Described herein is a fuel processor that produces hydrogen from a fuel source. The fuel processor comprises a reformer and burner. The reformer includes a catalyst that facilitates the production of hydrogen from the fuel source. Voluminous reformer chamber designs are provided that increase the amount of catalyst that can be used in a reformer and increase hydrogen output for a given fuel processor size. The burner provides heat to the reformer. One or more burners may be configured to surround a reformer on multiple sides to increase thermal transfer to the reformer. Dewars are also described that increase thermal management of a fuel processor and increase burner efficiency. A dewar includes one or more dewar chambers that receive inlet air before a burner receives the air. The dewar is arranged such that air passing through the dewar chamber intercepts heat generated in the burner before the heat escapes the fuel processor.