Eli5: how am I producing more than double the tons of carbon emissions than the technical weight of the fuel burned during a flight?

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I recently flew from Frankfurt to Cape Town and used an online calculator to see what my carbon emissions were in order to offset them.

I went through a few websites and the range of CO2 emissions results was anything from 0.6tons all the way to 2.2t for one person for a one way trip.

This spread annoyed me, so I did my own basic research, and found out that the plane I flew on (Airbus A340-300) has 335 seats, fuel burn of 6.5t per hour, and a flight time of 12 hours. So a rough total of 78t fuel burn for the flight, and therefore per seat 0.232t.

Now assuming the flight was full (335 seats) and everyone wanted to offset their emissions and used one of these calculators, even the lowest website result (0.6t -ICAO) would mean that the CO2 emissions for everyone on the flight were 201t. On the other end of the scale using the 2.2t from Atmosfair, it would be a whopping 737t!

But… the entire fuel burn is only 78t for the flight, and the max fuel the A340-300 can carry is 148t?!

This is obviously a very basic calculation and I have no understanding of either travel carbon emissions or science really, but I just don’t get what I’m missing in my understanding and how the weight of CO2 emissions seems to double compared to the fuel used?

In: Earth Science

You’re missing the oxygen. The airplane doesn’t carry the oxygen with it, it gets it from the atmosphere as it flies.

The weight of fuel is (almost) all hydrogen and carbon. That’s combined with oxygen from the atmosphere inside the engine to become water (H20) and carbon dioxide (CO2). So for every two hydrogen in the fuel (atomic weigh 2) and carbon (atomic weight 12) you pick up 3 oxygens from the atmosphere ( atomic weight 48).

So weight of fuel = ~14, weight of oxygen = 48, the weight of emissions is about 3x the weight of the fuel. We generally ignore the water fraction but the CO2 is still considerably heavier than the fuel by itself.

Edit:typo

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The only time the products of a chemical reaction have exactly the same mass as the components is if it’s completely self-contained. For example, if I mix baking soda and vinegar, if I could measure the mass of all the carbon dioxide produced and add it to the remaining stuff, I’d get the same total mass.

Burning fuel is not generally that kind of reaction. The fuel can’t really burn by itself, it has to be mixed with air, so that air is part of the total mass produced by the reaction. That’s how carbon dioxide gets produced: the carbon mostly comes from the fuel and the oxygen comes from the air.

So you’re just accounting for part of the reaction!

It may account for energy and fuel that is needed by ground services, control towers, etc. I am not sure for exact numbers but someone needs to pump that fuel, repair and maintain the plane, etc.
So a total of all things needed to make that flight and not only emissions from the flight per se.

CO2 is made up of carbon (C) and 2 oxygen (O).
Fossil fuels are basically long chains of Carbon with hydrogen (H).

When fuel is burned de hydrogen is exchanged for oxygen from the air. Oxygen is much heavier than hydrogen.
So all the carbon molecules in fuel add up to was more weight in CO2 after its burned.