As feasible as a cement-filled airframe
When scouting for carbon capture proposals, look for simplicity, scalability, versatility. But stay away from vanity projects created for news cycles.
When we started looking for feasible technologies for a sustainable carbon economy, we started by defining our requirements.
It has to be low-tech and scalable enough to be deployable anywhere, even in small-batch processes in non-industrial countries.
It should be at least energy-neutral, or even better, energy-producing.
It has to offer an economic opportunity even if we don't have negative carbon credits.
And most of all, it has to take unwanted inputs and turn them into wanted outputs, to create the economic incentives for many participants to want to join in.
With these goals in mind we quickly zoomed in on solid carbon and carbon sludge technologies: pyrolysis, torrefaction, leaching — hydrochar first and foremost among them.
Sadly, there are still too many technologies that sound good in press releases, but fail some or all of these criteria. “Something with carbon and climate” is the new “something with startups and innovation”. Too many poorly thought-out technologies competing for news cycles. Case in point: the cement-to-aviation fuel hype cycle.
Of course it sounds great on paper. Take the CO2 that is produced during cement production and turn it into aviation fuel with a little hydrogen from electrolysis. The fuel is now renewable and replaces fossil fuel in this way, making flying environmentally friendly. Certain to get lots of social media engagement and lifestyle blog writeups.
Sadly also too good to be true.
The catch?
We don't have the hydrogen in the quantities needed to convert this low-energy substance CO2 into jet fuel.
Then we'd have to extract the CO2 from the exhaust gases — a very complex process —, or use pure oxygen in advance (oxy-fuel combustion), to get the purest possible CO2 together with the carbon burning in the cement kiln. Producing this pure oxygen is also quite energy-intensive and makes the whole process infeasible. To top it all off, you have to invest heavily in air splitting plants, electrolysis plants and wind turbines to make the whole process viable.
Any alternatives?
Hydrothermal carbonization offers two alternative processes.
If you use hydrochar — biomass turned into brown coal-like carbon via hydrothermal carbonization — as fuel for the cement plant, then you can already work almost carbon neutral. Some residual CO2 from the limestone remains, which has to be compensated. But the main part of the CO2 produced by hydrothermal carbonization to make the cement clinker is already climate-neutral.
And now for jet fuel.
Producing jet fuel from CO2 is extremely inefficient. Just burn the hydrogen directly and save on the fuel production plants. (See also our newsletter on the hydrogen economy.) It is smarter to use hydrothermal carbonization here as well.
A short, very hot hydrothermal reaction can turn biomass into oil with good yields. This oil can be converted into biofuel via hydrogenation with renewable hydrogen. The fuel can be distilled, and then be used like diesel or jet fuel.
This skips the cumbersome and energy-intensive conversion of CO2 into more energetically valuable feedstock, which can be converted into this oil with hydrothermal carbonization. Because nature does that for us, and has done so for eons. Simply with sunlight via photosynthesis.
And best of all, we can also take waste biomass that's hard to discard otherwise . The hydrogen can be produced either from splitting biogas, from hydrochar and water or by electrolysis. Since the starting material, the oil is not too low in energy like CO2, however, you need much less valuable hydrogen.
Much simpler and more energy-efficient.
Please stop trying to produce energetically valuable substances from CO2. The only way to do this via an inverted methanol fuel cell. Using CO2 from cement plants and hydrogen to produce jet fuel is pointless.
Better use the hydrogen as fuel; stop toying with CO2 based processes, and start thinking about solid carbon and carbon sludge.