Steam reforming

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Steam reforming, or catalytic oxidation, is a method of producing hydrogen from hydrocarbons. On an industrial scale, it is the dominant method for producing hydrogen. Small-scale steam reforming units are currently subject to scientific research, as way to provide hydrogen to fuel cells.

Contents

1 Industrial Reforming
2 Fueling fuel cells
3 The process
4 References
5 See also

Industrial Reforming

Steam reforming of natural gas, sometimes referred to as steam methane reforming (SMR) is the most common method of producing commercial bulk hydrogen. It is also the least expensive method.George W. Crabtree, Mildred S. Dresselhaus, and Michelle V. Buchanan, The Hydrogen Economy, Physics Today, December, 2004 [link] At high temperatures (700 – 1100 °C) and in the presence of a metal-based catalyst, steam reacts with methane to yield carbon monoxide and hydrogen.

CH₄ + H₂O → CO + 3 H₂

The United States produces nine million tons of hydrogen per year, mostly with steam reforming of natural gas. This process is different from catalytic reforming, an oil refinery process that also produces significant amounts of hydrogen along with high octane rating gasoline.

Fueling fuel cells

Steam reforming of liquid hydrocarbons is seen as a potential way to provide fuel for fuel cells. The basic idea is that for example a methanol tank and a steam reforming unit would replace the bulky pressurized hydrogen tanks that would otherwise be necessary. This might mitigate the distribution problems associated with hydrogen vehicles

This approach to power generation yields several benefits:

Steam reforming can utilize CO₂-neutral liquid hydrocarbon fuels, such as bio-ethanol and bio-diesel, to produce green hydrogen from fuels that can be used during a transition from fossil-based fuels.
Small scale reformers could be distributed within our current hydrocarbon infrastructure at a relatively low cost, to convert gas stations into hydrogen stations.
When combined with a fuel cell, such refueling stations could provide backup power to neighbourhoods in addition to providing hydrogen for vehicle refueling.

There are also several challenges associated with this technology:

The reforming reaction takes place at high temperatures, making it slow to start up and requiring costly high temperature materials.
Sulphur compounds present in the fuel poison certain catalysts, making it difficult to run this type of system from ordinary gasoline. Some new technologies have overcome this challenge, however, with sulphur-tolerant catalysts.
The carbon monoxide (CO) produced by the reactor poisons the fuel cell, making it necessary to include complex CO-removal systems.
The thermodynamic efficiency of the process is between 70% and 85% (LHV basis) depending on the purity of the hydrogen product. However, despite these energy losses, the hydrogen product can then be used in a fuel cell more efficiently than the original hydrocarbon could be used in an internal combustion engine, typically by a factor of two or more.
The biggest problem for reformer based systems remains the fuel cell itself, in terms of both cost and durability. The catalyst used in the common polymer-electrolyte-membrane fuel cell, the device most likely to be used in transportation roles, is very sensitive to any leftover carbon monoxide in the fuel, which some reformers do not completely remove. The membrane is poisoned by with the carbon monoxide and its performance degrades.
The catalyst is frequently very expensive.

Even with these problems, the reformer–fuel-cell system is still being researched as a system that will power cars, homes and businesses in the future. An ideal system would be capable of running on existing fuels, such as natural gas or gasoline or diesel, but in the long run a renewable liquid fuel like bio-ethanol or bio-diesel may be preferable. The overall cost of making, transporting and storing the hydrogen fuel is the key issue.

The process

For most hydrocarbons, including methane, temperatures in excess of 700 °C are necessary. Methanol, however, can be converted at significantly lower temperatures (around 350 °C).

The chemical reactions that take place are:

C_nH_m + n H₂O → n CO + (m/2 + n) H₂

CO + H₂O → CO₂ + H₂

The produced carbon monoxide can combine with more steam to produce furher hydrogen via the water gas shift reaction.

The process is endothermic (consumes heat).

Steam reforming

Industrial Reforming

Fueling fuel cells

The process

References

See also