The Physics of Energy, page 137
A full treatment of seismic waves involves some details from the theory of elasticity, but the basic structure is quite simple and intuitively clear. A solid is characterized by two elastic moduli, which are conceptually similar to the spring constant k of an oscillator, in that they measure the restoring force that develops in response to a deformation of the material. The two elastic moduli give the ratio of stress to strain for compressions and shear deformations. As discussed in §29, pressure and shear stress have units of force/area. Strain is a fractional deformation in a dimension of a solid; for example, a rod that is uniformly compressed to 99% of its original length has a strain of along the axis of the rod. The bulk modulus K and the shear modulus μ give the ratios of pressure and shear stress over the corresponding strains. The elastic modulus governing the motion of P-waves is a combination of the two moduli K and (Problem 33.5). The wave equation for P-waves analogous to eq. (4.2) essentially follows directly from Newton’s law ,
and describes waves with velocity . A seismic P-wave in limestone travels at roughly 5.5 km/s, in sandstone at roughly 3.6 km/s, and in hard coal at between 1.8 and 2.8 km/s.
Like the light waves described in §4.5.2, when seismic waves pass from one medium to another medium with different properties, part of the wave energy is reflected and part continues to propagate into the second medium. When a seismic wave propagates from sandstone or limestone into coal, the amount reflected is determined by the difference in acoustic impedance between the two media, which is quite substantial. This property is used in coal exploration; a seismic wave with a gradually changing frequency may be produced on the surface, for example by a VibroseisTM truck, as shown at right. The truck lifts into the air and vibrates, producing powerful sound waves that are reflected by underground structures to receivers arrayed across the surface. The vibration begins at a low frequency and proceeds continuously to higher frequencies, so that the part of the waveform measured by the receivers can easily be identified and matched to the source to determine the time of propagation of each component of the reflected wave. This information is used to locate underground coal deposits (Figure 33.7).
(Image credits: (Top) US Geological Survey; (Bottom) NEES hub)
Geophysical Borehole Logging A more precise picture of the strata in a given location is given by borehole logging. After a borehole is drilled, a measuring device called a sonde is lowered into the hole. As the sonde is gradually lifted through the borehole, it can make a variety of measurements including determination of geophysical parameters using sonic, resistivity, and caliper (borehole diameter) probes. Among the most useful measurements in coalfield exploration are radiation-based measurements. A gamma ray log records the level of γ-rays coming from radioactive isotopes, particularly , in the surrounding rock. Since coal is generally low in radioactive isotopes, a low γ-ray count can be indicative of the presence of a coal seam. Within the coal seam, an increase in γ-ray count can correlate with increased ash content, though this is highly dependent on the type of inorganic material mixed into the coal; the radioactive isotope content of different impurities can vary significantly.
A density log is produced using a sonde with an energetic γ-ray emitter on one end and two scintillation detectors at different distances from the emitter. The detectors measure γ-rays that are scattered back to the sonde by electrons in the rock around the borehole. The two detectors together make it possible to calibrate for the borehole configuration and to measure the density of the surrounding rock. The number of γ-rays that reach the detectors varies inversely with the density of the surrounding rock since higher density gives rise to more scatterings, which reduce the γ-ray energy below the detectors' threshold. Since coal has a much lower density than most other rock types, coal seams are easily identified as local regions in the log with high counts of returning gamma rays.
33.1.4 Coal Extraction Methods
One reason why coal is inexpensive relative to other energy sources is that highly efficient extraction technologies have been developed over the last century. In general, coal extraction methods can be divided into underground and surface methods.
Underground methods are generally used for extracting coal more than 100 m beneath the surface. The main underground methods are longwall mines and room and pillar mines; these approaches can also be combined in a single mine. Underground mines are often the only way to access seams of the highest quality black coal.
Figure 33.7 Seismic profile showing two coal seams, Wyodak, Wyoming, USA. Depth beneath the surface can be estimated from round-trip travel time. From [226]. (Credit: John Wiley and Sons (2002))
Longwall mining In a longwall mine, all coal is removed from a roughly rectangular “panel” within a seam of coal (Figure 33.8). This method is usually used for seams 1.5–4 m thick, though variations are possible for thicker seams. The panel typically has a width of 100–300 m, and may extend for as much as several kilometers. Longwall mining is highly automated; a single longwall shearer can mine at a rate on the order of 50 t/h, and production rates in longwall mines are on the order of 10 kt/employee-year.
Figure 33.8 A longwall mining operation is highly automated. A longwall shearer moves back and forth across the face being mined, cutting coal that falls onto a conveyer system for removal. As the shearer advances, supports are moved in tandem, and the mine is allowed to collapse in the region behind the shearer. (Credit: Peabody Energy Corporation)
Room and pillar mining In this approach, a pattern of tunnels is dug leaving substantial intact pillars between the tunnels to support the roof (Figure 33.9). This method is cheaper and easier to implement than longwall mining, but less productive. Room and pillar mining has also been largely automated in some places.
