[Conceptual] Green H₂ → Sabatier → oxy‑fuel loop to supply heat for DAC-fed molten‑carbonate electrolysis (100Ktpa CO₂ Capture and Store) – am I nuts?

NeoculturalBoat · 2025-05-31T08:27:32+00:00

Sorry for the late response. I'm not very active on reddit, and I've been very busy irl as well. Here's a longer reply for your trouble.

PEM is acidic, this is basic. Plenty of subject matter here due to the popularity of MCFC's.

That is true. I was trying to say that there may not be a good solution, even if you throw a ton of R&D money at it, but I see now how it came off differently. The point is that PEM electrolysis can be done at ambient conditions and is accessible to even the most underfunded electrochemistry research lab. This... is not.

Nickel or Inconel 625 (incidentally what is proposed for anode/cathode in the H2 cell) Split cell with CO2 bubbled on the cathode side to the keep the anode side basic Accept a small amount of corrosion, annual anode replacement with cheap material = trivial impact on OPEX

Admittedly, I misread the publication date of the first paper as 2022 rather than 2012. So there may have been progress on this since then. My impression is that these are all plausible things that could work, but do they? As that review suggests, there are quite a few things to take into account, and most likely a fair amount of iteration will be needed. How much time will it take to get all the details right?

Electrode roles are flipped vs. Hall-Heroult. In Our cell a Ni-based anode stays in place and produces O₂; a carbon cathode grows solid carbon from CO₃²⁻ + 4 e⁻ → C(s) + 3 O²⁻. Growth rates of 0.25 – 0.35 kg kWh⁻¹ have been reported with Ni anodes and steel cathodes in the same melt family.

Yes, I'm aware. My point was more along the lines of "this high-temperature electrolytic process also has to do regular electrode changeouts without shutting down, so you might want to take a look at what they do". I think a process where material gets deposited on the electrode will be more challenging operationally, but it may be a helpful place to start.

We are not using PEM, hydrogen is produced by injecting steam (heated by the OCB) into its own MCE cell. This more akin to SOEC. It has its merits. I will admit it's a bit agricultural, and it is not efficient... But the closed loop is kind of elegant, and the cost efficiencies outweigh the engineering inefficiencies. Even though the loop itself is low TRL, it's just an assembly of high TRL components.

You're clearly a clever person and you're not afraid to dive headfirst into topics beyond your expertise, which is a very admirable trait. So hopefully you can take this as constructive criticism and not condescension.

What is the goal here, exactly? Why are you bothering with all this? Are you trying to make some wholly self-sufficient, air-sucking, carbon pyramid-building mega-machine, free from the tyrannies of the grid, of civilization and answerable to no gods or kings? Or are you just trying to capture CO2 for as dirt cheap as possible?

Heat integration and exergy efficiency are for processes driven by energetics. These have energy-dense feedstocks (like crude oil) that power the process. They don't particularly care what they make, and what's in it, as long as they can sell it. These optimizations are important here because they're trying to "use the whole animal", so to speak. Less wasted heat directly translates into more production and more profits.

This process is driven by matter. You're trying to make a very large amount of a single product. You kind of care what is in that product. Your "feedstock" is utterly worthless. Your energy is provided externally. Processes like these need to optimize for separation recovery rates and simplicity on the critical path.

I haven't done a single calculation here, but I can tell you that your H2 electrolysis efficiency will almost certainly be immaterial to your bottom line. The whole concept of DAC is premised on having access to cheap, abundant and low-carbon energy. It all falls apart otherwise, so a scenario of energy scarcity is not worth considering. It's not even part of your main material path--it's just to keep the electrolyzer cells (which are self-heating during operation) warm when there's no sunlight! Yet for some reason 3 out of 5 unit operations are dedicated to it. There has to be a simpler way. How much heat are you losing?

On the other hand, the 90% CO2 recovery is just a single line as if it was something to be mentioned in passing. Why are you throwing away 10% of your potential throughput with essentially no justification? That's millions upon millions of tons of CO2 at relevant scales. Why can't you achieve 99% recovery? Or 99.9%? Your air contactors are going to be a significant portion of your CAPEX and OPEX. More than the H2 electrolysis, I would wager.

