Abstract: A heat exchanger 500 for transferring heat between a first gas flow 28, such as flue gases, and a second gas flow 34, such as air or oxygen, includes a housing 514 having a first inlet plenum 520 for receiving the first gas flow 28, a first outlet plenum 522 for discharging the first gas flow 28, a second inlet plenum 526 for receiving the second gas flow 34, and a second outlet plenum 528 for discharging the second gas flow 34. The heat exchanger 500 further includes heat exchange elements 512 disposed within the housing 514. Radial seals 224, 226, 228, 230 are disposed between the housing 514 and the heating elements 512 that define a radial plenum 535, 536. Axial seals 220, 222 are further disposed between the housing 514 and the heating elements 512 to define an axial plenum 530. A third gas flow, such as recirculated flue gas, is provided in the radial plenum 535, 536 and the axial plenum 530 to reduce the leakage between the first gas flow 28 and the second gas flow 34.