SCR equipment won’t work unless the gases can contact the catalyst. Therefore, keeping the catalyst clean is a critical part of the design process, and involves substantial cost considerations. Two competing technologies are used for removing deposits from SCR catalyst beds: steam-operated sootblowers and acoustic generators (often called horns).
Experience with rake-style sootblowers, commonly used to clean boiler tubes, has shown that they are usually very effective at removing buildup from catalyst, but often at a significant cost. Each sootblower costs $17,000-$20,000, and several are required over each layer of catalyst. Total cost of a sootblower installation will depend on the size of the SCR, how many layers of catalyst are installed, and on the physical size of the sootblowers, since they must be supported with structural steel both inside and around the perimeter of the SCR. Sootblowers are comparatively difficult to install, require high-pressure steam to operate (including insulated steam lines run to each unit), and have many moving parts that tend to wear and require maintenance.
In contrast, acoustic horns are relatively inexpensive (one-fourth the cost of a steam sootblower), don’t require structural steel for support, and have only one moving part, a titanium diaphragm that might need to be replaced after three to five years. The acoustic horns operate on standard plant compressed air, and 70-90 psi air plumbing is all that is required to make them operational. (Solenoids are used to fire the horns; from the solenoid to the horn, flex hose is usually used.)
The SCR reactor at Mirant’s 250 MW Birchwood Power Facility in King George, Virginia initially started up with one-and-a-half layers of plate-to-plate catalyst, cleaned by three rake-style sootblowers over each layer. The sootblowers operated every 12 hours and performed well. In the Fall of 2000, Birchwood installed four BHA Powerwave DC-75 horns above the first layer of catalyst to assess their cleaning effectiveness. The acoustic cleaners operated from October 2000 through May 2001, with the sootblowers idle. During the May 2001 outage, plant personnel inspected the SCR catalyst and found it to be just as clean as when the sootblowers were being used. Birchwood decided to purchase a total of eight Powerwave horns to clean the SCR, a decision justified by the steam cost savings alone. “The steam sootblowers cost $40.50 per day to operate, while the eight acoustic horns cost $3.76 per day, resulting in an annual operating savings of $13,409,” said Jake Shelton, BHA Group’s Acoustics Technical Director. “The reduced maintenance saves several thousand dollars more.”
A more thorough comparison was conducted at TVA’s Paradise Fossil Plant in Drakesboro, Kentucky. On Unit #2, the plant installed an 80-ft by 50-ft SCR with four layers of honeycomb catalyst, with each layer cleaned using four rake-style sootblowers. “We decided to experiment with acoustic horns as an option to sootblowers to see if we could reduce our costs,” said TVA’s Joe Giles. Paradise intermingled six Powerwave acoustic cleaners with the sootblowers to provide a true side-by-side comparison of cleaning efficiency. The test was conducted on the top layer of catalyst, with one third using sonic horns and two thirds using soot blowers. All other layers were cleaned exclusively by sootblowers.
The reactor operated for six weeks before being taken off-line for a short outage and inspection. Plant personnel found no difference between the areas cleaned with horns and those cleaned with sootblowers on the first layer. TVA engineers believed turbulent gas flow may have been contributing to the cleanliness of the first layer, according to Giles, so the acoustic horns were removed and installed between the first and second layers of catalyst for another trial.
This time, the reactor operated from May to October, and was then re-inspected. The section of the catalyst layer cleaned with acoustic horns mounted external to the SCR module was free from any buildup. Horns mounted internal to the SCR, and exposed to the 650-700 F gas temperature, did not perform adequately because there was no access on the far wall to mount them outside as recommended. Based on the test results, TVA installed 40 Powerwave D-75 acoustic cleaners on the identical Paradise Unit #1 SCR. The six horns were removed from the Unit #2 SCR, and the sootblowers reactivated. Both units were then operated during the 2001 ozone season (May 1 through September 30) months, one with Powerwave horns, the other with sootblowers. The following Fall, both units were again inspected, and there were no significant differences in the cleanliness of the catalyst layer. TVA plans to use acoustic horns mounted external to the SCR module on future SCR projects.