Petrographic and geophysical assessment of coal quality as related to briquetting: the Miocene lignite of the Lower Rhine Basin, Germany

International Journal of Coal Geology 60 (2004) 17 – 41 www.elsevier.com/locate/ijcoalgeo

Petrographic and geophysical assessment of coal quality as related to briquetting: the Miocene lignite of the Lower Rhine Basin, Germany J. Naeth a,1, S.C. Asmus b,2, R. Littke a,* a

Institute of Geology and Geochemistry of Petroleum and Coal, Aachen University (RWTH), Lochnerstr. 4-20, D-52056 Aachen, Germany b RWE Rheinbraun AG, Bereich Tagebaue, Abteilung Markscheidewesen und Lagersta¨tte, Stu¨ttgenweg 2, D-50935 Cologne, Germany Received 15 January 2004; accepted 16 April 2004 Available online 20 July 2004

Abstract In the Lower Rhine Basin, Germany, Tertiary lignites are mined primarily for electrical power generation and briquetting purposes. Until 2000, when the mine was closed, coals from the Bergheim open pit mine were used for briquetting. After 2000, briquettes were produced from the Hambach open pit mine but did not show the same quality and hardness as those from the Bergheim pit. In this context, the macropetrographic and microlithotype composition of the Hambach lignites was studied in detail. Samples were taken from the three profiles, S1, S2 and S3, and a well, R1. All of the samples were macropetrographically described, analyzed in their microlithotype composition, and coals from profile S3 were analyzed for their briquetting compression strength. Three horizons of high gelite content were detected in the Main Seam, two of them with negligible thickness and one with a thickness of up to 1 m. Briquettes produced from these horizons are characterized by significantly reduced hardness. By careful interpretation of subtle changes in geophysical log data, it was possible to map the occurrence of these gelite horizons. As a consequence of these studies, an excavation plan was set up by the RWE Rheinbraun to avoiding mining of critical strata for briquetting purposes. D 2004 Elsevier B.V. All rights reserved. Keywords: Coal facies; Coal petrography; Gelite; Briquetting; Hambach Mine; Lower Rhine Basin; Log facies

1. Introduction * Corresponding author. Tel.: +49-241-805748; fax: +49-2418092152. E-mail addresses: [email protected] (J. Naeth), [email protected] (S.C. Asmus), [email protected] (R. Littke). 1 Present address: Robertson Research International, Tyn-YCoed Site, 583416 Llanrhos, Llandudno, North Wales, LL30 1SA, UK. Fax: +49-331-288-1782. 2 Fax: +49-221/480-22784. 0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2004.04.002

1.1. Objectives In the large open pit mines of the Lower Rhine Basin, lignites are mined mainly for electrical power generation although 2 Mt of lignite per year are processed into briquettes (RWE, 2000/2001). From 1988 to 2000, coal for this purpose came from the Bergheim opencast mine, close to Cologne. This mine

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was closed at the end of 2000. In 2001, briquetting of lignites from the nearby open pit mine of Hambach began. Initially, this was problematic as the briquettes did not show the expected hardness. In this context, it was our objective to study the macro- and micropetrographic composition of the lignites and their influence on briquetting to define an excavation plan that would avoid mining of strata unsuited for briquetting. In particular, the approach was to: (i) quantify the occurrence of the lithotype gelite, (ii) map the different organic facies within the large outcrop and (iii) evaluate geophysical measurements in order to find out if gelified horizons can be identified in the logs. 1.2. Geological background The Lower Rhine Basin is a large northwest – southeast trending sedimentary basin which separates the northern parts of the Variscian western and eastern Rhenish Massif (Fig. 1) (Walter, 1992). This subsidence trough is characterized by steep normal faults, trending northwest – southeast. These faults separate

areas of different tectonic subsidence and temperature histories (Karg, 1998); sedimentary thickness can differ significantly in different structural units. The major structural units are the Ko¨lner Scholle in the east, the Rur-Scholle in the northwest, and the Erftand Venlo Scholle in the central part (Hager, 1993). The basement is composed of folded Devonian and Carboniferous rocks, including coal-bearing Upper Carboniferous (Pennsylvanian) strata (Littke et al., 1994). Mesozoic rocks are only present in the southwestern and northern part of the Lower Rhine Basin and unconformably overlie the older basement (Hager, 1993). The basin has been actively subsiding during the Tertiary and Quaternary; the sedimentary fill is shown in Fig. 2. During the Early Tertiary, most of the Lower Rhine Basin remained sediment-starved. Significant sedimentation only started in Middle Oligocene times (Fig. 2). Marine transgressions were most pronounced during the Late Oligocene and the facies became more continental after the Middle Miocene. Fluvial sedimentation predominated during the Pliocene, when thick gravels and sands were deposited by the palaeo-Rhine river.

Fig. 1. Location of the Hambach open pit mine and other lignite mines in the Lower Rhine Basin showing major structural elements.

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Fig. 2. Stratigraphy and lithology of Tertiary sediments in the Lower Rhine Basin.

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In the overall paralic environment, humid warm climatic conditions favoured peat growth in swamps and raised bogs (von der Brelie and Wolf, 1981a,b). In particular, during the Mid- and Late Miocene, thick peats accumulated in the southern part of the Lower Rhine Basin. These peats were later buried by a few hundred metres of younger clastic deposits (Fig. 3) and converted to lignite. The present-day thickness of the main lignite seam can reach up to 100 m, corresponding to an original peat thickness of almost 300 m (Hager, 1993). The lignite seams of the Lower Rhine Basin are commonly classified in three members. The Oligocene includes the Cologne member (Ko¨ln-Schichten), which has only thin seams. In the Miocene, the Ville member includes the Main Seam of the Lower Rhine Basin. The Main Seam can be subdivided into the Morken; Frimmersdorf a and b; and Garzweiler III, II and I seams from bottom to top (Fig. 2). The Frimmersdorf and Garzweiler seams are separated by the Neurather Sand. The uppermost group of seams is present in the Inden member (Indener Schichten), which is of latest Miocene age (Fig. 2), and can reach a maximum thickness of 35 m. During the Pliocene, the annual mean temperature dropped significantly and the peat production ceased (Gebka et al., 1999). The total thickness of the sedimentary sequence is quite variable (Fig. 3). In the deepest parts of the Lower Rhine Basin, 1200 m of Tertiary sediments and 100 m of Quaternary deposits accumulated. The average subsidence rates in these deepest parts of the basin are on the order of 0.1 mm/year. The climate of the Eocene was tropical with average yearly temperatures of about 21 jC decreasing to 15 jC in the Miocene (Schwarzbach, 1966; Lu¨cke et al., 1999). The deposition of the Main Seam is interpreted to have taken place at a temperature ranges between 9 and 28 jC (Utescher et al., 2000). The youngest Tertiary sequences are characterized by fluvial deposits from the Rhine and Maas rivers and loess deposits. 1.3. Macropetrograpic classification scheme Based on cuticle analysis, Schneider (1995) developed a classification distinguishing seven facies types in the East German Lausitz lignite mining area (Fig. 4). These are namely the Alnus-Liquidambar-alluvial forest-facies (F-facies), Conifer-forest-facies (K-fa-

