Emission treatment system

An emission treatment system for removing NOx from flue gas includes a diversion member 34 that closes the stack 22 at a position intermediate the inlet and outlet ends. A major component module 26 includes a first sub-module, having an inlet and an SCR segment 28, a second sub-module, having a heat exchange segment 30, and a third sub-module, having an ID fan 32 and an outlet, forming a flue gas flow path extending from the inlet to the outlet. Inlet ductwork 40, which is in fluid communication with the stack 22 at a position intermediate the inlet end of the stack 22 and the diversion member 34, provides a passageway from the stack 22 to the inlet. Outlet ductwork 44, which is in fluid communication with the stack 22 at a position intermediate the diversion member 34 and the outlet end of the stack 22, provides a passageway from the outlet to the stack 22. An ammonia addition subsystem 54 injects ammonia vapor into the inlet ductwork 40.