NOVELTY - A primary reactor in a fuel processing system is operated by selectively injecting a water stream into a superheated steam stream to form a steam stream mixture. The water stream is selectively injected, based on an operating state of the primary reactor.    USE - Operation of a primary reactor in a fuel processing system to produce a hydrogen-containing reformate from a hydrocarbon fuel (claimed), useful for fuel cells producing electric power for use in electrical vehicular power plants and vehicular applications.    ADVANTAGE - The process provides control of the primary reactor temperature, improved overall production of reformate gas from the primary reactor, and increased steam volume in the fuel processor during transient operation. It provides for the ability to maintain a constant steam to carbon ratio, and to reduce the occurrences of carbon carryover/buildup in the primary reactor and to the downstream components of the fuel processor.    DETAILED DESCRIPTION - Operation of a primary reactor in a fuel processing system comprises:    (a) generating a superheated steam stream;    (b) selectively injecting a water stream into the superheated steam stream to form a steam stream mixture;    (c) introducing the steam stream mixture to an inlet of a primary reactor;    (d) providing a hydrocarbon fuel stream to the inlet of the primary reactor;    (e) connecting an air stream to the inlet of the primary reactor; and    (f) dissociating the steam stream, fuel stream and air stream in the primary reactor to form a hydrogen-containing reformate, where the water stream is selectively injected into the superheated steam stream based on an operating state of the primary reactor.    An INDEPENDENT CLAIM is also included for a fuel processing system for reforming a hydrocarbon fuel, comprising:    (i) a primary reactor having a reactor inlet and a reactor outlet;    (ii) a superheater including a first portion having a first inlet, a second inlet, and a first outlet in fluid communication with the inlet of the primary reactor, and a second portion having a third inlet in fluid communication with the reactor outlet;    (iii) a source of pressurized air in fluid communication with the second inlet, the superheater generating a superheated steam and air stream at the first outlet; and    (iv) a water supply system including a source of water in fluid communication with the first inlet, an injector in fluid communication with the first outlet, and a controller coupled to the injector for selectively injecting water into the superheated steam and air stream, based on an operating state of the primary reactor.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram of the fuel processor system incorporating the method of liquid water injection.