Low carbon process improves production of ultra-pure hydrogen

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Demand for high purity hydrogen is increasing thanks to its use in fuel cells for heat and power and transportation, but producing the gas can be expensive.

Now a new method for increasing the yield of ultra-pure hydrogen from cheaper feedstocks such as coal and biomass could help to bring costs down.

Hydrogen is most commonly produced from natural gas in a process known as steam reforming, in which methane is heated in the presence of steam, to convert it into a hydrogen-rich mixture known as synthetic gas, or syngas.

The hydrogen is often then separated from this syngas in a step known as Pressure Swing Adsorbtion (PSA), in which the gas is attracted to a solid surface, or adsorbent, under high pressure.

Reactors that use coal or biomass to produce hydrogen have previously been unable to match the high yields possible with natural gas, according to Dr Hyungwoong Ahn, a senior lecturer in Chemical Engineering at Edinburgh University.

But the new method, developed alongside researchers at Yonsei University, South Korea, can improve their yield, while also capturing carbon emissions.

To produce hydrogen from methane, heat is needed to operate the reactors, said Ahn. To provide this heat, the waste tail-gas from the PSA process is often recycled as fuel for the reactor.

“But if you use solid fuel such as coal or biomass instead of natural gas, the reactor does not require heat, so the PSA tail-gas can be utilised for other purposes,” he said.

The researchers found that the tail-gas can be split into three streams and used to improve the hydrogen yield from the reactor.

One stream can be used as a supplementary fuel for a carbon capture unit, to remove carbon dioxide from the synthetic gas stream.

A second stream of the gas can then be recycled to the reactors to improve the hydrogen yield by converting more carbon to hydrogen. Finally, the third stream can be used as a fuel for drying the coal or biomass, rather than using clean synthetic gas.

The process could improve the hydrogen yield of solid reactors by 2 per cent, above that expected from simply adding a carbon capture unit, the researchers have found.