Nov-2024

Key to scalable, sustainable hydrocarbon fuels

A novel iron-based catalyst and process directly convert CO2 and green H2 from water into jet fuel range hydrocarbons in one step, reducing Capex and Opex.

Andrew Symes
OXCCU

Article Summary

To prevent global temperatures from surpassing the critical two-degree threshold, a rapid transition away from using fossil fuels must occur within the next 30 years. This has led some to assume that all refineries and petrochemical plants will have to shut down, but this is incorrect. While they need to change radically, and some may be replaced, they will not all be replaced entirely, as demand for hydrocarbons in certain sectors will remain.

Wherever possible, renewable electricity should be used directly due to efficiency, but only hydrocarbons will suffice in some sectors. The critical change will be the inputs. Refineries and petrochemical plants must rapidly move from using fossil fuels as their feedstocks to using a source of recycled carbon with green electricity, as articulated in a recently published paper in Nature (Vogt and Weckhuysen, 2024).

Continued need for hydrocarbons
Aviation will continue to need hydrocarbon fuels due to its energy density requirements. While some reduction in aviation fuel demand may be necessary, particularly where there is excessive flying – such as short flights – significant demand will remain. It is unrealistic to expect people to stop flying or demand that politicians ban it. In fact, rather than seeking to ban aviation, many want to ensure that flight is available for future generations and people in developing countries.

Flights without hydrocarbons remain a distant prospect. Electric or hydrogen planes for long-distance flights face huge challenges with safety, refuelling, and range due to energy density, and lighter-than-air flights (airships) will always be limited in terms of speed. Additionally, hydrocarbon fuels will always be needed in military operations. With the urgent need to reduce emissions, a source of sustainable hydrocarbon aviation fuel, SAF, will play a vital role in meeting these needs with far fewer emissions.

In the same way, hydrocarbons are essential for the production of plastics and chemicals. It is unrealistic to expect a total ban, especially where they are critical for medicine, healthcare, food production, transport, and electronics. Reducing the excessive use of plastic is important, particularly as many unnecessary single-use applications are causing a huge waste problem and the increasing issue of microplastics. Likewise, excessive chemical use should be stopped, as it can contaminate soil, water, and air, harming human health and biodiversity. However, some sectors will need to continue, or even increase, their use of chemicals and plastics to improve life expectancy, human prosperity, and economic growth. These must be made with fewer emissions to reduce their climate impact.

To supply these critical hydrocarbons with fewer emissions, fossil carbon, crude oil, coal, or natural gas must be replaced by a source of recycled or surface carbon and varying amounts of low carbon intensity hydrogen, such as that derived from electricity generated from renewable sources. The three options are biomass, plastic waste, or carbon dioxide (CO₂). Despite requiring the most green electricity input via green hydrogen, CO₂-based hydrocarbons are predicted to be the largest component over time due to the challenges with biomass or plastic waste as feedstocks.

Feedstock challenges
First-generation biomass feedstocks, such as oil 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 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, 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.