Producing Renewable Liquid Fuels from Atmospheric Carbon Dioxide

[KurtLudwig]

TL;DR Summary

Will it ever be done? It is technically possible to produce synthetic fuels which are carbon neutral. After having done some reading, the cost of one gallon of renewable diesel or kerosene is estimated to be $6 per gallon.

The Sasol plants in South Africa are producing liquid fuels using coal and water as the source of energy and syngas, CO and H2. Some refineries are using natural gas as the starting fuel to produce liquid fuels. Both of these processes are not renewable.
The US Navy has a process to extract CO2 from seawater and use electricity generated by the nuclear power plant to produce kerosene for aircraft.
Solid Oxide Electrolysis Cells can convert water and carbon dioxide into syngas. Renewable energy can power the Fischer-Tropsch process to produce renewable kerosene and diesel fuel.
Although it is technically feasible, it is not feasible economically without long term government support.
The question is: Will there ever be sufficient government support?

 

[hutchphd]

I think the place where it is most likely to be used is for aviation.
Do those cost estimates include pulling existing CO2 from the air?? I would guess the true carbon neutral cost to be far higher. What does feasible mean here?

 

[Astronuc]

Summary:: Will it ever be done? It is technically possible to produce synthetic fuels which are carbon neutral. After having done some reading, the cost of one gallon of renewable diesel or kerosene is estimated to be $6 per gallon.

The Sasol plants in South Africa are producing liquid fuels using coal and water as the source of energy and syngas, CO and H2. Some refineries are using natural gas as the starting fuel to produce liquid fuels. Both of these processes are not renewable.
The US Navy has a process to extract CO2 from seawater and use electricity generated by the nuclear power plant to produce kerosene for aircraft.

I'm familiar with Sasol's program and the US Navy program. Both use a Fischer-Tropsch synthesis, the system using CO+H₂ + energy to produce alkanes, but one used CO₂ as a feedstock. Besides alkanes, on could produce alcohols.

https://www.netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/ftsynthesis

It would be more practical (or sustainable/renewable) if the CO₂ capture was placed at the discharge of the combustion process.

 

[russ_watters]

Summary:: Will it ever be done? It is technically possible to produce synthetic fuels which are carbon neutral. After having done some reading, the cost of one gallon of renewable diesel or kerosene is estimated to be $6 per gallon.

Will it ever be done? Probably. But there's no way it could be anywhere near that cheap.

The Sasol plants in South Africa are producing liquid fuels using coal and water as the source of energy and syngas, CO and H2.

Water, "syngas", CO and H2 are not sources of energy.

The fundamental problem here is that to covert CO2 into fuel you have to "unburn" it; Conservation of energy. So you need to put exactly as much energy into "unburning" it as you got when you burned the fuel that created it. That means it can never-ever be as cheap as the fuel that created it, unless we've already stopped using that fuel because something else (solar, nuclear?) is less expensive by a factor of two.

 

[Melbourne Guy]

The question is: Will there ever be sufficient government support?

There is government support already, whether you think it is 'sufficient' depends on your objective, I guess.

The US DOE has financing specifically for DAC in the $20M range the last couple of years, though it is not linked to creating fuel. There is a DAC-to-fuel project in Chile, HIF’s Haru Oni eFuels Pilot Plant, and Norsk's e-Fuel project that plans to use Sunfire and Climeworks tech to build a site in Herøya to provide 10M liters of renewable fuel annually for the Norwegian and European fuels market.

Other projects are happening, but I think @russ_watters comment on thermodynamics will see these companies create an expensive product that outside of subsidies will never be profitable.

 

[russ_watters]

Other projects are happening, but I think @russ_watters comment on thermodynamics will see these companies create an expensive product that outside of subsidies will never be profitable.

Yes, outside of subsidies not anywhere in the foreseeable future. But there may be a time in the not quite foreseeable distant future when we run out of easily recoverable fossil fuels and have abundant electricity that manufacturing it may become profitable.

 

[Melbourne Guy]

when we run out of easily recoverable fossil fuels and have abundant electricity that manufacturing it may become profitable.

I have a story idea for the reverse of global warming, where DAC becomes profitable and greedy corporations suck all the CO2 from the air, leaving plants gasping and we humans starving

 

[chemisttree]

Maybe that’s why Elon Musk really wants to go to Mars?

