To zero and beyond! Technology for a carbon-negative future

22 April 2021 marks the 51^st Earth Day, an annual event seeking to diversify, educate and activate the environmental movement worldwide, with events taking place in more than 193 countries. It is therefore a fitting day for the launch of another initiative: the XPRIZE Carbon Removal competition.

The competition, which has Elon Musk as its benefactor, is providing a $100-million prize pool for the development of atmospheric carbon dioxide removal solutions that are able to economically and practically scale to remove gigatons of CO₂ per year.

Capturing CO₂ and storing it in a way that it cannot readily escape into the atmosphere (and in turn contribute to global warming) is not a new concept. In fact, the Earth has been doing this for billions of years! Biomass absorbs CO₂ through photosynthesis and the oceans are an excellent sink of CO₂ – they are estimated to absorb around 30% of human CO₂ emissions (although the effects of this are not all beneficial).

Artificial carbon capture and storage (CCS) technology – where CO₂ is captured, compressed and stored, usually in geological structures – has also existed for the best part of 50 years. It was initially developed by the fossil fuel industry, where it was realised that CO₂ captured from point sources where CO₂ concentration is high, such as flue gases from refineries, could be pumped into oil fields to allow more oil to be extracted (a process known as Enhanced Oil Recovery). More recently it has not just been the fossil fuel industry that has been making use of CCS, but also other players in the chemical industry with considerable CO₂ emissions, such as fertiliser and steel producers.

However, whilst able to reduce greenhouse gas emissions associated with certain industrial processes, these applications do not count as ‘negative emissions’ – that is, the removal of excess CO₂ already present in the atmosphere. The IPCC special report on global warming of 1.5°C notes that all the pathways that limit global warming to 1.5°C with limited or no overshoot project the use of negative emissions technology at the scale of 100-1000 giga tonnes of CO₂ removed over the course of the 21^st century (in comparison, global CO₂ emissions currently sit at around 37 giga tonnes a year^[1]).

Providing negative emissions carbon capture solutions that can scale to the required levels is a substantial challenge. Energy use, cost, land use, possible environmental side effects and many other factors all need to be considered. So, what are the most promising carbon-capture technologies that we might see being rapidly advanced to capture the XPRIZE prize pool alongside a lot of CO₂?

Giant Air Sieves (Direct Air Capture)

Direct Air Capture (DAC), as its name suggests, is focussed on processing the air that you and I breath on a daily basis to directly separate out the CO₂ it contains for storage – imagine a giant sieve able to separate out CO₂ from the air and you’re not a million miles away. This is a much bigger technical challenge than point source capture because the concentration of CO₂ in the air, although problematic, is actually very low – just 400 parts per million – providing a very weak driving force for its separation. Whilst not the only technologies suitable for DAC, two of the most favourable techniques effective at the low CO₂ concentrations in ambient air are absorption and adsorption.

Absorption

Several of the pilot DAC facilities currently in operation achieve CO₂ separation using an absorption system, whereby air is usually drawn into units using fans and contacted with a liquid that selectively absorbs the CO₂ – typically an amine-based solution. The CO₂-rich amine solution can then be transported to a second processing unit where it can be heated to reverse the absorption reaction and release a pure CO₂ gas stream. Having been cooled, the now CO₂-lean solution can be recycled to absorb more CO₂.

These absorption-based technologies are attractive due to their relatively proven technology (similar systems are often employed when treating flue gases from industrial processes) and relatively good performance at low CO₂ concentrations. However, a major challenge that needs addressing is the heating-cooling cycle for the regeneration of absorption solutions, which makes the process very energy intensive. They use far more energy than the minimum dictated by the change in entropy from separating the gases, making the process expensive on a cost-per-tonne basis and meaning any CO₂ removed must be offset against emissions associated with the generation of the energy required to power the process.

Nevertheless, pilot plants from several organisations using this technology are already in place. CarbonEngineering has been sucking up CO2 at its pilot plant since 2015, with its first large-scale commercial plant, capturing up to a million tonnes of CO2 per year, slated for construction in 2022.

Adsorption

Another promising avenue that is already seeing encouraging progress is adsorption. This is perhaps the most widely adopted DAC technology, with Climeworks already boasting 14 facilities using their adsorption technology, including the world’s first commercial facility.

Although similar sounding, adsorption differs significantly from absorption in that the fluid to be captured (in our case CO₂) does not enter the bulk phase of the adsorbing substance. Instead, it adheres to its surface in a layer normally only one or two molecules thick.

Certain porous materials exhibiting very high surface-area-to-mass ratios can have their chemistry tailored to optimise their adsorption properties. Ideally, they are made highly selective towards CO₂ molecules, allowing CO₂ to adsorb to their surface whilst the oxygen, nitrogen and other gas molecules continue to pass through. Adsorbed CO₂ is usually then released for compression and storage using a pressure or temperature swing that promotes the desorption of the CO₂ from the adsorbent’s surface.

