Capturing Carbon in Anaerobic Digestion

In anaerobic digestion (AD), waste materials such as food waste, can be processed to generate energy, enabling a circular system in which waste is diverted from landfill to power or heat homes, or be used in transport. Carbon capture and storage (CCS) and utilisation (CCU) technology can be used alongside AD to utilise carbon dioxide which would otherwise be released into the atmosphere. There is a significant opportunity for AD plants to not only reduce carbon emissions by engaging CCS technology, but to generate revenue from the captured CO2 as a by-product that can be used in the food and drink manufacturing industry or in pharmaceuticals.

Adapting the AD process to capture carbon

For an AD plant to produce a gas which is high enough in quality for the national grid, the biogas which is initially produced in the AD tanks needs to be upgraded to biomethane, the chemical equivalent of methane in natural gas. The upgrading process involves removing CO2 from the biogas mixture, which is where there is an opportunity for CCS technology to be used.

Using CCS, 1.9kg CO2 can be captured and stored per cubic metre of CO2 which would normally be released. Methane slip is also returned to the digester, meaning gas which historically would have been lost can now lead to further revenue.

Worldwide, there are already approximately 50 AD plants using CCS, with ten based in the UK. At Privilege Finance, we see a significant opportunity for both new and existing AD plants to adopt the technology and start recovering CO2. We are actively funding upgrades to existing plants, and aim to fund multiple projects going forward.

It takes approximately ten months to install the equipment as an upgrade, and there are a range of different equipment providers for CCS technology, for example Pentair.

Uses for captured carbon

As well as keeping carbon emissions out of the atmosphere, the CO2 captured can be used in various ways. For example, in the food and drink sector it is used to produce carbonated drinks and for packaging and transporting meat and fresh produce to prevent deterioration of quality or safety. Currently, there are just seven AD plants in the UK supplying food grade CO2 into food and beverage and greenhouses.

Since we became involved with CCS projects, a number of people have asked about the safety of CO2 produced from waste. I think we need to help people to understand that the method in which CO2 is produced does not affect its quality and should not affect its use. It certainly makes sense to use a product from renewable energy, rather than manufacturing it from scratch, as we are all acutely aware that there is an excess of CO2 on the planet overall.

Achieving negative emissions

The other advantage of using CCS in conjunction with AD is that in the right system it has potential to contribute towards achieving negative emissions, once all factors are taken into consideration.

Imagine all the food waste in a town going to landfill. With time, it would break down and release a high proportion of methane, as well as CO2, directly into the atmosphere. Processing food waste in an AD plant reduces emissions, as the biomethane which is used to heat homes replaces demand for fossil fuel derived gas for heating, helping to keep carbon in the ground.

The use of CCS in the AD process further reduces associated emissions, as it avoids the release of that CO2 into the atmosphere when upgrading biogas to biomethane, while also replacing demand for manufactured CO2 in the food, drink or pharmaceutical industry.

Finally, the digestate by-product from AD can be used as a nutrient-rich organic fertiliser in farming systems, replacing the need for manufactured fertilisers which are produced using an energy intensive process with high levels of greenhouse gas emissions associated with it.  These manufactured fertilizers are then transported to a large distance to be delivered to the end user, therefore increasing the carbon footprint further whilst digestate can be delivered locally.


Image courtesy of Pentair: