What Can We Do About Aviation-Related Warming? An Exploration Via Methanol

Over on Quora, someone asked me if methanol were a viable fuel replacement for aviation kerosene. I explored the question to assess the potential value of it.

There are several points to make. To start with, it’s entirely possible to synthesize methanol from biological or molecular sources. I assessed that as part of my deep dive on the Carbon Engineering air-to-fuel nonsense. [1] Synthesizing methanol has a long history, so there’s no magic there. Oddly, I’ve actually been in the head office of Methanex, the 800 lb gorilla in the global methanol space. They are Vancouver-based and were changing some of their automation, and I was assisting them with some strategic options.

However, there are multiple challenges with aviation, not just CO2. Let’s look at each.

The first is direct emissions of CO2 into the atmosphere by burning fossil fuels. That’s step one. Biologically or synthetically created methanol would certainly reduce that, but not eliminate it.

Chart of synthetic methanol CO2e debt from CleanTechnica Carbon Engineering Case Study by author

In the assessment, I worked up the carbon debt of methanol synthesis. The Carbon Engineering process, if they made methanol, would start with a ton of CO2 and end with 0.73 tons of methanol. Very little math tells us that each ton of synthetic methanol, given the best possible numbers to the synthesis process, is in the range of 0.27 tons CO2e as the carbon debt. That’s before distribution, which we’ll ignore.

When burnt, the CO2 that had come from the atmosphere would return to the atmosphere, so the effect of burning a ton of methanol in a plane would be a net emissions of 0.27 tons CO2, which is better.

However, the problem is that the CO2 is captured at ground level and emitted mostly at around 30,000 ft. And the effect of CO2 emitted high in the atmosphere is about double that of CO2 emitted at ground level for a variety of atmospheric mixing and chemistry reasons. So while we are taking a ton of CO2 from ground level air or flue stack, we are putting it 30,000 feet up in the air where its impact is doubled.

Basically, this takes current aircraft emissions from the equivalent of two tons CO2e to 1.27 tons CO2e. Better, but not best.

The second challenge with air travel and warming is contrails. They trap outgoing infrared more than they reflect insolation, so cause warming by themselves. Methanol power systems by themselves do nothing for contrail generation.[2] Contrail-related warming is in the range of 10% of total aircraft-related warming, so the 1.27 tons CO2e turns into about 1.77 tons CO2e.

The third problem is nitrous oxides production from burning kerosene. NOx at altitude is also a global warming forcer.[3] Burning pure methanol does not emit NOx so a methanol power system dodges this bullet.[4]

But wait, there’s more. Methanol has an energy density that’s lower than kerosene, the primary jet fuel. Kerosene runs at 830 kg/m3 while methanol is at 787 kg/m3.[5] What that means is you have to burn more methanol to get the same energy, about 5% more. So that 1.77 tons CO2e turns into about 1.86 tons CO2e.

So methanol would cut global warming from aircraft by around 50%. About 5% of total global warming forcing comes from kerosene-fueled aircraft today per studies, so we’re down to 2.5% instead.[6] That’s not enough. The target is 100%.

Luckily, contrail impacts can be reduced by some operational changes, mostly flying lower and having fewer overnight flights reduces a surprising amount of the impact. But flying lower also increases fuel consumption and no one is going to give up nighttime flying, so that’s potentially a bit of a wash.

I don’t think methanol is the answer, in other words. In my opinion, battery-electric will be the answer for all local and regional flights, and I don’t have a great answer yet for long-haul flights. Mark Z. Jacobson, when I spoke to him about the subject, thinks a hydrogen power train has to be developed. ZeroAvia is a startup that has already demonstrated a hydrogen fuel cell powered small plane.[7] It has investment backing from Bill Gates and Jeff Bezos, both of whom have tons of money but at least in Gates’ case, frequently make bad technology bets (see Carbon Engineering as a prime example).

Long-haul flights are in the category of transportation where I say “hydrogen has not yet proven itself to be a waste of time,” unlike all ground-based transportation and most energy storage solutions.[8]

Footnotes