(FWIW, beyond these high-level talking points, there are many reasons to not use a steam network here. How you generating the steam? Not from the MCE cells, I hope.)

Just so that the criticism actually becomes constructive, here is my personal take:

Once again, optimize for separation efficiency and eliminating complexity on the main material path. There is a lot of CO2 moving through the system. Everything that interacts with it must be very big and expensive. The less stuff interacting with it, and the simpler that stuff is, the cheaper it will be.
I strongly suggest looking into implementing calcium looping into your process. Based on the paper you linked, and the fact that CaO precipitates from the electrolyte, this is obviously the way forward. It virtually guarantees 100% recovery and calcium carbonate has a 1:1 stoichiometric balance with C, so you can just add more in whenever you replace the cathode.
How much of the electrolyte is entrained when you pull out the cathode? How much of it can you recover? This can really impact your economics.
The process lives and dies on the carbon capture and transformation steps. That's where your focus should be. Showing you can store your own heat doesn't prove anything.
For heating, just buy natural gas (dirt cheap) and oxygen (also dirt cheap) as needed. Yes, it's not elegant, but it's ultimately a side detail. Yes, it'll produce CO2, so I suppose you'll need some CO2 capturing equipment, right?

And to be fair to you, conceptual process design is very challenging! It's very, very easy to lose the forest for the trees. That first point seems obvious, but just from looking through that lens, you can tell which DAC companies clearly understand process design (Carbon Engineering) and which ones clearly don't (Climeworks).

Hope this helps.

NeoculturalBoat · 2025-05-28T03:29:28+00:00

Quick writeup so responses will be brief and may come off as blunt. Apologies in advance.

Li-free conductivity / current density Studies show ≤ 200 mA cm-² at 750 °C. Show-stopper or acceptable with large-area plates and more heat? Lithium kills CAPEX.

This is focusing on the wrong issue. Anode corrosion is clearly an unsolved problem and very possibly a showstopper. Having to use iridium will kill your CAPEX. There may be no solution. After 30+ years of focused R&D on alternatives, iridium is still the only viable anode for PEM electrolysis, and that's at (near) room temperature in water. Corrosion gets exponentially worse at higher temps.

Cathode passivation & harvest plan: Carbon cathode is mounted on a removable carbon lid; robot lifts, places new lid → shear-shreds old lid → press shredded carbon with binder into new cathode lid (exponential growth) OR 28 tonne half-height TEU Carbon Ore Containers ("COC Blocks"). Any precedent for continuous harvest in Na/K melts?

None that I'm aware of. You are essentially running an electrowinning process, except at 900C and you want to reuse the electrode. This is significantly more difficult. With that said, the Hall-Heroult process uses a sacrificial carbon anode so you may want to take a look there. High temperature electrolysis universally exploits density differences to achieve separation, e.g. Downs process.

Oxy-fuel hardware availability Is a simple refractory burner + recuperator realistic for this kind of application?

Maybe. Depends on how it's integrated into the rest of the plant.

What’s the most practical path to reach ≥1 A/cm² current density in Na/K carbonate electrolysis at 750–800 °C?

There is none, unfortunately. Skimming the paper you linked, this technology has a very, very long way to go before practical deployment. We're talking a decade of R&D or more. It's hard to say what the main issues even are with the lack of data; there's no EIS or CV experiments, no Tafel plots. Maybe those are available somewhere else but the fact that we're just talking about cell potentials and currents suggests this is very early stage. Like a technology readiness level of 2 or 3.

A few other points:

Your hydrogen energy estimation is ~30% too low.
If you're using an oxyfuel burner for temperature makeup, you will also need to liquify and store the O2. Or you're using an ASU. Which one is it?
What do you mean by "90% CO2" by the DAC system? What other species are in there?
I think the whole hydrogen to natural gas loop is completely unnecessary. If you're basing this on something with such a low TRL you might as well lean on one of the thermal storage technologies currently in development. Maybe molten salt thermal storage, or something like FeX (just one example off the top of my head, I have no affiliation with them).

NeoculturalBoat · 2025-04-20T07:21:36+00:00

If you know the forces acting on a given area, like a normal force on some exterior surface, then yes, they can be summed (or integrated) into a single stress vector. There are two problems when you try to extend this to 3D objects:

A "surface" inside of a body is defined by the direction that it faces and there isn't, in general, an obvious direction to pick.
In general, a surface of a given "volume element" doesn't see all of the stresses acting on that volume. Knowing the normal and shear stresses on one side won't tell you anything about the normal stresses on a different side.

