by Vaclav Smil
Most coal basins have multiple exploitable seams, and hence even with the fuel of lower quality their deposits will average tens of gigajoules under every square meter, and the richest basins will have stores an order of magnitude higher. US data show that the average for the five richest Appalachian coal beds-Pittsburgh, Upper Freeport, Fire Clay, Pond Creek, and Pocahontas 3-is almost 200 GJ/m2 (USGS 2013). The latest evaluation of coal resources in the Powder River Basin in Montana and Wyoming-where the Wyodak seam, with an energy density of 20.3 GJ/t, is between 9.1 and 24.4 m thick-showed original stores up to 400 GJ/m2 (Scott and Luppens 2013).
In Germany's brown coal-mining region, between Cologne and Aachen, the deposit exploited by the Fortuna-Garsdorf mine yielded about 400 GJ/ m2 during the 1980s, while the region's three mines operating in 2013, Inden, Garzweiler, and Hambach, extracted from seams with a combined thickness of up to 45 m and contained about 450 GJ/m2 (RWE Power 2013). World records belong to the Australian coal deposits: in the Latrobe Valley of Victoria's Gippsland Basin, brown coal seams that are up to 100 m thick store more than 1 TJ/m2, and at least 50% more when inferred resources are added (Geoscience Australia 2012). And the thickest parts of the Number 3 seam of high-quality bituminous coal (32 m; 24.5 GJ/t) in Queensland's Blair Athol mine, whose production should cease in 2016 (Rio Tinto 2013), also stored at least 1 TJ/m2.
Underground Mining
The actual power densities of coal production depend not only on the energy stored in seams but also on the modes of extraction. Traditional underground room-and-pillar technique had to leave about half of all coal in place to support the tunnel roofs, but longwall mining has made it possible to nearly double that rate. This technique, pioneered in Europe and adopted in the United States only during the 1970s, uses large shearers (drum-shaped cutting heads) to cut coal from the length of a seam (some longwalls are now 300 m long) and dump it onto conveyors while the miners stay protected under a jack-supported steel roof that advances along a seam as the cutting progresses, and the roof behind it is left to cave in (Osborne 2013). Longwall mining can recover more than 90% of coal in place as long as the seams are reasonably level or slightly inclined. This technique boosted productivity to such an extent that the two largest underground US coal mines, Bailey and Enlow Fork, in Pennsylvania, now produce close to 10 Mt/year (USEIA 2011a).
Coal mining land claims fall into two obvious categories. Land occupied by permanent structures includes all buildings (with stationary operating equipment, maintenance shops, and parts stores), parking lots, facilities to process (wash, sort, crush) raw coal before marketing, and load-out arrangements to transport the fuel. The extent of this claim may change with time as a particular operation grows or declines, and in the case of underground mining it can be fairly limited. The aboveground structures of underground mines include buildings to operate hoisting and ventilation machinery, offices, machine shops, and storage spaces. For a mine producing highquality coal that need only be crushed to uniform sizes before marketing, the only other permanent structures could be a crusher and a silo to store coal and loading facilities for trucks or railroad cars; all of these could occupy only about 1 ha for a mine producing 1 Mt/year.
But most coals need preparation (washing, rock removal, and crushing to specific sizes), and most underground mines have on-site facilities to perform those tasks (Arnold, Klima, and Bethel 2007; Leonard and Leonard 1991). The resulting waste makes up the second, incremental category of land claims as more space is taken every year by its disposal (in the past, it was often stored in tall conical heaps) and by often extensive tailing ponds. Surface mining may have the same kind of need to store waste products, but most of its incremental space demand results from the need to remove and reposition often large volumes of the overburden (sands, clays, and rocks) that covers coal seams.
Even so, large underground coal mines make relatively limited land claims, an obvious consequence of concentrating the output from a large area of underground corridors into a fairly small area of the mine's aboveground operating and fuel-processing facilities. The total for the Bailey Mine in Waynesburg, southwestern Pennsylvania, is about 60 ha, and with an annual output of 9.8 Mt (at 30 GJ/t), that translates into a very high power density of about 15,000 W/m2. Similarly, even after counting all land claimed by past storage of mining waste and tailings, the Enlow Fork Mine, the second largest underground operation in the United States, also in Pennsylvania, occupies roughly the same area, and its extraction power density is about 14,000 W/m2. Rates for smaller mines with poorer coal in thinner seams are an order of magnitude lower, but commonly more than 2,000 W/m2.
