Fossil Fuel Power Plants

Summary

Fossil fuels are the organic residues of geological processes and include the various grades of coal, natural gas, petroleum, and crude oil. By definition, all fossil fuels are nonrenewable resources. Fossil fuel power plants all function in fundamentally the same manner and rely on a principle that has not changed significantly since the earliest applications of steam power. In short, the fuel is combusted to generate heat, producing pressurized steam that, in turn, drives the turbines of large electric generators.

Coal was the first major fossil fuel to be exploited as an energy source for the steam-powered plants that drove the Industrial Revolution of the eighteenth and nineteenth centuries. When steam technology was applied to the large-scale generation of electricity, coal became the fuel of choice because of its ready availability. Coal remains the major fuel source for fossil fuel power plants, although natural gas and, to a much lesser extent, petroleum-based fuels are used as well.

Significant costs, both economic and environmental, have been identified in the use of fossil fuels for the production of electricity. Economically, the prices of fossil fuels have been driven upward by the market economy, and environmentally, the combustion of large quantities of carbon-based fossil fuels releases a great deal of carbon dioxide and other gases into the atmosphere and contributes to global climate change.

Definition and Basic Principles

Fossil fuel power plants are generating stations that rely on the combustion of fossil fuels to produce electricity. Only three fossil fuels—coal, petroleum, and natural gas—are used for this purpose. The term power plant does not refer only to facilities that generate electricity but rather to any facility whose function is to produce usable power, whether electrical, mechanical, hydraulic, pneumatic, or another type. In common usage, however, power plant generally refers to those facilities that are used to generate electricity.

All fossil fuels are the remnants of organisms that existed, in most cases, many millions of years ago. Time and geologic processes involving heat and pressure chemically and physically altered the form of these organisms, turning them into mineralogical fossils (such as mineralized bones found in sedimentary rock formations) and the carbonaceous forms of coal, crude oil, and natural gas. When these carbonaceous materials are refined, they can be used as combustion fuels in fossil fuel power plants.

The combustion process is carried out in a variety of ways, from standard internal combustion engines using natural gas, gasoline, or diesel oil, to fluidized bed combusters using pulverized coal powder. Internal combustion engines are used to drive a generator directly, while other combustion methods are used to heat water and produce pressurized steam through heat exchange. The steam is then used to drive a turbine that in turn drives electric generators. The spent steam is generally recycled through the system. The exhaust steam from combustion is passed through a treatment process to reduce or eliminate contaminants formed from materials that were in the fuel.

Ideally, the combustion process would produce only carbon dioxide—from the combustion of coal—or carbon dioxide and water—from the combustion of hydrocarbon fuels such as natural gas and refined petroleum fuels. In practice, however, fossil fuels contain a percentage of materials other than carbon and hydrogen, such as sulfur, metals (including iron, mercury, and lead), and nonmetals (including phosphorus, silicon, and arsenic). In addition, air used to supply oxygen for combustion also contains about 78 percent nitrogen and about 1 percent of other gases. At the temperatures of combustion, these impurities can react with oxygen to produce a variety of pollutant by-products such as sulfur dioxide, nitrogen oxides, and fly ash.

The combustion of fossil fuels results in a very large quantity of carbon dioxide being released into the atmosphere, where it can act as a greenhouse gas. A greenhouse gas traps heat that normally would be radiated out of the atmosphere and into space. Researchers have concluded that the carbon dioxide released by the burning of fossil fuels is a primary factor in global climate change, which refers to an increase in the mean annual temperature of the planet.

Background and History

Coal has been used as a fuel for combustion for thousands of years. It was reportedly used by native North Americans when the first European settlers arrived, and it was undoubtedly used by other peoples throughout the world because of the ease with which it could be extracted from the ground in certain areas. It became the fuel of choice beginning in the eighteenth century and had almost completely replaced wood as the dominant fuel of industry by the early 1900s because of its more favorable energy density. Coal's increasing popularity as a fuel also drove the growth of the coal-mining industry, in turn increasing its availability.

With the development of the large-scale generation of electricity and its many applications, coal-fired power plants were used to drive electric generators where suitable water power, such as at voluminous waterfalls, was not available. The convenience and versatility of a common electric grid resulted in the growth of the electric generation industry. Small and localized generation systems ranging from low-output gasoline-powered home generators to large diesel-powered industrial generating stations can provide emergency and local service if the common grid is not available. Large generating stations using fossil fuels have been built and continue to be built in areas where coal or other fossil fuels are readily available.