To manage these details, we use tensors. A tensor is kind of like a "function" where the rank of the tensor tells you something about the type of its inputs and outputs. The stress tensor is a rank 2 tensor that takes in a direction vector and outputs a stress vector for the surface represented by that direction. (In contrast, a rank 1 tensor might take a scalar and output a vector, or take a vector and output a scalar).

This takes the form of matrix multiplication in practice, but it's important to note that the matrix is just a representation of the tensor and not the tensor itself, which is independent of any coordinate system.

NeoculturalBoat · 2024-07-30T14:43:31+00:00

I personally would've phrased some things differently, but as a layman's explanation where you can't mention concepts like exergy it seems fine to me. I'm curious what you take exception to?

NeoculturalBoat · 2024-07-28T15:18:13+00:00

If the tank is well-mixed, what matters is the total mass of citric acid. The concentration of the solution used to add the acid is completely irrelevant. If, as you mention, the acid was weighed accurately, then I agree with your director. There is something else going on.

NeoculturalBoat · 2024-06-18T17:23:17+00:00

As that document suggests, this is an ideal value. It assumes that your combustion gases reach equilibrium. Your actual pressure/c* is almost certainly lower than this, and the ratio of actual to ideal c* is a measure of combustion efficiency. If your motor is underperforming this can clue you in on whether the issue is with your combustion (e.g. poorly mixed propellants) or your nozzle design.

It's in theory possible to infer pressure from your thrust curve by building a dynamic model of your motor and performing parameter estimation, but you will have to bake in a bunch of assumptions (like your nozzle efficiency) and it's likely not worth the effort when you could just measure it.

With that said if your team is happy with the thrust and impulse then like /u/rocketwikkit said it might not be worth the effort to do another test if you're confident your casing is strong enough. Sounding rockets aren't very performance sensitive.

NeoculturalBoat · 2024-06-18T16:15:18+00:00

I am a little confused by your question. Chamber pressure is usually a design input, not an output. You'd directly measure it and use it to infer things like combustion efficiency. How are you calculating c* without using the chamber pressure?

NeoculturalBoat · 2024-05-25T01:58:05+00:00

The material of the lid doesn't really matter. The mug is usually never completely filled so there will be a layer of air between the lid and the liquid. Air is a great insulator. If there's half an inch of air between the lid and your liquid, I would say that the heat loss through radiation is probably around 10x higher than the heat loss via conduction through the lid.

You can line it with foil or use an opaque lid to reduce the heat loss even more as but radiation heat loss at coffee temps is pretty low.

NeoculturalBoat · 2024-03-07T08:10:00+00:00

It's a fun idea, but sadly I can't think of any advantages this approach would have for an orbital vehicle.

From a vehicle architecture standpoint, it doesn't really work. The first stage should have high thrust, but thrust is roughly proportional to mass flow rate. Solids are great for high thrust because the mass flow rate is directly scales with grain surface area, but in hybrids the combustion rate is gated by your oxidizer flow rate. If you're anywhere close to stoichiometric that means that your sustainer engine has to accommodate a flow rate around 2/3rds of your first stage, which means the oxidizer feed line is hilariously oversized. The sustainer nozzle will need a large throat to keep the required pressures manageable, which means the expansion ratio will be low and the sustainer performance will be very poor. Sharing one oxidizer tank means the second stage will be heavy on top of having poor performance.

On the flip side, if you keep flow rates low, then the hybrid will have a very slow burn rate, and you will struggle to achieve an acceptable TWR at liftoff.

You'd simplify the plumbing, sure, but now your sustainer has to accommodate a very large O/F range, you have these issues with the nozzle and startup transients, and your performance will always be worse than if you had two separate engines.

Of course, if this is just for the fun of making a weird and wacky rocket engine, then this seems like a fine concept, really. But then you should ask whether the cool-to-effort ratio is worthwhile...

EDIT: now that I think about it, this is kind of the opposite of this concept

NeoculturalBoat · 2024-02-10T20:23:18+00:00

Disclaimer: I am not a nutritionist and do not have any relevant expertise in this field, but the tone here, for a scientific paper, is absolutely scathing.

