Thoughts on the combustion institutes online seminar on SAF: Electrofuels as aviation's green bridge to hydrogen

I thank the combustion institute for organising a talk on Sustainable Aviation Fuels (SAF) discussing in detail their composition and their climate impact. 

Nearly 300 million tonnes of fuel is used by airlines which is a small percentage of nearly 4 Billion tonnes of global annual fuel consumption by the entire transportation sector. 

Although the exact figures are hard to obtain, some estimates suggest around 210 Billion tonnes(.14 tonne per barrel x1.5 Trillion barrels) of oil are recoverable as per our current capacity [1]. At the current rate of use all available oil will be exhausted in about 50 years if no new discovery is made. 

But new discoveries are continuously made & it is possible more discoveries are made that would add to the list of about 100 or so countries that have proven oil reserves or are already extracting. 

It is quite reasonable to assume that easily recoverable oil has already been tapped and further extraction is going to be more difficult. Moreover the annual global consumption of oil is so large that we will need to essentially discover oil fields that could match the total oil reserves in libya every 1.5 years just to keep up with the demand. To put that into perspective Libiya is the highest oil producer in Africa. 

Maybe that's possible.But it would not be such a bad idea to start thinking about alternatives. 

Hydrogen is the most obvious answer. But as a fuel it suffers from 2 problems: one is the low volumetric energy density which is solvable through compression. But specific to airlines the bigger challenge is to store this pressurised fuel effectively in tanks which requires significant modification to airplanes plus tweaks to the engine itself. Doable but costly and risky.

Amonnia alleviates some of the density problems. At high altitudes production of NOx is not as big of a concern because these products are dissociated by UV from the sun. But NH3 too comes with its own problems with engine and tank redesign besides its toxicity which introduces additional restrictions. 

So the current research is focussing on drop in replacements for jet fuels(essentially kerosene). Sustainable Aviation Fuels is an attempt by airlines to tackle this immense challenge of somehow finding green alternative fuel that can serve as a stop gap measure until aircraft redesigns are feasible. 

One approach is via the HEFA (Hydroprocessed Esters and Fatty Acids) process. It works but it requires vegetable oil as feedstock. The annual combined production of vegetable oil is 300 Million tonnes today. So airlines alone will consume all of it. It's possible to obtain fuels from biomass waste (crop residues etc which alone is around 800 Million Tonnes) and that does satisfy airline requirements but still falls short of the amount needed for the entire transportation sector in addition to consuming valuable lignocellulose feedstock that is critical for sustainability in the materials industry. 

Electrofuels are far more attractive as compared to biofuels

At .043% Total mass of CO2 is 2.21 × 10^15 kg (out of total 5.15 × 10^18 kg of air) . Today direct air capture technologies can extract .05kg of CO2 from air per kg of amine grafted activated carbon sorbent through chemisorption. It is costly but it can be a lot cheaper[2,3]. This, when combined,with the Fischer-Tropsch process can produce synthetic kerosene. Although it's got much less energy than hydrogen the entire production process can be around 60% efficient with combined heat and power recovery because the FT process is exothermic (hydrogen burns to form water and long hydrocarbon molecules). Even with its lower energy value synthetic kerosene is still order of magnitude better than any battery in the market today and it's highly unlikely that any innovation in the future would be able to match or even come close to the energy density of fuels through electric energy storage route. 

As a raw fuel nothing beats hydrogen & in the long run there is no doubt that it's far better to use hydrogen directly as a fuel. But for that to happen it might be useful to rethink how we make our engines. Fuel agnostic engines like Stirling engines could have far greater impact than they had in the past. 

FT has many important applications in the materials industry especially as it could be used in the production of important compounds like elastomers and adhesives. And this is where the expertise of the oil and gas industry is going to come in handy. Oil and gas is not just an energy supplier. It is also critically important for materials. Through greener feedstocks most of the problems with the industry simply disappear. 

Ofcourse without green energy it's not possible to make green fuels. It's estimated that about .3% of total land mass would be needed to cover all of our current energy needs. That is about the size of Spain. The dry desert of the Sahara occupies 8% of the land mass. The challenge is not the availability of land to make the transition happen. The challenge is to build infrastructure on a monumental scale. To find people who can deploy and manage that infrastructure. It will take a collective effort to make this work. 

REFERENCES 

1 List of countries by proven oil reserves

https://en.wikipedia.org/wiki/List_of_countries_by_proven_oil_reserves

2 Achieving net zero: Why costs of direct air capture need to drop for large-scale adoption

https://www.weforum.org/stories/2023/08/how-to-get-direct-air-capture-under-150-per-ton-to-meet-net-zero-goals/

3 Considering technology characteristics to project future costs of direct air capture

https://www.cell.com/joule/fulltext/S2542-4351(24)00060-6

Akshat Jiwan Sharma

Strategy Consultant--Innovation/ Materials science/International relations/Telecommunications/Digital Transformation/Partnerships 

Mobile/whatsapp:+919654119771 

email:getellobed@gmail.com