Co-Electrolysis of CO2 and H2O for the Production of Liquid Fuels
23 July 2010
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| Diagram of the proposed closed-loop fuel cycle. CO2 is recycled into hydrocarbon fuels in a process based on: capturing CO2 from the atmosphere, high-temperature co-electrolysis of CO2 and H2O in a solid oxide cell to yield syngas (CO/H2 mixture), and catalytic fuel synthesis from the syngas. Source: Lenfest Center. Click to enlarge. |
Researchers at Columbia University’s Lenfest Center for Sustainable Energy, in collaboration with Risø National Laboratory for Sustainable Energy, DTU, are investigating the high-temperature co-electrolysis of CO2 and H2O using solid oxide electrolysis cells (SOECs) to produce a syngas for conversion into liquid hydrocarbon fuels.
The Columbia/Risø team currently has a paper in press in the journal Solid State Ionics describing their experimental results on performance and durability of the solid oxide cells. In May, the Lenfest Center and Risø DTU hosted a one-day conference—“Sustainable Fuels from CO2, H2O, and Carbon-Free Energy”—addressing technologies that can be used to recycle CO2 into carbon-neutral liquid hydrocarbon fuels using renewable or nuclear energy. The co-electrolysis process was featured in several of the talks.
A process to produce liquid fuels from CO2 would comprise three basic stages:
- CO2 capture
- Dissocation of CO2 and/or H2O using renewable or nuclear energy
- Fuel synthesis using the dissociation products.
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| Possible pathways for the conversion of CO2 to hydrocarbons and alcohols. Source: Lenfest Center, Christopher Graves. Click to enlarge. |
A number of different methods for the dissociation—i.e., the conversion of renewable/nuclear energy to chemical energy—are feasible: thermolysis; a thermochemical cycle; high-temperature electrolysis; low-temperature electrolysis; and photoelectrolysis/photolysis. Lenfest Center and Risø DTU determined that dissociation by electrolytic methods is currently the most feasible.
Of those, they identified high temperature co-electrolysis of CO2 and H2O to produce the syngas (CO/H2 mixture) as a promising method. In one of the papers presented at the May conference, Carl Stoots from the Idaho National Laboratory noted that while it is possible to produce syngas by separately electrolyzing steam and CO2, there are significant advantages to co-electrolysis, including lower cell resistance and the reduced possibility of further reduction of CO to C. However, he noted, co-electrolysis is still not well understood.
The Lenfest/Risø team notes that high temperature electrolysis makes very efficient use of electricity and heat (near-100% electricity-to-syngas efficiency), provides high reaction rates (no need for precious metal catalysts), and the syngas produced can be catalytically converted to hydrocarbons in well-known fuel synthesis reactors (e.g. Fischer-Tropsch). There is no need for a separate reverse water-gas shift reactor to produce syngas, and the waste heat from exothermic fuel synthesis is useful in the process.
An analysis of the system energy balance presented by Christopher Graves at the May conference showed a 70% electricity to hydrocarbon fuel efficiency. Using solar photovoltaic energy at 10-20% efficiency, that would result in an overall 7-14% solar energy to liquid fuel efficiency, he said.
Their analysis of the economics of a co-electrolysis-based synthetic fuel production process, including CO2 air capture (earlier post) and Fischer-Tropsch fuel synthesis, determined that the price of electricity needed to produce competitive synthetic gasoline (at $2/gal wholesale) is $0.02 - $0.03 per kWh.
Dominant costs of the process are the electricity cost and the capital cost of the electrolyzer; this capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind).
The core of the process is the solid oxide cell for co-electrolysis. Risø has been developing and testing Ni/YSZ based Solid Oxide Cells. Although high initial performance was observed, long-term durability needs to be improved. Testing showed significant structural degradation at high current densities.
The team’s research involves studying reaction mechanisms at the negative-electrode that limit performance and durability, using simplified-geometry electrodes. They have also recently developed new all-ceramic nano-structured molybdate-based electrode materials that exhibit exceptional electrocatalytic performance and could significantly improve the overall energy use and economics of the CO2-to-fuels system.
Resources
Christopher Graves, Sune D. Ebbesen and Mogens Mogensen (2010) Co-electrolysis of CO2 and H2O in solid oxide cells: Performance and durability. Solid State Ionics In press. doi: 10.1016/j.ssi.2010.06.014
Sustainable Fuels from CO2, H2O, and Carbon-Free Energy (presentations from the conference)
If we reuse CO2 we reduce overall emissions. This could advance SOFC methods enough to use them in other applications as well.