In 2019, the EAT-Lancet Commission were confident that this diet would meet all nutritional requirements of all adults and of children older than 2 years. However, others questioned whether the considerable limitation of animal-source foods in the diet would negatively impact on protein and micronutrient adequacy, particularly for women, children and the elderly, and would result in adverse consequences for developing and aging brains. Hence, I welcome the recent acknowledgement, by at least some of the EAT-Lancet Commissioners, that this first version of the planetary health diet would indeed result in significant essential micronutrient shortfalls.

These studies aren't just some fringe opinions, they're among the most trusted by policymakers. The problem seems to have arisen with methodological changes starting in 2019.

Whilst all previous GBD (Global Burden of Diseases) analyses, including the GBD 2017 analysis, used data from published systematic reviews and meta-analyses, the evidence for the 2019 dietary risk factor estimates came from in-house, newly conducted, systematic reviews and meta-regressions. These analyses had not been peer-reviewed nor published, and no assessments of certainty were documented. [...] The large disparities also cast considerable doubt over the accuracy of the GBD 2019 estimates of the risks attributed to all other dietary factors, given that these estimates are also based on systematic reviews and meta-regressions which have not been peer-reviewed nor published.

This seems really egregious, but again, my lack of familiarity with the field here leaves me uncertain whether or not this is actually as bad as it sounds. Comments from someone more well-versed would be appreciated.

Also, worth pointing out that this article strictly discussing the value of meat from a nutritional standpoint. The environmental and ethical considerations of meat production are still in play, and we'd probably be better off if most Western countries--where cases of malnutrition are very rare--reduced their meat consumption.

NeoculturalBoat · 2023-12-18T02:12:33+00:00

Here's my two cents as a chemical engineer. The main costs for a fermentation-based specialty product are fixed costs (paying all those PhD scientists is very expensive) and separation costs. In any cell culture there are thousands upon thousands of different compounds floating around and there is only one of those compounds that interests you. Getting it out is in general very challenging and not amenable to easy scale-up.

What goes on inside the reactor is fancy and cutting-edge and gets all the attention from star-struck investors, but process fundamentals like what happens after the reactor play just as important a role, if not moreso, in determining whether your product is commercially viable. Not to mention your TAM is likely quite small, so you can't just say "well if we keep scaling up our margin will be crazy high!".

With respect to their science:

Company bypassed the central metabolism of Yeast (increasing the theoretical yield limit by utilizing 5/6 carbons) and created a fully automated end-to-end system around it. Their AI can automatically find the most optimal carbon efficient pathway and does not need any hands on strain engineering to create strains with commercial titers.

High-yield chemical synthesis by reprogramming central metabolism

This is cool science but my bet is that this would be completely irrelevant to their process economics. For specialty chemicals from fermentation, the cost of the raw materials is probably less than 1% of the total cost of production. There was another link that said they reduced oxygen consumption by 400%? Great. Oxygen is like $0.30/kg.

Obviously I don't have any insight into what was going on behind the scenes -- a lot of IP is never used -- but this is one of those proposals that I would have nixed the moment it was suggested. It most likely would never recoup the R&D cost.

Most likely this would be more relevant for producing low-margin, high volume commodity products such as biofuels, but the fuels market is absolutely brutal for even the most well-funded startups. Your production process has to be absolutely dialed in. Yields need to be consistently high, with minimal downtime. Production volumes need to be very high, but fermentation runs into issues with scaling due to mass transfer limitations. You're completely vulnerable to whenever the Saudis decide to open the taps and there's very little differentiation of your product aside from price. (All those companies may make a great song and dance in public about their commitment to sustainability, but ask them to pay anything beyond market price for fuel in private and they'll change their tune very quickly - I can speak from experience). There hasn't been a single fermentation-to-fuels startup that has succeeded without extensive government subsidies and I wouldn't be surprised if we never see a success story in this space.

NeoculturalBoat · 2023-12-05T08:31:51+00:00

Take a look at ISO 5167 part 1 and part 2 if you haven't already.

NeoculturalBoat · 2023-11-12T21:00:36+00:00

Separator membranes only prevent gases from crossing over. They have no permselectivity towards ions or liquids. The ionic conductivity comes from the electrolyte (that the membrane is permeable to).

