Fig. 1. Superheated steam generator system. This system can produce high-quality steam at temperatures in excess of 1300°C (2372°F) at atmospheric pressure. The system is fully programmable for obtaining specific steam temperatures, and temperature profiles, for a variety of applications.[1]
Superheated steam – steam heated above the boiling point for a given pressure – becomes increasingly important in the move toward green technologies. Energy-efficient systems that can produce high-temperature steam are increasingly being used in the aforementioned areas as well as materials processing, semiconductors, biomass processing, nanotechnologies and fuel cells. As steam becomes more prominent and important in research and industry, new methods of achieving higher temperatures while increasing the safety of users and researchers are being discovered (Fig. 1).
Commonly Recognized Forms of Steam
Wet steam –contains both condensed hot-water particles and steam and is considered a two-phase mixture. Wet steam is difficult to measure with any of the technologies currently available, and accuracies of the water content can vary with the testing method.Saturated steam –occurs at a particular pressure and corresponding temperature when a state of equilibrium exists between liquid and vapor states.
Superheated steam –is at a higher temperature than the saturation temperature at a constant pressure. In a boiler, steam is generally saturated. As steam leaves a conventional boiler, a pressure drop is created by the flow. This drop in pressure, along with a relatively constant temperature of the steam leaving the boiler, results in superheated steam. This superheated steam conforms to ideal gas laws.
Novel technologies are entering the marketplace that produce steam at very high temperatures (in excess of 1300°C / 2912°F) while remaining extremely energy efficient (more than 90%) and operating at atmospheric pressure. This safety component alone will allow many more researchers, labs and industry to operate and integrate superheated steam where it was difficult (or impossible) to do so before. Figure 1 depicts such a system that is currently available.
Fig. 2. A typical one-atmosphere superheated steam generator that produces high-temperature steam (up to 700°C).[1]
High-Temperature Applications of Steam
Steam is used in a variety of areas in its various forms. Specifically, steam is used in the following areas of heat treatment and other high-temperature processes.Steam and Superheated Steam for Chemical Cracking
Steam cracking is used in industrial petrochemical processes for breaking down saturated hydrocarbons into smaller hydrocarbons (mostly unsaturated). This method is used in the production of olefins (and other light alkenes), propene (or propylene) and ethene (or ethylene).
In the process of steam cracking, a gaseous or liquid hydrocarbon is diluted with steam and briefly heated to very high temperatures [ethane, naphtha or LPG (liquefied petroleum gas) are examples]. The heating takes place in a furnace without the presence of oxygen. These reactions are only permitted to occur for a very short time (milliseconds), and the reaction often is performed in temperatures as high as 850°C (1562°F). Once the gas reaches the cracking temperature, it is quenched in a heat exchanger to stop the reaction at the desired gas properties.
The resulting products can be varied based on the constituents, processing, composition of the feed, the amount of steam present and the cracking temperature used. This process also results in the small deposits of coke, a form of carbon, on the furnace reactor walls. The carbon deposits will lower the efficiency of the reactor. The process of removing the carbon deposits, or decoking, requires the furnace to have a high-temperature steam (or air/steam mix) flow through the affected furnace and reactor system. This will result in the carbon being removed by gasification, converting the deposits to carbon monoxide (CO) and carbon dioxide (CO2).[2]
Gasification by Steam
The gasification of organic waste has been specifically studied to reduce the weight and volume of the waste and to produce the gas (H2+ CO). For organic materials, a wet or water-gas reaction is simply:
C + H2O®CO + H2(1)
The heat source can be superheated steam, IR or plasma where steam is injected to react with carbide. The treatment of plasma with steam reduces the weight of charcoal and produces hydrogen from charcoal. The use of such processes could be an attractive heat source for the disposal of carbonaceous wastes.[3]
Other gasification or reforming reactions yield promising methods of producing usable hydrogen. Metal-steam reforming (MSR) using iron is a safe and economically viable method of hydrogen generation from inexpensive raw materials and for producing nano-scale Fe2O3. The use of iron and iron waste for hydrogen generation via MSR is known. For example, the reaction that occurs around 600°C (1112°F) with superheated steam has long been known as one of the promising ways of generating H2.[4]
3 Fe + 4 H2O®Fe3O4+ 4 H2(2)
Fig. 3. Enthalpy of Superheated Steam vs. Air – Steam has a large capacity to do work with the more than five times the enthalpy versus air at the same temperature and pressure. New novel devices can achieve these high steam temperatures at atmospheric pressure.[5]
High-temperature steam is used for sterilizaton in autoclaves. An autoclave is a device that uses pressurized steam to sanitize and sterilize materials and equipment. Since the steam is under pressure, it can reach these and higher temperatures. Autoclaves are commonly found in microbiology laboratories, hospitals, dental offices and similar environments for sterilization and other uses.
