Lots of facts and figures were presented in the November 9 Hot Topic Hour on dry scrubbers. Utilities operating these systems, utilities retrofitting them, and utilities considering dry scrubbers for new plants were among the 40 participants. While much of the data was supplied in semi-formal presentations, a significant amount was volunteered.  For example, Niels Jacobsen of Niro in Denmark gave some insights on rotary atomizer performance generally and spray drier efficiency in Europe. Mike Schantz of Chemical Lime and Shiaw Tseng of Graymont addressed aspects of lime quality and performance. The cost per ton of SO₂ removed depends greatly on the slaking operations and on the size of the systems. Removal efficiencies of 95 percent (a day-to-day number) or higher at certain units with SDA systems were stated. Questions arose when comparing performance or lime usage between the various units using SDA systems because so many factors are involved.

In general the spray drier absorber (SDA) technology has been used longer and more widely and it is capturing a big market share for new applications, e.g., the TXU eight pack.  The circulating dry scrubbers may have a unique place for medium efficiency and medium sulfur coal and are sandwiched in between the SDA and wet limestone systems. There are some large circulating fluidized bed (CFB) scrubbers being installed in China and now probably the U.S., so the experience hurdle is being crossed. In the United States, Westmoreland Coal Roanoke Valley has two units, one uses a CFB for SO₂ control (50 MW Unit 2), the other uses a SDA (182 MW Unit 1). Likewise, Black Hills operates both FGD systems. Black Hills will be using a SDA for its new 90-MW Wygen 2 unit.

Black Hills has become the focus of lots of a debate between the SDA and CFB suppliers. The SDA people point to the fact that the utility chose SDA for the new unit even though they are operating a CFB and an SDA. The CFB people argue that the CFB is actually doing better but that the decision on the SDA for the new unit was driven by the desire to buy a boiler/scrubber package. Thus the plant was unable to make the scrubber selection.

Stewart Nicholson of Primex reported that Primex works with SDA owners and operators to optimize performance of their systems. In general the key factors in optimizing performance are the method by which the slaked lime is prepared and the spray drier absorber temperature control. Primex is bullish on dry FGD. Stuart showed the wide variation in performance among specific units. This is measured in tons of lime per ton of SO₂ removed. He provided this data for a number of dry and wet systems and showed the averages and extremes.

Rich Abrams made a case for the Turbosorp CFB scrubber licensed by Babcock Power Environmental from Austrian Energy. According to Rich CFB is suitable for units burning up to three percent sulfur coal, with sizes ranging up to 250 to 300 MW (in China), where 95 to 97 percent SO₂ removal is required. The lime is hydrated (not slaked) on site, lime usage is less than with SDA systems, power usage is less, and the water used to hydrate the lime can be poor quality water. In a CFB absorber the solids are internally recirculated and also recirculated from the baghouse. The Turbosorp system at AES 115 MW Greenidge (burning 2.9 percent S coal) will start operating soon. Even with the steel necessary to elevate the baghouse, the construction costs for the Turbosorp System were less than for a SDA system.

Bill Ellison of Ellison represented Wulff. Wulff is supplying 350 MW CFB systems in China. Therefore China will be the most experienced user of CFB scrubbers over the next five years. However, there are already units providing up to 99 percent SO₂ removal with Ca/S stoichiometry no greater than 1.5.  Efficiencies of 92-95 percent are being achieved at ratios of 1.3-1.4.