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Energy resources: Coal

Introduction

There are many environmental reasons why coal is a rather undesirable source of energy. Burning it introduces large amounts of gases into the atmosphere that harm the enviironment in a variety of ways, as well as other, sollid waste products. Coal extraction leads to spoil heaps and mines that scar the landscape, land subsidence that affects roads and buildings, and in some cases water pollution.

With apparently so little going for it, why do we rely so much on coal to meet our energy needs? In this course, it will become apparent that the most appealing quality of coal is that there is plenty of it. Coal is twice as important globally as any other fuel in generating electricity, and could remain so for the next 200 years. That is reassuring for a future where energy demands continue to increasde and when the alternatives to coal are currently looking less dependable. The downside is that continued burning of coal could have dire consequences for the environment inthe coming centuries, unless 'cleaner' ways can be found to harness energy from it.

This course explores the basics: what coal is, how and where found, and how it is extracted at a variety of depths below the surface. Another important theme concerns the distribution of coal reserves and resources, and the control exerted on them by both economics and politics.

This OpenLearn course provides a sample of level 2 study in Science

Learning outcomes

After studying this course, you should be able to:

  • explain how coal is formed

  • explain how coal is found and extracted by either surface or underground mines

  • discuss how geological problems and environmental issues surrounding extraction affect mining

  • explain the reasons for the decline in the UK’s coal industry.

1 The origins of coal

1.1 Introduction

There are many environmental reasons why coal is a rather undesirable source of energy. Burning it introduces large amounts of gases into the atmosphere that harm the environment in a variety of ways, as well as other, solid waste products. Coal extraction leads to spoil heaps and mines that scar the landscape, land subsidence that affects roads and buildings, and in some cases water pollution.

With apparently so little going for it, why do we rely so much on coal to meet our energy needs? In this course, it will become apparent that the most appealing quality of coal is that there is plenty of it. Coal is twice as important globally as any other fuel in generating electricity, and could remain so for the next 200 years. That is reassuring for a future where energy demands continue to increase and when the alternatives to coal are currently looking less dependable. The downside is that continued burning of coal could have dire consequences for the environment in the coming centuries, unless 'cleaner' ways can be found to harness energy from it.

Before these issues are explored in more detail, it is necessary to explore the basics: what coal is, how and where it is found, and how it is extracted at a variety of depths below the surface. Another important theme concerns the distribution of coal reserves and resources, and the control exerted on them by both economics and politics.

1.2 The origins of coal

If you examine a piece of coal, at first sight it appears black and rather homogenous. However, closer inspection generally shows a series of parallel bands up to a few millimetres thick. Most obvious are shiny bands that break into angular pieces if struck. Between them are layers of dull, relatively hard coal and thin weak layers of charcoal-like carbon. Coal splits easily along these weak layers, which crumble to give coal its characteristic dusty black coating.

Microscopic examination of these bands shows that the bright coal represents single fragments of bark or wood, often showing a well-preserved cellular structure. The hard coal is an assemblage of crushed spores, fine wood debris and blobs of resin. The charcoal-like layers comprise charred fragments of bark and wood probably produced by oxygen-starved burning beneath the surface while dead vegetation was accumulating. These features suggest that coal forms from the highly compressed remains of land plants. Further evidence for this is provided by the preservation of individual plant fragments in sediments associated with beds of coal (Figure 1).

Figure 1
Figure 1 (a) A fossilized leaf. (b) Fossilized leafy branches and reproductive organs of the tree-like plant Lepidodendron (a giant club moss).

1.3 Coal-forming environments today

Coal formation begins with preservation of waterlogged plant remains to produce peat and then slow compression as the peat is buried. About 10 m of peat will compress down to form about 1 m of coal; clearly large amounts of plant debris must be available for preservation. Even so, for a significant thickness of peat to accumulate there must be a balance between the growth of plants and the decay of underlying dead material to form peat (a process known as humification).

Such a balance occurs in areas of poorly drained land known as mires (swamps).Whilst there are different types of mire, they all require the water table to be at or above the land surface for most, if not all of the year. This waterlogging of mire soils restricts the supply of oxygen. Such anoxic conditions prevent the complete decay of plant matter to carbon dioxide and water by aerobic bacteria. Instead, anaerobic bacteria convert some into methane, thereby reducing the hydrogen content of the decayed matter and increasing its carbon content, which is essential for the formation of coal. Rapid burial ensures that plant material decays and compacts fast enough to accommodate new plant growth in the mire above. The process of humification is fast in hot, humid tropical areas, but peat also accumulates in cooler, higher latitudes providing a humid climate is maintained. In regions such as Siberia and Canada, mosses rather than trees are the primary source of plant material.

Currently, 3% of the Earth's surface is covered by peat. However, not all of this is likely to form coal in the future.

  • Consider the extensive areas of peat which typify many upland districts of the UK and Ireland (where peat has been extracted for use as a fuel). Is this peat a good candidate for future coal formation?

  • Almost certainly not. Such peat was formed during a much wetter period of the geological past and in today's drier climate, large areas are above the water table undergoing oxidation. Therefore, this peat is likely to be eroded, rather than buried and turned to coal.

The next two sections look at different types of potentially coal-forming mires that occur today and are thought to have been significant in the geological record.

1.3.1 Peat formation in deltas and coastal barrier systems

Since mires require poor drainage, low-lying land close to coastal areas might provide the right conditions for peat to form. Most extensive areas of modern peat formation are indeed situated not far above sea-level, and as Figure 2 shows, they are commonly associated with river deltas and coastal barriers. Such environments would also have been significant areas of peat production in the geological past. However, the flooding of an area alone does not guarantee significant accumulations of peat; high productivity of organic matter is also required.

Figure 2
Figure 2 (a) The location of mires in deltas and coastal barrier systems. When river water spills through a breach or crevasse in a levee, a deposit known as a crevasse splay is formed. (b) A modern-day example of a coal-forming mire from the Okeefenokee Swamp, Florida. (c) Reconstruction of a late Carboniferous low-latitude swamp forest.

Deltas act as conduits bringing sediment via distributary river channels out into an open body of water (often the sea). In between these river channels are areas that receive less sediment, and it is here that high rates of subsidence and organic matter production may promote the development of mires (Figure 3).

Figure 3
Figure 3 Satellite image showing dark-coloured mires on the Mississippi delta south of New Orleans. These mires are forming around small lakes, and both are surrounded by a network of stream channels. The main river channel is approximately 500 m wide, and is flanked by natural levees that have been strengthened to reduce flooding. As a result the mires grow unchecked by annual deposition of sands and silts during floods. (This image was taken in 2001 — well before the major floods in this area caused by Hurricane Katrina in 2005.)
  • What is likely to happen to these areas when adjacent rivers are in flood?

  • Floodwaters will inundate the area and deposit a layer of sand or mud, which will contaminate the peat. This will (temporarily at least) halt the development of peat and, as we shall see later, contribute to the impurity of coal so formed.

As Figure 2a shows, a coastal barrier is a ribbon-like beach that protects a lake (or lagoon) on the landward side of the barrier from the sea. In this protected environment mires can develop along the fringes of the lagoon, or adjacent to tidal channels that cross the area.

  • Considering the proximity to the sea, what process is most likely to contaminate the mire with sediment?

  • Storms will tend to wash sand off the coastal barrier and redeposit it on the mire.

1.3.2 Peat formation in raised mires

Mires can also form inland within low-lying depressions, provided the rate of precipitation exceeds the rate of evaporation (Figure 4a). Peat is impermeable and so its accumulation progressively impedes drainage. This attribute gives mires the ability to maintain a water table independent of the area surrounding them. Therefore, the water table rises as the peat layer increases in thickness, thus elevating the surface level of the mire (Figure 4b-d). Such raised mires are now thought to have been very significant environments for coal formation in the geological record.

Figure 4
Figure 4 The development of a raised mire: (a) a lake forms in a depression with floating dead vegetation encroaching from the margins; (b) a mire is formed by the lake filling with peat and establishing luxuriant vegetation: (c, d) the mire surface is progressively raised by accumulation of peat, accompanied by a decline in the vegetation.

Having identified the modern environments in which the coals of the future may currently be forming, the next section will look at the evidence from ancient coal-bearing rock sequences, to see whether they formed in environments similar to those in which peats accumulate today.

1.4 Coal-forming environments in the geological record

Figure 5 simplifies a typical vertical succession of sedimentary rocks found in many coalfields. The sequence from the base of the section upwards reveals the following:

  1. When a mire starts to form, the first plants take root in underlying clays or sands that form the soil. Their rootlets are preserved beneath the coal seam as black carbonaceous markings, and the fossil soil in which they are found is called a seatearth (Figure 6). The presence of rootlets shows that the peat formed in situ, rather than being transported into the area by water currents.

  2. The coal seam itself consists largely of plant material with small but variable amounts of mud. The seam itself can vary in thickness from a few millimetres to tens of metres.

  3. Immediately above the coal seam there may occasionally be a mudstone containing rare but distinctive marine fossils (for example, brachiopods and cephalopods). This is unusual because most of the other fossil remains associated with coal-bearing sequences are normally of freshwater or land-based species. Where present, marine beds suggest that peat formation ceased as the sea flooded the area.

  4. The muddy sediments overlying the coal seam pass upwards first into siltstones and then into sandstones. This sequence of rocks usually gets steadily coarser upwards, but variations are common. Fossils within this part of the sequence invariably indicate non-marine conditions. The sandstones and siltstones may show sedimentary features that indicate action of waves and currents in relatively shallow water. Some sandstones show evidence of river channels.

  5. The sandstones often pass upwards into seatearths and another coal layer.

Figure 5
Figure 5 A vertical section through a typical coal-bearing sequence, showing a variety of sediments, and some typical fauna and flora. As is conventional in diagrams like this, the vertical axis represents sediment thickness (here schematic) and the horizontal scale represents maximum size of grains within the sediment. The numbers 1 to 5 on the left of the figure refer to sediment layers 1 to 5 listed in the accompanying text.
Figure 6
Figure 6 A seatearth showing fossil rootlets as black carbonaceous lines towards the top.

These vertical successions are thought to typify sediment deposition in certain areas of deltaic and coastal barrier environments similar to those shown in Figure 2. They are interpreted as representing initially flat low-lying sandy (or muddy) areas covered by vast freshwater lakes containing a variety of land plants growing in mires. Mire formation is then terminated by flooding of these areas, either by adjacent rivers bursting their banks, or by the sea flooding into the area. In both cases, these submerged areas would have filled up with mud, silt, and sand or, as depicted in Figure 5, mud grading into sand. Exactly what type of sediment overlies the mire depends on the source of the sediment (i.e. river or sea) and the processes involved in depositing it. Individual sedimentary successions can vary considerably, both laterally and vertically from one succession to the next. Eventually, this sediment pile will fill the submerged area enabling recolonization by plants and the establishment of a new mire.

