It is not that I had found a "fundamental flaw". Those eco-fuel things simply don't scale up to realistic levels, and the people who are behind it know that their small-scale experimental systems will never power the current level of aviation fuel demands.

Yes, human poo has some energy left. But it is way less than the same amount of fuel, I.e. you need several tons of poo to create a ton of fuel.

OK, lets have a look at this poo idea. Human faeces have an energy density of 8kJ/g. Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1793018/#:~:text=As%20the%20energy%20content%20per,measurements%20of%20stool%20wet%20weight.

Aviation fuel has an energy density of 43.5kJ/g. Source: https://s2.smu.edu/propulsion/Pages/energyex.htm#:~:text=The%20energy%20density%20of%20aviation,about%20820%20kg%2Fm3.

So if it was possible to get a lossless conversion of human poo to aviation fuel, you would need more than five tons of poo to create one ton of fuel.

A 747 from NY to LA burns about 60 tons of fuel. Source: https://www.quora.com/How-much-fuel-is-needed-to-fly-a-Boeing-747-from-New-York-City-to-Los-Angeles#:~:text=New%20York%20to%20LA%20would,be%2016%2C716%20gallons%20of%20fuel.

So you would need over 300 tons of shit to power that flight - if the conversion was lossless. It most likely is way worse.

Now a human produces between 125 and 500g of faeces per day. Source: https://www.healthline.com/health/do-you-lose-weight-when-you-poop#how-much-does-it-weigh

So you would have to collect the days worth of shit of way over half a million people to power this flight. And all this - again - with a lossless conversion. The reality is probably more like a 10% conversion productivity, meaning you would need ten times the poo.

I leave the question if this technology is actually sustainable to the reader.

And yes, my calculation of rapeseed oil based fuel was similarly funded in facts.