Cool SCRs Come to America Next Generation SCR Choices - High-Dust, Low-Dust and Tail-End

摘要

The great majority SCR installations in the US power plants are of the High dust (high temperature) type, treating the flue gas exiting the economizer. European experience includes two more variations of SCRs: Low-Dust version that treats flue gas after the particulate collection device, and Tail-End version, which treats the saturated flue gas exiting a wet FGD absorber. Retrofitting coal units with high dust SCRs is getting increasingly problematic; the easy installations have been already retrofitted. Going forward, utilities are seriously considering low dust and tail-end locations when the high-dust alternative is considered impractical. The most significant aspect of low dust and tail-end SCRs is the requirement for reheating the flue gas to 550- 650°F, then recuperating the heat energy by cooling the gas back to about 250-350°F. This type of heat exchanger is termed Gas-to-Gas Heater, GGH in short. The most common heat exchangers used are the regenerative (Ljungstrom) type; although zero leakage tubular types may be considered, depending mostly on capital cost. Both heating alternatives require additional heat input, via steam coil or fired natural gas, to provide the differential temperature rise from the GGH exit to the needed temperature for effective catalyst operation. Tail end installations have certain advantages regarding the catalyst type: less volume, longer life span and lower cost, as compared to both high and low dust. On the other hand, flue gas reheat energy for the Tail-End SCR normally present a significant penalty, which is higher than the Low-Dust heating requirement. In comparison, High-Dust SCR configuration requires no supplemental heating at high loads. This paper investigates power plant conditions that would exclude high dust SCRs and favor Low-Dust or Tail-End, describes the design characteristics of both types and summarizes European experience. In addition, the paper presents an economic evaluation of the Low-Dust and Tail-End SCR installations.