cies), Angiosperm-forest-facies (A-facies), Glumifloren-facies (G-Facies), Pinus-Myricaceen-Palm Treefacies (P-facies), Marcoduria-facies (M-facies), and bright layer-facies (HB-facies) (Fig. 5). The F-facies is characterized by a dark, unstratified or poorly stratified, and partly xylitic coal at the seam base. Leaves from Alnus (alder) and Acer (maple) trees are present, as are the remains of ferns and numerous seeds of the species Dystylium. The presence of leaf fragments with dentate margins characterizes an eutrophic environment of deciduous alluvial forests. The K-facies comprises unstratified or poorly stratified coals and contains xylitic tissues of various sizes from Taxodiaceae, e.g. swamp cypress. The xylites can be divided into the fractured, fibred and structured parts of stumps, trunks and branches (Fig. 5g). In addition, remains of bark and roots, as well as leaf fragments from Taxodium and Glyptostrobus, can be found. Needles of Taxodiaceae may appear as resin conduits. Gelification of the matrix, numerous and large accumulations of precipitated humic substances, and gelified xylites are typical facies indicators, as are layers of fusite. The A-facies is a nearly unstratified coal, which is free of texture, and fractures into shaly polygonal fragments (Fig. 5b). It contains almost no xylites and the colour is lighter than that of the other facies types, with exception of the HB-facies. Characteristic plant remains are Querus (oak) leaves and other smooth edged leaves and crushed Querus, Magnoliaesperumum geinitzi, and Magnolia burseraca seeds. Crescent shaped and finely toothed needles and needles from Cathaya roselti are also present. The G-facies is horizontally well-stratified and very fissile coal consisting mainly of Glumiflores (grass) without the presence of xylites (Fig. 5e). The leaves are about 5 –10 mm in width, striated, black and glossy. The matrix of this coal is highly gelified and dark black when dried. Additionally, 3-mm-long Aldrovandia seeds and insect wing fragments are present. The presence of Pinus needles and Myrica crenata leaves indicates a mixed facies P/G. The P-facies is well to poorly stratified (Fig. 5c). Roots, barks with the typical Wisbar cells (a type of cork tissue), needles and their resin conduits, together with other fragments from Pinus (pine) are the main components of the facies. Additionally, fine structured and indented leaves from Myrica crenata, fragments

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Fig. 3. SW – NE trending profile through the Tertiary of Lower Rhine Basin. The Hambach open pit mine is situated in the Erft block close to the Rurrand fault system.

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Fig. 4. Coal facies related to plant communities in the Lausitzer brown coal (modified from Schneider, 1980).

from palm trees, Aldroviandia, xylites, and accumulations of resins are present. The M-facies is a poorly stratified coal and shows a sauerkraut-like structure based on the intertwined roots (Fig. 5d). Main indicators are roots and bark fragments, xylites and Sequoia stumps. Additionally, palm leaves, fragments from Sciadopitys (Japanese fir; Fig. 5d), and Arctostaphyloides fruit remains are also present. Due to the high number of roots, this coal is a dense unit with almost no gelified parts. The HB-facies is described as a bright unit which can be easily recognized in outcrops (Fig. 5f). Fusites are the only macroscopically recognisable parts of this

massive, unlayered coal. They are good marker horizons with an average thickness of about 0.1 – 0.2 m. The origin of this facies is discussed in detail by Heinhold (1909), Hagemann and Hollerbach (1980) and Dehmer (1988). The bright layers are most likely to be related to diminished bacterial activity in an acidic and oligotrophic environment during a high water table. 1.4. Microlithotype classification scheme Microlithotype analysis is the means of microscopically quantifying the associations of macerals within

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Fig. 5. View of the Hambach open pit mine (a) and macropetrographic facies types: (b) A-facies coal, (c) P-facies coal, (d) M-facies coal with Sciadopitys, (e) G-facies coal, (f) bright layer and (g) xylite.

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Table 1 Overview of microlithotypes used in this study (modified from Schneider, 1980) Microlithotype group

Microlithotype

Textite

eu-xylo-textite medio-xylo-textite gelo-xylo-textite Marcoduria-textite Peridermo-textite Phyllo-textite texto-detrite eu-detrite gelo-detrite texto-gelite detro-gelite eu-gelite

Detrite

Gelite

the coal seam layers (Taylor et al., 1998). Sontag et al. (1965) suggested a classification scheme of microlithotypes for lignites, which was used by Schneider (1980) for the Lausitzian lignites. The microlithotype classes are listed in Table 1 and are based on the major groups of microlithotypes, which are textite, detrite and gelite, with several subgroups. The three groups are shown in Fig. 6 in their typical appearance under the microscope.

Textites are characterized by a telinitic appearance (Fig. 7a– d). They consist of more or less distinct, though often deformed, cell walls preserved from the initial plant material. The cells can be filled, partly or completely, with primary, precipitated humic acids or secondary, migrated humic substances. Fine structures in the cell walls are still recognisable and the topography of these elements enables a distinction between wood, bark, leaf and root fragments leading to the microlithotypes xylo-textite (wood fragments), peridermo-textite (bark fragments), marcoduria-textite (root fragments) and phyllo-textite (leaf fragments). Within the xylo-textites, which consist mostly of Conifer and palm fragments, a distinction based on the rate of gelification is possible. Non-gelified xylites are labelled eu-xylo-textites, partly gelified parts medio-xylo-textites and completely gelified xylites gelo-xylo-textites. Marcoduria-textite originates from the primary roots of conifers (Fig. 7d). The most striking characteristic of Marcoduria root tissue is their thickwalled, zigzag-shaped tissue septa which separate the poorly preserved outer cell layers of the rhizodermis to form a scaffolding-like tissue structure. Only the spindle-shaped transversal sections, not larger than 5

Fig. 6. Texture of different microlithotypes at a magnification of 600  (modified from Schneider, 1980).