But in many relatively shallow mines the tunneling has additional surface impacts far away from a mine shaft as a result of ground subsidence. For example, by the end of 2009 the underground operations of the just mentioned Bailey and Enlow Mines in Pennsylvania extended over respectively 12,600 ha and 14,000 ha, with further areas intended for expanded longwall mining (Schmid & Company 2010). Surface subduction caused by underground mining is sometimes a continuous slow process that damages streets and buildings; in other cases it results in a sudden catastrophic collapse. The impact on local water resources is another surficial effect of underground mining that is of obvious concern to communities relying on those water supplies.
Surface Mines
The removal of the overburden in surface (open-cast) mines has become an increasingly ambitious enterprise. Only in exceptional cases are thick coal seams hidden beneath a thin layer of soil and rocks: in Australia's Latrobe Valley, thick brown coal seams lie under only 10-20 m of easily removed sandy overburden (Minerals Council of Australia 2011). Technical advances have made it possible to exploit reserves whose overburden to coal seam ratio is higher than 3:1, in a few cases even surpassing 6:1, and the deepest mines (such as the Rhineland's Hambach) now extend to as much as 370 m below the surface.
Topsoil from fields is put aside for future reuse or is used in ongoing recultivation projects on the site, and in Europe, entire villages or small towns may have to be relocated or their inhabitants resettled elsewhere (Michel 2005). The overburden (usually sedimentary clays and rocks) is removed by some of world's largest electric machines: stripping is done either by giant bucket excavators (common in German mines and deployed to create terraced cuts) or by walking draglines (used in the United States). The German practice is to deposit the overburden by spreaders behind the cut or carry it farther away by conveyor belts and use it to backfill older mined-out sites or to create artificial tabletop accumulations. In the United States, surface mining in the West, including the world's largest open-cast mines in Wyoming, is done by terraced trenches or pits in ways similar to German operations.
In Appalachia, miners follow the contours of hills with outcropping seams, or they proceed in a much more destructive way: since the 1980s, mountaintop removal has leveled more than 500 peaks, mostly in Kentucky and West Virginia (Copeland 2014; McQuaid 2009; Perks 2009). Summits or ridges above coal seams are removed by blasting away layers of rock often more than 150 m thick. Laws require returning this overburden to the mined areas, but the spoils are commonly dumped into adjacent valleys, creating massive fills that bury streams and may be more than 300 m wide and more than 1.5 km long, with a volume of more than 200 Mm3. The extensive land claims of mountaintop mining are also illustrated by averages of land occupied per mine in West Virginia: for underground mines it is just over 16 ha, whereas for mountaintop operations it is about 140 ha.
Coal is usually mined by giant electric shovels (with a bucket able to scoop more than 100 t), and its recovery rates are high: in the world's largest mines, in Wyoming, they average 91% (WSGS 2013). Cut coal is either transported from the coal face by conveyor belts (in Europe) or it is hauled away by huge off-road Liebherr, Komatsu, or Caterpillar trucks capable of carrying loads of more than 300 t. In some mines coal is moved by conveyor belts directly from the mining face to a nearby mine-mouth electricity-generating plant, but in m
ost cases more land is claimed by loading facilities to transport the fuel by rail: for long-distance deliveries it is rapidly loaded into railway cars coupled in unit trains of up to 150 cars (Khaira 2009).
The trains' total loads are usually in excess of 10,000 t, and loading is done in less than two hours. In large open-cast Wyoming mines these loading arrangements consist of a railway loop, tall coal silos, and automatic loading dispensers that operate round the clock. In the Black Thunder Mine the loading facility (including the railroad loop) occupies less than 70 ha, in the Cordero Mine less than 20 ha. Unit trains run constantly between a mine and their destinations, large thermal electricity-generating plants that might be hundreds, even thousands of kilometers away, and coastal loading terminals, where coal is transferred to large bulk carriers for intercontinental shipping (mostly from Australia, Indonesia, South Africa). Some long-distance transport is also done very inefficiently by trucks (a common practice in China, as railroads are already overburdened by coal shipping) and very efficiently by river barges (in the United States on the Mississippi).