This cycle of mire-flooding-sediment infill-mire, is repeated time and time again, which explains why there may be many seams stacked vertically in a coalfield. The sedimentary succession in coalfields can reach hundreds or even thousands of metres in thickness, even though all the sediments were deposited in shallow water.

  • What does this suggest about the stability of the land surface on which such sediments accumulated?

  • Great thicknesses of coal-bearing rocks are clear evidence that the sedimentary basins in which they formed were subsiding. The compaction of peat and mud (sand is relatively less compactable) would have been contributory factors, but cannot alone account for such sediment thicknesses.

Question 1

Estimates of the current rate of subsidence for the Ganges and Nile deltas vary between 1 and 5 mm yr−1. Using a rate of 1 mm yr−1, calculate the time needed to deposit sufficient peat to form a 3 m coal seam. What assumptions does this calculation make?

Answer

As 10 m of peat are required to produce 1 m of coal (Section 1.2), a 30 m thickness of peat would eventually produce a 3 m coal seam.

A subsidence rate of 1 mm yr−1 is equivalent to 10−3 m yr−1.

So, 30 m of peat would be deposited in

This calculation assumes that neither sea-level nor the delta subsidence rate change during this time. It also assumes that all 30 m of peat are deposited before any further compaction occurs. In reality, the earliest formed peat will compact, making the calculated time span an overestimate.

Subsidence would not always have been uniform, so whilst mires existed in one part of the delta, sands, silts or muds were burying mires elsewhere. This variability results in seams that split laterally into two or more beds separated by bands of sandstone or carbonaceous shale, or that converge with an adjacent seam to become a single, thick one.

  • How do you think the vertical succession observed within raised mires might differ from the succession in Figure 5?

  • Most importantly, as raised mires can form far inland, flooding is less likely and so the cyclicity of mire-flooding-sediment infill-mire will not be seen. Instead, unimpeded development of the mire will mean that individual seams of greater thickness will develop. With flooding less likely, these coal seams will be less contaminated by sediment in comparison with deltaic and coastal barrier-formed coals.

The downside to coals formed in inland raised mires is that subsidence rates are not likely to be as high, and so total coalfield thicknesses are in general unlikely to be as great.

1.5 The physics and chemistry of coal formation

Coal is a type of sediment made up mainly of lithified plant remains. But how does spongy, rotting plant debris become a hard seam of coal? As discussed earlier, plant material growing in mires dies, and then rots under anoxic conditions to form peat (by the process of humification). With time, the mire becomes covered with layers of sediment, the weight of which squeezes water and gas out of the pore spaces and compacts the vegetation. As subsidence allows deposition of further mire-sediment cycles, the process of compaction continues. The vegetation matter interbedded with sand, silt and mud progressively increases in density to become indistinguishable from coal.

The first stage in the chemistry of turning plant material into coal is one of biochemical decomposition. Bacterial breakdown of the more soluble components, principally the cellulose, results in enrichment of the more resistant, waxy leaf coatings, spores, pollen, fruit and algal remains. Decomposition also expels some gases originally contained in the rotting matter — chiefly water, carbon dioxide and methane — leaving organic residues rich in carbon.

The second phase starts when the plant deposits are progressively buried beneath substantial amounts of mud, sand and silt. As depth of burial increases so too does pressure. Because of the Earth's internal heat flow, temperature also increases with depth. Coalification of the deposit involves progressive physical and chemical changes brought about by the increased temperature and pressure. The degrees of change result in distinguishable stages of coal quality, or rank, which reflect the maturity of the coal. The different rank stages are listed in Table 1, together with some of the parameters used to define them. Changes in rank are gradual and so the boundaries of the rank categories are somewhat arbitrary.

Compaction under pressure progressively increases the hardness of the coal. The continuous variation of chemical composition through the rank series is shown in Figure.7. Low-rank coals (b, c) contain more volatiles than do high-rank coals (d, e), reflected by the variation in their oxygen and hydrogen content in Table 1. Anthracites, for example, usually contain less than 10% volatile matter. To form, they require high pressures associated with tectonic deformation or high temperatures near to igneous intrusions. Metamorphism at very high temperatures and pressure transforms coal to graphite deposits, which are not energy resources, although valuable in their own right.

Figure 7
Figure 7 The relationship between carbon, oxygen and hydrogen contents for the six stages in the rank series in Table 1. (Rank increases from a-f.)

Question 2

Using Table 1 and Figure 7, describe how the chemistry of coal changes with increasing rank.

Answer

The most important chemical change shown by increasing rank is the increase in the amount of carbon at the expense of oxygen. The proportion of hydrogen present remains relatively constant at 6-9% by weight over much of the rank series until about 90% carbon, where a significant reduction in hydrogen occurs.

Changes in rank involve the expulsion of water, carbon dioxide and methane (CH4). Only small amounts of methane are liberated during the early stages of coalification, but during the transition from bituminous coal to anthracite (particularly over the range 85-92% carbon), expulsion of methane and other hydrocarbons removes hydrogen, whereas the emission of carbon dioxide declines. As the complex organic compounds in buried vegetation are slowly transformed to simpler, more carbon-rich compounds, coal changes colour from brown to black.

  • Would you expect the density of coal to increase or decrease with increase in rank?

  • As rank increases, the porosity and water content of the coal decrease, so density increases.

The density of coal is reflected in its structure, hence the arbitrary distinction between 'hard' and 'soft' coals used in Table 1. In fact all coal is soft compared with most rock forming minerals, and the terms refer to how coal breaks: soft coal is crumbly, whereas hard coal breaks in a brittle fashion, and remains in lumps when transported.

The relative proportions of carbon and volatiles in coals affect their physical properties and their uses.

  • Low-rank coals rich in volatile matter (more than 30%) are easy to ignite and burn freely but with a smoky flame. Low-volatile (high-rank) coals are more difficult to ignite, but they burn with a smoke-free flame; they are natural smokeless fuels.

  • For industrial use, coal is usually heated to expel remaining volatiles. The carbonized residue that remains after the volatile matter has been driven off in the absence of air is called coke. High-rank bituminous coals become partly fluid on heating and swell up to form a porous coke, especially valuable for the iron and steel industries. Coke makes a useful artificial smokeless fuel because it is free of volatiles.

  • The calorific value of coal (i.e. the amount of heat liberated under controlled conditions) generally increases with rank. Nevertheless, coals with a high volatile content (>30%) are usually burned in power stations for the ease with which they burn, even though they give out less heat than higher ranked coals.

1.6 Impurities in coal

Coal rank reflects the maturity of a coal, but another variable is the ratio of combustible organic matter to inorganic impurities found within the coal. As discussed earlier, impurities result mainly from clay minerals washed into the mire prior to its eventual burial. In addition, some impurities are formed from the plant material itself during coalification.

These inorganic impurities are non-combustible and therefore leave an inert residue or ash after coal combustion. High-ash contents increase the volume of particulate matter ejected into the atmosphere following combustion. So coals used as boiler fuels or for coking require less than 10% ash content.

A second important group of impurities are carbonate minerals. During the early stages of coalification, iron carbonate is precipitated either as concretions (hard oval nodules up to tens of centimetres in size) or as infillings of fissures in the coal. Interestingly, it was probably the breakdown of such iron carbonates to molten iron when coal that contained them was burned, which led to the accidental discovery of iron smelting.

Other impurities are nitrogen and sulphur that are chemically reduced during coalification to the gases ammonia (NH4) and hydrogen sulphide (H2S), which become trapped within the coal. However, most sulphur is present as the mineral pyrite (FeS2), which may account for up to a few per cent of the coal volume. Burning coal oxidizes these compounds, releasing oxides of nitrogen (N2O, NO, NO2, etc.) and sulphur dioxide (SO2), notorious contributors to acid rain.

In addition, relatively high concentrations of sodium chloride (NaCl), deposited by seawater floods into coal-forming mires, make the coal virtually unusable in power plants because salt causes severe boiler corrosion. Lastly, trace elements (including germanium, arsenic and uranium) are significantly enriched in coal and are released by burning it, contributing to atmospheric pollution.

1.7 How old is coal?

Not surprisingly, the distribution of coal deposits through time corresponds closely to the origin and distribution of land plants. (This is discussed further in Section 4.) Coals are commonly found in rocks from Carboniferous times onwards, Devonian coals are rare, and pre-Silurian true coals are never found. This coincides with evidence for the evolution of land plants, which first appeared in Silurian times about 400 Ma (million years) ago, colonized the land surface rapidly through the Devonian and became abundant by Carboniferous times. Not all post-Carboniferous terrestrial sediments contain coals, however. Coal-bearing rocks tend to be concentrated in rocks of specific ages in certain locations, reflecting the development of ancient mires under specific climatic conditions.

1.7.1 Carboniferous mires

During the late Carboniferous, mires developed over vast areas of the UK. Much of today's land area was an extensive, low-lying plain bordering a sea to the south (a sea that was soon to be the site of a mountain-building episode). Any mountains that existed lay hundreds of kilometres to the north. Large river systems meandered southwards across these plains.

At that time, the UK lay in tropical latitudes, almost on the Equator (see Figure 33). The low plains were covered by extensive forests: the Carboniferous equivalent of the present-day tropical rainforests (see Figure 2c). However, most of the 'trees' were hollow, not solid, and more closely resembled modern horsetails than modern trees. No flowers or birds existed, but insect and reptile life was abundant in the forests.

Tropical storms were probably as common then as they are today. Such storms would devastate vast areas of forest, reducing trees and plants to jumbled leaves, branches and crushed hollow logs. The same storms would also have caused extensive flooding. After this devastation, the forest would quickly reestablish itself, only to be devastated again by subsequent storms. The forest floor was probably often metres deep in rotting vegetation destined to become peat and, much later, coal.

In the late Carboniferous, cycles of global sea-level rise and fall resulted from the melting and re-growth of continental ice sheets in the Southern Hemisphere. Consequently, numerous mire-flooding-sediment infill-mire episodes occurred on low-lying coastal plains, which led to thick sedimentary successions with numerous coal seams.

Question 3

Use Figures 2a and 5 to help you match the following sediments in a coal-bearing sequence A-E with their likely environment of deposition (a)-(e).

  • A.Mudstone with freshwater shells

  • B.Siltstone

  • C.Coarse sandstone

  • D.Seatearth

  • E.Coal

  • a.Peat accumulations on the swampy areas of a delta plain

  • b.Distributary stream channels cutting through the delta plain

  • c.Fossil soil beneath the mire

  • d.Deposits laid on delta plain in times of flooding

  • e.Shallow lakes and lagoons on the delta plain

Answer

A (e); B (d); C (b); D (c); E (a).

2 Finding and extracting coal

2.1 Finding and extracting coal

Coal is often regarded as the principal fossil fuel, and with good reason. There is almost three times more energy available from the global proven coal reserves as there is from proven oil and gas reserves taken together. Therefore, it is unsurprising that even today much time and effort is spent locating it.

