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| 07/11/18

Researchers demonstrate the potential of carbon-negative hydrogen fuel


Making hydrogen today is at best a carbon-neutral process, but a trio of US researchers have devised a process that goes one step beyond.

Could there be a fuel that is not merely low carbon, or even carbon neutral, but actually carbon negative? Could vehicles one day be powered by a fuel that even after combustion permanently sucks carbon dioxide from the atmosphere?

It seems fantastical, but this is the dream of a trio of American researchers. In a Nature Climate Change paper published at the end of June, they sketched out a number of potential production processes they are calling “electrogeochemical” that generates hydrogen that does exactly this.

The majority of scenarios that scientists have produced for the UN that allow us to keep within 2C of warming assume that we will overshoot our remaining budget for atmospheric CO2. As a result, these same scenarios include a huge role for technologies that allow us to go carbon negative, actively sucking the pollution out of the atmosphere.

Up to now, the main option for this has been bioenergy coupled to carbon capture and storage (BECCS), but this presents substantial challenges as production of bioenergy at the large scales required could compete for arable land with production of food.

The US researchers think they have an alternative negative emissions option: giving a tweak to hydrogen production. While their suite of techniques have been demonstrated in the lab, the researchers stress that more research would be needed to test economic and technical feasibility at commercial volumes.

Currently, clean generation of hydrogen employs a direct electric current to split water, which is made up of two hydrogen atoms and one oxygen atom. And this process, electrolysis, must be powered by electricity from renewable or nuclear energy. (The majority of hydrogen production however is not clean in that it is generated via “cracking” of natural gas, emitting CO2 as a byproduct)

But even here, the best that has been thought to be achievable is carbon-neutral hydrogen, which can be used in a fuel cell to create electricity that can run things like cars. Internal combustion engines can also run directly on hydrogen, but at lower overall system efficiencies. No CO2 is drawn down from the air in the process, but also none is released when the hydrogen is combusted.

In order to make the product carbon negative, one option devised by the researchers is that seawater, instead of freshwater, is split in an electrolytic cell into hydrogen gas and hydroxide ions (a hydrogen and oxygen atom bonded together and carrying a negative electric charge). The salt in the water is also split, producing sodium ions (that carry a positive electric charge), and chlorine gas.

The negative part comes from the hydroxide ions and the sodium ions forming a solution that is absorptive of CO2, ultimately forming sodium bicarbonate, better known as baking soda, that locks away the carbon. Because of its alkalinity, adding sodium bicarbonate to the ocean would also help counteract ocean acidification while providing vast carbon storage potential.

This technique also produces chlorine gas, which could be combined with the hydrogen in a fuel-cell to produce electricity, with a strong acid as a by-product. While this acid does have industrial uses, it could also be neutralized with inexpensive alkaline minerals to form benign salts.

Variations on this scheme use minerals in the electrolysis process or generate oxygen rather than chlorine. Oxygen can have several beneficial uses including substituting it for air in conventional fossil fuel combustion. This makes the combustion more efficient while producing a more concentrated and hence more inexpensively captured CO2 waste stream.

Beyond the requirement to be powered by clean electricity to make sense, another challenge is that electrolysis is very energy intensive, and thus expensive. So the researchers wanted to know what was the global potential for carbon-negative hydrogen, given these constraints. They considered the current global renewable electricity production from solar, hydro, wind, geothermal, biomass and ocean (wave and tidal) sources, and the associated costs. They found that carbon-negative hydrogen could increase energy generation and carbon removal by more than 50 times compared to BECCS, at the same cost or cheaper.

In depth

Can there really be carbon-neutral gasoline? A BC firm reckons it might arrive sooner rather than later

The Climate Examiner speaks to BC-based Carbon Engineering about the technology, the business and the policies that could make direct air capture, synfuels and carbon sequestration work.

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