Coal

Wet, Dry and In Between

Issue 3 and Volume 111.

By Brian Schimmoller, Contributing Editor

In 2004, as part of a least-cost resource plan filing with the Colorado Public Utility Commission, Xcel Energy proposed constructing a 750 MW supercritical coal-fired unit at the Comanche Station near Pueblo, Colo. To win support for the plan from environmental groups, Xcel agreed to a number of renewable, conservation, emissions control and economic development initiatives. A less-publicized part of the plan concerned water availability in the Arkansas River Basin, where the plant is located. Based on regional economic growth projections, the Pueblo Board of Water Works preferred to limit the amount of water allotted to the new Comanche unit.

“Water purchases from the Arkansas Basin for use outside the area are very controversial,” says Tim Farmer, Xcel’s project manager for Comanche Unit 3, citing rising demand from places as far away as 150 miles. “Since we would have kept the water in the Valley, we probably could have acquired enough water rights for a completely wet cooling system.” Xcel, however, decided to contract for a lesser volume of water and install a parallel condensing system from GEA Power Cooling Systems to reduce water requirements for cooling.

Parallel condensing systems unite conventional wet cooling technology with dry cooling technology to reduce water use; the steam exhausted from the steam turbine is split between a steam surface condenser (tied to a conventional wet cooling tower) and an air-cooled condenser (ACC). At Comanche, the split will be about 50-50, resulting in estimated water requirements of 4,750 to 5,550 acre-feet for Unit 3 (750 MW), versus about 9,500 acre-feet for existing Units 1 and 2 (660 MW) at 90 percent capacity factor.

Water concerns are strongest out West, where an arid climate and population growth accentuate competition for water supplies. Many of the dry cooling systems installed in recent years have been west of the Mississippi River. The Eastern half of the country, however, is not immune from water availability concerns. Most new power plants in Massachusetts are dry-cooled, as are many in New York. Further, suburban sprawl in many Eastern states has precipitated demand-driven drought conditions, in which the narrow balance between water supply and demand triggers near-term emergencies and limits longer-term industrial development.

As pressures mount to reduce water consumption or to deal with lower water availability, alternate cooling schemes become more palatable. Parallel condensing systems are one option, but others exist. “Whether you put in a two-cell wet tower as part of a parallel condensing system or 40 cells as part of an all-wet cooling system, you still need air and water permits,” says Bill Wurtz, general manager of Dry Cooling, Americas, for SPX Cooling Technologies. “If the permits could invite significant opposition, it might be simpler to absorb the higher up-front cost of the air-cooled condenser and go all dry.”

Moreover, the penalties associated with dry cooling may not be as high as some think. Industry consultant Bill Powers, P.E., with Powers Engineering, believes comparisons between wet and dry cooling are often done on an apples-to-oranges basis. For example, in its Technical Development Document for Section 316(b) regulations under the Clean Water Act, EPA applied lesser performance capabilities to ACC systems than to wet systems, resulting in higher heat rate penalties. In general, the lower the design initial temperature difference (ITD) for an ACC system, the greater the heat transfer capabilities, resulting in a larger ACC but a lower heat rate penalty. “The ITDs used for air-cooled coal plants in the EPA analysis are much higher than the ITDs being specified for new coal plants today,” says Powers. “Current state-of-the-art ITD for a coal plant is 40 F, and 35 F ITDs are becoming more common.”

In analyzing ACC systems, Powers found a much smaller delta between wet and dry systems when compared using similar criteria and when not focusing solely on performance at peak ambient temperatures. Based on publicly available data for Weston Unit 4, a coal plant being built by Wisconsin Public Service, Powers compared wet and dry performance at various temperatures. For an ITD of 40 F, the annual and peak (at 90 F ambient) heat rate penalties were 2 percent and 3.6 percent, respectively, compared to a wet system with an approach temperature of 12 F. For an ITD of 35 F, the annual and peak heat rate penalties fell to 1.5 percent and 2.8 percent, respectively. Because Weston Unit 4 is equipped to fire 3 percent more fuel than rated throughput if necessary, it could sustain its 515 MW rated capacity with an ACC at temperatures up to 90 F.

Inventive Options

For existing plants facing a water crunch, more inventive options may be necessary. One large coal-fired power plant in Wyoming, for example, obtains its makeup water from a reservoir; but growing demand from farmers and residential areas is reducing water availability. There is not enough room to route the steam flow from the turbine to an ACC system, so SPX’s Wurtz offers another solution: “By taking the hot water off of the condenser and running it through a set of air coolers in series with the existing wet cooling tower, water use could be trimmed significantly.”

Parallel condensing systems and all-dry cooling systems are more expensive than conventional wet cooling systems. The parallel condensing system at Comanche – which includes 45 air-cooled condenser cells, each almost 80 feet from grade to fan deck – will be three times the capital cost of a conventional wet cooling tower system, but 40 percent less than the cost of an all-dry cooling system, according to Xcel’s Tim Farmer.

Overall project cost impacts, however, are less severe. Powers pegs the bottom-line impact at around 5 percent of total project cost for an ACC system relative to a wet cooling system. Plant developers siting new plants may be increasingly willing to absorb that 5 percent to remove the risk associated with public opposition and future water availability concerns.

It’s a brave new world out there in terms of securing water rights. Wet, dry or in between, your plant can still stay cool.