NOVELTY - A fuel cell system comprises a reformer to produce a hydrogen-containing gas for use in a fuel cell stack; and an emulsion composition being fed to the reformer at start-up. The composition comprises at least40 wt.% hydrocarbon, 30-60 wt.% water and 0.01-5 wt.% surfactant. The surfactant is alkyl sorbitan ester and/or alkoxylated alkyl sorbitan ester.    USE - As a fuel cell system.    ADVANTAGE - Using an emulsion at start-up achieves a well-mixed water/fuel injection. This improves the efficiency of start-up of the reformer. The fuel-water mixture can be sprayed into the reformer. The composition has the ability to form emulsions at low shear and the ability of the surfactants to decompose at temperatures below 700degreesC. The viscosity of the emulsion is such that it is easily pumpable. The emulsion viscosity decreases with decreasing temperature. The emulsion is stable to freeze thaw cycles at -54 to +50degreesC. The emulsion composition also exhibits detergency and anti-corrosion function to keep clean and clean up of metal surfaces comprising Group IIIA-IIB elements, e.g. catalyst surface or internal surface of fuel cell system.    DETAILED DESCRIPTION - A fuel cell system comprises a reformer to produce a hydrogen-containing gas for use in a fuel cell stack; and an emulsion composition being fed to the reformer at start-up. The composition comprises at least40 wt.% hydrocarbon, 30-60 wt.% water and 0.01-5 wt.% surfactant. The surfactant is alkyl sorbitan ester of formula (I) and/or alkoxylated alkyl sorbitan ester of formula (II).    R = 5-18C alkyl;    R' = H or (M-O)n-H;    n = 2-20;    M = CH2-CH2, CH2-CH2-CH2, CH2CH-CH3, CH2-CH2-CH2-CH2, and/or CH2-CH-(CH3)-CH2.    The total number of (M-O)n-H groups per molecule is 2-60.    INDEPENDENT CLAIMS are also included for:    (a) a method to prepare a bi-continuous emulsion by mixing at an energy of 0.15x10-5 - 0.15x10-3 kW/liter of fluid; and    (b) a method for preventing corrosion of a metal surface by contacting the metal surface with the emulsion for 1 second to 3 hours at -20 to 100degreesC.    (c) a bi-continuous emulsion.    DESCRIPTION OF DRAWING(S) - The figure shows a diagram of a fuel cell system wherein a start-up system is operably connected to a reformer.