Labadie Power Plant, St. Louis, MissouriJune 18, 2008 at 2:12 am | Posted in Power Plant | 1 Comment
Owned by AmerenUE, St. Louis, Mo.
Last November, for the fourth year in a row, Units 1 through 4 of the Labadie Power Plant took the top four places on the EPA’s list of U.S. plants with the lowest NOx emissions. Each unit produces just 0.11 lb/mmBtu of the pollutant. In fact, the top 19 pulverized coal-fired boilers on the list all burn PRB coal and use Alstom low-NOx burners without selective catalytic reduction. Labadie’s microscopic NOx figures didn’t come overnight; they took several years of hard work, beginning with upgrades of everything from mills to the low-NOx burners. The last dash to the finish line was the addition of a Pegasus neural net combustion control system.
By David Fox, Plant Manager, and Mike Fiala, Outage Supervisor
AmerenUE’s Labadie plant and Alstom Power have joined forces in a long-term commitment to plantwide improvements that has produced remarkable results in the facility’s four 600-MW coal-fired boilers (Figure 1). The transition is remarkable because the two companies, working together, have:
1. Labadie Power Plant.
Labadie Power Plant made the switch to PRB coal in 1995 to reduce its SOx emissions. Since then, the plant has increased its availability to 90% and extended the time between major overhaul outages from one to four years. The plant was the PRB Coal User’s Group 2002 Large Plant of the Year.
Increased maximum individual unit capacity by 10% (from 580 MW to 630 MW).
- Increased total production by 35% (from 12.2 MWh to 16.7 MWh).
- Increased availability by 15% (from 75% to 90%).
- Increased efficiency while lowering NOx to 20% of previous levels and SOx to 10% of previous levels.
- Extended the time between major scheduled outages from one to three years.
All of this was accomplished while the units were converted from a high-Btu bituminous coal to a low-Btu subbituminous Powder River Basin (PRB) coal with 20% less heating value. As a result, the four Labadie units are now the lowest NOx- producing coal-fired units in the country—without using selective catalytic reduction—and the facility was named the PRB Coal User’s Group 2002 Large Plant of the Year (Table 1).
Labadie Power Plant’s operating statistics
Labadie was converted to 100% PRB coal in the late 1990s to cut its SO2 emissions. Such conversions often lead to operation, maintenance, and safety problems. But the 2,400-MW station has overcome those challenges and for the past five years has set production records—each year surpassing the previous year’s performance. The station also installed low-NOx burners, an overfire air system, and advanced computer controls to dramatically cut its NOx emissions.
Plant upgrading is a long-term process requiring a long-term commitment from the plant owner. At AmerenUE’s Labadie Plant, located just outside St. Louis, all four tangentially fired units were included in a long-term commitment to plantwide improvement and environmental stewardship made in the early 1990s.
Labadie’s tangentially fired, controlled-circulation, radiant reheat boilers were originally supplied between 1970 and 1973 by Combustion Engineering (now part of Alstom Power Inc.). All four units are essentially identical. Each has a boiler that supplies steam at a maximum flow of 4,080,000 lbs/hr at 2,400 psig and 1,005F/1,005F to a 600-MW steam turbine. The original combustion controls were Combustion Engineering CFS burners; in 1990 offset air produced about 0.620 lb of NOx/mmBtu.
To take full advantage of the switch to PRB coals and to reduce NOx emissions, each unit’s firing system was retrofitted with an Alstom LNCFS-III (Low NOx Concentric Firing System, Level 3). This system (Figure 2) includes overfire air, flame front control coal nozzle tips, and concentric firing through offset, secondary air nozzle tips. The plant’s distributed control system controls the auxiliary air, fuel air, and overfire air dampers. Dampers for SOFA, CCOFA, AAS, and Fuel Air are controlled by individual drives.
2. Unit 4, retrofitted with LNCFS-III burners.
Units 1 and 2 began burning blended fuel (70% eastern coal and 30% PRB coal) in 1992. Units 1 and 3 were retrofitted with LNCFS-III burners in 1993; Unit 2 was retrofitted in 1994. Between 1992 and 1994, combustion engineers focused on tuning the boilers to further reduce NOx.
Overfire air (OFA) is the industry’s most established and effective method for decreasing NOx emissions from pulverized coal-burning, tangentially fired steam generators. The use of OFA stages the combustion process by redistributing a portion of the secondary air above the main firing zone to decrease the amount of available oxygen within the main firing zone. Two types of OFA configurations are used in the LNCFS technology: close-coupled overfire air (CCOFA) and separated overfire air (SOFA).
