By Steve Blankinship, Associate Editor
Most utilities scrub in the shower, but scrubbing in the tub has its advantages.
Taking a shower and taking a bath might be two ways of accomplishing the same goal. But when it comes to flue gas desulfurization (FGD) system design, a shower versus a bath means two distinctly different means to an end.
Most wet scrubber technology has been designed to direct coal plant flue gas through spray towers containing limestone reagent. But an alternate approach is currently employed at about 65 plants worldwide supporting 17,000 MW, including 10 units at five sites in the United States that plunges stack gases into a bath of reagent. In addition to the 10 submergent FGD systems supporting 5,500 MW installed to date in the United States, another 12 units are under construction that will support more than 9,100 MW of generation when completed. As a result, bathing may become an attractive alternative to showering.
In a submergent FGD system flue gasses are forced into a tank of limestone solution. Illustration courtesy of Chiyoda Corp. and Black & Veatch Corp.
The original “bath” approach was developed in the 1970s by Chiyoda Corp. of Japan. The submergent technology is currently licensed in North America exclusively to Black & Veatch (B&V) and to Atlanta-based Southern Co., the latter arrangement for use in Southern’s power plants only. B&V provides the technology under the name CT-121. While B&V actively represents the Chiyoda scrubber technology in North America, B&V may also participate in any project in which its client chooses an alternate technology.
The first CT-121 designed by B&V went into a wet limestone flue gas desulfurization (WFGD) system in May 2007 at Dayton Power & Light’s 635 MW Killen Unit 2. Since then, B&V and DP&L have completed construction on four more WFGD processes at the Ohio utility’s nearby JM Stuart Station. The five, 600 MW DP&L units can capture and eliminate more than 97 percent of the sulfur dioxide (SO2) emitted through their smokestacks. The systems also remove a significant portion of remaining fine particulate matter and mercury from the plants’ flue gases.
“To date, the CT-121 technology at our Killen and Stuart facilities has outperformed our expectations,” said Paul Barbas, DP&L president and CEO. The addition of the CT-121 technology is enabling DP&L to use lower-cost coal that has higher sulfur content and is available from mines that are closer, thus reducing transportation costs.
“The projects at DP&L started out to be a compliance issue but they soon moved the schedule up as the decision became an economic one,” said Mike Perry, senior vice present B&V. “The fuel flexibility and the potential sale of SO2 credits prompted a joint effort to bring the units on line one year earlier than first planned.”
B&V had been working with DP&L since 1998 installing selective catalytic reduction (SCR) systems on their five large coal units. “When we began working on the DP&L scrubber program, we jointly decided that instead of going with an emissions compliance model we had initially been using, we decidedwould look more specifically at what their overall business model might yield, including fuel costs, disposal costs and potential gypsum sales.”
In the case of DP&L, Perry said the utility had narrowed its interest to two scrubber technologies, but became more interested in CT-121 because it had switched the units to blended and/or pure compliance coal. “Their coal costs had increased dramatically and they were having some operational difficulties from the switched fuel, so we did studies for them looking at all the various options,” he said. “In the course of that, it became clear to them that CT-121 offered some advantages.”
One of the biggest advantages is the ease with which it can be retrofitted. “When you are dealing with very limited space and other problems inherent with adding on to an existing plant, flexibility with size, shape and configuration becomes very important,” Perry said. “This scrubber technology is very flexible.”
That’s not to say those aren’t advantages for new builds as well. Perry said that many new plant applications look at a single boiler-to-stack contract. On new units this gives the owner a single source responsibility for cost, schedule and performance for the whole air quality stream with a single source OEM approach. In a retrofit application, Perry said he believes the CT-121 submergent technology approach is a good one and can see it attracting a good market share.
Perry has been installing scrubbers on power plants since the early 1970s. What he likes about the submergent technology is that it is a more positive means of insuring that all of the flue gas molecules come in contact with reagent with sufficient reactivity to promote consistent SO2 removal. As removal guarantee levels have risen above 95 percent to the 97 percent to 98 percent range, insuring no flue gas bypass of the removal process is critical.
Turn Down Ability
Perry also likes submergent technology’s turndown ratio. With spray tower technology, “if you have a 500 MW unit that you want to operate at 125 MW, you either have to leave all your spray levels on—depending on how long you’re going to be down—or, if you shut some spray levels and start flushing out the nozzles, all the clean water used can significantly change tank chemistry.” At that point, the operator has to stabilize at a new operating point. As a result, for short- to medium-duration load reductions, most plants simply left all the spray levels on, creating a fairly substantial parasitic load loss on the percentage of gross generation.
By contrast, the submergent approach operates so that the flue gas is bubbled down through the medium. The system does have a higher total draft, or fan, loss than spray towers, but enjoys a lower pumping loss because it’s not constantly circulating slurry against high head to spray levels with high head conditions. Relatively small pumps are needed for the submergent technology to recirculate the slurry. “We keep a huge bath in the base of the tank and a very stable chemistry is maintained in the slurry below,” he said. All that’s required for turn down with the CT-121 is to change a few inches of submergence to match the removal set point with the level of generation. As a result, the system follows load very well.
