=energy =global warming =technology
The theoretical energy required
to purify something is only logarithmic with the concentration ratio. The
problem with extracting something with a low concentration is that you have
to process a lot of material.
If you want to dig something out of the
ground (like coal or limestone) and put it in a truck, that's about $20/ton.
Desalinated water is extremely cheap, about $0.50/ton. Making it
involves taking seawater (which is free), filtering it, pressurizing it, and
running it past a relatively cheap type of membrane.
If you made a
lithium separator that processes seawater for $0.50/ton, your lithium would
cost $2.5 million / ton. Realistically, lithium from seawater would be over
10x that.
Now, suppose you could process air for $0.50/ton and remove
all the CO2. There's about 400 ppm of CO2 in air now, so that's about
$1250/ton of CO2 removed. Air can be scrubbed of CO2 for significantly less
per ton than desalination costs, but this is actually a decent starting
point; actual costs would probably be between 1/2 and 1/10 that.
To collect dilute stuff, sorbent
needs to hold it pretty strongly. You also want to use liquids if possible.
So, an obvious (to chemical engineers) and reasonable approach is NaOH
or KOH in water, transferring carbonate to Ca(OH)2 with precipitation of CaCO3,
and then recovering CO2 by calcination of CaCO3. There are 2 relatively influential papers on CO2 capture from air using this approach:
paper 1 (2011) estimates $430 to $550 / ton CO2
paper 2
(2018) estimates $94 to $232 / ton CO2
Paper 2 claims it reduces costs over paper 1 by:
- using
cross-flow air exchangers instead of counterflow, increasing airflow vs fan
cost
- using cheaper PVC packing instead of steel
- using KOH instead
of NaOH
As techno-economic analysis goes, they're both rather mediocre. (That's to be expected: the people who do the most sober and well-informed analysis tend to avoid things that seem far from viability.) So, what's wrong?
paper 1:
- Stainless
steel exposed to strong alkali in air will corrode.
- As paper 2 notes,
the concern about spray from the alkali solution isn't necessary, and
cross-flow is probably better.
paper 2:
- Salty water
won't wet PVC or polypropylene very well; you can't just assume your full
packing material surface area is used.
- KOH isn't a huge improvement
over NaOH; their basis for claiming that is bad.
- We know how much
calcination of portland cement costs. You can't just assume you have a
cheaper and more efficient way of doing that.
With this approach, $200 to $300 / ton CO2 seems correct to me. By the way, both papers have calcination of CaCO3 being done with natural gas. Trying to do that with electricity would be considerably more expensive.
What about "accelerated
weathering" for CO2 sequestration?
Digging stuff up is about
$20/ton, and you'd need to dig up 3 tons of Mg silicate to absorb 1 ton of
CO2.
But suppose you do that. Great, you exposed some fresh magnesium
silicate to the CO2 in air, and now a very thin layer of carbonate will form
on the surface as it very slowly reacts. If you crush it to fine particles
and spread it over a large area, you can get it to actually react, but
obviously that's more expensive.
The US government decided to
spend a bunch of money on development of CO2 capture and sequestration from
coal plants. This accomplished nothing. The
Kemper Project (a
pilot plant for coal power with CO2 capture) cost $7.5 billion, and never
operated.
People say the US doesn't have industrial policy like China
does, but one could argue that it's more accurate to say that the US
government is just so bad at industrial policy now that it doesn't seem like
it's doing anything. In other words, the US used to spend its industrial
policy money on stuff like the
Heavy Press
Program, and now the US spends it on debacles like the Kemper Project
and obviously stupid ideas like
cylindrical solar cells.
Separating CO2 from a mixture is
cheaper when the concentration is higher. The optimal process for CO2
separation also varies with concentration: to remove very dilute CO2
(perhaps from air) you need a something that binds to it strongly (such as
hydroxides).
In order of increasing cost, some reasonable sources of
pure CO2 are:
1) Separation
from syngas during ammonia production, maybe with Selexol. This is basically
a free side product.
2) Separation from gas made during fermentation
of sugar (to eg ethanol) with cryogenic
separation. Maybe $25/ton.
3) Amine scrubbing of flue gas from
boilers. Maybe $50/ton.
4) Gas turbines using pure oxygen and
recycled CO2. Maybe $60/ton...with very large cryogenic air separation
facilities.
5) Amine scrubbing of exhaust from gas
turbines. Maybe $80/ton.
The cost of amine scrubbing is largely from steam to heat the liquid sorbent for CO2 stripping, so it varies depending on the cost of steam.
CO2 from any of those sources is,
for a pure chemical, a very cheap material. The problem
is...what do you do with it? Note that liquefying it and trucking it around
significant distances will make it several times as expensive. Cryogenic
separation is probably the cheapest approach if CO2 gets transported by
truck to, say, greenhouses.
CO2 is
mostly used to make urea, which is basically a way of stabilizing ammonia;
the CO2 just gets released again.
You can do "cyclic carbonate
synthesis" with epoxides and CO2. You can
make
salicylic acid from phenol and CO2. If you really want to, I guess you
can deprotonate acetone and make acetoacetate. These are all niche reactions
that are very expensive per mass of CO2.
So, the US government decided to
give a bunch of grants for scientific research into chemical uses for CO2.
This is a smart approach, unless you know about chemistry, in which case
it's dumb.
Some people thought: "How about we turn CO2 and
electricity from solar panels into chemicals, maybe fuels?" There were a
bunch of grants for doing that. A bunch of time was spent by PhDs and
professors on it. People wrote papers where they tried to pretend that
converting CO2 to cheap stuff with inefficient electricity use and expensive
metals is a promising line of research, but nothing useful resulted, because
the premise was bad.
The best current approach for converting CO2
with electricity is running a solid oxide fuel cell backwards. Not only are
SOFCs too expensive, but that makes a product that's worth less than the
inputs, which is why SOFCs are usually meant for generating
electricity. But still, there are startups doing that now. Perhaps they're
hoping for subsidies.
If you want to use CO2 to make chemicals, it's
better to combine it with hydrogen and make syngas than to try to make
products directly. Syngas is
well-understood and heavily used. Of
course, hydrogen from electrolysis isn't currently viable
either. It's ~2.5x the cost of purified hydrogen from natural gas - but that's a
lower multiple than many other approaches for renewable chemicals!
"But in the
future we'll have cheap solar power for that."
Yeah? Why? Solar power
has stopped getting cheaper. If anything, renewable power with storage will
be more expensive than what we have now.
"Because there will be
excess PV capacity during the day, and you can do electrolysis then."
No, you don't want to run electrolysis plants for a 4 hours a day. You
need to run them most of the time for the economics to work.
So, what else can you do with
CO2?
Well, it's used for
enhanced oil recovery. (That's not what the "direct air capture" people
want to do!)
You could potentially send it to
greenhouses. It's generally been too expensive to do that; it's been cheaper
for greenhouses to connect to existing natural gas pipes and burn methane to
make CO2. But you could build greenhouses and CO2 pipes.
If you want to actually collect CO2 from air economically, you'd need some kind of...nanobots. Self-replicating nanobots, maybe powered by sunlight. Using some kind of molecular machines powered by sunlight to collect CO2. And maybe they could use the CO2 for the self-replication. Yep, that's what you'd need.