layering in lignite seams, proceeding to characteristics of lithotypes and macerals, and then to details of structure at the molecular level. This Chapter follows in part from the discussion in Section 2.4 on the coalification of plant matter. It then sets the stage for the discussion of organic reactions of lignites in Chapter 4. 3.1 P E T R O G R A P H Y OF LIGNITES 3.1.1 Lithologic Layering Early studies of northern Great Plains lignites recognized the existence of distinct layers discernable in the beds [1]. More recent work has focused on the lithologic layers, also called lithobodies, observed in the Beulah-Zap lignite (North Dakota) at the Freedom Mine [2,3]. The subdivision of the seam into distinct lithologic layers is based on megascopic characteristics: appearance of the broken surfaces in the highwall; luster; fracture and hardness; and the presence of lithologically distinct units such as thin layers of clay or silt. The pattern of layering observed in the Freedom mine persists at least into the neighboring Beulah mine, a distance of 16 km [2]. The top three layers are similar petrographically, with the fourth layer substantially different. The relationship of these petrographic differences to pyrolysis behavior is discussed in Chapter 4. Table 3.1 provides the petrographic and chemical analyses of the four lithologic layers in the Freedom mine [4]. Lithologic layering is also evident in both the Beulah and Center (North Dakota) mines (which are in the Beulah-Zap and Hagel beds, respectively). In both cases, huminite- and liptinitegroup macerals are more abundant near the base of the seam, and inertinite content is inversely related to huminite content [5]. The huminite-group macerals are more abundant in the Hagel lignite (83%) than in the Beulah-Zap (65%). Fusinite macerals, particularly semifusinite, are more abundant in the Beulah-Zap than in the Hagel bed. Lithotype abundance is the basis for classification of lithobodies in the Beulah-Zap seam at the Beulah, Indian Head, and Freedom mines; six major types are recognized on this basis [6]. This classification scheme was established on the basis of principal factor analysis and cluster analysis. The classification system is shown in Table 3.2 [6]. The lithobodies range from 5 to 85 cm in thickness [7]. As a rule, attritus-dominated lithobodies occur more frequently near the top of

80 TABLE3.1 Petrographic and chemical analyses of the lithologic layers in the Freedom mine [4]. Layer: Maceral Analysis (a) Huminite Group Ulminite Humodetrinite Gelinite Corpohuminite Liptinite Group Sporinite Cutinite Resinite Suberinite Alginite Liptodetrinite Fluorinite Bituminite Inertinite Group Fusinite Semifusinite Macrinite Sclerotinite Inertodetrinite Micrinite Minerals Pyrite Quartz Clays Proximate Analysis (b) Volatile Matter Fixed Carbon Ash Ultimate Analysis (b) Hydrogen Carbon Nitrogen Sulfur Oxygen (c) Calorific Value MJ/kg (b)

1 (Top)

2

3

4 (Bottom)

35.5 25.2 0.5 1.0

38.9 23.1 0.4 2.6

38.2 21.9 1.4 2.0

42.8 18.0 1.3 6.2

0.9 0.7 2.7 0.0 1.2 5.0 0.0 0.0

2.4 O.5 1.9 0.5 0.4 5.9 0.4 0.0

1.4 O.5 0.9 0.4 0.9 3.8 0.0 0.0

3.3 O.5 1.7 1.5 1.2 5.3 0.0 0.0

4.5 6.8 0.7 0.3 8.9 1.3

4.6 8.1 0.5 0.5 7.6 0.7

8.5 7.2 0.2 0.4 8.5 1.8

2.7 5.3 0.0 0.3 4.2 1.3

1.2 2.7 0.5

0.4 0.2 0.2

1.1 0.4 0.4

0.8 1.0 0.5

38.7 46.8 14.6

43.4 45.4 11.1

43.5 50.1 6.5

42.5 50.9 6.5

3.8 60.9 1.0 1.8 17.9 23.2

4.1 62.9 1.1 0.7 20.0 24.1

4.5 66.2 1.1 0.6 21.2 25.8

4.3 66.3 1.5 0.8 20.6 25.9

Notes: (a) percent volume, (b) moisture flee basis, (c) by difference. the lignite, whereas vitrain-dominated bodies occur more frequently in the center and near the base. Lithobody 6 has a very high ash value (32%, dry basis), and, on an maf basis, has higher volatile matter (54.8%) and oxygen (24.0%), and lower fixed carbon (45.3%), carbon (68.8%), and calorific value (25.6 MJ/kg) than the other lithobodies [6]. The proximate and ultimate analyses of the other five lithobodies are remarkably similar. For example, on an maf basis the volatile matter is 44.8--48.7%; carbon, 71.1-72.6%; and calorific value, 27.2-27.7 MJ/kg [6]. Division of the Beulah-Zap bed into ten lithobodies has also been proposed [7]. Lithologic layering in the Gascoyne mine (Harmon bed, North Dakota) but has not been

81 TABLE 3.2 Average lithotype abundance (volume percent) in Beulah-Zap lithobodies [6]. Lithobody 1) Attritus dominated, abundant fusain 2) Vitrain dominated 3) Vitrain dominated, with attritus 4) Vitrain and attritus equal 5) Attritus dominated, with vitrain 6) Attritus dominated

Fusain 19.0 1.5 3.4 6.3 3.1 0.6

Vitrain 25.O 91.5 76.0 50.7 32.3 18.0

Attritus 56.O 7.7 21.5 43.0 64.7 81.4

studied in as much detail as the Beulah-Zap bed at the Freedom mine. Five layers were identified, and were partially differentiated on the basis of fusain content [8]. The ranges of fusain are Layer 1 (bottom), 2-6%; Layer 2, 3.5-13%; Layer 3, 3.5-10%; Layer 4, 19%; and Layer 5 (top), 5.5-7.5% [8]. Fusain forms horizontal planes of weakness in lignite seams. Since horizontal fracture helps differentiate layers in the seam, a classification scheme for distinguishing layers on the basis of fusain content could be developed in the future. 3.1.2 Lithotypes Lithotypes are megascopically observable components of the lithologic layers, and are distinguished on the basis of physical characteristics. For North Dakota lignites, vitrain and fusain have been described according to the Stopes-Heerlen system, and attritus by the Thiessen-Bureau of Mines system [9]. Fusain is composed of fragmental chips and fibers. It occurs in lenses up to 2 cm thick on bedding plane surfaces [2,8,10]. Typical dimensions of fusain are 2--4 mm thickness and about 2 cm2 in area [8]. Thicker lenses may be continuous for up to 3 m [8,10]. Discontinuous fusain fragments are typically 2-5 cm in length, although one fusain horizon was traceable for 30 m [10]. Fusain displayed a characteristic extreme friability, which was responsible for many horizontal partings in the seam. Crumbling fusain in the hand produces a very fine, "dirty" powdery residue [11]. Fusain is opaque in thin sections but easily recognized by the porous structure remaining from its woody origin. Generally fusain is less important than the attritus or vitrain. Vitrain occurs as discontinuous layers within the attritus. The distinguishing characteristics of vitrain are a bright luster, smooth surface, and fractures 90 ~ to the bedding plane (i.e., bidirectional cleating); the most typical physical property was its brittleness [2,10]. A fracture pattern along two perpendicular planes produces regular, blocky fragments. Vitrain does not produce much dust on fracturing. The minimum thickness of vitrain lenses is about 5 mm, typically forming fiat lenticular bodies about 10-30 mm long [8]. The maximum horizontal extent of vitrain lenses is less than 25 cm [10]. Discontinuous lenses 5-30 mm thick are contained within a dull granular matrix [ 10]. Some vitrain lenses, usually less than half of the total vitrain, contain obvious plant structures, such as concentric growth tings. Such structures are typical of a second form of

82 vitrain, anthraxylon [ 12], which has a silky luster, conchoidal fracture, and is very hard but brittle. The anthraxylon form of vitrain often occurs in bodies of 5--50 mm thickness. A type of vitrain with a dull texture in the Beulah-Zap lignite occurred most frequently in massive attrital lithobodies and usually contained significant amounts of well-preserved plant structures. Among the Fort Union lignites, vitrain bands > 12 mm in thickness may make up more than 15% of the lignite [ 13], with a total huminite content of about 80% or more. Large vitrain lenses can be seen in the working faces of mines, where they may occur in the form of compressed tree stumps or logs of length and diameter each > 1 m [ 13]. Attritus is a collective term for layers of dull to moderately bright lignite interlaminated with vitrain and fusain lenses [10]. Attritus has characteristic granular texture [2,14]. Attritus flakes if scraped, but shows extreme resistance when struck. Attrital layers 10 cm thick can often be traced laterally for several meters [ 10]. Attritus occurs as granular massive layers of 5--40 mm thickness with dull luster, irregular fracture along horizontal planes, and some 90* cleating (though not as extensive as in vitrain) [8]. This behavior is particularly noticeable in the field, in that lithologic layers in which attritus is the major lithotype appear as bulges or protrusions from the exposed lignite face, showing fewer ~ffects of weathering and the absence of desiccation cracks. Attrital lignite breaks easily along irregular horizontal planes when struck with a hammer. Woody vitrain is much more resistant. Attrital lignites are mainly composed of plant debris, much of which was macerated or otherwise decomposed (e.g. by bacterial action), probably during the peat stage of coalification [ 15]. Most of the attritus is translucent, often with a yellowish coloration, when examined in thin section, but up to 15% may be opaque [15]. The plant debris derives from plant fibers; decayresistant reproductive parts such as pollen, spores, and seeds; and cuticles of leaves and fruit. These materials are generally translucent orange in thin section. (Completely opaque attritus has not been related to specific plant components [ 15].) Attrital lignites have a uniform grainy texture and light to dark brown coloration. They are not banded in appearance. Translucent and opaque attritus are differentiated on the basis of optical behavior in thin section [16]. Translucent attritus generally consists of pollen, spores, and leaf cuticles. Opaque attritus is amorphous or may show some cellular structure. (Fusain is also opaque and may show a woody cellular structure, but is differentiated from opaque attritus by being thicker in vertical section in the seam [ 16].) Attrital lignite originated in a swampy environment that provided for very rapid decay of plant material [15]. In such an environment, only the most decay-resistant portions of plant material could be preserved reasonably intact. Any woody tissue that would survive in this environment would be very finely macerated. Some attrital lignites may be allochthonous; that is, they accumulated in shallow water by the influx of water- or air-borne plant debris. Arkansas lignites may have accumulated in this fashion [15]. Lignites that give high yields of extractable waxes are predominantly attrital [ 15]. Woody lignites, in contrast, give low yields of waxes. Nonbanded attrital lignites having high concentrations of waxy plant debris may be "canneloid," i.e., the low-rank homolog of cannel

83 coals [ 1]. The average concentrations of the lithotypes in Beulah-Zap lignite are 35-45% vitrain, 5% fusain, and 40--60% attritus [3,17]. Attritus is more abundant directly overlying clay partings and at the base of the seam, whereas fusain was essentially absent at these locations [3,10,17]. This distribution is generally comparable to observations made on bituminous coal seams [ 18]. Vitrain occurs as discontinuous lenses less than 5 mm thick within the attritus. Lithobodies dominated by vitrain or fusain occur more frequently near the top of the seam. The horizontal variation of lithotypes in six samples from various locations in the Gascoyne mine shows vitrain ranges of 28-37%, attritus 59-66%, and fusain 2-7% [8]. The results are fairly consistent from one sampling location to another, and show no obvious trend with horizontal direction. Characteristics of the lithotypes separated from Beulah-Zap lignite are summarized in Table 3.3. The data in Table 3.3 are averages calculated from more extensive tabulations in [10]. Proximate and ultimate analyses for one set of lithotypes of Beulah lignite are given in Table 3.4 [ 19]. The petrographic analyses of the samples used to obtain the data in Table 3.4 are given in Table 3.13. TABLE 3.3 Average analyses of Beulah-Zap lithotypes (moisture-free) Vitrain Attritus Fusain Proximate (a) Volatile Matter Fixed Carbon Ash Ultimate (b) Hydrogen Carbon Nitrogen Sulfur Oxygen Calorific Value, MJ/kg (b)

48.2 45.4 6.4

46.5 45.4 8.2

39.9 45.1 15.0

4.42 64.66 0.94 0.81 22.79 25.5

4.09 64.17 1.10 0.67 21.89 24.9

3.40 58.12 0.75 0.69 20.48 22.9

(a) Based on five fusain and seventeen attritus and vitrain samples. (b) Based on four fusain and sixteen attritus and vitrain samples.

The proximate analysis of unseparated lignite agrees fairly well with a proximate analysis calculated from weighted results of the proximate analyses of the separated lithotypes. The proximate analysis of lithotypes separated from Beulah lignite, and the agreement with the reconstructed analysis of the unseparated lignite, are shown in Table 3.5 [ 13,20]. The data in Table 3.5 were obtained by thermogravimetric analysis and are shown on a moisture-free basis.

84 TABLE 3.4 Proximate and ultimate analyses of Beulah lithotypes, moisture-free basis [19]. Vitrain Proximate Volatile Matter Fixed Carbon Ash Ultimate Hydrogen Carbon Nitrogen Sulfur Ash Calorific value (MJ/kg)

Attritus

Fusain

45.9 47.5 6.6

43.9 46.5 9.6

40.3 48.4 11.3

4.70 66.93 0.81 1.05 6.6 26.1

4.01 65.14 1.03 0.76 9.6 24.8

3.85 65.30 0.92 1.61 11.3 25.1

TABLE 3.5 Proximate analyses of lithotypes and unseparated Beulah lignite [13,20].

Volatile Matter Fixed Carbon Ash

Vitrain Attritus Fusain 41 43 33 52 50 59 7 7 8

Whole Lignite Calcd.* Exptl. 42 42 51 49 7 8

*Calculated from the average of the values for the lithotypes, weighted for the amount of each li thotype in the unseparated lignite.

A compositional relationship is found among the medium dark, medium light, and dark lithotypes of Victorian brown coal [21] and attritus and fusain in Beulah-Zap lignite [2] by expressing elemental compositions on a ternary bond equivalence diagram [21,22]. The five lithotypes are related in order of decreasing quantity of cellulose (or, in other words, by increasing loss of cellulose): medium dark > medium light > dark > attritus > fusain. Further, a linear correlation (r = 0.85) exists between the aromaticity of the lignites and the carbon bond equivalence [21. The most thorough investigation of separated lithotypes has been performed with those from Beulah lignite. Some properties of hand-separated samples of lithotypes from the Beulah mine are provided in Table 3.6 [2]. Vitrain shows evidence of significant biochemical activity. The very low cellulose content suggests bacterial degradation during diagenesis. The principal ulminite maceral in the vitrain is euulminite, in which very little cell wall structure remains. The attritus is less well compacted than the vitrain, which accounts in part for its much higher relative mechanical friability. Fusain is charcoallike in appearance, indicative of an origin in bog or forest fires. The large amount of residual

85 TABLE 3.6 Characteristics of lithotypes of Beulah lignite [2]. Proper t3, Carbon, %maf Hydrogen, %maf Nitrogen, %maf Sulfur, %maf Aromaticity (fa) Cellulose, %maf Methoxy, %maf Carboxyl, meq/g maf Inorganics, % mf (a) Minerals, %mf (a) Mechanical friability, %(b) Fusinite, % Semifusinite, % Ulminite, % Humodetrinite, % Inertodetrinite, %

V i train 71.66 5.03 0.87 1.12 0.65 0.005 2.08 2.39 2.90 1.35 18.4 1 1 69 10 2

A ttri tus 72.06 4.44 1.14 0.84 0.66 0.025 0.68 2.83 4.01 3.17 36.1 2 5 31 34 10

Fusain 73.62 4.34 1.04 1.82 0.80 0.023 0.72 2.47 3.34 4.06 32.3 45 20 10 5 8

(a) follows Australian brown coal practice (e.g., [24]) (b) discussed in Chapter 7 cellulose, relative to vitrain, suggests interruption of biochemical processes by the fire. Since decarboxylation occurs readily during pyrolysis, the similarity of carboxyl contents among the lithotypes indicates that they formed by later oxidation of the coal and are not relics of the original plant material. A simplistic approach based on separation of Beulah lignite into light and dark lithotypes has produced some insights on differences between lithotypes [23]. The darker lithotype was the more aromatic (fa = 0.71) and contained fewer carboxyl groups, as estimated from X-ray photoelectron spectra. The dark lithotype also had the more pronounced 1600 cm-1 band in the Raman spectrum, indicative of a somewhat better ordered or more graphitic structure of the dark lithotype. Consistent with a higher carboxyl content, the light lithotype had a higher sodium content. The better structural ordering in the dark lithotype was due to a lack of steric effects that would be associated with the relatively bulky carboxyl groups and their sodium counterions. Six classes of lithotypes have been determined for Texas lignites, shown in Table 3.7 [25]. 3.1.3 Macerals (i) Within-seam variation. In the Beulah-Zap bed inertinite macerals are more abundant in the top-most meter of the seam [26]. Inertodetrinite, desinite, gelinite, and semifusinite concentrated near the top of the seam [27], although in some locations semifusinite is relatively constant throughout the seam [17]. The dominant inertinite maceral at the top of the seam is inertodetrinite [ 17]. Thicker fusain lenses contain fusinite and semifusinite. (In some cases discrete fusain lenses are observed toward the base of the seam.) Megascopically the lignite in this region