NOx reduction is achieved in the LNCFS-III by delaying or staging the mixing of secondary air. Burners incorporate two forms of staging, horizontal and vertical. Vertical staging is achieved by the use of overfire air. Horizontal staging is accomplished by introducing a portion of the secondary air concentrically around the primary air and fuel. The tuning strategy at Labadie was to minimize the NOx emissions by staging as deeply as possible while maintaining CO emissions at less than 50 ppm.
The LNCFS-III installed at Labadie uses a combination of CCOFA and SOFA, with SOFA introduced into the furnace via assemblies located above each main windbox. A major advantage of the Level 3 system is its flexibility in adjusting the distribution of overfire air between CCOFA and SOFA. This helps to decrease CO emissions, unburned carbon, and the potential for upper furnace slagging. Post-retrofit results included a reduction in NOx emissions from the original design fuel—Illinois coal—from 0.6 lb/mmBtu to 0.3 lb/mmBtu. NOx emissions were further reduced to 0.18 lb/mmBtu on the PRB coal, and reduced further to the current level of 0.11 lb/mmBtu by new operating strategies and tuning (Figure 3).
3. Lowest emissions.
The Labadie plant remains the NOx leader due to more than a decade of intense effort focusing on that one goal. Last November, for the fourth year in a row, Units 1 through 4 took the top four places on the EPA’s ranking of lowest NOx-emitting plants in the U.S. Each unit produces just 0.11 lb/mmBtu of the pollutant. Data for 2002 and Q1 2003 are unofficial.
The additional reduction must be attributed to the efforts of the full-time dedicated tuning team, which worked over the course of several years to optimize unit operation and minimize NOx emissions. The team used many tools, including a neural network system that it swears by. The Pegasus Optimization Suite (Figure 4) remains a critical part of the team’s operational strategy for improving efficiency and reducing emissions and is currently being installed on other AmerenUE coal-fired plants.
4. Pegasus Optimization Suite.
The Pegasus neural net combustion control system optimizes 29 parameters every 15 minutes. Pegasus was installed on Units 1 and 3 in 1998 and on Units 2 and 4 in 2000.
People make the difference
Cumulatively, AmerenUE has spent more than $300 million since the early 1990s to reduce stack emissions from all of its facilities. The investment has paid off in systemwide emissions reductions of 70% for SO2 and 50% for NOx.
Although top-of-the-line equipment played a major role in Labadie’s achievement, Plant Manager David Fox says that maintaining top performance year after year would have been impossible without a dedicated and capable plant staff. Fox is so high on his O&M staff’s contribution to the plant’s success that, once he gets started talking about them and their HWP (hard work pays) philosophy, it’s difficult to get him to change the subject.
Fox is also quick to point out the contributions made to the project by others in the plant. In 1996, Fred Pope, then-manager of power plant design engineering, sent Tom Ziegler and Kevin Kersting to New Castle, Pa., to investigate a system that, through programmed scenarios, could modify the boiler’s flow of air and fuel to maximize generation efficiency. “We wanted to use this surrogate ‘brain’ to get more megawatts out of every ton of coal we burned,” recalls Kersting, an engineer for Consulting Power Plant Maintenance at AmerenUE. “We had low-NOx burners already in place, so that was not our initial concern. Of course, with the more stringent emissions requirements we are facing now, it is.”
Ameren Services’ Consulting Project Engineer Tom Ziegler adds, “We show the system how the boiler operates based on a variety of conditions or scenarios, and it then comes up with tiny modifications that can make a difference. In 1997, Labadie became the laboratory for testing this—it had never been tested on a plant the size of Labadie. And clearly this became the most complex project any of us had ever worked on.”
But did operators embrace the system? “They were incredibly receptive,” says Kersting. “Because we took the time to explain how it works, the operators now go out of their way to apply the lessons they learn from the system to their units. The system really helps the operators, because they must worry about monitoring everything from feedwater pumps to coal belts, and they don’t have time to go tweak a damper in the midst of all that. The system relieves the operator of needing to intensely monitor NOx burners so he can concentrate more on all the other variables.” Ziegler adds that “the systems also allow the operators to mimic its software. So even without it, they can achieve lower NOx emissions.”
It has been a demanding, time-consuming journey. Ziegler recalls that, initially, he was given nine months to get the system installed. It was part of a $35 million Labadie control room computer upgrade managed by AmerenUE’s Fred Pope. “I have to credit Fred and Labadie Plant management (David Fox and the now-retired Mel Sanazaro) for sticking with it and allowing us to create a highly customized system that really has made a difference,” says Ziegler.