The Future of Coal: Efficiency Over Politics?

These conventional types of power plant take the heat that is generated by burning fossil fuel, including coal, and use that heat to boil water and raise steam. This steam is used to drive a steam turbine that rotates an electric generator to produce alternating current to feed the grid.

While these plants can reliably produce large amounts of electricity, many new coal-fired power plants have been postponed or abandoned in recent years because of their relatively lower thermal efficiency and the cost of the required pollution controls.

The older U.S. power plants have efficiencies averaging between 33% and 35%. These plants have steam conditions between 2000 and 3000 psi and temperatures in the region of 1000°F (537°C).

Newer designs of these plants fall into the HELE category (high efficiency low emissions). The more efficient a plant, the less fuel is needed to generate electricity and the fewer pollutants are emitted. Supercritical and ultra-supercritical plant designs are considered HELE. The steam conditions of a typical supercritical steam cycle are pressures greater than 3200 psi and temperatures from 1000 to 1050 °F (537°C to (565°C). The ultra-supercritical (USC) conditions have similar pressures but temperatures above 1050°F (565°C). The first USC plant built in the U.S. is the John W. Turk Jr. 600MW plant in Arkansas. It has an efficiency of 42%.

The disadvantage of HELE coal plants is the additional cost of construction, which us estimated at between 20% and 30% over a conventional plant. Part of this increased cost comes from the specialized materials needed to contain the higher temperatures and pressures. Boiler tubes, superheat and reheat tubes require special alloys. These components as well as the associated valves also must resist the heightened erosion and corrosion associated with the steam conditions. Beefier valves demand higher torques to operate and that means bigger actuators.

A typical 2,000 MW coal-fired power plant has about one automated valve for every 2MW of generation capacity. This translates into about 1000 automated valves per power plant.

The additional pollution controls demanded for coal plants contribute to the number of automated valves and dampers. The major pollutants, sulphur dioxide, nitrogen oxides, hazardous air pollutants (HAPs) and fly ash all require removal. Dry or wet scrubbers are used to remove Sulphur from the flue gas. These require valves to manage the water-based reagents that react with the Sulphur in the scrubber. Dry scrubbers also use water, but in smaller quantities. Nitrogen oxides are removed by catalytic reduction, which also requires valves and actuators. Additionally, the combustion flue gas flow must be controlled into and out of these pollution controls, so automated dampers are required as well as valves.

Comparing Efficiencies

In recent years, natural gas has seen a major increase in power generation use. The impact of shale gas on availability and fuel pricing has enhanced the attraction of gas turbine generation. Gas turbines are directly coupled to an electric generator for primary electric power generation. The hot exhaust gases from the turbine are then used to raise steam in a heat recovery steam generator (HRSG), a specially designed boiler. This steam is then used to drive a steam turbine coupled to a secondary electric generator.

A coal-fired generating plant may have an overall efficiency of about 35 to 42% but a gas-fired combined cycle plant may reach efficiencies up to 54%—a significant increase.

A typical 750 MW gas turbine combined cycle plant will use about one automated valve per 4 MW, so only about 190 automated valves per plant. The valves associated with the gas turbine section are few, but there is automaton on the main air inlet damper and chiller valves. The HRSG uses similar valves to those used on any thermal steam cycle, boiler feed pump valves, bled steam and preheater valves, etc., as well as blowdown and pressure relief valves.

However, because natural gas is cleaner burning than coal, the extensive pollution controls are not required and so there are far fewer valves and dampers on the back end of the boiler.

Compared to conventional coal-fired power plants the combined cycle plants have some distinct advantages in today’s circumstances.

1. Greenhouse gas emissions and other pollutants are lower.
2. They produce more energy for a given space, so their footprint is smaller.
3. It is easier to obtain permitting for them, partly due to items 1 & 2.
4. They are more thermally efficient than coal-fired plants. Thus, for a given amount of fuel they produce more electricity.
5. Gas turbine can run up to full generation very quickly so it can be online faster.

Conclusion

With the current advantages of the gas turbine combined-cycle technology, it seems unlikely that even the latest ultra-supercritical coal-fired power plants can reach their high levels of thermal efficiency. When the other considerations, both economic and environmental, are factored into the equation, it does not bode well for a resurgence of demand for coal in the power generation industry in North America—even with the easing of environmental constraints from the new administration.

Chris Warnett is vice president of engineering at Upstate Valve and Control and principal of CPLloyd Consulting Inc.