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Fig. 7. Microlithotype documentation of different microlithotypes (see Schneider, 1995): (a) eu-xylo-textite, (b) gelo-xylo-textite, (c) peridermotextite, (d) marcoduria-textite, (e) eu-detrite, (f) gelo-detrite, (g) eu-gelite and (h) texto-gelite.

Am, of adjoining cell edges are recognizable (Schneider, 1995). However, in many cases the lumen of the ‘‘scaffold’’ is filled with infiltrated gelite.

Peridermo-textite is indicated by cork tissue (Fig. 7c). Each cell of these stack-like aggregates consists of a very thin cell wall that encloses corpohuminite. Phyllo-textite originates from leaf tissues which are

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easily identified by their cuticules. The partly lignified epidermal cell walls are either ungelified or transformed to textinite B during diagenesis. The cells are commonly filled with corpohuminite. The stomatal opening can be clearly recognized regardless of its sectional cut during sample preparation and, thus, Phyllo-textite can be attributed to a Conifer origin. The cells of Gymnosperms and Pinus are recognized by the shape of the guard cells and can be detected by cuticular analysis. Detrites do not show coherent plant structures, except for inclusions of palynomorphs and fungal remains (Fig. 7e– f). Biological structures are almost completely destroyed. The presence of small structures lead to the texto-detrite group, the transitional microlithotype between textites and detrites. Eu- and gelo-detrites do not show any structure, but can be distinguished by the overall amount of gelification. The gelite group originates from compaction and/ or filling of cells with humic substances (Fig. 7g,h). Under the microscope, gelites do not possess a porous structure. Dehydration processes cause a shrinkage of volume and lead to sharp-edged fractures which is also characteristic to this microlithotype. Gelites with recognisable cell structures are called texto-gelites, and have textite as a precursor. Accordingly, a detrital structure is visible in detro-gelites. Eu-gelites are newly precipitated humic substances in pores, fractures and cavities without any structure.

2. Methods Samples for this study were taken from the Hambach open pit mine of the mining company RWE Rheinbraun. Three profiles, S1, S2 and S3, and one core from well R1 were sampled for both macro- and microlithotype analysis. Samples from profile S3 were also used for testing the compression strength of briquettes generated under laboratory conditions. For the analysis, a representative sample quantity of about 2– 3 kg was taken for each stratigraphic section. After the description of the samples, they were wrapped in aluminium foil, labelled and stored in plastic bags for the macro- and microlithotype analysis. In conjunction with previous studies undertaken by the Laubag (Bo¨nisch et al., 1998), the three profiles, S1, S2, S3, and well R1 were macroscopically de-

scribed, the composition of microlithotypes determined and correlated to borehole measurements. The profiles cover parts of the Main Seam, whereas well R1 penetrates it completely. The used petrographic methods, essentially described by Schneider (1980) and Bo¨nisch et al. (1998), vary from the ones suggested by the ICCP (1993). They are based on cuticule analysis and are sensitive to changes in the biological communities found in immature lignites. Several studies were carried out regarding the quality of briquette-coal and their composition and processing (e.g. Kurtz, 1970; Vogt, 1970; Krug et al., 1977; Azahari, 1996). However, Bo¨nisch et al. (1998) showed that the quality of briquettes depends on the botanical content in the Lausitzian and Rhenish lignites and presented data for 1997’s mining front in the Hambach open pit. Data and results from these studies were used as a basic data set, correlated to geophysical logs and extrapolated to the exploration area. 2.1. Macropetrography The facies description—or macropetrographic description—is based on the plant content of the coal (Bo¨nisch, 1984). Samples were split parallel to the layering of the seam. Contents of plant remains, fusinitic materials, xylites and inorganic material, colour, texture and gelifiction were described. Blocks of 5  1  1 cm were sawed out, freeze-dried and then again described under a binocular microscope with low magnification (10  ). 2.2. Microlithotype analysis Microlithotype analysis provides information on the volume fraction of different lithotypes in a chosen interval (Taylor et al., 1998). For this analysis, the freeze-dried coal was ground to a particle size of less than 3.15 mm. Ten grams of this material was fixed in cold mounting epoxy resin and cut vertically to bedding to prevent separation due to different settling rates of the particles. These pieces were ground flat with sandpaper (300 – 1200 grit SiC paper) and afterwards polished with a diamond suspension of decreasing grain size ranging from 6 to 1 Am. These polished mounts are used for point-counting (Glagolev, 1934). The Swift point-counting instrument gen-

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erates a uniformly spaced grid of points where lithotypes are determined at every grid point. In this study, 1000 points per mount were measured (Taylor

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et al., 1998). The petrographic microscope was set up with a 50  oil immersion objective and a 10  ocular, giving a total magnification of 500 .

Fig. 8. Location of profiles S1, S2 and S3, two cross sections based on 30 wells (see Figs. 9, 13 and 14) and well R1 (see Fig. 9) in the Hambach open pit mine.

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The microlithotype analysis was modified slightly from the original method. The microscopically similar groups of eu-xylo-textite and medio-xylo-textite and the groups eu-detrite and texto-detrite were combined into one category as they were very rare. Furthermore, counts for bituminite and mineral matter were also combined due to the same reason. Additionally, three other facies indicating textite groups were added: marcoduria-textite, peridermotextite and phyllo-textite. The following twelve microlithotypes were differentiated for compositional analysis: eu-medio-xylo-textite, gelo-xylo-textite, marcoduria-textite, peridermo-textite, phyllo-textite, eu-texto-detrite, gelo-detrite, texto-gelite, detro-gelite, eu-gelite, inertite and bituminite-minerals. 2.3. Geophysical data Exploration wells are regularly drilled in the Lower Rhine Basin (Fig. 8). During and after drilling geophysical logs, resistivity, density and natural radioactivity are measured (Mach, 1997; Podder and Majumder, 2001). Logs used for this study were taken from 30 wells acquired by various companies using different tools during 1963 – 1999 (Fig. 8). This resulted in different log qualities but the trends within the logs were identical. Log measurements were available in the form of paper copies which were digitised, scaled and arranged along both cross sections according to their location (Fig. 8). However, due to the poor quality of the database, changes in the log characteristics were recognized but not qualified. This method was therefore strictly used for a relative comparison within the lignites rather than an interpretation based on the measured absolute values. 2.4. Briquette processing Every year 2 Mt of lignite from the Lower Rhine Basin are processed into briquettes (RWE, 2000/ 2001). The quality of the briquettes depends on the mined lignites (Kurtz, 1970; Vogt, 1970; Krug et al., 1977; Azahari, 1996). Coal from the Hambach open pit resulted in variable briquette quality even though the technique used (Kurtz, 1970) remained the same. To test the quality of the briquettes, RWE Rheinbraun used a Toni-Pru¨fmaschine (TGL 9491, 1975) to evaluate the compression strength. This enables one to

