A fuel processor for generating hydrogen rich gas or cleaned hydrogen rich gas from hydrocarbon fuel includes an inner housing and an outer housing defining a mantel space between them, wherein at least one fuel reformer unit for reforming hydrocarbon fuel to a hydrogen rich gas and optionally a gas-cleaning unit for cleaning the hydrogen rich gas from unwanted by-products are arranged in the inner housing. The fuel processor further includes a processor inlet for introducing hydrocarbon fuel into the inner housing and a processor outlet for releasing cleaned hydrogen rich gas from the inner housing. The outer housing further includes a fluid inlet for introducing a heat transporting fluid into the mantel space. The inner housing includes at least one opening for providing a fluid-connection between the inner housing and the mantel space. A method for operating such a fuel processor is also provided.