Embodiments of a series-coupled fluidized bed reactor unit are provided. In one embodiment, the reactor unit includes primary and secondary reactors. The primary reactor includes a reaction vessel, a gas distributor fluidly coupled to the reaction vessel, and a cyclonic plenum assembly. The cyclonic plenum assembly includes a plenum assembly housing, which is fluidly coupled to the gas distributor and which has an annular sidewall; and a gas/solids inlet pipe, which fluidly couples a partially-reacted gas outlet of the secondary reactor to the plenum assembly housing. The gas/solids inlet pipe is tangentially positioned with respect to the annular sidewall of the plenum assembly housing to induce vortex flow within the plenum assembly housing of the partially-reacted gas received from the secondary fluidized bed reactor through the gas/solids inlet pipe to promote the cyclonic separation of entrained solids from the partially-reacted gas prior to entry into the gas distributor.