Posted by: SJC | 23 July 2010 at 08:33 AM
the goal of H2 + CO syngas is a good one. Can it be done efficiently though? We have commented before there are more sound ways to approach these problems.
Posted by: Reel$$ | 23 July 2010 at 09:25 AM
If it takes 20 kWh of electricity to make 1 gallon of methanol at 4 cents per kWh producer cost, that is 80 cent per gallon methanol. It would still cost more than making methanol from natural gas, coal or biomass, but we have plenty of CO2 around...so we will see.
Posted by: SJC | 23 July 2010 at 04:21 PM
Once this technology is developed, the next obvious step is to produce simple molecules (like ethanol or aceto-acetate) chemically like this, and then feed them to heterotrophic organisms to transform them to food (you could already give feed ethanol to pigs to transform it to meat)
Producing food this way will soon be much more economical and ecological than the 'old fashioned agricultural way'. no more need for land areas, pest control, ...
It is also a much more secure way for food production, much less vulnerable to weather conditions or fertiliser availability. One big windmill can produce the food today produced on hundreds of acres. One big nuclear reactor can produce the food for a small country.
Posted by: Alain | 23 July 2010 at 07:29 PM
If it takes 20 kWh of electricity to make 1 gallon of methanol and then we use the mthanol to power something using the methano to drive a generator to drive a motor; why not just use the electricity directly in step 1.
Oh - it is more convenient to transport the methanol?
OK, maybe, in some cases - maybe not.
Either way I am underwhelmed.
Posted by: ToppaTom | 23 July 2010 at 09:50 PM
Well, it certainly is not our job to make sure that you are not "underwhelmed".
Posted by: SJC | 23 July 2010 at 10:33 PM
Thought-experiment: 5 MW nameplate wind turbine yields 1.7 MW average (34% capacity factor). Electrolysis of CO2+H2O followed by methanation yields CH4 at 70% efficiency (1.2 MW avg). Methanotrophic bacteria consume CH4 to make biomass at 70% efficiency, filter-feeding shrimp consume bacteria to make shrimp at 70% efficiency. Net yield is about 600 kW of food calories as shrimp, or 12 million Cal/day. This would be enough to supply 2500 Cal/day to 4800 people, roughly 1 person per kW of nameplate.
It's not so impressive as motor fuel. A gallon of gasoline is 33.7 kWh of thermal energy. If you generated it at 70% efficiency, it would require 48 kWh input. That gallon might drive an efficient hybrid 55 miles, but the 48 kWh would drive a typical electric vehicle 192 miles at 250 Wh/mi.
Electric-to-chemical looks good as a dump load for spot surpluses of electricity and a high-tech last-ditch food source. Not so good for anything else.
Posted by: Engineer-Poet | 24 July 2010 at 09:52 AM
@ToppaTom - If you can use the electricity directly, obviously just do that. There is clearly no reason to convert back and forth if you don't need to. The purpose of converting electricity to a liquid hydrocarbon is mainly to obtain a high energy density for application as transportation fuel which can be used in existing fuel infrastructure and vehicles.
@Engineer-Poet - You are fully correct in that storing electricity in batteries used to drive an electric motor is more efficient than storing it as liquid fuel and converting it to vehicle motion. However, efficiency is certainly not always the deciding factor - the cost of the battery can easily outweigh the cost of energy wasted, and the battery electric vehicle is less convenient with a limited range and slow recharging.
Not sure why people are focusing on making synthetic food ?? It is much easier and more sensible to make synthetic fuel for machines to consume than to make synthetic food for natural creatures! Hydrocarbon fuel production by electrolysis is better than biofuels - if we do not use biomass for machine fuel production then we can dedicate it to food and preserving nature. Keep the machines feeding the machines and life feeding life.
Posted by: energy del sol | 24 July 2010 at 10:57 AM
I agree that putting the electricity into batteries and running EVs is more efficient and if we had 100 million of them on the road we would be in good shape. But after 10 years we have less than 1% of the cars on the road HEVs getting 30% better mileage. I don't think we can wait many decades longer for full adoption of EVs. We need to promote EV/PHEV/EV while we are doing plan A...FFV/M85.
Posted by: SJC | 24 July 2010 at 11:00 AM
A scheme like this works much better with PHEVs. The cost/year of PHEVs goes up as the battery size increases, as you have to amortize the battery even if it isn't all being used. At some point the incremental mile is cheaper to run using electrolytic fuel in a range-extender than a bigger battery. The losses of the system may be acceptable if they only have to be taken for 20% of total miles driven.
