Operation Principle

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.

Meanwhile, after preheating in a recuperator, air enters the module and passes through the air manifold into air feed tubes that take it to the bottom of the cells on the inside. 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 at a maximum of 1000° C. Depleted fuel then enters the recirculation plenum where a fraction of it is recirculated and the balance flows into the combustion 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 passes into the recuperator, and then into another heat exchanger to make us of the valuable heat.

The Tube

Because they rely on electrochemical reactions instead of combustion fuel cells need hydrogen, but some like solid oxide fuel cells (SOFC) can also utilize carbon monoxide (CO). This makes them more fuel flexible and also generally more efficient with available fuels, such as natural gas or propane. Hydrogen and CO can be produced from natural gas and other fuels by steam reforming, for example. Fuel cells like SOFCs that can reform natural gas internally have significant advantages in efficiency and simplicity when using natural gas because they do not need an external reformer.

The electrochemical principle of fuel cells

Fuel cells work with the movement of ions across their electrolyte when air and fuel are supplied to the cell. Using SOFC as an example under the right temperature and other conditions the oxygen in the air is ionized at the cathode and sets up a flow of oxygen ions through the electrolyte.

When the ions reach the fuel at the anode they oxidize the hydrogen to H2O and the CO to CO2. In doing so they release electrons, and if the anode and cathode are connected to an external circuit this flow of electrons is seen as a dc current. This process continues as long as fuel and air are supplied to the cell.

To supply the air and the fuel an air blower and a fuel pump are usually needed. Other equipment such as a heat exchanger to use the exhaust to heat incoming air, startup and shutdown systems, as well as controls, instrumentation and safety equipment are also needed.

Source: Siemens web site 8/01; TechPro DTE Energy Bob Fegan 2002