86 TABLE 3.7 Lithotype classes, lithotypes, and inclusions for Texas lignites [25]. Lithotype Class Pure coal, detrital

Lithotypes* Banded

Pure coal, non-xylitic

Unbanded Finely Moderately Moderately Highly Unbanded

Pure coal, xylitic Impure coal, detrital Impure coal, non-xylitic Impure coal, xylitic

Unbanded Finely Moderately Highly Moderately

Inclusions Gel particles Resin bodies Gelified groundmass Resin bodies Charcoal Gelified tissues Cuticles Gel particles Resin bodies Gelified groundmass Charcoal Resin bodies Charcoal

*Banding is classified as follows: finely, <2mm; moderately, >2mm, <5mm; and highly, >5mm.

of the seam appears to be more fragmental than lignite lower in the seam, and has a dull luster. Huminites dominate throughout the middle and lower portions of the Beulah-Zap lignite. Ulminite that is not extensively gelified associates with inertinites near the top of the seam, and may incorporate corpohuminite in the preserved cell lumens. The more highly gelified huminites are more abundant near the base of the seam. Gelinite and gelified ulminite occur in association with attritus above the underclay and clay partings [ 17]. Attrinite and desinite are more abundant near the base of the seam. The huminite macerals become more abundant, relative to the inertinites, toward the base of the seam, with ulminite, corpohuminite, and fusinite concentrating near the center [27]. The seam becomes more massive and displays a brighter luster with increase in huminites. No obvious pattern of liptinite distribution occurs [ 17,26], although attrinite, sporinite, and cutinite concentrate near the base [27]. Liptinites generally show very little vertical variation [ 10]. Maceral composition does not vary significantly in lateral directions [10]. Distributions of huminite and inertinite maceral groups in Beulah-Zap bed at the Freedom mine show a similar pattern to the Beulah mine, with inertinites more abundant near the top of the seam and the huminites more abundant toward the bottom [28]. A vertical variation in mean reflectance values in two stratigraphic sequences of Beulah-Zap lignite [29] shows the influences of the original environment of deposition. In Estevan (Saskatchewan) lignite, fusinite and semifusinite averaged 19% for the seam, but were concentrated in the lower two-thirds of the seam [30]. Exinite was distributed throughout the seam; the highest concentration was 13-20 cm from the top. Reflectance of eu-ulminites A and B increases downward for both the Estevan and Willow Branch (Saskatchewan) lignites [31 ]. This trend is not observed for corpohuminite [31]. Eu-ulminites of low reflectance also tend to have low

87 calorific values [31]. In Sandow (Texas) lignite, attrinite and desinite contents exceed ulminite in the upper part of the seam [25]. The upper portion also includes high concentrations of sporinite and thin-walled cutinite. Fusinite and semifusinite contents are low everywhere except in the upper portion. The middle of the seam contains abundant attrinite, desinite, liptodetrinite, and resinite. In the lower portion of the seam, a high huminite content is due to attrinite, desinite, and levigelinite. Sunrise (Louisiana) lignite consists of bright lithotypes and vitrite and clarite microlithotypes near the base, while the upper two-thirds is semibright with exinite, inertininte and mineral matter increasing while vitrinite decreases [32]. Column sections from the Dakota Colleries mine (Mercer County, North Dakota) show wide variations of petrographic composition with respect to depth [33]. (ii) Association with lithotypes. In Beulah-Zap lignites, huminite and inertinite macerals are more abundant in the vitrain and fusain lithotypes, respectively [10]. Liptinites are not predominantly associated with any particular lithotype, but tend to associate with the huminitegroup macerals. Generally ulminite is most abundant in vitrain; fusinite and semifusinite are abundant in fusain; and macerals with detrital origin (attrinite, desinite, liptodetrinite, and inertodetrinite) are common in attritus [6]. Petrographic data on the Freedom lignite has been shown in Table 3.1. Petrographic analysis of the lithotypes from the Beulah mine is shown in Table 3.8 [3,19]. TABLE 3.8 Petrographic analysis of Beulah lithotypes [3,19]. Vitrain Huminite Textinite Ulminite Attrinite Desinite Gelinite Corpohuminite Total Huminite ................. Liptinite. Sporinite Cutinite Resinite Suberinite Liptodetrinite Total Liptinite ................... Inertinite Fusinite Semifusinite Sclerotinite Inertodetrinite Micrinite Total Inertinite ................. *tr - trace

Durain

Fusain

4 0 tr* 64 34 27 10 16 9 4 5 3 1 2 1 4 2 2 84 .............. 59 .............. 42 3 2 1 1 1 9 ..............

2 1 1 2 2 7 ..............

1 1 1 2 2 5

tr 2 tr 2 1 5 .............

tr 10 1 20 tr 31 ..............

15 25 1 9 tr 49

88 As an indication of the variability in these same lithotypes, average maceral contents have been reported as follows: total huminite, 87% in vitrain, 71% in attritus, 16% in fusain; total liptinite, 7% in vitrain, 12% in attritus, 5% in fusain; and total inertinite, 5% in vitrain, 17% in attritus, and 63% in fusain [10]. Relative standard deviations in the range of 36-83% for the major macerals such as ulminite, attrinite, desinite, liptodetrinite, fusinite, semifusinite, and inertodetrinite [ 10], and relative standard deviations in many cases over 100% for the minor macerals [34], were observed for a suite of 96 samples of Beulah-Zap lignites. Comparable differences in maceral composition of lithotypes of Savage (Montana) lignite have been observed [35]. Ulminite is the major constituent of the vitrain lithotypes in the Beulah-Zap lignite, the vitrain containing up to 70% ulminite and 93% of all the huminite group macerals [ 10]. Eu-ulminite predominates among the huminite group macerals. Both the A and B varieties were observed, but A was the major constituent. Corpohuminite associates closely with ulminite. In a few cases, gelinite and highly gelified ulminite were the most abundant macerals. Gelinite occurs in thin, vitreous vitrain layers associated with a humodetrinite matrix. Most vitrain lenses have low concentrations of macerals other than the huminites [17]. Fusinite exhibited very well preserved cell structure. Inertinite macerals are the dominant constituent of fusain. Fusinite and semifusinite make up 61% of the total maceral content [10,17]. Ulminite occurs in all fusain samples, often with semifusinite at the boundaries of the fragments. Liptinite macerals associate more intimately with the huminites than with the inertinites. Humodetrinite macerals contribute the majority of the macerals that represent the detrital fragments in attritus [17]. Detrital macerals make up about 55% of the attritus [10,17], with humodetrinite constituting the majority [10]. However, the single most abundant maceral associated with attritus is ulminite [17]. Inertodetrinite occurs as elongated, angular fragments. (iii) Characteristics of lignite macerals. Low-rank vitrinites having a reflectance less than 0.5% are called xylinoids [36]. Several xylinoids may develop from a single chip of wood [36]. Xylinoids are the coalified remains of wood or bark. They belong to the vitrinite group of macerals [35]. (The term xylinoid has been used, mainly in the early literature, to refer to the macerals of the vitrinite group normally found in lignites [35]). In the lignite and subbituminous ranks, vitrinite reflectance varies relatively little as a function of rank. The maximum reflectance in oil changes from about 0.3% at 16.3 MJ/kg (m,mmf basis) to about 0.5% at 25.6 MJ/kg [37]. Consequently, vitrinite reflectance is questionable as an effective method of rank differentiation for the low-rank coals. The maxima in the fluorescence spectra of sporinite show a much greater change with rank, shifting from 40(0500 nm for peats to 630-670 nm for high volatile B bituminous [38]; sporinite fluorescence spectra provide a useful means of rank differentiation. In Canadian low-rank coals, fluorescence intensities decrease to a minimum at about 0.40% Rrandomand then increase again (as a function of reflectance) [39]. Eight northern Great Plains lignites from North Dakota, Montana, and Wyoming showed random reflectances ranging from 0.27 to 0.38% [40], the median value was 0.32%; the mean, 0.32%; and the standard deviation, 0.046. The ulminite reflectance of Freedom lignite was 0.36

89 (Rmax) [8]. Although this value was said to be low [8], it is in fact at the high end of data measured elsewhere [40]. Average random reflectance values for Texas lignites are higher than for North Dakota lignites [41]. Saskatchewan lignites show Rrandom of 0.27-0.33% [39]. At vitrinite reflectance values higher than 0.6% it is no longer possible to distinguish suberinite [42]. Similarly, as rank increases it becomes increasingly difficult to distinguish attrinite and desinite from collinite or desmocollinite [42]. Vitrinites can be characterized as structured, structureless, or groundmass vitrinite [30]. Structured vitrinite has, as the name implies, as well-defined cell structure. It is sometimes difficult to distinguish from semifusinite. Structured vitrinite may grade into the structureless form. Structureless vitrinite has few or no cell structures. The texture of structureless vitrinite is smooth, though sometimes cracked, and it has a higher reflectance than structured vitrinite. Groundmass vitrinite forms a groundmass for particles of other macerals. Groundmass vitrinite has a lower reflectivity than structured vitrinite. Vitrinite makes up, on average, 64% of the whole seam of Estevan lignite [30]. The range of vitrinite contents in individual samples was 48-80%. The vitrinite was about evenly divided among structured, structureless, and groundmass types. Structured vitrinite was most abundant at the top and bottom of the seam, as well as an interval about 43-71 cm from the top. Groundmass vitrinite was more abundant in the top half of the seam. Structureless vitrinite was about evenly divided in all seam intervals. Xylinite, detrinite, and dopplerinite are the counterparts of the vitrinite in higher rank coals [30]. As rank increases, these macerals become less diverse in both optical and chemical properties; eventually they form the vitrinite group. Xylinites are humic constituents of lignites whose most important characteristic is a well preserved cell structure. In detrinites the cell structure is discernable, though not as prominent as in xylinites. Detrinite may sometimes be observed as a ground mass containing spore or pollen remains, resin bodies, or particles of inertinites. Dopplerinite has undergone gelification and consequently has little or no discernable cell structure. Thus dopplerinite is more homogeneous than xylinite or detrinite. Ulminite in Hagel (North Dakota) lignite has a higher degree of gelification than Beulah-Zap lignite [43]. This difference suggests that these two Paleocene lignites may have had somewhat different depositional origins. The abundant woody material in the Hagel lignite, together with corroborative palynological data [44], suggests that the environment of deposition of the Hagel lignite was dominated by arboreal vegetation. The Beulah-Zap depositional environment was dominated by more abundant herbaceous plants. Ulminite dominates maceral the huminite group macerals, and in Beulah-Zap lignite accounts for 62% of the huminites [ 10 ]. The submaceral varieties of ulminite depend on the degree of gelification of the colloidal humic mass of degraded cellulose and lignin. Most ulminite in Beulah-Zap lignite is eu-ulminite, which is highly gelified and contains few preserved plant structures. Texto-ulminite is found in minor quantities near the top of the seam. In comparison, Martin Lake lignites (Texas) show well-preserved cell structure as texto-ulminite [25]. Many

90 huminites in deep (70-315 m) Wilcox (Texas) lignites have experienced partial or complete gelification [45]. Some are granular with a low reflectance and a weak orange fluorescence, possibly representing an early stage in the formation of micrinite. Micrinite is indeed found in close proximity to this type of huminite. The humodetrinite subgroup contains attrinite and desinite. These two macerals account for 35% of the huminite group and 20% of all macerals in the Beulah-Zap lignite [ 10]. The distinction between the two is based on size; humodetrinite <10 0rn is classed as attrinite. Attrinite is the most abundant humodetrinite maceral in Beulah-Zap lignite [ 10]. Attrinite, desinite, and levigeliniteare the principal huminites in San Miguel (Texas) lignite. Other huminite macerals in Beulah-Zap lignite include gelinite and corpohuminite, which together account for about 2% of the huminites [ 10]. Corpohuminite has a higher reflectance than normal for a huminite group maceral, due to its high hydrogen content, and is distinguished from inertinites by a characteristic bright oval or circular morphology in cross section. Corpohuminite is commonly associated with ulminite; differentiation between them is difficult in highly gelified samples. Anthraxylon derives from bark and woody tissues. During coalification, anthraxylon develops a homogeneous appearance and smooth texture. Anthraxylon is also characterized by a bright luster, conchoidal fracture, and brittleness. In thin section, anthraxylon has a translucent reddish color and often shows the structure of the woody tissue [ 15,16,46]. In thin sections cut normal to the bedding planes, anthraxylon appears as well-defined homogeneous bands running across the section. The so-called "woody" lignites are composed mainly of anthraxylon. The two most abundant inertinite macerals in Beulah-Zap lignite are fusinite and semifusinite [10]. Together they account for 55% of the total inertinites [10]. The reflectance of fusain from Beulah lignite is 1.73 [47]. Semifusinite differs from fusinite by having a lower reflectance and thicker cell wall structures. Both are extremely brittle. North Dakota lignite is richer in semifusinite than Texas lignite [41]. Fragments of detrital inertinites too small to determine a maceral type are classified as inertodetrinite. In the Beulah-Zap lignite most inertodetrinite particles are <25 ~tm in diameter [ 10]. Inertodetrinite comprises about 40% of inertinites in Beulah-Zap lignite [ 10]. Both macrinite and sclerotinite occur in trace amounts ( i.e., less than 1%) in Beulah-Zap lignite [ 10]. In Martin Lake lignite, fusinite and semifusinite show well preserved cell structure [25]. These two macerals, along with inertodetrinite, account for most of the inertinites in this lignite. Some inertodetrinites show rounded or corroded grain boundaries, indicative of transportation from outside the site of deposition. In comparison, inertinites in Big Brown (Texas) lignite are a mixture of sclerotinite, inertodetrinite, and macrinite. The mean inertinite reflectance for Martin Lake lignite exceeds that of other Wilcox Group lignites in Texas, suggesting the possible deposition of this lignite in a more highly oxygenated environment. Sporinite is distinguished from other liptinites by a characteristic elongated morphology. In Beulah-Zap lignite sporinite commonly occurs in the humodetrinite matrix and may range up to 200 ~tm in length [ 10]. Cutinite occurs as long, thin layers in a matrix of attritus. It comprises less than

91 2% of Beulah-Zap lignite [10]. Resinite also occurs in the attrital matrix, in bodies ranging from tens to hundreds of microns in diameter [10]. Texas lignite is richer in liptinites than North Dakota lignite [41]. In Martin Lake lignite the principal liptinites are sporinite and cutinite. Sporinite and cutinite are also the principal liptinites in San Miguel lignite, along with liptodetrinite. However, in Big Brown lignite, liptodetrinite is equal or higher in content to the sporinite and cutinite. Resinite occurs in Big Brown lignite in globular form and in exsudatinite form. (iv)

Comparative petrographic compositions of lignites. Lignites

are more petrographically

heterogeneous than higher rank coals [41]. The petrographic compositions of six North Dakota lignites are summarized in Table 3.9 [10]. Compilations of petrographic analyses of lignites prior to 1950 have been published [46,48], including some data on the vertical variability of petrographic composition within a seam [46]. In addition, the report [46] is an excellent guide to the literature published prior to 1950 on the occurrence and petrography of lignites. TABLE 3.9 Bulk maceral analyses of North Dakota lignites (Vol. pct.) [ 10]. Lignite*

CEN

FAL

FRE

GAS

IND

VEL

52 24 tr tr 1

41 29 2 1 2

37 25 3 1 1

42 22 3 1 1

38 35 2 1 1

39 27 3 1 2

Sporinite Cutinite Resinite Suberinite Liptodetrinite

1 1 1 0 3

3 1 1 0 3

2 1 1 0 7

1 1 1 0 4

2 1 1 tr 6

tr 1 2 tr 4

Fusinite Semifusinite Macrinite Sclerotinite Inertodetrinite

1 10 0 tr 3

1 6 0 0 7

3 4 0 tr 10

1 2 0 tr 4

3 2 0 tr 7

7 5 tr tr 8

3

3

3

3

2

tr

Ulminite Attrinite Desinite Gelinite Corpohuminite

Minerals

*CEN - Center; F A L - Falkirk; FRE- Freedom; GAS - Gascoyne, Blue Pit; IND- Indian Head; V E L - Velva; tr- trace.