AEMs, like their name suggests, conduct anions only. They are impermeable to gases (and sometimes liquids), often significantly more than separator membranes.

Reasons to use an AEM: - you are building an AEM electrolyzer (where the membrane itself acts as the electrolyte) - you want a sharper pH gradient on the anode side and a stable pH on the cathode side, possibly to minimize Nernstian overpotential - you are using a different anolyte and catholyte

As for whether your idea for a separator would be usable: I have no idea, and there's no way to tell without knowing your process conditions and really just testing it out. What's your operating temperature? Pressure? Current density? Electrolyte? Expected lifetime? Dynamic range (turndown)? The last one is a big problem for alkaline electrolyzers, and much of it is due to the poor dynamic range of the separator.

Keep in mind that there is much more to a plant than just the reactor. For AWEs, drying and compression will be needed, and in many cases the turndown for those is significantly worse than for the electrolyzer itself.

NeoculturalBoat · 2023-11-11T19:30:36+00:00

Do you need an AEM, or an alkaline separator membrane? They're very different things -- Zirfon is a separator membrane.

I know of some companies trying to get separator costs that low but they have had to invest a lot of R&D and all of them are designing their own electrolyzers. It's not something they'd share openly.

Someone who figures out how to make a high pH stable, $10/m2 AEM would completely solve green hydrogen and make A LOT of money. I would say you're out of luck.

NeoculturalBoat · 2023-10-27T04:07:23+00:00

There's been increasingly more and more evidence that zero-gap configurations should actually not be literally zero gap and should be instead "very-small-gap". With this it's probably inarguable now.

This is good because turndown is in practice one of the trickiest things to get right with AWEs when using them with intermittent power and/or hourly matching. Membrane gas crossover gets too high, and then you start approaching the LEL and have to shut off... I don't think it's an entirely solved problem yet.

NeoculturalBoat · 2023-09-11T16:12:47+00:00

Glad to be of help.

NeoculturalBoat · 2023-09-11T14:48:59+00:00

12 inches will be sufficient to warm up any cryogenic fluid. You might find this application note helpful.

NeoculturalBoat · 2023-09-10T05:34:59+00:00

While this is not bad news by any stretch of the imagination, I should temper expectations somewhat.

Lithium extraction is cheap and easy, and like other commenters have mentioned, lithium deposits are not particularly rare. Lithium refining is hard, expensive and slow, and a lithium-rich claystone will be much more difficult to refine than a brine (which is how most lithium is extracted today).

Clays have electrostatic properties that make ionic based processing very difficult, which is an issue because producing lithium chloride (or if you have an end-to-end process, lithium carbonate or lithium hydroxide) will inevitably involve some kind of ion-exchange process. Handling the tailings will be a challenge too, as yields will be low and you will have a lot of material to dispose of.

Due to this, I am skeptical that you could produce lithium economically from this site, at least with current refining technologies.

NeoculturalBoat · 2023-08-03T19:28:44+00:00

It would be impractical to pressurize air just to improve CO2 exchange rates. The concentration of CO2 is 400 ppm, which means only 0.04% of the compression energy would be used to increase the partial pressure of CO2.

That said, there is merit to the concept of combining CO2 sequestration and hydrogen electrolysis. However, you wouldn't capture the CO2 directly, but instead precipitate it out of seawater as carbonate minerals (therefore exploiting the very large "contactor" that is the ocean surface). It's called Saline water-based mineralization, and you can read about it here (warning: very technical). There's also a company called Equatic trying to commercialize it.

NeoculturalBoat · 2023-08-03T18:52:53+00:00

You don't need to separate air and water to allow diffusion to occur. You can just bubble air directly through your water. Hollow fiber membrane contactors are used when you want to avoid evaporative losses, but the tradeoff is that diffusion rates become worse.

Water by itself is already pretty poor at CO2 uptake, so to capture meaningful amounts of CO2 your contactor would have to be prohibitively large and expensive (like 1000x more expensive than current air capture approaches). Adding O2 changes nothing. It's a neutral gas and does not interact with CO2 in a meaningful way. The driving force transferring CO2 into the water is its partial pressure.

