The simplest fuel cell 'burns' hydrogen and oxygen in order to generate power. But hydrogen is not a readily available fuel. Fortunately hydrocarbon gases such as natural gas or gas generated from biomass (this contains a large quantity of methane) can easily be converted into a mixture of hydrogen and carbon dioxide. This will also form a suitable fuel for a fuel cell.
The conversion is usually carried out as a two-stage process. In the first stage methane is mixed with water vapour and passed over a catalyst at high temperature where it reacts to produce a mixture of hydrogen and carbon monoxide. A second reaction, called a water shift reaction, is then carried out during which additional water vapour reacts with the carbon monoxide to produce more hydrogen and carbon dioxide.
The second stage is extremely important for fuel cells because the catalysts in low-temperature cells are sensitive to carbon monoxide poisoning. In consequence, virtually all the carbon monoxide must be removed from the fuel before it is fed into the fuel cell. Fuel cells are sensitive to any sulphur impurities too and these must be scrupulously removed.
While natural gas is the most convenient source of hydrogen for a fuel cell today, other fuels can also be exploited. Methanol can also be converted into a hydrogen-rich gas using a reforming process, as can gasoline, though the latter requires an extremely high temperature (800°C). Both these processes are of interest to the automotive industry.
Since reforming of all these fuels takes place at a relatively high temperature, low-temperature fuel cells usually need an external reformer to supply their fuel. The conditions inside a high-temperature fuel cell are sufficient for the reforming to take place within the cell, simplifying system design.
It is important to note that while a fuel cell burning hydrogen and oxygen produces no carbon dioxide, most fuel cells will generate carbon dioxide because they derive their hydrogen from a fossil fuel. When methane is converted into hydrogen it generates exactly the same amount of carbon monoxide as it would have generated if it had been burned in a gas turbine. What can be claimed for the fuel cell is that its high efficiency means that less carbon dioxide is produced for each kilowatt-hour of electricity generated than would be the case for a lower-efficiency process.
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