The hard-coal deposits of the Ruhr area are thus dominated by layers of sedimentary rocks and hard-coal seams, and also by elements of constricted tectonics and extending fracture tectonics. They principally comprise a folded structure attenuating in a north-westerly direction, consisting of anticlines and synclines running south-west to north-east, the vertical levels of which are controlled by their axis corrugation, distorting folds and accompanying overthrusts and, more or less perpendicular to these, grabens and horsts striking from south-east to north-west, with their causal faults (Fig. 19).
The Variscan Foreland with its practically continuously coal-bearing Upper Carboniferous strata extends across virtually the entire northern part of Germany and the territory of the Netherlands and then under the North and the Baltic seas (Fig. 1). The strata dip toward the north, i.e. the further north the coal seams are encountered, the deeper they are. This dip below the increasingly thick overburden is interrupted by local tectonic structures, as at the Osning Fault, at the Ibbenbüren Horst or at the Ochtrup Anticline, for example. The Ruhr‘s hard-coal deposits are thus only a small portion of the total coal reserves of the Variscan foreland.
The orientation of these seams can be stated in terms of their angle of dip and of strike. Angle of dip is critical for recovery of coal, with the result that seam inclinations are subdivided, for technological reasons, into dip groups. Classification of angle of dip is not uniform, however. One classification, for example, defines a low inclination of the seams as a dip of up to 20 gon, a moderately dipping seam with an angle of dip of 45 gon and steep deposits with an inclination of more than 45 gon . Other classifications categorize low inclined deposits as from 0 to 20 gon inclination to the horizontal, moderately dipping as from over 20 to 40 gon, severely dipping as from more than 40 to 60 gon and then steep dip as from above 60 to 100 gon (equivalent to 90 °) .
Tectonic processes following the deposition of the coal-bearing strata resulted in differing dipping of the originally horizontal seams (Fig. 24). In view of the last happened limitation of mining to seams of low inclination, the recovery of hard coal is thus restricted to the higher zones of the anticline and the bottom of the syncline and left the sides and folded zones largely untouched. Such a restricted scope of mining resulted correspondingly in inadequate exploitation of the deposits .
The geometry of strata is determined by the dimensions in three directions. Due to the seam-shaped configuration of the coal-bearing strata, the interfaces with the country rock in the floor and in the roof largely run in parallel, with the result their seam thickness remains approximately constant across large distances. The seams exhibit flexing in the vicinity of anticlines and synclines, as a result of folding, and elevated dips in the vicinity of the sides. As a result of faulting, a seam is, in addition, subdivided into individual sectors, which, in some cases, exhibit significant offsets to each other.
The seams of the Ruhr region occur in strata ranging from a few millimetres to several metres in thickness. Seam thicknesses greater than 4 m are rare, however. A seam of around 6.3 m was once encountered at the “Sachsen” mine, for example, but consisted in fact of several individual seams separated by thin bands .
Not all of the more than two hundred seams in the Ruhr area were economically exploitable. Exploitability depended on market situation, the available mining technology and the prevailing geological conditions. These interdependences also changed with time. The average thickness of coal seams mined in the 1930s was around 1.15 m pure coal, or around 1.25 m coal when the included and co-extracted mineral matter is included, but then rose to 1.25 m/1.41 m by 1960, and reached 1.42 m and 1.74 m in 1990 [6, 26, 28].
The continuity of the strata is also disrupted by fracture tectonics, with the result that faults divide the seams into part-seams and thus restrict their lateral extent. The tectonics of the hard-coal formation thus restricts the part-seams of constant stratum thicknesses. The intensity of the fracture tectonics varies greatly, however (Fig. 25). The number of faults and lateral faults per unit of length increases from south-east to north-west and thus behaves inversely proportionally to the confinement of the Ruhr Carboniferous, i.e., the more greatly the rock formation is folded, the fewer the transverse and diagonal faults that occur. In addition the large trench zones and main synclines tend to exhibit an elevated level of fracture tectonics, unlike the zones of axial depressions and main anticlines. Part-seams undisturbed for several 1000 m of their length and width as well as of a thickness of at least 1 m tend to be a rarity .
