Chemical reaction

Technology borrowed from large-scale chemical production could help reduce the cost of stationary fuel cells, according to BASF researchers.

Stationary fuel cells are tipped to replace boilers for generating heat and power for homes, offices and factories. They generally convert natural gas into hydrogen, then react this with oxygen from the air to generate electricity and heat. However, the main bar to their adoption has been their cost.

Engineers at the German company’s catalysts business unit are working to simplify the processes which take place inside the fuel cells, and to reduce the costs of the materials needed to catalyse the reactions.

Many of the problems associated with existing fuel cells concern the pretreatment of the gas needed to generate a clean, pure hydrogen feed.

In conventional systems, this is done by a six-step process called autothermal reforming. This involves desulphurisation of the gas feed; reformation of the methane in the gas to hydrogen and carbon monoxide using a platinum/ rhodium catalyst; three catalytic reaction steps at decreasing temperatures to convert the carbon monoxide into carbon dioxide using platinum, copper and platinum/ rhodium catalysts; and a final stage to incinerate waste gases over a platinum/palladium catalyst.

Most of these stages are necessary to remove compounds which can contaminate the catalysts in the fuel cell itself, said Markus Hölzle, product manager for fuel processing catalysts. Even trace amounts of carbon monoxide would rapidly render the cell inoperative.

Not only does the conventional process require relatively large amounts of extremely expensive precious metal catalysts, but it also needs complex engineering, said Hölzle.

‘This reaction concept requires numerous valves and control units, since the natural gas, air and water need to be dosed in precisely-defined proportions,’ he said. ‘It also needs reactors, blowers and ventilators. To be honest, it’s never really worked that well.’

The BASF team realised that the process could be simplified if it was switched from autothermal reforming to steam reforming. ‘It’s the same reaction as the one we use in our ammonia process, and we’ve been doing that for a hundred years,’ said Hölzle.

The process works by reacting the methane with steam rather than air, and has two main advantages: it requires less expensive catalysts, and it no longer requires precise dosing of air. The reformation itself needs a nickel/rhodium catalyst; the three-stage CO removal is reduced to two, with the initial high-temperature phase no longer needed and the final stage again using a nickel/rhodium catalyst; and the final waste incineration phase needs no catalyst at all.

‘We’ve found that all you actually need for that is two reactors with a heat exchanger in between them, rather than the five separate vessels with attendant equipment you needed before,’ he said.

But the company still has further work to do to reduce the costs further and improve the fuel cell’s reliability, said Hölzle. For example, platinum is still required in the fuel cell itself.

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