 

[KurtLudwig]

Do those cost estimates include pulling existing CO2 from the air?? I would guess the true carbon neutral cost to be far higher.

The CO2 is removed from seawater, not from the atmosphere. It was stated in the report, that CO2 is much more concentrated in seawater than in air, and therefore cheaper to extract. It was stated in the report that the cost of purchasing and delivering aviation fuel to an aircraft carrier is more than producing it on board. The officer in charge of the program stated that the fuel is currently used by un-manned drones. CO2 is trapped in resins or amines and then released, maybe by increasing its temperature. The electricity and heat needed for the Fischer-Tropsch process are supplied by the nuclear power plant.

 

[Melbourne Guy]

The CO2 is removed from seawater, not from the atmosphere.

That is certainly how the US Navy is demonstrating it, @KurtLudwig, but it is not the method that most investment is going to. (Also, I'm not sure what report you are referring to? Is there a link to that?)

The oil and gas majors are aggressively promoting carbon capture and storage (CCUS) at primary generation source (gas fired electrical plant, for instance) which is a direct air capture method. Storage is another matter; we've not seen geologic CO2 sequestration proven at scale yet, and the poster child Gorgon project in Western Australia shows how difficult - and expensive - it is to catch and push CO2 back underground.

https://www.theguardian.com/environ...ssing-carbon-capture-target-at-wa-gas-project

Firms like Carbon Engineering are hooking into the dream of reducing CO2 in general, so the conversion equipment does not need to be co-located with the CO2 generation. Their capture cost was estimated at $100 per ton of CO2 "at large scale" in 2018, but that has not been validated as far as I can find, and I recall the actual not-at-scale cost being closer to $200 per ton a few years ago.

None of these schemes are simple. All of them are expensive. And every one of them is worse than not burning fossil fuels in the first place, but I expect regulators and investors will need to see many more DAC / CCUS projects fail before the concept is put aside as providing no real value.

 

[anorlunda]

None of these schemes are simple. All of them are expensive. And every one of them is worse than not burning fossil fuels in the first place, but I expect regulators and investors will need to see many more DAC / CCUS projects fail before the concept is put aside as providing no real value.

Good point. No doubt it is politically necessary to sponsor failures to avoid being labelled as anti environment.

I remember that in the 80s, Norway was going to capture CO2 (and maybe methane) at the offshore oil platforms and send it right back down the hole. Does anyone know if they succeeded?

 

[hutchphd]

And every one of them is worse than not burning fossil fuels in the first place,

Except for the very niche application of making fossil fuels for the aircraft on a Nuclear aircraft carrier at sea as mentioned above! I hadn't previously considered that one... But for the most part carbon capture is absolutely bogus and dishonest.

 

[Melbourne Guy]

Norway was going to capture CO2 (and maybe methane) at the offshore oil platforms and send it right back down the hole

Is this the project you were referring to, @anorlunda?

https://www.ice.org.uk/knowledge-and-resources/case-studies/sleipner-carbon-capture-storage-project

Interestingly, Norway is at it again with Carbon Removal AS, a Norwegian Direct Air Capture development company that is using Carbon Engineering to suck CO2 out of the air. I'm not confident that even with Norway's renewable electricity resources that this will be profitable...or impactful. We need to remove a lot of CO2 from the atmosphere to make a difference and with this plant planning 0.5 to 1 megaton of CO2 a year, it is literally a drop in a very big CO2 bucket.

The other aspect of CCUS that makes me doubt effective outcomes is that these projects are essentially mechanical engineering, with spinning parts and pumps and fluids and all the wear and tear that entails. Many people assume the scale factor of 'digital' applies everywhere and that's going to "save" CCUS and DAC, not understanding that anything mechanical comes with ongoing maintenance, repair, and overhaul (MRO) costs that increase with equipment age, irrespective of economies of scale.

 

[anorlunda]

Is this the project you were referring to, @anorlunda?

https://www.ice.org.uk/knowledge-and-resources/case-studies/sleipner-carbon-capture-storage-project

Yes. Thank you. The linked article is interesting, and it portrays the project as a success. It makes good reading for anyone interested in this topic.

But a critical factor is that this project began with a concentrated source of CO2 and thus skipped the expensive and difficult step of sucking CO2 from the atmosphere or seawater.

