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Key takeaways: Each repair method presents problems and varying degrees of effectiveness, although weld repair has the greatest potential for long-term service. Temporary repairs are often made to return casings to service and can be good for up to 5 years of service, although in many cases the repair is only good for a much shorter period. In some cases, several temporary repairs can be made before a casing is replaced. A major repair generally involves welding, stress relieving and machining. The repair/replacement decision is both a technical and an economic decision, especially when new casing costs can be in the millions of dollars.
Environmental controls
Case study: Tampa Electric, at Big Bend, controls SO3 from the SCR using
ammonia injection
The expanded use of multiple pollution control processes and devices on coal-fired units, brought about by the advent of more stringent environmental regulation, has led to
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To prevent this from occurring, several processes have been developed, or are in development. One such process is the injection of ammonia vapor into the flue gas to reduce the SO3 concentration prior to the FGD system. Tampa Electric Company installed ammonia injection systems downstream of the air preheaters on coal-fired Units 3 and 4 of its Big Bend Station. Field testing evaluated the effectiveness of the ammonia injection systems in reducing SO3 concentrations in the flue gas of these two units, which burn eastern medium- to high-sulfur bituminous coal. The secondary impacts of the injected ammonia on the marketability of the resulting fly ash and the environmental implications of the increased nitrogen content of the FGD chloride purge stream discharge were other issues to be addressed.
Key takeaways: The use of ammonia for control of sulfuric acid mist or blue plumes is very effective. Its effects on ESP performance appear to be minimal to date, as are its effects on FGD performance, gypsum quality and FGD purge stream quality. Its effects on fly ash, however, are very significant and will eliminate it from consideration as a commercial byproduct unless treated to remove the ammonia to acceptable levels. The untreated ammonia levels in the fly ash also could prove problematic for fly ash landfill disposal operations.
All units at Big Bend have added or are adding an SO3 mitigation system that uses ammonia and an accompanying fly ash beneficiation system to remove ammonia from the fly ash. A controls upgrade was implemented or is planned for each of the 4 units. Units 3 and 4 also both had innovative economizer gas temperature control systems installed to provide greater flexibility and reliability while operating the units at lower loads.
Alan Grunsky holds a dual role at EPRI, as the senior program manager of the Major Component Reliability (MCR) group and the program manager of Program 65, Steam Turbine Generators and Auxiliary Systems, both in the Generation Sector. Prior to joining EPRI in 1999, Grunsky’s experience included 23 years with Duke Energy in the Charlotte, N.C., area. During this time, he managed the turbine-generator maintenance program for the Duke system for both fossil and nuclear units, and spent 5 years at the Duke Marshall Steam Station as the station maintenance superintendent. Later, he managed a combined fossil/nuclear/hydro machine shop organization and then managed a combined valve testing, metallurgical lab and welding program group. He has a bachelor’s in Mechanical Engineering and an MBA from the University of South Carolina.
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