Figure 33.9 In a room and pillar coal mine, pillars to support the mine roof are left behind as the coal is removed. Often, the pillars are later worked as the roof collapses behind. (Credit: Peabody Energy Corporation)
Box 33.2 Extraction of Coal Energy
One way to appreciate the highly compact and effective nature of fossil fuels is to consider the rate of extraction of energy in the form of coal from a standard coal mining operation. In a typical US longwall mining operation, the rate of coal extracted per employee is on the order of 10 kt/employee-year. Assuming that the coal being mined is anthracite with an energy density of 30 MJ/kg, this represents some 300 000 GJ of energy extracted in one year from the efforts of each person involved. It is illuminating to consider the efforts needed to extract this quantity of energy from other systems (see Problem 33.4).
Opencast and strip mining Surface extraction methods known as opencast and strip mining proceed by excavating all material from the surface downward, including material above and between coal seams (Figure 33.10). Surface mining is generally only viable to depths of less than 100 m. Surface mining has fewer technical complications than underground methods, though extra costs are involved in even partial restoration of areas impacted by surface mining. Surface mining is almost exclusively used for extraction of brown coals. As some easily accessed black coal reserves have been exhausted and technology for surface mining and using brown coal effectively has advanced, new mining has largely shifted to surface methods in recent years. Large brown coal surface mines can produce up to 40 Mt/yr.
Figure 33.10 An opencast lignite coal mine at Garzweiler in Germany. The mine covers 48 km2. (Credit: ©Raimond Spekking ICC-BY-SA 4.0 license via Wikimedia Commons)
Both underground and surface mining methods can have numerous environmental impacts. Surface mining can affect large areas of land, though in many developed countries reclamation efforts are mandatory that can at least in part restore affected landscapes. Underground mining can also impact substantial land areas through subsidence. Surface and underground coal mining produces a substantial amount of liquid effluents (acid mine drainage) that can affect surface and ground water. Coal mining releases trapped methane, which acts as a strong greenhouse gas (see §34). There are also many health and safety issues associated with coal mining. For a much more detailed discussion of health and environmental issues related to coal extraction, see [227].
33.1.5 Uses of Coal
Coal has been burned as a source of heat for warmth and cooking for thousands of years, and has been used in forges and for metalworking since about 1200 AD. The primary contemporary uses of coal in industrialized nations are for electrical power generation, steel and iron production using coke, and in cement production. In 2015, 92% of US coal use was in the production of electrical power. Globally, however, other uses of coal (including ongoing direct use in many countries for domestic heating and cooking) represent substantial energy use and contribute significant amounts to global CO2 emissions. As mentioned in §9, some 5% of global CO2 emission is from cement production, of which 40% comes from coal combustion. Coal can also be converted to a gas (§33.1.7) or liquid hydrocarbon fuel, as discussed in §33.1.7 and §33.4, though currently coal-to-liquid fuel conversion is not economically favorable or widely practiced. In the past, coal was used as a feedstock for production of various chemicals. Petroleum now has replaced coal in this role in most situations, though a resurgence of coal as a chemical feedstock may occur as oil supplies become less plentiful.
Figure 33.11 World coal use in 2014 (includes peat and oil shale) [236].
33.1.6 Coal and Coke in Steel and Iron Production
Coke is produced by heating coal without oxygen to a temperature in the vicinity of 1000℃ for an extended period of time, generally from one to several days. In this process, volatiles are released and the coal increases in strength and carbon density, producing the material known as coke.
Coke is primarily used in steel and iron production. In the smelting of iron ore, coke acts not only as a heating fuel but also as a reducing agent (see §9.5.3) for the iron. Because most of the volatiles are removed in coke production, burning coke produces very little smoke; for this reason, coke is often used in furnaces and other combustion environments where smoke needs to be minimized.
There are a number of constraints on the types of coal that can be used in coke production. Coal used for coke must be low in ash and sulfur content, and should have a relatively high () vitrinite content; this type of maceral fuses well into a stronger coke material. The rank of coal used is also fairly constrained, coke is generally made from bituminous coal but not from lignite or anthracite.
33.1.7 Coal and Electrical Power Generation
In 2014, coal produced nearly 40% of the world’s electricity. Typical coal power plants currently use pulverized coal as a fuel. Reducing the coal to a fine powder increases the surface area to volume ratio of individual particles, allowing the coal to burn more quickly. Before the widespread adoption of pulverized coal, stoker, or fixed-bed, designs were used, with much larger fuel particles having a size of a centimeter or more. Coal of all ranks, from lignite to black coal, is used in power plants, though most coal used for electricity production is in the middle of the range, with energy content typically from 20–25 MJ/kg. The thermal energy from the combusted coal is transferred to steam that drives a turbine in a Rankine cycle as described in §13. As of 2014, average coal plant efficiency worldwide was roughly 33%, though ultra-supercritical (USC) plants (see below) have efficiencies of up to 45%. A number of features of the coal supply must be considered for use in any given plant. Ash and volatile content are important, as is moisture content; greater ash and volatile content reduces the energy density of the fuel, and moisture increases the difference between higher and lower heating values of the fuel. Levels of trace elements such as sulfur affect emissions, as discussed below.