How It Works

Fossil Fuels. Coal and petroleum are the remnants of plants and animals that lived millions of years ago. Over the years, geologic processes compressed and chemically altered the plants and animals in such a way that coal consists almost entirely of pure carbon, crude petroleum consists almost entirely of a vast assortment of hydrocarbons, and natural gas consists almost entirely of methane, ethane, and propane, which are simple hydrocarbon gases. Coal can be found at various depths in the earth's crust, in veins ranging from only a few centimeters to hundreds of meters thick. It is mined out as a solid, rocky, and relatively lightweight material and used in forms ranging from crude lump coal to a fine powder that is fluidlike in its behavior.

Crude oil and natural gas are found only at depths of hundreds and thousands of meters. As liquids or fluids, these materials have migrated downward through porous rock over a long period of time, until further progress is prevented by an impervious rock layer. There, they collect, often in large pools of oil and gas that can be recovered only after being found through careful exploration and deep borehole drilling. Natural gas requires no further processing before being used as a combustion fuel unless it is classed as sour gas, meaning it contains an unacceptably large proportion of foul-smelling hydrogen sulfide and other poisonous gases that must be removed. Petroleum, or crude oil, must be heavily refined before it can be used as a fuel. The crude oil is subjected to thermal cracking, which breaks down and separates the various hydrocarbon components into usable portions from light petroleum ethers such as pentanes and hexanes to heavy tars such as asphalt. The most well-known fractions refined from petroleum are gasoline, kerosene, diesel fuel, and waxes, and various grades of lubricating oils and greases.

Combustion. Combustion is a chemical reaction between a material and oxygen. The reaction is an oxidation-reduction process in which one material becomes chemically oxidized and the other becomes chemically reduced. In the context of fossil fuels, the material that becomes oxidized is coal, fuel liquids, or natural gas. Combustion of these carbonaceous materials converts each atom of carbon in the fuel molecules to a molecule of carbon dioxide, according to the general equation:

C + O₂ → CO₂

This conversion (greatly simplified here) releases an amount of energy that can then be transferred to and captured by a moderator—typically water—through the use of heat exchangers (devices that facilitate the transfer of heat from one material to another material). Combustion of hydrocarbons also produces water as an output of the reaction, in which two atoms of hydrogen combine with one atom of oxygen to produce one molecule of water. Other reactions corresponding to the combustion of impurities in the fuel stream also take place, and their products are ejected in the exhaust flow from the combustion process.

Turbines and Generators. Steam under pressure, produced by heating water via the combustion of fuels, is directed into a mechanical device called a turbine. Turbines can basically be described as high-tech versions of the ancient water wheel. The pressure of the flowing gas (steam) pushes against a series of vanes attached to an armature (electric component) in the structure of the turbine, driving them to spin the armature with force. This converts the linear fluid motion of the steam into the rotary mechanical motion of the turbine. Turbines are coupled to an electric generator so that their rotation results in the generation of electricity.

A generator is another rotary device that, in its most basic concept, consists of a magnet spinning inside a cage of conducting wires. The magnetic field of the magnet also spins at the same rate that the magnet spins. The movement of the magnetic field through the conductors in the surrounding cage produces an electromotive force (EMF) in the conductors, which is measured in volts. If the generator is connected to a circuit, this EMF causes current to flow in the circuit. Strictly speaking, generators produce direct current (DC) electricity, while alternators produce alternating current (AC) electricity. AC electricity is the standard form of electricity used in national power grids around the world.

Both generators and alternators are available in various output capacities and are driven by many types of rotary engines, from small internal combustion engines to large industrial steam turbines.

Applications and Products

Fossil fuel power plants, in the context of electric generating stations, produce only one product: electricity. Any and all other materials that come from them are considered ancillary or waste by-products. The ultimate goal of operating a fossil fuel power plant is to maximize the output of electricity from each unit of fuel consumed, while also minimizing any and all undesirable outputs. To that end, the efficiencies of control design, the data feedback process, economics, fuel processing, and a host of other aspects of the electric power generation industrial complex are examined each year. Not the least of these considerations is the placement and construction of new facilities and the maintenance of older facilities.

At one time, the competitive cost and the availability of natural gas and crude oil nearly spelled the demise of the coal-fired power plant. However, the prices of natural gas and crude oil were driven upward, both artificially and naturally, making these choices less attractive. Nuclear power plants were initially welcomed by the public, but their popularity declined because of radioactivity concerns. Because of these circumstances, the continuing demand for electricity caused fossil fuel power plants—especially coal-fired plants—to regain their position of prominence in electric power generation. The operation of fossil fuel plants spawned related industriesthe development of coal-mining methods and machinery, oil and gas exploration and recovery, fossil fuel transportation and preprocessing, specialized construction and trades, environmental assessment and maintenance operations, financial and administrative companies, plant operations and control technology, industrial maintenance, and grid supply and service.