Some open-cast coal mines publish their cumulative and annual land claims, and satellite imagery makes it easy to assess not only the extent of the areas directly affected by ongoing open-cast extraction but usually also of the previously exploited areas that have been simply abandoned, replanted with grasses and trees, or converted into water reservoirs. Calculations based on the company's data (RWE Power 2013) show the following annual power densities of lignite extraction in the three Rheinland opencast mines: Inden, 2,400 W/m2; Garzweiler, about 2,800 W/m2; and Hambach, approximately 5,000 W/m2 (fig. 4.1).
For the world's two largest open-cast mines, the North Antelope Rochelle Mine, a complex of two mines (operated by Peabody Powder River Mining, near Wright, Wyoming; fig. 4.2) and the Black Thunder complex (Arch Coal Company, operating six pits), recent power densities of coal extraction have been almost 12,000 W/m2 (Arch Coal 2013; Wyoming State Geological Society 2013). In 2010 and 2011, Queensland's Blair Athol Mine extracted coal with densities around 2,400 W/m2 if only incremental land claims were counted (Rio Tinto 2013). China, the world's largest coal producer, extracts most of its coal from underground mines, but its largest open-cast operation, Heidaigou Mine, south of Junggar in Nei Monggol, produces subbituminous coal (at an energy density of 16 GJ/t) from a site that now covers about 15 km2, with a power density of less than 1,000 W/m2 (Google Earth 2014; Zhang and Cotterill 2008).
Figure 4.1
Garzweiler lignite mine, Germany. © FEDERICO GAMBARINI/epa/Corbis.
Mountaintop removal is unquestionably the most space-demanding method of coal extraction (also causing great harm to streams, water quality, and biodiversity), as many of these operations cover more than 500 ha and as the largest mountaintop mines can claim more than 2,500 ha and result in dumping some 750 Mm3 of spoils. Extraction in some of these mines-where massive volumes of rock are blasted away to get at some thin seams-proceeds with power densities lower than 200 W/m2, and even less than 50 W/m2, the same order of magnitude as some PV conversions. I have chosen extreme cases for these calculations in order to establish the full range of power densities of coal production, and there is a helpful way to verify the results by using aggregate US data on land disturbed by coal mining.
According to the annual reports of the US Office of Surface Mining and Reclamation, between 1996 and 2009 surface mining in Wyoming disturbed about 25,700 ha, for a cumulative output of 4.37 Gt (SourceWatch 2011). Converting the output with an average energy density of 20.3 GJ/t yields a high power density of almost exactly 11,000 W/m2 for Wyoming coal extraction. In contrast, in 2009 the power density for surface coal mining in Tennessee was only about 350 W/m2, a very low rate caused by the state's extensive mountaintop extraction. Land claims in other states fall between these two extremes, and nationwide totals of 624,400 ha and 8.69 Gt for the years 1996-2009 translate to just over 1,000 W/m2.
Figure 4.2
North Antelope Rochelle Mine Complex, Wyoming. NASA Earth Observatory.
Those open-cast operations that follow the strictest operating rules cause only temporary disruption. Once the mining stops, the land can be contoured to approximate its natural state and replanted with grasses, shrubs, and trees or converted to water surfaces within three to five years after the mining ends. On the other hand, every coal-mining country has large areas of old open excavations, overburden heaps, rock waste from underground mining, and mine tailings that have never been reclaimed. Since 1977, when the US Congress passed the Surface Mining Control and Reclamation Act, the program has reclaimed nearly 100,000 ha of land affected by coal mining, but that still leaves almost 5,200 coal-related abandoned mine sites to be fully reclaimed (Abandoned Mine Lands Portal 2013).
Coal Transportation
Adding land needed for coal transportation outside mine loading facilities and unloading structures at power plants of ports could be done easily only in the case of a dedicated line used for no other shipments-but then it might not turn out to be a significant addition. For example, a double-track line used only to move coal between an open-cast mine (extracting 10 Mt/year with a power density of 2,500 W/m2) and a 4-GWe power plant 500 km away would occupy (even under the liberal assumption of a 20-m right-of-way for two tracks) 1,000 ha-but during the 30 years of extraction the mine would disturb more than 11,000 ha.