determine the storage life of the briquettes. Coals from the profile S3 were dried for 3 h at a temperature of 100 jC and ground to a particle size of 0– 2 mm. One hundred grams of this material was then pressed into a briquette and subjected to the Toni-Pru¨fmaschine. Punch diameter for this test was 50 mm, with a pressure 236 kN for a duration of 20 s.

3. Results and discussion 3.1. Facies evolution Profile S1 covers the Garzweiler II – III and Frimmersdorf b seams, profile S2 and profile S3 cover Frimmersdorf a and b seams, and well R1 penetrates the entire Main Seam (Fig. 8). Small parts of the well samples were unusable because the core spalled during drilling. Fresh coal samples were described at centimetre intervals. The macropetrographic description of the facies sequences of the four profiles is shown in Fig. 9. Correlation between the profiles is verified by the sandy layers in between the seams (e.g. Neurather Sand) and the layers with typical, unique botanical remains, e.g. Cyrilaceen leaves and subordinate layers with high content of leaves, needles, fusite, macrinite and cork tissue. Cyrillaceen leaves are unambiguous indicators and are characterised by smooth margins, visible venation and a tapered drip-tip. They typically occur in horizon 21 (Fig. 9) which has an average thickness of about 2 m. This horizon is also found in well R1 which was drilled in an exploration area north of the present-day Hambach mine. The facies determination led to a good correlation of horizons over the entire seam, although lateral facies changes occur and horizon thickness is variable. The base of the Main Seam starts with coal of the K-facies, which is marked by conifer fragments. A typical succession of the Lausitz Miocene brown coal also starts with K-facies due to a shallow groundwater table (Schneider, 1978). Ideally, the rising water table would change the vegetation from a coniferous forest to an alluvial angiosperm forest recognisable as an Afacies type coal (Fig. 4). Steady rise of the water table enables the growth of Monocotyledons (grass and reed-like vegetation) and result in the G-facies, which has the highest water table in this model. The falling

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Fig. 9. Macropetrographic facies in different vertical profiles (see Fig. 8 for location) revealing the general stratigraphic sequence.

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Horizon

Facies

Eu-medioxylo-textite

Gelo-xylotextite

Marcoduriatextite

Peridermotextite

Phyllotextite

Eu-textodetrite

Gelodetrite

Textogelite

Detrogelite

Eugelite

Inertite

Bituminiteminerals

Degree of gelification

34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13

P A P/G A P/M A P A P M A P/G A G HB P A M A P A P

3.6 2.3 2.9 0.9 0.9 0.2 0.9 0.1 0.5 4.8 1.0 1.1 0.1 1.9 0.0 1.8 0.0 0.1 0.0 0.5 0.1 0.0

5.4 17.8 1.2 0.5 2.6 4.6 2.9 1.3 3.0 8.4 2.2 5.0 1.7 6.2 0.2 4.2 1.8 7.4 3.9 7.4 1.8 3.1

1.3 0.6 0.3 0.3 0.8 0.6 0.6 0.1 1.4 7.1 2.0 0.1 0.1 0.0 0.0 0.0 0.0 7.0 4.9 0.3 0.1 0.0

8.7 10.4 14.9 6.7 22.8 8.6 21.5 8.6 17.9 6.7 5.6 14.6 11.5 13.3 0.5 10.0 4.0 4.3 3.8 6.9 3.1 10.1

0.3 0.4 2.7 1.1 2.2 1.2 0.9 0.1 2.8 0.3 0.2 1.1 0.4 0.3 0.0 1.7 0.2 0.2 0.7 1.4 0.0 1.1

34.3 18.7 5.1 13.3 17.6 18.1 38.6 40.6 28.5 23.3 15.2 5.3 16.9 2.3 78.9 48.1 25.6 51.0 62.1 34.8 17.9 23.8

32.6 34.3 44.0 69.4 37.8 53.3 28 43 38.8 43.1 51.6 45.3 36.9 16.9 15.1 31.1 47.3 24.8 16.4 27.5 70.8 52.1

2.3 2.8 6.5 1.9 6.0 5 4.2 2.0 4.8 3.7 2.3 18.2 19.8 17.9 0.6 1.3 1.1 4.7 3.0 7.2 1.8 3.0

4.2 8.8 21.3 4.7 6.8 6.2 0.2 1.2 1.7 1.1 17.5 7.6 9.4 39.2 1.0 0.1 17.7 0.0 3.1 12.5 3.7 5.0

0.6 1.4 0.5 0.6 0.5 1.4 0.8 0.5 0.1 0.8 0.9 0.5 0.9 0.4 0.4 0.6 0.0 0.1 0.3 0.6 0.0 1.3

5.1 0.8 0.3 1.9 0.2 0.7 0.7 1.8 0.3 0.6 0.8 1.0 1.7 1.5 2.8 0.6 1.1 0.4 0.7 0.8 0.5 0.4

1.6 1.3 0.3 0.3 1.8 0.1 0.7 0.8 0.2 0.1 0.7 0.2 0.6 0.1 0.5 0.5 1.2 0.0 1.1 0.1 0.2 0.1

2.9 4.2 7 2.5 6.5 6.4 4.9 2.5 4.9 4.5 3.2 18.7 20.7 18.3 1.0 1.9 1.1 4.8 3.3 7.8 1.8 4.3

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Table 2 Microlithotype composition of samples from profile S3 (vol.%)