 

[sophiecentaur]

I have a story idea for the reverse of global warming, where DAC becomes profitable and greedy corporations suck all the CO2 from the air, leaving plants gasping and we humans starving

. . . .plus severe global cooling? Taxation and regulations would need to be applied appropriately and continuously for a feedback control which takes us back to pre-industrial revolution conditions and keeps us there. But then - why not close all the production systems and wait for the population levels to fall to something more reasonable than they are today? See A Modest Proposal (Swift)

 

[Melbourne Guy]

Taxation and regulations would need to be applied appropriately and continuously

Well, surely that's the problem, isn't it, @sophiecentaur. The assumption that any current government / regulatory regime could effect either of those outcomes seems very unlikely! Thanks for the reference to A Modest Proposal, also, I'd not seen that before

 

[Baluncore]

A Modest Proposal

I do like children, but I could never eat a whole one.

 

[sophiecentaur]

Well, surely that's the problem, isn't it, @sophiecentaur Thanks for the reference to A Modest Proposal, also, I'd not seen that before

Apparently, at the time, many people missed his irony. (Possibly some today too.)

 

[KurtLudwig]

Except for the very niche application of making fossil fuels for the aircraft on a Nuclear aircraft carrier at sea as mentioned above! I hadn't previously considered that one... But for the most part carbon capture is absolutely bogus and dishonest.

After having done some research on carbon sources for renewable synthetic fuels, I would like to report on four sources:

1 Flue-gas emissions from fossil-fuel power plants. The cost here is $7.50 per ton. This is a bridge technology, since it still will emit fossil carbon dioxide. However, it will reduce carbon dioxide emissions 50% for the power plant and 50% for the synthetic fuel. Source: Wikipedia, Carbon-neutral Fuel.

2 As already mentioned in this discussion, carbon dioxide extracted from seawater is estimated to cost about $50 per ton. Source: Wikipedia, Carbon-neutral Fuel. This extraction need not be done on a nuclear powered aircraft carrier and use electricity and heat from its nuclear power plant. Electricity generated by wind turbines or photo voltaic cells will be used. The separation plant will be located on shore. Carbonic acid in seawater is in chemical equilibrium with atmospheric carbon dioxide. A company called PARC developed a membrane separation system where seawater is pumped through alternating bipolar and anion exchange membranes. A voltage is applied to the electrodes, producing H+ and OH- in alternating channels, with the result of releasing carbon dioxide into the ambient air.

3 Carbon dioxide capture from air is much more costly, estimated to be between$94 and $232 per ton. This is due to the carbon dioxide concentration in ambient air of only 420 parts per million. Carbon dioxide in seawater is much more concentrated. It is always cheapest to pollute and let posterity worry about the consequences. Costs to address the result of global warming will also be huge and recurring.

4. There are interesting partial solutions demonstrated at two plants in Austria. One is at Guessing where waste biomass is used to produce electricity, biodiesel and district heat. Search for "Biomass CHP Plant Gussing: Successful Demonstration of the Steam Gasification Process". The other is in Simmering, Vienna, Austria: "Green Fuel from Residual Waste". Residual waste from agriculture and forestry are used in a thermochemical gasification process to produce syngas, CO and H2. Using Fischer-Tropsch synthesis various liquid fuels are produced. Waste biomass, instead of decomposing and emitting carbon dioxide, is used to produce carbon-neutral fuels. Website: www.best-research.eu

 

[Melbourne Guy]

where waste biomass is used to produce electricity, biodiesel and district heat

We are doing a waste to hydrogen project at work, it is one use case where, assuming the hydrogen is consumed locally, might make sense, but it is not sufficiently advanced that I can report on the CAPEX / OPEX aspects. I remain sceptical that hydrogen generation will ever be sufficiently cheap to displace using electricity directly, or that without carbon taxes, it will ever be cheap at all.

We are not doing district heating though, I don't think that's much of a thing Down Under.

 

[TeethWhitener]

Personally, I think using CO2 capture and electroreduction to hydrocarbons is probably one of the best ideas long-term for energy storage (for instance: to store extra energy generated from wind or solar off-peak).

I’ve been beating a very unpopular drum for a while, which is that batteries and hydrogen both are kind of terrible for energy storage. Batteries don’t have anywhere near the energy density that hydrocarbons do, they’re made from toxic conflict-sourced minerals, and they are not nearly as safe as hydrocarbons (e.g., your gas tank doesn’t burst into flames if you open it, but your lithium computer battery sure might).