92 3.2 THE CARBON SKELETON

This section begins a discussion of the molecular architecture of lignites, with specific concern for the carbon framework. The two subsections treat the aromatic carbon and the aliphatic carbon, respectively. In each of these subsections the discussion is divided, for convenience, according to the principal experimental technique used. The macromolecular or three-dimensional structural arrangements in lignites are treated separately in Section 3.4. 3.2.1 Aromatic carbon (i) Oxidation studies. Oxidation of lignite with a variety of oxidants indicates that the predominant aromatic structures are benzene and phenol rings [49,50]. Hydroaromatic structures, such as tetralin fragments, may also be important [49,50]. Structures containing more than one ring are fairly rare. Polycyclic and heterocyclic ring systems are not important components of the structure [49]. Oxidation of Decker (Wyoming) lignite with 0.4 M sodium dichromate at 250~ for 40 h produced no aromatic products containing more than two fused rings [51,52]. (In contrast to some other oxidizing agents, sodium dichromate tends to preserve polynuclear aromatic systems, rather than degrading them to benzenecarboxylic acids.) Naphthalenecarboxylic acids were only about 10% as abundant as benzenecarboxylic acids from the dichromate oxidation of Sheridan (Wyoming) lignite [52]. Zap lignite produced benzenecarboxylic acids as the major products (accounting for about 82% of all acids identified) with very minor amounts of naphthalenecarboxylic acids (about 3%) and no phenanthrenecarboxylic acids [53]. In terms of relative abundance, the major aromatic units (identified as methyl esters of carboxylic acids or as neutral compounds) were benzene, 100; naphthalene, about 11; anthraquinone (1), about 2; and 9fluorenone (2), about 5 [54]. O

O

O 1

2

Oxidation of Estevan lignite with aqueous potassium permanganate at 60-70~

produced

47.3% yield of benzoic acids, with 21.4% oxalic acid and 6.8% acetic acid [55]. Expressed in terms of the carbon content of the original lignite, 1.7-3.8% of the carbon appeared as acetic acid, 6-8% as oxalic acid, and 45-50% as benzoic acids [55]. The benzoic acids identified, but not quantified, included phthalic (3), isophthalic (4), terephthalic (5), hemimellitic (6), trimellitic (7),

93 COOH

COOH

~L~COOH

4

~

~

COOH

COOH

COOH COOH COOH 7

~

5

COOH COOH

HOOC ~

COOH

COOH

~,

ZCOOH 8 COOH

COOH COOH

COOH

COOH HOOC

HOOC ~COOH HOOC~

COOH

COOH 10

~COOH COOH

11

trimesic (8), mellophanic (9), pyromellitic (10), benzenepentacarboxylic, and mellitic (11) acids. Formation of polycarboxylic acids having adjacent carboxyl groups (i.e., all of the polycarboxylic acids except trimesic) implies existence of larger fused ring systems. Potassium permanganate oxidation of lignite from Oliver County, North Dakota produced mainly benzenedi- and tricarboxylic acids as the aromatic oxidation products [56]. Permanganate oxidation of sporinite from North Dakota lignite produced methoxy- and hydroxybenzene carboxylic acids [57]. Phthalic acid was the most abundant aromatic acid in the permanganate oxidation products of methylated Sheridan lignite [58]. Aromatic acids from the oxidation of Beulah lignite with ruthenium tetroxide are listed with the aliphatic acid products in Table 3.14; tetra- and pentacarboxylic acids predominated among the benzenecarboxylic acids [59]. The relatively large yields of benzenetri-, tetra-, and pentacarboxylic acids suggest a preponderance of fused ring systems [60], in general agreement with permanganate oxidation of Estevan lignite, although such interpretation conflicts with other oxidation studies [49,50] and calorimetry studies [61 ]. The relative unimportance of naphthalene rings is indicated by a low yield of phthalic acid. Oxidation of Rockdale (Texas) lignite with ruthenium tetroxide produced about 10-15 mole percent of benzenecarboxylic acids among the products [62]. The most abundant was phthalic acid, in contrast to the results obtained with Beulah lignite. It has not been established whether this distinction has any implications for the organic geochemistry of these two lignites. Neither 4 nor 5 were found in the Rockdale lignite oxidation products, nor was 8.

94 The product ratios of the acids 3, 6, 7, and 10 were 20:20:10:1. Some methylbenzenecarboxylic acids were also produced. These results indicate that ortho-substitufion patterns on the aromatic rings are important in the structure of this lignite, and that biphenyl-type structures are insignificant. The di-acids and tri-acids result from oxidation of such structures as naphthalene, naphthols, and heterocyclic compounds of oxygen. The low yield of the tetra-acid relative to the diand tri-acids indicates that condensed aromatic structures, such as anthracenes and phenanthrenes, are not important components of the structure of this lignite. The high yield of phthalic acid (3) relative to the 1,3- and 1,4- diacids is evidence for 1,2- fusion patterns in this lignite [63]. This type of ring fusion is further corroborated by the higher yields of 6 and 7 relative to 8. The formation of trimellitic acid (7) suggests that 2-arylnaphthalene structures might be important in the structure of this lignite. Oxidation of Sheridan lignite with acetic acid and hydrogen peroxide at 400C for 10 days caused 80% conversion to methanol-soluble acids [54]. The active oxidant in this system is peroxyacetic acid, which oxidizes naphthols to phthalic acid, but does not convert phenols to muconic acids (oxidation products of phenols that form by oxidative destruction of the aromatic ring; e.g., phenol itself is converted to the parent muconic acid, 2,4-hexadienedioic acid). Of the aromatic products, 69% were benzenecarboxylic acids and 31% methoxybenzenecarboxylic acids [541. Oxidation of Sheridan lignite with 70% nitric acid at reflux for 16-24 hours resulted in a 70% recovery of benzenecarboxylic acids [52]. The approximate percentages of the acid products were di-acids, 3%; tri-acids, 37%; tetra-acids, 39%; benzenepentacarboxylic acid, 15%, and mellitic acid (11), 4%. The low yield of mellitic acid, whose formation indicates extensive ring condensation, suggests that highly condensed ring structures are not important in this lignite. In comparison, nitric acid oxidation of three Chinese lignites formed alkylated aromatic dicarboxylic acids, in which the aromatic moiety was benzene, naphthalene, anthracene, or phenanthrene [64]. Benzenecarboxylic acids were the primary products from oxidation of Sheridan lignite by bubbling air through an HC1 solution while irradiating with ultraviolet light [52]. Photochemical oxidation of Decker lignite for eight days with a mercury lamp also produced a series of benzenecarboxylic acids as the primary products [51]. Benzenepolycarboxylic acids were the major aromatic products of oxidation of Wyoming lignite with, six oxidizing agents: potassium permanganate, alkaline cupric oxide, nitric acid, sodium dichromate, peroxytrifluoroacetic acid, and cesium fluorosulfonate [49]. Reaction with alkaline copper(II) oxide produced phenolic acids similar to typical oxidation products of lignin. Formation of the pyridinium iodide of lignite, followed by alkaline silver oxide oxidation, yields aliphatic- and aromatic-rich fractions [65]. Application of this reaction sequence to a Wyoming lignite forms alkylbenzenes as the major aromatic components. Lesser amounts of naphthalenes, aromatized sesquiterpenoids, indanes, tetralins, and fluorenes were produced. No evidence was found for aromatics of more than four tings. Among the phenolic compounds, phenol, cresols, anisole, and xylenols were abundant.

95 (ii) Nuclear magnetic resonance. The distribution of aromatic carbon types in two samples of Beulah-Zap lignite is summarized in Table 3.10 [66]. TABLE 3.10 Distribution of aromatic carbon types in Beulah-Zap lignite* [66] Sample Carbon type Total aromatic carbon (Ira) Aromatic carbon in an aromatic ring (fa') Carbonyl carbon (fac) F'rotonated aromatic carbon (faH) Nonprotonated aromatic carbon (faN) Phenolic or phenolic ether carbon (faP) Alkylated aromatic carbon (fas) Aromatic bridgeheads (faR)

A 0.61 0.54 0.07 0.26 0.28 0.06 0.13 0.09

O 0.66 0.58 0.08 0.21 0.37 0.08 0.16 0.13

*A = Argonne premium sample; O = sample presumed to be oxidized,

The value for protonated aromatic carbon, faH, observed in this experiment is similar to that for high volatile bituminous coals [66]. This behavior is unusual in that generally, for coals ranging from lignite through low volatile bituminous, faH increases with rank. The North Dakota lignite is anomalous in this respect. Considerable debate exists about appropriate methods and techniques for obtaining accurate, quantitative NMR data on coals. Values of fa and other structural parameters are clearly sensitive to the experimental method used, even when different methods are applied to portions of the same sample of a given lignite. For Beulah-Zap lignite, fa determined by cross-polarization magic angle spinning (CPMAS) at a magnetic field strength of 2.3 Tesla is 0.66. The same sample, analyzed at 4.7 Tesla in an experiment designed to suppress spinning side bands in the spectrum, showed an fa of 0.69 [67]. CPMAS fa measurements for Beulah-Zap lignite made at another laboratory were 0.67-0.70 [68], with some dependence on frequency, measurements being made at 25 and 75 MHz. Single-pulse excitation spectra of the same lignite indicated fa of 0.74-0.76 at 25 MHz and 0.77 at 75 MHz [68]. About 95% of the carbon in the lignite was observed in singlepulse excitation at 25 MHz. Treatment with samarium iodide gave fa of 0.68-0.70 in a CPMAS experiment, with 55% of the carbon observed [69]. Applying the Bloch-decay method to the SmI2treated lignite allowed observation of 65% of the carbon, with fa found to be 0.74. Depending on the method of choice, fa of Beulah-Zap lignite can be measured as 0.61-0.77. In somewhat similar work, the aromaticity of a Powder River Basin lignite was 0.55 when measured by cross polarization, and 0.58 determined by Bloch decay [70]. Using hexamethylbenzene as an internal standard, the Bloch decay measurement observed 65% of the carbon, while the cross polarization

96 experiment observed 55%. Distribution of carbon types in two Canadian lignites, an Ontario lignite and Bienfait (Saskatchewan) lignite, is summarized in Table 3.11 [71]. The data were obtained by dipolar dephasing techniques. Some data on the aliphatic carbons are shown also for completeness; aliphatic carbon is discussed in the following subsection. TABLE3.11 Distribution of carbon types in Canadian lignites [71]. Carbon type Total aromatic carbon (fa) Fraction of aromatic carbon nonprotonated (faa,N) Fraction of aromatic carbon protonated (faa,H) Fraction of all carbon protonated and aromatic (faH) Methyl carbon mass fraction (fMe) Lowest value reported Highest value reported Mobile methine and methylene; quaternary carbon, rotating methyl (fw) Rigid methyl, methine and methylene (fs)

Ontario 0.61

Bienfait 0.70

0.59 0.41

0.85 0.15

0.25

0.11

0.19 0.31

0.18 0.28

0.14 0.86

0.39 0.61

For comparison, the fa of Estevan lignite is 0.58-0.60 [39]. Values of fa of 0.61 and 0.62 are observed for an Ontario lignite, the higher value obtained by CPMAS at 2.3 Tesla, and the lower in an experiment at 4.7 Tesla to suppress spinning side bands [67]. Structural parameters for a variety of lignites, as determined by cross polarization - magic angle spinning NMR, are shown in Table 3.12 [39]. The table headings are defined as follows: fa, fraction of aromatic carbons; fall, protonated aromatic carbons; faN, non-protonated aromatic carbons; fsH, protonated aliphatic carbons; and fsN, non-protonated and methyl carbons. TABLE3.12 CP/MAS NMR Structural parameters of lignites [39] Sample* 1 2 3 4 5 6

%C 67.5 69.9 70.3 68.0 72.9 ....

H/C 0.94 0.89 1.12 0.65 0.87

f_a__ 0.64 0.55 0.37 0.43 0.64 0.57

fall 0.17 0.40 0.30 0.29 0.49 0.38

0.47 0.15 0.07 0.14 0.15 0.19

0.25 .... .... 0.46 .... ....

0.11 0.11

*Samples are identified as follows: 1, Beluga, Alaska; 2, Chester, Mississippi; 3, Henning, Tennessee; 4, King Cannel, Utah; 5, Oxbow, Louisiana; and 6, Thermo, Texas. Table headings are defined in text.

97 Although there is certainly a spread in values of fa, for the ten lignites characterized by the data in Tables 3.10 to 3.12, eight of the ten fa values lie in the range 0.55-0.70, and six are in the narrower range 0.57-0.66. The values for faH and faN vary more widely, although seven of the ten values of faHlie in the range 0.20--0.40. A variety of other lignites has been studied at least sufficiently to obtain a value of fa. Reported values include 0.57 for both a Canadian lignite [72] and Pust (Montana) lignite [73], 0.55 for a Wyoming lignite [74], and 0.44 for Rockdale [73] and Big Brown lignites [75]. In comparison with the data on North American lignites, fa values of 0.58 and 0.64 have been determined for, respectively, Central Otago and Southland (both New Zealand) lignites [76]. (iii) Calorimetric measurements. The estimation of aromaticity, fa, can be performed using a pressure differential scanning calorimeter (PDSC) [61,77]. A 1-1.5 mg sample o f - 1 0 0 mesh coal is heated in a 3.5 MPa atmosphere of oxygen between 150 and 600~

at a linear rate of

20*/min. The thermograms usually consist of two exothermic peaks; the lower temperature peak is assigned to the aliphatic carbon and the higher temperature peak to aromatic carbon. An example of the PDSC thermogram for Ledbetter (Texas) lignite is shown as Fig. 3.1 [78]. The assignment of the low-temperature peak to aliphatic carbon and the high temperature peak to aromatic carbon is based on model compound studies [61]. A wide variety of model compounds were studied by PDSC, ranging from the purely aliphatic to purely aromatic, nTetracontane (n-C4oH82) has a single, low-temperature peak at 215~

a temperature lower even

than the low temperature peak of a sample of Minnesota peat. In contrast, phenanthrene produces a single peak at 410~

in the temperature range characteristic of the high temperature peak in the

thermograms of subbituminous coals. Retene (7-isopropyl-l-methylphenanthrene, fa = 0.78) produced a thermogram with a small low-temperature peak and a more prominent high temperature peak, remarkably similar to that of Rosebud (Montana) subbituminous coal as shown in Fig. 3.2 [61]. An "apparent aromaticity" is determined by taking the ratio of the high temperature (presumably aromatic) peak height to the sum of the heights of both peaks. For run-of-mine coals, the aromaticity is then calculated from the equation fa = 0.263 + 0.868x where x is the apparent aromaticity [77]. The linear least squares coefficient of determination r2 is 0.85. The original basis on which this relationship was established required knowledge of the "true" aromaticity of the samples; this was determined from the Teichmuller plot of fa vs. maf carbon content [79]. Thus for example an aromaticity of 0.60 was derived from PDSC data for a Beulah lignite [23]. This value is at the low end of those obtained from NMR measurements discussed above, but measurements were not made on the same samples. Separated lithotypes of Beulah lignite gave values of 0.94 for fusain, 0.64 for vitrain, and

98 100

80

60

40

20

0

,

100

I

200

,

I

300

,

I

,

400

I

500

600

Temperature, *C Figure 3.1. PDSC thermogram of Ledbetter lignite [78].

0.63 for attritus [3,77,79]. The remarkably high value for the fusain lithotype of this sample was not observed for fusains of other lignites, but the data for vitrain and attritus are generally comparable with other observations shown in Table 3.13 [80,81]. TABLE 3.13 Aromaticities of lithotypes of Fort Union lignites [80,81].

Lignite Beulah Gascoyne Glenharold Indian Head Velva

Vitrain Attritus Fusain 0.65 0.66 0.80 0.54 0.71 0.78 0.63 0.62 0.67 0.71 0.71 0.77 0.66 0.65 0.86

Whole Coal Calcd.* Exptl. 0.66 0.66 0.64 0.57 0.63 0.63 0.71 0.68 0.66 0.70

*Calculated from the aromaticities of the individual lithotypes, weighted for the amount of each present in the unseparated samples. The aromaticities of the vitrain and attritus are very similar, with the single exception of Gascoyne, and are always lower than the value for fusain. Core samples of Ledbetter (Texas) lignite showed a slight decrease in aromaticity with

99 200

I

'

'

I

'

I

'

I

,

P.

,/" "'~,

I00

0 100

,

I

200

,

I

300

/

,

!

