Hydrogen’s biological options

The latest issue of E&T looks at the future of hydrogen and how its economics might play out. One of the main assumptions in the economics is that production of green hydrogen, which does not create carbon dioxide as a by-product, will rely on electricity generated from the environment. There’s a simple reason behind that: the technology has already demonstrated that it is scalable if not exactly cheap right now. It’s not necessarily the only answer to generating clean hydrogen.

Biology has spent hundreds of millions of year finding ways to turn solar energy into usable materials. By shifting to biological photosynthesis you can cut out the electrical middleman. You could, in principle, have carpets of algae and bacteria pumping out hydrogen in place of solar panels that feed power directly to chemical tanks. That’s the dream. Making it happen is a bit more tricky.

The first bit of bad news is that biology is not geared up to mass-produce hydrogen. Though there are quite a few micro-organisms to choose from that can be coaxed into splitting water into hydrogen and oxygen, few organisms rely on the ability to make hydrogen gas. They mainly want the element in the form of carbohydrates and other organic molecules.

Green microalgae would great candidates for photosynthesising hydrogen but the oxygen produced by the reaction winds up inhibiting the enzyme responsible for the reaction. Blue-green microalgae are perhaps more reliable but it’s a process that only really works if you keep them lit up all day, which means putting them into reaction chambers lit by LEDs. It’s possible but it’s not the same as simply spreading algae over large artificial beds that you can float out at sea. Some other organisms produce hydrogen as a by-product of other processes but that points to another key issue with biosynthesis in the hydrogen business: scaling up.

If you go back to the enthusiasm for petrol-like biofuels before the 2008 financial crash, which helped nail the lid on the coffin of many start-ups in that area, the big issue with many of the projects was that they worked on a small scale. But numerous problems emerge as you try to build up production levels that would make the process commercially viable. They are both technical and financial.

One issue is that a process that works in a small reaction chamber does not necessarily work well in an industrial-size tank. Half the time, the organisms wind up poisoning themselves if the product is not removed quickly or simply grinding to a halt because too much of the chemical in their environment leads them to stop making it, as with green microalgae and hydrogen. That means finding ways to design systems that can extract the product quickly and cheaply without disturbing the organisms too much.

Food is also an issue. Some of these organisms only produce high yields if fed on pure sugars, which increases your cost, though genetic engineering can make them less fussy. But, you also have to deal with the problem of unwanted algae or bacteria turning up to eat the grub and leaving slime everywhere.

Financially, there is a bigger issue, as outlined by Bioeconomy Capital managing director Rob Carlson at the SynbeCite conference late last year. Fuel needs to be cheap. This is not a good match for an R&D-intensive area where much of the technology needed to improve yields and develop novel reaction chambers at scale does not yet exist. Trying to raise funds for a project that might beat petroleum costs by 20 per cent is not all that attractive to the financiers, he pointed out. “They will say they can invest in real estate and earn that return.”

For this reason the biofuel start-ups from the 2000s who are still around today switched to materials synthesis. And not just any old materials: they are using synthetic biology to manufacture relatively complex compounds. Carlson says: “What I can get investment for is for materials that can't be made easily other ways.”

There is one biological avenue for hydrogen that might be more promising, as it deals with the problem of feeding costs: generating hydrogen from waste. Take micro-organisms and set them to work on chewing through biological waste. Or simply find chemical and industrial processes that can work with ready sources of hydrogen. This is the target of a project researchers at the University of Coventry are running with Severn Trent and The Organics Group. The water company is looking for ways to use the tonnes of ammonia its sewage plants produce daily and which today has to be destroyed.

One possibility is to split the ammonia into nitrogen and hydrogen, which is a relatively simple process. In principle, Severn Trent’s sewage plants could generate up to 450 tonnes of hydrogen every year from the 10,000 tonnes of ammonia the treatment beds emit. The key questions for this kind of process is what will work out to be the most commercially viable approach. The ammonia itself is potentially valuable as a fuel because, despite being toxic and pretty noxious, it may well become one of the main ways in which hydrogen is shipped around the world. Potentially, ships can run on ammonia directly.

The ammonia is also a source of fertiliser, which may provide Severn Trent with a by-product that is more readily sold, not least because it comes from a greener source than most of the industrial ammonia produced today. The team’s main project in the short term is to develop an efficient mechanism for recovering reasonably pure ammonia from the waste gases.

Ammonia may also be the easier option in the short term. John Graves, associate professor at the Institute for Future Transport and Cities at Coventry University, says if the project goes well the ammonia-recovery aspect will be at a more advanced stage than the hydrogen conversion step.

Peter Vale, technical lead for innovation at Severn Trent, says, “My view is that there is considerable potential for both green ammonia and green hydrogen.” The question is “whether the optimum approach for Severn Trent, and other water companies, is to sell green ammonia for fertiliser or as an energy store or green hydrogen. [That] probably depends on how the market for both develops. For now we are keen to explore all approaches.”

One other option for bio-hydrogen is to simply use the bio part as inspiration and try to find molecular analogues of the enzymes that perform photosynthesis to take on the energy-intensive challenge of splitting water into its component parts. This kind of 'artificial leaf' is the focus of people such as Harvard professor Daniel Nocera. However, as the former biofuels companies have done, the main target is to couple hydrogen production with biosynthesis to produce foods and materials.

There is the possibility that efficiency breakthroughs will make biology a better bet for hydrogen generation. But in the absence of that, the chances are electrolysis will be the mass-market option for some time yet.