I have always had some concern about fuel cells that require cracking hydrocarbon fuels. These still emit CO2, and I have not yet seen direct comparisons about the efficiency and pollution of this type of fuel cell vs. a high-efficiency internal combustion engine. There are so many variables, that the comparison is anything but obvious. An internal combustion engine can burn fuel directly. However, it loses much more energy to heat than a fuel cell. But then we have to look at what the fuel cell uses to crack the hydrocarbon. Is it heat?

Obviously, many of these variables are not an issue for a pure hydrogen cell.

One of Larry's links led me to a very detailed reference on fuel cell technology which is useful. Some excerpts:

...Processing hydrocarbon fuels to generate hydrogen is a technical challenge and a relatively demanding operation.
Hydrocarbon fuels require processing temperatures of 700 C - 1000 C. Sulfur, found in all carbon-based fuels, and carbon monoxide generated in the fuel processor, must be removed to avoid poisoning the fuel cell catalyst. Although the reformate/air fuel cell lacks the zero emission characteristic of the direct hydrogen fuel cell, it has the potential of lowering emissions significantly vs. the gasoline internal combustion engine. The near-term introduction of reformate/air fuel cells is expected to increase market acceptance of fuel cell technology and help pave the way for the widespread use of direct hydrogen systems in the future.



...As long as hydrogen is difficult to store on a vehicle, on-board fuel processors will be needed to convert a hydrocarbon fuel, such as methanol or gasoline, to a H2 rich gas for use in the fuel cell stack. Currently, steam reforming of methanol to H2 is the conventional technology, although partial oxidation of gasoline to H2 is attractive because of the gasoline infrastructure already in place in most countries. Both types of fuel processors are complex
systems.

The steam reforming of methanol involves the reaction of steam and pre-vaporized methanol at 200°C (gasoline requires temperatures over 800°C) to produce a mixture of H2, carbon dioxide (CO2), carbon monoxide (CO), and excess steam. This mixture passes through another reactor, called a shift reactor, which uses catalysts and water to convert nearly all of the CO to CO2 as well as additional H2. There can be a third stage in which air is in-jected
into the mixture in a third type of reactor, the preferential oxidation reactor. Oxygen in the air reacts with the remaining CO over a Pt-containing catalyst to convert CO to CO2. The final gas mixture contains about 70% H2 , 24% CO2, 6% nitrogen (N2) and traces of CO.

With the partial oxidation reformer system, liquid fuel is first vaporized into a gas. The gas is then ignited in a partial oxidation reactor which limits the amount of air so that primarily H2 , CO and CO2 are produced from the combustion. This mixture is passed through a shift reactor to convert the CO to CO2 and then through a preferential oxidation reactor to convert any remaining CO to CO2. Conventional partial oxidation takes place at ~1000°C and catalytic partial oxidation takes place at ~700°C. The final reformate composition is about 42% N2, 38% H2,
18% CO2 , less than 2% CH4 and traces of CO.



Some questions come to mind when discussing hydrocarbon-using fuel cells that are portable or in vehicles:

Where does the steam come from for steam reforming? CO can be removed catalytically, but sulfur would have to be removed at the factory, if it is possible at all?

In partial oxidation, how much room is this chemical factory going to use up inside a car?

I think that unless some tech breakthrough occurs, cells that use hydrocarbon fuels may be more trouble than they're worth. Storing pure hydrogen in hydrides and in other ways is probably less of a hassle.

However, for stationary systems like backup generators, where size and complexity is not so much of an issue, a hydrocarbon fuel cell generator piped directly into the standard natural gas pipes (with a backup tank in case of busted pipes) could be a big seller for hospitals and other places that need efficiency and quiet. It's got to be better than those diesel jobs they're using now.