The hard coals of the Ruhr area played a decisive role in the industrialisation of Germany. Not only the magnitude of the deposits, but also the various ranks of coal, in other words, the material features, were decisive. Some ranks were suitable to meet energy requirements in the form of steam coal for power-generating plants, town gas for private and industrial use and briquettes for domestic heating, while others supplied carbon as a metallurgical reductant, in the form of coke, for the iron and steel industry, and others, for their part, were used in coal chemistry for the production of basic chemical feedstocks, such as crude benzole and ammonia.
A parallel development occurred in coal analysis, for the determination of the geological, chemical and physical properties of coals. Coal, due to the differing input materials and geological conditions of its formation, is an extremely complex material resource, with the result that it cannot, even now, be characterized adequately from a purely scientific viewpoint. For this reason, additional material tests which supply technological characteristics (i.e. grindability, wear index) are performed, depending on the processes and machinery used or intended for use. Another complicating factor is that differing test methods and parameters have developed and are used in the “traditional” coal countries, such as Germany, Great Britain, Poland, France or the USA (e.g. USA: ASTM D 388; Australia: AS K 184; Poland: PN-68 / G-97 002). These problems remain apparent even today in the parallel validity of national standards which may, for example, complicate the export of machinery. There are, nonetheless, efforts at the development and definition of globally uniform criteria for characterization of the visibly and increasingly internationally traded commodity of coal, so-called “world coal” (e.g. DIN 23 003, which itself is no longer valid). It is, for these reasons, not possible to rigorously derive and compile coal‘s material features on a scientific basis.
Such geological characterization is based on extremely diverse methods, however. Not only geological, biological, chemical and physical analyses and terms are used, but also technological criteria which have been developed in the course of long years of practical mining of hard coal. The scientific derivation of the geological system for coals is therefore also not satisfactory, but is the result of historical development.
In view of the complications described above, coals are characterized below on the basis of various criteria. The features derived from this result in various classifications of coals and of their constituents [1, 2, 4, 19, 20, 24, 26, 27, 41, 44, 54].
The volumetric percentage of inorganic constituents is used for classification of coal as a rock (Fig. 26). Up to a content of 8 Vol-%, we have coal, and above this up to 20 Vol-%, impure coal. Above an inorganics content of 20 Vol-%, the rock is classified as carbonaceous shale, and at above 60 Vol-% as gangue .
184.108.40.206.2 Coal types2
Externally, coals can be differentiated into banded and non-banded coals. The cause of the differing external appearance of these two types of coal can be found in their origin.
220.127.116.11.3 Coal ranks2
Lumps or strips of coal which exhibit a uniform external appearance can now be separated out of the seams. These are used for macroscopic characterization. In the case of the humic coals, the visual impression is used for designation. The mining terms “bright”, “semibright”, “dull” and “fibrous” coal are adopted for this purpose. The internationally used names are based, in some cases, on other properties of the lithotypes, however. “Bright” coal, also known as vitrain, is black and exhibits a glassy lustre. Anthracite is this coal‘s purest form.
Unlike “bright” coal, “dull” coal generally has a grey undertone and exhibits a matt to dull lustre. The international term “durain” refers to another property of this coal, namely its hardness, however. “Semibright” coal occupies in visual terms an intermediate position between “bright” coal and “dull” coal. Its technical designation “clarain”, refers to the sound generated when the coal is struck.
“Fibrous” coal is grey to black, and has a silky, velvety lustre, and exhibits a fibrous appearance. These coals are also known as “fusain”, due to their highly colorative property.
