Rejuvenate your controls
The plant control system reaches across all the system boundaries to harmonize plant operations. A well-behaved combustion system relies heavily on system instrumentation and controls, as well as accurate controls tuning and final control element calibration. When calibrated well, most control variability and inaccuracy in the system controls can be eliminated.
Optimizing the controls is the final step after all the process measurement and control devices are correctly calibrated. Some older control systems may not have migrated to the latest control schemes available, so now is the time to do so. Many improved functional control schemes are available that will improve unit response and ramp rates and provide for finer control of combustion and primary airflows, fuel flows, and excess oxygen.
Challenges = opportunity
Many plants have installed flue gas desulfurization systems over the past few years, and many have a scrubber in their future. Some utilities are looking toward a future in which fuel flexibility might give them a market advantage; these utilities may have installed an oversized scrubber to enable the burning of higher-sulfur fuels later. On the surface, that logic makes perfect sense to the accountants. The challenge will always be whether the boiler can handle the higher-sulfur fuels that always seem to yield heavy slagging and increased waterwall tube wastage that will chew away at plant reliability. The boiler is the weak link, not the scrubber.
Carefully examine the combustion system improvements made at SEC Unit 2 when considering moving away from your design fuel spec. SEC was able to continue operating Unit 2 with a wider range of fuels because of a series of equipment and process changes implemented over several years. Today, Unit 2 continues to operate an average of just under 8,000 hours per year, as it has since 2000, with an average heat rate of approximately 9,900 Btu/kWh. That’s about 500 Btu/kWh better than the national average heat rate—quite an achievement given that the unit uses substantially poorer fuel than it was designed for. That result was made possible by modifications that now enable the unit to handle a wide range of high-sulfur fuels without degrading the performance of the boiler.
The authors would like to acknowledge Jim Czarniecki and Tony Engelmeyer of the Orlando Utilities Commission for their support and contributions to this project. We would also like to acknowledge the engineering staff of Storm Technologies Inc., especially Stephen Hall and Shawn Cochran.
—Stephen K. Storm (stephen.storm@stormeng.com) is executive vice president of Storm Technologies Inc. and is responsible for technical field service. Jack Lyons (jlyons@ouc.com) is a senior engineer at Stanton Energy Center.
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