Converting coal and nuclear plants into combined cycles

With the Environmental Protection Agency (EPA) reigning down on aging coal plants, one option for power plant operators is transforming the facility into a combined cycle (CC) plant, harnessing both steam and natural gas. By doing so, they can make use of all the steam turbines as well as a large portion of the remaining plant.

The same applies to nuclear facilities, which already have the bulk of infrastructure in place for natural gas CC operation. Faced with the prospect of being edged out of the U.S., European and Japanese markets, some utilities and independent power providers may decide to switch from coal and nuclear to natural gas as the best route ahead. And the economics are favorable. Many anticipate a boost in combined cycle construction in the U.S. and other regions of the world.

Tim Xie, Lead Power Plant Performance Evaluation Group at Worley Parsons said, “Upcoming EPA rulings on greenhouse gases (GHG) and maximum achievable control technology (MACT), as well as the uncertainty around renewables have created an expectation of many new future natural gas units in the U.S.” He laid out the math in favor of CC. It has the lowest construction cost among all types of generation technologies, on top of a relatively short construction duration, lower emissions, and low water consumption compared with other fossils. Financing, too, he said, is easier on CC compared to others.

“In March 2012, a combined cycle gas turbine power plant had lower $/MWh fuel costs compared to coal-fueled power plants for the first time in history,” said Xie. 

However, low gas prices pose a dilemma for CC plant design as fuel savings over the plant’s lifecycle may struggle to offset the increased capital costs.

Therefore, GT selection is the key to success. Those specifying power plants are advised by Xie to investigate fuel costs and capital costs for the various GTs being considered to find the best option, which varies from site to site. He cited the trend in Asia and Europe for increased orders for H- and J-class GTs, where high natural gas costs make it easier to justify higher upfront costs. Those types of turbines have far fewer U.S. installs because of cheaper natural gas. As a result, there appears to be more focus on up-rating the proven F Class gas turbine models.

New reality

As operators retire coal-fired plants, they will be faced with the challenge of cycling, according to Bill Siegfriedt, project manager at Sargent & Lundy. Most of the recent combined cycle plants were originally built for base load operation, he said. Yet they find themselves tasked with cycling duty to fit the needs of a grid holding an abundance of variable renewable resources. Unfortunately, many of the design techniques to equip CC plants for cycling duty are not suited for retrofit.

More in January/February 2013 issue of Turbomachinery International