In reality, a large mine usually supplies many customers, and unit trains travel over many lines and share them with other traffic (Kaplan 2007). For example, coal from the Black Thunder complex is transported via Burlington Northern Santa Fe and Union Pacific railroads to some 115 power plants in more than 20 states, as well as to Europe (BNSF 2013). While it might be possible to calculate the share of the ROWs attributable to coal that has originated from that mine since it began coal extraction (on the basis of a detailed breakdown of annually carried cargo), it would, when prorated over many decades, represent a negligible addition to incremental space claims made by that large open-cast mine. Similarly, land occupied by large railway terminals where coal is received and loaded for overseas export is a small fraction of the land claimed by mining. The Lamberts Point Coal Terminal in Norfolk, Virginia, for example, covers more than 150 ha, but because it can handle up to 44 Mt of exports a year (Dinville 2013), its throughput power density prorates to at least 24,000 W/m2, a very high rate, adding a negligible amount to the overall land claim.
Perhaps the most notable conclusion of this survey of power densities of modern coal extraction is their wide range. The most productive underground mines using longwalls produce the fuel with power densities in excess of 10,000 W/m2, while many underground mines exploiting thinner seams of poor-quality coal and storing incombustible waste and tailings produced by the requisite coal cleaning have power densities well below 1,000 W/m2. And the differences are even greater for surface mines: some mountaintop removals in Appalachia rate well below 100 W/m2 (fig. 4.3), while extraction in some of the world's largest mines exploiting the world's richest coal seams in Wyoming's Powder River Basin or Australia's Latrobe Valley exceed 10,000 or even 15,000 W/m2. The power densities of coal extraction thus span two orders of magnitude, being nearly as low as those of PV-based electricity generation and as high as those of rich hydrocarbon fields.
Figure 4.3
Mountaintop removal coal mining, in West Virginia. © Daniel Shea/Galeries/Corbis.
Crude Oils, Refining, and Long-Distance Deliveries
Crude oils are much more energy dense than coals (their energy densities cluster tightly around the mean of 42 GJ/t) and their share of incombustible constituents is negligible, but their sulfur content is often high (more than 2%) and serves to divide the liquids into sweet (low-sulfur) and sour (highsulfur) streams (Smil 2008). Another key classification is according to specific density, with light oils containing a higher share of lighter fractions and heavy oils requiring expensive catalytic cracking to produce higher shares of the most valuable transportation fuels, ga
soline and kerosene. The worldwide supply of crude oil has been facilitated by relatively inexpensive ways of long-distance delivery, by pipelines on land and by tankers for the intercontinental trade.
Prorating the best reserve estimates per unit of surface area that corresponds to the underground extent of the reservoirs results in power densities that are very similar to those of coal deposits. For many smaller fields the rates are less than 1 GJ/m2, and even some giant oil fields (the designation applies to any field that contains 500 million barrels, or 59,000,000 m3, of ultimately recoverable oil) store less than 10 GJ/m2. Only the richest fields store 101_102 GJ/m2. As many fields also contain substantial volumes of associated natural gas, their original storage should be reported for the combined content of the two hydrocarbon fuels. The next paragraph lists a few well-known examples of original storages in ascending order of energy density (Li 2011; Nehring 1978; Robelius 2007).
The Algerian Hassi Messaoud oil field, known for its exceptionally light and sweet crude, rates 35 GJ/m2. The Shayba oil field, a supergiant in the desolate sands of Rub' al-Khali/Empty Quarter desert, Saudi Arabia, stores about 130 GJ/m2. Samotlor, Russia's largest oil field and number six in the global ranking, which covers 1,752 km2 in the Tyumen region of western Siberia, had original stores of about 180 GJ/m2. The al-Ghawar oil field, the world's richest oil field and a true supergiant that extends over 220,000 ha in Saudi Arabia's Eastern Province, originally contained about 260 GJ/m2 (220 GJ/m2 of crude oil and 40 GJ/m2 of natural gas). Alaska's Prudhoe Bay (North Slope) oil field, the largest (85,417 ha) and richest hydrocarbon field in North America, had originally in place about 145 EJ of crude oil and 44 EJ of natural gas (BP 2006), for an aggregate energy storage density of about 220 GJ/m2. And the Greater al-Burqan, Kuwait's largest field and the world's largest petroliferous sandstone formation, just south of Kuwait City, which covers 780 km2 and had about 440 EJ of oil and 65 EJ of natural gas originally in place, stored about 560 GJ/m2.
