Sustainable Fuels Question 1

I occasionally get questions about sustainable fuels. I'm happy to answer them, but I hate losing track of answers on Facebook, so from now on my answers go on my blog!

Today's question comes from an old high school jazz band friend:

Hey Logan. There’s layers to this question. Here goes.

I know for sure you’re not electrolyzing water since methane (CH₄) has a carbon atom in it, and water (H₂O) doesn’t. That methane's carbon has to come from somewhere, so it either (A) started off as in a fossil fuel (and ended up in the atmosphere when you burned it in the rocket), or else (B) started off in the atmosphere as CO₂ and got converted into methane (and thus the burning it to make CO₂ was part of a cycle that didn’t contribute to the total amount of atmospheric carbon).

As a brief aside, electrolysis is the process of zapping things like water or methane to get hydrogen gas (H₂) and leave all the carbon behind. This is useful for folks (with which I disagree) who envision a “hydrogen economy.” To use carbon (as opposed to just hydrogen) sustainably takes more steps, but it’s worth it since carbon-based fuels like gasoline are so much more energy dense and easier to transport than hydrogen. To get the hydrogen to have the same energy in the same space requires dangerously high pressures.

The rest of this blog entry discusses how to accomplish the sustainable option of making fuel from atmospheric CO₂. In order accomplish this it's absolutely necessary that we start with concentrating carbon from the atmosphere. This can either be done with plants like grass and trees (which concentrate atmospheric carbon in the form of a solid chemicals such as cellulose) or big solar-powered gizmos that filter out atmospheric CO₂ for storage in tanks (such gizmos probably won’t ever be as land-space efficient as a plants, but they could work in the desert where plants can’t grow).

After concentrating the carbon, we then need some method of converting it into fuel. Thermochemical conversion methods such as gasification/pyrolysis and several varieties of catalysis can all use some form of solar energy to convert both biomass and bottled CO_2­ into pretty much any carbon-based fuel (be it methane, gasoline, or diesel). Biochemical conversion methods are another set of techniques that all have the disadvantage of not converting CO₂ in tanks, meaning they only work on biomass. Many biochemical methods (such as fermentation to make ethanol) are really inefficient at producing fuel and only work on things we nominally consider food (like cane sugar).

Since you asked about methane, one biochemical method I like quite a bit is anaerobic digestion. Anaerobic digestion works on practically any kind of biomass (not just food) and is much more efficient at making fuel than other biochemical methods. The process makes gaseous methane which is both more convenient than hydrogen since it’s more energy dense, but less convenient than liquids like gasoline or ethanol. Here’s an article I wrote about how Washington D.C. is using anaerobic digestion to get a portion of their power from poop: http://www.thedailybeast.com/articles/2015/10/14/america-s-capital-powered-by-poop.html

Since all these methods take carbon from the atmosphere and turn it into fuel, the burning of that fuel in a rocket is just a part of a cycle that goes plant (or tank) -> fuel -> atmosphere -> and back to plant (or tank). Fuel gathered in this way is sustainable carbon fuel, and it doesn’t contribute to the total amount of carbon in the atmosphere.