A modified air heater gas inlet plenum for a tubular air heater which permits retrofit installation of additional flue gas environmental treatment equipment. A divider plate is located within the inlet plenum to subdivide it into first and second flue gas passages. The first flue gas passage created by the divider plate merely conveys the hot flue gas through the inlet plenum and into flues which convey the flue gas to the new equipment. Return flues convey the flue gas back to the second flue gas passage created in the inlet plenum which, in turn, conveys the flue gas into the tubular air heater heat exchanger tubes. By taking advantage of the large size of a conventional air heater gas inlet plenum, the first and second flue gas passages created by the divider plate still have sufficient cross sectional area so that acceptable flue gas velocities are preserved. One particular application of the modified air heater gas inlet plenum is to permit retrofit application of selective catalytic reduction (SCR) systems for treating flue gas to existing steam generators or boilers.