Decarbonising the UK: Anaerobic Digestion and Carbon Capture

As the industry for managing organic wastes has grown, so has the potential for capturing the CO2 byproduct and either reusing or storing it.

So far, this has been more of an idea, but new initiatives are starting to make good on the potential. Although anaerobic digestion (AD) already plays an important role in tackling climate change by capturing methane, the use of biogas still results in CO2 emissions. Now innovations in how to capture this are opening up new markets for the AD sector, both in terms of selling CO2 for other applications or for payments for carbon capture.

And it could potentially be huge. The Committee On Climate Change estimates that there will be around one million tonnes of CO2 that could be captured annually by 2050 and many industry stakeholders suggest that this figure is actually a conservative estimate.

CO2 extraction from biogas

Just as there are many different approaches to the design and build of digesters, there is a range of techniques for the capture of carbon dioxide.

One such process is to separate the carbon dioxide, hydrogen sulphide and nitrogen gases that are produced in a biomass facility, and then treat them with a group of chemicals called amines. The mixture is put through an amine solution that in turn ‘grabs hold’ of the CO2 and releases the nitrogen.

Another option is to tackle the issue further on in the commercial process in the production of biomethane, according to John Baldwin, Managing Director of biogas industry consultancy CNG Services. A simple and cost-effective way to do this is by retrofitting existing biogas plants with the facilities for carbon capture: “When you make biogas in an anaerobic digester you’re basically making two things - carbon dioxide and methane. So if you can imagine a big tank full of a mixture of CO2 and methane. What you want to do then is separate the methane out so you have almost a pure stream of CO2 so you’ve not got any of this nitrogen… You’re starting after the amine stage - you don’t need that stage with biogas because you’ve already separated it out of the methane and there’s no nitrogen.”

A novel approach, pioneered by the French firm Air Liquide, captures the CO2 produced during a hydrogen-making process that is used to produce the sulphur-free fuels used by the oil and gas industries. Applying patented techniques (trademarked as Cryocap), hydrogen and CO2 are separated using a cryogenic process. The hydrogen is used to power fuel cells and is used in clean mobility applications like forklift trucks, while the carbon dioxide is used to meet the needs of local industrial markets such as the food industry and water treatment.

Another innovation, at an earlier stage of development, is a process developed by the EU-funded NTPleasure project, which uses an ultra-thin zeolite membrane to separate the CO2 from the biogas.The researchers have also created a number of highly active catalysts which can convert CO2 to methane using an electrical discharge which ‘excites’ the CO2 and hydrogen molecules causing them to break their bonds and interact with the catalyst surface. After testing, the team involved achieved a carbon capture efficiency of 91.8 per cent alongside a carbon utilisation efficiency of 71.7 per cent. The project has provided a proof of principle for the technology, with the researchers seeking additional funding to scale its development.

How can captured carbon be used?

So, once captured, what happens to the CO2? One option employed by Oxfordshire waste management company Grundon takes fly ash that you find in an incinerator and reacts it with CO2 to make artificial limestone pellets, which effectively lock the carbon away. The finished Carbon8 aggregate is used mainly in the manufacture of lightweight concrete masonry blocks.
However, the amounts of carbon involved in these projects are small compared to those dealt with at the higher end of the carbon offsetting market. A major player here is Future Biogas, a Surrey-based firm that recently announced plans to build 25 new biogas plants with carbon capture and storage facilities by 2028. At present they operate 10 plants in the UK for a range of clients, including green financial investment portfolios for JLEN Environmental Assets Group and Aviva.

The biogas that these AD plants produce is put into the grid and sold through to a corporate customer. After the separation, the CO2 that is left is liquified by compressing and chilling it. “Technically it’s relatively straightforward,” claims Future Biogas CEO Philipp Lukas. “You get a volume of carbon dioxide [approximately] 30 tonnes or so in a big tanker at -20 to -25 degrees celsius. You take that to a port-side facility, collect it from a variety of locations there, stick it in on a ship and it’s sent to the [carbon capture] facility.”

In June 2021, Future Biogas announced that it had signed an agreement to supply CO2 to Northern Lights, a joint venture between Equinor, Shell and Total Energies to use an area 2600 metres below the North Sea for carbon capture and storage. 

“What they do is pump it down a well similar to the ones from which you would have extracted oil and gas,” explains Lukas. “When you get to two or three kilometres down in the North Sea there are lots of caps of impermeable rock, underneath which, in some cases, has collected oil and gas. One of these capped areas that don’t have any oil or gas is where Equinor has chosen for the repository. Effectively, the carbon is being put back to where it should be.”

You might assume that piping CO2 back below the North Sea would be inordinately expensive but Lukas maintains that with projects like these huge economies of scale kick in. “All forms of geological sequestration are between £400-1000 per tonne of CO2, so it’s considerably higher than the EU Emissions Trading Scheme or the UK Emissions Trading Scheme pricing which is around €80/90. But the premium is paid by people who want to buy permanently sequestrated negative carbon. If you go out and look at people like Swisslife Reims, Microsoft - they have all been buyers in that market.”

The carbon offsetting market is certain to grow over the next few years as businesses seek to fulfil their sustainability obligations. For many, rapidly decarbonising their operations quickly represents an enormous challenge, so purchasing carbon offsets is necessary if they are to hit net-zero targets. For responsible corporations, the ability to accurately measure the amount of CO2 captured and have confidence that this has been permanently sequestered makes projects like the partnership between Future Biogas and Northern Lights very attractive.

How large might this market become? “McKinsey put out a report last year saying that it could be £50 billion by 2030,” says Philipp Lukas. “Personally I think it’s going to have to be if we’re going to stand any chance whatsoever of sticking to two degrees, let alone 1.5. We’re not going to be able to decarbonise fast enough, which means we’ll have to do everything we can to remove CO2 from the atmosphere as well.

“So it’s part of the solution, but a tiny cog in an enormous problem. You can definitely see it removing several million or tens of millions of tonnes of CO2 but compared to the hundreds of thousands of millions of tonnes that we currently emit, it’s no silver bullet. Though nor, frankly, is anything. But it has huge potential, particularly because you’ve got that double benefit - it’s making clean energy and removing as a processed byproduct CO2 from the atmosphere.”