Systematic analysis of the coals of the Ruhr area indicates a correlation between the coal ranks and the amounts of the individual lithotypes which they contain . Here, fifty-eight seams from fifteen mines were sampled in a defined program (channel samples), the coal ranks being classified for these analyses into gasflame coal, gas coal, fat coal and lean coal and the lithotypes into “bright”, “dull”, “fibrous” coal and gangue(Fig. 28). The “fibrous” coal content varies only little between the individual coal ranks. The average in the Ruhr coal seams is 3.3 %. The “bright” and “dull” coal contents exhibit a contrary tendency. “Bright” coal contents in the gasflame coal and gas coal are around 65 % and increase to above 80 % in the fat and lean coal. The “dull” coal contents of gasflame coal and gas coal correspondingly decrease from 25 % to 13 % and to 7 % in the case of fat and lean coal. The jump in coalification between the gas coal and fat coal can be clearly recognised by the “bright” and “dull” coal contents . The gangue contents are between around 3 and 6 % on average. Higher contents – frequently above 50 % in ROM coal – occur if it was not possible to keep out gangue during winning or when gangue from the roof and the floor entered the coal.
The lithotypes of sapropelic coals include cannel coal and boghead coal. Both coals have a black-brown, matt appearance, and are similar to the “dull” coal. Their names derive from their combustion properties and the place at which they were first described. In small fragments, cannel coal can be easily ignited and then burns like a candle. The designation “boghead coal” comes from the Scottish town of Boghead.
The quantities of cannel coal and boghead coal in the Ruhr region are of no importance. They nonetheless constitute not insignificant reserves locally, however. Cannel coal is found in seams and seam groups of between a few centimetres and 1.4 m and in horizontal extents of a few 100 m and several kilometres, as in the case of Seam 12 at the Schlägel und Eisen 5/6 colliery and in the Zollverein 6 seam at the Helene-Amalia Colliery (Helene Shaft). Boghead coals occur more rarely. They are found in seam groups, as in Seam Z at Auguste Victoria Colliery and in Seam 2 at the Friedrich Thyssen II/V Colliery. Mixed types consisting of cannel and boghead coals also occur, at the Lohberg Colliery, for example .
The lithotypes examined macroscopically are composed at the microscopic level of microlithotypes with differing maceral paragenesis and mineral paragenesis in each case. The microlithotype represents the intergrowth of the maceral groups, with minerals contents ranging up to 60 % (on this, see Sections 18.104.22.168.6 and 22.214.171.124.8). Where minerals content is low (< 8 Vol-%), with the result that the density of the hard coal does not exceed a figure of 1.5 g/cm³, the microlithotype is designated on the basis of its maceral composition . Three microlithotypes are differentiated in this context, namely vitrite, liptite and inertite, which can occur in isolation and also in combinations of two and three of these types (Fig. 29). The combinations of two types are assigned separate designations, as in the case of clarite, the combination of vitrite and liptite, of durite, consisting of liptite and inertite, and of vitrinertite, consisting of vitrite and inertite. The nomenclature of the tri-maceral microlithotypes is orientated around the prevailing percentages of the maceral groups making them up. Where the vitrinite contents dominate, with subordinate amounts of inertinite, we have duroclarite, and with the reverse proportions clarodurite [1, 24]. Where, on the other hand, inertite dominates above vitrite and liptite, the term “vitrinertoliptite” is used.
Microlithotypes with an elevated minerals content (8 ÷60 Vol-%) increase the density of the hard coal. Where this is between 1.5 to 2.0 g/cm³, these microlithotypes are referred to as carbominerites. The designation of the coal/mineral intergrowth is then accomplished as a function of the included minerals and on the basis of the predominant mineral group, although this is not rigorous, scientifically speaking, since differing percentages of other minerals are permitted. Not only these primarily mono-mineral coal/mineral intergrowths with a dominant percentage of one mineral group but also intergrowths including several mineral groups, occur. These poly-mineral intergrowths are known as carbopolyminerites  (Fig. 29). These minerals are referred to as gangue if the minerals content exceeds 60 Vol-% or the density of the coal 2.0 g/cm³ (Fig. 26).