There's plenty of CO2 emitted by landfill decomposition, manure fermentation and such. All of this CO2 is a relatively easy-to-capture byproduct potentially ready for electrolytic, photochemical or other conversion to useful products.Because it's interesting, and the opposite of "food into fuel".
Posted by: Engineer-Poet | 24 July 2010 at 11:40 AM
If a process can be secured to provide sufficient liquid fuels from CO2 and H2O, I can't see what the problem is vis a vis the EV debate.
People should, and I dare say will have a choice between EVs or liquid fuelled vehicles in the future. I don;t think it will be a case of either/or. Unfortulately there does seem to be a lot of politics behind this debate.
Personally I prefer liquid fuels. My short trips that the EV market will aim at are done on foot or by mass transit. My trusty old Audi A6 comes into its own for long roadtrips - 900 miles on a single tank.
Posted by: Scott | 25 July 2010 at 07:44 AM
And of course, air transport. There's no alternative in the near term to liquid hydrocarbon fuels for powering aircraft.
Posted by: richard schumacher | 25 July 2010 at 08:37 AM
@Engineer-Poet:
Then no one will buy the synthetic fuel :)
Exactly, I agree completely. Use electricity to drive vehicles as much as you can and use liquid fuels for range extension. PHEVs are nice because as you suggest, a large fraction of miles can be powered from the battery with a minimal (expensive) battery size. Liquid fuels will still be needed (especially in aircraft, ships, etc) and producing them in a method like the above, without using biomass or fossil fuels, is surely the most sustainable way.
Even if you cut liquid hydrocarbon fuel demand by 80% as you suggest might happen with full adoption of PHEVs, the CO2 sources you're talking about will only supply a very small fraction of what is needed to produce fuels by recycling the CO2 at transportation-sector scale. To really scale up CO2 recycling you need bigger sources of CO2. For a while one could use industrial process emissions but the fuels would not be carbon-neutral since you recycle the carbon only once - ultimately you need to form a closed-loop cycle by capturing CO2 from the atmosphere (see the work of Klaus Lackner who pioneered CO2 "air capture" and recently described the latest low-energy-demand technology, links 1 2 3).
I'll give you that it is interesting, but food and nutrition is so much more complicated than fuel for machines - I cannot see us being able to make synthetic food and support human health in any reasonable time frame, whereas making synthetic liquid hydrocarbons identical to gasoline/diesel is very straightforward.
Also, that not everyone has enough food is not a technological problem.
Posted by: energy del sol | 25 July 2010 at 10:16 AM
There's a lot of CO2 from non-wastes produced today. Fermentation of corn to ethanol produces 1 mole of CO2 per mole of ethanol (roughly 1.7 pounds of elemental carbon per gallon of EtOH). All of that is potential feedstock for CO2-to-hydrocarbons processes. The CO2 from a 10-billion-GPY ethanol industry could produce another 3.2 billion gallons of gasoline. It's no replacement, but it's enough to do much better than E85 with no fossil inputs.
It's quite simple, actually. If you can use electricity to fix carbon in a form chemotrophs can eat, you can select organisms to make a food chain ending in something humans like. This isn't too likely to be used in lieu of crops in fields, but on something like a spacecraft it could fit nicely/It's quite simple, actually. If you can use electricity to fix carbon in a form chemotrophs can eat, you can select organisms to make a food chain ending in something humans like. This isn't too likely to be used in lieu of crops in fields, but on something like a spacecraft it could fit nicely.Posted by: Engineer-Poet | 25 July 2010 at 01:11 PM
If the farmers in South America and Africa not only have a domestic market for food crops, but have a market for the farm waste for fuel, OPEC pricing power will diminish. This would go a long way towards making those countries more prosperous with the added benefit of becoming world customers for goods and services.
Posted by: SJC | 25 July 2010 at 04:12 PM
Well done folks!
This is one of the best discussions I've seen in some time.
Keep up the good work.
Posted by: Lucas | 25 July 2010 at 07:22 PM
Thank you Lucas, we have our moments.
Posted by: SJC | 25 July 2010 at 11:23 PM
Why we should make synthetic food ? :
Because agriculture is the industry that is by far the most responsible for habitat destruction and biodiversity loss.
Simply use google-earth and look at your own country. look at the total area, and how much percentage is used for 'real' industry, how much for housing, how much for agriculture and how much of it is left for the other organisms inhabiting our planet. It is very clear that if we want to preserve nature, the farmost important thing we must do is give them the ecosystems and areas. In every discussion about biofuels, the strongest objections are about land use. This also should count for food in general.
The price of (renewable and nuclear) energy will keep comming down by orders of magnitudes as technology evolves.
