Decarbonisation Technology - November 2024 Issue

the process substantially reduces total emissions, as fewer fossil fuels need to be dug up to supply the same amount of end product. In this scenario, CO₂ is recycled, which would otherwise have been emitted. This means getting two uses out of some of the carbon that has been dug up before it ends up in the atmosphere. It improves the world’s carbon efficiency. It is progress, but not perfection. The priority is still to eliminate the 38 billion tonnes of fossil CO₂ emitted annually, and being able to use it should not be an excuse to keep infrastructure, such as coal power plants, when these can be replaced by zero-carbon alternatives. However, in sectors like cement production, it is very unlikely that all CO₂ emissions will be eliminated in the short term. In such cases, reusing a fraction of that CO₂ to reduce oil demand and overall emissions is a logical step, while working towards a future where only biogenic or DAC CO₂ is used. Hence, while there must be a shift from fossil carbon to surface carbon (biomass, DAC CO₂, and biogenic CO₂), realistically this will take time and investment, and recycling fossil CO₂ during the transition represents a step in the right direction. Ultimately, no fuel, chemical or plastic will ever be perfect in terms of emissions. Even if DAC and 100% renewable electricity are used, there are emissions embedded in the production of the equipment used, land use impacts of DAC in particular, non-CO₂ effects associated with aviation emissions due to contrails, and the potential for warming from hydrogen leakage. These will all need to be factored into a good life-cycle analysis (LCA). LCAs provide a comprehensive and accurate assessment of the total greenhouse gas (GHG) emissions associated with the fuel or product across its entire life-cycle. They cover all life-cycle stages, use consistent boundaries and reliable data, account for direct and indirect emissions, and convert all GHGs into CO₂e, enabling transparency and comparability. The source of CO₂ and H₂ will directly affect the overall environmental score. To qualify as SAF or a low-emissions chemical or plastic, emissions must be below a certain threshold. Currently, the barrier to scaling PtL is cost. While over the longer term, the more difficult constraint could be access to surface CO₂, currently there are numerous biogenic CO₂ sources to use. The bigger

challenge currently is accessing low-cost green hydrogen. The good news is that the electrolyser industry is starting to scale, and the cost of green electricity continues to fall as more and more renewable electricity is rolled out. An efficient industrial process is needed to convert CO₂ and hydrogen into hydrocarbons. We want the minimum number of steps, the lowest energy input, and the highest selectivity, so the maximum amount of carbon and hydrogen goes into the product rather than byproducts. The production of water is inevitable because both oxygens have to be taken off the CO2 and turned to water to make a deoxygenated hydrocarbon fuel. The goal is to have all the remaining hydrogen not making water, and all the carbon transformed into valuable hydrocarbon products. The good news is that if enough hydrogen is used in the 3:1 ratio, the reaction is thermodynamically favoured (negative change in Gibbs free energy), and the reactor will release heat (exothermic) rather than requiring energy input. The challenge is twofold: the kinetic stability of the CO 2 , and being able to direct the reactions that occur towards making longer deoxygenated hydrocarbon chains rather than methane, light hydrocarbon gases, or alcohols. OXCCU’s direct hydrogenation process This ‘direct hydrogenation’ of CO₂ to long-chain deoxygenated hydrocarbons in a single step is a fairly new area of research. The vast majority of Fischer-Tropsch (F-T) research over the last 100 years has focused on syngas (CO and H₂) to fuels in countries that have coal or gas and want to reduce oil imports rather than using CO₂ and H₂. Hence, the main focus of PtL processes to date has been the ‘two-step approach’. Here, CO₂ is first converted to CO via the reverse gas shift reaction, and then combined with H₂ to get to syngas, which can be used with conventional F-T catalysts (normally cobalt-based) (see Figure 2 ). The challenge is that the first reverse gas shift step is expensive from both a Capex and Opex perspective, and it does not match well with the F-T process. This is because it is an endothermic reaction that operates at 700-1,000°C, while the F-T reaction is a highly exothermic reaction that is normally kept down at 280°C. Reverse water gas shift (RWGS) requires a large energy input, which cannot be efficiently provided by the low-