Plastics
Chemicals
Aviation fuel
CO
OXCCU
Product use
H
Circular c arbon u tilisation
CO + H O
CO Capture
Figure 1 Circular carbon utilisation
green hydrogen are the feedstocks (see Figure 1 ), and this has some key advantages despite the significant requirement for green electricity. Most importantly, in utilising CO₂ as the feedstock along with green hydrogen from renewable energy, e-fuels or PtL have the potential for scale with minimal impact on land use. The fuel can be circular if the CO₂ has recently originated from the atmosphere. CO₂, which was recently in the atmosphere, is made into a fuel and then returns there as CO₂ when burned. There are two types: direct air capture (DAC) CO₂, where a machine captures the CO₂ from the air using green energy, or biogenic CO₂, where a plant captures the CO₂ from the air to make biomass. The biomass is harvested and used to make a product, producing waste CO₂ in the process. This waste ‘biogenic’ CO₂ is captured and used to make a fuel that releases the CO₂ to the atmosphere when burned. Assuming the plant can regrow fairly quickly to make more biomass, capturing more CO₂ from the atmosphere, the process achieves circularity. Ethanol production and anaerobic digestion are continuous sources of biogenic CO₂ that is destined for the atmosphere anyway, derived from crops that will regrow. Transition from fossil carbon to surface carbon If fossil CO₂ is used, the byproduct of processes that use fossil fuels (or mineral CO₂ in the case of cement), a low-carbon fuel is created. It is not circular, as carbon originally trapped underground still ends up in the atmosphere as CO₂. However,
crop-based biodiesel and SAF, produced through the hydrogenated esters and fatty acids (HEFA) process, and ethanol from corn fermentation, dominate biofuel and biochemical production today. However, their growth is severely limited by competition with food crops, land use constraints, and, in the case of ethanol, the costly requirement of additional units to convert ethanol into more valuable long-chain hydrocarbons through olefins and oligomerisation. Second-generation carbon waste-based fuels form a diverse category with a wide variety of feedstocks and conversion processes but most commonly involve a type of lignocellulosic waste (biomass waste), municipal solid waste (rubbish), or plastic waste. The processes generally entail heating the waste without oxygen via pyrolysis to convert it to a liquid or turning the waste into gas through gasification and then converting that gas into a liquid. Crop waste fermentation to ethanol is also possible but still has technical challenges despite efforts over the last 20 years. All these processes can play a role in the biofuel and biochemical landscape. However, they all suffer from the same challenges: securing, aggregating, and sorting the feedstock and ensuring the intermediate liquid or gas in the process is free of the contaminants in the feedstock. Power-to-liquids This has led to excitement around the newest option, CO₂-based fuels, chemicals, and plastics, often called power to liquids (PtL). Here, CO₂ and
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