Simple-Cycle Atmospheric SOFC Cogeneration System
Desulfurized natural gas is fed directly into an ejector (also known as a jet pump). The ejector creates a partial vacuum that pulls spent fuel from a recirculation plenum near the top of the stack. This captures water vapor needed for the reforming of the natural gas. This fuel mixture passes over a catalyst (the pre-reformer) where any higher hydrocarbons are converted to methane, hydrogen and carbon monoxide. The fuel mixture is then passed through in-stack reformers where the methane is completely converted to hydrogen and carbon monoxide before it reaches the cells. This reformed fuel mixture then enters a fuel manifold at the bottom of the stack where it is distributed to the outside surface of the cells, flowing upwards.
Simplified sketch of a SOFC module: Between each row is placed an in-stack reformer that is radiantly heated by the adjacent rows of cells
Meanwhile, air enters the module, passes through the air manifold and down into air feed tubes. These first pass the air through a recuperator to raise its temperature and then take it to the inside bottom of the cells. Exiting the feed tube the air flows up the inside surface of the cells. With fuel on the outside and oxygen from the air on the inside the electrochemical reaction takes place along the length of the cells, consuming ~85% of the fuel in the process.
The temperature inside the module varies somewhat but is generally kept below a maximum of 1000°C. Depleted fuel then enters the recirculation plenum where a fraction of it is recirculated - to provide the steam needed to reform the fuel - and the balance flows into the oxidation plenum to mix with the excess air. A small amount of combustion takes place here that helps to preheat the air flowing down the feed tubes. The hot gases now make up the exhaust, which leaves the module and then goes into a heat exchanger to transfer its valuable heat to a heating or chilling system.
Flow diagram of a typical CHP SOFC system