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water table would then result in a Pine forest and eventually in a Sequoia forest, giving a sequence of Pand then M-facies type coals. This ideal model, established by Schneider (1978), was not recognized in complete sequences of the Rhenish lignites, which might be due to different evolutions of water level in the basins which is known to be the decisive factor in peat evolution (Moore and Shearer, 2003). Taking the facies description of well R1 into account, the coal of the K-facies is followed by an A-facies coal and then by another K-facies coal, leading to the assumption that after deposition of the A-facies coals, the water table dropped again and the swamp dried out. This trend is visible over the entire seam and suggests a non-continuous, possibly fault controlled subsidence of the Lower Rhine Graben system. Plant remains are still easily recognisable as the lignite is highly immature. Huminite (vitrinite) reflectance values are at 0.3% Rr. Mechanical destruction of organic material is visible, but empty and well-structured cells point to good preservation of the tissues, which have been affected by strong compaction. The microlithotype composition of horizons 13 – 34 of profile S3 are summarized in Table 2. Average assemblages of microlithotypes have been correlated with coal facies (Table 3) using the 148 measured samples from profiles S1 – S3 and well R1 (Table 4). Accordingly, horizons containing both marcoduria-textite and peridermo-textite of at least 2– 3 vol.% are often classified as M-facies coals. A-facies type coals contain only minor amounts of textites, whereas P-facies coals are characterized by pine fragments combined with high detrite contents. G-facies coals are characterized by high gelite contents. When the results from the microlithotype analysis using the average assemblage described in Table 3 are compared to those of the macropetrographic descrip-

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tions, a good correlation is observed for the three profiles S1 –S3 and the well R1. Data for profile S3 are presented in Table 1. In the case where macropetrographic analysis was not possible due to the absence of facies indicators or spalled core material, microlithotype analysis yielded more precise classifications of the lithotypes. Thus is especially evident for distinguishing the P/G mixed facies. For example, deviations are found in horizon 30 in well R1 (Table 4). A coal, macropetrographically classified as P/A facies, was recognized clearly as M-facies in the microlithotype analysis due to the presence of marcoduria- and peridermo-textite. The latter finding resulted in a better fit with the observations from nearby profiles. Horizon 21 of well R1, which was badly spalled, was determined to contain coal of the G-facies using microlithotype analysis. An even better correlation was achieved, when additional microscopic facies indicators such as the occurrence of marcoduria- or conifer cells were taken into account. Well R1 served as a control point for the observation of lateral facies changes as it is located at least 300 m away from the studied profiles S1, S2 and S3 in the open pit mine. In general, the horizon sequence known from the sampled profiles was found in the well with minor lateral changes in facies and thickness (Fig. 9). An important result of the microscopic study is the presence of gelite in the coal, especially the large amounts of texto-gelite and eu-gelite in the G-facies. Both microlithotypes are characteristic of lignite in which high rates of gelification have occurred. The degree of gelification is of importance for the briquetting properties of lignites and in particular for the correlation of the compression strength of the briquettes, which will be discussed later. Commonly, gelification is regarded as a biochemical process during which cellulose is progressively

Table 3 Mean microlithotype composition of different macroscopically defined facies types (in vol.%) for samples from S1, S2, S3 and R1 (vol.%) Facies Eu-medio- Gelo-xylo- Marcoduria- Peridermo- Phyllo- Eu-texto- Gelo- Texto- Detro- Eu-gelite Inertite Bitumitexylo-textite textite textite textite textite detrite detrite gelite gelite mineral K A P G M

5 1 2 0 1

18 5 15 10 13

0 0 1 1 2

1 1 1 2 1

0 0 0 0 0

19 28 18 11 20

44 60 56 59 59

8 2 4 11 2

2 1 1 3 1

0 0 0 1 0

1 2 1 1 1

2 0 0 0 0

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Table 4 Microlithotype composition of samples from well R1 (vol.%) Facies

Eu-medioxylo-textite

Gelo-xylotextite

Marcoduriatextite

Peridermotextite

Phyllotextite

Eu-textodetrite

Gelodetrite

39 39 39 39 39 39 39 39 39 38 38 38 38 38 38 37 37 37 37 37 37 35 34 33 33 32 32 31 31 31 30 30 30 30 29 28 28 27 27

K K A A K A A A A HB A A A K A A A HB A P A A P A A P/G P/G A A A P/A P P P A P P A A

24 16 0.7 26 14 1.1 13 8 8.2 0.4 3.5 0.7 3.3 27 0.9 28 5.7 3.3 1.8 0.9 2.9 0.8 3.4 2.3 0.6 0.2 0.6 0.3 2.3 0.2 0.1 1 0.4 0.3 0.3 1.1 0.2 0.6 1.1

13 70 5.4 28 24 2.2 18 76 6.2 2.8 2.1 14 5.1 9 3.9 41 6.8 2.6 4.3 6.2 11 5 9.8 9 9.4 11 12 8.2 10 2 7.6 62 17 13 8.6 14 16 13 7

0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 2 5 1 2 0 1 1 0 0 0 1 0 3 0 3 1 0 1 1 1 1

0 7 0 0 2 0 0 0 0 0 0 0 0 0 0 0 1 0 2 2 1 0 0 0 0 0 2 1 1 0 1 0 1 1 1 1 4 2 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0

20 0.2 15 26 4.2 26 9.3 2.7 29 76 31 25 15 35 36 15 18 64 25 5.5 12 65 56 11 18 15 7.3 4 12 6.5 8.1 13 2.7 4.1 50 8.1 35 40 43

36 4.9 77 18 54 67 56 9.7 51 17 58 59 73 25 55 15 66 24 57 73 66 23 24 73 60 70 74 83 72 89 77 21 73 75 37 67 40 38 45

Textogelite

Detrogelite

5.8 1.2 0.1 0.2 0.7 0.1 1.1 1.7 0.4 0.2 3.8 1.1 1.4 0.4 0.1 0.1 0.4 0 2.1 3 2.4 0.3 5.6 2.2 9.1 1.1 1.4 0.3 0.2 0.2 0.5 0.4 1.8 4.3 0.8 2.8 2.6 3.9 1

0 0.5 0.5 0.3 0.7 0.4 0.5 1.2 0.8 0.3 0 0.1 1 1.3 0.7 0.7 1.2 0.4 2.6 2.1 3.1 1.4 0.4 0.4 0.9 0.7 0.7 0.7 0.7 0.4 1.1 0.8 0.9 0.2 0.6 2.5 1.1 1.2 0.4

Eugelite

Inertite

0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 2 1 0 0 0 0 0 0 1 0 0 1 1 1 1 1 0 1 1 1 0