I

400

,

I ~

500

600

Temperature, ~ Figure 3.2. PDSC thermogram of retene (solid line) and Rosebud subbituminous coal (dashed line) [61].

depth, ranging from 0.65 for the Brown seam at 21-28 m to 0.56 for the Green seam at 55-58 m [20,77,79]. The thermograms were very similar in shape and positions of the peak maxima on the temperature scale to those of poly(vinyltoluene), for which fa is 0.66. A comparable study of Baukol-Noonan (North Dakota) lignite showed less variability, albeit over a much narrower vertical span. Over 2 m the aromaticity varied from 0.66 to 0.71 with a standard deviation of 0.02. Nevertheless it did decrease slightly with depth, from 0.71 two meters above the base of the unit to 0.68 at the base. Petrographic data were not available for either set of samples. (iv) Other methods of determining aromaticity. An aromaticity of 0.60 was estimated for a Montana lignite by X-ray diffraction studies [82], based on a resolution of two peaks, the (002) band typical of the interplanar spacing in highly condensed aromatic systems (most notably graphite) and the y band observed, for example, in paraffin and attributed to the spacing between disordered aliphatic or alicyclic structures. An innovative but seldom-used method involving reaction of coal with fluorine indicated a value of 0.52 for a Wyoming lignite [83]. Fourier transform infrared (FFIR) spectroscopy of Beulah lignite indicated fa of 0.84 [84]; well above the aromaticities usually found for lignites, unless the sample happened to be fusain-rich. FTIR examination of Titus (Texas) lignite showed 0.7% aromatic C-H and 4.1% aliphatic C-H [85]. (v) Ring condensation. For an aromatic ring system with a given number of condensed rings, the H/C atomic ratio will vary depending on the type of condensation. That is, for

1 O0 pericondensation, where the rings are as condensed as completely as possible, the H/C ratio will be lower than for a cata- condensed system, in which condensation is minimal. Examples of peri- and catacondensed four-ring systems would be pyrene (12) and chrysene (13), respectively. The relationship between the H/C ratio and the number of rings for peri- and catacondensation has been shown graphically [86]. The relationship between atomic H/C ratio and carbon content has also been shown graphically [87]. Thus for a known carbon content it is possible to estimate the H/C ratio and, from that, the number of condensed rings. In the case of coals having 70--83% carbon, the average ring number found in this way is two [86]. (For two condensed rings there is no difference between peri- and catacondensation, since the only possible structure is naphthalene.) Indeed, most studies of ring condensation suggest that the aromatic ring number of "younger coals" is 1-2 [88]. The average aromatic unit in lignites is estimated to contain nine atoms [89], again implying a mixture of one- and two-ring systems.

12

13

Many oxidation studies also suggest that the aromatic ring systems in lignites are small, with benzene and naphthalene structures predominating. Degradation of lignite with alkaline copper(II) oxide, a reagent used for lignin depolymerization, showed that benzene and phenol structures are the predominant aromatic components [49]. NMR of Beulah-Zap lignite indicates that the aromatic cluster size is nine carbon atoms [66]. The mole fraction of bridgehead carbons is 0.17, with three attachments per cluster and an average molecular weight of 277 [66]. NMR of pyridine extracts from North Dakota lignites that had been treated with sodium hydroxide and ethanol at 300"C for up to two hours shows a total ring number per structural unit of 3.0-3.5, with an aromatic ring number per cluster of 1.5-2.0 [90]. There are on average about 1.5 naphthenic rings per cluster. The aromaticities of these extracts were in the range 0.60-0.66. NMR of Nakayama (Japan) lignite showed that the petroleum-ether-soluble portion of the pyridine extract had single aromatic or quinone tings [91]. The portion of the pyridine extract soluble in chloroform but insoluble in petroleum ether had both single and double ring systems. The temperature of the maximum in the aromatic (high-temperature) peak in the PDSC thermogram was determined for model compounds containing one to five fused aromatic tings.

I01 Generally the temperature maximum of the high temperature peak increases with increasing ring condensation. Similarly, the high temperature peak maximum increases for coals of increasing rank. Although the data have sufficient scatter to warrant plotting as a band rather than as a line, nevertheless in the rank range peat-bituminous the band for the coal samples is remarkably congruent with the band for the model compounds [61]. This similarity of behavior was used to infer that the average ring condensation in lignites is about 1.5 to 2 fused rings per aromatic cluster [23,61]. X-ray diffraction shows that the average diameter of aromatic layers in lignites is about 0.5 nm, consistent with, on average, 7 to 8 atoms per layer [92]. (Considering the differences in techniques used, this is in good agreement with other estimates [66,89] of 9 atoms per aromatic cluster.) The average bond length between atoms in the layers is about 0.1387 nm, for comparison, the length of the C-C bonds in benzene is 0.139 nm [93]. The spacing between layers in lignites is 0.37-0.435 nm, compared with the interlayer spacing in graphite of 0.335 nm. Lignites at the upper end of this rank range show the beginnings of the characteristic (002) band of graphite in the x-ray diffractograms. Pyrolysis/mass spectrometry of Texas lignite indicated the presence of naphthalene-derived structures such as tetralin, dihydronaphthalene, and alkylated naphthalenes, as well as tetrahydroanthracene [94]. Single ring structures predominate among the aromatic compounds in a supercritical fluid extract of lignite [95]. 3.2.2 Aliphatic carbon This subsection discusses the principal forms of aliphatic carbon in lignites, the methods by which they have been studied, and estimates of the amounts of the various aliphatic structures. (i) Oxidation studies. Oxidation of Beulah lignite with ruthenium tetroxide gave relatively high yields of succinic and glutaric acids (totalling 18.3 area % of the gas chromatogram of the methylated products) [60]. This result implies that the major links between aromatic units in this lignite are dimethylene and trimethylene bridges. (Some keto structures, such as tetralone, could also contribute to the formation of these acids.) Succinic acid is the aliphatic dicarboxylic acid produced in greatest amount [96]. This suggests that either dimethylene bridges or hydroaromatic structures such as 4,5-dihydropyrene are major contributors to the structure of this lignite. Oxidation at 27~

in the presence of a phase-transfer catalyst yielded 2.7% succinic, 2.5%

glutaric, and 1.2% adipic acids (maf basis) [97]. The percentage of the carbon in this lignite present in each of the types of aliphatic bridging groups is dimethylene (and possibly 2-tetralone structures), 1.5%; trimethylene, 1.5%; and tetramethylene (and possibly tetralin), 0.7% [3]. Only small amounts of aliphatic monocarboxylic acids were produced from this lignite during ruthenium tetroxide oxidation. A complete analysis of acids observed in the ruthenium tetroxide oxidation of Beulah lignite is shown in Table 3.14 [59,98,99]. The major products are aliphatic dicarboxylic acids and benzene polycarboxylic acids [ 100].

102 TABLE 3.14 Yield of acids from ruthenium tetroxide oxidation of Beulah lignite, % maf [59,98,99]. Acid Succinic 2-Methylsuccinic Glutaric 2-Methylglutaric Adipic 2-Methyladipic Pimelic Suberic Azeleic Sebacic Octanoic Nonanoic Lauric

Yield 2.38 0.41 0.59 0.26 0.11 0.07 0.04 0.12 0.04 0.01 0.02 0 <0.01

Acid Yield Myristic 0.01 Palmitic <0.01 1,2,3-Propanetricarboxylic 0.70 1,2,4-Butanetricarboxylic 0.31 Phthalic 0.07 1,2,3-Benzenetricarboxylic 0.23 1,2,4-Benzenetricarboxylic 0.30 1,3,5- B enzenetricarboxyli c 0.01 1,2,3,4-Benzenetetracarboxylic 1.5 1,2,4,5-Benzenetetracarboxylic 1.53 1,2,3,5-Benzenetetracarboxylic 1.5 Benzenepentacarboxylic 1.5 B enzenehexacarboxylic 0.01

Some ambiguity arises because certain of the aliphatic diacids could be produced from two structures: polymethylene bridging groups or hydroaromatic structures. A suspension of Beulah lignite in xylene was dehydrogenated by refluxing with dichlorodicyanoquinone [59,98]. The yield of succinic acid from subsequent ruthenium tetroxide oxidation decreased from 2.38% in the untreated lignite to 1.42% in the dehydrogenated lignite. This result suggests that about half the succinic acid may derive from dihydrophenanthrene or dihydropyrene structures. Further corroboration is an increase in yield of phthalic acid, from 0.07 to 0.22%. The increased yield of phthalic acid from the dehydrogenated lignite is attributable to dehydrogenation of dihydrophenanthrene or dihydropyrene structures. In comparison, the yields of adipic acid from the untreated and dehydrogenated lignite were 0.11 and 0.12%, respectively. Thus very little of the adipic acid arises from tetralin-like hydroaromatic structures; only about 10% of the C4 units are present in tetralin structures [101]. Dehydrogenation of a fairly aliphatic Big Brown lignite (fa of 0.44) with dicyanodichloroquinone also reduced the succinic acid yield by about 50%, from 2.06% in the untreated lignite to 1.01% in the dehydrogenated lignite [75], again suggesting that about half of the aliphatic precursors to succinic acid are present in hydroaromatic structures based on dihydropyrene or dihydrophenanthrene moieties. Similar reductions, of about 50%, were observed in yields of glutaric acid and adipic acid, suggesting that about the half the total yield of these acids results from the oxidation of indan and tetralin (respectively) moieties. Comparison of the yields of major acids before and after dehydrogenation suggests that tetralin-like structures

(i.e., tetrahydroaromatics) are only about 10% as plentiful as the dihydroaromatics in this lignite. Interpretation of data from the ruthenium tetroxide oxidation is complicated because a given acid product many not be related unequivocally to a particular structure in the lignite. For example, 1,5-pentanedicarboxylic acid arises from at least four types of structures [62], trimethylene bridges between aromatic units, indan, 1-aryltetralins, and structures of the type Ar(CH2)3(CH)(Ar)2.

103 Malonic acid is a very minor product in the ruthenium tetroxide oxidation of diphenylmethane. Consequently, absence of malonic acid from the aliphatic dicarboxylic acid products is not necessarily an indication of the absence of single methylene carbon bridges from the lignite structure. The formation of benzene polycarboxylic acids implies extensive crosslinking of the structure, though some carboxyl groups are already present in the lignite even before oxidation. Methylation of Beulah lignite with methyl-d3 iodide prior to oxidation with ruthenium tetroxide showed that about 10% of the aliphatic diacids were produced from the oxidation of arylalkanoic acids [59]. An example would be the formation of succinic acid from 3-arylpropanoic acid structures. During oxidation with ruthenium tetroxide acetic, propionic, and butyric acids (and, in principle, higher acids) are produced from alkyl side chains. Since oxidation preserves the ring carbon to which the side chain was attached, a methyl side chain produces acetic acid; ethyl, propionic acid, etc. This method showed the following percentages of carbon incorporated in each type of side chain in Beulah lignite: methyl, 1.8%; ethyl, 0.14%; propyl, 0.04% [3,97]. Oxidation of Rockdale lignite with ruthenium tetroxide produced about sixty aliphatic acids [62]. All of the dicarboxylic acids from four to eleven carbon atoms were found. Virtually every isomer of the diacids containing five, six, or seven carbon acids was found, as well as a variety of tri- and tetraacids. The predominant acids among the products included butanedioic, pentanedioic, 2-methyl-l,4-butanedioic, hexanedioic, 2-methyl-1,5-pentanedioic, heptanedioic, octanedioic, nonanedioic, 1,2,3-propanetricarboxylic, phthalic (3), and 1,2,4-benzenetricarboxylic (7) acids [63]. These results show that branched aliphatic structures are relatively abundant in this lignite. About twenty aliphatic tri- and tetracarboxylic acids were observed in the oxidation products from Rockdale lignite [63]. Although unambiguous structures were not assigned, the results suggest the presence of branch points such as

~

H 2 ---CH ---CH 2 - - - ~

D Oxidation of various lignites with ruthenium tetroxide provided evidence for methylene bridges of two to twelve carbons [ 102]. As discussed above, the method is not a satisfactory test for single methylene carbons. The relative amounts of the various methylene bridges decrease with increasing length of the bridge. Oxidation of various lignites with ruthenium tetroxide in the presence of a phase-transfer

104 catalyst [100] was used to compare the yields of six acids: succinic, glutaric, adipic, 1,2,3propanetricarboxylic, phthalic (3), and 1,2,4,5-benzenetetracarboxylic (10). The results are shown in Table 3.15 [59,98,99]. The results of ruthenium tetroxide oxidation of lithotypes of Beulah lignite are shown in Table 3.16 [59,98,99]. TABLE 3.15 Comparative yields of acids* from ruthenium tetroxide oxidation of lignites, % maf. [59,98,99] Lignite Beulah Gascoyne Velva Martin Lake SanMiguel

SU 2.38 2.58 2.71 2.50 2.94

GL 0.59 0.56 0.56 0.96 0.61

AD 0.11 0.13 0.16 0.27 0.12

l:rI" PH 0.70 0.07 0.62 0.07 0.73 0.05 0.76 0.09 0.62 0.04

BT 1.53 1.60 1.37 1.55 0.73

*SU, succinic; GL, glutaric; AD, adipic; FrF, 1,2,3-propanetricarboxylic; PH, phthalic; BT, 1,2,4,5-benzenetetracarboxylic.

TABLE 3.16 Yields of acids* from ruthenium tetroxide oxidation of Beulah lithotypes, % maf [59,98] Lithotype Vitrain Attritus Fusain

SU 2.41 2.10 1.38

GL 0.60 0.50 0.33

AD 0.08 0.12 0.10

PT 0.58 0.52 0.27

PH 0.02 0.08 0.15

BT 0.71 1.38 1.40

*See note for Table 3.15 above.

Comparison of Tables 3.15 and 3.16 shows a wider variation in the amounts of these six acids among the lithotypes of a single lignite than among "whole" samples of different lignites. This finding highlights the great variability in samples from a single mine, and indicates the need for reporting petrographic data (whenever it exists!) with other analytical results. Furthermore, correlation of data from different experiments with the same lignite may be questionable unless all of the experiments have been done with the same sample. Reaction of a North Dakota lignite with peroxytrifluoroacetic acid indicated that 5.5% of the hydrogen originally in the lignite was identified in succinic acid, and 1.2% in malonic acid [103]. Succinic acid is produced from bibenzyl-type linkages, including 9,10-dihydrophenanthrene- and acenaphthene-type structures, -CH2CH2CO- linkages, and indans [103]. Malonic acid presumably arises from single methylene bridges, such as diphenylmethane-type structures. Oxidation converted 3.9% of the hydrogen originally present in the lignite to acetic acid [ 103]. This

105 reaction should convert methyl groups attached to aromatic rings to acetic acid. Reaction of Hagel lignite showed that 0.20% of the carbon was present as arylmethyl groups and 0.06% as arylethyl groups [ 104]. There was no evidence for the existence of arylpropyl structures [ 104]. In contrast, sodium hypochlorite oxidation indicates that about 1% of the total carbon in low-rank coals is present in n-butyl groups and 3-5% is in n-propyl groups [ 105]. Sheridan lignite, methylated with dimethyl sulfate to protect phenolic rings from attack, was oxidized with sodium dichromate. No long-chain acids were produced; but they were formed from a cannel coal containing 19.2% exinite [ 106]. (Sheridan lignite contains about 1% exinite.) The difference in behavior between the cannel coal and Sheridan lignite suggests that the lignite structures responsible for the formation of long-chain acids during oxidation are not the same as the aliphatic structures in cannel coal [ 106]. The latter derive from spores or pollen. Lignite can be separated into aliphatic- and aromatic-rich fractions by treatment with iodine in pyridine, followed by alkaline silver oxide oxidation of the resulting pyridinium iodides [65]. The aliphatic-rich fraction contained materials of molecular weights 60(05000, and was similar in composition to alginite or Type I kerogen, in agreement with results from dichromate oxidation [ 107]. The major fragments observed in solid probe mass spectrometry were alkanes and alkenes up to C 20, alkylated monocyclic alkanes and alkenes, sterane (14), sterene (15), 4-methylsterene (16), tri-, tetra- and pentacyclic terpanes, pentacyclic terpenes, and benzenes and phenols having long chain alkyl substituents.

C 14

15

16 Sheridan lignite contains aliphatic-rich materials similar to Type I kerogen, but not directly

106 related to exinite macerals [ 108]. Oxidation with potassium permanganate produces large amounts of aliphatic dicarboxylic acids in the range C4 to C21, with the C9 acid being most abundant [49,58,108]. Comparable results were obtained in the oxidation of Soya (Japanese) lignite [49]. Branched dicarboxylic acids in the range C5 to C10 and tricarboxylic acids in the range C6 to C8 were also identified in the oxidation products from the Sheridan lignite, but in much lower concentrations than the dicarboxylic acids [58,108]. These compounds might derive from material similar to Type I kerogen; that is, from lipid-rich organic material. Phthalic acid and 1,2,4- and 1,2,3-benzenetricarboxylic acids were abundant among the aromatic products of oxidation [ 108]. Oxidation products from methylated Sheridan lignite are similar to those produced from Green River oil shale [ 107]. Specifically, straight-chain aliphatic diacids were abundant in the range C3 to C15, with a maximum at C9 [107]. Diacids with branched chains or cyclic structures were observed only in much lower quantity. Treatment of Sheridan lignite with 15% nitric acid at 80"C for 14 h yields 11-13% aliphatic diacids in the range C4_C12[49]. Aliphatic diacids in the range C4-C13 were produced by oxidizing this lignite with CsFSO4 [49]. Potassium permanganate oxidation of sporinite isolated from a North Dakota lignite yielded straight-chain dicarboxylic acids as the major class of product [57]. Branched-chain diacids and tricarboxylic acids were minor products. Oxidation with nitric acid showed diacids in the range of C a to C13 [57].

