Coal-fired power plants can and should play a starring role in the integration of renewable power.
That was the conclusion of researchers who prepared an eye-opening report for the Department of Energy’s National Renewable Energy Laboratory. The December 2013 report, Flexible Coal: Evolution from Baseload to Peaking Plant, showcased the transformation of a 1970s era coal-fired plant, which was modified to cycle on and off at low generation levels like a peaking plant.
“This study proves that coal can be part of a power system with high levels of renewable energy,” said NREL’s Jaquelin Cochran, the report’s lead author. “Coal plants can be modified to respond to the changing output of renewable energy and run at low levels when renewable electricity generation is high but demand is low, such as at night.”
The case study offers a glimpse at the future of coal-fired generation, a world where coal is used to offset the fluctuations in renewable power through greater flexibility. It also provides a road map for incentivizing and modifying similar projects.
The report summarizes the hardware and operational modifications at an unnamed coal generating station in North America, which was originally built to run as a baseload plant with an 80 percent annual capacity factor. The plant was designed to run near full capacity most of the year, but the addition of nuclear capacity displaced most of the plant’s coal-fired generation.
Today, the plant cycles on and off “as many as four times a day,” the report shows. “It is one of a few coal plants worldwide to accomplish this level of flexibility.”
The steam generator and supporting equipment – boilers, rotors, condensers, turbines and pulverizers - were modified to enable frequent cycling. But giving the plant the ability to cycle on and off at lower output required few modifications in hardware. The most extensive modifications centered around the plant’s operational practices. In fact, the plant’s owner estimated “90 percent of future savings in costs came from adjustments to operating procedures,” according to the report.
The plant cycled on and off as many as four times a day to meet peak demand. The increased cycling and the rapid changes in temperature and pressure led to several issues, including thermal fatigue, turbine corrosion, boiler tube failures, cracked rotors and wear and tear on auxiliary equipment.
“There is a cost to this flexibility,” Cochran said. “But these costs can be minimized with strategic modifications and maintenance.”
Despite the wear and tear to equipment, the plant continues to operate profitably, thanks to increased inspection, monitoring and training.
“The plant owner has achieved what few coal plant operators have been able to do,” NREL said in its report. “Key to the owner’s success is changing operational practices.”
The point is this: Power plants equipped with combined cycle gas turbine (CCGT) technology are highly efficient and flexible, but CCGT technology isn’t the only option available to power producers and grid managers who are struggling to maintain a balanced load amid a growing source of intermittent electricity. The NREL report shows that older baseload coal units can be reinvented to support the use of cleaner-burning power.
The NREL report serves as compelling evidence that the key to suppressing climate change and preserving this nation’s most abundant and reliable source of generation is greater flexibility for coal-fired plants.
The name of this column, “The Evolving Coal Plant,” is also one of seven conference tracks being offered at COAL-GEN 2014 in Nashville, Tenn., Aug. 20-22. This issue will be discussed in detail during one of the sessions at COAL-GEN, where panelists will examine ways to improve the flexible generation of coal-fired plants to accommodate the integration of more renewable power.
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