The Clean Air Interstate Rule (CAIR) is pushing utilities in 28 eastern states and the District of Columbia to retrofit many of their coal-fired power plants with selective catalytic reduction (SCR) systems to minimize NO
x, and with flue gas desulfurization (FGD) systems (scrubbers) to do the same for SO
2. The U.S. Environmental Protection Agency estimates that by 2015, when the CAIR regulations have been fully implemented, SO
2 emissions will have been cut to 2.5 million tons a year, 73% below 2003 levels. By then, NO
x will have been reduced by a similar amount—61% from 2003 levels. Because CAIR's NO
x and SO
2 caps will begin to be enforced in 2009 and 2010, there's currently a boom in construction of air quality control systems (AQCS).
Project overview
The environmental improvement project at Pleasant Prairie Power Plant (P4) was undertaken as part of We Energies' plan to meet current and pending air quality improvement commitments and environmental regulations. The plant exemplifies the challenges of retrofitting complex SCR and FGD systems to a working power plant. Cost is one of those challenges. AQCS upgrades often cost many times the original capital cost of the plant due to competition for limited vendor shop capacity, shortages of qualified field labor, and rising material costs.
In many ways, P4 (Figure 1) is representative of the challenges facing utilities seeking the least-cost path to CAIR compliance. Its two 617-MW units were designed and built in the early and mid-1980s to burn pulverized Powder River Basin coal. Unit 2 was retrofitted with a hot-side SCR system in 2003. The current project added a hot-side SCR system to Unit 1 and a wet-limestone, forced-oxidation FGD system to both units. The configuration of the plant required installation of new booster fans and ductwork between the existing induced-draft (ID) fans and the new absorber vessels. A new, dual-flue stack (to serve both units) made of concrete and fiberglass-reinforced plastic (FRP) had to be added. The AQCS upgrades are capable of reducing P4's NOx emissions by 90% and removing 95% of the SO2 from a design coal with a sulfur content of 0.5%.
1. CAIR package. We Energies' twin 617-MW units at Pleasant Prairie Power Plant received a major air quality facelift. Unit 1 was retrofitted with an SCR system and a wet scrubber. Unit 2, which received an SCR system in 2003, received a new scrubber. Both units also share a new stack (shown at left). Courtesy: Washington Group International
All of the FGD system equipment (for absorption, reagent preparation, and gypsum dewatering) was supplied by Wheelabrator Air Pollution Control (www.wapc.com). The SCR system was provided by Riley Power Inc. (www.babcockpower.com). Overall project coordination and management were handled by Washington Group International (www.wgint.com). Washington Group's scope included balance-of-plant engineering, procurement, construction, and commissioning. Emerson Process Management (www.emersonprocess.com) supplied the new systems' distributed processing units, operator and engineering workstations, and network periphery gear, and integrated those elements into existing plant controls. Last but not least, Pullman Power (www.structural.net) built the new stack under a turnkey contract.
Unit 1's SCR and FGD systems were placed into service in November 2006. The commissioning of Unit 2's scrubber followed five months later, in April 2007. Exorcising the inevitable gremlins before those dates was challenging but necessary. Because both P4 units operate in baseload mode, the three new actors and their supporting casts need to be at least 98% available.
The SCR system for Unit 1 (Figure 2) is designed to reduce NOx emissions by 90% (a 30-day rolling average) when burning
the design fuel. That removal rate assumes a NO
x concentration of 0.5 lb/mmBtu in the flue gas entering the system. The hot-side system consists of two reactor modules, each with two initial layers of catalyst and provisions for two more in the future. The catalyst type is Ti-V-W honeycomb, supplied by Cormetech Inc. (
www.cormetech.com). Because P4's air permit requires the SCR system to operate continuously, a reactor bypass is not included.
2. Clearing the air. The SCR system for Unit 1 under construction. The hot-side system is designed to remove 90% of NOx emissions, using aqueous ammonia injection. Courtesy: Washington Group International
NOx reduction in an SCR is achieved by mixing flue gas with ammonia in the presence of a catalyst. In the P4 SCRs, aqueous ammonia is injected at eight locations per reactor module. Effective mixing is ensured through the use of a Delta Wing static mixer supplied to Babcock Power by the Balcke-Durr division of SPX Corp. Both sonic horns and steam sootblowers are used to clean the SCR catalyst of ash buildup. This enhances the catalyst's performance, reducing ammonia consumption and ammonia slip while still achieving high rates of NOx reduction.
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