FUEL CELLS:HYDROCARBON GAS REFORMATION

HYDROCARBON GAS REFORMATION

A fuel cell is designed to “burn” hydrogen and oxygen to generate electricity. Hydrogen is not generally available, but hydrocarbon gases such as natural gas or even gases generated from biomass can be converted into hydrogen in a process known as reformation. The reformation reaction generates a gas that contains a mixture of carbon dioxide and hydrogen that can then be supplied as a reactant for the fuel cell. (The carbon dioxide will be inert and so will not interfere with the reaction other than by acting as a diluent for the hydrogen.) The main constituent of natural gas and of most biogas is methane, and this is the main target for the standard reformation process, although other hydrocarbons can also be reformed and even coal can be converted into a hydrogen-rich fuel if necessary (see coal gasification in Chapter 3).

The conversion is usually carried out as a two-stage process. In the first stage the methane-rich gas is mixed with water vapor and passed over a catalyst at a high temperature where the gases react to produce a mixture of hydrogen and car- bon monoxide, a process called steam reforming. A second reaction, called the water shift reaction, is then carried out during which additional water vapor is added to the new mixture where it reacts with the carbon monoxide, again in the presence of a catalyst, to produce more hydrogen and carbon dioxide.

The degree to which this second stage reaction is carried to completion 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. Natural gas can also contain some sulfur impurity. This is normally removed by cleaning before the fuel is reformed, but if not, then any remaining sulfur must be removed since low-temperature fuel cell catalysts are extremely sensitive to its presence.

While natural gas is the most convenient source of hydrogen for a fuel cell today, other fuels can also be exploited. For example, methanol can also be converted in to a hydrogen-rich gas using a reforming process, as can gasoline, though the latter requires an extremely high temperature (800 oC). 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 have to be equipped with an external reformer to process the fuel before it enters the fuel cell. However, heat generated within the fuel cell may be used to help drive the reforming process. The conditions inside the two main types of high-temperature fuel cells are sufficient for the reforming to take place within the cell itself, simplifying system design.

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 natural gas or another carbon-containing 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. However, if hydro- gen can be generated without the need for fossil fuel combustion—by using hydropower to electrolyze water, for example—then burning the gas in a fuel cell provides an efficient and emission-free source of electricity.

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