The Physics of Energy, page 136
The high energy density of fossil fuels derives from the enthalpy of combustion of hydrocarbon molecules. As discussed in §11, energy is released from hydrocarbons through the combustion reaction
(33.1)
A table of lower and higher heating values for a variety of simple hydrocarbons is given in Table 33.1. These combustion energies can be roughly understood in terms of the constituent bond energies within the molecule (Problem 33.2).
Table 33.1 Enthalpies of combustion: lower and higher heating values for a variety of hydrocarbon molecules. (HHV from [22], LHV computed from HHV as in Example 9.2.)
Although fossil fuels are a finite resource, they will continue to dominate human energy use for years to come. Existing petroleum reserves combined with anticipated future discoveries seem adequate to supply at least a substantial fraction of world oil demand for many decades into the future. Alternative sources of oil including tar sands and oil shale are also abundant, though their use produces substantially greater carbon emissions per joule of usable energy than use of oil from conventional petroleum resources. There are tremendous reserves of coal, which could continue to power global electrical systems at the current rate for over a century. Natural gas, which emits the least carbon dioxide per unit of energy released of all the fossil fuels, is playing an increasingly significant role in power production, and is likely to increase further in importance in the coming decades. Technologies that enable the conversion of coal to liquid or gaseous fuels, could in principle eventually replace oil and natural gas, which are less abundant than coal. Natural gas can also be converted to liquid fuel. Thus, were it not for the impact on climate of carbon emissions from extensive fossil fuel use (§34, §35), and other environmental effects and economic considerations, it would be possible for fossil fuels to continue to dominate human primary energy consumption for at least many decades into the future.
Given the role that fossil fuels have played in past and present energy systems, and their likely continued importance in the future, these energy sources must play a central part in any discussion of energy policy or economics. There is a tremendous body of scientific and engineering knowledge relating to the discovery, extraction, and utilization of fossil fuels. Over the last two centuries a sophisticated set of technologies have been developed by fossil-fuel-related industries. Many physicists and other scientists devote their careers to identifying new deposits of fossil fuel resources using a variety of probes of underground structure such as seismic surveys and well logs, modeling the details of reservoir dynamics and the extraction process, and optimizing the extraction of energy from fossil fuels in systems from power plants to automobile engines to fertilizers. We touched on some of the issues involved in exploration and reservoir modeling in the related context of geothermal energy in §32. Modeling of oil fields is now an extremely sophisticated and computation-intensive enterprise. We do not go deeply into the physics of fossil fuel discovery or extraction in this book. We give a scientific overview of these topics, as well as of the origins, energy content, uses, and transformations of fossil fuels. In §11 and §13 we analyzed the physics behind the transformation of thermal energy from fossil fuel combustion into kinetic energy (in automobile engines) and electromagnetic energy (in power plants). This chapter puts those analyses into the context of available fossil fuel resources.
We describe in this chapter each of the main categories of fossil fuels: coal (§33.1), petroleum (§33.2), and natural gas (§33.3). Emerging fossil fuel resources such as light tight oil and shale gas are included in the discussion, as are unconventional fossil fuels such as tar sands and oil shale. In §33.4 we describe conversion from solid and gas fossil fuels to liquid hydrocarbons. We conclude in §33.5 with a brief summary of the essential energy, carbon, and resource information for fossil fuels.
For further reading and a more in-depth technical study of the science of fossil fuels, there are number of good texts covering each of the major fossil fuel sources. Thomas [226] contains a clear description of the geology of coal, including geophysical methods used in exploration and coal mining. Miller [227] gives a comprehensive treatment of coal energy systems, including emission issues. A concise introduction to modern developments in coal-fired power plants, along with a brief history of power plant designs is given by Termuehlen and Emsperger [228]. Gluyas and Swarbrick [229] gives an introduction to petroleum geoscience, with emphasis on exploration, while Bjorlykke [230] takes a more physics-based approach. Smil [231] gives a good non-technical overview of the oil industry, including the history and geology of oil, as well as political and economic commentary. An overview of the current status and future prospects for natural gas is given in an MIT report [232]. A particularly useful source of statistics on fossil fuel use can be found in [9].
33.1Coal
Coal (Figure 33.3) is an abundant and easily accessible fossil fuel that is widely distributed around the world. Coal currently plays a central role in electric power generation worldwide. In 2014, coal provided 30% of the energy consumed worldwide, roughly 160 EJ. The global rate of coal production2 since 1980 is shown in Figure 33.4. Proven reserves of coal would be sufficient to sustain the current level of use for over 100 years.3 Thus, without a substantial change to the status quo, coal will continue to play a central role in power generation for the foreseeable future. Current statistics regarding coal can be found, for example, on the website of the World Coal Association (WCA) [233].
Figure 33.3 A sample of the chemical structure of coal. From [50].
Figure 33.4 Global coal production since 1980 [12]. 1 tonne of coal equivalent GJ.
33.1.1 Origins of Coal
Fossil fuels are formed when organic material, deeply buried under sediments and deprived of oxygen, is subjected to high temperature and pressure at increasing depth. Over tens or hundreds of millions of years, the complex organic molecules that compose living organisms break down, release moisture, oxygen, and other volatile substances, and recombine into simpler hydrocarbons.