1 0 1 1 1 2 1 1 4 2 1 1 1 1 3 1 1 6 2 1 1 2 1 1 1 2 1 1 1 1 1 1 0 1 1 1 1 0 1

Bituminiteminerals 0 0 0.1 0 0.1 0.4 0 0 0 0.5 0.2 0 0.1 0 0 0.1 0.2 0.5 0.7 0.2 0 0.6 0 0.1 0.3 0.1 0.1 0.1 0 0.4 0.1 0 0 0 0.1 0.1 0.1 0.2 0.1

Degree of gelification 6.2 1.2 0.2 0.5 1 0.5 1.2 1.8 1.3 0.3 4.2 1.1 1.8 0.7 0.4 0.2 0.6 0.3 4 3.6 2.6 0.7 5.8 2.3 9.5 1.4 1.9 0.6 0.2 0.8 1.4 1.1 2.3 4.9 0.8 3.7 3.1 4.5 1.4

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

Horizon

M M A A P/G A A A G G G G G HB P P A M M M A P P P A A A P P P P A A P/G P/G P/G P/G A K K K K K

2.1 0 4.4 0.7 1.3 0.1 43 0.3 1.2 1 0.6 2.6 0.8 0 0 0.1 0.6 0.4 0 0.6 0.6 0 0.2 0.2 1.2 0 0.4 0 1.2 0.2 1.2 0.4 0 0.4 0.4 0 0.2 0 1.2 0 0 0.4 0.4

13 18 19 11 11 6.2 7.8 2.6 13 10 10 10 13 7.6 14 13 6.6 8.4 7.6 11 5.7 4.2 4 8.6 15 5.4 5.2 7 16 12 12 3.6 26 8.4 14 3 5 1.2 16 17 14 24 37

0 1 3 1 0 1 0 1 1 3 3 2 1 0 0 1 0 0 0 1 0 1 0 0 3 1 1 0 0 0 2 0 0 1 0 0 1 0 0 0 0 0 0

0 1 1 0 0 1 0 1 1 1 1 3 1 0 2 1 1 2 3 4 1 1 0 1 1 1 1 4 3 1 3 1 0 3 3 1 1 1 1 0 0 0 0

1 0 0 0 0 0 1 0 0 0 1 0 0 0 0 0 2 0 1 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

38 7.4 7.4 5.3 13 8.1 16 5.6 3.5 5.8 27 8.9 15 19 11 2.6 29 24 12 17 5.4 7.8 19 31 15 25 12 10 17 4 9.2 20 1.8 2.8 3.2 7 32 29 7.6 6.8 41 10 12

40 66 62 73 70 81 26 87 73 65 50 61 51 64 65 45 54 56 68 53 82 82 73 53 59 64 74 73 53 66 57 71 66 82 76 87 58 62 49 44 28 58 45

2.8 4.2 3.6 8.8 2.1 2.1 1.2 0.5 2.3 9.4 4.8 8.4 13 3.8 3.8 20 4.4 6.8 6.4 9.8 4.3 1.4 0.6 3.6 2.8 0.4 3.2 2.4 7.6 15 12 2.4 5 1.2 3 1 2 3 20 26 1.2 3.8 4.6

2.9 1.9 0.6 0 2.1 0.4 4.7 0.3 0.7 2 1.6 1.7 1.2 0.2 1.4 12 0.2 0.8 0.4 1.4 0.3 0.2 0.6 2 1.2 0.4 1 1.6 1 0.8 2.2 0.6 0 0.8 0.4 0.4 0.2 1.2 3.8 5.8 1.2 2 0.6

0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 2 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

1 1 0 0 1 1 0 2 4 2 1 2 3 4 2 2 1 1 1 1 1 2 1 1 1 3 2 1 1 1 0 1 0 0 0 1 1 2 0 0 2 0 0

0 0.3 0 0 0 0.2 0 0.5 0.2 0.2 0 0.1 0.2 0.2 0.3 0.2 0 0 0 0 0.5 0.4 0 0 0.2 0.2 0.2 0 0.2 0 0 0.4 0 0 0 0 0 0.2 0 0.2 14 0.2 0.8

2.9 4.6 3.6 9.1 2.9 2.3 1.3 0.9 2.7 9.6 4.8 9.1 13 4.4 3.9 22 5 7.6 6.8 10 4.4 1.8 0.6 3.6 3 0.6 3.6 2.8 8 15 13 2.4 5 1.2 3.2 1.2 2 3 21 26 1.2 3.8 4.8

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

25 25 24 24 23 22 22 22 21 21 21 21 21 20 19 19 18 17 17 17 16 15 15 15 14 14 14 13 13 13 13 12 12 11 11 11 11 10o 10u 10u 10u 10u 10u

(continued on next page) 33

34

Table 4 (continued) Facies

9 9 9 9 9 8 8 8 8 7 7 7 7 6 6 5 5 5 4 3 3 2 1 1 1 1

A A A A A A P A A M M M M P P K K K A K K M K K K K

Eu-medioxylo-textite 0.2 0 0 0 0 0 0 0 0 0 0 0 0.2 0.2 1.4 0 0 0 0 0 0 0 0 0 0.2 0.2

Gelo-xylotextite

Marcoduriatextite

Peridermotextite

Phyllotextite

Eu-textodetrite

Gelodetrite

Textogelite

4.6 7 13 0.6 1.8 8.2 8 5 5 5 4 12 12 2.8 20 9.8 10 15 9.4 6.6 18 2.2 9 17 22 12

0 0 0 0 0 0 0 6 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 0 1 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

43 16 38 51 55 57 11 40 14 69 21 33 4.4 9.2 13 13 13 5.4 63 51 11 13 7 16 33 5.6

44 71 44 45 40 34 74 42 76 23 67 48 65 81 59 61 51 68 20 34 64 75 69 55 7.6 61

1.2 2 3.8 0.6 0.2 0.4 5.2 6 0.4 1 1.2 1.8 11 0.6 1.8 8.8 16 8.6 3.4 5 4.4 7.4 12 7 36 11

Detrogelite 0.8 1.6 1.2 1 0.6 0 1 0.6 2.2 0.4 2.8 1.6 4.6 4.4 2.2 5 7.6 1.6 3.8 1.8 1.2 0.6 1.6 2.8 0 7.8

Eugelite

Inertite

0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 0 1 0 0 0 0 2

1 0 0 2 1 0 0 1 1 1 4 3 1 0 1 1 0 1 0 0 0 1 0 2 1 0

Bituminiteminerals 5 1.6 0.4 0 0 0 0 0 0 0 0 0.2 0.4 0 0.2 0 0 0 0 0 0 0.2 0 0 0 0.4