And hydrogen is just a mess. Gravimetrically, it has maybe a few percent of the energy density of hydrocarbons max, and to up that, you have to go to hydrogen storage, which is wildly inefficient in terms of preparation and often also dangerous (e.g., boron hydrides which are pyrophoric, compressed hydrogen or cryogenic slush which are going to be nasty in a car crash).

Hydrocarbons are reasonably safe, as energy dense gravimetrically and volumetrically as any chemical system, and we have a massive infrastructure already in place to process them. The hardest part of implementation is the carbon capture (from air specifically—as previously mentioned, seawater is much easier) and the electroreduction to specific products (e.g., turning CO2 into something like methanol or jet fuel specifically). IMO a lot of progress could be made by investing research dollars into solid oxide fuel cells that can run efficiently on a mix of hydrocarbons.

But no one listens to me because they hear “hydrocarbons” and think “fossil fuels” or more specifically, “primary energy source,” when in reality, I’m talking specifically about energy storage for transportation and to even out the intermittency of renewables. That and we’ve already sunk billions of dollars and decades into battery and hydrogen research and it is admittedly a little presumptuous of me to come along and say: hey maybe this wasn’t the best idea. Dunno, just my $0.02.

 

[Melbourne Guy]

I’ve been beating a very unpopular drum for a while,

I don't think that drum is so unpopular, @TeethWhitener, but it is not just thermodynamics that is against any scheme to 'reconstitute' hydrocarbons and replace fossil fuels with a carbon neutral equivalent. Internal combustion is noisy and noxious and cities are banning them not just because of CO2. BEV for urban transport is considerably less polluting and health benefits will accrue. (I'd not worry about FCEV too much, the CAPEX and OPEX of the hydrogen supply chain makes them unlikely to ever displace BEV for transport.)

Medium term electricity storage remains an issue, no matter the option, and we're seeing all manner of ideas being proposed. Pumped hydro is a likely mainstream contender, but most of the other methods are so much more expensive than wind / solar + gas generation that they are unlikely to be adopted without considerable carbon taxes. I include nuclear fission in this, despite claims from NuScale and similar small reactor proponents, meeting regulatory requirements is a considerable cost impost that prices them out of the market.

As for batteries, there are a number of cost-effective, non-lithium architectures that might scale up to production requirements, and if they do, biofuels should suffice for the hard-to-decarbonise sectors such as aviation and long-distance shipping.

(Sodium is an example, such as this: https://source.wustl.edu/2021/05/bai-lab-develops-stable-efficient-anode-free-sodium-battery)

 

[TeethWhitener]

BEV for urban transport is considerably less polluting and health benefits will accrue.

It’s considerably less polluting if you live in a developed nation. It’s considerably more polluting if you’re actually mining the stuff. I’m holding out hope for the wide variety of direct carbon fuel cells.

If we’re talking about NIMBY, I can imagine people would rather have a giant vat of hydrocarbons nearby to run a generator than a giant vat of molten sodium. And yes, hydrocarbons still beat sodium-air on the thermodynamics front.

The backlash against hydrocarbons for energy storage reminds me a lot of the backlash against nuclear for clean energy generation. It’s a frustratingly non-scientific outlook on a technology that is ultimately a net positive environmentally.

 

[Melbourne Guy]

It’s considerably more polluting if you’re actually mining the stuff

Well, that's true for most mining, should BEV be singled out in that regard, @TeethWhitener?

If we’re talking about NIMBY, I can imagine people would rather have a giant vat of hydrocarbons nearby to run a generator than a giant vat of molten sodium. And yes, hydrocarbons still beat sodium-air on the thermodynamics front.

I do not understand what this refers to. What's the vat of molten sodium doing? But yes, hydrocarbons beat most options for energy packaging. However, with a 'good enough, cheap enough' battery we can start replacing coal and gas fired power plants. Li-ion is currently too expensive per MWh for anything but hours of storage and supply cost arbitrage. Some of the other architectures - sodium, iron, aluminium - may provide an order of magnitude cheaper MWh compared to Li-ion and then we're into cost-effective days and weeks of storage. They are more flexible and faster to deploy than pumped hydro, and other gravity types (cranes, trains, and mine shafts) are likely to suffer higher OPEX costs than batteries.

