Give me a bio break: why biofuels have a limited future in aviation’s clean flight journey  - ZeroAvia

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    March 21, 2024

    Give me a bio break: why biofuels have a limited future in aviation’s clean flight journey 

    Contrails can be beautiful, finely traced lines of pure white against a blue sky. They are also the most visible manifestation of the climatic impact of aviation. Those long thin white scars in the atmosphere serve as a daily reminder that the era of zero-carbon aviation is well overdue. 


    Kerosene-fuelled combustion engines in jet and turboprop aircraft leave an unpleasant chemical cocktail in their wake containing carbon dioxide, sulphur oxide and nitrogen oxide, unburned fuel, particulates, and small particles of metal. Add to that the fact that combustion at altitude has a greater atmospheric warming effect than at sea level. 


    In 2019, just before the Covid pandemic began, there were more than 4.5 billion annual passenger air journeys1 – equivalent to more than half of the world’s population getting on a plane once a year. Aviation has transformed humanity. But it has done so at a price we are only now beginning to recognise. Global greenhouse gas emissions from aviation have quadrupled since the mid-1960s when the era of mass jet travel began2 and at current growth rates could reach three times today’s levels by mid-century3. 


    How we fly must change, and quickly. Policymakers have begun to respond to that challenge. However, for the most part, their preferred approaches rely on a global pivot to biofuels that is unrealistic and unachievable. Many believe that sustainable aviation fuel (SAF) is the ideal drop-in replacement for kerosene-based aviation fuels. SAF can be blended with jet fuel to reduce overall emissions, much as biofuel is blended with gasoline to create the E5 and E10 petrol sold to motorists. Under the EU’s ReFuelEU Aviation proposal4, aviation fuel suppliers would be required to supply aircraft operators with blended fuel containing 2% SAF from 2025 with blending to rise to 70% SAF by 2050. Similar initiatives are underway or under discussion in other countries and regions. 


    Does SAF ease the path towards a zero-carbon future for aviation? It is certainly crucial on the journey, but there are significant ecological and economic obstacles that seem to have been overlooked in the rush to adopt SAF as the saviour of the sector. We want to address these not to counter SAF adoption – which is the only certified methodology for reducing flight emissions today – but rather that we don’t rest on our laurels and accept SAF as the panacea.  


    Firstly, the environmental improvements are important, but not as far reaching as is ultimately needed. According to Fly Zero, if biofuel SAF were powering the full global fleet, the climate effect of CO2, NOx, water vapor emissions and fuel production would be around three times higher and when compared to hydrogen used in fuel cells.5 


    The biofuel equation seems simple. Carbon dioxide is absorbed by the plant as it grows and then emitted when fuel derived from the plant is burned. There is a natural balance between input and output. But, often, it isn’t that simple. Energy crops that involve change in land use can have harmful ecological consequences (including deforestation and wetlands loss) with net carbon impact outcomes that are worse than the continued use of conventional jet fuel6. One study of current aviation decarbonisation strategies estimated that SAF could consume 30% of total global sustainably available biomass in 20507. In a warmer world with increasingly scarce water resources, energy crops will also compete with food crops. The UK Government’s Jet Zero Strategy8 restricts production incentives to SAF produced from wastes or other non-food sources. But many other countries may be less discerning. The history of cash crop adoption, from coffee to soy, is far from encouraging. 


    Waste cooking oils, crop waste and food waste are also an important SAF feedstock. Making good use of waste is desirable and sustainable. Organic waste dumped in landfill breaks down to produce methane that is released into the atmosphere. It is far better to utilise those wastes for SAF production. But the challenge is scale. Only around 17% of municipal solid waste across the EU is diverted into anaerobic digestion plants9. The EU is a world leader in municipal recycling capacity; much of the rest of the world is a great deal worse. 


    It is well understood that synthetic kerosene (or PtL) will involve hyrogen as a key ingredient, but it is important to note that hydrogen is also essential to the majority of approved biofuel production methodologies for aviation (and using cleaner forms of hydrogen has a big impact on the “well-to-wake” emissions savings that biofuels can offer aircraft). It begs the question of the efficiency of this process as and when hydrogen can be used as a direct fuel instead. At the least, it should impress upon policymakers further the need to scale up drastically green hydrogen production for aviation.  


    We also need to look at the economics of biofuel. Aviation is not the only sector turning to biofuels as the solution. Several industries reliant on middle distillate fuels will be competing with airlines for scarce biofuel capacity. Annual marine diesel volumes are much greater than jet fuel10, and the shipping industry faces very similar challenges to aviation in identifying viable zero-carbon alternatives. Biodiesel for certain types of heavy goods vehicles will be another competing market destination, as will other hard-to-decarbonise applications such as heating oil in rural locations. 


    The net effect is that biofuel SAF volumes will struggle to get anywhere above a fraction of aviation sector needs, and a significant proportion of what’s available will be far less sustainable and emissions-reductive than it appears. This is why many predictions see majority of SAF in 2050 coming from Power-to-Liquid (PtL) SAF, also known as eKerosene. Here, we have theoretical feedstocks in abundance, but also an inefficient process with crushing economics (which we will cover in our next blog post).  


    That’s the bad news. The good news is that UK and EU policymakers’ ambitions can be achieved – potentially swiftly – by focusing on a different kind of energy source altogether: hydrogen. 


    Aviation hydrogen fuel cell technology11 is advancing at remarkable speed. Hydrogen-electric powertrains do not emit greenhouse gases or other pollutants (the main discharge is water), involve far fewer moving parts than a jet turbine and offer much greater energy density than lithium-ion batteries. They are truly zero-carbon across the whole energy chain when fuelled with ‘green’ hydrogen produced using renewable energy. 


    This is not a scientific theory. It is an engineering fact, today. At ZeroAvia, we are building a hydrogen-electric powertrain for 40 to 80-seater regional turboprop aircraft with a range of up to 1,800 kilometres – and we expect first deployment just three years from now. That maximum range is more than sufficient for almost three-quarters of all flights within Europe in 202212, and there are similarly sizeable short-haul and mid-haul regional markets in the US and Asia. 


    The technology is proven. The economics are compelling. The sustainability advantages are unambiguous including far lower noise pollution. Two things now need to follow. First, infrastructure investment by airport operators and related parties. Second, action by policymakers. Creating hydrogen energy systems – for aviation, and for other sectors for whom H2 is the future will require smart proactive policy decisions. 


    For all the reasons listed above, SAF may play a role, but it will be neither panacea nor long-term solution for aviation’s climate impact reduction. This is the dawn of the new hydrogen era, and energies need to be directed here.