All liquid hydrocarbon fuels (gasoline, diesel fuel, kerosene, and methanol), as well as natural gas, contain significant hydrogen, which is chemically bound to carbon. Both diesel fuel and gasoline contain about two hydrogen atoms for every carbon atom, while natural gas contains four.
“Reforming” of a hydrocarbon fuel is a chemical process that converts the natural gas or liquid fuel into a hydrogen-rich gas. The product of this process is called “reformate,” and when used to fuel a PEM fuel cell, it is typically composed of a mixture of hydrogen gas, carbon dioxide, nitrogen, and water vapor. Reformate used to fuel an SOFC can also contain carbon monoxide. Depending on the fuel being reformed and the process used, the reformate could be anywhere from 40 to 75 percent hydrogen by volume (College of the Desert, 2001a).
There are a number of processes that can be used to reform different fuels. Fuel reforming often requires several different steps, each of which involves flowing the fuel or partially processed reformate across a catalyst bed in a closed vessel, or “reactor.” These reactors are generally constructed like heat exchangers. with the working fluid flowing through one set of channels coated with some kind of a catalyst, and another fluid (thermal oil or water-ethylene glycol)
flowing through another set of channels to either add or take away heat. Each process step may also require the addition of air or water to the inlet flow stream. The catalyst coating promotes chemical reactions in the vessel, which usually occur at relatively high temperatures and pressures. The necessary process heat may be produced by combusting some of the liquid fuel and/or depleted reformate after it leaves the fuel cell stack, in a burner. As a whole, the fuel reformer unit is close to being a “solid state” device, with very few moving parts.
Natural gas and alcohol fuels, like methanol, are easier to reform than gasoline or diesel fuel and also yield a reformate with higher hydrogen content. Both gasoline and diesel fuel are a mixture of different hydrocarbons, including aromatics and olefins that tend to form polymer gums and carbon during reforming, which can block the reformer catalyst sites (College of the Desert 2001a). Reforming of gasoline and diesel fuel, especially for use in a PEM fuel cell, usually requires additional processing steps.
Hydrogen is often produced at a centralized hydrogen fueling station by reforming natural gas on site. If so, additional processing steps are used to remove the carbon dioxide and other impurities from the reformate to produce very pure hydrogen gas. This hydrogen is then compressed for on-site storage and delivery to vehicles.
Different reformer designs are possible, but most will likely be packaged into a “hot box” that incorporates all of the process steps, including the process heater or burner, into a relatively compact unit housed in a single enclosure (see Figure 11). The plumbing inside this box may be very complicated, with the different systems feeding each other. The device will likely also have interconnections with the fuel cell stack outlet (for depleted reformate), the fuel cell water recovery system, the fuel cell cooling system, and the liquid fuel storage system.
The reformate leaving the fuel reformer is generally at approximately the same temperature and pressure at which the fuel cell stacks operate.
For SOFC APUs, the fuel reformer and SOFC stacks may be packaged into a single unit in a common enclosure, with only external fuel line, process air intake, exhaust outlet, and electrical connections to other vehicle systems.
Onboard reformers can also be used with fuel cell vehicles so that compressed or liquid hydrogen does not need to be carried on the vehicle. For example, Georgetown University has fielded a fuel cell transit bus operated on methanol fuel that is reformed onboard. The methanol fuel processor on this bus uses low temperature steam reformation and selective oxidation to make the hydrogen-rich reformate, which is fed to a PEM fuel cell.
In steam reformation, the methanol must first be vaporized and mixed with steam. The steam/methanol mixture then passes across a heated catalyst bed in the steam reformer, which converts the methanol and water to hydrogen gas, carbon dioxide, and carbon monoxide. Because PEM fuel cells cannot tolerate carbon monoxide, the reformate must go through a second catalytic process called selective oxidation, which converts the carbon monoxide into carbon dioxide. The final reformate is approximately two-thirds hydrogen, with the balance CO2, water, nitrogen, and less than 20 parts per million CO. The required heat for the process is provided by oxidizing the depleted reformate in a catalytic burner after it exhausts from the fuel cell stacks. See Figure 11 for a picture of this methanol fuel processor.
At least two companies are also working on a fuel reformer/processor to reform diesel fuel to power an SOFC APU. Unlike a PEM fuel cell, an SOFC can tolerate CO, so this fuel processor is based on catalytic partial oxidation and does not require the second, selective oxidation processing step.
Compared to onboard storage and use of compressed or liquid hydrogen in a PEM fuel cell engine or SOFC APU, onboard reforming of hydrocarbon fuels creates more tailpipe emissions. In particular, the vehicle will emit carbon dioxide, as well as small amounts of nitrogen oxides created during fuel reforming.
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