This section considers the techniques used in coal exploration and how coal is produced from surface and underground mines. But first, a brief look at a few of the historical aspects of coal mining will put subsequent developments into context.

2.2 Winning coal in former times

Coal was probably first used as a fuel by early Chinese civilizations, and there is evidence for coal working in the UK since Roman times. However, early approaches to mining were limited by the available technology, and left much of the coal behind.

At first, coal was dug from seams exposed at the surface in shallow excavations into valley sides that followed the coal seam. The amount of coal that could be extracted from these trenches and from adits (short horizontal tunnels) was small, even when wooden props were used to stop the overhanging roof from collapsing. The first true underground mines, which signified some knowledge of hidden coal seams, were bell pits. Miners would dig a vertical shaft down to a coal seam less than 10 m below the surface. Once the underground seam was reached, the coal was worked outwards in all directions from the bottom of the shaft. When the bell pit became unsafe, the shaft was simply abandoned and a new one started nearby (usually 20-30 m distant).

Digging vertical shafts through overlying strata over 10 m thick is somewhat unproductive, and so pillar-and-stall working was later adopted. As Figure 8 shows, a rectangular grid of tunnels was driven horizontally into the coal from the base of a vertical shaft. Later, the wide pillars of coal within the grid were systematically removed from the furthest limits of the mine back towards the shaft, allowing the roof to collapse (Figure 8b).

Figure 8
Figure 8 (a) and (b) Pillar and-stall workings involved driving a right-angle grid throughout the whole area to be worked, leaving support pillars. In (b) the roof has subsided after removal of the pillars. (c) Longwall workings that removed all the coal as working advanced became common in the 19th century.

For coal seams at depths greater than 300 m, the support pillars were often crushed by the weight of overlying rock and so longwall mining was widely used after 1850. This technique involved a 'wall', usually about 30 m long, being cut into the coal seam in one continuous process, advancing away from the shaft (Figure 8c). Any stone available (for example, sandstone within the seam) was built into support walls 2-5 m wide running at right angles to the working face. These walls and wooden props were used to support the roof after the coal was cut and to prevent crushing of the coal at the working face.

Poor ventilation and drainage limited the size of these early mines. As soon as shafts were sunk beneath the water table, mines began to flood and had to be abandoned if they flooded faster than they could be drained. Flooding ceased to be a major problem following the introduction of steam pumps as early as 1712 in UK mines.

As mines extended further underground, ventilation became a significant problem. Fires were lit at the foot of an 'upcast' shaft — one of a pair of shafts close together. The upward movement of hot gases drew a corresponding draught of fresh air into the mine through the nearby 'downcast' shaft. This method continued to be used until the advent of mechanical air pumps in the 1830s.

Coal extraction became more efficient in the mid-19th century with the invention of rotary cutters powered by compressed air. However, even in the mid-20th century miners still relied on picks, shovels and crowbars to win coal from seams inaccessible to machines. By the early 20th century, electric conveyor belts were used to transport the coal away from the coalface. Even though coal could be cut and loaded onto conveyor belts in one continuous automated operation, the manual loading of coal was commonplace until the 1950s and 1960s.

2.3 Exploring for coal

Early miners would have found it easy to trace the distinctive black colour of coal along an outcrop (for example, a coastline or river valley), and surface trenches were used to locate less obvious outcrops. However, tracing an outcrop underground was problematical as the only means of exploration was by digging costly trial shafts. The development of exploratory steam-powered drilling in the early 19th century improved matters, but it was not until the mid- to late- 20th century that more advanced techniques made it possible to significantly reduce the uncertainties associated with estimating the size of a coalfield.

Modern exploration techniques are aimed at accurately assessing the location, quality and quantity of coal in a coalfield. In order to achieve this there are three broad categories of tools available to geologists: mapping, geophysical methods and drilling. They are considered here in the order in which they are likely to be employed.

2.3.1 Geological mapping of coalfields

Coalfields can be divided into two categories: exposed coalfields, where the coal-bearing strata outcrop at the surface, and concealed coalfields, where they are hidden beneath younger rocks. Exposed coalfields can be defined with considerable precision by surface geological investigations; indeed geologists recording field data still represent the cheapest exploration 'tool' available to the coal industry.

In populated regions, the locations of coal outcrops are well known and mapped. However, in remote areas new finds are still possible. In such areas data acquired from satellites or aircraft are assessed before geologists start exploring on the ground. The Global Positioning System (GPS), now increasingly used for navigation, uses signals from satellites to pinpoint locations precisely, so that geologists can more easily create accurate maps in the field. Field data are increasingly processed using spatial analysis software to create digital maps of coal outcrops and to model the likely extension of the coal beneath the surface.

2.3.2 Geophysical methods — seismic surveying

Geophysical survey methods use measurements made at or near the Earth's surface to investigate the subsurface geology. The most widely used geophysical method is seismic reflection surveying; a rapid and highly cost-effective way of gathering data.

A seismic source (produced either by the explosive release of compressed air in a shallow borehole, or a heavy pad vibrated hydraulically at the surface) generates seismic waves that travel through the ground (Figure 9). These are reflected at buried geological boundaries and return to the surface where their amplitude and time of arrival are recorded by an array of detectors (Figure 9). Data produced by moving the positions of both source and detectors along a surface traverse is processed by computer to produce a seismic section through the Earth along the line of the survey, which take the form of a vertical cross-section. Further details of seismic surveying follow in Section 3.3.3, where its principal use in petroleum exploration is discussed.

Figure 9
Figure 9 The layout of a seismic survey, showing the reflection of seismic waves by two reflecting layers.

Figure 10 shows an example of a seismic section. A series of light and dark lines (or reflectors) approximately show the orientation of the sedimentary strata at depth. Coal has very different seismic characteristics from those of most common rocks. This results in large variations across coal-sediment boundaries that produce strong reflections. Seismic sections can also indicate geological problems that might be encountered in a coalfield. In particular, the displacement of a number of reflectors might indicate the presence of a fault, of which there are several in Figure 10.

Figure 10
Figure 10 An example of a seismic section. The (vertical) arrival time axis in milliseconds (ms) is roughly equivalent to increasing depth. Towards the top of the section a pair of dark lines indicate major coal seams. They are displaced by a fault near the centre of the traverse (marked by the dashed red line). Many other features show up, including greater complexity in the deeper part of the section, and towards the left of the section deep, more steeply dipping reflectors are truncated by the simpler ones at shallower depths: this is an unconformity (solid blue line).

The data in a seismic section enables the subsurface geological structure to be visualized. However, eventually, the interpretation has to be related to reality by drilling vertical boreholes to sample inferred rocks directly.

2.3.3 Drilling

Drilling is expensive, so this next phase of exploration only begins when all the data have been gathered from pre-existing geological and topographic maps, aerial/satellite photographs, geological mapping and from seismic surveying.

The thickness and quality of a coal seam in an area are first determined by drilling boreholes a few kilometres apart using a grid pattern. Mobile drilling rigs (Figure 11) use a powerful motor to rotate a drill bit attached to a series of steel rods within the hole. The bit, made of tungsten carbide or studded with diamonds, grinds away the rock, cutting a cylindrical hole through the rock sequence as pressure is applied to it. Specialized drilling fluids are used to lubricate the bit. The same fluids bring small fragments of rock, or cuttings, to the surface, where they can be examined by the geologist.

Figure 11
Figure 11 A mobile drilling rig in operation.

If substantial samples are required when a coal seam is penetrated, a cutting barrel can be used in place of the solid drill bit to drill out a cylinder of rock, called a core (Figure 12). Coring sequences of strata is slow and expensive, and is only undertaken when details from cores are essential.

Figure 12
Figure 12 Core samples being removed from a cutting barrel in the field.
Question 4

Given that drilling a borehole currently costs around £190 per metre, and that coring is two and a half times more expensive than this, calculate the cost of drilling a borehole 800 m deep, which includes 130 m of core.

Answer

If drilling a borehole costs £190 per metre, coring must cost £190 per metre × 2.5 = £475 per metre. The cost of drilling the borehole is therefore:

The cost of coring the remainder is therefore:

The total cost is therefore £61 750 + £127 300= £189 050

Although expensive, cores do make it possible to constrain the thickness and depth of the coal seam. Furthermore, detailed analysis of other sediments in the cores can reveal the environment in which the rocks were originally deposited. Recalling Figure 5, this may permit geologists to predict parts of the rock sequence where coal is either more or less likely to be found. Coal samples are usually recovered from the core for chemical analysis, to measure its carbon (i.e. rank), sulphur and ash content.

2.3.4 Geophysical methods — borehole logging

If a core is not recovered from a borehole, another way to assess the types of rock that it penetrates is to measure their physical properties. Mounting a string of electronic instruments behind the drill bit most conveniently does this: it allows the properties of the rock to be monitored as the borehole is drilled. An alternative is to lower instruments down the completed borehole by cable; hence the name wireline logging.

Such logging measures several physical properties of the rocks surrounding the borehole. These include the velocity at which sound travels through each rock type, their density and their emission of natural gamma radiation (from unstable isotopes of uranium, thorium and potassium that vary a great deal between different rock types). Because a single physical property cannot define a rock type, the logging geologist must compare a range of properties at each depth to interpret the rock type present.

As the location, quality and quantity of coal in a coalfield will affect the profitability of a mine, mining will not commence until the mining company and its financial partners are satisfied that these crucial parameters have been sufficiently constrained. Even after mining has commenced, further exploration (including the drilling of more boreholes) periodically allows initial estimates to be refined.

2.4 Modern mine planning

Once the geological data gathered during the exploration phase has been evaluated, geologists will estimate the quality and quantity of coal present. Coal reserves (in tonnes) are calculated from volume × density (Section 5). The volume of coal is controlled by seam area and seam thickness.

Hence:

tonnage = seam area × seam thickness × coal density

Seam area is not the same as surface land area, as the coal seam may dip. However, the surface land area will define the land area required for mining. Other considerations are the purity and rank of the coal, the likelihood of encountering any geological problems (which will be discussed a little later), and whether the seam has been worked in the past.

The first decision to be made is whether to open a surface mine, or an underground mine. In surface mines, exposed and/or shallow coal seams are accessed by removal of waste rock or overburden from the surface (Argles, 2005). They are therefore rarely deeper than 100 m below ground level.

  • What do you think controls the maximum depth to which coal seams can be worked economically using surface mining methods?

  • There is no profit to be made in removing overburden, so the maximum economic depth for surface mining depends on the value of coal produced compared with the cost of removing the waste material. In addition, surface mining machinery is unable to operate below a certain depth.

The ratio of the amount of overburden to the total amount of workable coal is therefore of critical importance. Within the coal industry, this is known as the stripping ratio (or overburden ratio in Argles, 2005). Stripping ratios can be calculated either in tonnes or in thickness ratios (commonly used in coal operations). The maximum economic stripping ratio for surface mining has steadily increased over the years to around 20:1, helped by improvements in the productivity and life of the plant and equipment.

