Steam Energy Technology

Summary

Steam energy technology is concerned with converting chemical energy in fuels into the mechanical energy of a rotating shaft. Steam energy is used to propel ships, drive electric generators, and power pumps. Components such as boilers, turbines, pumps, heat exchangers, and piping systems are involved. The majority of electric power in the United States is generated with steam. Steam is less popular for ship propulsion than in the past, but it is still used for nuclear-powered ships and ships transporting liquefied natural gas. In combined cycle technology, the hot exhaust gas from a gas turbine is used to produce steam that is used to provide additional mechanical energy or to heat buildings.

Definition and Basic Principles

Modern steam energy technology may involve producing superheated steam at relatively high pressure and using that steam to drive a mechanical device. It may also involve producing saturated steam at relatively low pressure and using it to heat buildings or industrial processes.

Most of the electric power in the United States is produced through high-pressure, superheated steam driving a turbine. In turn, the turbine drives the electric generator that produces electricity for use in homes, businesses, and factories. This type of steam is also used to drive turbines aboard ships, and those turbines drive the ships' propellers. Because turbines operate best at several thousand revolutions per minute, and propellers operate best at a few hundred revolutions per minute, a speed-reducing gear is used between the turbine and the propeller.

Many industrial processes in the chemical industry and elsewhere require large amounts of low-pressure saturated steam to heat the processed materials. For instance, crude oil is made to boil in a distillation column to separate volatile components such as gasoline from nonvolatile ones such as tar. Steam produced by a boiler in the basement may also heat large buildings in cold climates.

In ancient times, energy was provided by human or animal strength. Steam energy technology was one of the first technologies that humankind used to produce greater force, higher speed, and greater endurance than living things could produce.

Background and History

Hero of Alexandria is credited with inventing the first steam engine about 200 BCE, although the device does not seem to have been put to practical use. Records indicate that in 1543, Spanish naval officer Blasco de Garay attempted to propel a ship with paddle wheels driven by a steam engine. In the late 1600s, groundwater needed to be pumped out of English mines, and Thomas Savery patented a steam-powered pump on July 25, 1698. By 1767, fifty-seven steam engines were used in mines near Newcastle, England, with a combined power of about 1,200 horsepower. James Watt was granted a patent for a much-improved engine in 1769.

Robert Fulton launched his Hudson River steamboat in 1807. This vessel traveled from New York City to Albany, New York, a distance of 150 miles, in thirty-two hours. The Brush Electric Light Company in Philadelphia built the first electric generating station in the United States in 1881.

Sir Charles Algernon Parsons of England is regarded as the inventor of the modern steam turbine. He first built a small turbine and used it to drive an electric generator. In 1894, he built the first steam turbine–powered watercraft. This vessel, the Turbinia, achieved an astounding speed of 34.5 knots (just under 40 miles per hour). George Westinghouse acquired the American rights to Parsons's invention in 1895. During the twentieth century, applications of steam energy technology expanded rapidly, and it became the dominant global energy source.

How It Works

Steam is produced in a boiler, where fuel is burned and the heat released is transmitted to water, which boils to form saturated steam. The fuel can be almost anything that burns. Solid fuels include peat, wood, and coal, while liquid fuels range from residual fuel, which is so thick that it must be heated to about 200 degrees Fahrenheit to make it flow easily, to kerosene and gasoline. Natural gas is also used as boiler fuel. Heat is produced as these fuels react with oxygen in the air.

Boilers may operate at pressures from slightly above atmospheric pressure to 3,500 pounds per square inch. Boilers at electric generating plants may produce 10 million pounds of steam per hour. There are two basic boiler types: fire tube boilers, where hot gases produced by combustion pass through tubes surrounded by large quantities of water, and water tube boilers, where water-filled tubes are exposed to hot combustion gases. Fire tube boilers are suitable for low-pressure boilers, and water tube boilers are used for pressures above about 300 pounds per square inch.

Superheating Steam. After steam is produced, it may pass through additional tubes that are exposed on the outside to hot combustion gases. This process, called superheating, raises the temperature of the steam above its boiling temperature. Superheating is done to increase the energy content of the steam without changing its pressure. The steam used in turbines is usually superheated to 950 to 1,000 degrees Fahrenheit. Special steel alloys must be used in superheater tubes to endure such high temperatures.

Combustion. Liquid fuels are sprayed into a cavity in the boiler called a furnace. Fuel is mixed with air and burned. In a water tube boiler, the hot gases produced by combustion flow first over the superheater tubes and then over tubes containing liquid water, where steam is produced.

Turbines. Superheated steam leaves the boiler and flows to the steam turbine. Here, the steam is directed against blades mounted on disks attached to the rotating shaft. The shape of the blades deflects the steam, and the steam causes the blades to move in the opposite direction. The process is similar to what happens when someone blows on a pinwheel. In a typical steam turbine, there may be twenty or more disks attached to the shaft. Each disk has many blades arrayed around its rim.