A dark horse could be geothermal, which has proven frustratingly difficult, and I am discounting wave power of any kind as the sea seems an inimical environment that is too expensive to cope with.

The backlash against hydrocarbons for energy storage reminds me a lot of the backlash against nuclear for clean energy generation.

I presume you are talking about carbon neutral hydrocarbons? The issue seems more one of cost than backlash, because surely the O&G industry would promote it - and politicians would favour it - as it's "least change" and who doesn't love that? But so far the cost is like hydrogen generation from electrolysis: physics puts a lower limit on efficiency and while scale can reduce costs, there is a 'floor price' due to thermodynamics that sets how low we can expect to go.

 

[hutchphd]

The backlash against hydrocarbons for energy storage reminds me a lot of the backlash against nuclear for clean energy generation.

What is the maximum efficiency of the closed cycle process? Say I burn LPG to produce the energy necessary to reconstruct that LPG from the atmosphere. Is there a good treatment of this you can recommend (or create) ? At first blush this seems very lossy to me but I don't feel like re-inventing the wheel here and my thermo is rudimentary.

 

[TeethWhitener]

What is the maximum efficiency of the closed cycle process? Say I burn LPG to produce the energy necessary to reconstruct that LPG from the atmosphere. Is there a good treatment of this you can recommend (or create) ? At first blush this seems very lossy to me but I don't feel like re-inventing the wheel here and my thermo is rudimentary.

This is a very good point—operationally, the system is quite lossy. Commercial methanol fuel cells, for instance, only get to maybe 40-50% efficient (though you can do much better in the lab). Electroreduction of CO2 can be very efficient if you aren’t focused on a single product, but that’s where most of the research has been. Still, the efficiencies are quite impressive (50+%—again, in the lab). The important thing is that, because (e.g.,) lithium batteries are only 10% as energy dense as hydrocarbons, you really only have to do better than that. @Melbourne Guy has brought up a number of other important issues as well.

Of course, currently a closed carbon cycle is not operationally ready. But it’s my belief that with concentrated R&D, it could be done fairly quickly. One of the biggest hurdles is simply the fact that batteries work “well enough” for the immediate future, so there’s a lot more attention being paid to those issues, R&D and deployment-wise.

Edit: reading over this again, I need to point out that lithium-air batteries are comparable to hydrocarbons in energy density (as opposed to lithium ion rocking chair batteries). That said, the lithium supply chain is rather tenuous, which has been one of the driving factors for development of sodium batteries (among others). But once you start using elements that are heavier than carbon, you start rapidly losing energy density.

 

[TeethWhitener]

Well, that's true for most mining, should BEV be singled out in that regard, @TeethWhitener?

Carbon capture doesn’t require mining for the source material, and there are significant metal-free carbon capture and electroreduction developments taking place at the research level right now to potentially cut down on mining for catalysts for those applications.

What's the vat of molten sodium doing?

Storing energy. You pointed this out yourself.

Some of the other architectures - sodium, iron, aluminium - may provide an order of magnitude cheaper MWh compared to Li-ion and then we're into cost-effective days and weeks of storage.

I agree, but these are roughly in the same developmental stage as direct carbon fuel cells and CO2 electroreduction. Silicon batteries are also an early-stage candidate that I think we’ll hear a lot more from in the future.

As you pointed out, and as I mentioned to @hutchphd above, the big hurdle for a closed carbon cycle is the fact that we’ve already done so much work getting batteries to a good place.

They are more flexible and faster to deploy than pumped hydro, and other gravity types (cranes, trains, and mine shafts) are likely to suffer higher OPEX costs than batteries.

Rapid power generation is one area where batteries really shine right now, and I agree that anything mechanical (pumped hydro, wave, etc) is going to have a costly (and often swept under the rug) maintenance budget.

I presume you are talking about carbon neutral hydrocarbons? The issue seems more one of cost than backlash, because surely the O&G industry would promote it - and politicians would favour it - as it's "least change" and who doesn't love that?

