W.A. Parish Electric Generation Station, Thompson, Texas
Owner/operator: Texas Genco Holdings Inc.
Texas Genco has invested heavily in upgrading its W.A. Parish coal- and gas-fired plant southwest of Houston. Although this nine-unit, 3,653-MW plant is the largest fossil-fueled plant in America, its NOx emissions have been reduced to microscopic levels. Based on those levels, W.A. Parish could rightly claim that it is among the cleanest coal plants in the U.S.
Texas Genco's W.A. Parish Electric Generation Station (WAP) is the largest coal- and gas-fired power facility in the U.S. based on total net generating capacity. It and its owner, Texas Genco Holdings Inc., operate in the Electric Reliability Council of Texas (ERCOT), one of the largest electric power markets in the nation. Over the past few years, the majority-owned subsidiary of Houston-based CenterPoint Energy Inc. has met the challenge of adding emissions control equipment to these baseload units while maintaining the availability and reliability required by ERCOT's competitive market.
In the process, Texas Genco has emerged as an industry leader at reducing emissions and demonstrating new NOx-control technologies. The company's fleet of plants operates at one of the lowest NOx emission rates in the country, and WAP likely emits less NOx on a lb/MMBtu basis than any coal-fired plant of any size in the U.S. Cleanliness is costly; the company has spent more than $700 million on new emission controls since 1999.
With the commissioning of another round of emissions-control equipment this year, NOx emissions from Texas Genco's Houston-area power plants—including WAP—will be 88% lower than 1998 levels. These actions play a major role in the Houston/Galveston Area Ozone State Implementation Plan and are helping to clean the air in the greater Houston area. To honor the accomplishment, the W.A. Parish plant was recently given the Facility Award by the Power Industry Division of the Instrumentation, Systems, and Automation Society (Research Triangle Park, N.C.) for installing equipment to reduce emissions and improve reliability while minimizing operational costs.
WAP (Figure 1) is actually two multiple-unit plants located on the same 4,664-acre site. Units 5 through 8, with a total capacity of 2,462 MW, are located in the southwest quadrant of the site; they are drum-type units that burn Powder River Basin (PRB) coal and operate in baseload mode. Units 5 and 6, each rated at 650 MW, entered commercial service in 1977 and 1978, respectively. Units 7 and 8—rated at 560 and 610 MW, respectively—were commissioned in 1980 and 1982. The ninth unit is a 13-MW gas turbine installed in 1967.
1. Big and clean. Texas Genco's nine-unit, 3,653-MW W.A. Parish Electric Generation Station is the largest fossil-fueled plant in the U.S. and one of the lowest NOx producers as well.
Courtesy: Texas Genco
The coal-fired units (Figure 2) are cooled by Smithers Lake, a semi-artificial body of water almost wholly within the site's confines. Its volume is 19,000 acre-feet and its average depth is 7.6 feet. Cooling water enters the plant directly through screen intake structures and is returned to the vicinity after passing around a series of dikes.
2. Lake effect. The plant's four coal-fired units, which have a total capacity of 2,462 MW, are cooled by water from Smithers Lake (rear). Whereas the condensers of Units 5 and 6 are directly cooled, both Units 7 and 8 have a cooling tower on their condenser-cooling loops.
Courtesy: Texas Genco
Whereas Units 5 and 6 use the lake water to directly cool their condensers, both Units 7 and 8 have a 16-fan cooling tower on their condenser-cooling loops. Fresh water from four deep wells is used for station service; it is treated to serve as boiler makeup water and as makeup for the auxiliary cooling systems of Units 5 through 8. Baghouses on all four units limit emissions of particulates; Unit 8 also has a flue gas desulfurization (FGD) system to remove SO2.
Units 1 through 4, on the west side of the site, are a combination of intermediate- and cycling-service gas-fired units with a total capacity of 1,191 MW. Units 1 and 2, each rated at 174 MW, are drum-type reheat units that entered commercial service in 1958. In 1990 and 1991, they were upgraded to enable them to be cycled daily, improving their ability to respond to market conditions. Unit 3 is a 278-MW drum-type reheat unit that was commissioned in 1961. Unit 4, a 552-MW supercritical unit, entered commercial service in 1968. Like Units 5 and 6, Units 1 through 4 use water from Smithers Lake to cool their condensers.
