A duct burner assembly for a HRSG having a casing that defines a combustion chamber with a liner for communicating an exhaust gas. A firing runner attaches to the liner and extends through the combustion chamber. The firing runner defines a plurality of orifices for emitting combustible gas and sustaining a flame. A flame stabilizer attaches to the firing runner and is configured to at least partially shield the plurality of orifices from the exhaust gas. A guide plate attaches to the firing runner and is configured to define a slot between the liner and the guide plate. The guide plate has an upstream end and a downstream end wherein the downstream end is closer to the lining than the upstream end to reduce turbulent flow of the exhaust gas through the slot and cool the liner.