Question 5

Figure 13 shows a cross-section through coal-bearing rocks. By counting the squares in the overburden and coal, and by assuming a maximum stripping ratio of 20:1, determine whether the coal seam could be extracted profitably using surface mining techniques.

Answer

The coal seam covers 76 mm2 squares, whereas the overburden covers 21 cm2 squares, i.e. (21 × 100) mm2= 2100 mm2 squares.

So, the ratio of overburden: coal is 2100:76, or 27.6:1. This stripping ratio is higher than the 20:1 limit, so this coal seam would have to be extracted by underground mining.

Figure 13
Figure 13 A cross-section through coal bearing rocks. For use with Question 5. Note that areas on this cross-section are proportional to volumes of rock. The lines showing the mine limit are at angles where bare rock surfaces are stable, so that the mine walls do not collapse during operations.

Mines working high-rank coal, which is more valuable, can operate higher stripping ratios than those working lower-rank coal; the surface anthracite workings in south Wales for example, operate a stripping ratio of 35:1. Note also that because coal is a moderately high place-value resource (Sheldon 2005), it is economic for some present-day surface mines in the UK to extract coal at greater depths than underground mines in other parts of the world that are remote from points of sale.

Once it has been decided whether to use either a surface or underground mine, engineers will begin planning the optimal mine layout and the processes that will be used to extract and remove the coal from the coalface. As Figure 14 shows, computers are often used to model the continually changing layout of the mine with time.

Figure 14
Figure 14 Three-dimensional computer model showing two stages in the development of a surface mine. The pink area shows the projected development of the coalface through time.

2.5 Surface mining

In surface mining (sometimes called 'opencast' mining in the UK), the coal seam is accessed by removing the rock overburden; a process that benefits from economies of scale by using some of the world's largest machines (Figure 15).

Figure 15
Figure 15 A dragline operating to remove overburden above a shallow coal seam being cut by a mechanical shovel.

Figure 16 shows schematically how surface mines are organized. Topsoil is removed and stored or used immediately for land restoration elsewhere. Shallow or soft overburden is removed by draglines, hydraulic shovels and dump trucks, but at deeper levels with harder layers of rock explosives may have to be used. When the first coal is reached, seams are usually worked by bench mining methods (Figure 16). The top surface of coal exposed on each bench is carefully cleaned to remove any adhering waste rock. This careful exclusion of non-coal material enables high-quality coal to be produced consistently by surface mining. The clean coal is then broken up by diggers or with the aid of explosives, and loaded into trucks using mechanical shovels. Bench methods allow every seam to be worked, not just the thickest. Seams as thin as 0.1 m can be worked where they form part of the overburden to a lower, profitable seam.

Figure 16
Figure 16 A diagrammatic section through the surface mining process showing bench mining methods. The ultimate depth of excavation can exceed 100 m. Benches are cut where coal is exposed, but also at other levels to allow access. The benches are often left in place to ensure stability of the mine walls.

The precise sequence of overburden and coal removal in a surface mine depends on the area covered by the mine, and the thickness and vertical spacing of the coal seams. In some cases, a single large pit is opened and the overburden is then moved around within it to gain access to different parts of the coal seam (Figure 17). In mountainous areas, benches may be cut into the slopes and the removal of coal and overburden may result in the mountain being levelled.

Figure 17
Figure 17 Surface mining using bench mining methods at Orgreave, Staffordshire. Nine seams, ranging in thickness from 0.1 to 1.35 m, were worked in this surface mine.

The critical factor is the angle at which the seams dip beneath the surface. In the case of horizontal strata (Figure 13) and an almost flat land surface, wherever the coal seam is present beneath the surface, the overburden is roughly the same. Consequently, the potential size of a surface operation to mine a horizontal coal seam is limited only by environmental legislation, ownership of the land and any areas of rising ground that would increase the overburden ratio. Finding seams that remain horizontal over vast areas is uncommon, however. Where the strata dip beneath the surface, overburden increases as the seams dip to deeper levels, thereby posing a limit on mine size.

Question 6

Figure 18 shows a cross-section through coal-bearing rocks that dip at 45° to the horizontal. By counting the squares in the overburden, and by assuming a maximum stripping ratio of 20:1, determine whether it would be economic to strip mine the coal seam down to depth A and additionally down to depth B. You can assume that the coal seam covers 125 mm2 squares down to depth A and 150 squares to depth B.

Answer

Down to depth A

The coal seam covers 125 mm2 squares, whereas the overburden covers 25 cm2 squares, i.e. 25 × 100= 2500 mm2 squares.

So, the ratio of overburden: coal is 2500:125, or 20:1. This stripping ratio is equal to the 20:1 limit, so this coal seam could be strip mined to this depth.

Down to depth B

The coal seam covers 150 mm2 squares, whereas the overburden covers 33 cm2 squares, i.e. 33 × 100= 3300 mm2 squares.

So, the ratio of overburden: coal is 3300:150, or 22:1. This stripping ratio is higher than the 20:1 limit, so surface mining of the coal seam to this depth would not be economic.

Clearly, at greater depths, proportionately more overburden has to be removed per volume of coal, and this steadily increases the stripping ratio.

Figure 18
Figure 18 A cross-section through coal bearing rocks. For use with Question 6. Note that areas on this cross-section are proportional to volumes of rock.

Your answer to Question 6 shows how the extent of surface mining is limited laterally in the direction of dip, the limits of a mine becoming narrower as the dip increases, because the stripping ratio increases more with depth of excavation. A dip of 45° is at or above that where unconfined rock becomes unstable and begins to slip: surface mining of coal with steeper dips is too dangerous to be undertaken, and, if at all, the seams must be won by underground operations. At lesser dips, surface mining takes the form of operations that are along and parallel to the outcrop of coal seams: this is termed strip mining. Such operations are widespread in the western USA (Box 1) and are particularly appropriate for single, horizontal coal seams at shallow depths beneath flat topography, as in some Australian coalfields and the low-grade brown coals of Germany. Only a narrow strip is 'sterilized' from agricultural use for a year or so as the strip mine progresses across country. Bench mining, as shown in Figure 16, is favoured where there are a number of seams within 100 m of the surface.

In the UK, surface mines are now located in areas of shallow coal where underground mining has ceased, and include districts where the old mining methods had extracted only a proportion of the coal. Typically, surface bench mines are favoured in the UK where access to large areas is highly restricted. They usually operate for between three to five years, cover about 2 km2, and contain about two million tonnes of coal in a dozen or so seams. In 2004, just under half of UK coal production came from 42 surface mines (Figure 19). This is half the number of operating surface and underground mines just eight years before.

Figure 19
Figure 19 The location of surface and underground mines in the UK in 2004. (Underground mining in the UK is considered in Section 2.2.5.)

The UK has approximately 43 × 106 t of coal reserves in operational and nonoperational surface mines. (75% of this coal is in Scotland, the rest being shared equally by England and Wales.) It is becoming increasingly difficult to estimate how long this coal will supply the UK demand, as cheaper imports of coal from around the world have forced the European coal industry, both in surface and underground mines, into rapid decline since the mid-1990s. As a direct consequence, 53% of the coal that is consumed in the UK is now imported and this figure is likely to increase in the future.

The decline in the production of surface mined coal in the UK is not reflected elsewhere in the world where in general, production from surface mines has increased in recent years. Despite the fact that most coal reserves are only exploitable through underground mining, globally some 40% of coal is now surface mined. The USA produces 50% of its coal from such mines (see Box 1); Australia, 70%; and Venezuela and Columbia close to 100%. However, China — by far the largest producer of coal — is dependent on deep coal seams and only mines 7% of its coal at the surface.

Box 1 Surface coal mining at the Black Thunder Coal Mine, Powder River, Wyoming

In the USA, three states (Montana, Illinois and Wyoming) possess over 56% of the country's coal reserves (Figure 20a). Until recently, Wyoming laid claim to the largest single coal mine in the USA; the Black Thunder surface mine. Black Thunder, located in the Powder River Basin (Figure 20b), opened in 1977. In 2003, the mine produced 51.5 × 106 t of coal.

Figure 20
Figure 20 (a) Coalfields in the USA. (b) Map showing coal reserves in the Powder River Basin, Wyoming, and the location of the Black Thunder mine. Note that the zone of strippable coal, of which the mine is a part, extends for almost 2° of latitude and 2° of longitude in Campbell County alone.

To place this in perspective, the whole of the UK produced just over half this amount of coal in the same year.

The coal at Black Thunder is of Early Tertiary age (56.5-65 Ma), so it is significantly younger than the Carboniferous coal of the UK. (The ages of the various coal deposits are discussed further in Section 4.) The principal Wyodak seam is an impressive 22 m thick (Figure 21a). This single coal seam covers some 35 000 km2 of the Powder River Basin, making it the most important in the USA. Its great thickness and the fact that it is composed of the remains of woody material suggest that the coal formed in a stagnant tropical mire where the rate of subsidence was sufficient to allow peat to accumulate over thousands of years. This extensive mire was associated with a delta that moved westward during the Palaeocene.

At Black Thunder, the seam is gently dipping, and locally splits into two beds separated by up to 18 m of waste. The coal is low-sulphur and sub-bituminous and has an ash content of around 5%.

Strip mining techniques are used at Black Section 2 Thunder. They start with the removal of topsoil by mechanized scrapers ahead of the pit and this soil is trucked and placed on areas undergoing restoration. Then, the overburden is stripped away to a depth of 15-75 m to reach the Wyodak seam in a two-stage process.

Around 20 to 30% of the overburden is removed by blasting it directly into areas where the seam has already been mined. Moving such large volumes of rock in this way requires an enormous amount of explosive. The remaining overburden is removed by four large draglines. The largest, called 'Ursa Major', has a 110 m-long boom and carries a 122 m3 bucket, which can clear 150 t of overburden in one scoop (Figure 21b).

Once the coal seam is exposed, it too is blasted to ease its removal. Electric mining shovels then load the coal onto trucks, which transport it to a nearby crushing plant. A 3.5 km-long conveyor belt takes the coal to a further crusher and then into silos from which it is loaded onto trains.

Figure 21
Figure 21 (a) The Wyodak seam, which at the Black Thunder mine is 22 m thick (Photo © Arthur Meyerson Photography). (b) The dragline 'Ursa Major' at the Black Thunder mine. Note the trucks for scale.

With a low-sulphur and ash content, Wyoming's coal is well suited to fuel power stations without any preparation except crushing. Indeed, 96.7% of coal produced in Wyoming is used to generate electricity in 22 US states. With reserves of over 900 × 106 t, the Black Thunder mine could continue to produce coal at its current rate until 2022.

The quantity of coal extracted per worker-day in surface mining can be many times that in underground mines — a factor that is reflected in the generally lower cost of surface mined coal (helped also by lower capital and operating costs). Other advantages of surface mining are that geological problems are more easily resolved and the working environment is safer for mining personnel.