The Parish plant's 360 employees, who average 20 years of experience in the power industry, have overcome many challenges through their skill, hard work, and ingenuity. Among their most significant accomplishments have been the environmental improvements detailed in the remainder of this article.
NOx modifications at WAP began with the gas-fired units. Low-NOx burners were retrofitted to Unit 2, and induced flue gas recirculation systems were installed on Units 3 and 4. Next up were the four coal-fired units, each of which was equipped with a selective catalytic reduction (SCR) system (Figure 3). Unlike many plants that operate their SCR systems only during the ozone season, the Parish plant must operate its systems year-round to comply with its incredibly low NOx emission target of 0.03 lb/MMBtu at all times, starting this year. Texas Genco's approach to balancing the business and technical requirements of the project was to minimize the production of NOx by the combustion process, thus minimizing the size and cost of the SCR systems.
3. NOx killer. Unit 5 was retrofitted with a selective catalytic reduction system in April 2003.
Courtesy: Texas Genco
Major modifications to the coal-fired units began in 1999. Units 5 and 6—which are powered by opposed-wall-fired boilers from Babcock & Wilcox Co. (Barberton, Ohio)—were retrofitted with 56 state-of-the-art, low-NOx DRB-4Z burners from Babcock & Wilcox. Forty of these new burners are dual-fuel burners, enabling the units to reach full load on gas if necessary. The new burners and 12 overfire air ports reduced the coal-fired units' NOx emissions from 0.35 lb/MMBtu to 0.16 lb/MMBtu.
The final modifications were to Units 7 and 8, which are powered by tangentially fired boilers from Combustion Engineering, now a subsidiary of Zurich-based ABB. They were retrofitted with TFS 2000 firing systems from Paris-based Alstom. The systems—which consist of new burners, close-coupled overfire air ports, and two rows of separated overfire air—reduced the NOx emissions of Units 7 and 8 from 0.40 lb/MMBtu to 0.15 lb/MMBtu.
Lowest-NOx coal plant in America
As part of the project, Texas Genco hired Chicago-based Sargent & Lundy to design the SCR systems to operate year-round in two-year cycles. That was a daunting task, given that the systems work in a high-dust environment (upstream of the baghouses) and must maintain their inlet temperature above the minimum reaction temperature at all times. Sargent & Lundy also gave the SCR systems full bypass capability, to help protect their catalyst during start-ups and shutdowns and in the event of an economizer tube leak.
As for the catalysts themselves, after evaluation of competitive bids and extensive testing of various types using slipstream test reactors, Texas Genco chose the Houston office of Denmark's Haldor Topsoe A/S as the supplier. The SCR systems serving Units 5 and 6 were designed to use four layers of the company's DNX 484, with three installed initially and the fourth to be installed after the first two years of operation. The systems serving Units 7 and 8 were designed to use three layers of Haldor Topsoe's new, higher activity catalyst—DNX 684—with two installed initially and the third to be installed after two years. Plans are to add DNX 684 to the SCR systems on Units 5 and 6 when their catalysts need replenishing.
The ammonia system supporting the SCR systems uses aqueous ammonia with two vaporizer vessels per unit. Aqueous ammonia was selected for its safety advantage over anhydrous ammonia. There are four dilution air fans per unit, with two running lead and two running backup at all times. The dilution air fans use heated, secondary air to vaporize the ammonia and transport it to the ammonia injection grids (AIGs). One static mixer upstream and two downstream of the AIGs ensure adequate mixing of the ammonia and flue gas prior to its passage through the catalyst layers.
The SCR systems for Units 5 through 8 went into service in April 2003, January 2003, April 2004, and December 2003, respectively. Since the retrofits, the four coal-fired units have been able to maintain NOx emission levels of 0.03 lb/MMBtu.
