Purge Considerations for Seasonal Selective Catalytic Reduction (SCR) Systems

摘要

Selective Catalytic Reduction (SCR) systems are being retrofitted to the flue gas paths of pulverized coal-fired boilers as post-combustion, seasonal means of reducing the amount of NOx emitted from the boiler exhaust stacks. Due to the seasonal nature of SCR use, a means is also included as a part of the process to allow the flue gas to bypass the SCR catalyst when the flue gas is not at the appropriate temperature or when operation of the SCR is not required. This bypassing of the SCR extends the life of the SCR catalyst and may also allow some SCR maintenance to be performed with a fire in the boiler. There are mixed interpretations as to what the National Fire Protection Association (NFPA) boiler hazards code requires, or what the intent of NFPA 85 is, in regards to purging of the SCR box before tie-in to the flue gas stream. This paper will discuss the issues associated with SCR box purging and the author's opinion as to a procedure to be followed in this regard. It is hoped that this paper will cause others to investigate their seasonal SCR installations to determine the appropriateness of their operational procedures.