Concern about greenhouse gases resulted in the birth of an industry aimed at capturing and storing the carbon dioxide produced by power plants. Different approaches are being developed, but among the most promising technologies is the sequestering of carbon dioxide in deep underground water formations. This engendered a new area of research and development in compression and recovery technology modeled on nature's carbon sequestration process. Like natural carbon sinks formed by oceans, forests, and grasslands, geological carbon sequestration captures and stores carbon dioxide underground. Through the 2020s, companies explored the development of carbon-capturing technologies, though some environmentalists criticized carbon recapture and sequestering as a distraction from ongoing efforts to reduce carbon emissions. Graphene production, engineered molecules, and carbon capture and storage are among the most successful technologies.

The operation of fuel fossil power plants generates chemicals, some of which have been recovered during preprocessing and exhaust gas scrubbing procedures. Sulfur recovered from preprocessing and from entrapment of sulfur dioxide in the exhaust gas stream is used to produce sulfuric acid, an important industrial chemical, as well as numerous other sulfur-containing compounds. Similarly, nitrogen oxides recovered from the combustion process provide nitric acid and other nitrogen-containing compounds. Interestingly, the entire plastics industry grew out of research to find uses for compounds recovered from coal tar, a by-product of the coal processing industry in the nineteenth century. In modern times, however, essentially all plastics are derived from petroleum.

Careers and Course Work

Students undertaking any program that will lead them into a career related to fossil fuel power plants, whether directly or indirectly, will be required to exit high school with an understanding of physics, chemistry, mathematics, and business and technology. Biology will also be required if the career path chosen is directed toward environmental studies. College and university-level coursework will depend greatly on the area of specialization chosen, as the range of options at this level is immense. At a minimum, students will continue studies in mathematics, physical sciences, industrial technologies (chemical, electrical, and mechanical), and business as undergraduates or as trade students. More advanced studies will be specialist courses in a chosen field. In addition, as technologies and regulations change, those working in the field of fossil fuel power plants can expect to be required to upgrade their working knowledge almost continually to keep abreast of changes.

Social Context and Future Prospects

Beginning around the turn of the twenty-first century, coal- and oil-fired power plants lost dominance to natural gas plants, as well as renewable energy sources, such as solar and wind power. Natural gas plants' combined-cycle generators recover and recycle waste heat, making them more efficient for producing electricity, and gas-fired turbines start up faster than coal-fired ones. Because of hydraulic fracturing (fracking) innovations and the US shale oil boom that began in the late 2000s and continued in the 2020s despite a crash during the global COVID-19 pandemic, natural gas dropped in price. Meanwhile, solar and wind power developed significantly, costing less than natural gas in many communities. Another threat to the fossil fuel power industry comes from increasing water stress, given the role of steam in running electric turbines. However, despite increasing investments in renewables, many governments worldwide continued to subsidize the fossil fuel sector. Conversely, in April 2024, the US government released an array of new environmental rules for power plants, particularly coal-burning plants, that many called unprecedented. The regulations included reducing carbon emissions by 90 percent within ten years, requiring plants to capture an amount of their emissions depending on the type of plant, and limiting mercury limits by 70 percent.

The world's reliance on the ready availability of coal and the industrial convenience of fossil fuel power plants, especially in developing and middle-income economies such as China and India, likely ensures that those facilities will be part of the landscape for many years to come, despite those intensive efforts to develop alternative power sources. Advances in existing technologies, such as pulverized coal combustion systems and new oxygen-separation and heat-recovery techniques for coal gasification, can improve the efficiency of fossil fuel power plants and thereby decrease greenhouse gas emissions. Technologies being developed, such as carbon dioxide capture and storage (CCS) and cogeneration power systems combining geothermal or biomass energy and fossil fuels, will require fossil fuel power plant workers to understand both the older and the newer technologies as fossil fuel power plants are managed toward a low-cost, high-efficiency, zero-emissions platform (ZEP). It may also become possible to recover carbon dioxide from fossil fuel power plants for use in plastics, petrochemicals, concrete, oil and gas extraction, or fuel cell generators. Another issue facing fossil fuel power plant workers is the wear and tear on fossil fuel power plants from increased cycling, load following, and ramping as these plants shift to providing backup to renewable energy sources.

Emission-abatement plans also include elimination of the heavy metals in fossil fuels, as the combustion of those materials has the effect of greatly concentrating any heavy metals, including radioactive trace elements such as uranium and thorium, in the ash residues. It is, therefore, not inconceivable that future work in a fossil fuel power plant will also require training in working with radioactive materials.

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