=chemistry =energy =economics =global warming =technology
The economics of turning electricity into chemical energy are questionable, but there is one new process I like.
chloralkali vs water electrolysis
Today, the main way electricity is used directly for chemical production
is the
chloralkali process. Electrolysis of NaCl in water makes H2, NaOH, and
Cl2. That typically uses a
Nafion-type membrane and
mixed
metal oxide anode, both of which are fairly expensive.
Chloralkali electrolysis requires 2.1V, and is done at a +0.9V overpotential
over that to balance the cost of equipment and electricity. The voltage for
water is lower (~1.23V) but for the same overpotential, water electrolysis
is probably a bit more expensive: NaOH is corrosive, but Cl2 is easier to
make at an electrode than O2.
The reverse of water electrolysis is a
hydrogen fuel cell, but voltage drop from 1.23V is even worse than voltage
increase from it, so you absolutely need expensive catalysts, which makes
fuel cells more expensive. I don't see them going below $1000/kW even with
more research and large-scale production, which by itself is more expensive
than batteries for a car.
Electricity for water electrolysis is too
expensive. The equipment for water electrolysis is too expensive. Put them
together, and it's too expensive. The median competent cost estimate for
large-scale electrolytic hydrogen production seems to be ~$7/kg. There are
many papers on this - here are
some
examples - and you can read them if you want to see math, but that's my
conclusion.
If the goal is to avoid natural gas usage when making
hydrogen, you also have to consider the cost of that on power. Electrolyzers
are expensive, so you want to run them all the time to amortize capital
costs, but using some natural gas is the cheapest way to get reliable power,
and if you don't, then power costs probably go up.
Hydrogen-related
research has gotten large amounts of funding. The chloralkali process is
used on a large scale. I don't expect much progress on low-temperature water
electrolysis soon.
subsidies
The US Department
of Energy set targets of $2/kg for green hydrogen by 2025 and $1/kg by 2030.
A lot of people assume those targets were based on what's plausible, but
they were based only on, respectively, what prices would make some uses
competitive and what would make replacing natural gas competitive. Those
targets have no chance of being met.
We can tell the US government
doesn't really believe in those targets, because the oddly-named
Inflation Reduction Act has
subsidies
(technically, tradable tax credits) for hydrogen from electrolysis that are
higher than those prices - up to $3/kg of hydrogen from electrolysis.
Despite the restrictions, that's almost enough subsidy to make
electrolysis competitive with conventional sources of pure hydrogen, and
some rich
investor-philanthropists will probably take the residual loss of
building plants. That's bad, because it's doing something inefficient, using
taxes that create more economic losses, in a way that could create a
persistent vested interest like how corn ethanol has.
Hydrogen-based
energy already has a sizeable vested interest - all the professors and
bureaucrats who built a career on it as a topic. That's often necessary to
make things happen, but it keeps going regardless of whether it should.
gratuitous quote
The Army told me everything, all the weapons they had, and all the gadgets, everything and they asked me all kinds of questions. Then they had cocktail parties, meeting generals and so on; informal things. A general would tell me that what they need is a tank that uses sand for fuel, because it would be great if you could just scoop up the sand and turn it into power, because they could keep on going. The problem is refueling the damn things when they go too far, and so on and so on. They apparently thought that science could do anything. There probably is a way of solving this tank problem, but not by scooping up sand and making energy out of the sand.
what if the power is free?
Suppose solar
panel costs come down much more. There are physical limits here, but let's
just suppose they're free. Well, the supports + land + installation +
conversion already cost more than the actual panels, but let's suppose those
are all cheaper too, and there's free electricity available for 6 hours a
day.
It still wouldn't make sense to run current water electrolysis
systems - again, you want to run them all the time to amortize capital costs
- but is there some way to sacrifice efficiency to make them much cheaper?
Not really.
Why is Nafion/Aquivion used as a membrane, despite being
quite expensive? There are other polymer membranes with good conductivity,
but they degrade over time under conditions oxidizing enough to produce
oxygen or chlorine.
Can you use a cheaper electrode than MMO? Most
metals form an insulating oxide layer. How about graphite? As Wikipedia
notes for chloralkali electrolysis:
Graphite will slowly disintegrate due to internal electrolytic gas production from the porous nature of the material and carbon dioxide forming due to carbon oxidation, causing fine particles of graphite to be suspended in the electrolyte that can be removed by filtration.
The graphite anodes oxidize at the edges of graphite molecules; it's not something you can manage just by careful voltage control.
high-temp is better
- Water
electrolysis is endothermic, and the higher the temperature, the more of the
energy for it comes from heat. Electrical resistance makes heat, and
operating at a high temperature means that waste heat is contributing to the
process, increasing efficiency.
- Low-temperature water electrolysis
requires expensive catalysts to keep the voltage low. Operating at a high
temperature makes reactions happen without expensive catalysts.
