By Brian Wheeler, Associate Editor
One of the nation’s first 100 percent lignite-fired plants with selective catalytic reduction (SCR) for nitrogen oxide (NOx) reduction and activated carbon sorbent injection for mercury control is now being operated by Texas-based Luminant. The Oak Grove Power plant, some 100 miles northwest of Houston, includes two supercritical lignite-fueled power generation units that deliver 1,600 MW.
Fluor, the engineering, procurement and construction (EPC) company at Oak Grove, completed the lignite coal-fired plant in 35 months. But the Oak Grove story begins in the late 1970s when Luminant (then known as TXU) had plans for two separate power generation facilities. The current Oak Grove site was planned as a two-unit site and the other project location as a one-unit site. Over the years the two projects faced setbacks due to a reduction in load demands and TXU scaled back construction on both sites, even though some large components had already been purchased. Design for the boiler buildings and steam turbine table top foundations were completed and steel was partially erected on Unit 1. As a result, when Luminant and Fluor re-initiated work in 2007 after receiving new air permits from the state of Texas, they had a running start. However, the two project sites were combined into a single project with some major equipment from each site composing the new two-unit configuration.
After re-certifying and re-analyzing 30-year-old steel and concrete designs, Fluor engineers brought the structures up to code and confirmed that the foundations met current-day requirements for static, vibration and dynamic analysis. Adding to the difficulty, engineers had to work within the site boundary laid out in the 1970s. And in some cases, equipment purchased for the earlier plants had to be upgraded.
“Technology advancements, code changes, more stringent emissions requirements and different suppliers for some of the major equipment; it was quite a task to meet all the elements of this unique and challenging project,” said Jim Mackey, vice president of Solid Fuels for Fluor.
One example was upgrading the controls on the General Electric steam turbine three generations from Mark II to Mark VI technology. In addition to advancing the equipment controls, the plant-wide control system architecture also incorporates some of the most advanced broad use of digital control communications.
“The plant has a significant amount of digital bus controls used where previously plant communications would have primarily been hard-wired signals,” said James Brown, director of engineering for Fluor Power.
Using digital bus communication technology not only led to advanced technology in data communication, but also to reductions in the amount of copper used for wiring. That, in turn, led to labor savings, which was critical as many skilled laborers were drawn to the Gulf Coast for recovery work following hurricanes Katrina and Rita.
With a peak total of 2,400 workers on site, Fluor completed the work with nearly 11 million safe man-hours on-site and with less than one-half a percent in change orders.
And on Dec. 23, 2009, Fluor handed over care, custody and control to Luminant for Unit 1 followed by Unit 2 on June 2, 2010
The two Oak Grove units use different steam generator boiler technologies, not surprising since this equipment for each unit was originally purchased decades ago for two separate projects. Unit 1 contains an Alstom tangential-fired supercritical steam generator with a GE single reheat steam turbine. The second unit is a Babcock and Wilcox wall-fired supercritical steam generator. Unit 2 also had a GE single reheat steam turbine. As part of the plant’s NOx control, both original equipment steam generators were upgraded with low NOx burners and over-fire air. Both units are equipped with SCR for additional NOx control, fabric filter baghouse for particulate control, wet flue gas desulfurization system for sulfur dioxide reduction and activated carbon sorbent injection for mercury control.
The plant burns lignite, which is bountiful in Texas. With the latest technology in air quality control systems, Luminant expects the completed units to have lower emission rates than any other lignite-fired plant in the state and at least 70 percent lower than the national coal plant average.
“We have the proven technology that lignite coal can be burned cleanly,” said Cliff Watson, Oak Grove’s generation director.
Oak Grove’s lignite comes from the neighboring Luminant-owned Kosse Mine. Working 24 hours a day and 7 days a week, the mine’s 300 employees help move 30,000 tons of lignite per day by train to the Oak Grove plant. Luminant anticipates the mine will produce roughly 9.5 million tons of lignite annually.
The main steam conditions on the 2×800 MW net lignite-fired units are 3,500 psig with 1,000 F on the main steam and 1,000 F on the reheat. To cool the plant, Oak Grove uses a reservoir (that also dates from the 1970s) and once-through cooling technology.
With day-to-day operations in Luminant’s control now, Watson said the plant is progressing well and has had very good run days. Unit 1 is currently in commercial operation under Electric Reliability Council of Texas (ERCOT) control. Unit 2 is still going through the final on-line testing phase, which when completed, will make the unit available for reliability dispatch.
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