Reduction Gears. Often, the desired speed of a steam turbine is much greater than the speed of the device to which it is connected. For instance, a ship's turbine may rotate at several thousand revolutions per minute, while the propeller should rotate at about one hundred revolutions per minute. A reduction gear is used to convert the high speed of the turbine shaft to the low speed of the propeller. In a reduction gear, a small gear turning at high speed meshes with a large gear that turns at lower speed. Because there is a limit to the ratio of gear sizes, it is often necessary to perform the speed reduction in two steps.

Condensers. Leaving the turbine, the steam enters a condenser. This is a specialized heat exchanger that has cooling water flowing through thousands of tubes. The cooling water may come from a river or the sea. As the steam comes into contact with the outsides of these tubes, it condenses back to liquid water and drops to the bottom of the condenser. Condensers often operate at 12 or 13 pounds per square inch below atmospheric pressure. This pressure is determined by the cooling water temperature. The lower the condenser pressure, the more energy the turbine can extract from the steam.

Pumping and Air Removal. Once the steam has been condensed, a series of pumps transfers the water back to the boiler, where the process begins again. These pumps may be driven by electric motors or by small steam turbines. Because the condenser operates below atmospheric pressure, small amounts of air may leak into the water. Most of this air is removed by a vacuum pump, but some leaking air may dissolve in the condensing steam. This air is removed either by chemicals or by heating the water to its boiling point without boiling it after a pump has raised its pressure above atmospheric pressure.

Reciprocating Engines. Early steam engines had pistons that moved up and down, much like the pistons in an automobile engine. These engines are known as reciprocating engines. The famous Liberty ships of World War II were powered by reciprocating engines. Turbines are much more efficient than reciprocating engines, so relatively few reciprocating engines remain in use.

Applications and Products

Electric Power Generation. Most of the electricity used in the United States is generated in steam plants powered by coal, oil-based fuels, natural gas, and nuclear power. Regardless of the fuel type, these plants tend to be very large. In 2022, the United States sourced about 40 percent of its electricity from natural gas, 20 percent from coal, 18 percent from nuclear power, and 22 percent from renewable sources, according to the Energy Information Administration (EIA). Over 40 percent of electricity generation in the United States was produced using steam turbines. All coal-fired power plants, almost all nuclear plants, and most oil- or gas-fired plants use steam engineering technology.

A relatively new technology for electric power generation is a combined cycle plant. These plants use gas turbines to drive electric generators for most of their power production. However, gas turbines alone have relatively low efficiencies (about 35 percent) because the exhaust gas contains a lot of unused heat energy. In a combined cycle plant, this hot exhaust gas produces steam, which drives a turbine that powers another electric generator. The efficiency of combined cycle plants is very attractive (50 percent or more).

Nuclear Power. Nuclear power is used extensively worldwide for electric power production. In the United States, nuclear plants produce 18 percent of the country's electric energy. There are several types of nuclear power plants, but nearly all of them make use of steam energy technology. In these plants, the nuclear reactor replaces the boiler furnace as a heat source. Most of the rest of the plant closely resembles a conventional steam plant. Unlike conventional plants, nuclear plants use saturated steam rather than superheated steam because it is not feasible to superheat steam utilizing a nuclear reactor. Nuclear fission produces electricity using steam turbines.

Ship Propulsion. For most of the twentieth century, steam was the dominant energy source for ship propulsion. Early in the century, reciprocating engines were used, but turbine engines soon took over. The famous liner SS United States had four turbine engines producing about 250,000 horsepower. This ship set and held the transatlantic speed record for nearly fifty years. In 1970, most of the ships in the US Navy were steam-powered, and the world's merchant fleet was also primarily steam. Later, diesel and gas turbine propulsion became more popular than steam. In the early twenty-first century, most ships built to carry liquefied natural gas to Europe and the United States were steam-powered. However, for all other modern ships, diesel is consistently more efficient and has a lower operating cost.

On ships propelled by steam, the electric generators are also driven by steam turbines. Some pumps on these ships are driven by small steam turbines. Steam is used as a heat source for boiling seawater to produce freshwater for human use and replenishing the water in the steam system.

All nuclear-powered ships are powered by steam turbines. The nuclear reactor produces very hot water. This water exchanges heat with water at lower pressure in a steam generator, and the lower-pressure water turns to steam. This steam is then used to drive a turbine.

Proposals for the propulsion of liquefied natural gas ships have called for combined cycle systems much like those used for electric power generation. Such systems are much more efficient than conventional steam systems.

Industrial Processes. Many industrial processes, such as oil refineries, steel mills, chemical plants, and paper mills, require steam. In oil refineries, steam is used to heat various liquids, and in the cracking process, it breaks large molecules into smaller ones. A by-product of the cracking process is carbon monoxide. The blast furnace in a steel mill also produces large amounts of carbon monoxide. The carbon monoxide produced is used as fuel in a boiler to create steam. A paper mill uses roughly 10,000 pounds of steam per ton of paper produced. Much of this steam is generated using waste products such as tree bark as fuel. In the chemical industry, steam is used to make ethylene and a plastic styrene. It is also used to heat chemicals that would turn solid in their tanks at room temperature.

