Ultra-supercritical generation
In general, an ultra-supercritical (USC) steam generation unit represents an advancement in conventional coal-fired steam generation.  It involves the use of supercritical steam pressure and the use of steam temperatures at or above 1100 degrees Fahrenheit (593° C).

The terms supercritical and ultra-supercritical are derived from the definition of the temperature and pressure at which water vapor and liquid water are indistinguishable – known as the Critical Point.  The Critical Point of water occurs at 705 degrees Fahrenheit under pressure of 3208 pounds per square inch (psia).  At the Critical Point, the bubbling formation associated with boiling no longer occurs.  Instead, with the addition of heat or increase in pressure the fluid experiences a continuous transition from water-like to steam-like characteristics. 

Pressure is said to be “supercritical” when the pressure exceeds 3208 psia. A conventional supercritical unit operates at a steam pressure of 3500 psi or higher and steam temperatures of 1000 –1050F.  By contrast, a subcritical unit operates below the critical pressure, typically 2400 psi at similar temperatures. 

By operating at elevated steam pressures and temperatures, the turbine cycle is more efficient.  This reduces fuel (coal) consumption, and reduces emissions in the process.

Supercritical Technology
In a conventional sub-critical steam generation unit the system operates at pressures that convert water to steam through the process of boiling.  At supercritical pressures, water is heated to produce steam through a gradual expansion without boiling. Due to improved thermodynamics of expanding higher pressure and temperature steam through the turbine, a supercritical steam generating unit is more efficient than a sub-critical unit.

Diagram

 

Early supercritical units –such as AEP’s Philo Unit 6 and Philadelphia Electric’s Eddystone Unit 1 – initially operated at ultra-supercritical levels.  However, due to the unavailability of metals that could tolerate these high temperatures for extended periods of time, operation at these levels could not be sustained.

Today, companies can use recently developed chrome and nickel-based super alloys in components of the steam generator, turbine and piping systems that are exposed to the higher temperatures. The new metals can perform under these prolonged operating conditions, rendering USC no longer a goal, but practical design basis.

Improved Efficiencies and Environmental Performance
With efficiencies in the 39 percent range, USC technology means that electricity can be produced using less fuel. Because less coal is consumed, emissions of sulfur dioxide (SO2), nitrogen oxide (NOx), mercury (Hg), carbon dioxide (CO2) and particulate and solid waste byproducts are reduced.  And USC technology is compatible with all types of coal.

For example, a 600 megawatt USC unit burning a typical Powder River Basin coal would consume 2.0 million fewer tons of coal over its lifetime than a comparably sized supercritical unit at steam temperatures of 1000°F.  Again comparing these same two units, a three percent improvement in the heat rate (a measure of the fuel consumed to produce a unit of electric energy) would reduce CO2 emissions by nearly 4 million tons over a 30-year.

USC has the potential for even greater efficiencies when paired with newer steam generator technologies.

In addition to improved environmental performance due to reduced coal use, USC technology will be paired with state-of-the-art emission control technologies:

  • Selective catalytic reduction system for NOx reduction
  • Dry flue gas desulfurization system for SO2 reduction and
  • Baghouse technology for particulate removal

USC and AEP
AEP’s tradition of advancing generation technology began in 1917 with the Windsor Plant in West Virginia, the first major mine-mouth generating unit coupled with long-distance high-voltage transmission to connect the plant with a major load center.

The world’s first supercritical unit, AEP’s Philo Unit 6 in Ohio, operated from 1957 until 1975.  This unit pioneered use of supercritical-pressure steam (4,500 psi), use of super-high temperature steam (1,150°F) and use of double-reheat steam.

Today, with more than 17,000 megawatts of supercritical generation, AEP owns and/or operates North America’s largest fleet of high efficiency supercritical coal units.

The company’s focus on generating efficiency continues with its pursuit of integrated gasification combined cycle (IGCC) and ultra-supercritical technologies.