Origins of Oil, Coal and Gas

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Short version: Growing plants use the sun’s energy and simple chemicals to make more plants, and animals “burn” the plants to get that stored energy from the sun. Almost everything that grows is burned, but in special cases some plants are buried without oxygen, escaping burning. Time and heat turn these buried plants into fossil fuels.

Friendlier but longer version: Recall that energy is the ability to do things. And, living requires doing things—fighting against randomness to put particular chemicals in particular places to make cells and cell walls, to protect oneself and reproduce.

Living things on Earth could tap into many energy sources. Heat flows out of the Earth beneath our feet, for example. But, the energy from the sun reaching the Earth’s surface exceeds that from inside the planet by more than 2000 times, so it is clear that harnessing the sun gives greater opportunities for living things. (This is also why you will never hear a weather forecaster worrying about the effects of the Earth’s heat!)

Please watch the following video:

Photosynthesis and Respiration (:51)

Click here for a video transcript of "Photosynthesis and Respiration".

DR. RICHARD ALLEY: This is the simplest version of photosynthesis and respiration that we can come up with. This is a plant. It stands in for all the green plants.

And it takes CO2 from the air, and it takes water, and it puts those together. But that requires energy from the sun to make the chemical bonds which give us plant, shown here as CH2O. They're just sort of the formula of plant. And it releases oxygen to the air.

Now, animals and bacteria and fungi and all these other things take plant, and they take oxygen, and they burn them to get energy so they can run around and do things. And that releases the CO2 and H2O. Fires also do this, but they don't do it in quite so controlled a manner. 

Source: Richard B. Alley

Photosynthesis is the process by which plants grow more of themselves, using simple chemicals and the sun’s energy to make more-complex chemicals that store energy. Respiration is the process by which animals, fungi, etc. run photosynthesis backward, “burning” plants to release the stored energy for use by the animals, and releasing simple chemicals, ready to be used by plants again.

You probably have seen the equations for photosynthesis, the process by which plants harness the sun. The simplest statement of the commonest type of photosynthesis goes something like this: water + carbon dioxide + solar energy → plants + oxygen. Or, if you prefer chemical symbols saying the same thing: H20 + CO2 + hν → CH2O + O2

(Don’t worry if you had a class sometime in which this equation was written with 6 waters plus 6 carbon dioxides making 6 oxygens plus the chemical glucose, C6H12O6; that’s the same story simplified in a slightly different way, and either way you write it is close enough for our purposes.)

Almost all of the biological activity on the planet depends on this pathway to capture the sun’s energy. When the sun isn’t shining, plants run this backward, and animals and bacteria and fungi all run it backward, combining oxygen with plants to release water, carbon dioxide, and the sun’s energy that the plants stored chemically. Fires do this, too. Depending on whether it happens in a fox or a fire, you may see this energy release called respiration, or burning, or oxidation, or combustion, or perhaps other words, but all serve to combine oxygen with plant material to release carbon dioxide, water and energy.

Learning Checkpoint

Please watch the following video:

Respiration (:39)

Click here for a video transcript of "Respiration and Conflagration".

PRESENTER: It may not be immediately obvious that this forest fire down below and this moose in Glacier National Park up above are in some sense doing the same thing biochemically, but they are. The moose-- it's called respiration. And down here it's called a fire-- conflagration. But what they're doing is taking plant, adding oxygen. And using that to cause chemical reactions that release energy, which you see is the heat and light here, or you see is the moose able to stand up, and chew, and run around and do moose things.

Source: Moose photograph by Dr. Richard B. Alley. Forest Fire: Fire Weather, National Weather Service website
Respiration is slow burning, or, a burning fire is fast respiration. Both the moose (above right, in Glacier National Park) and the fire (above left) are converting plants back to CO2 and H2O, releasing energy. The respiration in the moose clearly is more controlled than the conflagration of the forest fire. Photosynthesis is the process by which plants grow more of themselves, using simple chemicals and the sun’s energy to make more-complex chemicals that store energy. Respiration is the process by which animals, fungi, etc. run photosynthesis backward, “burning” plants to release the stored energy for use by the animals, and releasing simple chemicals, ready to be used by plants again.

Averaged around the planet and over a year, roughly 0.1% of the energy from the sun that reaches Earth is stored as chemical energy by plants. (This is called net primary production, if you want the technical term.) Clearly, plants capture more of the sun’s energy in some places and times than in others, and agricultural experts have worked hard to find ways to make plants especially productive for us in our gardens and farms, but plants are still not very efficient. Even so, the world’s plants capture about 10 times as much energy as humans use.

If plants would jump into our fuel tanks and liquefy, we would have far more energy than we needed, but things don’t work that way. And, because everything alive on Earth wants to burn plants for energy, we face large difficulties in harvesting plants and burning them for our use before something else beats us to it. Almost all plant material is burned rather quickly after it grows, sometimes being eaten by caterpillars or cows while still alive, other times by fungi or bacteria after dying.

But, it is a very large and very old world, so even a very small difference between what grows and what is burned will eventually add up to a very large store of energy. And, that is what fossil fuels are.

video iconEarth: The Operators' Manual

Video: Formation of Fossil Fuels: A 2+ minute clip on Fossil Fuels - How they are made and why they are ultimately unsustainable.

Click here for a video transcript of "Formation of Fossil Fuels".

So why are fossil fuels such a powerful, but ultimately problematic, source of energy? Conditions on the waterways of today's Louisiana help us understand how fossil fuels are made and why they're ultimately unsustainable. Oil, coal and natural gas are made from things, mostly plants, that lived and died long ago. It's taken hundreds of millions of years for nature to create enough of the special conditions that save the carbon and energy in plants to form the fossil fuels that we use.

Here's how it works... Plants, like these tiny diatoms encased in silica shells, grow in the upper layers of lakes and oceans, using the sun's energy to turn carbon dioxide and water into more plants. When they die, if they are buried where there's little oxygen to break them down, their chemical bonds retain the energy that began as sunlight. If enough carbon-rich matter is buried deeply enough, for long enough, the Earth's heat and pressure turn it into fossil fuel, concentrating the energy that once fed the growing plants.

Vary what goes into Earth's pressure cooker, and the temperature, and you end up with the different kinds of fossil fuel. Woody plants make coal. Slimy plants, algae, will give you oil, and both of 'em give rise to natural gas. The fossil fuels formed over a few hundred million years, and we're burning them over a few hundred years, and if we keep doing that, sooner or later, they must run out.

But there's a bigger problem with fossil fuels. As we've seen, they're made of carbon primarily, and when you burn them, add oxygen and that makes CO2 that goes in the air. We're reversing the process by which they formed. And if we keep doing this, it must change the composition of Earth's atmosphere. (jet engine roaring)

Credit: Earth: The Operators' Manual