(ii) Nuclear magnetic resonance. NMR of Big Brown lignite shows it to be more aliphatic than typical North Dakota lignites, having an aromaticity of 0.44 [75]. NMR suggests that much of the aliphatic carbon is present is polymethylene chains. Polymethylene structures containing at least four --CH2-- units have been observed by liquefaction in tetrahydroquinoline followed by NMR analysis of the deuterochloroform-solubles [ 109,110]. Application of this technique to a series of Texas lignites showed contents of --(CH2)n-- of 6.53-10.52% for Wilcox Group lignites, 7.83-14.51% for Yegua Formation li gnites, and 8.05-13.89% for Manning Formation lignites (weight percent, maf basis) [ 109]. The average for ten Wilcox Group samples was 8.23%, with a standard deviation of 1.14. For Alcoa (Texas) lignite, 6.6% of the lignite is found as polymethylene groups by this technique [110]. Despite the fact that the Wilcox stretches for hundreds of miles, the uniformity of polymethylene content suggests a relatively constant paleoflora and environment of deposition over a large area [ 109]. NMR of Rockdale lignite of (fa 0.44) showed that the amount of methylene groups present as part of the macromolecular network structure of the coal is greater than that trapped in the coal as extractable fatty acids [111]. Proton NMR spectroscopy of the 3:1 benzene:methanol extracts from seven Texas lignites showed an average CH2/CH3 ratio of 3.49 [111]. (The extract yields were in the range 2.3-7.6%.) By comparison, a German brown coal treated in the same fashion yielded 3.3% extract with a CH 2/CH3 of 5.63. This difference might be attributable to a shortening of aliphatic chains with increasing maturation, or a greater amount of chain branching or methyl substituents in the Texas lignites [ 111]. In comparing these two studies [ 109,111 ] it should be borne in mind that the

107 former observes polymethylene units liberated from the macromolecular structure by breaking covalent bonds, whereas the latter is a characterization of compounds produced by solvent extraction. Supercritical toluene extracts of lignites have sharp bands at 1.3 ppm in the 1H spectra and at 29.7 ppm in the 13C spectra, assigned to methylene groups in chains containing at least eight carbon atoms [95]. The average chain length of the extracts is between 2.5 and 4 carbon atoms [95]. Aromatic structures are predominantly single-ring moieties. Brandon (Vermont) lignite shows a sharp, intense peak for aliphatic carbons indicative of material derived from algal residues [ 112]. This observation compares with results of oxidation studies which suggested the existence of material like Type I kerogen in lignite [65,107]. Two samples of Beulah-Zap lignite provided the 13C NMR data on aliphatic carbon summarized in Table 3.17 [66]. TABLE 3.17 Aliphatic carbon types in Beulah-Zap lignites* [66]. Sample Carbon type Total aliphatic carbon (fai) Aliphatic carbon as CH or CH2 (falH) CH3 or non-protonated aliphatic carbon (fai*) Aliphatic carbon bonded to oxygen (faio)

A 0.39 0.25 0.14 0.12

O 0.34 0.21 0.13 0.10

*A = sample from Argonne National Laboratory premium coal sample bank; O = non-premium sample believed to have been oxidized. (iii) Other instrumental methods. Fourier transform infrared spectroscopy (FTIR) of Bienfait lignite shows an absorption at 693 cm-1 attributed to a double bond in a side chain, or possibly in a trapped long-chain compound [ 113]. This band could also arise from substitution of dissimilar groups onto an aromatic ring [ 113]. Examination of the asymmetric C-H stretch in the FFIR spectra shows very few --CH3 bands in Beulah lignite [114]. T h e - C H 3 / - C H 2 - r a t i o increases with rank. FTIR of Titus lignite showed 4.1% by weight of aliphatic C-H, compared to 0.7% aromatic C-H (both on a dmmf basis) [85]. Secondary ion mass spectrometry (SIMS) shows that, in general, low-rank coals contain more methyl groups than higher rank coals, on the basis of comparison of intensities of the peak at 15 daltons [ 115]. The complete SIMS spectra of low-rank coals are similar to those of wood. The material considered to be amorphous in X-ray diffraction includes aliphatic side chains on aromatic ring systems, phenolic oxygen, and other functional groups [92]. The amount of amorphous material decreases with increasing carbon content. In the range of 60-70% carbon, the amorphous material decreases from about 38% of the coal to about 34% [92]. (iv) Studies of lignite extracts. Henning lignite, Seyitomer (Turkey) lignite and three British

108 bituminous coals, were studied by a series of pyrolysis and extraction methods [ 116]. Extractions were done using supercritical toluene and a hydrogen-donor solvent. After separation of the products into preasphaltenes, asphaltenes, and pentane-solubles, analysis was done using 13C NMR and GC/MS. About 30% of the total carbon in the two lignites is present as aliphatic carbon. Less than half of the aliphatic carbon is present in hydroaromatic structures or bridges between aromatic ring systems. About 20% is present as methyl groups. Alkyl chains of eight or more carbons account for about 35% of the aliphatic carbon (equivalent to 10% of all the carbon in the lignite). Long-chain n-alkanes and high-molecular-weight aliphatic acids or waxes are minor components. This suggests that many of the long aliphatic chains are bonded to aromatic structures, and contrasts with the view (e.g., [ 112]) that aromatic and long-chain aliphatic materials represent separate domains in lignite. Aliphatic structures of 23-25 carbon atoms, possibly remnants of the original plant material, have been isolated from Novodmitrovskoe (Ukrainian) lignite [ 117]. Sequential solvent extraction of Beulah-Zap lignite shows terpenoid hydrocarbons to be the major components of the extract, with tricyclic diterpenoids the most abundant [118. n-Alkanes are relatively minor components. In the range C16-C32the alkanes with odd numbers of carbon atoms predominate, with a maximum at C27 [118]. (v) Characterization of reaction products. Straight-chain hydrocarbons in the C24-C28 range were isolated from the neutral fraction of lignite low-temperature tar [33]. The largest alkyl substituent on an aromatic ring was found to be C4. The largely aromatic structures were present in a dispersed phase in an aliphatic matrix of high-molecular-weight hydrocarbons, forming a solid colloid. Additional structural integrity is provided by crosslinking in the dispersed aromatic phase and by hydrogen bonding. This structural model accounts for the existence of long-chain aliphatic molecules. Compounds making up the aliphatic matrix, which is not necessarily the major portion of the lignite structure, could derive from the waxes and resins of plant debris. A contribution of the phytyl chain from chlorophyll was also suggested [33]. Flash pyrolysis indicates that lignites may contain up to 10% polymethylene units of the type --(CH2)n-- [ 119]. Pyrolysis at 600~ drives off some of these units as alkanes or alkenes in the range C17 at least to C24 [119]. Flash pyrolysis of Alcoa lignite produced various light hydrocarbon gases, including 3.2% ethylene, 0.74% propylene, and 0.41% butadiene [120]. These products arise from polymethylene precursors having more than four --CH2-- units in the chain [120]. The amount of --(CH2)n--, where n>4, in this lignite amounted to 5.3% based on the flash pyrolysis results [ 110]. This is somewhat lower than the estimate of 6.6% obtained from NMR analysis of products of liquefaction of this lignite in tetrahydroquinoline [ 110]. Lignite liquefaction products contain a homologous series of alkanes, n-Alkanes with up to 32 carbons are obtained in yields of 3-5% (maf basis) during liquefaction with synthesis gas. Since there seems no plausible way in which these compounds could be synthesized in situ or be an artifact of the experiment, these alkanes are present in the lignite and are liberated during liquefaction [121,122].

109 Reaction of Texas lignite in pyrolysis, in hydrogen, or in the presence of a hydrogen donor solvent (tetralin) produces in each case products composed predominantly of long-chain aliphatic compounds [ 121 ]. The reaction temperatures were kept below 400~ to avoid secondary reactions of the products. The relative proportions of the alkanes from each type of experiment were essentially the same, although pyrolysis in inert gas gave the smallest yield of alkanes and reaction in tetralin the highest. The constant proportion of alkanes released in each experiment suggests that these products are indeed constituents of the lignite structure, and presumably derive from the original plant material. Soxhlet extraction of Beulah-Zap lignite with tetrahydrofuran yields cyclic and branched alkanes, diterpanoids, tetracyclic tel-panes, and pentacyclic triterpanes [ 122]. Subsequent reaction with hydrogen at or below 290~ in methanol (290~

followed by reaction in a 10% solution of potassium hydroxide

nitrogen atmosphere, 1 h) gave a product containing n-alkanes, alkyl

aromatics, and heterocyclic compounds [122].The aliphatic portion is 4.5% of the product, equivalent to 3.2% of the lignite [122]. The results indicate that alkyl groups and methylene bridges are part of the macromolecular network of the lignite. A smooth distribution of n-alkanes in the range C 12-C40 occurs, with no preference for odd- or even-numbered carbon chains. Some branched alkanes and alkenes were also observed. Dehydrogenation of vitrains has shown that 25-30% of the hydrogen content of lignites is present in hydroaromatic structures [123]. The reactions were carried out using 1% palladium dispersed on calcium carbonate as the dehydrogenation reagent in phenanthridine as the vehicle [123,124]. After depolymerization with phenol and boron trifluoride at 100~

the yield of phenol-

soluble products was proportional to the number of cleaved methylene bridges [ 125]. Lignite gave a 72.8% net yield of phenol-solubles, 2.5 to 8 times as great as yields observed from bituminous coals. 3.3 H E T E R O A T O M S

3.3.1 Introduction Oxygen is by far the most important of the heteroatoms in lignite. North American lignites can contain over 20% oxygen on an maf basis. Most of this section is a discussion of the distribution of oxygen among various functional groups. Much less is known about the organic sulfur and the nitrogen in lignites. Each accounts for about 1% of the lignite (maf basis). Few thorough investigations have been conducted on the contribution of these two elements to the structure of the lignite, or on the functional groups in which they are incorporated. 3.3.2 Carboxylic acid groups The carboxylic acid group is one of the most important of the oxygen-containing groups in lignites. In North Dakota lignites up to 65% of the oxygen occurs in these groups [ 126]. However,

110 this value is quite high compared to most of the measurements reported in the recent literature. A Beulah-Zap lignite in which the total organic oxygen content was 17.21% (dry basis), as measured by neutron activation analysis, contains 3.81-3.94% oxygen as carboxyl oxygen [127]. This is equivalent to 22-23% of the total oxygen being present in carboxyl groups. A value of 18% was observed for Estevan lignite vacuum-dried at 100*C [128]. Some estimates give values even lower, 10-13% of the oxygen as carboxyl, for example [129]. The carboxylic acid group is labile under mild thermal conditions, decomposing to carbon dioxide well below 500* C. Although the volatile matter content of lignites is higher than of bituminous coals, much of the material lost from lignite as volatiles is carbon dioxide rather than hydrocarbon gases, oils, or tars. Thus only about 17% of the calorific yield of North Dakota lignite occurs in the volatile products whereas about 30% occurs in the volatiles from bituminous coals [ 130]. The ability of the acid groups to bind cations on ion exchange sites is important in the inorganic chemistry of lignites (Chapter 5). A carboxyl-metal cation-carboxyl linkage may also have a role in crosslinking of the lignite structure [89]. A general procedure for the determination of carboxylic acid groups has been developed [131,132]. The basis of the method is that the carboxyl groups, which have all been put into the acid form by a preliminary demineralization step, are reacted with barium acetate solution. Barium ions displace acidic protons from the carboxyl groups. Titration of the now-acidic solution with standard base determines the quantity of protons released, equivalent to the number of acidic functional groups in the coal. Experimentation with several variations of the general method led to the recommendation of a single reflux of 4 h with barium acetate at pH 8.25 followed by titration of the acetic acid formed as the most reliable method [131]. North Dakota and Texas lignites contain 2.2-2.3 meq/g (dmmf) carboxyl [131]. Total acid group contents (i.e., carboxyl plus phenolic hydroxyl) range from 7.5 for a South Dakota lignite and 6.9 for a North Dakota lignite, to 6.1 for a Texas lignite, all reported as meq/g, dry basis [132]. Demineralization preliminary to the determination of carboxyl groups involves reaction with 5M hydrochloric acid, concentrated hydrofluoric acid, and then concentrated hydrochloric acid, all at 55--60~ for 1 hour, followed by extensive washing with distilled, CO2-free water [133]. The carboxyl content can then be determined by reaction with 1N barium acetate at pH 8.25-8.30, followed by titration with 0.05N barium hydroxide to restore the pH to the initial value [ 133]. Carboxyl contents of various lignites are shown in Table 3.18. The percentage of the total oxygen content of the coal incorporated in the carboxyl groups is also given, when that information was provided in the original literature. A modification to the barium-exchange method involves ion exchange with barium chloride solution at pH 8.3 in a blender under a nitrogen, followed by titration [75]. For Big Brown lignite, the carboxyl content was found by this method to be 3.6 meq/g (maf basis). Data for Beulah lignite highlight the variability of carboxyl content [23,134,135,138]. Six samples show carboxyl contents ranging from 1.46 to 2.97 meq/g with a median value of 2.31 meq/g. Four samples identified as "Seam 2" Beulah lignite ranged from 2.25 to 3.67 meq/g, with a median of 2.72. It is likely that some of the spread in the data may be accounted for by variations

111 TABLE 3.18 Carboxylic acid contents for lignites Source of lignite Carboxyl State Seam or Mine meq/~ (dmmt) Alabama Choctaw 2.83 Alabama Pike County 2.04 Montana Fort Union 3.00 N. Dakota Baukol Noonan 2.74 N. Dakota Beulah 2.78 N. Dakota Beulah 2.30 N. Dakota Center 3.58 N. Dakota Freedom 2.60 N. Dakota Gascoyne Blue 2.68 N. Dakota Gascoyne Red 2.46 N. Dakota Gascoyne White 2.62 N. Dakota Gascoyne Yell. 2.67 N. Dakota Hagel 3.13 N. Dakota Indian Head 2.45 N. Dakota Kincaid 2.89 N. Dakota (unident.) 2.2 N. Dakota Velva 2.55 Texas Big Brown 2.26 Texas Bryan 2.25 Texas Darco 2.11 Texas Ledbetter 1.82 Texas San Miguel 1.4 Texas (unident.) 2.3 (unident) 1.1

Percent of oxygen 42 38

46

37

Ref. [ 134,135] [134,135] [133, 136,137] [134,135] [138] [23] [134,135] [139] [138] [26] [26] [138] [133,136,137] [134,135] [33] [131] [134,135] [ 134,135] [134,135] [ 133,136,137] [ 134,135] [ 134] [ 131 ] [ 138]

in experimental technique; furthermore, no information was provided on how long the samples might have been exposed to air or on the petrographic composition of the samples. (The carboxyl contents of vitrain and fusain from Beulah lignite are 2.28 and 2.49 meq/g, respectively [140], and attritus, 2.83 meq/g [139].) Nevertheless, the carboxyl content likely varies within a seam. Certainly the amount of inorganic material in the lignite, including the ion-exchangeable cations (Chapter 5) varies throughout the seam. The variation in carboxyl content may relate directly to the observed variation in inorganic composition. Values for two samples of Gascoyne "Blue Pit" lignite are 2.71 and 2.90 meq/g; for two samples of "Red Pit" lignite, 2.46 and 2.57 meq/g [26]. (These designations refer to different working pits within the mine.) The data for Gascoyne lignite in Table 3.18 also indicate how carboxyl content may vary from one working pit to another within the same mine. The total oxygen incorporated in carboxyl groups for three lignites [ 136] is lower than the reported maximum of 65% of total oxygen incorporated as carboxyl groups [ 126]. For these three samples, the percentage of carbon in the carboxyl groups are 7.6% for Hagel, 3.4% for Darco (Texas), and 5.2% for Montana lignite [133]. Approximately 50% of the total surface area measured by CO2 adsorption is covered with carboxyl groups, of which about half are in the