One disadvantage for mine operators is that some major coal-producing countries, including the UK and US, have legislation requiring rehabilitation of the mined area, or laws prohibiting surface mining on land where rehabilitation would not be possible. In such cases, and where the coal is deeper than the economic stripping ratio, underground mining is the only option.

2.6 Underground mining

Coal extraction is of course less straightforward using underground mining techniques. The associated costs are higher, and these begin with the sinking of two shafts, an 'upcast' and a 'downcast' shaft for ventilation (Figure 22). Sinking these to a depth of a kilometre may take a few years and during this time, no coal is extracted. However, not all underground mines involve deep, vertical shafts. Coal seams less than 350 m deep may be reached by inclined tunnels, or drifts. In hillsides, horizontal adits may be opened up directly into the coal seam, producing an earlier return on investment.

Figure 22
Figure 22 Winding houses above coal mine shafts at Harworth Colliery in Nottinghamshire, UK (now mothballed). The inclined structures are covered conveyor belts.

The coal seams exploited in most European mines are typically about 1.5 m thick, but can vary from 0.5 m to 3 m. Modern underground mines use longwall extraction methods (Figure 8c), relying on highly mechanized extraction techniques.Acutting machine works its way along the start of the planned extraction to develop a roughly 250 m long coal face (Figure 23). Temporary hydraulic jacks support the roof at the face. Cut coal falls onto a conveyor belt laid out parallel to the coal face, and is carried away.

Figure 23
Figure 23 A cutting machine working its way along a coal face. A large rotating drum armed with steel picks cuts coal from the seam to fall on the conveyor belt, which moves to the left in this case.

In the most recent installations the conveyor is articulated, and advances automatically, together with the roof supports, as the cutter moves along. By the end of each traverse, the face is ready for the next run in the opposite direction. As the face advances, supports are removed and the roof is allowed to fall into the cavity (goaf) left when the coal is removed (Figure 24).

Figure 24
Figure 24 Schematic block diagram of a typical modern underground mine.

Tunnels or gates at either end of the face link it to the colliery's permanent 'road' system. These roads are used for access of miners and machinery, to ventilate the face and remove the coal. In both new and established mines, roads have to be driven to new areas of coal working. Cutting gates and roads is not a profitable operation in itself, but it does provide valuable geotechnical information to assist the mining operation. Like faces, roads and gates require support, using concrete linings, steel arches or bolts secured in the rock with resin. Wire mesh, secured by the latter, prevents minor rock falls.

The road network links the faces (most modern mines have several working at any one time) to the two shafts (Figure 22 and 24) that allow coal and stale air to be removed, and fresh air, workers and supplies to enter the mine.

Figure 25 illustrates the two common types of underground mine layout used in the UK. At an advance face, the coalface is advanced into a block of coal. The access gates at either end of the face are also advanced to keep pace with the face. At a retreat face, the two access gates are first driven to the far boundary of the block of coal before the coalface is opened at their extremities. The face is then worked back towards the main roadways and the roof collapses in the same direction.

Figure 25
Figure 25 Two types of modern coal face layout in plan view. In the advance face, the coalface and gates advance into a block of coal. In the retreat face, the access gates are first driven to the boundary of the block of coal. The coalface is then opened at their extremity and worked back towards the road network.

Until the late 20th century, most underground mines in the UK were separate operations, dating from the period before nationalization in 1945, although some adjacent mines were linked. The last major underground coal development in the UK focused on newly proven major reserves in North Yorkshire, well away from the existing coalfield of South Yorkshire. It involved mine planning on a huge scale, involving several linked mines (Box 2).

Box 2 The rise and fall of an underground coalfield: the Selby complex in North Yorkshire

The UK's most recently developed underground coalfield, below Selby in North Yorkshire, was initiated between 1972 and the first production of coal in 1983. The Selby complex provides not only a good illustration of the development of a modern coalfield, but also how economic forces can dictate the future of a prospect.

Coal seams in the Yorkshire coalfield dip eastwards under a cover of younger Permian rocks. As demand for coal increased and the exposed coalfield became exhausted, the working area extended progressively eastwards to the deeper levels of the concealed coalfield.

In the 1960s, exploration drilling showed that the principal seam (the Barnsley seam) is between 1.9 m and 3.25 m thick under the Vale of York, north of the town of Selby (Figure 26). This represented an exciting prospect of workable coal covering an area of 260 km2. A major drilling programme commenced in 1972, and from 68 deep boreholes, drilled approximately 1.5-2.5 km apart, reserves of 600 × 106 t were calculated in the Barnsley seam. The coal was found to be very clean, with 2-8% ash content (ideal for the power station customers who specified no more than 18% ash).

Figure 26
Figure 26 Exploration of the Selby coalfield. The planning limits of the Selby project are shown, together with the locations of the Gascoigne Wood mine entrance and underground drift, and the locations of the Wistow, Stillingfleet, North Selby, Riccall and Whitemoor mines. The exploration boreholes are also shown as dots.

Stripping ratios of up to 550:1 at North Selby were far too high for surface mining, so the coal could only ever be extracted by underground methods. The mining complex consisted of two parallel 12.2 km long tunnels that provided the main coal outlet to the railway at Gascoigne Wood. Other tunnels connected working areas in different parts of the coalfield to this main outlet. Shafts served each working area, by providing ventilation and allowing access for workers and machinery (Figures 26 and 27).

Figure 27
Figure 27 The Selby coalfield in 1992, showing the mine shaft, roadway and tunnel layout. The depth from the surface to the Barnsley seam is shown at each mine.

The drilling programme had proved nine other seams of workable quality lying at depths between 300 m and 1100 m. However, the land in the Vale of York is only some 7 m above mean sea-level, so surface subsidence with a consequential risk of flooding was an important consideration. Because of this, planning permission limited extraction to just the Barnsley seam, thereby limiting the surface subsidence to less than a metre, with no extraction around the towns of Selby and Cawood (Figure 26) and some industrial areas. Leaving nine seams behind, as well as a substantial part of the Barnsley seam itself reduced the total amount of recoverable coal from 2000 × 106 t to just 224 × 106 t.

  • What proportion of the coal in the Selby coalfield was recoverable? How long would it take the miners in the Black Thunder strip mine to extract a similar volume of coal?

  • The proportion of recoverable coal at Selby was

    produced compared with the cost of removing the waste material. In addition, surface mining machinery is unable to operate below a certain depth.

    The Black Thunder mine produced 51.5 × 106 t of coal in 2003. Therefore, to produce 224 × 106t would take

The £400 million colliery complex started production under very strict environmental controls in 1983 and output peaked in the early 1990s at 10 × 106 t yr-1. At the height of production, coal from ten retreat faces was transported along the network of conveyor belts to the coal-handling point at Gascoigne Wood. There the coal was washed and blended (by mixing coals of different ash contents from different mines) before being transported by rail to the nearby power stations at Ferrybridge, Eggborough and Drax A and B. Although Selby coal then represented about 15% of all coal used for electricity generation in the UK, it also supplied the domestic 'house coal' market too.

Output at the Selby complex fell to 6 × 106 t yr−1 by 1999. This decline in production resulted from an over-reliance on the power stations as customers, with contracts agreed at a time of low global coal prices. In the end, Selby was unable to maximize its potential, despite long-term contracts with the power stations, the expansion of their market beyond power station fuel, and despite repeated initiatives to maximize production and cut costs. The last mine at Riccall closed late in 2004, some five years short of that originally envisaged for the complex.

2.7 Geological problems in coal mines

A modern coalface is a very complex operation that represents a large investment in terms of capital, labour and planning. Cutting machines and lengthy conveyors are inflexible and require uniform geological conditions to maximize output. What then are the effects of geological variations on such a mining system?

Geological factors control the selection of working areas. The two principal geological conditions that affect mining operations are, first, the nature of the coal-bearing rocks and lateral variations in rock type. The second concerns geological structure: the dip of the strata and the presence of faults.

Variations in thickness and type of rock may occur both within the seam and in the strata forming its roof and floor. The thickness of a seam has a considerable bearing on the profitability of mining operations. Seams 1-2 m thick are particularly suited to mechanized longwall mining, whereas seams less than a metre thick suffer from the law of diminishing returns, since to obtain the same volume of output from a thin seam as a thick one, a greater area of extraction (and hence more rapid face advance) is required. The presence of layers of mudstone within a seam reduces its value, because the quality and marketable value of coal is determined by its ash content. Such layers may also indicate the beginning of a split within the seam.

Of equal importance in achieving rapid face advance is the nature of the roof and floor strata. Mudstones and siltstones usually provide good working roofs for a coalface, but a soft seatearth below the seam will cause heavy equipment to gouge out several centimetres of floor material, reducing the quality of the coal. To counteract this, coal may have to be left at the bottom of the seam.

The presence of sandstones can also result in operational problems at the coalface. The most serious of these arise where channel-fill deposits are encountered in the mudstones above a coal seam. These structures formed when erosive drainage channels on the original delta plain cut down into the underlying sediments and filled with coarse sand. If such channels cut down into the mudstones above a seam, they usually result in unstable roof conditions, and serious roof falls can occur. Sometimes the channels cut down into the coal seam itself and extreme examples may locally have eroded the seam away completely (Figure 28). Where these washouts occur they will bring a working face to a standstill.

Figure 28
Figure 28me of the geological factors that influence coal-mine planning and reserve assessment.

The stripping ratio for the two seams in Figure 28 is less than 7:1 for most of the area to the NW of the fault: the two seams there are economic for surface working and can be considered as reserves. If only the thin seam were present, that would be economic only in the western part of the area. To the SE of the fault, the stripping ratio rises to 23:1, due to the displacement and the angle of dip. This is uneconomic for surface mining and the downfaulted block cannot be considered a reserve for a surface operation. The coal seams in that area would possibly be a prospect for underground mining. However, the features revealed to the west of the fault — washouts, old workings and seam splitting — that pose no great problem for surface mining, would considerably reduce the reserves, were they to be encountered underground. The split might make both branches of the thicker seam too thin to work. Old workings and washouts present hazards from flooding as well as the absence of coal. Large volumes near them would not be workable. Consequently the reserves underground would be considerably reduced.

Whereas the sedimentary setting of a seam and its associated strata determines the profitability of the mining operations, the structural setting imposes physical constraints on the layout of mine workings. Faults form major problems for mechanized faces, as any displacement greater than the seam thickness may bring production to a halt with consequent loss of output (Figure 28). Substantial displacements may also result in the stripping ratio beyond the fault becoming too great for the coal to be economically recoverable by surface mining. Such faults may therefore mark the boundary of a prospect's reserves.

In addition, in some areas of the world igneous rocks intruded into coal-bearing strata will adversely affect the quality of the coal adjacent to the intrusion.

2.7.1 Recognizing geological problems

The most profitable coal mines are those that possess unbroken, horizontal seams of constant thickness and quality. In mines where this is not so, profit levels will depend on the ability of the mine geologist to predict changes in the seam before they are encountered at the face.