B&V is involved in projects where more than one boiler is being put into the same scrubber vessel to utilize the turndown ratio flexibility. One such multiple-boiler-into-one-scrubber approach is now in service at Power South’s Lowman Plant. As part of an overall air quality modifications for this three unit site, SCRs were added to the two larger units (units 2 and 3) which had older scrubbers for partial flue gas scrubbing. As part of this project, the two older scrubbers are being modernized and upgraded to scrub the entire Unit 3 flue gas to a level of 98 percent removal. A Chiyoda CT-121 scrubber was also installed to scrub the combined flue gas stream from both units 1 and 2. This scrubber was designed to operate effectively at the minimum load of Unit 1 with Unit 2 off line.
Southern Co. first began researching scrubbers in the 1970s and selected Chiyoda as its preferred technology several years later, said John Huggins, general manager of Environmental Strategy for Southern Company Services. Southern Co. installed the technology on the 100 MW Yates Unit 1 as pilot project with the U.S. Department of Energy in 1991. Yates was the first use of the submergent FGD technology in the United States.
Southern uses the same chemistry as B&V, but with a single gas riser vessel design instead of a multiple gas riser. “We saw an opportunity to improve the constructability on some of our units by going to a single gas riser,” said Huggins. “It was easier to construct that way.”
A second Southern CT-121 went into service this past May on the 900 MW Bowen Unit 3 in Georgia. Three more CT-121 units are being built at Bowen, all of which will be operational by 2010. Two more are being built at Plant Wansley in Georgia, one at Plant Gaston one at Plant Barry for Alabama Power and one at Plant Crist in Florida for Gulf Power, where the balance of plant is being designed by Black & Veatch.
Southern has another fleet of scrubbers using the Mitsubishi design contracted to Advatech. In the Mitsubishi/Advatech “fountain tower” design, the slurry flows upward into the air within a fountain from the bottom of the tower. Southern currently has eight fountain tower units in construction and two in operation. The units in operation are at Alabama Power’s Plant Gorgas and Georgia Power’s Plant Hammond.
Huggins said that in addition to Southern looking at a plant’s needs and matching the attributes of the particular technology to those needs, the utility also wanted a two-technology strategy to reduce the risk of a systemic problem across the fleet.
Ed Healy, consulting engineer, said that Advatech units are deployed more on lower sulfur coal units and Chiyoda’s on the higher sulfur end.
“I think that technology-wise, Chiyoda is better at the high end of the sulfur scale,” said Healy. “I think the chemistry is better for plants where you might have a higher ash loading to the scrubber or need to run when there are some process upsets. Both technologies have met performance expectations. We have seen that the multi-pollutant aspect of the Chiyoda has been better in terms other than SO2, such as metals, mercury, ash and aerosols. There has been a measurable difference for us.”
Huggins saids that where the company thinks it needs the most fuel flexibility (all other things being equal) the CT-121 is a better fit; be it SO2 content, Btu content or ash content.”
In addition to plant operators controlling how deep the flue gas is bubbled into the solution, which controls contact time to achieve the efficiency needed, they also control the pH of the solution and the calcium sulfate content in the slurry to a much greater level than is achieved with spray towers. Calcium sulfate is the result of limestone capturing SO2, and is the basis for the saleable gypsum product generated by wet limestone scrubbers.
The spray pumps and nozzles in a spray tower tend to break apart the gypsum crystals formed during removal. With CT-121, the crystals can continue to grow to a larger size thus making them easier to separate from the slurry and enhancing overall gypsum quality. In addition, there is slightly better use of the reagent and less undesirable material in the gypsum byproduct.
“With the CT-121 you end up with much larger gypsum crystals and a better purity,” said Perry, “And compared to gypsum from other scrubbers, it meets or beats the quality from a sales standpoint.”
DP&L System Design Challenges
Black & Veatch faced unique design and construction challenges at both Dayton Power & Light (DP&L) Ohio sites when the CT-121 system.
The first was constructing fiberglass vessels and internals in the field and attaching ducts of high nickel metal alloys (including internal and external alloy and fiberglass piping) to the fiberglass vessels.
The heart of the process at all five DP&L units is a jet bubbling reactor (JBR) scrubber vessel. The vessel stands approximately 75 feet tall and measures approximately 87 feet in diameter. Moving the process slurries and water throughout the system entails 30,000 feet of pipe at Killen and 90,000 feet of pipe at the four Stuart units. Black & Veatch’s design team worked along DP&L and with Chiyoda’s experts to complete the installation.
Another major challenge at Killen was converting a 900-foot-tall stack designed for dry, hot gases into one that could withstand moist, lower-temperature flue gases after they had passed through the scrubber. This chimney work was balanced to accommodate scheduled outages that were part of normal plant maintenance.
“The outstanding performance at both DP&L plants proves the technology in North America and demonstrates Black & Veatch’s ability to deliver industry-leading technology,” said David Harris, Black & Veatch vice president of Air Quality Technology.—SB