Carbon dioxide is rarely associated with happy thoughts in the power industry. The remote threat or possibility of future regulations has kept many a utility executive from a sound night’s sleep. Carbon dioxide does have its redeeming qualities, however. Beyond its value in keeping the world supplied with oxygen and its use in enhanced oil recovery efforts, carbon dioxide can also be used to aid in removing deposits from boiler, HRSG and generator winding surfaces.
In HRSGs, for example, debris and corrosion products commonly settle on fin tubes, thereby impeding heat transfer. Water and sandblasting can be effective, but present a few disadvantages, according to Randall Martin, vice president with Environmental Alternatives Inc. Water interacts with tube deposits, promoting corrosion that shortens the life of the unit. Grit blasting erodes the surface metal of the fin tubes and can also lead to grit entrapment in the center of the tube bank. Water and sandblasting also create a significant quantity of secondary waste that requires disposal. Finally, water and sand lose their energy upon initial impact with the first row of tubes, reducing their ability to flush additional debris behind the initial deposit.
The CO2 process is a surface cleaning technology that utilizes small cylindrical dry ice pellets to remove fouling, rust and scale from various substrates. Cleaning is performed without creating any secondary waste. The CO2 pellets sublimate to a gas and the only waste to dispose of is the actual debris that is removed from the boiler surfaces. Because the CO2 process is dry and inert, it does not promote corrosion. Also, the cleaning is performed without damage or erosion of the fin tube or boiler surfaces.
The process involves converting liquid CO2 to a solid dry ice pellet. An on-site pellet conversion device ensures that sufficient pellet is available and that pellet density/quality is controlled to maximize cleaning efficiency. A high-pressure compressor propels the pellets through a hose to a specially designed nozzle that delivers the pellets at supersonic speeds of up to 1,000 ft/s. Upon exiting the nozzle, the pellets penetrate the debris layer on the surface being cleaned and sublimate to a gaseous vapor. At the sublimation phase, the pellets transform from a solid to a gas, expanding to 750 times the original solid volume. This transformation creates a “mushroom” effect that lifts the debris from the metal surfaces and removes it. Vacuum equipment then removes all of the debris loosened during the cleaning.
Before (top) and after (bottom) photos of heat exchanger tubes, demonstrating the effectiveness of CO2 cleaning. Photos courtesy of Environmental Alternatives Inc.
EAI has used the CO2 process to clean hundreds of HRSGs over the past 10 years. TransCanada/Ocean State Power has employed the technology at its two-on-one 500 MW combined-cycle power plant in Harrisville, Rhode Island. Traditional cleaning methods had been used to remove accumulated combustion byproducts, but were time-consuming, generated secondary waste and damaged the tube fins, according to Joe Collard, the plant’s maintenance supervisor. OSP has used CO2 cleaning several times to remove debris from its HRSG tube bundles. During a 2004 maintenance shutdown, for example, OSP evaluated all of the boiler tube bundles and determined that only the feedwater heater required cleaning. The feedwater heater has eight headers, 2-inch OD tubes, with a surface area of almost 61,000 ft2.
EAI dispatched a self-contained trailer to perform the cleaning. The cleaning was designed to remove all plated fouling (ammonia salts) and accumulated rust, debris and insulation from within the fins to aide in increasing heat transfer efficiency and open up any bridging across the gas lanes of the unit to reduce the overall backpressure. The feedwater heater cleaning process took 24 to 36 hours per HRSG. When Ocean State Power came back online, the differential pressure in the HRSG dropped by three inches and the plant recovered 1.1 MW in lost capacity.
The only on-site services required are connection to plant power for the equipment, both 480 volt, 3 phase and 110 volt, 1 phase. EAI obtains its CO2 by recycling gaseous CO2 from other manufacturing sources, condensing it to a liquid for use in its cleaning process. The volumes of carbon dioxide used for a typical cleaning are small; the cleaning at OSP, for example, required less than 4,000 liquid pounds.
To expedite cleaning, Collard recommends that all lock-out/tag-out procedures be in place, ready for signing, and that all access doors be opened for immediate access. Also, because much of the cleaning is required to remove accumulated ammonia salts, OSP now pays greater attention to its SCR operating procedures, especially for monitoring ammonia flow, dilution air flow and temperatures. p