Geological problems fall into two categories — gradual changes and sudden changes. Where a change is gradual, such as a seam thinning or splitting, data from boreholes in advance of the workings, supplemented by observations on working faces and development roads can be used to identify those areas where a seam becomes too thin or split to be worked.

Where the changes are sudden, for example with washouts or faults, other methods have to be employed. This may involve the detailed study of the strata exposed in roadways and in borehole sections in order to plot the paths of distributary channels. Forecasting fault positions is more difficult. In particular, unforeseen small faults with a displacement of less than 5 m may require major changes to the mine layout, threatening the profitability of the mine.

On an advance face, information about the geological conditions that lie ahead is limited to that which can be deduced from any nearby workings or boreholes. Therefore, an advance face is a high-risk layout, though this risk has now been mitigated to some degree by using geophysical techniques to 'see' horizontally into the rocks ahead. On a retreat face, the access roads expose any geological problems in the block of coal to be extracted, and the face can be worked back towards the main roadways without risking interruptions. Consequently, the average daily output from retreat faces is some 25% higher than that from advance faces.

  • Why do geological disturbances result in so many problems for underground mining, yet have very little effect upon surface mining?

  • Underground mining is inflexible and needs uniform geological conditions to maximize its potential. Such mines are incapable of negotiating any serious variations in the thickness of the seam. Surface mining is extremely flexible and can strip away all the non-coal rocks regardless of geological variations, leaving the coal to be extracted easily.

For sound geological, economic and safety reasons, surface working will always be preferred to underground mining, so long as the stripping ratio is favourable. Over 95% of all near-surface coal can be recovered in this way, and it is relatively straightforward to exclude all the valueless roof and floor sediments during production. By comparison, underground mining rarely recovers more than 70% of coal in thick seams to ensure exclusion of roof and floor rock. Thin seams, even within the mined sequence, cannot be economically extracted. Areas where the seam is disturbed geologically may not be workable, and support pillars may need to be left to prevent roof collapse and subsequent damage to underground roadways or surface buildings.

Question 7

When a coalface first enters a new area of reserves, it may encounter a variety of geological hazards, which may halt production. Which hazards may be described as gradual changes, and which are sudden changes? What would be the effect on production of each of these two classes of hazards?

Answer

Gradual changes include seam thinning and splitting, whereas sudden changes include faulting and washouts. Gradual changes result in the deterioration in the quality of coal produced because more impurities are extracted as the seam thins. Sudden changes result in the face being halted because the seam is suddenly absent.

3 Environmental aspects of coal mining

3.1 Environmental aspects of coal mining

Coal produced by both types of mining is used either to fuel electricity generation or for industrial and domestic heating, both of which result in atmospheric pollution, but here we are concerned with direct environmental impact on the land. Surface and underground mining operations cause significantly different environmental problems. Those that surround surface mining are common to any large quarrying operation: sterilization of the land and restoration of quarry sites; dust; and noise while operating. Mining waste is not a problem since it helps to fill the hole created. Underground mines produce less noise and dust at the surface, but cause land subsidence and generate long-lasting waste tips, even though some waste is used as backfill. As you will see, both operations can potentially cause water pollution.

3.2 Surface mining

Many environmental issues arise when surface mining is considered, and such mines regularly arouse local opposition. By their very nature, surface mines have a major impact on the landscape, involving the digging of enormous pits with accompanying noise, dust and traffic movements, and destruction of mature landscape. Increasingly, in recent years the environmentally conscious public has used the planning processes to oppose and sometimes prevent mining on sites where the environmental impact would be severe.

Many steps can be taken to minimize the nuisance of surface mines. Topsoil is commonly stored in graded embankments around the boundaries of mines as a baffle against visual intrusion, noise and dust. On-site vehicles can be fitted with effective silencers. To prevent dust being raised on site, water bowsers spray haulage roads. Lorries leaving the site pass through wheel washers and their loads are often covered. Furthermore, individual mines have lives limited to 5-10 years and operators are required to restore former sites to productive farmland, forestry or recreational use by re-spreading the original topsoil.

3.3 Underground mining

Underground mining operations have four significant environmental impacts — spoil heaps, methane build-up, subsidence and water pollution. Spoil heaps have always been the principal surface feature of underground mining operations. However, legislation and technical advances have brought improvements in modern mines, and the closure of many of the UK's older mines has often been followed by successful rehabilitation of mine sites and spoil heaps by landscaping and tree planting.

Coal seams naturally generate methane, and as this is an explosive gas, build-up in old mine workings has to be prevented by venting to the atmosphere. Such methane has been successfully used to generate saleable electricity or diverted into household gas supplies, which lessen its effect as a powerful greenhouse gas.

3.3.1 Mining subsidence

Subsidence is an inevitable hazard wherever underground mining is carried out.

The major factors affecting the extent of subsidence are seam thickness and its depth beneath the surface.

The amount of subsidence can be calculated roughly by using the formula:

where s is the amount of surface subsidence (in m), t is the thickness of the worked seam (in m), and d is the depth to worked seam (in m).

Activity 1

(a) Plot a graph on Figure 29 to investigate the subsidence produced at depth by mining a seam of 3 m thickness under the Selby area.

Hint: You will need a calculator to do this. You will have to work out, using Equation 2.1, the amount of subsidence s for a seam thickness of 3 m for a range of working depths. Use a range from 50 m to 750 m, at 100 m intervals, and then record your calculated values of s and d in a table. Then plot your values of s and d on Figure 29, and draw a curve through them.

Figure 29
Figure 29 For use with Activity 2.1.

(b) From your graph, what is the minimum depth at which a 3 m seam can be worked to produce less than the 0.99 m of surface subsidence demanded at Selby?

Answer

(a) Using Equation 1, you should have been able to calculate the values in Table 2. Your graph of s against d should be similar to Figure 30.

(b) The graph shows that a 3 m seam can be worked at depths of about 60 m and deeper if it is to produce less than the 0.99 m of surface subsidence demanded at Selby.

Figure 30
Figure 30 Graph of estimated subsidence against depth to worked seam for a seam of 3 m thickness.

Roof collapse will often start within 24 hours of coal extraction, but the full effects are transmitted rather slowly upwards, eventually resulting in subsidence at the surface (Figure 31). It may be over 10 years before the surface is completely stable again. Vulnerable structures, such as conurbations, dams, viaducts, and historical buildings are protected by leaving coal unworked beneath them, but such protection may be extremely costly where it significantly affects the layout of the mine. The Selby mine highlighted a particular problem of subsidence by affecting the land surface in a low-lying area. At Selby, government-imposed restrictions minimized both the visual impact of mine site buildings and the effects of ground subsidence in areas particularly liable to flooding or subsidence damage. The mine operator also had to finance flood protection measures and the relocation of the main east coast railway line.

Figure 31
Figure 31 Surface subsidence effects above an abandoned coal mine in Sheridan, Wyoming, USA. The mine was in operation between 1904 and 1921. The indistinct, rectangular-shaped depressions occur where the coal has been removed; their arrangement suggests that pillar-and-stall mining was used (see Figures 8a and 8b). The old workings are approximately 10 to 15 m below the surface. Circular pits at the margins of the rectangular depressions are caused by localized collapses into open stalls.

3.3.2 Water pollution from coal mines

Most underground and some surface mines lie well below the water table. Both therefore have the potential to pollute any groundwater that flows through them. The root cause of the problem is the action of aerobic bacteria on pyrite (FeS2) within the coal sequence. This process releases metal and sulphate ions into solution, which in turn causes the acidity of the mine water to increase:

Before mining, groundwater flow through coal sequences is usually sluggish and in general chemically slightly reducing. During mining, most of the original water is extracted by pumping to keep the mine dry. Pumping exposes the coalbearing rocks to moist air, leading to oxidation of pyrite within the sequence, helped by the catalytic action of bacteria. When mines close and pumping stops, the water table can rise again. Soluble products of pyrite oxidation will pollute groundwater with sulphuric acid, iron and manganese cations, sulphate anions, and sometimes highly dangerous arsenic ions. The roadways of abandoned coal mines form a very effective artificial network of underground watercourses, which can channel polluted water to the surface. Such acid mine drainage ( AMD; see Smith, 2005) is highly damaging to most forms of aquatic life and to humans who come into contact with it. Where this water emerges, surface streams show the telltale sign ofAMD: ochreous slimes (Figure 32).

Figure 32
Figure 32 Polluted waters from acid mine drainage.

Surface mines represent only a transient problem. Water is pumped out of them during working into settling ponds, which ensure that only clean water leaves the site. The mined area is backfilled after use, and the water table generally equilibrates to that within the surrounding area. Preventive measures usually centre around excluding oxygen, water or bacteria from underground exposures to prevent pyrite oxidation. In old mines, products of pyrite oxidation have already accumulated over centuries of exposure, and sealing old mine workings to reduce water flow is impractical.

A solution to AMD is not easy to find. Continued pumping after a pit has closed down is expensive, and routing polluted mine waters into special treatment areas is impractical because it is almost impossible to predict the movement of groundwater in anything other than general terms. Environmental scientists have looked at other ways of improving the quality of mine waters, such as the use of wetlands in which some plants can permanently absorb pollutants, and neutralizing mine waters by adding alkaline calciumcarbonate or sodium hydroxide. Neutralization also decreases the solubility of dissolved metal ions, so that they precipitate and settle out of solution.

4 Global coal reserves

4.1 Global coal reserves

Having looked in detail at how coal is mined, this section focuses on those areas of the world that produce it. It begins by looking at how and why reserves of coal are distributed throughout the UK and Europe, before reviewing the current global reserves of coal.

4.2 Coal distribution in the UK and Europe

The UK and Europe were fortunate in having extensive coalfields that powered the Industrial Revolution. Figure 33 shows the distribution of the major Carboniferous mires which became coal-bearing rocks across Europe, either outcropping at the surface or buried beneath younger rocks. The first thing that is evident from this map is that not all countries shared the same good fortune; either such rocks were deposited, but were subsequently eroded away, or they were never deposited in the first place.

In deciding which of these two possibilities is correct you need to look more closely at Figure 33. It shows the geography of Europe as it might have appeared in Late Carboniferous times, about 300 Ma ago, when the entire region lay close to the Equator. It is clear that the Carboniferous landmass bears little resemblance to the present-day coastlines, which are superimposed on the map for reference.

Figure 33
Figure 33 A map of the present-day North Atlantic region showing the palaeogeographical distribution of mires in Late Carboniferous times. Most of central Europe was mountainous, bounded to the north by low-lying plains and coal-forming mires. Areas where the major Carboniferous coal bearing rocks are preserved (both at the surface and at depth) are superimposed in black.
  • Why are the outlines of Greenland and Norway so close together in Figure 33?

  • The North Atlantic Ocean did not exist in Carboniferous times.