Macerals and minerals are more or less intimately intergrown with one another, depending on their genesis. Here, the syngenetic and epigenetic inorganic constituents are counted as mineral inclusions . Other minerals (including oxide ores and sulphates, for example) may also be present as inclusions, however, without any special names applying here. The term “salt coal” is used when salts are included, on the other hand.
126.96.36.199.6 Maceral groups2
Coals are not homogenous, but consist, instead, of various structural constituents, the “macerals”, which are collated into “maceral groups” depending on their initial substance and their conversion during the coalification process. The subdivision of the maceral groups is based on the three components of the vegetable substrate. The humous constituents consist primarily of protein, cellulose, lignin and tannoids, which form the macerals of the vitrinite group in the course of the coalification process. The fat components comprise, primarily, resins, waxes, fats, oils, latex and cork, the coalification products of which are assigned to the exinite macerals group. The third group, the inertites, are the result of the conversion of the inert constituents of the plants, such as chitin and charcoal-like components.
The various coal types and lithotypes, and also the microlithotypes, consist of individual maceral groups (Fig. 30). Some coals, such as “bright” coal, consist predominantly of only one maceral group, namely more than 95 % of vitrinite, whereas “semibright” coal is formed from not less than two maceral groups of differing compositions.
The macerals of the exinite or liptinite (a term more widely used today) groups (Fig. 31) predominantly comprise the bitumen-containing elements of the coals. They originate from the reproductive organs of the plants of the Carboniferous, and of other resistant plant parts. There were as yet no angiosperms during the Palaeozoic, instead, the predominantly coal-forming plants reproduced by means of spores. The inner dermis of the spores was destroyed after the death of the plants, whereas the epidermis (exine) is extremely durable and largely survived throughout the process of coalification. For this reason, the word “exine” provided the name, exinite, for all bitumen-containing elements in coal. The initial material for the sporinite maceral is the skin of the spores, the cuticles in the case of cutinite, the durable membrane of the epidermis of the plants, and the leaves, in particular, the oleaginous algae in the case of alginite, the waxes and resins in the case of resinite. Where the initial material of exinite is no longer recognisable, and is, therefore, present as “detritus”, these constituents of the coal are grouped together as the liptodetrinite maceral. The macerals of the exinite group form inclusions in the matrix of the humic coals. The matrix does not, on the other hand, constitute the main component of the sapropel coals; the inclusions predominate, algenite in the case of the boghead coals, for example, and sporinite in the case of the cannel coals .
In addition to organic macerals, coal also consists of inorganic substance. This is subdivided into four groups.
One part originates from plants which stores biogenic, inorganic minerals in their tissue. The percentage of these minerals is low, however, and does not exceed figures of 0.1 % to 0.2 % . Sulphur is also present in the coal, biogenically, and thus organically, fixed. Sulphur contents in Ruhr coals are generally between 0.6 % and 0.8 % .
The third group, the epigenetic constituents, are precipitated from solutions during coalification only after the diagenesis of the coal in tectonically generated cracks and crevices and in fissures as a result of the shrinkage in volume of the coal. The minerals deposited are composed primarily of carbonates, silicates and sulphides. The percentages of these admixtures fluctuates greatly locally, due to their genesis [15, 30]. Inorganic impurities, which fall into the coal during winning from the intermediate rock or from the country rock of the roof or the floor as a result of the technology used, must be differentiated from these constituents. In this case, there is generally no intergrowth with the coal. The winning technologies used in the Ruhr area in some cases cause significant by-winning of country rock, with the result that the average gangue content of the ROM coal were frequently above 50 % .
The intensity of intergrowth with the coal thus rises from the technogenic, via the epigenetic and syngenetic to the biogenetic inorganic substances. Separation in the processing plant is thus becoming ever more difficult, or even impossible.
It is necessary, in order to estimate the full potential of a deposit, to take into account total resources content. A total coal content of 204 x 109 m3, equivalent to around 270 x 109 t, was determined for the Ruhr within the boundaries stated in Section 188.8.131.52.1, using the methodology examined in Section 4.1.2, Fig. 21 [7, 11, 23].