When it comes to the materials being used, zeolites, a class of super-porous materials that have been used for years as catalysts in the petrochemical industry, have found favour in CO₂ adsorption technology. They have surface areas of hundreds of square meters per gram and certain zeolites have also been shown to have high CO₂ uptake. However, their sensitivity to moisture and the resultant loss in capacity is problematic given the quantities of water vapour usually present in air, necessitating either development of less water-sensitive chemistries or the use of a drying pre-treatment step for zeolites to be deployed in large-scale DAC facilities.

More recently, excitement around other materials, including metal organic frameworks (MOFs) and solid amine-based sorbents, has been growing. MOFs have even higher surface-area-to-mass ratios than zeolites, ranging in the 1000s m² g^-1 (the area of multiple Olympic-sized swimming pools per gram!) and both technologies may offer even better performance than zeolites.

The distinct advantage that all these solid-state adsorbents have over traditional liquid absorption technology is their considerably lower heat capacities, which greatly reduces the energy input required for regeneration of their CO₂ separation capacity, and possibly the cost per tonne of CO₂ captured. Moreover, solid sorbents offer greater tunability with regards to their interaction with CO₂ molecules than liquid absorbents, potentially providing greater scope for their development. However, these benefits currently come at the cost of lower CO₂ loading in comparison to absorption technologies.

Back-to-Basics – Trees as Capture Technology

Another technology in the toolbox of CCS engineers has existed for some 370 million years: trees! Trees, absorbing CO₂ as they photosynthesise, are a key carbon sink in the Earth’s carbon cycle. Thus, is an obvious solution not just to keep planting trees until we have enough to suck up all our CO₂ emissions?

Unfortunately, simply planting trees and leaving them there is unlikely to be successful at the scale we need – a quick back-of-the-envelope calculation suggests that half the world’s landmass would need covering in trees just to absorb the CO₂ emissions of the USA^[2]! Even using seagrasses, whose CO2 uptake capacity far exceeds land-based carbon sinks such as rainforests, would likely still not be sufficient. We need to be a little more inventive in our biomass management.

This is where Bio Energy with Carbon Capture and Storage (BECCS) enters the frame. The basic concept comprises four key steps:

1. Sustainably managed forests are grown to absorb CO₂ from the atmosphere
2. The trees are harvested and processed into biomass pellets
3. Biomass pellets are used to generate renewable electricity^[3]
4. Carbon produced in the energy generation process is captured, transported and permanently stored.

BECCS has become a major avenue of development in the field of negative emissions technologies thanks to two critical attributes. Firstly, it has the potential to be a carbon-negative energy generation technology, rather than simply being carbon neutral (or nearly carbon neutral) like other sources of renewable electricity are, or being a carbon removal technology that requires an energy input. This is particularly attractive given energy demand is projected to continue growing as the world population swells in size and develops. Secondly, unlike DAC, BECCS does not have to grapple with the problem of the very low concentration of CO₂ in ambient air. Instead, it can use technology intended for CO₂ capture from point sources, be it pre-, post- or oxy-fuel combustion technologies, significantly reducing the energy requirements associated with CO₂ capture and widening the range of viable technologies.

Drax, a power station in North Yorkshire originally constructed and operated as a coal-fired station in the 1970s, has been refocussing to biomass firing over the past 15 to 20 years. As recently as early March this year, Drax applied for permission to install carbon capture technology that it suggests would capture 95% of the CO₂ that would otherwise be emitted during its electricity generation process. Their CO₂ intensity (tCO₂/GWh) has been falling rapidly over the past decade and with CCS installed they are forecasting carbon-negative operation by 2030.

However, BECCS is not without its challenges either. For one, in comparison to DAC technologies there are far more intermediate steps in the BECCS process. This includes biomass harvesting, processing to pellets and transport (Drax currently source the majority of their pellets form the USA), the emissions from which must be offset against the CO₂ captured. Careful life-cycle assessment is crucial. Moreover, even with repeated harvesting and regrowth of forests, land and water use remain an issue when considering tree growth on the scale that would be required for BECCS to play a major role in reaching our climate targets.

To Zero and Beyond!

There seems to be little doubt that reducing our CO₂ emissions to zero, and then likely going beyond this into negative emissions, will be necessary to meet the world’s climate targets. Whilst this blog has only scratched the surface of the technologies that could be deployed, by October 2022 we will have a clearer picture of which technologies are leading the way in the XPrize competition. 18 months from now the 15 most promising teams will receive $1 million each in funding to assist them in reaching the competition’s targets in time for its closure in four years’ time on 22^nd April 2025.

What we can be sure of is that rapid development of giga-tonne-scale negative emission technologies will require collaboration between experts from a wide range of fields, including IP professionals, who will be critical in gaining protection for new materials, processes, equipment and designs that emerge. This is already being reflected in the patenting rate of technology relating to CO2 capture or disposal, which has risen almost threefold from the early 2000s. Patent offices are also eager to assist inventors in obtaining protection for these technologies, for example via the UKIPOs “green channel” that offers accelerated processing of patent applications for environmentally beneficial technologies.

Watch this space, or maybe even get involved – after all, it’s only the earth’s future that’s at stake.

References

1. Excluding emissions associated with land-use changes
2. How to Avoid a Climate Disaster – Bill Gates
3. Traditional combustion, or more advanced chemical processes, may be used for energy generation from the biomass