Carboniferous Britain and north Europe formed a low-lying plain backed by newly formed mountains to the south and a shallow sea to the north, beyond present-day Scandinavia. Tropical waterlogged mires developed across Britain and Ireland, through the southern North Sea into Belgium, the Netherlands and northern Germany and east into Poland, as well as between Greenland and Scandinavia. It is likely that coal formed across the whole of this area.

So, why aren't Carboniferous coal measures found over all the area that was land during that period? Consider the following.

  • Is it likely that coal-bearing rocks would have formed in the mountainous areas?

  • Coals form from mires found mostly (though not exclusively) in low-lying areas (see Section 1.3). Therefore, it is unlikely that Carboniferous coal would have formed in the mountainous areas marked on Figure 33.

  • Why are no Carboniferous coals found over the plains area marked in light green on Figure 33?

  • Although relatively low-lying, these areas (including Scandinavia, the Baltic States, south-eastern England and west Wales) were hilly enough to prevent the development of mires.

  • Why aren't Carboniferous coals preserved over all the area once covered by mires?

  • In many areas that have undergone uplift due to tectonic activity, erosion may have removed the coal-bearing sequence. (This happened in the Pennine area of the UK.)

Erosion also helps to expose Carboniferous coals by removing overlying, younger rocks. As a result, surface mining is common in the Ruhr area of Germany, in Poland, in the Midland Valley of Scotland, and in parts of Lancashire and Yorkshire.

4.3 The UK's coal reserves

Production of large quantities of coal in the UK during the 19th and 20th centuries led to the progressive depletion of reserves. In 2005 underground mining was limited to the Carboniferous coalfields of Yorkshire and the East Midlands, with only one underground mine operating in South Wales. However, surface mining sites still work coal in most of the coalfields (Figure 19).

As Section 5 will show, what is considered to be a reserve (i.e. the amount that is thought to be recoverable in the future under existing economic conditions — Sheldon, 2005) changes with time. The coalfields of northern England and the Midland Valley of Scotland that extend eastwards under the North Sea are examples of this. Although coal has been worked there in the past, it is not currently recoverable at a profit.

In 2004, the UK's coal reserves were estimated to be 1.0 × 10 9 t of anthracite and bituminous coal, and 0.5 × 109 t of sub-bituminous coal and lignite. Together, these two figures represent 0.2% of total global coal reserves.

4.4 Coal in the European Union

The EU's coal reserves in 2004, after enlargement to 25 member states, stood at 100 × 109 t. Table 3 shows the eight European Union Member States with the most significant reserves ranked in order of greatest tonnage. With a little over 100 × 10 9 t of coal of all ranks, the EU possesses approximately 10.2% of total global coal reserves. Germany has by far the largest reserves (dominated by 'hard' brown coal and lignite), rivalled only by Poland for coals of higher rank. By contrast, the UK's reserves make a minor contribution to the European total.

Of course, for a more complete picture, the coal resources (i.e. the amount currently (reserves) and potentially profitable to recover in the future, given reasonably foreseeable changes in economic and technical conditions — Sheldon, 2005) of these countries (Table 3) should also be considered. The resource figures (calculated four years before those of the reserves) should be treated with some caution as countries often adopt different criteria (in terms of seam thickness and depth) when calculating them. However, a comparison of the last two columns in Table 3 shows that there are huge quantities of coal currently considered uneconomic to mine. This is especially true in the UK, an issue that will be considered in more detail later on.

Table 3 also shows that many EU countries produce low-rank brown coal in large quantities. Germany, the Czech Republic, Greece and Hungary have greater reserves of brown coal than higher rank coals. Much of this coal formed during Tertiary times, about 20 Ma ago. Tertiary lignites in the UK are limited to small areas around Bovey Tracey in Devon and Lough Neagh in Northern Ireland.

Figure 34 illustrates the production of coal in Europe since 1981 for those countries listed in Table 3. With the exception of Greece, production has declined, especially among the major producers. This trend reflects the ending of generous EU-supported government subsidies, which had allowed otherwise lossmaking mines to remain in operation. As European coal is around three times as expensive as imported coal, it cannot compete in the global market without such subsidies. By switching to fuels other than coal, the European domestic market is unlikely to come to the aid of its ailing coal industry.

Figure 34
Figure 34 Coal production in Europe during the period 1981 to 2002. (The miners' strike accounted for the drop in UK production in 1984.)

Even so, in 2004, the EU produced approximately 593 × 106 t of coal. By contrast, it consumed 749 × 106 t yr−1, making it a net importer of coal. Some countries (e.g. France and Spain) rely more heavily on imports than others.

4.5 Global distribution of coal

Figure 35 shows the global distribution of coal deposits. The major areas are principally in the Northern Hemisphere; with the exception of Australia, the southern continents are relatively deficient in coal deposits.

Figure 35
Figure 35 A map showing the global distribution of known coal deposits of all ages.

This relatively uneven distribution is the result of peat formation at different times in the geological past in predominantly tropical latitudes, and the subsequent drift of the continents to their present-day positions. As Figure 36 shows, the oldest coals of any economic significance date from the Middle Carboniferous Period — the earliest geological strata in which coal has been identified are of Devonian age but they are of little economic significance. With the exception of parts of the Triassic Period, major coal deposits have been forming somewhere in the world throughout the last 320 Ma. Sedimentary sequences of the last 2-3 Ma do not contain coals, simply because there has been insufficient time for them to develop from plant debris. Possible future coals exist today in the form of peats that are accumulating in modern mires.

Figure 36
Figure 36 Global distribution of major coal deposits by geological age. Some coals are known from the Devonian Period, but have minor economic significance.

A broad chain of large coalfields of Carboniferous age extends from the eastern USA, through Europe, the Russian Federation and south into China. A second chain of Permian coalfields is found in the southern continents — South America, India, southern Africa, Australia and Antarctica (the latter is not shown on Figure 35). The vast coalfields of the western USA (see Box 1) and Canada are of Cretaceous-Tertiary age (Figure 35). Mesozoic-Cenozoic lignites are also important global sources of coal.

Figure 35 shows that not all regions have coal reserves. They either do not possess coal-bearing rocks, or they do, but they are not considered reserves.

There are several geological reasons for this:

  • Regions where post-Devonian sediments were not deposited or have been completely eroded have no coal whatsoever, such as Scandinavia and much of Africa.

  • Regions may have no coal-bearing rocks — although they were once deposited they have now been eroded away. Ireland once contained a substantial sequence of Upper Carboniferous coals; probably as much coal as the UK, but only a tiny fraction now remains.

  • Some regions have coal-bearing rocks that are buried so deeply under younger sedimentary sequences that they cannot be worked economically. The Amazon basin is a good example.

  • Few preserved sedimentary rock sequences formed under terrestrial conditions with which mires were associated. Some terrestrial sediments formed at high palaeolatitudes and others in arid climatic belts, neither of which are conducive to peat formation, whilst other sediments formed on the sea floor. The Carboniferous sediments of South Africa and the Permian of the UK (dominated by terrestrial glacial and desert deposits respectively) are good examples, as neither is coal bearing, whereas Permian terrestrial sediments of South Africa and those of the Carboniferous of the UK both contain substantial coal sequences formed in a humid climate.

4.6 Global coal reserves and their life expectancy

In 2003, global proven coal reserves were estimated at 984.5 × 109 t, of which slightly over half (52.7%) was anthracite and bituminous coal and the rest (47.3%) was sub-bituminous coal and lignite.

Figure 37 shows the breakdown of global reserves by continental regions. North America has 26% of total global coal reserves, South Asia and the Pacific (mainly Australia) have 30%, Europe 20%, and the Russian Federation 16%. The relatively sparse distribution of coalfields elsewhere (Figure 35) is borne out by far less significant reserves in Africa and the Middle East (6%), and in South and Central America (2%).

Figure 37
Figure 37 Global coal reserves proven by the end of 2003. Amounts are in 109 t, with the total of anthracite and bituminous hard coal in brackets.

Table 4 shows known global reserves as of 2003, ranked in order of the countries with the highest annual production. However, current reserves represent a snapshot in time; what is considered a reserve depends on current extraction technology, and reserves are depleted as coal is mined. Moreover, coal is a high-volume, low-value resource, and so reserves are highly sensitive to economics, the more so because at the start of the 21st century there is a glut of easily mined coal worldwide.

Resource figures are also given in Table 4, however such data are not available for all countries, including many of the world's major producers.

  • Compare the USA and China in terms of reserves versus coal production in 2003.

  • China has less than half the reserves of the USA, and yet annually it produces 1.5 times more coal. Clearly if this rate is maintained, China's coal will be exhausted before that of the USA.

This is in fact a recent phenomenon. Since 2000, China has increased production of coal by over 40%, overtaking the USA, which was the world's largest producer between 1999 and 2001.

Comparisons between the sustainability of coal production in different countries are made easier in Table 4 by calculating the ratio of proved reserves in the ground (R) to the annual production (P). This R/P ratio has dimensions of tonnes divided by tonnes per year, which is equal to years (t/(t yr−1) = 1/yr−1 = yr). The R/P ratio is therefore a measure of how long the coal reserve (or that of any other fossil fuel or non-renewable resource) will last, if the current annual production figure is maintained into the future.

  • Would you expect the R/P ratio to remain reasonably constant for decades?

  • The pace of exploration and evaluation continually adds to reserves, so they are often maintained at a constant level for many years. However, economics also plays a role — if costs of mining go up or the price of coal goes down some reserves will be lost from the inventory (Sheldon, 2005).

Annual consumption figures and R/P ratios for 2003 are also given in Table 4. Some interesting facts underlie the data given in this table.

  • Despite producing 21.9% of the world's coal, the USA has to import coal to satisfy its demand, even though coal supplies less than a quarter of its primary energy requirement.

  • By contrast, China has four times the population of the USA, yet coal supplies two-thirds of its primary energy requirement.

  • The very high R/P ratio of the Russian Federation (>500 years) is a consequence of the recent fall in industrial coal consumption following the break-up of the Soviet Union.

  • India has the world's fourth largest reserves, but needs to import about a quarter of its coking coal for the country's steel industry.

  • Australia is by far the world's largest exporter (as you can judge from its high production yet low consumption), selling coal to over 30 countries. About half the exports go to Japan (the world's largest coal importer).

  • The figures for the UK reflect the widespread closures of mines in the 1990s that effectively reduced reserves enormously (Section 5).

In years to come, as reserves begin to run low, global coal prices may increase. More coal resources would then become economic to mine, to be reclassified as reserves. This would in turn raise the R/P ratio. It is therefore important to include the future conversion of resources to reserves when considering how long coal will last as a source of energy. However, the current lack of coal resource data for many countries, together with uncertainties associated with predicting future changes in global coal prices make such calculations rather speculative. Of course, such speculations presuppose that coal continues to be favoured despite its contribution to global warming.

