Above: Paving the way to CCU: carbon-dioxide-cured concrete (Photo: Solidia)
It is easy to see why the idea of turning CO2 emissions into useful products has captured imaginations. By promising a commercial opportunity in place of a waste disposal service, carbon capture and utilisation offers a more palatable alternative to the ‘carbon capture and storage (CCS)’ concept of pumping the greenhouse gas deep underground. CCU or ‘CO2 conversion’ has ridden high on the more recent wave of renewed interest in carbon capture, perhaps most clearly evidenced by the fact the technologies have now been collectively rebranded as ‘CCUS’. Often linked to popular sustainability concepts like ‘the circular economy’, CCU has notably grown in importance in the research funding programmes and climate strategies of the USA, Europe, China and Japan. At the same time, the challenge of finding an economically feasible, large-scale application for carbon dioxide has been widely cast as a kind of ‘grand challenge’ for society, generating various high-profile, competition-based funding schemes such as the Carbon XPrize.
However, enthusiasm in the scientific and engineering community for CCU is considerably more muted, and the approach has even been decried by some CCS-advocates as a dangerous distraction from the real business of mitigating CO2 emissions. As early as 2005, an IPCC special report on CCS concluded that CO2 conversion technologies would not significantly contribute to climate change mitigation, due to their limited scale, high energy demand, and generally short-term nature of the carbon storage. These fundamental limitations have remained at the heart of CCU scepticism, even as the technology’s profile has risen, along with estimates of the potential market for CO2; some figures suggest CCU could theoretically account for up to around 5 Gt in CO2 demand.
Aside from its direct use in applications like enhanced oil recovery, CO2 is already chemically converted into a number of products, of which urea – used as a fertiliser – is by far the largest, consuming around 120 Mt of the greenhouse gas per year. More recent commercial uses include polycarbonate plastics and CO2-cured concrete. However, there is no dispute that very few CO2-based products offer long-term storage of carbon. Most chemical applications re-release CO2 upon use, while even plastics will break down after a few decades. Alternative transport fuels, such as methanol, offer by far the largest potential market for CO2-based products, but they also represent the most obviously temporary carbon store. Unless these applications make use of CO2 originally removed from the atmosphere, either by direct air capture or via a biofuel, their climate benefit relies on a ‘substitution effect’ – that is, the reduction in fossil fuel use elsewhere in the economy as non-CCU products are replaced.
Unsurprisingly, this effect is very difficult to assess, requiring a full life cycle analysis and identification of a ‘counterfactual’ scenario in which the CCU product is not used. As noted by the IPCC, the enormous energy demand for most CCU applications creates a challenging hurdle for many technologies when assessed in these terms. This is particularly associated with the common need to react the CO2 with hydrogen, which must itself be obtained via a green method such as electrolysis using renewable energy. The IEA has illustrated the scale of this demand by estimating that the power required to replace the world’s primary chemical demand with CO2-derived alternatives would be of the same order of magnitude as current global electricity consumption.
Regardless of questions of scale, while clean energy can still be usefully used to decarbonise a fossil-based grid, this alternative generally significantly outweighs the savings of the CCU scenario. Consequently, CCU concepts usually call on the substantial ‘excess’ clean power that could arise in a wind and solar rich future, but this has also raised concerns: if there is a new demand for that power to create fuels, then it will no longer truly be surplus or ‘free’.
Some proponents of a role for CCU in decarbonisation highlight the future, less controversial role it can play in CO2 removal – using CO2 from the air or biomass to create hard-to-replace hydrocarbons like aviation fuel. In the meantime, using industrial emissions could act as a kind of technological bridge to this fully decarbonised world, even if the mitigation is less effective. Critics point to the limited time remaining for such transition solutions, and the fact that by that stage, CCU technologies will be competing with potentially lower cost alternatives such as biofuels or ‘negative emissions’ (through geological storage of atmospheric carbon) used to offset conventional fuel production.
Although CCU is a rising star in national policy, it is still usually cast as something of a complementary extra to CCS, rather than a revolution for the chemical industry. Its ability to create a commercial value for CO2 may help kickstart medium-sized CO2 capture facilities and help develop technologies, while the appeal of stimulating new industries has made it a popular target of funding. The idea that consumers will not balk at paying a premium for CO2-derived products – particularly as part of high-value items like cars – is also presented as a powerful means of placing a high proxy value on carbon.
However, quantifying and allocating the mitigation effect of CCU technologies for the purposes of emissions accounting is a complex task with no universally agreed solution, and their role has yet to be recognised by most carbon pricing schemes. Until greater consensus develops on the true value of CO2 conversion, we should not expect to see a surge in products made from pollution.
