With the holiday season in full swing, many people are traveling quite far to celebrate with their families. Airplanes are a great choice for passengers looking to cover large distances in a short amount of time, but releases large amounts of CO2 into the atmosphere. Aviation is responsible for “~2-3% of total emissions” globally, as it requires an extremely large amount of fuel to lift the “over 8 billion passengers” that flew in 2023 off the ground, along with all their luggage and the heavy aircrafts that held them. Naturally, environmentally-concerned individuals are interested in decreasing the emissions released from flight, but obstacles stand in their path. Since aircraft must be as small and light as possible to reduce fuel consumption, airplanes have always been designed to utilize kerosene, which is energy-dense, so existing technology and infrastructure is not compatible with alternative fuel sources.

One of the most developed alternatives to kerosene is SAF, which stands for sustainable aviation fuel, which is usually made from biomass and processed with hydrogen in a way that makes its chemical composition the same as kerosene. Biomass-derived SAF (bio-SAF) is the most affordable and developed alternative to kerosene, but its production uses a lot of land and energy, so it is not fit to entirely replace kerosene in aviation.

There are two main categories of solutions underway for decreasing the immense quantities of GHGs emitted through air travel, which include fuels that reduce carbon emissions and fuels that avoid them entirely. Low-carbon synthetic fuel utilizes syngas, which is comprised of carbon monoxide and hydrogen, so it requires sustainable sources of carbon and hydrogen. In the case of direct air carbon capture (DAC), carbon is extracted from the air using compounds that bond specifically to CO2. This technology can be implemented anywhere but requires large amounts of energy to filter enough air due to low concentrations. Similarly, point-source carbon capture also collects carbon from the air, but it does so by essentially recycling carbon from areas that release large quantities of CO2 in high concentrations, such as a power plant. This option is better than emitting twice the amount of CO2, but less clean than sequestering the collected CO2 and using a zero-carbon fuel source for aircraft. Researchers have also looked into gathering CO2 in places other than ambient air, such as oceans, which have “about 120–150 times” higher concentrations of CO2, but there are risks of technology, costs, and impact on ocean ecosystems. Hydrogen can be isolated for syngas production in many ways, such as separating oxygen and hydrogen from water by reducing hydrogen through electrolysis, which produces no GHGs, but requires energy. Using clean energy makes this a good option, but expensive. Another way to generate syngas is through biomass gasification, which uses biomass to produce syngas, which is ideal in regions where cheap biomass or waste sources are available near cheap and sustainable energy sources. The main issue with low-carbon synthetic fuel is cost, as it is “currently estimated at 3-5 times more than fossil jet fuel” in price.

The other alternative to kerosene are the options that don’t emit any carbon at all, which include hydrogen, ammonia, and electric propulsion. Engineers are currently working on planes that use hydrogen fuel, but they won’t be released until after 2035 because the territory is new: engineers must create designs for an entirely new fuel source for technology that has solely used kerosene for generations. Additionally, there is a lack of infrastructure for hydrogen fuel, so even if efficient and affordable designs were produced, there would be issues employing them. Additionally, they would be smaller and go shorter distances, prompting the investigation of ammonia as a fuel source, as it “has 49% more energy per volume than liquid hydrogen,” and could be used for longer flights. However, its toxicity and lack of infrastructure have discouraged investigation, putting necessary technology ten years behind that of hydrogen. Fully electric aircraft are being researched as well, but the heavy weight of the respective batteries only allow their utilization on small aircraft such as drones, preventing them from being widely implemented on a commercial scale.

Since aviation makes up such a significant portion of global emissions, it’s good that research is being done on alternative fuel sources, though it looks like lack of development and increased costs mean that it will take some time for them to be implemented on a significant scale. Although many of these options require further development, it’s encouraging to know that work is being done to reduce the environmental impact of air travel.

I can’t believe I hadn’t heard about electric aircraft before, or rather the lack thereof, considering how huge the transition to EVs have been. I didn’t really consider what that would look like in planes, but it makes sense that heavy batteries make it an unrealistic change. Good to hear, though, that other alternatives are under investigation, especially considering how much energy is required for flight and how common it is.

This week marked the end of the semester, meaning I finally have time to write these articles in real time! This will probably mean more recent news in science, which I’m enthusiastic about. Let me know if you have any suggestions or concepts you want to learn more about, and stay tuned to learn with me!

https://www.catf.us/2024/04/making-sense-options-decarbonize-aviation/