An emission treatment system for removing NO.sub.x from flue gas includes a diversion member that closes the stack at a position intermediate the inlet and outlet ends. A major component module includes a first sub-module, having an inlet and an SCR segment, a second sub-module, having a heat exchange segment, and a third sub-module, having an ID fan and an outlet, forming a flue gas flow path extending from the inlet to the outlet. Inlet ductwork, which is in fluid communication with the stack at a position intermediate the inlet end of the stack and the diversion member, provides a passageway from the stack to the inlet. Outlet ductwork, which is in fluid communication with the stack at a position intermediate the diversion member and the outlet end of the stack, provides a passageway from the outlet to the stack. An ammonia addition subsystem injects ammonia vapor into the inlet ductwork.