NOVELTY - Multiple stage reactor/gas washing apparatus (10) comprises a reactor having a chamber, where a reactant can be mixed with an input fluid to be treated such that sulfur compounds in the input fluid form sulfuric acid, and atoms of the reactant form compounds with hydrogen; and a reversible fuel cell coupled to an output (22) of the reactor to receive an output comprising the hydrogen compounds.    USE - The invention is used for treatment of fluids, particularly for sulfur removal from fluids. It can be used in conjunction with fossil fueled electric power generating plant(s). It is useful in stack gas cleaning process in fossil fueled utility plant.    ADVANTAGE - The invention reduces parasitic loads to the power producing function of the power plant.    DETAILED DESCRIPTION - Multiple stage reactor/gas washing apparatus comprises a reactor having a chamber, where a reactant can be mixed with an input fluid to be treated such that sulfur compounds in the input fluid form sulfuric acid, and atoms of the reactant form compounds with hydrogen; and a reversible fuel cell coupled to an output of the reactor to receive an output comprising the hydrogen compounds, and constructed and arranged to use electricity to selectively split the hydrogen compounds to produce hydrogen and reactant, or to generate electricity by combining hydrogen and reactant atoms to produce hydrogen compounds.    An INDEPENDENT CLAIM is also included for a method of using a multiple stage reactor/gas washing apparatus comprising supplying a halogen reactant to a reaction chamber; supplying an input fluid comprising sulfur compounds; reacting the reactant with the input fluid to form sulfuric acid and HX; supplying HX to a reversible fuel cell; and selectively running the reversible fuel cell in a first direction to produce hydrogen and consume electricity, and in a second direction to consume hydrogen and produce electricity.    X = halogen.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a multiple stage reactor/gas washing apparatus.    Multiple stage reactor/gas washing apparatus (10)    Recirculation loop (20, 28)    Output (22)    Pump (24)