Over the last 100 years, the efficiency of conversion of thermal energy from the combustion of coal into electrical energy has increased steadily, from steam turbine generators with thermal-electric conversion efficiencies of 15% in the early 1900s to modern USC plants operating at three times that efficiency. The principal advances that have made this improvement possible are a significant increase in steam turbine efficiency (from 60% or so 100 years ago to around 92% currently), and an increase in Rankine cycle efficiency due in large part to an increase in the temperature and pressure at which the steam cycle operates. The improvement in turbine efficiency, like the improvement in wind turbine efficiency discussed in §28, has come in part from an increased ability to understand, model, and design complex 3D turbine geometries. Rankine cycle efficiency was improved in the early part of the twentieth century by implementing systems to preheat the air used for combustion and the water feeding the boiler using waste heat from the system, as well as by the shift to pulverized coal. We review briefly some of the different coal plant technologies that have enabled a further increase in overall efficiency through improved operating conditions.
Supercritical and Ultra-supercritical (USC) Plants As discussed in §13.3.6, Rankine cycle efficiency is improved when the pressure is sufficiently high that the heat transfer phase of the cycle is supercritical, bypassing the saturation dome. To handle pressures above 22.1 MPa and temperatures of 550℃ and above, improved steel alloys were developed in the 1950s and 1960s. Modern ultra-supercritical (USC) power plants operate at pressures of 25 MPa or higher and at temperatures of 600℃ or above, with a conversion efficiency of up to 45%. Further developments in material technology, for example using nickel alloys, could make it possible to operate plants at pressures of >30 MPa and temperatures of 700℃ or higher, which could raise coal plant efficiencies to 47–48%. Efficiency as high as 50% may be attainable with further advances in turbine technology [228]. Note that the steam temperatures relevant for efficiency are generally much lower than the temperature at which the coal itself is combusted, which can exceed 1200℃ in pulverized coal combustion systems.
Fluidized Bed Combustion (FBC) A new technology for coal plants known as fluidized bed combustion has been under development since the 1980s, and began to see widespread commercial deployment in the 1990s. In a fluidized bed combustion system, strong jets of air are passed upward through the coal or other fuel, keeping the fuel particles in a turbulent “fluid-like” motion during the combustion process. This technology was originally developed to reduce power plant emissions. Because heat transfer is enhanced in the fluidized bed, combustion can occur at a lower temperature than in a standard pulverized coal operation, generally 800–900℃. This suppresses the formation of nitrogen oxides, which form at substantially higher temperatures. Limestone placed in the fluidized bed can absorb sulfur, reducing emissions of sulfur dioxide as well. A further benefit of the lower combustion temperatures is that a variety of fuels can be used. Fluidized bed plants can operate on a mixture of coal with wood, biofuels, coke, petroleum, or other combustible fuels. Current efficiencies of FBC plants are over 40%. This technology is not extremely expensive, and is a promising direction for future development, though the low combustion temperature in current FBC systems limits the efficiency that can be attained without further development of the technology.
Integrated Gasification Combined Cycle (IGCC) In §13, the combined cycle plant was described, in which a high-temperature gas turbine is combined with a lower-temperature standard Rankine turbine in a high-efficiency power plant. Coal cannot be used directly in a combined cycle plant, since even if it is pulverized, the combustion products contain fly ash (see below) that would destroy the turbine. As mentioned in §13, coal can be combined with superheated steam to produce a combustible synthetic gas known as syngas, which contains principally a mixture of hydrogen and carbon monoxide and may also include carbon dioxide. Syngas can be used as a fuel in a combined cycle plant. Beginning in the 1970s, coal gasification and combined cycle technologies have been combined into a single power plant design. In an integrated gasification combined cycle (IGCC) plant, a coal gasification system is integrated into a combined cycle plant, and optimized to fit the parameters required by the gas turbine. IGCC systems have been built that achieve efficiencies as high as the best standard steam cycle plants. This technology is quite expensive, but has the potential for reaching higher efficiencies than standard steam cycle plants.
Coal gasification begins with steam reforming, in which high-temperature steam is passed over a bed of coke forming a mixture of hydrogen and carbon monoxide known as water gas. The chemical reaction
(33.3)
is endothermic kJ/mol and has positive reaction free energy kJ/mol, so it does not occur spontaneously at room temperature. goes negative in the neighborhood of 1000 K, so the reaction requires energy both because it is endothermic and because the reactants must be raised to a high temperature for it to proceed. This energy can be provided by allowing into the gasifying reactor a limited amount of oxygen (or, at the cost of dilution with nitrogen, air), which reacts exothermically to produce more carbon monoxide. If additional steam is present, the water gas will react exothermically to produce hydrogen and carbon dioxide, in the water gas shift reaction
(33.4)