Controls upgraded to boost flexibility
Because efficient combustion generates less pollution, Texas Genco also has significantly upgraded the controls of the boilers at the W.A. Parish plant over the past few years. For example, Unit 4—a 552-MW supercritical Combustion Engineering gas-fired unit—was upgraded in 2001 to help it compete in the demanding ERCOT market. A Siemens AG PROFI (FlexPlant) package was purchased and interfaced to the unit's existing ABB Bailey Infi-90 controls to help improve its operating flexibility. To make the connection, all process I/O lines from the Infi-90 system were wired into the Siemens system's programmable logic controller. The PROFI system monitors the thermal stress of various boiler components and predictively calculates the optimal process conditions at the current operating point, including the setpoints for optimal control of the unit.
Installation of the FlexPlant Control modules improved Unit 4's operations immediately and significantly. For example, its load ramp rate improved from 12 to 30 MW/min, and its low sustainable load plummeted from 125 MW to 30 MW. What's more, start-up fuel costs were cut by 70% while the duration of start-ups fell by over 50%. Notably, these improvements were in addition to reduced thermal stresses on Unit 4's turbine. The success of the upgrade to Unit 4 led Texas Genco to install the same system on Unit 2, and there are plans to duplicate the project on Unit 1.
Another project to upgrade boiler controls at the Parish plant took place in 2000 and 2001. A Symphony distributed control system from ABB Bailey based on the Conductor VMS operating platform was installed on each of the four coal-fired units. Each system controls its unit's boiler, main turbine, boiler feed pumps, burners, and SCR system. As it displays readings on eight CRTs (Figure 5), the system automatically operates all control loops and fine-tunes the boiler across its load range. Among the Symphony system's other features are full controller redundancy and engineering tools for trending, troubleshooting, and the building of logic and graphics. The four systems have given operators of the Parish plant much more information on the condition of their units even as they have improved their responsiveness, ramp rates, and reliability.
5. Plant nervous system. This is the ABB Bailey Symphony distributed control system (DCS) for Unit 5. The same system is now running on Units 6, 7, and 8. Plant operators programmed all the DCS graphics.
Courtesy: Texas Genco
At Parish, the distributed control systems are interfaced to a plant data acquisition system and a plant data historian, both from Honeywell Industrial & Control (Phoenix, Ariz.). Plant engineers use the data to continuously monitor both the performance of the units as a whole and the performance of individual components that may be of concern.
Train for success
In addition to the new control systems, Texas Genco also purchased and installed a full-size simulator capable of replicating the operation of any of the four coal-fired units at W.A. Parish (Figure 6). This simulator is used both to train new control room operators and to give experienced operators practice coping with challenging operating scenarios. Among the tasks that can be simulated are handling a boiler feed pump trip at full load, the routine shutdown or start-up of a unit, and putting an SCR system into service. The simulator also has proven very valuable for testing logic changes prior to downloading them into the unit's control system.
6. Learning the ropes. Before a new hire takes the controls of any of the plant's coal-fired units, he must respond to challenging scenarios generated by a simulator. The simulator also is used to test logic changes before they are downloaded into the unit's control system.
Courtesy: Texas Genco
Finally, Texas Genco and the staff of WAP have been installing and operating intelligent soot-blowing systems on the four coal-fired units in collaboration with EPRI (Palo Alto, Calif.). Now in service on Units 7 and 8 are systems that use heat-flux sensors and thermocouples placed throughout the boiler to measure slagging. The instruments are tied into a computer running software from Applied Synergistics Inc. (ASI, Lynchburg, Va.) that controls the water cannons, water lances, retracts, steam wall blowers, and sonic horns used to clean the boilers. Units 5 and 6 already have the majority of the system installed, and plans are to install water cannons from Diamond Power Specialty Co. (Lancaster, Ohio) and the ASI software within a year. Since the intelligent soot-blowing systems were installed on Units 7 and 8, inspections reveal that quench cracking has pretty much stopped.
WAP also has a top-notch emergency response team. Its 51 members, representing all levels of the plant, are trained in medical, fire, high-angle rescue, and emergency incident command and in confined-space rescue. Membership is divided evenly among crews to ensure rapid response 24/7. The group has been recognized for its expertise by the Emergency Services Training Institute at Texas A&M University as well as by local emergency response agencies.