Boilers, Clean Coal Technologies, Coal

Clean Coal at Polk Power Station

Issue 3 and Volume 115.

By Brian Wheeler, Associate Editor

Built with $140 million of support from the U.S. Department of Energy (DOE) as part of its Clean Coal Technology program, the Tampa Electric Co.-owned Polk Power Station is one of the United States’ first greenfield commercial integrated gasification combined-cycle (IGCC) power plants. Completed in 1996, the 260 MW Polk IGCC station sits on 4,300 acres of land that was once used for phosphate mines.

The IGCC unit, Polk 1, is the focus of the plant’s clean coal technology and is one of the first-of-its-kind combinations of coal gasification and combined-cycle. The gasification technology was developed by Texaco in the 1940s but has since been acquired by General Electric. Gasification is the process of combining coal or other fuels, such as petroleum coke, with oxygen to create a clean-burning gas known as syngas. Syngas is the primary fuel at Polk although it can run on distillate oil, if needed.

Polk Power Station is one of the nation’s first greenfield IGCC plants. Photo courtesy Tampa Electric Co.

“Gasification is a fairly common process, however, IGCC is not a technology that is in widespread use for power production,” said Mark Hornick, director of Planning, Engineering and Construction for TECO.

The IGCC process at Polk uses coal that is stored on-site in two 5,000-ton silos and mixes it with water to create a slurry. That slurry is then pumped into the 40-foot-tall GE gasifier, which combines with oxygen to produce syngas. After processing, the clean-coal gas is used in the 1:1 GE 7FA combustion turbine in combined-cycle with a GE D11 steam turbine. This combined-cycle design takes the turbine’s waste exhaust heat, recovers it in the heat recovery steam generator and passes it through the steam turbine to put out even more electricity to the grid.

The plant functions as one of the world’s cleanest power plants, according to DOE. The Polk station operates at 90 percent availability with over 98 percent SO2 reduction, 90 percent NOX control and particulate emissions of less than 0.004 lb/MMbtu.

“With gasification, CO2 could also be removed prior to combustion,” said Hornick. “And that could be a big advantage.”

The IGCC offers the plant fuel flexibility. Although designed to burn Northern Appalachian coal, the plant has operated on over 20 different coal types and blends with petroleum coke.

The Polk Power Station is also considered zero process water discharge. Hornick, who served as Polk plant manager from 2000 to 2008, said that compared with conventional coal plants, IGCC plants use less water due to their combined-cycle makeup, rather than using an all-steam cycle. TECO also adapted existing mine cuts at the site to act as the plant’s cooling reservoir.

In March 2009, Tampa Electric established an agreement with the City of Lakeland and the Southwest Florida Water Management District to supply Polk Power Station with up to five million gallons of treated reclaimed water daily that would otherwise be discharged to surface waters.

Continuing with the trend of environmentally friendly operations, all slag from the plant is sold locally for cement or as aggregates to be used for blasting.

“There is almost no by-product disposal from this plant,” Hornick said. “We sell it for beneficial re-use.”

For TECO to meet peaking demand, the utility built four simple-cycle units at Polk between 2000 and 2007 that operate using GE 7FA combustion turbines. Unit 2 and Unit 3 are 180 MW dual-fuel turbines fired with natural gas and distillate oil. The newest 160 MW units, Unit 4 and Unit 5, operate using only natural gas.

Hornick said the future of IGCC plants in the U.S. depends on the cost of fuels and new technology to capture CO2. At Polk, the 10-person operation and maintenance team operates the IGCC unit to provide “low conventional-emission” power as part of Tampa Electric’s 4,450 MW of total capacity.

The Polk team continues to make improvements to the plant. Polk 1 had its best ever availability and heat rate in 2010. “And at Polk, we have generated more electricity from syngas than any plant in the world,” said Hornick.

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