Burning Buried Sunshine (2003)

criticaltinker 2021-08-17 21:10:55 +0000 UTC [ - ]

> to produce one litre of petrol it takes 1.29 kg of oil, of which 85% (1.1 kg) is carbon. And as only 1/10,750 of the carbon remains from the plants that were buried millions of years ago, our one litre of petrol is the result of 1.1 x 10,750 = 11,825 kg of carbon from ancient plants. Finally, as plants are approximately half carbon, that means that 23.65 tonnes of plants were required to make just one litre of the petrol available at your local station

Wow, and this is a lower bound because it doesn't even consider manufacturing and distribution.

My cognitive dissonance at the gas station is approaching an all-time high.

dredmorbius 2021-08-18 05:34:55 +0000 UTC [ - ]

I very strongly recommend reading the underlying article itself. I've submitted that numerous times to HN, unfortunately PDFs seem not to have much uptake, in part because many readers arrive via mobile devices.

https://www-legacy.dge.carnegiescience.edu/DGE/Dukes/Dukes_C...

I'd stumbled across this through my own research, trying to find literature with a detailed description of fossil-fuel formation. I've since found it referenced by numerous other works --- yes, one of my forms of entertainment is looking for familiar citations amongst footnotes. Smil's Energy and Civilization being one work citing Dukes.

It's not just the material conversion that's staggering, there's the time of accumulation. For petroleum, humans burn in one year a quantity which took five million years to accumulate.

version_five 2021-08-17 22:09:35 +0000 UTC [ - ]

Maybe a pedantic point: I think there is a difference between saying 24 tonnes of plants were "required" and saying 1 L represents 24 tonnes of plants. The same way 1 kg of iron represents some, I imagine incomprehensible amount of fusion. But if you want 1 kg of iron, you don't need to go out and expend that energy, any more than you need to go out and expend 24 tonnes if plants. Both are basically finite, with geological or astrophysical origins, rather than something we can make again on human timescales

Robotbeat 2021-08-17 22:12:30 +0000 UTC [ - ]

The difference being that iron is recyclable and can be re-smelted (if oxidized) without needing to be fused in stars whereas with fossil fuels we aren’t so lucky. (However, synthesizing hydrocarbons IS feasible.)

throwaway09223 2021-08-18 01:09:48 +0000 UTC [ - ]

Recycling carbon is easy, we just use plants.

Soybeans are a common way of capturing carbon from the atmosphere, using solar energy, to produce diesel.

_3u10 2021-08-17 22:54:13 +0000 UTC [ - ]

Yeah, its almost as annoying as when people talk about "using" water. The cycle time is 9 days, in 9 days it comes back down as rain, very little water ever gets "used".

yetihehe 2021-08-18 06:59:19 +0000 UTC [ - ]

Because they typically omit "easily available". There's lot's of water in the ocean, why do people die from lack of water in the middle of big body of water?

_3u10 2021-08-19 08:44:02 +0000 UTC [ - ]

Honestly, because they don’t drink it soon enough / slow enough. There’s a pretty cool study on how you can drink seawater and remain hydrated if you do it right.

I meant freshwater cycle time.

Yes there are certainly places that run out of water, I meant more places like Vancouver where the city just doesn’t build a bigger reservoir and instead bemoans an annual water shortage.

pvaldes 2021-08-18 09:35:10 +0000 UTC [ - ]

> 23.65 tonnes of plants were required to make just one litre of the petrol available at your local station

The correct number would be closer to 3.54 tonnes of plant material. The other 18.91 tonnes aren't spent. They are instead fully recycled and re-enter in the system (or go away out of the oil processing chain).

lovecg 2021-08-17 20:52:14 +0000 UTC [ - ]

A pedantic point, but hydrocarbons don’t actually “store energy”. The energy from the combustion actually comes from the oxygen! We just don’t think of it as the “fuel” since it’s so abundant.

A better intuition is this: plants, etc. used energy to create two products, oxygen (which went into the atmosphere) and hydrocarbons (buried in the ground). When we recombine them and add heat, we reconfigure some bonds and release energy in the oxygen bond. The hydrocarbons act as a “sponge” for the oxygen atoms, and the denser the sponge the more energy we can release.

