Technology

The Technology

Genesis reformers are designed to produce high purity hydrogen from fuels such as alcohol. Fuel is mixed with water, pumped into the reformer, heated, and converted to hydrogen and carbon dioxide in a catalyst bed. Most of the hydrogen is removed from the reformed gases using a purification membrane. The leftover hydrogen and the carbon dioxide (referred to as raffinate) is sent to a burner to provide heat.

The reforming reaction for methanol and ethanol alcohols are:

Methanol

CH₃OH + H₂O→3H₂ + CO₂

Ethanol

C₂H₆O + 3 H₂O→6H₂ + 2CO₂

Some carbon monoxide and other products may also be produced; only the basic reactions are illustrated above. Both reactions are endothermic, that is, they are energy absorbing. Diesel, gasoline, propane, and methane (natural gas) can also be reformed, but increasingly large volumes of water are required as the hydrocarbons become heavier, i.e., as the number of carbon atoms increase. Steps must be taken to remove the sulfur from these fuels, as the sulfur will poison catalysts in reformers and fuel cells.

There are a variety of methods used for reforming fuels, including autothermal reforming, steam reforming, pyrolysis, plasma reforming, and purification reforming (the Genesis approach). These methods are summarized below:

• The Genesis Process (PURIFICATION REFORMING) is a higher pressure steam reforming process where the hydrogen is removed from the reformed mixture through a hydrogen-selective membrane. The extra pressure is needed to "push" the hydrogen through the selective membrane. Purification reforming does not require a carbon-monoxide cleanup step since the hydrogen-selective membrane allows only production of purified hydrogen. This makes the reformer simpler, and will prevent poisoning of the fuel cell. Fuel cells are easier to design and operate on pure hydrogen, and produce more power than when running on a diluted hydrogen stream.
• AUTOTHERMAL OR PARTIAL OXIDATION REFORMING Requires multiple and difficult-to-control process stages. Air is mixed into the fuel and the mix is burned, which supplies heat for the endothermic reforming reaction. The partially burned (or oxidized) fuel is mixed with steam and processed into hydrogen, carbon dioxide, water, carbon monoxide and other reaction products in the presence of a catalyst. The mixture is then cooled and sent to a water-gas shift stage to convert most of the carbon monoxide into hydrogen and CO2. Then, the mixture is again cooled and the remaining CO is preferentially oxidized by injecting air into a 3rd catalyst bed. Finally, excess water content is condensed, and the gas mixture is sent to the fuel cell. The downside of the method is that CO and fuel dewpoint management can be problematic, and process control (especially when the system is not baseline loaded) is extremely difficult. Process temperatures must be kept within very precise limits, and failure to maintain these limits will cause the overall system to become inoperative.
• LOW PRESSURE STEAM REFORMING works much like partial oxidation, except that the heat for the reaction is not supplied by burning the fuel in the presence of the catalyst, but the heat is instead supplied from an external source. Transferring heat to the reaction catalyst is thus an added challenge with this method compared to partial oxidation reforming. However, a benefit to steam reforming is that the product is not diluted by the nitrogen and carbon dioxide from the partial oxidation air.
• PLASMA ENERGY REFORMING can be used to break hydrocarbons down, but the gases exiting the plasma must still be sent to a water-gas shift bed, and then to a preferential oxidation step. A plasma processor thus ends up looking much like a conventional steam reformer, except for the first stage.
• PYROLYSIS can be used to create hydrogen. Pyrolysis involves heating a hydrocarbon to a high temperature until the carbon and hydrogen bonds break apart, leaving carbon and hydrogen. Process heat can be supplied by burning the leftover carbon, or the carbon itself can be used for catalysis of certain fuels, such as gasoline. The drawbacks are low efficiency, carbon buildup and the need to clean up the hydrogen stream before feeding it to a fuel cell.