Steam Heating. Steam was historically an important energy source for space heating, and it continues to be used in some areas. Many cities throughout the United States and Europe have steam utility systems, but New York City's system is the largest. Consolidated Edison of New York (Con Ed), New York City's electric power utility, sells steam through miles of pipes under Manhattan streets. At three of its electric generating plants, steam leaves the turbines at 350 degrees Fahrenheit and at a pressure of about 115 pounds per square inch higher than atmospheric pressure. Instead of being condensed in the plant, steam flows through large pipes throughout Manhattan. About 100,000 buildings, including landmarks such as the United Nations building, Rockefeller Center, and the Metropolitan Museum of Art, buy this steam and use it for heating. Altogether, Con Ed sells about 30 billion pounds of steam for heating each year. A church in lower Manhattan became one of the first steam customers in 1882, and it has been buying steam ever since.

Careers and Course Work

Electric power plants are designed by architecture and engineering companies. Many people involved in designing the plants are mechanical engineers with either bachelor's or master's degrees in mechanical engineering. Students in mechanical engineering study advanced mathematics, thermodynamics, fluid mechanics, heat transfer, machine design, and other technical subjects. The design process also requires people with expertise in civil engineering and electrical engineering. The actual equipment—the boilers, turbines, pumps, and so on—are designed by companies that specialize in particular components, while the designers at architecture and engineering firms design the systems that connect these components and make them work together.

The people who manage these plants are usually employees of large public utilities that own the plants. They often have mechanical engineering degrees. In addition to an engineering degree, a manager may have a business degree. A college degree is optional for those who operate and maintain these plants.

Designers of shipboard steam energy systems may have degrees in mechanical engineering or marine engineering, two closely related fields. They are typically employed by firms specializing in naval architecture and marine engineering.

Operating personnel on US Navy ships are, of course, naval officers and enlisted personnel. Officers on merchant ships must be licensed by the US Coast Guard as operating marine engineers. Many of the officers in the US Merchant Marine are graduates of state maritime academies or the US Merchant Marine Academy. Still, a college degree is not required to obtain a license. Some people obtain the knowledge required for a license through experience and specialized training. They gain this experience by serving in unlicensed positions, such as oilers or qualified members of the engineering department on merchant ships.

Social Context and Future Prospects

Global electricity demand continues to grow, and steam power will likely remain an important form of electricity generation. Prospects in the energy industry are generally closely tied to global socioeconomic and political trends. In the first decades of the twenty-first century, China's economy grew rapidly, increasing its electricity demand. The United States has large reserves of coal. Increased use of coal could reduce its dependence on foreign oil, but coal produces more emissions than any other fuel. In particular, coal produces large amounts of carbon dioxide. Ongoing research investigates ways to make steam energy more efficient with increased power output and fewer emissions. Recapturing carbon dioxide and lowering its contribution to global warming are primary concerns in the twenty-first century. If this research is successful, there may be growth in the use of coal.

Supplies of oil and natural gas are finite, and renewable fuels are being sought. A significant advantage of steam energy technology is that boilers can burn all fuels—solid, liquid, and gas. Increased use of solid fuels produced from agriculture may also cause growth in steam engineering technology. Because the combustion of fuels produces carbon dioxide, a major greenhouse gas, efforts are being made to reduce fuel usage. Wind and water power have a clear role here, but nuclear power, which produces no airborne emissions but produces radioactive waste, remains controversial. Since nearly all nuclear plants use steam, growth in nuclear power may cause growth in the use of steam energy technology.

Bibliography

Babcock & Wilcox Co. Steam: Its Generation and Use. 42nd ed., Babcock and Wilcox, 2015.

Bloch, Heinz P., and Murari P. Singh. Steam Turbines: Design, Applications, and Rerating. 2nd ed., McGraw-Hill, 2009.

"Civil Engineers." U.S. Bureau of Labor Statistics, 17 Apr. 2024, www.bls.gov/ooh/architecture-and-engineering/chemical-engineers.htm. Accessed 6 June 2024.

"Electrical Engineers." U.S. Bureau of Labor Statistics, 17 Apr. 2024, www.bls.gov/ooh/architecture-and-engineering/mechanical-engineers.htm. Accessed 6 June 2024.

"Electricity Explained." US Energy Information Administration , 30 June 2023, www.eia.gov/energyexplained/index.cfm?page=electricity‗in‗the‗united‗states. Accessed 6 June 2024.

Gardner, Raymond, et al. Introduction to Practical Marine Engineering. Society of Naval Architects and Marine Engineers, 2002.

Gülen, S. Can. Gas Turbine Combined Cycle Power Plants. CRC Press, 2020.

Kehlhofer, Rolf, et al. Combined-cycle Gas and Steam Turbine Power Plants. 3rd ed., Penn Well, 2009.

Komarov, I. I., et al. “Comparative Analysis of the Efficiency of Using Hydrogen and Steam Methane Reforming Storage at Combined Cycle Gas Turbine for Cogeneration.” Journal of Physics: Conference Series, vol. 2053, no. 1, 2021. doi.org/10.1088/1742-6596/2053/1/012007.

Milton, James H., and Roy M. Leach. Marine Steam Boilers. Butterworth, 1980.