Fuel from Carbon Dioxide 3/22/19

By Richard E. Bleil

Once again, I’ve read of a plan to produce liquid fuel from carbon dioxide. I love this. It’s modern alchemy.

It feels like the perfect solution to two of our major problems. It scrubs the greenhouse gas carbon dioxide from the air which should help reduce global warming. Second, it produces a new liquid fuel reducing the need for drilling or mining.

Unfortunately, it will fail on both counts.

No, this is not opinion. This is an application of the first and second laws of thermodynamics. It’s science.

Let’s start with a little background. Potential energy, as most of my readers will know, is “stored energy”. So if I hold this computer above the ground, it has the potential to fall. I have but to release it to convert that potential energy into kinetic as it hurtles to its doom. Similarly, chemicals have stored potential energy in the form of kinetic energy. The foods that we eat have stored within them energy, that our body must metabolize in order to release it into chemical energy to keep us alive.

The byproduct of organic fuels tends to be carbon dioxide. When we metabolize food, or burn fuel, we are oxidizing the organics, the end result being carbon dioxide. When my tablet smashes upon the floor, it stops moving because the floor is the lowest energy it can obtain. Similarly, food and fuel becomes carbon dioxide because it is the lowest energy it can obtain. The total energy released from oxidizing this fuel is the difference between the energy it had and the energy of the carbon dioxide, just as the energy released from the falling tablet is the difference between the energy at the height it was being held and the floor.

Pretty straightforward, don’t you think?

Now comes the first law of thermodynamics. This is the law of conservation of energy. So to create a fuel, we have to input energy to get the carbon dioxide from its current energy level to a higher energy level. If it’s not at a higher energy level, it would not be useful as a fuel. But to get it to this energy level means we have to input energy.

Now that we’ve dropped our tablet (and the energy generated has destroyed it), we can pick it back up again. We can increase the potential energy, but when we release it once again in a repeated pattern of violence against the tablet, the maximum amount of energy we can get back out of it when we drop it again would be the amount of energy we put into lifting it to the maximum height before, once again, releasing it like the psychotic anger mongers that we are. But the best we can expect is zero net energy release, as we started from the floor, and ended back at the floor. The energy we got out is no more than the energy we put in to lift the computer back off of the floor.

When we use our newly minted fuel from carbon dioxide, in releasing that energy once again, it will go back to carbon dioxide. The energy released in the net process, then, is zero at best, as we started from carbon dioxide, and ended up back at carbon dioxide. The first law of thermodynamics precludes the possibility of ever releasing more energy than we put in to the process to produce the fuel: the law of conservation of energy.

As far as carbon dioxide removal, the law of conservation of matter tells us that we will release as much carbon dioxide back into the atmosphere as we’ve removed to produce the fuel. As such, by removing carbon dioxide from the air and returning it once again, the net effect will be, at best, no change.

But, it gets worse. The second law of thermodynamics tells us that we can’t even break even. When I picked up that broken tablet that is only still working because it cannot read and understand this discourse of violence against it, the energy to do that had to come from somewhere. That energy came from ATP in my system, converted to ADP. The ATP was created in metabolic processes that, even within my own body, are not perfect. Energy is lost to keep my system warm, foods are not processed perfectly creating energy loss, and other processes have to occur for me to pick the tablet remains up beyond muscle contractions. This means that more energy is put in to lifting the computer back off of the ground than the energy that will be released when again it hurtles to the ground in this senseless act of shame.

The energy required to convert carbon dioxide to a fuel has to come from somewhere, and will most likely rely, at least in part, on the power grid that still utilizes fossil fuels that are drilled or mined. Because this process is not perfectly efficient, it will cost more energy to produce the fuel, than the energy we can actually get out. What’s more, the carbon dioxide released from the burning of the new fuel will be supplemented by the carbon dioxide created in the generation of the electricity needed to convert the carbon dioxide to fuel in the first place.

Thermodynamics classes teach a process called the “Carnot Cycle”, a hypothetical heat engine that physicist Nicolas Carnot used to demonstrate why perpetual motion machines cannot exist. This proof was mathematical in nature, and put to bed, once and for all, the possibility of creating a perpetual motion machine. In the cycle, a “perfect” engine produced enough energy to keep the cycle going indefinitely, but the proof showed that internal energy loss would eventually bleed too much energy for the process to continue forever. A cycle of producing fuel from carbon dioxide should, in principle, be able to fuel itself, with the fuel from the plant being used to run the plant. But even if none of this fuel left the plant (which at best would be of no practical value), internal energy requirements means that more energy would be needed to run the plant than the energy obtained from it. Consider the fans alone, drawing air in to the plant. The energy to keep the fans going, and the internal friction of the fans working against their operation would make this process less than perfect.

So do we give up? No, that’s not the point. We have problems. Excess carbon dioxide in the atmosphere has been demonstrated (beyond reasonable doubt) to be the cause of global warming, and further reliance on fossil fuels is only accelerating the damage already done. But, by the same token, we have to be smart about our approaches.

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