The exact process by which organic material is transformed into coal depends both upon the nature of the original material and the sequence of conditions to which the material is subjected. Coal is classified in several ways. Because it is closely linked with the process by which coal is formed, we focus first on the classification by rank. The rank of a particular coal specimen refers to the extent to which the original organic material has undergone coalification, and ranges through peat to lignite, sub-bituminous, bituminous, and anthracite. Note that the precise definition of the different ranks is not universally standardized or agreed upon; thus the numbers we give here for carbon and energy content for the different ranks are intended as a rough characterization of the different ranks, and may differ from values used by specific organizations.
Peat The first stage in this series of transformations is the formation of peat. Peat is organic matter with elevated carbon content, produced from mosses or other plants. While plant matter is typically 50% carbon, peat typically contains 60% carbon by mass. Peat forms most readily in wet areas such as bogs or marshes, where dead plant material is often submerged in an environment without adequate oxygen for complete decomposition by aerobic bacteria, which would release the carbon again to the atmosphere. When dried, peat burns readily, and is used for heat and cooking in many places around the world. The energy content of completely dry peat composed purely of matter of organic origin with minimal mineral content is somewhat below 20 MJ/kg. While this energy/mass ratio is comparable to some coals, peat is much less dense, so that the energy content per unit volume is lower than that of any coals by a factor of four or five. Most peat is not dried completely, and the residual moisture content lowers the calorific value by a factor as large as two, giving a typical energy content of 10–15 MJ/kg. Furthermore, in most locations with large quantities of peat, the organic content is not 100%, further lowering the energy density. There are substantial deposits of peat throughout the world (estimated at ~4.5 km2 [234]) ranging up to 20 m in thickness. Finland, for example, which has one of the highest proportions of wetlands of any country in the world, has substantial peat resources; there cogeneration is used to provide both electricity and heat from burned peat, which provides 6% of the country’s total energy supply.
Lignite When peat is buried more deeply, pressure and temperature increase. This leads to dehydration and compaction of the material, with associated loss of oxygen. As the O/C ratio and moisture content decrease, the material begins to form the lowest grade of coal, known as lignite. Lignite ranges from 60–80% carbon, and has an energy content below roughly 18 MJ/kg.
Bituminous coal At greater depths and pressures, the coalification process progresses further. As the O/C ratio decreases further and the structure of the organic molecules continues to break down, bituminous coal is formed, with 80–92% carbon and an energy content ranging up to 35 MJ/kg; coal with energy content in the range 18–28 MJ/kg is generally described as sub-bituminous coal. The process of bituminization is believed to occur at temperatures around 100–150℃. In most locations, the degree of coalification increases progressively with depth beneath the surface, with the rate of increase correlated with the geothermal gradient. For a geothermal gradient of 30℃/km, lignite is formed in the two kilometers directly beneath the surface, and increasing bituminization occurs from 2 to 6 km beneath the surface.
Anthracite Bituminous coal that is subjected to substantially higher temperature and pressure undergoes further metamorphic change in which the molecular structure breaks down further, and remaining oxygen, sulfur, and other volatiles, as well as some hydrogen, are released. The remaining carbon-rich material forms the most dense type of coal, known as anthracite. Anthracite is the purest type of coal, with a carbon fraction ranging from 92.1% to 98%. The energy content of anthracite can be in the range 28–33 MJ/kg, though sometimes the term anthracite is reserved for coal with energy content >32.5 MJ/kg. Anthracite and bituminous coal are often referred to as black coal, while brown coal refers to sub-bituminous coal as well as lignite.
Graphite If coal is subjected to even higher temperatures and pressures, the remaining hydrogen atoms are released, mostly in the form of methane. Under sufficiently high pressure and temperature and subject to other geological stresses, the carbon atoms begin to align into a regular lattice, through the process of graphitization. Graphite, a crystalline form of pure carbon, can be considered the end point of the evolution of coal – although graphite can be formed in other ways and is even found in meteorites. As the number of hydrogen atoms decreases, coal becomes less easy to ignite and the energy content asymptotes to 33 MJ/kg for pure graphite.
Coal is generally found in seams, typically ranging from less than a meter to several meters in thickness. Particularly thick seams can be 10–20 m or more thick. Irregularities in seams arise from many geological processes. In particular, nearby intrusions of magma can cause more rapid coalification in localized regions, leading, for example, to isolated deposits of anthracite in regions generally dominated by brown coal.
The oldest substantial coal deposits date back to the Carboniferous period around 300 Ma (Ma = million years ago, see Appendix D.2). Before this time terrestrial plant cover was not sufficient for large-scale coal formation. During the Carboniferous period, the eastern US and much of Europe were located near the equator, and great swamp forests known as coal forests gave rise to large coal deposits.
Figure 33.5 A rendering of a coal forest from the Carboniferous by the German illustrator Heinrich Harder.
