Fuel processing and fuel cell systems with feedstock delivery systems that are designed to deliver a mixed component feed stream to a hydrogen-producing region for the production of hydrogen gas therefrom and to selectively deliver the feed stream to a heating assembly for use as a combustible fuel stream for heating at least the hydrogen-producing region. The feed stream contains water and a carbon-containing feedstock, and may contain at least 31 vol % water. In some embodiments, the feedstock delivery system may be adapted to mix the components of the feed stream at a determined mix ratio and to deliver this feed stream to the fuel processor(s). The fuel processing system may also include one or more fuel cell stacks that are adapted to produce an electric current from the product hydrogen stream produced by the fuel processing system.