112 cationic form [137,141]. Acidic hydrogen in six Texas lignites was determined by introducing a pulverized sample into a solution of lithium aluminum hydride in diethyl ether, and measuring the total volume of hydrogen gas evolved. This method will liberate hydrogen attached to any group acidic enough to react with lithium aluminum hydride; thus it does not differentiate between carboxyl and hydroxyl groups. Active hydrogen ranged from 1.52 to 2.77 meq/g (daf basis), averaging 2.22 meq/g [111]. Assuming that the active hydrogen represents 2.22 meq/g of carboxyl, this carboxyl content is equivalent to 71 mg/g of oxygen, or about 7.1%. The total oxygen content of these six samples, as determined by neutron activation analysis, was 26.09%-36.15% [111]. Thus a significant fraction, roughly four-fifths, of the oxygen in these lignites must be present in carbonyl, methoxyl, and other ether groups. The reaction of lignite with sulfur tetrafluoride has been suggested as a method for determining carboxyl groups [ 142]. Carboxyl groups are converted to acyl fluorides, which can then be determined by high-resolution solid-state NMR. IR evidence exists for the conversion of --COOH to --COF by reaction with sulfur tetrafluoride at 110~ for 1 h [143]. The FFIR spectrum of Beulah lignite shows a major band at 1562 cm-1, assigned to carboxylate groups [114]. Major bands at 1655 and 1520 cm-1 in the spectrum of Beulah-Zap lignite are assigned to conjugated carbonyl and carboxyl groups [ 114]. A concern about reliance on the barium-exchange method for carboxyl determination is that the exchange reaction might not be complete, and therefore the results might underestimate the total quantity of carboxyl [144]. Conversion factors have been developed to relate the intensity or area of the 1700 cm-1 FFIR peak to the percentage of oxygen in the sample present as --COOH [144]. X-ray photoelectron spectroscopy of Beulah lignite, using poly(ethylene terephthalate) as a calibration standard, estimated the molar ratio of carboxylate to all C-O single bonds to be 0.625 [23]. The spectroscopic data could not be refined to distinguish between phenolic and ether C-O. Little work has been done on applying XPS to oxygen functional group analysis of lignites. However, this result is high relative to the ratio of [-COOH]/{[-C-OH] + I-C-O-C-]} calculated from data obtained from other methods. The value of this ratio is 0.33 for a Beulah-Zap lignite calculated from the data in [127], and 0.40 calculated from the data in [128]. Ruthenium tetroxide oxidation provides information on the structural arrangement of carboxyl groups [97]. The products include three tricarboxylic acids, oxalomalonic (17), 7.7% of the organic products of the reaction; oxalosuccinic (18), 3.2%; and oxaloglutaric (19), 1%. The benzoic, phenylacetic, and phenylpropionic acid structures of this lignite are in the proportion 65:27:8. Oxidation of Beulah lignite methylated with methyl-d3 iodide indicates that about 10% of the aliphatic diacids produced in the reaction derive from arylalkanoic acids such as 3arylpropanoic acid [59]. This result indicates that at least some of the carboxyl groups in lignite are at the ends of aliphatic chains connected to aromatic rings. The 1600 cm-1 band in the Raman spectrum (assigned to the E2g mode of a graphite-like structure) of two Beulah lignites differing in carboxyl contents is less intense in the sample of higher carboxyl content [23]. This suggests that

113 HOOC CHCOOH I CO I COOH 17

HOOCCHCH 2COOH I CO I COOH 18

HOOCCH ,,CHCH 2COOH "1 CO I COOH 19

the steric requirements of the relatively bulky carboxyl groups, with their associated cationic counterions, may prevent alignment of aromatic ring systems, affecting long-range structural order in the lignite. Fatty acids have been isolated from lignites [145]. In contrast to modern biological systems, in which fatty acids with even numbers of carbon atoms predominate, the isolated acids include ones with odd numbers of carbon atoms [145]. Microbes may preferentially use the evennumbered acids, so that even if they far exceed those with odd-numbered chains in modern systems, microbial consumption of the even-numbered chains increases the proportion of oddnumbered acids among the total [ 146]. Waxes isolated from lignites also contain both even- and odd-numbered fatty acids and alcohols [145]. Acidic fractions isolated by sequential extraction of Beulah-Zap lignite consist mainly of even-numbered acids in the range C22-C30, with a maximum at C26 [118]. Field ionization mass spectrometry signals from Beulah-Zap lignite also demonstrate the existence of fatty acids [147]. 3.3.3 Humic acids Extraction of fresh lignite with dilute aqueous alkali yields 2-4% of so-called natural humic acids [33]. Up to 90% of the original dry, ash free organic material can be extracted from Dow (Texas) lignite after reaction with 0.25N sodium hydroxide solution for 5 h at 433 K [148]. Extraction of German brown coals with 1% ammonia, followed by precipitation with HCI, yields 0.5-3% humic acids [ 149]. Since mild atmospheric oxidation of the base-extracted residue yields more humic acids on repeated extraction, it is not clear whether the natural humic acids were present in the lignite or formed as a result of initial oxidation as soon as the lignite is first exposed to the atmosphere. Nevertheless the extracted humic acids represent only a minor modification of the lignite structure. Lignites can be converted to sodium humates by high speed blending in aqueous sodium hydroxide [150]. The yield increases in the presence of small amounts of air (i.e., air leaking into the apparatus--a kitchen blender--which had initially been purged with nitrogen). Humic acids from Big Brown and Beulah lignites and Wyodak (Wyoming) subbituminous coal show similar aromatic/aliphatic peak intensity ratios in 13C NMR spectra, suggesting similarities in the chemical nature of the acids. High speed blending (about 24,000 rpm) of Beulah lignite in 5% aqueous sodium hydroxide gave 89% conversion to humic acids. The acids had a weight-average molecular weight of 4.3 x 105. The weight-average molecular weight is inversely proportional to the rotational speed

114 of the blending, suggesting that higher shear rates effect greater disruption of the coal structure, giving higher yields of a lower molecular weight product. The effect of blending conditions on Russian low-rank coals has been demonstrated [151]; amorphous humic acids are recovered, but the properties of the products depend on the conditions of blending (i.e., choice of reagents, and use of an air or inert atmosphere). Very low conversions were obtained when the reaction was attempted with 30% aqueous ammonia (12%) or with pyridine (8%), suggesting that the liberation of humic acids is neither the result of cleavage of hydrogen bonds nor of ester hydrolysis [ 150]. Humic acids isolated by high speed blending of Beulah lignite in nitrogen have a composition of 67.5% C, 4.2% H, 0.86% N, and 26.7% O (maf basis) [150]. The inorganic content of the humic acid is also low, reported as 0.83% ash [ 150]. The integrated peak intensity for the carboxyl carbons in the 13C NMR spectrum was about 11% of the total integrated intensity. Some differences in the elemental composition and the carboxyl content are observed depending on the specific isolation technique. Blending Big Brown lignite with 5% sodium hydroxide gave a 81% yield of a humate latex [75]. The humic acids were more aromatic than the original lignite. The molecular weights of reduced, methylated derivatives of the acids were 1,300,000, similar to humic acids from North Dakota lignites. (Humic acid extracted from Beulah lignite had a molecular weight of 1,300,000, as measured by low angle laser light scattering [ 152].) Alkaline digestion of Australian brown coal results from disruption of the structure by electrical double layer repulsion and migration of water molecules into the hydrogen-bonded gel structure of the coal [153]. The mechanism involves interaction of hydroxide with the acidic functional groups, followed by chemisorption of alkali metal cations by the coal structure. 3.3.4 Methoxyl groups Arylmethoxyl groups are important in the structure of North Dakota lignite, but may be insignificant in Wyoming lignite [103]. Oxidation of North Dakota lignite with peroxytrifluoroacetic acid indicated that about 15% of the total hydrogen was present in arylmethoxyl groups, comparable to lignin [103]. This oxidation converts arylmethoxyls to methyl trifluoroacetate, which was measured by NMR. A simplified methoxyl determination involves refluxing lignite in concentrated sulfuric acid, followed by distillation and gas chromatographic measurement of the liberated methanol in the distillate [102], a value of 0.71% being found for Indian Head (North Dakota) lignite [154]. Kincaid (North Dakota) lignite has a methoxyl content of 0.90%, or 0.29 meq/g, as determined using the Zeisel method [33]. The methoxyl content of Indian Head lignite was determined to be 0.7% (as-received basis) by oxidation with peroxytrifluoroacetic acid, followed by hydrolysis of the methyl ester products and gas chromatographic measurement of the liberated methanol [155]. This gives quite good agreement with the method using sulfuric acid reaction [102]. The methoxyl groups are very labile in hot water drying (Chapter 10) and extraction with supercritical water. The methanol yield during

115 hot water drying of the same lignite at 325~ was 0.5%, and during supercritical water extraction at 390"C, 0.6% (as-received basis) [ 155]. Oxidative degradation with sodium periodate is specific for o-methoxyphenols (the products are methanol and o-benzoquinone). Determination of the methanol provides a measure of guaiacol groups not etherified at the 4-0 position. This procedure applied to Beulah lignite showed at least a third of the methoxyl groups present in free guaiacol structures [3,155]. A similar proportion is observed in sodium periodate oxidation of wood [ 155]. Under reflux, the methanol yield increases from 0.25 to 0.44% (as-received basis), due either to the depolymerization of the lignite structure making more guaiacol groups accessible to periodate, or to hydrolysis of omethoxyphenyl linkages generating new guaiacol units [155]. The methoxyl contents of Beulah lignite lithotypes, determined by the peroxytrifluoroacetic method, were 1.3% for the vitrain and 0.7% for attritus (maf basis) [3]. The composite sample

(i.e., not separated into lithotypes) contained 1.3%. A value of 1.8% (maf basis) has also been reported for methoxyl in Beulah vitrain [156]. Methoxyhydroxybenzene structures in the vitrinitic components of lignite derive from fossil lignin moieties [ 147]. Fusain contains 1.1% methoxy on an as-received basis [155]. A composite sample of Indian Head lignite treated in a similar manner had 1.2% methoxyl. The methoxyl content, determined by this method, for a coalified tree stump from the Beulah mine was found to be 0.2% on an as-received basis [ 155]. (Fusain also had the highest amount of residual cellulose, discussed below. Whatever geochemical process interrupted the degradation of cellulose also interrupted lignin degradation. The low value obtained for the coalified tree stump may reflect an easier access of fungal enzymes to the isolated stump than to highly compressed lignite in the interior of a seam.) Potassium permanganate oxidation of sporinite, followed by methylation of the products with dimethyl sulfate-d6, produces both methoxy-d3- and methoxybenzenecarboxylic acids [157]. This confirms the existence of both hydroxy- and methoxybenzene structures in sporinite. 3.3.5 Phenolic groups Many of the aromatic tings in lignites have an --OH group, and some have either a second --OH group or one or two methoxyl groups [158]. Beulah-Zap lignite contains 9.16% oxygen (dry basis) as phenolic oxygen, amounting to 53% of the total organic oxygen in the sample [ 127]. For Estevan lignite, the corresponding data are 7.2% and 38%, respectively [128]. A phenolic content of 2.97 meq/g has been determined in Kincaid lignite [33]. Extraction of Estevan lignite with benzene at 4.8 MPa (approximately 280~

for 3-10 days

gave an extract separable into acidic and non-acidic portions by treatment with 5% aqueous potassium hydroxide. Phenols were liberated by acidification and subsequently identified as their brominated derivatives. The total phenolic content of this lignite was 2.55%, the principal phenols being phenol, p-cresol, and catechol [55]. The same phenols, in the same proportion, were extracted from Morwell (Victoria, Australia) brown coal. Extraction of mono- and dihydric phenols from lignite with benzene suggested that the phenols exist as such in the lignite, either as free

116 phenols or in some weakly bonded association [33]. The yield of phenols increases with increasing pressure at least to 280~ [33]. Extractions at high temperature are ambiguous in that one is not sure whether the increased yield is due to improved extraction of species already existing in the lignite or is due to species liberated by thermal decomposition. In this case, 280 ~was presumed to be below the onset of thermal decomposition. Reaction with triethylborane has been proposed as a means of determining hydroxyl groups [ 159]. The reagent reacts with hydroxyl groups to form the O-diethylborylate and liberate ethane. The ethane can be determined volumetrically. No data on North American lignites have been published, but for three German brown coals containing 67% carbon and 26-27% oxygen (daf basis) the hydroxyl contents determined by O-diethylborylation ranged from 6.20 to 7.83 mmol/g, equivalent to the incorporation of 36 to 46% of the total oxygen in hydroxyl groups [ 159]. This is in reasonable agreement with results mentioned above, but obtained by greatly different methods, for Beulah-Zap [ 127] and Estevan [128] lignites. Potassium permanganate oxidation of sporinite from North Dakota lignite and methylation of the oxidation products with dimethyl-d6 sulfate produced a mixture of methoxy-d3 and natural methoxybenzenecarboxylic acids [57,157]. This indicates that the aromatic units contain both phenolic and methoxyl functional groups. GC-MS analysis of the product mixture indicates that the phenolic groups are more prevalent than the methoxy by a factor of about 9.4 [57]. 3.3.6 Carbonyl groups The carbonyl oxygen content of Kincaid lignite was determined to be 4.4% (maf basis) [33], equivalent to 2.7 meq/g of oxygen in ketone, quinone, and aldehyde groups. Beulah-Zap lignite contains 1.96% oxygen in carbonyl structures, equivalent to 11% of the total organic oxygen [ 127]. Results for Estevan lignite are somewhat higher, 3.6% oxygen in carbonyl groups, or 19% of the total oxygen [128]. NMR of Estevan lignite suggests 29-32% of the structural groups in the coal contain C=O structures [39], presumably including carbonyl and carboxyl. The existence of carbonyls in Beulah lignite has been suggested on the basis of the band at 1655 cm-1 in the FFIR spectrum [114]. 3.3.7 Ethers other than methoxyl Ethers in Beulah-Zap lignite, estimated by difference after determination of carboxyl, carbonyl, and phenolic groups, account for 2.28% oxygen, or about 13% of all the oxygen in the lignite [ 127]. The comparable figure for Estevan lignite is 25% of the total oxygen in "residual" oxygen functional groups [ 128]. 4-Nitroperbenzoic acid cleaves benzyl ethers [160]. Benzyl ether linkages are an important component of lignin structures and may persist from the lignin into coals. Reaction of Beulah lignite with 4-nitroperbenzoic acid in refluxing chloroform, followed by extraction of the residue with base, gave yields of humic acids of up to 90% (maf basis) [152,161]. The soluble material resembled the waxy material removed from lignite by solvent extraction prior to oxidation.

117 Cleavage of benzyl ether crosslinks generated base-soluble carboxylic acid or phenolic functional groups. (This reaction could involve other structural featrues which are reactive toward 4nitroperbenzoic acid; carbon bridges between aromatic units, as in diphenylmethane, are unreactive, but phenanthrene and anthracene structures could be oxidized at least as far as quinones.) The molecular weight of the base-soluble product was determined by low angle laser light scattering to be 1,300,000 [152,161], comparable to that of humic acids extracted directly from the lignite by base. Thus oxidative cleavage of benzyl ethers releases structural units of very large size, comparable to humic acids. Since the yield is much larger from the 4-nitroperbenzoic acid oxidation than from base extraction without prior oxidation, the results imply that large macromolecular humic acid-like structural units are present in the lignite, held in place by benzyl ether groups. Reaction with phenol and boron trifluoride, a reagent pair well known to depolymerize coal [ 125], showed that lignite produced the highest yield (75%) of phenol-soluble products of any of the ranks of coal tested [ 162]. Substitution of p-toluenesulfonic acid (pTSA) for boron trifluoride converted over 90% of low-rank coals to pyridine-soluble products [162]. For lignites, a major contribution to this extensive depolymerization is cleavage of aliphatic and benzyl ethers. Extensive depolymerization of lignite in these reagents suggests the presence of abundant ether linkages. However, aliphatic bridges linked to single phenolic rings are also reactive enough to participate in depolymerization by phenol-BF3 or phenol-pTSA. The well-known ability of aqueous alkali to solubilize large amounts of lignite has been attributed to the cleavage of ether linkages [163]. The benzyl aryl ethers of the 1~-O-4, ~x-O-4, andy0-4 type appear to be especially labile. (The structural notation for lignin is given on page 70.) The reaction occurring upon blending lignite in aqueous sodium hydroxide proceeds by base-catalyzed guaiacol ether cleavage similar to the alkaline pulping of lignin. Humic acids have been isolated from brown coals by treatment with sodium in liquid ammonia [ 164], a reagent sufficiently basic to convert ethers to alkenes [165]. Benzyl ethers in lignin-derived structures may be sterically hindered, and thus high base concentrations and high shear rates may be important in breaking the lignite structure down to colloidal dimensions. Humic acid derivatives obtained by high speed blending have a weight average molecular weight of 4.3 x 105, lower by a factor of two than obtained by medium-speed blending, and lower by a factor of three compared to magnetic stirring [150]. The yield was highest after high-speed blending. The more severe blending breaks more crosslinks in the lignite structure, giving a humic acid yield that is higher but in fragments of lower molecular weight. Oxidation of Sheridan lignite with potassium permanganate forms, among many other acids, furan-, benzofuran-, dibenzofuran-, and xanthonecarboxylic acids, indicating the presence of some oxygen in heterocyclic ring structures [ 108]. Oxidation with sodium dichromate at 2.50~ for 36-40 h produced cyclic ethers having the following approximate relative abundances: benzofuran (20), 7%; isochroman (21), 2.5%; dibenzodioxane (22), 2%; xanthone (23), 17%; benzoxanthone (24), 1%; and dibenzofuran (25), 2.5% [52]. Some oxidation studies suggest that