  • In Table 4, the R/P ratio for the USA is given as 258 years. If half of the coal currently considered a resource, but not a reserve, was to become a reserve in future, how long would this coal last (assuming no change in production figures)?

  • For the USA, reserves = 250.0 × 109t, and resources = 457.5 × 109 t. Converting half of the coal currently considered a resource only to a reserve:

    0.5 × (457.5 × 109 − 250.0 × 109) t = 103.8 × 109 t

    Therefore total future reserves = 250.0 × 109 t + 103.8 × 109 t = 353.8 × 109 t

    R/P ratio = 353.8 × 109 t/970.0 × 106 t yr−1 = 365 years

Table 4 shows that global coal reserves could last into the 23rd century without any further reserves being identified, if production were to be stabilized at present-day levels. However, coal is mined in over 100 countries, and in the absence of any cartels that might control production (as OPEC does with oil), constraining future global coal production is unlikely. Therefore, this R/P ratio will most likely change with time. As you will see in the next section, R/P ratios for individual countries can vary dramatically too.

5 Coal production in the UK early in the 21st century

This section examines the UK's coal industry in a little more detail, to see how the complex interplay of location, economics and politics has led to the rapid demise of an industry that was once at the heart of the UK's economy.

Figure 38 shows production and consumption figures for coal mined in the UK since 1945 as a number of categories. The decline in total consumption shows that the demand for coal in the UK fell steadily since 1955.

Figure 38
Figure 38 The UK's coal production and imports (blue lines) and consumption (red lines) figures for the period 1945 to 2003. (The miners' strike accounted for the drop in production in 1984.)
  • Which markets, shown on this figure, have contributed to this decline?

  • Between the early 1950s and 2003, both the coke and domestic coal markets declined markedly. In fact, the quantity of coal used for both fell by over 90%. Since 1990 the quantity used for electricity generation also fell sharply.

Coking coal was used extensively for steel-making: the decline in demand between 1955 and 1980 parallels the decline in the UK's steel industry.(Indeed as the globalization of coal and steel markets grew during this time, EU policies aided the decline of both industries in the UK.) The demand for domestic coal reduced over the same period as gas from the North Sea was offered as a cleaner and more convenient fuel from the early 1970s onwards.

Conversely, the demand for coal in electricity generation (Figure 38) grew, albeit erratically, and peaked in the 1980s. At first glance it appears that coal sales to the electricity generating market more than compensated for the decline in the coking coal and domestic markets. However, the fact that total UK consumption has exceeded UK production since the 1984-5 miners' strike bears witness to a global change in coal economics. The growth in size of bulk-transport ships and of surface-mining equipment resulted in coal being produced where surface mines work thick seams close to the surface, thousands of miles from potential markets: coal ceased to be a high place-value resource. Such coal can be transported to the UK for less cost than producing the UK's own coal from underground mines. The UK has been an importer of coal since 1970, but statistics suggest that the miners' strike triggered an increasing reliance on imports (Figure 38).

In the 1980s and 1990s, the UK's coal producers became progressively dependent on power stations for their survival. However, the lucrative contracts offered to the power companies when the coal industry was privatized in 1994 were insufficient to prevent the UK's electricity generation industry (itself privatized in 1990) from moving away from coal to natural gas. This initiative, at the time dubbed the 'Dash for Gas', was triggered by a change in EU regulations, which for the first time allowed gas to be used to generate electricity. Consequently, the amount of coal used for UK electricity generation dropped from over 84 million tonnes in 1990 to less than 42 million tonnes in 1999. That reduced amount of coal was also increasingly supplied by surface mines in the UK and by imports, rather than by the more expensive underground-mined coal.

Figure 39 illustrates how the demand for coal (and indeed oil) for electricity generation in the UK dropped sharply in favour of gas from 1990 to 2003.

Figure 39
Figure 39 The balance of fuels used to generate electricity in the UK in 1990 and 2003. Imports refer to electricity imported from Europe.

As a direct consequence of the switch to gas, the UK's underground-mined coal industry virtually closed down in the early 1990s. In 1984, there were 170 underground mines; by 1994 the rush to close down less profitable mines in the run up to privatization of the coal industry in that year helped to reduce this figure to just 17. By the end of 2005, there were only seven major underground mines still operating. (Small, independent mines still produce coal for domestic use in South Wales, The Forest of Dean, and even north Cumbria.) Ironically, rationalization of the industry now means that the 9000 workers still employed in it are twice as productive as those of the 1980s and 1990s.

Despite coal production being in private hands, government subsidies during periods of low coal price have prolonged the lives of some mines, though political and economic pressures continue to ensure an uncertain future for the seven mines that remained in 2004 (Figure 19). At the outset of the 21st century, the UK's major coal producers continue to depend almost entirely on the power companies. Power generators consume coal preferentially when the global gas price is high, with a knock-on effect for coal production (this is the cause of the fluctuations seen in Figure 38). With this variable in mind, it has been estimated that by 2010, coal-fired electricity generation could be anywhere between 35 and 80% of current levels, the huge range illustrating the uncertainties of prediction in the energy industry.

The quantity of imported coal continues to grow and in 2001, for the first time, more coal was imported than produced in the UK (Figure 38). In 2004 the UK imported about 50% of coal used here, mainly from Columbia and South Africa, together with some from Australia (a sea voyage of some twelve thousand miles), Poland and the USA. The low cost of bulk shipping, the quality of the coal and a competitive price clearly matter much more than the distance from the UK.

The short-term future of the UK's coal industry may ultimately be decided by its own government, which is committed to renewable sources of energy. New EU Directives aimed at reducing NOx, SO2 and CO2 emissions will force power companies to switch to low-sulphur imports and close older coal-fired power stations, further damaging the coal industry. The technology needed to capture NOx and SO2 during coal burning does exist, but has yet to be deployed widely. However, the UK's coal industry may ultimately be saved by the need for a secure domestic fuel source, especially with nuclear power facing an uncertain future and North Sea gas production in decline.

The plight of UK coal illustrates dramatically how vulnerable the concept of reserves is to economic and political change. In 1950, it was estimated that the UK's 'remaining coal reserves' were 172 × 109 t. The UK's total coal production during the period from 1950 to 2003 was about 7 × 109 t, which should have left more than 165 × 109 t in place. Why then does Table 4 show the UK's reserves to be just 1.5 × 109 t? Although most of the coal from the 1950 estimate is still in place, the country's 'technically and economically accessible' reserves have fallen to less than a hundredth of their mid-20th century value. Whilst most of the 165 × 109 t not currently considered reserves will be available in future, the coal remaining when underground mines close down is lost forever, through both flooding and subsidence. So, the proportion of the UK's coal resources that will constitute reserves in future is a technological, economic and political issue.

From today's perspective, the global electricity generation sector cannot manage without coal. Despite the demise of the UK's underground coal mines, the global future for coal looks more optimistic than for any of the other fossil fuels, simply because there is so much of it.

Conclusion

  1. Waterlogged organic matter accumulates in deltaic, coastal barrier or raised mires to form peat. Coal forms by the compaction and decomposition of peat. Chemical changes imposed by increasing temperature and pressure over time determine the coal rank.

  2. Coalfields can be classified as either exposed or concealed, depending on whether or not the coal-bearing rocks are hidden by younger strata. In most coalfields, mining commenced in the shallower exposed regions and has gradually extended into the deeper parts of the concealed coalfields.

  3. Surface outcrops of rock can indicate the likelihood of finding coal at depth. In remote areas of the world, initial surveys often use data acquired from satellites or aeroplanes, before detailed ground exploration takes place.

  4. Drilling boreholes is the only way to determine reserves and coal quality in concealed rock sequences. Geophysical logging records the nature of the strata located in the boreholes without the need to take expensive cores.

  5. Calculation of the stripping ratio indicates whether surface or underground mining is most economic.

  6. Surface mining is a flexible, cheap system that is currently producing most of the coal exported globally. However, it results in short-term local environmental disturbances; underground mining provides longer lasting problems, owing to spoil heaps, subsidence and acid mine drainage.

  7. Most underground coal is extracted nowadays using the longwall method and mechanized systems. Such coal-cutting systems are very inflexible and are incapable of negotiating geological variations in the coal seam. Continued exploration during coal production is therefore necessary to predict these variations and to maintain the continuity of operations.

  8. The oldest coalfields are of Carboniferous age, which formed in tropical latitudes. The Permian-Triassic coalfields of the Southern Hemisphere originated in temperate latitudes. Brown coals make up half of global coal reserves; they are mainly Jurassic to Tertiary in age. Extensive Tertiary deposits of low-rank coal are worked in Eastern Europe.

  9. Reserves of coal depend on whether or not deposits are economic to mine under current economic circumstances (costs of mining set against the price of coal). Technology, economics and political circumstances often change unexpectedly. So it is possible for reserves to become resources if they fail to be profitable, and vice versa.

  10. In 2003, global reserves of 984.5 × 109 t of coal were predicted to last into the 23rd century, at 2003 rates of production. China is currently the largest producer though it has a relatively low R/P ratio.

  11. Demand for coal has fallen in the UK over the last 20 years, because of technological, economic and political factors. The UK now imports more coal than it produces.

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Acknowledgements

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Figure 1a Department of Earth Sciences Image Library, The Open University

Figure 1b Professor R A Spicer, The Open University

Figure 2b Norman Tomalin/Alamy

Figure 2c John Watson, The Open University

Figure 3 NASA

Figures 4 and 35 Thomas, L. (2002) Coal Geology, John Wiley and Sons Ltd

Figure 6 Kevin Church, The Open University

Figure 7 Skinner, B.J. (1976) Earth Resources, Figure 13, Prentice-Hall, Inc, Englewood Cliffs, NJ

Figure 8 Cossons, N (1987) 'Coal', The BP Book of Industrial Archaeology, 2nd edn, David and Charles Publishing Inc

Figure 10 WesternGeco

Figure 11, 17, 22, 23 and 32 MRP Photography

Figure 12 and 15 James A Luppens, USGS

Figure 14 Scottish Coal Company Ltd

Figure 21a Arthur Meyerson Photography

Figure.21b Tejada Photography Inc

Figures 24 and 25 Guion, P.D. and Fulton, I.M. (1993) 'The importance of sedimentology in deep-mined coal extraction', Figures 31 and 32, Geoscientist, vol. 3, no. 2, The Geological Society

Figure 27 The Selby Coalfield, British Coal

Figure 31 Dunrud, C.R. and Osterwald, F.W. (1980) 'Effects of coal mine subsidence in the Sheridan, Wyoming area: a summary of geology, subsidence, and other effects of past and present mining as related to the environment, coal resource management, and land use', US Geological Survey Professional Paper, 1164, 49p

Figure 35 Based on map by Professor A.J. Smith

Figures 38 and 39 UK Energy Brief July 2003, Department of Trade and Industry. 'Crown copyright material is reproduced under Class Licence Number C01W0000065 with the permission of the Controller of HMSO and the Queen's Printer for Scotland'

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