184.108.40.206 Derived features
The characterisation of a deposit, including the hard-coal deposits of the Ruhr Basin, can be accomplished on formal geological criteria. The chemical, physical and technological features needed for design of machinery and systems can be derived from the geological features thus attained. Further supplementary investigations may be necessary, in cases in which the correlations between the features are not yet known.
The costs of the exploration of a deposit rise in parallel to the number and size of the samples to be taken. Without substantiated knowledge gained from sample-taking and analysis, for the purpose of selection and dimensioning of systems and machinery, it is not possible to quantify the commercial and technical risks involved, however. For this reason, the aim is always to derive from cost-effectively obtainable samples (of rock, coal, etc.) and the necessary analyses (macerals, minerals and their structure) the features relevant for practical purposes, such as, for example, floatability or density of materials, to permit the design of the sorting processes, and hardness, to permit selection of appropriate materials for winning machinery, chutes and silos.
The closure of the last hard-coal mine in this coalfield at the end of 2018 will mean that around 10 trillion t of coal were mined from more than 3500 collieries since the start of mining here [18, 34]. Given the current global production of some 8 trillion t/a, this quantity would just be sufficient to cover needs for slightly more than one year.
The total coal content of that part of the Ruhr Basin examined here, some 270 billion t, given the global coal production currently necessary of around 8 billion t, would suffice for just approximately thirty-five years. Even when the extra reserves of coal located in the Münsterland, an additional approx. 170 billion t, are included, the theoretical chronological range is extended only up to around sixty years . New mining technologies would need to be developed, in view of the total depletion of the deposits then necessary, however.
On present-day standards, the contributions made by the production of hard coal to the country‘s Gross Domestic Product must therefore be considered relatively modest, despite the fact that this ranking does not apply to the past. The reserves still remaining are nonetheless of a not uninteresting magnitude, however, with the result that it continues to be entirely rational, from a national-economic viewpoint, to invest money in the further development of existing and the evolution of new mining, winning and processing technologies.
The economic, and also the political, social, scientific and cultural development of Germany has been based for centuries on the extremely numerous – but generally poor – deposits of metallic, energy and industrial mineral resources. Only a few had and have international significance, those of lignite or potash salts, for instance. The most important deposits are those of the hard-coal reserves of the Ruhr area. Since the first documented mention of mining, in 1302, more than seven hundred years ago, they have frequently played a major role in German history, in the context of industrialisation and in times of war and of subsequent reconstruction .
In future, Germany, despite having its own adequate reserves, will obtain hard coal solely from abroad. Large deposits located at lesser depths, produce coal from open-cast operations (in the USA, Indonesia, South Africa and Columbia, for example) or from deep mines (e.g. Australia, Poland and Russia) at significantly lower costs. Germany, on the other hand, is no longer politically willing to bear the subsidies necessary for domestic mining of hard coal. A further aspect is climate policy, which sets the target of reducing CO2 emissions in Germany even further in the future. Such mining was stop in 2018. By the end of 2018, the Prosper-Haniel Mine, in Bottrop, the last colliery closed. It is today not possible to estimate the extent to which these political decisions will continue to be viable in the future. The reduction in domestic supply of energy resources and basic chemical feed materials, and a readjusted estimate of the climatic relevance of CO2 emissions, could result in different decisions and thus in different developments.
As sobering as these analyses and as relativising as these evaluations may be, they do not, of course, do justice to the historical, scientific, economic and technological importance of these deposits. Depending on the international political situation, the prospects for Germany in possessing adequate reserves of mineral resources for energy and chemicals in the Ruhr region for any future mining activities which may become necessary may be soothing. The precondition for this, however, would be the further development of existing and the evolution of new mining, winning and processing technologies and of corresponding machinery suitable for mining of the complex geological structures of these deposits.