If it was the oxygen buried underground we would probably call it “fuel”!

Robotbeat 2021-08-17 22:15:11 +0000 UTC [ - ]

Whether you consider the energy stored in the oxygen or the hydrocarbon doesn’t particularly matter. The energy is in them being split apart.

I read a proposal to send an aircraft to Titan and bring along oxygen as the energy source to burn with the methane in the atmosphere. (It works… sort of. In a methane and oxygen stoichiometric reaction, the oxygen is 4/5ths the mass so it’s not nearly as good of a deal as it is on Earth.)

blendergeek 2021-08-18 02:09:56 +0000 UTC [ - ]

> Whether you consider the energy stored in the oxygen or the hydrocarbon doesn’t particularly matter.

It truly doesn't matter which one you consider it to be, given that both are technically incorrect.

> The energy is in them being split apart.

This is a common misconception (likely) perpetuated by highschool chemistry textbooks. The energy is actually release by the combination of oxygen with hydrogen and carbon. Let me explain.

It takes energy to break chemical bonds. Some bonds take more energy to break, others take less. When a bond is created, an amount of energey (roughly) equal to the energy require to create the bond is released.

Here I will demonstrate the combustion of a simple hydrocarbon:

CH4 + 2 O2 -> CO2 + 2 H2O

When this reaction takes place, there is not energy "stored" in either the Carbon-Hydrogen bonds or the Oxygen-Oxygen bonds that is "given off". Rather, energy is actually required to break the bonds. It goes something like this:

CH4 + 2 O2 + (a little bit of energy) -> CO2 + 2 H2O + (a lot of energy).

Making hydrocarbons (or hydrogen from water) is an energy intensive process because breaking the bonds in CO2 and H2O requires a lot of energy.

lovecg 2021-08-18 03:47:53 +0000 UTC [ - ]

Yeah this is correct and I was oversimplifying. Though that begs the question of how that equation balances out - where does (a lot of energy) comes from?

The products on the right side sit at a lower energy level (they hold together “tighter”) overall. On the left, O2 needs less energy to break up while CH4 needs more, so in that sense the energy “comes” from O2 being easier to break apart. Conceptually I imagine these as a bunch of magnets with the two “magnets” in O2 held at a larger gap (so easier to take apart). With a jolt, things reconfigure and snap into a tighter pattern, and there’s an overall energy release.

Robotbeat 2021-08-18 03:39:07 +0000 UTC [ - ]

Yes, I didn't say the energy is stored in the bonds or is released by breaking bonds. I was referring to the fact that photosynthesis (or electrolysis) is used to separate the oxygen from the carbon/hydrogen in H2O or CO2. That's where the energy is stored (in the separation of the fuel from the oxygen).

mr_toad 2021-08-18 01:25:00 +0000 UTC [ - ]

Since Titan’s surface is mostly water ice you’d think it would be easier to use electrolysis to generate oxygen locally.

aidenn0 2021-08-17 23:04:55 +0000 UTC [ - ]

> Finally, as plants are approximately half carbon, that means that 23.65 tonnes of plants were required to make just one litre of the petrol available at your local station.

Surely plants are majority water by mass? A quick google claims 89-90% water[1]

1: https://www.toppr.com/ask/question/the-percentage-of-water-i...

wrytour 2021-08-18 01:00:32 +0000 UTC [ - ]

This was my first thought, too. I wonder if the author considered this?

aidenn0 2021-08-18 02:39:20 +0000 UTC [ - ]

Cellulose is almost 50% carbon by mass, so perhaps the dry weight is half carbon? That would mean you need to derate the authors numbers by about 75%; it's still a large input for a small output though.

User23 2021-08-18 00:10:03 +0000 UTC [ - ]

While it's basically certain as can be that anthracite is fossilized plants, on account of the leaf imprints, locations, and so on, the evidence for oil is far less conclusive. At the end of the day though it makes no difference. Whether or not the Earth's oil reserves are buried sunshine, primordial hydrocarbons with biological contamination, or both doesn't make much difference. More isn't getting made on human timescales.