Supercritical Boiler

A supercritical steam generator is a type of boiler that operates at supercritical pressure, frequently used in the production of electric power.

In contrast to a subcritical boiler in which bubbles can form, a supercritical steam generator operates at pressures above the critical pressure. Therefore, liquid water immediately becomes indistinguishable from steam. Water passes below the critical point as it does work in a high pressure turbine and enters the generator's condenser, resulting in slightly less fuel use.
The efficiency of power plants with supercritical steam generators is higher than with subcritical steam. Only with high pressure can higher temperature steam be converted more efficiently to mechanical energy in the turbine (as given by Carnot's theorem).

Technically, the term "boiler" should not be used for a supercritical pressure steam generator as no "boiling" actually occurs in the device.

History of supercritical steam generation

Contemporary supercritical steam generators are sometimes referred to as Benson boilers. In 1922, Mark Benson was granted a patent for a boiler designed to convert water into steam at high pressure.

Safety was the main concern behind Benson's concept. Earlier steam generators were designed for relatively low pressures of up to about , corresponding to the state of the art in steam turbine development at the time. One of their distinguishing technical characteristics was the riveted water/steam separator drum. These drums were where the water filled tubes were terminated after having passed through the boiler furnace.

These header drums were intended to be partially filled with water and above the water there was a baffle filled space where the boiler's steam and water vapour collected. The entrained water droplets were collected by the baffles and returned to the water pan. The mostly-dry steam was piped out of the drum as the separated steam output of the boiler. These drums were often the source of boiler explosions, usually with catastrophic consequences.

However, this drum could be completely eliminated if the evaporation separation process was avoided altogether. This would happen if water entered the boiler at a pressure above the critical pressure (); was heated to a temperature above the critical temperature () and then expanded (through a simple nozzle) to dry steam at some lower subcritical pressure. This could be obtained at a throttle valve located downstream of the evaporator section of the boiler.

As development of Benson technology continued, boiler design soon moved away from the original concept introduced by Mark Benson. In 1929, a test boiler that had been built in 1927 began operating in the thermal power plant at Gartenfeld in Berlin for the first time in subcritical mode with a fully open throttle valve. The second Benson boiler began operation in 1930 without a pressurizing valve at pressures between at the Berlin cable factory. This application represented the birth of the modern variable-pressure Benson boiler. After that development, the original patent was no longer used. The "Benson boiler" name, however, was retained.

1957: Unit 6 at the Philo Power Plant in Philo, Ohio was the first commercial supercritical steam-electric generating unit in the world, and it could operate short-term at ultra-supercritical levels. It took until 2012 for the first US coal plant designed to operate at ultra-supercritical temperatures to be opened, John W. Turk Jr. Coal Plant in Arkansas.

Two current innovations have a good chance of winning acceptance in the competitive market for once-through steam generators:

* A new type of heat-recovery steam generator based on the Benson boiler, which has operated successfully at the Cottam combined-cycle power plant in the central part of England,
* The vertical tubing in the combustion chamber walls of coal-fired steam generators which combines the operating advantages of the Benson system with the design advantages of the drum-type boiler. Construction of a first reference plant, the Yaomeng power plant in China, commenced in 2001.

On 3 June 2014, the Australian government's research organization CSIRO announced that they had generated 'supercritical steam' at a pressure of and in what it claims is a world record for solar thermal energy.

Definitions

These definitions regarding steam generation were found in
report on coal production in China
investigated by the Center for American Progress.

* Subcritical – up to and (the critical point of water)
* Supercritical – up to the ; requires ''advanced materials''
* Ultra-supercritical – up to and pressure levels of (additional innovations, not specified, would allow even more efficiency)

Nuclear power plant steam typically enters turbines at subcritical values – for ''Once Through Steam Generators'' 153 bar and 330 °C, lower temperature but same pressure for U-Tube Steam Generators type.

The term "advanced ultra-supercritical" (AUSC) or "700°C technology" is sometimes used to describe generators where the water is above 700 °C.

The term High-Efficiency, Low-Emissions ("HELE") has been used by the coal industry to describe supercritical and ultra-supercritical coal generation.

Industry leading (as of 2019) Mitsubishi Hitachi Power Systems charts it
gas turbine combined cycle
power generation efficiency (lower heating value) at well under 55% for gas turbine inlet temp of 1250 °C, roughly 56% for 1400 °C, about 58% for 1500 °C, and 64% for 1600 °C, all of which far exceed (due to Carnot efficiency) thresholds for AUSC or Ultra-supercritical technology, which are still limited by the steam temperature.

See also

* Supercritical water reactor
* Boiler

Notes

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External links

Thermopedia, "Benson boiler"

Boilers
Chemical equipment
Steam boilers
Steam engines
Steam generators
Power station technology