On the other hand, polymer
membranes aren't stable at high temperatures, so you need a
ceramic electrolyte instead. Those have to be thicker, which increases
resistance, and it's generally harder to manufacture ceramic tubes than
polymer sheets.
High-temperature water electrolysis is significantly
better than low-temp. I estimate it at ~$5/kg of hydrogen, as follows:
1)
SOFCs were
used in experimental power plants, and I think they only added ~$0.10/kWh to
net cost.
2) Voltage in is higher than voltage out, which multiplies
capital costs by ~0.6 for electrolysis vs power generation.
3) Hydrogen
requires ~40 kWh/kg at 100% efficiency. Add the cost of electricity, account
for efficiency, and do some conservative adjustment for tech progress and
manufacturing scale, and I get ~$5/kg.
Why, then, are most startups
not doing that? Because manufacturing is already established for chloralkali
electrolysis, and high-temp water electrolysis still doesn't make the
economic sense that justifies serious investment scales.
There are
some startups doing co-electrolysis of CO2 and H2 to make syngas
to make oil
that can be refined to fuels. Obviously the economics doesn't work out, but
some groups are willing to pay a lot for small amounts of renewable fuel.
that was all background
None of the above is new, and it's been said by other people before, people
with more influence than me. Why, then, am I posting this now?
I've
always considered water electrolysis and related processes to be too far
from viability to be worth thinking about, but I recently was obligated to
look into them anyway, and I found exactly one that's approximately
economically competitive.
current steam reforming
Steam reforming on methane is a major industrial process. Methane is reacted
with steam and sometimes oxygen to make CO and H2, which are purified.
To make ammonia, the CO is
mostly
converted to CO2 + H2 at a lower temperature, and remaining CO is
converted to methane. That's cheaper than trying to separate out the
hydrogen with membranes or adsorption. To make methanol, you want 1 CO to 2
H2, which usually involves adding oxygen and/or recycling CO2.
This
process is somewhat expensive, and has large energy losses. As a result,
methanol has generally cost more by mass than methane, despite being half
oxygen by mass.
electrochemical reforming
So, what was the single viable process for electricity to chemicals I found?
The net reaction is this:
3 CH4 + CO2 + 2 H2O
-> 4 CO + 8 H2
...yes, that's the same net reaction as
combined reforming of methane with steam and CO2, which doesn't require
electricity. What's different here?
Normally, that reaction requires
a large amount of excess steam, and either oxygen separation from air or CO2
recycling. Adding electricity shifts the equilibrium, so you can use less
steam, no added oxygen, and higher pressure. The term for that is
electrochemical reforming;
here's a
recent paper on it. Designing good cells for this process is still an
active area of research.
This is also similar to a
SOEC.
Why does this make sense, while direct electrolysis of CO2 + H2O to syngas
and O2 doesn't? Also, it's similar to a
SOFC
operating in reverse - don't those often have durability problems?
- This takes
most of the energy from methane, which is a cheaper source of energy than
electricity.
- Many high-temperature fuel cell designs have
problems
with durability due to corrosion on the air side, while an electrode in
a reducing environment is much easier. Electrochemical reforming of methane
has a reducing environment on both sides.
but does that help?
Oxygen
isn't produced in the above process, which is good because making oxygen
with electrolysis is economically bad - but that means all of the
"reduced-ness" of the products comes from methane. So, does it help at all,
in terms of reducing CO2 emissions or fossil fuel usage?
Rather than
reducing chemicals, what this process does is avoid burning methane.
In steam reforming, natural gas is burned to supply heat. In autothermal
reforming, the methane is reacted with oxygen. Either way, this replaces
combustion with electricity consumption. Not only that, but I believe it's
possible to get costs of this equipment down low enough for intermittent
operation on solar and wind power to be reasonable.
then what?
OK, so this
makes syngas with 2 H2 to 1 CO, which can be converted to methanol. What can
you do with methanol? Well, current world production is over 160 million
tons a year, so it must have some uses.
- The current
main use in the US is conversion to formaldehyde, for urea-formaldehyde
resins for plywood. However, there's already enough production for that, and
those can release formaldehyde - which is bad - so I'd rather see its usage
decrease.
- Methanol is easily converted to
dimethyl ether,
which is a good fuel for diesel engines that produces much less pollution
than current ones. However, it must be stored under pressure, like propane.
- Methanol can be used as a fuel for spark-ignition engines. It has a
very high octane rating and allows for high compression ratios and thus good
efficiency. However, it has low energy density, it dissolves some rubbers
often used in current cars, and its boiling point is somewhat low. China
currently adds some methanol to its gasoline.
- Methanol
can be converted to propylene and ethylene. However, this isn't
competitive with ethane dehydrogenation in the US.
The main use I envision for large amounts of new methanol production is fermentation. It's possible to engineer microbes to grow on just methanol and produce useful chemical intermediates that can be used for fuels and plastics. Some such processes would be profitable today, but the ones I've seen startups pursuing would not.