Degree of gelification 1.2 2.2 4 1 0.2 0.4 5.4 6 1.4 1 1.2 1.8 12 1 1.8 10 17 8.8 3.6 5.2 5 7.4 12 7 36 13

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

Horizon

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

removed from woody tissues (Teichmu¨ller and Teichmu¨ller, 1954). Lignin degrades, losing methoxyl groups and undergoes minor oxidation. The atomic ratios H/C and O/C decrease during gelification while aromaticity increases. Thus, gelified woods in soft brown coals are, so to speak, ‘‘pre-coalified’’. As wood decomposition is thought to occur mainly under aerobic conditions by fungi and to some extent by bacteria (Benner et al., 1987), gelification may be governed by different microorganisms (e.g. anaerobic bacteria) not involved in the destruction of cellulose (Bechtel et al., 2003). In the hard brown coal stage, the last remnants of lignin and cellulose disappear and the remaining humic acids condense to larger molecules, losing their acidic character to form alkaliinsoluble humins (Taylor et al., 1998). Colloidal dissolved humins are then precipitated into cavities. Gelified leaves are distinguished from ungelified leaves by the shape of the cell wall. Gelified leaves are swollen, then compacted, cemented, and homogenised to become the maceral ulminite (Taylor et al., 1998).

35

Gelification within the Main Seam is not uniformly developed, and is quite variable in different parts of the vertical profiles. For example, the upper part of the Frimmersdorf a seam contains about 17% of eu-textite (Fig. 10) and less than 10% of gelo-textite, whereas the lower part of the Frimmersdorf a seam contains up to 22% of gelo-textite and almost no eutextite. The general trend shows low gelification in the upper part of a seam and high gelification towards the base. This can be explained by the withdrawal and later precipitation of humic acids on impermeable clay layers, e.g. between the Frimmersdorf a and b seams (Fig. 10). Pathways for fluid migration are created by trunks and stumps that are more resistant to compaction than detrital plant fragments. In the pressure shadows of these xylites, detritus remains uncompacted, forming zones of high permeability for moving fluids. If humic acids become clogged and restrained in the conducting paths of xylites, they will be able to form gelites, explaining the high content of texto-gelite at the top of the seam.

Fig. 10. Contents of eu-textite (diamonds) and gelo-textite (squares) in seam Frimmersdorf at well R1.

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J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

The coal of the M-facies in horizon 4 in well R1 is described by small amounts of eu- and texto-detrite and high gelo-detrite contents. The absence of xylites and the felty structure lead to low permeability, limited fluid transport and high gelification of the matrix. Generally, gelo-detrites are relatively rare, indicating a good overall permeability for detritic lithotypes. The eu-gelite content of about 3% is relatively low. A high eu-gelite content combined with high contents of texto-gelites is typically found in the coal of G-facies, the P/G-mixed facies and the basal K-facies. The high degree of gelification (sum of eu- and texto-gelite) is characteristic for horizons 13, 21 –23 and 32. Horizon 21 is a G-facies coal, horizons 23 and 32 a P/G mixed facies and horizon 22 is an Afacies coal. G- and P/G facies are known for their high degree of gelification (Bo¨nisch et al., 1998) but the high degree of gelification in horizon 22 is prominent and unusual for an A-facies coal. This is because the G-facies coal below acts as an impermeable layer due to compacted, finely layered blades of grass and reed like plant material. Combined with the high primary content of gelified material, horizons 21 and 23 will act as aquitards, and hold back colloidal dissolved humic substances

in horizon 22. This may explain the unusual, high degree of gelification in horizon 22. Leakage of water on the fresh outcrop above horizon 21 supports this observation. 3.2. Comparison to briquettes Eleven samples from profile S3 were pressed into briquettes and analyzed for their compression strength (Fig. 11). Comparison of the results with those from the microlithotype analysis revealed that samples with a high degree of gelification (sum of eu-gelite and texto-gelite; originally proposed by Bo¨nisch et al., 1998) have the weakest compression strength. Gelification higher than 8% (Bo¨nisch, 1999, personal communication) results in compression strengths lower than the minimum threshold of 5.5 N/mm2. Eu- and texto-gelite react in a brittle way when stress is applied leading to the weak compression strength. While pressing the briquettes, the eu- and texto-gelites fracture and do not establish bonds with the surrounding particles. Sampled eu-gelites, which occur in nests at the base of the seams, exhibit brittle and crumbly character. Humidity uses these pathways to penetrate the briquette and react with single particles, and causes the coal to swell. This swelling causes

Fig. 11. Correlation between the degree of gelification and compression strength for individual coal samples of the Hambach open pit mine.

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

cracks and fractures diminishing the storage life of briquettes. The high degree of gelification is related to the Gand P/G-coal facies of horizons 13, 21 –23 and 32 in the Main Seam (Tables 2 and 3). Removing these layers before pressing briquettes or mixing them with other facies coal types can increase the compression strength and thus the storage life of the briquettes. This approach provides a useful technical application of implementing macro- and microlithotype analysis of a seam to categorize the coal into different qualities. 3.3. Log-facies In order to deliver coal of the same quality and standard to power plants and briquette factories, the planning of coal mining is important. For this purpose, wells are drilled outside the present mines to explore changes in facies, thickness, and mineral content of the seams. Core samples can be used for macro- and microlithotype analysis. However, cores are not always appropriate for macropetrographic description, as fast drilling techniques may destroy structure, texture and facies characteristics of the coal. Additionally, the preparation and evaluation of coal for microlithotype analysis is time-consuming. Therefore, an alternative technique to classify the coal and to locate the gelified horizons is to use geophysical well logs. For the Hambach mine, an initial correlation was possible by separating the coal from sand/clay-layers (Fig. 12). These horizons are marked by sharp peaks in the gamma ray, density and resistivity logs. Within the Main Seam of the Lower Rhine Basin, horizon correlation is limited by the wide spacing of the detectors on the wireline logs (0.15 – 0.25 m). Hence, thin horizons cannot be easily recognized. Additionally, measurements have been provided by a number of companies using a variety of tools, leading to different peak heights. The logs are also influenced by the drilling mud which penetrates the coal during drilling. In summary, the distinction of horizons within the seam depends on the relative changes in the logs. There are no absolute values which can be correlated to a facies as defined by the macropetrographic and microlithotype observations. Within the coal, a classification of different coal facies using

