The Montour Steam Electric Station, located about a mile northeast of Washingtonville, Pa., is a two-unit coal fired plant with a combined output of 1,525 megawatts. Originally built in the 1970s, it has been modernized several times with the most recent major modernization occurring in 2000. Replacement of plant equipment and instrumentation not only improved plant economics, but also helped reduce particulates and gaseous emissions.
The units are Combustion Engineering tangentially fired, supercritical, pulverized coal-fired boilers rated at 745 and 755 MW gross maximum continuous load. In the 1990s the boilers were retrofitted with Low NOx burners that combined concentric firing with close-coupled and separated overfire air.
The SCR project was approved in spring 1999 and the work on Unit 2 and Unit 1 was completed in the spring of 2000 and 2001, respectively. This article is focused on the activities related to Unit 2.
NOx Reduction Chemistry
The catalyst facilitates the reaction of NOx in the flue gas with ammonia. The result is nitrogen plus water. The basic chemical equation governing the removal of NOx is as follows:
2NH3 + NOx -> N2 + 3H20
The SCR design criteria included the following:
• Inlet NOx of 0.4 lbs/MMBtu
• 2 ppm ammonia slip (excess ammonia in flue gas) at 90% NOx reduction (at 7,600 hours)
• 12 month per year operation, initial operation is limited to five-month ozone season, May through September
• Two years between catalyst change-out
Major SCR Equipment Installed
Each boiler has two reactors. The reactors are essentially a mirrored set of gas-tight ductwork and support steel needed to house the catalyst modules and to provide the required flue gas mixing. Each boiler reactor is 72 feet wide by 41 feet deep by 66 feet high. They are located in the flue gas path between the boiler economizers and the air heaters. The precipitators are located further downstream. Temperature in the reactors varies from ambient to 750°F, depending on whether the SCR is in service or bypassed. To avoid damaging condensation, temperatures are maintained above 150 degrees when possible. Two catalyst layers are presently installed, and plans are underway for the installation of two additional layers. This provides for a staged, catalyst replacement program.
SCR Bypass Dampers
Each reactor has two very large bypass damper assemblies consisting of an inlet blade and an outlet blade. The hydraulically controlled blades measuring 34 feet long by 12 feet high direct flue gas into the SCR reactors or divert flue gas around the SCR for a "bypass mode." The SCR is operated with the dampers in the non-bypassed position, with 100 percent of the flue gas flowing through the SCR. Even though the full speed stroke time of the dampers is about one minute, during the warm up sequence it takes about five hours to fully close the bypass to satisfy catalyst and reactor steel temperature rate of change limitations. Damper positioning is based on the temperature ramp. All four dampers must travel in unison with less than 10 percent position deviation from each other to minimize imbalances to the flue gas flow. One of the priorities for the Metso Automation control system was secure, reliable damper control execution.
Ammonia Injection Dilution Air
A Dilution Air System was installed consisting of two steam-to-air heat exchangers and associated valves and piping. The Dilution Air System uses Forced Draft fan discharge air. The dilution air is used to convey the ammonia to the SCR for injection into the flue gas. The temperature and flow of the dilution air is controlled by the Metso DCS. Four in-situ flue gas NOx analyzers measure SCR inlet and outlet NOx on each reactor.
Ammonia Processing Plant
Anhydrous ammonia is injected into the flue gas to perform the NOx reduction. The Ammonia Processing Plant is common to both Units 1 and 2 and consists of two 60,000-gallon storage tanks and associated pneumatic valve system. Ammonia handling includes semi-automatic truck and rail car unloading, SCR feed pump operation, SCR ammonia injection flow control, and various safety monitoring and alarm functions, including leak detection.