Poo-Derived Jet Fuel Is Far From A Crap Idea
This could solve one of the most significant issues with biofuels.
Aviation is one of the most polluting industries out there, responsible for about 2–3% of global carbon emissions. It’s also one of the most difficult to decarbonise, as alternative power sources, such as hydrogen or batteries, currently have serious technological and logistical hurdles to overcome before we can realistically use them. One solution is carbon-neutral biofuels, which allow current jets to run with net-zero emissions. But even this technology has some major issues, as it dramatically impacts food supplies by using up food stocks or replacing them with inedible biomass crops. For example, even though global wheat supply slumps below demand, causing many to slip below the bread line, Europe currently burns the equivalent of 15 million loaves of bread daily with the wheat-based biofuel it mixes with its petrol. Needless to say, the current geopolitical climate makes this approach morally and practically questionable. It seems like aviation is doomed to pollute the world for years to come. But there might be an icky yet genius solution, ultra-low carbon jet fuel derived from poo! But can this really solve our flying dilemma?
This rather gross solution comes from Firefly Green Fuels in the UK. They have developed hydrothermal liquefaction technology that can turn sewage into bio-crude oil. From that, you can extract bio-kerosene, also known as aviation fuel, using fractional distillation. That’s all quite complex, so let me explain.
Hydrothermal liquefaction is a process where wet biomass is put under high pressures (10–25 MPa) and high temperatures (280°C–370°C). This causes a thermal depolymerisation process in which carbon-rich proteins within the biomass turn into hydrocarbons of varying lengths, making a bio-oil which is practically identical to crude oil. In other words, it dramatically accelerates the natural process that produced the crude oil we drill out of the ground from ancient biomatter.
But crude oil (geological or bio) is useless without refining it; that is where fractional distillation comes in. The different lengths of hydrocarbons vaporise at different temperatures, and this process uses this characteristic to syphon off the various petrochemicals one by one, enabling you to extract pure kerosene (as well as petrol, diesel and other fuels).
In theory, aviation fuel created in this way should be carbon-neutral, as the carbon they are made from originates from the atmosphere and is turned into biomass via photosynthesis, then turned into fuel. So even though it does produce carbon emissions when burned, the net effect on global carbon levels is zero. However, both hydrothermal liquefaction and fractional distillation can release carbon emissions, so this fuel isn’t 100% carbon-neutral. Nonetheless, it still produces 92% less net emissions than the equivalent fossil-derived aviation fuel we use today.
This is why poo-derived biofuels make so much sense! As they are chemically identical to our current fuel, they can be implemented incredibly quickly and cheaply. They have zero impact on the food supply, as they use food which has already been digested. This means there is no real risk to the supply of raw materials needed to make the fuel. What’s more, we are already struggling to process all the sewage mankind makes and are therefore damaging environments with raw sewage. This technology can, therefore, kill two birds with one stone, fighting climate change and halting mankind’s ever-growing poonami.
But this solution is far from perfect. There is a supply issue.
The average person produces around 145 kg of excrement per year (which is more than I was expecting). Firefly can turn this into 4–5 litres of aviation fuel. This means the entire UK population (67.33 million) can produce 303 million litres of poo-derived jet fuel annually. That might sound like a lot, but it is only 5% of the UK’s demand for aviation fuel. Simply put, the UK doesn’t have the supply necessary to entirely switch to Firefly’s ultra-low carbon fuel.
However, the UK has a higher number of high-mileage flyers. Countries which rely far less on aviation could meet a far higher percentage of their jet-fuel demands. As such, Firefly is in talks with officials in Mumbai to supply 80% of flights from their international airport using jet fuel from locally sourced sewage.
But sadly, Mumbai is an outlier here. If we converted the entire annual global supply of human sewage (all 8 billion of us), we could supply 36 billion litres of sewage-based aviation fuel annually. In 2023, the global aviation fuel demand reached 325.5 billion litres! So, even if we fully scaled and utilised Firefly’s technology, we could only supply 11% of the world’s jet fuel demand.
So, sadly, this isn’t a turn-key solution but rather a crucial small part of a broader push to make aviation sustainable. But even then, this technology on its own has the potential to save hundreds of millions of tons of carbon emissions each year, which is still a significant leap forward. But, there is also another side to this coin: reducing the fuel demand of the aviation industry. If the aviation industry is levied with a carbon tax (which isn’t an unlikely thing), then costs will go up, and demand for flights and fuel will reduce. Many ultra-fuel-efficient plane designs are also coming to market, like the weird-looking Celera 500L, which uses 8 times less fuel than an equivalent-size private jet. If such a design could be sized up to a passenger craft size, retain its fuel efficiency, and be used across the industry, it would allow Firefly to supply nearly the entire aviation industry with ultra-low-carbon fuel. So, it is at least possible for this bizarre technology to significantly impact aviation’s decarbonisation.
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