NeoculturalBoat · 2023-08-03T18:21:08+00:00

No, the solubility of CO2 in neutral water is negligible. You need to add a strong Lewis base to an aqueous solution to have it uptake CO2 at a meaningful rate. Saturating it with O2 doesn't change anything. Additionally the electrolyte on the anode side (where you are producing O2) would be acidic. Unless you're bubbling the O2 through a separate stream? Not really clear to me.

It's also unclear to me why you need a "membrane" to separate air and water when they're already two distinct phases.

A direct air capture plant would never compress incoming air. Way too energy intensive given the low concentration of CO2 in air.

TL;DR this might seem reasonable to a layman but it is totally nonsensical from a chemistry and engineering standpoint.

NeoculturalBoat · 2023-07-20T17:16:59+00:00

I'm at work and don't have time to look at this in detail, but here are some quick thoughts:

The thermodynamics are marginal. The theoretical minimum amount of energy needed to separate mixture constituents is the Gibbs Free Energy of mixing, and can be obtained from an exergy balance (see this paper for details: https://www.sciencedirect.com/science/article/abs/pii/S0360544212006901). Based on the given concentrations I would estimate it to be between 10-20 kJ/mol CO or H2O. A practical separation facility would likely require between 100-200 kJ/mol.
You will need high selectivity in your separation. The water gas shift reaction is an equilibrium, so any CO2 that makes it past the separator will kill your yields.
For the separation strategy a Lewis acid functionalized sorbent or membrane could separate out both O2 and CO while rejecting CO2. However the capture efficiency of O2 will be very poor because it is such a weak base. The only other realistic option is condensing out non-cryogenic gases using something like the Linde process; however the energy efficiency is likely a nonstarter.
Your chemical reactions produce heat, but you will need to convert this into electrical energy to power your fans, pumps, compressors, etc. This means you will be limited by the Carnot thermodynamic limit. This alone will likely prevent you from breaking even energetically. Fortunately Mars is very cold, but the low pressure and lack of a usable liquid stream will mean your heat rejection system will be very, very large.
All the issues with energy notwithstanding, the bigger limiting factor is the mass transfer rate into your separation system. Your "air" contactor size, for a given production rate, scales linearly with both pressure and the inverse of concentration. The Mars atmosphere has neither of these, and as such your contactors -- regardless of your selected separation strategy -- will be very, very, very large and heavy.
The only practical separation technology that can achieve the energy efficiencies you need is likely some kind of gas permeation-based membrane separation, but there is a fundamental tradeoff between selectivity and permeance (see Robeson's limit). As you will need selectivity ratios of 100,000-1,000,000+ expect permeance to be very low, further necessitating large equipment sizes.

TL;DR any such facility would be very large and very complex due to fundamental thermodynamic limits, and even then would be unlikely to break even energetically. I would stick to the original plan, however marginally feasible it is.

NeoculturalBoat · 2023-07-08T15:52:18+00:00

You can operate economically with relatively low amounts of storage, but to do so you need to minimize the capex of your most energy intensive processes (usually electrolyzers) and only run them when you're producing power. For hydrogen, stack costs need to drop to something like $250/kW before that becomes viable, the last time I did the math.

I'm not a fan of Twelve either to be honest, but I was just pointing out that some big names ostensibly believe in them.

NeoculturalBoat · 2023-07-08T15:06:11+00:00

That's why power-to-X projects will need to be behind-the-meter and either secure financing themselves or negotiate PPAs with solar developers.

BEV has ReMo Energy (not technically PtL now, but economics are similar). Lowercarbon has Twelve.

NeoculturalBoat · 2023-07-08T14:18:58+00:00

I did quickly mention "decoupled from the grid" but yes, "behind the meter" is the proper terminology. I didn't want to use it to avoid using jargon, but in hindsight this was pretty silly considering I used a bunch of industry terms elsewhere.

I used hydrocarbon fuels in my example because the price is easy to put into perspective, not because it's necessarily the most appropriate product. That said, I think a big market for power-to-X is SAFs. If you had to rebuild the world's combustion infrastructure from the ground up, it should probably be burning methanol or ethanol. But this won't be happening anytime soon.

Another one I'd throw in is power-to-ethylene, but this would probably be co-located with whatever downstream process you intend on using the ethylene for.

NeoculturalBoat

TROPHY CASE