Most of the known anthracite deposits are in coal dating to the Carboniferous and subsequent Permian periods. Coal deposits dating to the Jurassic and Cretaceous (206 Ma–66 Ma) are less extensive, and contain substantial amounts of bituminous coal. More recent coals from the Tertiary period (66 Ma–1.6 Ma) are dominated by lignites, or brown coal. These coals are more varied than the older coals, due in part to the increased diversity of plants and in part because newer coals have been subject to less metamorphosis, so that distinguishing characteristics associated with their different sources are still present. The seams of Tertiary coals are generally thicker than those of the older coals.
Coal is broadly distributed geographically, with the most significant deposits in North America, Asia, and Australia. As shown in Figure 33.6, fewer coal deposits are found in South America and north and west Africa.
Figure 33.6 Primary world coal deposits. From [235].
33.1.2 Coal Properties and Classification
In addition to rank, coal is classified according to a number of different properties. The type of coal refers to the materials of which the coal is composed. This is determined in part by the kind of organic material from which the coal was formed. The different types of basic organic units, called macerals, are somewhat analogous to the minerals that make up inorganic rock. Macerals are generally divided into three categories: huminite/vitrinite, woody materials such as tree trunks, roots, or bark; liptinite, algae, spores, resins and cuticles; and inertinite, oxidized plant material. Coal is often referred to through one of these types or a combination of types; vitrinite has the highest tendency to form coal, while liptinite has greater hydrogen content and favors petroleum production, and inertinite has little potential for fossil fuel production. Another important feature characterizing the type of coal is the fraction of inorganic minerals, such as clay minerals, that have mixed into the coal. The reflectance of coal, particularly vitrinite, generally increases with rank, and is often used as a rank indicator in coals with high carbon content such as anthracite.
The quality of coal refers to the overall utility of the coal, which depends upon rank, type, and other characteristics. The ash content of a coal refers to the fraction of the coal that remains as inorganic residue after the coal is combusted. The volatile content of coal represents the fraction that is liberated at high temperature, including oxygen and sulfur in addition to various volatile hydrocarbons. In general, the quality of the coal increases with its rank and with lower ash and volatile content. Anthracite contains only a few percent volatile matter and ash, while typical brown coal can have 50% or more volatile matter.
Coal is generally measured in tonnes (1000 kg), or in short tons (907.2 kg). The energy content of coal can vary widely, ranging from 15 MJ/kg to 35 MJ/kg depending upon the rank of the coal, ash content, and other features. As a unit of energy, a tonne of coal equivalent (TCE) is defined as
(33.2)
Note that a typical tonne of coal contains somewhat less than this amount of energy. The 924 Mt of coal produced in the US in 2013, for example, corresponded to 21.1 EJ of primary energy, averaging roughly 25.2 GJ/tonne.
33.1.3 Coal Exploration
Geophysical methods play an important role in exploration for and identification of new fossil fuel resources. Some of these methods were described in the context of exploration for geothermal energy resources in the previous chapter. Seismic surveys are of particular value in coalfield exploration, though other surface-based methods such as gravitational surveys, resistivity mapping, and magnetic surveys are also useful. Geophysical surveying or logging of boreholes also provides detailed information about the structure of rock and coal seams in a given area. There is a great deal of physics involved in all these techniques; we mention briefly just a few of the relevant issues involved.
Seismic Surveys A primary method used for coalfield exploration is seismic surveying. Coal is significantly less dense and has a lower seismic wave velocity than most other rock structures; seismic velocity in hard coal is generally between 1.8 and 2.8 km/s compared to ~3.6 km/s in sandstone and ~5.5 km/s in limestone. Thus, seismic waves reflect strongly when passing between coal seams and more dense rock (see Box 33.1). By producing seismic waves at the surface using either explosions or an earth compacter, and then measuring the reflected waves, coal seams can be accurately identified to a depth of 1–1.5 km. Additional features such as faulting, washed out regions of the seam, or changes in the thickness or structure of the seam can also be identified using this approach. As technology for seismic surveys and computer analysis software has progressed over recent decades, this has become a highly accurate method for determining coalfield structure from the surface. Seismic methods using horizontally propagating waves are also used to determine the structure of specific coal seams. Since the density and wave velocity within a coal seam are substantially lower than in the rock above and below the seam, the coal seam acts as a wave guide.4 By generating a seismic wave that propagates along the seam starting from an underground point in the seam, faults or other obstructions in the seam can be accurately identified. This technique is heavily used in longwall mining (see below).
Box 33.1 Geophysical Methods: Seismic Waves
Just as sound passes through the air as a wave of oscillating pressure, energy can be transmitted through liquid and solid material by the propagation of waves. Waves propagating within or on the surface of Earth are called seismic waves (see §32.1.1). As illustrated in the figure at the right, waves propagating in the bulk material beneath the surface are divided into primary P-waves, associated with variations in pressure, where the motion of the material is in the direction parallel to wave propagation, and secondary S-waves, associated with shear stress and motion transverse to the direction of wave propagation. The designation primary/secondary comes from the early observation that P-waves travel faster and arrive sooner than S-waves. Because fluids cannot support shear stress, only P-waves propagate in fluids. Seismic waves have played an important role in understanding the internal structure of Earth, as discussed in the previous chapter, and play an important role in coal and oilfield exploration and analysis.
