SCR Performance Enhancements for NO_X Compliance and Reduced Load Operation

摘要

The operating envelope for a coal-fired selective catalytic reduction (SCR) system is constrained by the presence of sulfur trioxide (SO_3) in the flue gas. SO_3 produced in the boiler limits the minimum operating temperature of the SCR reactor, which affects the ability to inject ammonia and control NO_X emissions during reduced-load operation. In addition, SO_3 produced in the boiler and by the SCR catalyst itself forces operation with low ammonia slip to avoid air heater fouling. Consequently, SCR reactors must be operated at a higher average reactor potential, which increases catalyst consumption and further elevates SO_3 production. As a result of the Cross-State Air Pollution Rule (CSAPR) and the Ozone National Ambient Air Quality Standard (NAAQS), utilities are faced with the need to achieve further reductions in annual and ozone-season NO_X emissions, which will require reductions in their NO_X emission rates during higher-load operation and the ability to maintain the SCR system in operation during reduced-load operation. SO_3 removal upstream of the air heater, and ideally upstream of the SCR reactor itself, is becoming an increasingly important tool for meeting these challenges. Several examples are provided here illustrating how utilities are evaluating the concept of reducing NO_X emissions by removing SO_3 upstream of the air heater, and then operating their SCR system at slightly elevated levels of ammonia slip. Duke Energy has used their "pre-SCR" SO_3 mitigation systems at the Gibson station to allow ammonia injection during reduced-load operation for over seven years. Duke is performing extensive testing in the summer of 2016 to extend this capability to allow even lower-load operation at Gibson and at other Duke plants. An overview of the testing at Gibson is provided here.