1) yes carbon neutral. 2) The fossil fuel industry is really really complicated. Overlapping government incentives provide a really difficult patchwork to sort out, and I’m not an expert on this by any means. But at least a few oil companies have their hands in carbon capture research as well as fuel cells and electroreduction. Of course, many of these technologies are multi-use, so it’s difficult to peg exactly where FF companies’ true interests lie (other than turning a profit). But for examples, you can take a look at some of the work funded by the petroleum research fund, which is a grant managed by ACS and funded out of a trust maintained by a consortium of oil companies.

 

[Melbourne Guy]

Storing energy. You pointed this out yourself.

Aha, I was referring to sodium ion battery, not molten salt style storage (CSP, for example). Sorry, I should have noted that.

But I feel I'm dragging us off topic, @TeethWhitener. Air-to-fuel is currently so expensive that the applications are extremely limited, and absent high carbon taxes, it is hard to see how that comparative cost can be bridged. I guess we'll see what uptake there is over this decade, but I expect a flurry of activity before it fades away as competition from other storage options undermines hydrocarbon fuels from any source.

 

[TeethWhitener]

Aha, I was referring to sodium ion battery, not molten salt style storage (CSP, for example). Sorry, I should have noted that.

I wasn’t referring to molten salt either. The most energy dense sodium-based battery is sodium-air. Sodium-sulfur is also promising. At scale, both are likely to use molten sodium (metal, not salt) to avoid the formation of dendrites which grow as the battery is cycled and tend to short the battery, limiting its lifetime. Rocking chair batteries (regardless of chemistry) are $\leq$ 1% as energy dense as hydrocarbons.

Air-to-fuel is currently so expensive that the applications are extremely limited, and absent high carbon taxes, it is hard to see how that comparative cost can be bridged.

You might be right, but we could’ve said the same thing about solar and wind a decade or two ago. We’ll see if a closed carbon cycle follows the solar PV trajectory or if it follows the nuclear trajectory.

 

[sophiecentaur]

I wasn’t referring to molten salt either. The most energy dense sodium-based battery is sodium-air. Sodium-sulfur is also promising. At scale, both are likely to use molten sodium (metal, not salt) to avoid the formation of dendrites which grow as the battery is cycled and tend to short the battery, limiting its lifetime. Rocking chair batteries (regardless of chemistry) are $\leq$ 1% as energy dense as hydrocarbons.

You might be right, but we could’ve said the same thing about solar and wind a decade or two ago. We’ll see if a closed carbon cycle follows the solar PV trajectory or if it follows the nuclear trajectory.

I have a feeling that the energy required to remove CO2 from air is likely to be far less than for obtaining a usable fuel from atmospheric CO2. I know plants are pretty good at doing the whole job but why not approach the removal job as a single problem and use anaerobic digesters and the like for producing fuel?
This thought may have already been expressed, higher in the thread, but it needs re-stating.

 

[TeethWhitener]

I have a feeling that the energy required to remove CO2 from air is likely to be far less than for obtaining a usable fuel from atmospheric CO2. I know plants are pretty good at doing the whole job but why not approach the removal job as a single problem and use anaerobic digesters and the like for producing fuel?
This thought may have already been expressed, higher in the thread, but it needs re-stating.

Actually, plants are really bad at it. Photosynthetic efficiency in nature tops out at a few percent. In the lab, artificial photosynthesis is closing in on 25%:
https://en.m.wikipedia.org/wiki/Artificial_photosynthesis
But yes, generally you need to break the problem into parts to solve it.

 

[sophiecentaur]

Bad or good? Depends on the criteria. Low efficiency is no bad thing when it avoids catastrophic changes in the environment. The worst you can say about Natural photosynthesis is that it’s inconveniently slow to deal with this man made disaster. Apart from their part in allowing our arrival, I would say they didn’t do too badly. Just imagine what would have happened if the numbers had allowed just a few more percent of atmospheric oxygen.

 

[Bystander]

few more percent of atmospheric oxygen.

..., and...?

 

[sophiecentaur]

Overdosing on O2 is not good for you or living things in general. Tho’ I have to admit that the level of O2 would not be a problem if all the CO2 were converted and then the plants would all die. We have to turn off the super processing machine at an appropriate time.

 

[TeethWhitener]

Yeah I guess I’m not seeing the point either. Atmospheric O2 was 50% higher 300 Mya:
https://en.m.wikipedia.org/wiki/Geological_history_of_oxygen
Also, plants don’t move (at least not very quickly), so they can afford to have low photosynthetic efficiency.