A reformer reactor is provided for converting hydrocarbon fuel into hydrogen rich gas by auto-thermal reaction process having a cylindrically shaped and double walled, housing with two side faces forming a reaction chamber of the reformer. Additionally, a fuel inlet is provided in one of the two side faces for providing hydrocarbon fuels into the reaction chamber, wherein further a fuel preheating means is provided which preheats the hydrocarbon fuel before the hydrocarbon fuel enters the reaction chamber.