37

gamma-ray logs is not possible except for the gelified horizons 21– 23 and the HB-facies (Fig. 12). These horizons are marked by a small increase ( f 1 API) in the natural radioactivity. Both facies types were influenced by a high groundwater table, possibly enabling the deposition and incorporation of radioactive elements (Henry, 1997). Correlations using the resistivity logs provide a distinction between sediment and coal, as well as good sensitivity within the coal. Resistivity logs can be used to distinguish between A- and G-facies on one hand, and P-, M- and K-facies on the other as shown in Fig. 13 for cross section 2 (Fig. 8). The Afacies is determined by a relatively low resistivity compared to adjacent facies because of the high content of detritic material with relatively low porosity. The P-facies, including xylites with higher porosity, is characterized by a very irregular log appearance with slightly higher resistivity than the HB-facies. No distinction between P-facies and M- or K-facies is possible. A typical peak of low resistivity followed by a peak of high resistivity is observed for the G-facies and the HB-facies. The G-facies is also very densely compacted and allows higher electricity flow than the A-facies. This signal is especially noticeable for horizon 21 and weaker in horizon 32 in most wells and is recognizable in form of an Sshape as presented in Fig. 12 for well SHU297 (for location, see Fig. 8). Low resistivity marks the Gfacies coal and higher resistivity the underlying HBfacies coal. As the thickness of the G-facies is 0.2 – 0.4 m, it has not been detected in all wells. Generally, this peak is a good marker for the gelified strata in the seam. The density logs show significantly stronger peaks than the resistivity log demonstrated for cross section 1 in Fig. 14. Again, a distinction between coal and clastic sediment is easily recognisable. Within the coal, high density indicates that the HB-facies is a significant marker horizon. The A-facies is also recognisable by low density values but not as easily as in the case of the resistivity logs. Generally, the facies distinction is more difficult than in the resistivity logs. Combined observations show a good correlation between the geophysical measurements and the A-, HB- and G-facies on the one hand, and P-, K- and Mfacies on the other. The P-, K- and M-facies are

38

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

Fig. 12. Gamma ray and resistivity log response in gelified horizons of SHU 297 well through coals of the Hambach area.

characterized by similar depositional environments with a low groundwater table. The botanical components yield similar coals which are therefore not distinguishable in the log measurements. On the contrary, the G-facies, if developed in an appropriate

thickness, can be detected. By using these geophysical measurements, the seam can be classified and the mining of coal can be controlled in such a way that mining of G-facies coal for briquetting purposes is prevented.

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

Fig. 13. Seam correlation using resistivity logs for cross section 2.

Fig. 14. Seam correlation using density logs for cross section 1.

39

40

J. Naeth et al. / International Journal of Coal Geology 60 (2004) 17–41

4. Conclusions This study shows that macro- and microlithotype facies description combined with geophysical borehole measurements enables classification of lignites for mining purposes. The classification is based on the contents of eu- and texto gelites, i.e. the degree of gelification, which can be used as a qualitative parameter for mining activities. Based on the botanical content, the seam can be subdivided into A-, K-, M-, P-, G-, F- and HB-facies sequences. The petrographic quality parameter for briquetting purposes, as identified by laboratory tests using lignite from the Main Seam (Hauptflo¨z) in the Hambach open pit mine, is the occurrence of the G-facies. This facies is characterized by high percentages of gelified material. The presence of gelified material reduces the quality of the briquettes. The G-facies is characterized by finely layered Glumiflores (Monocotyledons) without xylites. The large number of leaves and the high level of gelification result in a dense coal, probably of low permeability. Above layers of G-facies coal in the outcrops in the Hambach open pit mine, water springs were observed after heavy rainfall which suggest a low permeability of the G-facies coal. This observation is consistent with the microlithotype analysis of the Gfacies, which is characterized by a high content of euand texto-gelites inferring a dense coal structure. This dense structure is recognizable by typical Sshaped wave in the resistivity log with low resistivity for the G-facies coal and higher resistivity for the underlying HB-facies coal. The A-facies is also characterized by a comparatively low resistivity compared to adjacent facies, probably due to a relatively high content of detrital material. The use of geophysical borehole measurements enables mapping of the facies distribution based on information from exploration boreholes. Such facies information can later be used for mining plans. Microlithotype characterisation of the G-facies coal showed a relatively large content of eu- and textogelites. These microlithotypes are identified by sharp shrinkage cracks in the microlithotype investigations and brittle reaction on stress. During the pressing of briquettes, the eu- and texto-gelites do not adhere to the surrounding particles, thus forming microscale fissures. When stored, air moisture is able to enter the briquettes through these cracks and enlarge them

by penetrating single particles which then start to swell. This process triggers the development of large-scale cracks and disintegration of the briquette. The quality of briquettes is thereby lowered significantly. For high quality briquettes, the use of G-facies type coal should be avoided. G-facies type coal can be determined by geophysical borehole measurements and can be confirmed by macro- and microlithotype analysis. The borehole measurements provide information on the overall facies composition of the entire seam. These facies sequences, observed in the present-day mining front, were correlated to the geophysical measurements of nearby drilled wells giving good control on facies description and geophysical measurements. Exploration wells in the forefront show a similar facies composition of the seam with little lateral facies or thickness change. As every facies is assigned to a quality factor, the mining of the seam can be planned using these quality factors, making the geophysical measurements a powerful tool for qualitative assessment of the seam. Micropetrographic facies characterization is still necessary in order to confirm the geophysical observations and it is the only way to quantify the degree of gelification as a quality parameter for the briquetting properties of the coal. Acknowledgements This study was financially and logistically supported by RWE Rheinbraun, Cologne. The authors wish to express their gratitude to R. Spann, R. Hagenbrock and F. Bo¨ll from the Rheinbraun mining company. The authors also acknowledge the collaborative contributions of R. Bo¨nisch, Senftenberg, and Dr. J. Schneider and C. Niemz, Lauta. Finally, we thank reviewers Janet Dehmer, Jennifer O’Keefe and Jim Hower who thoroughly read an earlier draft of the manuscript and made numerous valuable suggestions to improve its quality. References Azahari, H.L., 1996. Untersuchung zur Brikettierbarkeit indonesicher Braunkohle und deren Umweltauswirkungen beim Einsatz als Energietra¨ger im Haushaltssektor und in gewerblichen Feuerungen. PhD Thesis (Dissertation), RWTH Aachen University. 171 pp.

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