Today, newer one-atmosphere devices can produce high-quality, high-temperature steam without the need of pressure chambers. Figure 2 depicts one such device that is highly efficient and generates high-temperature steam at one atmosphere pressure. This technology is expected to improve the safety concerns associated with high-pressure steam autoclaves.
Steam Used for Energy Transfer and Storage
In industrial applications, steam is used for energy storage. This energy is introduced and extracted by heat transfer, usually through pipes. Steam is an excellent reservoir for thermal energy because of the high latent heat of vaporization of water (2260 kJ/kg at 100°C). This high latent heat can be seen in Figure 3 depicting the enthalpy of superheated steam (SHS) versus air. Steam gas can hold more than five times more energy per kg at the same temperature as air. This provides an excellent medium for energy transfer.
Power Generation
Power-generation plants produce high-temperature steam from the combustion or reaction of a fuel to turn a turbine generator (creating rotational energy from thermal energy). These steam temperatures are quite high and can be as much as 600–800°C in the case of supercritical boilers. In the U.S., more than 86% of electric power is produced using steam as the working fluid, nearly all by steam turbines. Since the primary difference in these systems is the fuel for creating the high-temperature steam, the common types of power plants utilize steam similarly.
Coal- or gas-fired plants use high-temperature steam to activate a turbine generator. Coal-fired boilers are quite common. In this method, a fuel such as coal is burned to heat water to create steam that turns an electric turbine generator. A class of fossil-fueled power plants labeled supercritical employs very high pressures (more than 3,200 psi), along with the heating of the water, such that boiling does not occur and water becomes high-temperature steam very efficiently. The overall result is a more efficient power plant that uses less fuel to create more energy than a “subcritical” boiler power plant.
Nuclear reactors are used to heat water to high-temperature steam to then turn a steam turbine. There is no combustion component, making for a more clean option for creating power. However, many concerns about long-term environmental impact and safety of such plants still remain.
Other methods for creating steam, such as collection of heat energy from industrial processes, can be used to turn a turbine and create power. Technologies such as solar electric plants, which use solar energy to produce steam, are also in use. New materials are becoming available that can withstand such high temperatures and processes as required for the long life of components used with superheated steam.[7]
High-Temperature Materials Testing
Power generation is moving toward supercritical conditions of steam. Supercritical boilers and turbines that are used at these extreme conditions require the use of special materials such as 304H. Steam testing at very high temperatures above 1000°C (1832°F) is now becoming important in order to get accelerated test results. Devices like those shown in Figure 1 are able to provide for these very high temperatures.
Fig. 4. General phase diagram illustrating the supercritical point where liquid to gas transformation can occur without crossing the phase boundary.[6] Power stations that employ such techniques to produce superheated steam are more efficient and burn less fossil fuel.
Summary
Superheated steam carries more total energy than air at the same temperature and pressure. For exam-ple, air at 1 atm and 300°C has an enthalpy of 575 kJ/kg. Superheated steam at the same conditions has an en-thalpy of 3,070 kJ/kg. Therefore, superheated steam carries over five times the total energy of air at the same conditions. This gives superheated steam a much larger capacity to do work at the same temperature and pressure. As higher temperatures allow for higher efficiencies, superheated steam in the areas summarized will become increasingly important.Versatility, ease of temperature adjustment and safety will be paramount for a fast-changing environment and for accommodating new research, industrial and testing demands. Newly available systems are clean, low-pressure and have a small footprint, thus giving users the needed versatility to achieve results using high-temperature steam. IH
For more information:Contact Brian Kandell, customer solutions engineer, Micropyretics Heaters International, Inc. (MHI), 750 Redna Terrace, Cincinnati, Ohio 45215; tel: 513-772-0404; fax: 513-672-3333; e-mail: brian.kandell@mhi-inc.com; sales: sales@mhi-inc.com; web: www.mhi-inc.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: enthalpy, superheated steam, plasma, steam turbine
