SUCK it up. At a vast scale. That’s the challenge laid down by the IPCC if we’re going to limit global warming. Whichever of its climate pathways we take, we need to draw increasing volumes of CO2 directly from the atmosphere.
“Firstly, hard-to-abate greenhouse gas emissions will have to be balanced with removals in order to achieve net zero CO2 emissions in less than 30 years,” Artur Runge-Metzger, former director at the European Commission’s climate policy unit wrote in a study published last year by Oxford University on the state of carbon dioxide removal. “Secondly, from then onwards, vast amounts of CO2 will have to be captured from the air for many decades, cleaning up the atmosphere and returning atmospheric CO2 to climate-safe levels.”
Depending on the scenario, we’re looking at an 8bn t/y gap that needs to be bridged by 2050. Currently, we’re removing around 2bn t/y – the majority coming from natural methods of carbon absorption. More trees will be planted but technologies, and therefore engineers, are going to have to make a significant contribution. Currently, novel-engineered methods such as direct air capture (DAC), enhanced rock weathering, and biochar are estimated to contribute just 2m t/y of this removal.
DAC is among the most developed of these methods with Climeworks starting commercial operations of the largest facility in May (see p28). There are chiefly two DAC methods: a liquid-based capture process and a solid one.
The liquid one (L-DAC) involves two closed chemical loops. The first brings air into contact with an aqueous basic solution to capture CO2. The second loop then releases it from solution by heating at temperatures between 300°C and 900°C.
The solid method (S-DAC) uses solid sorbents at ambient temperature and pressure. A temperature-vacuum swing process is used to release the CO2 at low pressure and temperatures between 80°C and 100°C.
Data from the International Energy Agency (IEA) updated in April reports 27 DAC plants commissioned worldwide, capturing just 10,000 t/y in total. Climeworks’ Mammoth has since added 36,000 t/y of capture capacity. The CO2 is either being stored underground or used to produce fuels, carbonated drinks, or pumped into commercial greenhouses to fertilise vegetables.
A further 130 plants are planned but only 15 are in advanced development. If all 15 go ahead, the IEA estimates they would provide less than 5% of the 80m t/y capture capacity needed by 2030 to stay on a net zero pathway.
The US government is part-funding some of the largest. In August it announced US$1.2bn for two plants to capture and store emissions underground.
The first is the South Texas DAC Hub being built by Occidental subsidiary 1PointFive, Worley, and L-DAC process developer Carbon Engineering. It will be designed to remove up to 1m t/y. The second in Louisiana will capture more than 1m t/y using DAC technologies from both Climeworks and Heirloom.
Aside from burial, there are opportunities to manufacture products from captured carbon including methanol and plastics. Covestro currently uses CO2 from its own facilities in Europe and Asia to produce isocyanate and polycarbonate.
Meanwhile, tech startup ViridiCO2 has developed a heterogenous catalyst that could help decouple chemicals from fossil carbon by providing novel routes that use captured carbon to produce surfactants and polymer precursors. Speaking at the ChemUK trade show in May, commercial director Nick Smith noted the promise of setting up close to DAC plants.
“What we need overall for our technology to have a benefit and bring that benefit to the world is a plentiful supply of ready for use CO2.”
A key logistical challenge is the sheer amount of energy needed to operate DAC processes.
The Oxford Institute for Energy Studies (OIES) estimates that 2 TWh/y of power is needed to capture and store 1m t/y of CO2.
For the world to reach net zero by 2050, the IEA estimates that DAC needs to capture 980m t of CO2. From 2022, that means around 33m t/y of CO2 would need to be removed.
To put this into perspective, the largest hydroelectric power station in the world, China’s Three Gorges Dam, can generate 0.5 TWh of electricity per day, and has the surface water area of more than 1,080 m 2 .
The OIES said that renewable energies may be difficult to rely on to make up for this power requirement. Solar and wind cannot provide enough power alone to generate TWh, and hydropower would be limited by location and land availability.
S-DAC can operate at low to medium temperature and be powered by low-carbon sources like heat pumps and geothermal energy.
The IEA said for DAC to remain as low-carbon as possible, L-DAC could use biomethane and electrolytic hydrogen. However, L-DAC historically has been designed to use natural gas for heat and capture CO2 during combustion.
The volume of land needed for DAC depends on the type of system and energy needed.
According to the World Resources Institute, an L-DAC plant powered with natural gas and using CCS to capture 1m t/y of CO2 would require the smallest area of land at around 0.4 km 2 . While an S-DAC capturing 1m t/y powered by wind would need 66 Km 2 of land.
Though the land requirements may seem vast, DAC has the advantage of not needing to be located on arable land and not requiring the capacity of other CO2 removal (CDR) solutions including reforestation and bioenergy with carbon capture and storage (BECCS).
DAC projects around the world have enjoyed both public and private funding from the likes of petrochemical companies and tech moguls.
Most governments have lumped DAC funding in with overall CCS funding. The UK has committed £20bn to CCUS, of which DAC is considered a leading technology. Additionally, it has invested £70m specifically for DAC projects for its Direct Air Capture and Greenhouse Gas Removal programme, currently in phase 2.
On the side of private investment, petrochemical firm Occidental bought DAC startup Carbon Engineering last year for US$1.1bn, and is working with BlackRock to build STRATOS, a plant with the capacity to capture 500,000 t/y in Texas – with a total investment of US$500m.
Critics of DAC, and CCS as whole, have highlighted the issue of companies using carbon capture wholly to offset the same amount of CO2 they emit, without making any effort to reduce their emissions.
The carbon market is a form of regulation, pushed by the likes of the EU Emissions Trading Scheme (ETS), that caps the amount that companies can emit, forcing them to buy credits to cover any excess emissions.
Shell made headlines recently when it was found to have registered 5.7m tradeable credits for carbon removals that never took place.
This raised questions over the validity of the carbon market. However, experts say the solution lies in better verification practices.
Stacy Kauk, head of science at credit certification platform, Isometric, said: “The world needs CDR technologies to reach gigaton scale and that won’t happen without restoring trust in carbon markets.
“Measurement, reporting and verification is key to ensure buyers can trust the credits they’ve bought and that funds continue to flow into emerging CDR technologies.”
Editor, The Chemical Engineer
