In one embodiment, a fuel reformer can comprise: a mixing zone capable of mixing a fuel and an oxidant to form a fuel mixture and a reforming zone disposed downstream of the mixing zone. The reforming zone comprises a primary substrate and a secondary substrate. The primary substrate is disposed upstream of the secondary substrate and has a primary thermal mass that is greater than a secondary thermal mass of the secondary substrate. One embodiment of a method for operating a fuel reformer can comprise: mixing an oxidant and a fuel to form a fuel mixture, combusting the fuel mixture, heating the secondary substrate above its light-off temperature, changing an air to fuel ratio of the fuel mixture to a reforming mixture, producing an exotherm and a reformate at the secondary substrate, heating a primary substrate with the exotherm to above its light-off temperature, and producing a reformate.