118

20

21 O

23

22 O

24

25

the contribution of cyclic structures such as furan, 20, 23, and 25 is minor [50]. Reactions of Texas lignites with mixed carboxylic-sulfonic acid anhydrides indicate that few ether linkages occur [ 111]. 3.3.8 Ester groups Little seems to be known about the contribution of esters to lignite structure, although there seems to be a consensus that these groups are not of great importance. Reaction of Beulah lignite with sodium methoxide in methanol produced a waxy material with no spectroscopic evidence of incorporation of methoxide into the residue, indicating that no transesterification with methoxide had occurred. This observation suggests that esters are probably not an important component of the structure of lignite [150]. Some mono- esters and aromatic di-esters have been observed in BeulahZap lignite by field ionization mass spectrometry [ 147]. 3.3.9 Cellulose and lignin residues Plant lignin is first concentrated, and then modified, in the conversion to lignite [166]. Formation of methanol during pyrolysis indicates the presence of guaiacol units [3]. They may be present as plant lignin that has survived coalification as relatively unaltered lignin substructures, or as isolated guaiacols liberated by a decomposition process that did not affect the methoxy or side chain carbons. Since thermal alteration of lignins generally involves loss of methoxy and conversion of side chains to ketones or carboxylic acids, it seems more likely that relatively unaltered lignin is present. Guaiacol, 4-methylguaiacol, 6-methylguaiacol, 4-ethylguaiacol, and 4propylguaiacol have been identified as pyrolysis by-products in the gasification of lignite [167]. (Examples of several of these structures are shown in Chapter 2.) Lignin-like polymers are an important component of the structure of low-rank coals. Oxidation of low-rank coals with alkaline copper(II) oxide produced large quantities of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and 4-hydroxybenzenedicarboxylic acids [ 108]. Lignin

119 polymers are converted to lignin-like materials during early stages of coalification, and these ligninlike polymers become the foundation of the macromolecular structure of the lignite. Preferential concentration of these lignin-like polymers, along with phenolic materials and lipids, then results in the formation of peat and, eventually, lignite. Pyrolysis/mass spectrometry of Texas lignite shows lignin peaks diminished relative to modern biomass samples [102]. Similar experiments with lowmoor peats also show a diminution of lignin monomer peaks in the mass spectrum relative to modem biomass. Formation of p-hydroxybenzoic acid during lignite oxidation indicates the existence of lignin-like structures [50]. Oxidation of Sheridan lignite with alkaline copper(II) oxide produced large amounts of p-hydroxy- and 3,4-dihydroxybenzoic acids [ 168]. Both are known oxidation products of lignin. No o- or m-hydroxybenzoic acids were found. Conversion of the hydroxy acids to deutromethoxy ethers and subsequent mass spectral analysis showed that the phenolic acids derived from cleavage of alkyl ether structures via the reaction

ROCH 2@

O

R

'

[~ Hooc oH

rather than methoxyaryl ethers. No significant amounts of methoxyaryl ethers formed. Hydroxybenzene polycarboxylic acids and hydroxynaphthalene carboxylic acids in the products indicate that lignin-derived structures are more aromatic and more highly crosslinked in the lignite than in natural lignin. Taylerton (Saskatchewan) lignite exhibits thermal decomposition behavior (differential thermal analysis in a nitrogen atmosphere, heating at 6~

similar to that of a blend of 80%

Klason lignin and 20% cellulose [169]. The lignite has exothermic peak maxima around 300 ~and 455~ the cellulose:lignin mixture, around 320 ~and 420~ A lignite-like thermogram is also obtained from synthetic humic acids produced by the oxidative polymerization of hydroquinone with sodium peroxodisulfate [169]. Determination of methoxyl groups provides an estimate of the lignin content of lignite. A methoxyl content of 0.75% was determined for Beulah lignite by oxidation with peroxytrifluoroacetic acid, followed by hydrolysis of the methyl trifluoroacetate and gas chromatographic determination of methanol [ 170,171]. If an average structure for lignin equivalent to poly(coniferyl alcohol) were assumed, this result corresponds to a lignin content of 5% in this lignite sample [ 170,171 ]. For various lignites the calculated lignin concentration ranged between 5% and 10% [102]. These same lignites contained only traces of cellulose residues. The results again substantiate the concept that cellulose structures break down early in coalification whereas lignin endures for longer times. Decomposition of the partially coalified lignin in reactions such as in liquefaction produces a mixture of phenols and aromatic hydrocarbons in ratio of about 1:1 [1721.

120 Few arguments have been raised in opposition to the presence of lignin residues in lignite. Degradation of Texas lignites with thioacetic acid and boron trifluoride etherate (Nimz's procedure) produced no monomeric phenols typical of lignin degradation products [ 111 ]. This conclusion is based on a search for characteristic methyl ether and aromatic ring protons in the 1H NMR spectrum and methyl ether resonances in the 13C NMR spectrum of the products. These negative results suggest that polymeric phenolic linkages typical of lignin are not present in the Texas lignites; however, no palynologicat or petrographic characterization of the samples was published. The 180/160 ratios of New Zealand lignite suggest that the major source of oxygen in the lignite was cellulose, and that lignin and plant resins provided insignificant contributions [173]. The fibrous lignites of Central Europe are considered to contain relatively high proportions of cellulose [174], although that view has been challenged with the argument that the fibrous structure in fact represents lignin [149]. The presence of appreciable lignin residues in fibrous lignite increases the amount of phenols and pitch in the carbonization tars [149]. The dark color of euulminite A in Saskatchewan lignites may indicate the presence of residues from cellulose decomposition [31]. These products disappear early in coalification of the Canadian lignites [31]. Treatment with a solution of cadmium oxide in aqueous ethylenediamine (cadoxen) effectively extracted cellulose from lignite [175]. Extraction with cadoxen gave a dark humic material from which small amounts (e.g., 0.01%) of glucose and other monosaccharides were obtained by hydrolysis. Quantitative extraction could not be obtained, but the results probably give an order-of-magnitude estimate of the cellulose content of North Dakota lignites. The method applied to a sample from a coalified tree stump from the Beulah mine showed a cellulose content of 0.01% [98,176], despite the fact that the sample retained remarkable physical similarity to a modern tree stump. This observation is consistent with the idea that cellulose is destroyed early in the coalification process. Monosaccharides from the hydrolysis of cadoxen-extracted polysaccharides in Beulah lignite, expressed as ratios to glucose, are glucose, 1.0; galactose, 0.5; xylose, 0.2; arabinose, 0.03; and ribose, 0.02 [175]. These were determined by gas chromatography of the alditol acetates, formed by reduction with sodium borohydride and subsequent esterification with acetic anhydride [175]. Proof that the isolated glucose derives from cellulose was obtained by methylation of the cadoxen-extracted material. The O-methylated material subsequently was hydrolyzed with dilute sulfuric acid and the hydrolysis products were converted to alditol acetate derivatives. GC/MS of the alditol acetates showed that the product was 2,3,6-tri-O-methylglucitol triacetate, identical to the product obtained from the same sequence of reactions applied to cellulose [11]. That no tetra-O-methylglucitol diacetate was observed implies that the glucose chains in cellulose must be very long. The cellulose contents of lithotypes of Beulah lignite were determined by the cadoxen method: vitrain, 0.003%; attritus, 0.015%; fusain, 0.014%, on an as-received basis [98]; and, on an maf basis, are vitrain, 0.005%; attritus, 0.025%; and fusain, 0.023% [3,175]. The cellulose content of a composite (i.e., not separated into lithotypes) sample of the same lignite was 0.003%

121 (as-received) [98]. The dominance of vitrain is probably responsible for the value for the "whole" sample [ 170]. These very low values of cellulose content show that whatever chemical differences exist among the lithotypes must be due to structures other than cellulose residues. Cellulose has long been thought to be the more easily degraded component of wood [177]. Low amounts of cellulose in North Dakota lignites are consistent with the hypothesis that cellulose was broken down by microbiological oxidation at an early stage of coalification. The higher values for attritus and fusain suggest some interruption of the biochemical degradation process. Although the extraction of cellulose with cadoxen may be incomplete, and thus the absolute numbers are questionable, the relative rankings of the lithotypes may be useful. The existence of some low-rank coals with high cellulose contents may be indicative of alternative, anaerobic coalification processes [178]. (Lignin and cellulose contents in Polish lignites have been reported to be as high as 61% and 44%, respectively [179]; these results seem remarkably high in light of the work reported for North American lignites.) The nutrient supply in the early stages of coalification may be a factor in determining the extent to which cellulose is degraded. In low-moor peat samples, low cellulose levels result from more extensive microbiological degradations permitted by the higher nutrient levels [ 180]. 3.3.10 Nitrogen groups Small quantities of porphyrins can be isolated from lignites. Extraction of Canakkale-~an (Turkey) lignite with methanol and sulfuric acid, followed by ultraviolet and mass spectral analysis of the extract, showed the presence of iron, gallium, and manganese metalloporphyrins [ 181]. The iron porphyrins were etioporphyrin, carboxyetioporphyrin, and dicarboxyetioporphyrin, with 28-30 carbon atoms. Isolation of tetrapyrrole pigments is accomplished by extraction with methanesulfonic acid at 100~ neutralization with sodium carbonate, and re-extraction with ether. Texas, Montana, and Baukol-Noonan lignites contained these pigments at a concentration of 0.005 ~tg per gram [ 182]. Hagel (North Dakota) lignite contained 0.01 ~tg/g. Examination of 42 coals of all ranks showed that the highest concentration of these nitrogen compounds appears be in the hvB bituminous coals; however, the decreased yield in the lower rank coals may reflect a lower extraction efficiency resulting from the inability of the solvent to swell the coal. Extraction of a Mississippi lignite with 7% sulfuric acid in methanol at room temperature for 36 h, followed by separation of products by thin layer chromatography identified an iron complex of a dicarboxyetioporphyrin, C36I--~N404Fe, possibly mesoheme [183]. Pyridine and pyrrole tings, as well as nitrogen-containing aliphatic linkages, are minor contributors to the structure of lignite, as concluded from oxidation studies [50]. In some cases the presence of pyrrole rings could not be confirmed [50]. Oxidation of Sheridan lignite with aqueous sodium dichromate at 250~ for 36--40 h produced pyridinecarboxylic acids in relative abundance of about 2% [52]. In comparison, the relative yield of all acids with heterocyclic oxygen was about 32%. No evidence was obtained for pyrrole ring systems, nor for aliphatic amines, by oxidative

122 degradation of Wyoming lignite [49]. This work used six different oxidizing agents. Humic acids isolated from a Spanish lignite produced small amounts of thirteen amino acids upon hydrolysis in 6 N HCI [184]. 3.3.11 Sulfur groups Ultimate analyses of lignites from the Center, Falkirk, and Glenharold mines (Hagel seam) have shown an inverse relationship between sulfur and nitrogen [44]. This observation agrees with an earlier proposition that as the available nitrogen is biochemically fixed, the fixation of sulfur by microorganisms decreases [185]. Asphaltenes and preasphaltenes from liquefaction of coals of different ranks show a dependence of the sulfur content of these heavy products on rank [ 186]. The results suggest that sulfur is more tightly bound to the coal structure in higher rank coals. Sulfur forms determined in Beulah lignite by X-ray absorption fine structure (XAFS) spectroscopy are shown in Table 3.19 [187]. The sulfur forms in a Texas lignite and some of TABLE 3.19. Forms of sulfur in Beulah lignite, determined by XAFS [187].

Total sulfur,weight percent Sulfur forms, percent of total Pyrite Sulfide Thiophene Sulfoxide Sulfone Sulfate

Sample 1 (fresh) 0.80

Sample 2 0.80

29 28 30 2 0 12

37 24 30 2 0 7

its macerals, also determined by XAFS, are shown in Table 3.20 [187]. TABLE 3.20 Forms of sulfur in a Texas lignite and some of its macerals, determined by XAFS [187]. "Whole lignite" Sulfur forms, percent of total Pyrite Sulfide Thiophene Sulfoxide Sulfone Sulfate

49 18 30 0 1 2

Sporinite 46 54 0 1 0

Vitrinite 12 28 55 0 4 0

Inertinite 73 10 14 0 3 1

The data in Table 3.19 can be compared with the results of a programmed-temperature oxidation of

123 the same lignite, which indicated that, as a percent of the total sulfur (which itself amounted to 0.80%), pyrite is 31%, aromatic sulfur (thiophenic structures, aryl sulfides, and aryl sulfones) 20%, non-aromatic sulfur 34%, and sulfate not detected [ 143]. Organic sulfur species in Alcoa (Texas) lignite were examined by flash pyrolysis [188]. This lignite contained 1.30% total sulfur, with 0.73% organic sulfur. The observed distribution of sulfur types, expressed as a percentage of the organic sulfur, was 82% aliphatic sulfides and mercaptans, <1% aromatic sulfides and mercaptans, and 18% stable structures such as thiophenic compounds [ 188]. Thioethers represent up to 76% of the organic sulfur in Central European lowrank coals [ 189]. A general estimate is that 40-50% of the total organic sulfur in lignites is present in aliphatic sulfide structures [89]. Thiophene moieties, as well as sulfur groups in aliphatic chains, are not important contributors to the structure of lignite, on the basis of oxidation studies [50]. Thiophenic structures amount to about 24% of the organic sulfur in the low-rank coals of Central Europe [ 189]. A rule-ofthumb estimate for lignites suggests that about 30% of the total organic sulfur is present in thiophenic structures [89]. In contrast to the oxidation results [50], X-ray absorption near edge spectroscopy (XANES) indicates that 46 mol% of the sulfur in Beulah-Zap lignite is present in thiophenic structures [190]. X-ray photoelectron spectroscopy (XPS) of the sulfur 2p binding energies indicates that the environment of organic sulfur in a Polish brown coal is heterocyclic (binding energy of 163.3 eV) [191]. XPS of Beulah-Zap lignite indicates that 55 mol% of the sulfur in Beulah-Zap lignite is in thiophenic structures, which seems reasonable agreement with the XANES results, but is much higher than results from XAFS [ 190]. 3.3.12 Chlorine The chlorine content of most North American lignites is _<0.1%, which is considered to be negligible in terms of any impact on processing of the lignite [192]. XAFS examination of BeulahZap lignite indicated a chlorine content of 0.04%, which is possibly present as chlorine atoms bound to aromatic rings [ 193], though it may be premature to attach significance to these results. Most coals of higher rank examined by XAFS contain chlorine as aqueous chloride ion in microcracks in the coal particles [ 193]. 3.4 T H E T H R E E - D I M E N S I O N A L

S T R U C T U R E OF L I G N I T E

This section discusses aspects of the three-dimensional structure of lignites, particularly the crosslinking to form a macromolecular network, swelling of the structure, and molecules apparently trapped inside the network. 3.4.1 Evidence for a colloidal structure Several lines of evidence have been adduced to support the concept of a colloidal structure for lignite. Colloids carry an electric charge; passage of 110 v through an aqueous suspension of

124 lignite causes immediate flocculation [ 194]. Lignite is peptized by treatment with aqueous alkali. This property is shared with subbituminous coals but not with bituminous coals or anthracites [ 194]. The adsorption behavior of water vapor and other gases onto lignites is typical of a colloid. In this regard, lignite shows a strong similarity of behavior to silica gel. 3.4.2 Crosslinking in lignite structures The glass transition temperature, Tg, of Titus lignite is 580 K, as indicated by differential scanning calorimetry [195]. Swelling this lignite with pyridine caused a very large decrease of T g in the early portion of the swelling. With a pyridine uptake of 0.265 g per gram of lignite (48% of the pyridine uptake achieved at equilibrium), the glass transition temperature dropped to 498 K. When equilibrium had been achieved, T g had decreased to 423 K. This dependence of Tg on the weight fraction of the solvent (i.e., pyridine) is similar to the effects observed with glassy crosslinked polymers. Field ionization mass spectrometry of Beulah-Zap lignite showed a very low abundance of signals above m/z = 200 [196], indicative of a very highly crosslinked macromolecular structure of this lignite. Oxidation of Texas lignite with ruthenium tetroxide produces twenty aliphatic tri- and tetracarboxylic acids [63]. One source of such acids could be the oxidation of structures of the type

~

~/~----CH 2 ~CH

---CH 2

(as discussed previously on page 103) which, in this example, would form propane-l,2,3tricarboxylic acid. The large number of tri- and tetraacids formed implies that these branching or crosslinking points must exist in a variety of configurations. This work suggested the existence of one to two crosslinks per 100 carbon atoms [63]. Transalkylation

of coal using

1,1-diphenylethane

in toluene solution,

with

trifluoromethanesulfonic acid as catalyst, converts the methine carbon -CH- crosslinking site to tritolylmethane (26) [ 197]. The number of methine carbons was 0.38 per 100 carbon atoms in Hagel, and 0.104 in a Wilcox Group lignite [197]. In comparison, the value for a Pittsburgh seam (Pennsylvania) bituminous coal was 0.0076 [197]. The number of crosslink sites correlates roughly with solubility of the coal in tetrahydrofuran. Assuming a molecular weight of 130 for a hypothetical repeating unit in the structure (the molecular weight of naphthalene is 128 and of propylbenzene, 120), the number of repeating units between crosslinks was about 10, ranging from 9.7 for Titus to 10.8 for Darco (Texas) lignite,

