Can Flyash Still Be Sold After Hg Reduction?

FGD and DeNOx
NEWSLETTER

January 2010
No. 381

Many utilities presently sell the flyash collected in their ESPs and baghouses for by-product use, producing an environmental benefit and an additional positive revenue stream. With the introduction of activated carbon injection (ACI) systems for mercury (Hg) reduction there has been a growing concern that carbon contamination of the flyash could turn this by-product into a waste product.

Portland General Electric (PGE) operates the Boardman Power Plant, a 600-MW coal-fired power plant, which sells all of its flyash. The Oregon Utility Mercury Rule set emission limits that will require PGE to install and operate an activated carbon injection system at Boardman to reduce mercury emissions by 90 percent or to an outlet emission of 0.6 lb/TBtu before July 1, 2012. The team of PGE, EPRI, ADA-ES and Sargent & Lundy developed a test plan to determine if various powered activated carbons (PAC) could effectively remove mercury from the flue gas and still result in a flyash stream that was acceptable to PGE’s flyash broker (Boral Materials). Various carbons from three manufacturers (Calgon, Norit and Sorbent Technologies) were tested along with Norit’s Hg-LH carbon (as a baseline).

A series of injection rates from 1 to 7 lb/MMACF were evaluated for each of the carbons to determine the associated mercury (Hg) removal at each injection rate. For each test run, the PAC was injected through existing ports in the inlet duct to the ESP. Each manufacturer provided activated carbons with less impact on concrete properties when the flyash/activated carbon mixture is used as cement replacement—Sorbent Technologies’ C-PAC^TM, Calgon’s FLUEPAC^® CR PLUS Ultra, Norit America’s DARCO^® EXP 1 and EXP-224. Flyash samples were taken from Section C of the ESP (south side) and Section J of the ESP (north side)

During the testing of the different carbons their effectiveness at mercury removal and their effect on particulate emissions and flyash characteristics were evaluated. ADA evaluated the ash collected during each test run to determine whether the sorbents will impact PGE’s ability to sell their flyash. After the preliminary injection tests, a one-week system test was carried out to evaluate the effects of ACI on overall plant operation. Based on previous testing, the long-term system effects due to ACI are more a function of the amount of carbon being injected into the flue gas stream rather than the manufacturer of the carbon. Consequently, one supplier’s carbon was chosen as the activated carbon to be used for the system test, and the results were assumed to be similar for all suppliers’ PAC.

Results from the characterization and continuous tests indicate 90 percent mercury removal appears to be potentially achievable at an injection rate of 4 to 6 lb/MMACF with some of the activated carbons tested. However, these were short-term tests and it is uncertain if 90 percent removals can be sustained over time. Even though some of the novel concrete compatible sorbents did show minimal impact during foam index testing, the air entrained in the resultant concrete mix appeared to degrade over time and required adjustments to the amount of foam agents used over time. Given that the amount of carbon in the ash may vary during the long-term plant operation and the amount of concrete air may also vary over time, even for constant amounts of carbon in ash, it is uncertain if the ash sales at Boardman can be preserved with ACI before the ESP, especially for high mercury removals, reported Paul Farber and Danielle Flagg of Sargent & Lundy, Rick Tetzloff of Portland General Electric, Ramsay Chang of EPRI and Tom Campbell of ADA-ES at Coal-Gen 2009.

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