The issue
Current U.S. Generation Mix
Source: U.S. Energy Information Administration,
Form EIA-923, "Power Plant Operations Report.”
Coal represents almost 50% of today’s electricity generation and the United States has enough coal resources to last generations. Coal based generation is under pressure to reduce air emissions, including SO
2, NO
X, mercury and a host of other metals and compounds deemed to be Hazardous Air Pollutants (HAPs), as well as potential greenhouse gas legislation. However, because of its prevalence and relatively low cost for electricity generation, coal will continue to be a significant part of the U.S. generation mix. Technological advances continue to increase efficiency and improve environmental performance.
Supercritical technology
In a Pulverized Coal (PC) plant, the coal is ground into fine particles and blown into a furnace where combustion takes place. The heat from the combustion of coal is used to generate steam to supply a steam turbine that drives a generator to make electricity. Subcritical steam generation units operate at pressures such that water boils first and then is converted to superheated steam. At supercritical pressures, water is heated to produce superheated steam without boiling. Due to the improved thermodynamics of expanding higher pressure and temperature steam through the turbine, a supercritical steam generating unit is more efficient than a subcritical unit. The steam cycle for the pulverized coal-fired units, which determines the efficiency of generating electricity, traditionally falls into two categories, subcritical and supercritical:
Subcritical operating conditions are generally accepted to be 2,400 psig/1,000ºF superheated steam, with a single reheat to 1,000ºF.
Supercritical steam cycles typically operate at 3,600 psig, with 1,000ºF – 1,050ºF main steam and reheat steam conditions. Ultra-supercritical is a term applied to supercritical pressures and temperatures above 1100 ºF.
Ultra-supercritical technology
Ultra-supercritical (USC) steam generation currently is the most efficient technology for producing electricity fueled by pulverized coal. A USC unit operates at supercritical pressure and at advanced steam temperatures of 1,100ºF (593ºC). These temperatures and pressures enable more efficient operation of the turbine cycle. This increase in efficiency reduces fuel (coal) consumption, and thereby reduces emissions, solid waste, water use and operating costs.
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.
Rendering of AEP’s John W Turk Jr Plant
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 ºF) and use of double-reheat steam.
The company’s focus on generating efficiency continues with its pursuit of ultra-supercritical technologies.
Today, AEP is constructing the first USC plant in the U.S, the 600 MW John W Turk Jr Plant in Fulton, Arkansas.
New materials support USC technology
Today, recently-developed chrome and nickel-based super alloys are used in the components of the steam generator, turbine and piping systems that are exposed to high temperature steam. The new metals can perform under these prolonged high temperature operating conditions, rendering USC no longer a goal, but practical design basis. USC technology is compatible with all types of coal.
Improved efficiencies and environmental performance
USC technology is the most efficient steam cycle in existence today with an efficiency in the 39% range (approximately 39 percent of the thermal energy that goes into the power generation process comes out as electric power). As a result of that higher efficiency:
- The unit emits less CO2, NOx, and SO2 per MWh
- The USC unit consumes less coal per MWh, resulting in fewer coal train cars per year
- The unit consumes less FGD lime per year resulting in fewer lime deliveries per year
- Total ash + FGD waste is reduced resulting in a reduction in landfill size