125

26

CH3

based on pyridine swelling studies [198]. The average molecular weight between crosslinks for these two lignites was 1261 and 1404, respectively; for lignite from the Titus A Pit seam, 1313 [198]. These values were corrected for pore volume, mineral matter, and adsorbed pyridine. In contrast, the average molecular weight between crosslinks in Hagel lignite was estimated to be 70 on the basis of methanol sorption [ 199]. A structure with molecular weight in this range is --CH2--CH2--O--CH2--CH2-- (molecular weight 72). Solvent-swelling ratios measured in pyridine are Indian Head, 1.6; Gascoyne (Blue pit), 1.3; and Center, 1.3 [154]. Values of 2.1-2.2 were observed for three Texas lignites at 35 ~ [198]. The swelling ratios observed for the Texas lignites increased with temperature, so that at at 80 ~the values for the same lignites were in the range 3.2-3.5 [198]. This behavior with temperature indicates that calculated values of the number of repeat units between crosslinks and the average molecular weight between crosslinks both increase with temperature. Possibly at the higher temperature pyridine vapor more effectively breaks down the hydrogen-bonded structure of the lignite [ 198]. Solvent-swelling ratios of Big Brown lignite were measured perpendicular and parallel to the bedding plane for three solvents [200]. These ratios, perpendicular-to-parallel, are 1.08 in chlorobenzene, 1.07 in tetrahydrofuran, and 1.14 in pyridine. Thus the lignites, as is also true for coals of higher rank, are anisotropic. The anisotropy reflects geological pressures exerted during coalification. Because anisotropy observed by solvent swelling is a bulk effect, it must arise from molecular structures that have a greater bonding density in the bedding plane rather than perpendicular to the plane. One configuration that fulfills this requirement is disk-shaped micelles more strongly bonded around the edges than at the surfaces. Anisotropic swelling must reflect anisotropic distribution of crosslinks. Furthermore, if the crosslink density is anisotropic, it is reasonable to assume that mechanical properties will also be anisotropic. In the absence of gross heterogeneity and any major faults such as cracks or cleats, this lignite should therefore be stronger in the direction of the bedding plane than perpendicular to it. Chlorobenzene is not capable of breaking hydrogen bonds. On the other hand, pyridine probably breaks most of the hydrogen bonds in the lignite. Since the perpendicular-to-parallel swelling ratio is highest in pyridine and lowest in chlorobenzene, the distribution of covalent bonds must be highly anisotropic [200] because the anisotropy is greatest in pyridine, where the effects of

126 hydrogen bonding are least. It follows further that the density of hydrogen bonds must be greater perpendicular rather than parallel to the bedding plane. Hydrogen bonding of water to carboxyl or phenolic groups establishes a supramolecular structure in which the polymeric network has been expanded or plasticized by the water molecules [201]. Evidence for hydrogen bonding in Beinfait lignite has been obtained by FFIR [113]. Carboxyl groups might also participate in crosslinking if polyvalent metal cations are present as the counterions, in structures of the type -- COO--- M+2-- COO- [89]. Spin lattice relaxation times, determined by pulsed electron paramagnetic resonance, suggest that Beulah-Zap lignite has a more rigid structure, in terms of molecular flexing of the network, than bituminous coals [202]. A Wyodak subbituminous coal is similar to the lignite. 3.4.3 Molecules trapped in the lignite structure This subsection discusses some of the species that can be removed from lignite by mild solvent extraction. If extractions are carried out at temperatures below a point at which thermally induced cleavage of covalent bonds would occur, it can be presumed that the compounds isolated were trapped in the macromolecular network of the lignite, and were not part of the macromolecular structure itself. In a sense they represent a separate phase from the network. The isolation of waxes from lignite by mild solvent extraction has been carried out commercially; for that reason, a discussion of wax extraction is deferred to Chapter 12. The yields of extract from the Soxhlet extraction of vacuum-dried Indian Head lignite with various solvents are shown in Table 3.21 [203-205]. TABLE 3.21 Soxhlet extract yields from vacuum-dried Indian Head lignite [203-205]. Solvent Benzene Cyclohexane Heptane Hexamethyleneimine Hexane Methanol Octane Pentane Pyridine

Yield, % 0.75 0.51 0.21 27.0 0.17 1.5 0.86 0.26 8.3

Reference [203] [204] [205] [205] [205] [204] [203] [203] [203]

It was not determined whether the remarkable extraction yield achieved with hexamethyleneimine is an artifact reflecting incorporation of the solvent in the extract, or is a preliminary indication that this compound could be an outstanding solvent for the extraction of lignites. Beulah and Big Brown lignites were examined by sequential Soxhlet extraction, beginning with chloroform [206]. Each extract was divided into hexane-soluble and hexane-insoluble

127 fractions. The total chloroform-soluble material was 2.8% of the Beulah and 3.7% of Big Brown (maf basis). The hexane-soluble portions of the chloroform extracts represented 1.0 and 1.7% of the lignites, respectively. For Beulah lignite, the hexane-solubles are 36% of the chloroform extract, and for Big Brown lignite, 46%. The residue from the chloroform extraction was then further extracted with a chloroform:acetone:methanol azeotrope (in proportions 47:30:23). The azeotrope-soluble material was 1.6% of the Beulah residue and 2.4% of the Big Brown. The hexane-soluble portion of the azeotrope extract amounted to 0.13% of the Beulah and 0.19% of the Big Brown residues. Expressing the hexane-solubles as a percentage of the total azeotrope extract shows remarkable agreement: 8.1% for Beulah and 7.9% for Big Brown. Among the compounds identified in the extracts were sesquiterpenes tentatively assigned a bicyclic C 10 structure with five substituent carbon atoms. Compounds identified by capillary GC analyses of the extracts of Beulah lignite are listed in Table 3.22 [206]. TABLE 3.22 Compounds identified in extracts of Beulah lignite [206]. Compound GC Area Percent Pristane 0.15 n-Alkanes C-14 0.82 C-19 0.74 C-20 0.99 C-21 1.38 C-22 1.22 C-23 0.69 C-24 0.67 C-25 1.89 C-26 1.55 C-27 2.92 C-28 0.42 C-29 0.76 C-30 0.57 C-31 0.29

Compound GC Area Percent Cadalene 0.31 Alicyclic terpenoids C15H26 c 1.9 C15H26 d 1.12 C15H26 e 0.24 C15H26 g 2.3 C15H26 h 0.10 C20H36 tricyclic alkane 1.7 C17H30 tricyclic alkene 1.2 C33H58 0.07

Remarkable differences were found between the extracts of the two lignites for the terpenoid and related compounds. None of the C 15H26 sesquiterpenes, cadalene, the C20H36, nor C17H30 were found in the extract from Big Brown lignite. However, compounds that were found in the Big Brown extract included C31H54 and C32H56 hydrocarbons (3.3 and 1.1 area %, respectively) and an unidentified compound of retention time intermediate between C17H30 and C29H50 (0.66 area %). In addition, the C 33H58hydrocarbon occurred in the Big Brown extract at 3.5 area % (compared with 0.07% for the Beulah extract). Decker lignite was extracted with refluxing 3:1 benzene:methanol for 24-48 h [51,54]. The products were analyzed by GC/MS and MS, after separation into fractions by alumina column

128 chromatography. The major components of the hydrocarbon fraction were sesqui- and diterpenoids. Three compounds, eudalene, cadalene, and simonellite, indicate a significant input of coniferous plant material. Eudalene and cadalene have been isolated from Siberian conifers, and simonellite has been extracted from lignite of coniferous origin [51]. Diterpenoids found in this extract have been isolated from conifer resins [207,208]. (A German lignite extracted with benzeneethanol yielded 5-7.5% "bitumen" that consisted mainly of resins [ 149].) On a relative basis (with the predominant compound(s) in the gas chromatogram set equal to 100), the compounds identified were sesquiterpenoids, 75.1; diterpenoids, 100.0; indan/tetralin derivatives, 44.1; and naphthalene derivatives, 6.4 [54]. Compounds observed included C 15Hx, where x ranges from 28 to 30, and three C 10H2z-- derivatives of tetralin [54]. Other types of compounds identified included steranes, ~,-pyrones, triterpenoids, and oxygen-containing triterpenoids [51]. The benzene:methanol extract contained 67% of waxes containing 24-32 carbon atoms [51]. Benzene:methanol (40:60) extracts of Beulah-Zap lignite contained carboxylic acids as large as (:33, as well as a series of esters [209]. Field ionization mass spectrometry of Beulah-Zap lignite identified fatty acids and esters having m/z of 368, 396, 424, 452, and 480, which are typical of plant-derived compounds and exist in the lignite as extractable biomarkers [ 196]. Extraction of North Dakota lignite sporinite, using 3:1 benzene:methanol, yielded C 12-C30 aliphatic monocarboxylic acids as major products [57]. Carbon chains of even numbers of carbon atoms predominated; the maximum concentration was observed at C16, with a secondary maximum at C28. Rockdale lignite provided a 1.3% extract yield in hexane [73]. After methylating the extract with dimethylformamide dimethyl acetal in pyridine, gas chromatographic analysis indicated that the principal components of the extract were aliphatic carboxylic acids in the range C 24-C34. Acids in the ranges C13-C23 and C35-C36 were very minor components. The even-numbered carbon chains predominated over the odd-numbered chains. These characteristics resemble those of modern plant waxes. The total acids represented 7% of the weight of the hexane extract. Other prominent components were the C23-C33hydrocarbons. Three substituted aromatic dicarboxylic acids were observed in the gas chromatogram, but were not fully identified. Soxhlet extraction of Big Brown lignite with tetrahydrofuran provided a 7.2% yield of extract [210]. The major components included cyclic aliphatic compounds such as mortanes and hopanes, i.e., hydroaromatic isoprenes. Polycyclic aromatic compounds were not important components of the extract. Sheridan lignite was treated with 5% hydrochloric acid to release any organic acids associated with minerals, and then extracted with 2.5% sodium hydroxide solution at 35~ for 16 h [211]. The yield of acids from this experiment was 2.6%; yields under the same conditions for a bituminous coal and an anthracite were 0.23% and zero, respectively. The acids were identified by GC-MS and high resolution MS of their methyl esters; 36 acids were identified, ranging from succinic and glutaric through methoxymethyldibenzofurancarboxylic acid. Comparison of the acids

129 extracted from the lignite with similar extractions of shale, lignin, peat, and humic acids showed the following [211]: No benzenecarboxylic acids with C3 side chains were isolated from the lignite, but are common in extracts of the other materials. Benzene di- and tricarboxylic acids are abundant in the lignite extract, but not in extracts of the other materials. Phenolic acids extracted from the lignite contain a single hydroxy group and no methoxy groups, whereas the other materials yield acids with two or more hydroxy or methoxy groups. Two diterpenoid acids, dehydroabietic acid and its methyl homolog, presumably derive from abietic acid, which is a common constituent of the resins of conifers [211]. These acids may be the precursors of diterpenoid hydrocarbons isolated from benzene-methanol extracts of the lignite. Furan, benzofuran, and dibenzofuran acids extracted from the lignite may derive from two possible sources: from furoguaiacin and pinoresinol lignins in wood, or from degradation of dibenzofuran compounds formed from condensation reactions of phenols during coalification [211]. The acids trapped in this lignite are similar to acids produced during oxidation with alkaline cupric oxide [168], suggesting that the trapped acids are oxidation products from the degradation of ligninderived materials during coalification. Extraction of Boundary Dam and Coronach lignites (both Saskatchewan) with pentane, toluene, and tetrahydrofuran show removal of up to 20% of the organic substance, but no change in aromaticity (as inferred from measurements of mean random reflectance) of the extracted coal [212]. 3.4.4 Depolymerization reactions Phenol softens lignite [213]. Treatment with phenol and boron trifluoride at 100~ results in depolymerization via cleavage of aliphatic-aromatic carbon-carbon bonds and exchange of the aromatic structures with phenol. Depolymerization was substantially more effective for Velva (North Dakota) lignite than for five other coals of higher rank [214], based on the total of the yields of benzene-, benzene/methanol-, and phenol-soluble fractions produced in the reaction. This sum for Velva lignite was 75.2%. By contrast, comparable data for other coals studied ranged from 47.4% for Ireland (West Virginia) high volatile bituminous to 9.8% for Itmann (West Virginia) low volatile bituminous. A blank extract with phenol showed only 2.4% solubility, lower than all other coals except the Itmann lvb. These results were interpreted in terms of a structure having small aromatic units extensively crosslinked with bridging groups to result in a rigid polymer [214]. A rigid, highly crosslinked polymer should give a low extraction yield in phenol. Extensive depolymerization in phenol-boron trifluoride indicates both a large number of bridges and of their aliphatic character. Petrographic analysis of the residue after solvent extraction of the phenol-boron trifluoride reaction product showed 98% (semifusinite + micrinite) and 2% fusinite. The vitrinite and exinites had been completely depolymerized. Depolymerization of Beulah lignite with phenol, catalyzed by p-toluenesulfonic acid, was used to obtain structural information on the methanol-soluble and -insoluble products [215]. The results were not unequivocal, because of the incorporation of some phenol into the lignite

130 a structure characterized by high phenolic or hydroxy OH and low carbonyl content, with a predominantly aromatic carbon framework. Depolymerization decreases the oxygen content, attributed to dehydration of phenolic structures. Long-chain aliphatics, alkenes, or carbonyls were not detected in the 13C NMR spectra of the extracts. REFERENCES

10 11 12 13 14 15 16 17 18 19 20

D. White and R. Thiessen, The origin of coal, U.S. Bur. Mines Bull. 38, 1913. H.H. Schobert, F.R. Karner, D.R. Kleesattel, and E.S. Olson, Characterization of the lithologic layers of North Dakota lignites, Proceedings 1985 International Conference on Coal Science, pp. 608-611. H.H. Schobert, F.R. Karner, E.S. Olson, D.R. Kleesattel,and C.J. Zygarlicke, New approaches to the characterization of lignites: A combined geological and chemical study, in: A. Volborth (Ed.), Coal Science and Chemistry, Elsevier, Amsterdam, 1987, pp. 355380. S.F. Ross and D.R. Kleesattel, Pyrolysis and devolatilization, University of North Dakota Energy Research Center monthly report, February 1986. J.P. Hurley, D.R. Kleesattel, and E.N. Steadman, Distribution of inorganics, University of North Dakota Energy Research Center monthly report, September, 1985. F.R. Karner, J.P. Hurley, D.R. Kleesattel, E.N. Steadman, and C.J. Zygarlicke, Distribution of inorganics, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1846, (1986), pp. 15-1 - 15-17. D.R. Kleesattel, Distribution, abundance, and maceral content of the lithotypes in the Beulah-Zap bed of North Dakota, Proc. Rocky Mtn. Coal Symp., U.S. Dept. Energy Rept. CONF-841048, (1984). F.R. Karner and J.P. Hurley, Distribution of inorganics, in: G.A. Wiltsee (Ed.), Lowrank Coal Research, U.S. Dept. of Energy Rept. DOE/UNDERC/QTR-85/2, (1986), pp. 15-1 - 15-29. International Committee for Coal Petrology, Handbook of Coal Petrology, Centre National de la Recherche Scientifique, Paris, 1975. D.R. Kleesattel, Petrology of the Beulah-Zap lignite bed, Sentinel Butte Formation (Paleocene), Mercer County, North Dakota, M.A. Thesis, University of North Dakota, Grand Forks, ND, 1985. E.S. Olson, Organic structure, University of North Dakota Energy Research Center monthly report, January 1986. R. Thiessen and G.C. Sprunk, Microscopic and petrographic studies of certain American coals, U.S. Bur. Mines Tech. Paper 564, (1935). F.T.C. Ting, Petrographic and chemical properties of selected North Dakota lignite, in: F.T.C. Ting (Ed.), Depositional Environments of the Lignite-bearing Strata in Western North Dakota, N.D. Geol. Surv. Misc. Set. 50, (1972). J.M. Schopf, Field description and sampling of coal beds, U.S. Geol. Surv. Bull. 111 lB, (1960). B.C. Parks, Petrography of American lignites, Econ. Geol. 46 (1951) 23-50. R. Thiessen, G.C. Sprunk, and H.J. O'Donnell, Preparation of thin sections of coal, U.S. Bur. Mines Inf. Circ. 7021, (1938). S.A. Benson, J.P. Hurley, and E.N. Steadman, Distribution of inorganics, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1574, (1984), pp. 15-1- 15-27. R.R. Dutcher (Ed.), Field Description of Coal, American Society for Testing and Materials, Philadelphia, 1976. S.A. Benson, J.P. Hurley, and E.N. Steadman, Distribution of inorganics, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984), pp. 15-1 - 15-15. K.S. Groon, Peak temperatures of argon endotherms, Unpublished report, University of North Dakota Energy Research Center, August 1983.

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