Since human food consumption only has the energy value of about 10 MJ per day (=250ml gazoline), it is only a very small proportion of the total human energy consumption. As soon as we are able to produce all of our energy needs renewably, it will only be a very small step to also produce all of our food needs synthetically. Even if the conversion of wind-energy to food only has an efficiency of 30%, it will still be a very worthy task. Immagine how much habitats can be restored, created and protected if we don't need the areas for food production anymore.
Of course, it would also solve enourmous problems considering food-safety and prevent water-wars, since conversion of wind-energy to food would only use minute amounts of water compared to agriculture.
Actually, making synthetic food is already relatively simple. It is easy to make synthetic molecules that can fed to animals : acetic acid, ethanol, fatty acids can be made chemically and fed to algae or directly to animals to turn them into proteins or meat directly.
If not feeding animals purely by synthetic molecules, a significant part of their calory intake kan be provided by synthetic molecules. It will surely be possible to combine the genes of acetate-eating micro-organisms with plant-genes to produce a different kinds of starch-sources that can be transformed to bread, spaghetti, 'rice'. Just like we now transform 1000 litres of milk in 1000l of yoghurt overnight, we will transform 1000000kg of acetic acid (or some other synthetic molecule) to 800000kg of starch overnight.
Posted by: Alain | 26 July 2010 at 03:07 AM
Yes a very thoughtful discussion.
In the UK North Sea gas was once converted into Methanol to feed organisms to make Pruteen, a cattle feed. If it had been used, the whole Mad Cow disease would have never happened, and instead the farmers fed the cows badly cooked sheep brains.
In Norway the oil company was producing pet food from North Sea gas.
In the UK, Quorn is organism produced proteen sold in many edible forms, and it can likely be trained to feed on hydrogen and CO in various combinations as some are fermenting ethanol with it for fuel, but ethanol is also a human food used for many calories by many persons.
Some organisms in a cow's guts might be able to use hydrogen injected direcly. Can any animal use hydrogen dissolved directly in the bloodstream for energy??
The sodium sulphur battery in large quantities can become the most cost effective electric energy storage device in the near future. Several installations are in operation in the US now in addition to the ones in Japan. The device also could be modified into a large scale flow battery with large tanks of sodium and large tanks of sulphur and large tanks of the product with a smaller set of reaction cells. Perhaps it is nearly as cheap just to build many cells. France should fund such a project.
General Electric's version of the ZEBRA battery can be used, at lower costs, with nearly equal or better energy density to lithium batteries in large regularly used vehicles and many automobiles. If you don't drive your car every day and do not wish to use it for grid connection service for cheaper home power, use lithium or lithium-zebra-Nimh hybrids. A Prius with an extra ZEBRA battery makes a long range electric vehicle.
The US coal industry should get its money together to make a coal to diesel conversion factory going on a foreign remote pacific rocky island. The manufacturing CO2 release will not count against the US CO2 releases because Australia does not count its export of coal against itself. Perhaps floating coal to diesel converting ships can be built.
All new natural gas burning power plants should be made with a number of small engines or turbines that can be stopped and started quickly to match the energy produced from wind and solar, and Direct current transmission lines can carry such solar-wind-natural gas power almost any distance more efficiently than conversion of electricty to fuel.
Natural gas not used because of available windpower can be converted to liquid fuels very cheaply and efficiently. What is really needed is an electrolysis cell that converts oxygen and methane to methanol directly.
Coal burning power plants should be all converted to synthesis gas from coal fueled small turbines or engines which likewise can start and stop rapidly to match demand. Making the synthesis gas will nearly eliminate ash and pollutant release to the air and it will be available to make methanol at times of low demand. Synthesis gas can be used in high efficiency engines and combined cycle units. Coal can be converted and transported in the form a pump able water slurry.
The avoiding of the use of natural gas and coal when wind and solar are available is the cheapest and most efficient way to store electricity and makes hydrocarbons available for liquid fuel making.
The energy density and future very low cost of Sodium Nickle Chloride or Sodium Sulphur batteries allow for the complete electrification of all railroads in countries with a power grid and without installing any overhead catenaries. Many roads in such countries can also be partially electrified at a small fraction of the cost of the total cost of the roadway.
Glycerol is a simple molecule that can be used as food but there are a few other very small ones that also can supply human energy. Vinegar is one. What are the others? Subcritical wet air oxidation can produce vinegar and its relatives from paper or sawdust. ..HG..
Posted by: Henry Gibson | 27 July 2010 at 11:57 AM
I don't see anyone bringing up the obvious question. If the busbar cost of nuclear power is in the 1.5-3 cent per kilowatt hour range and the economics of fuel syn plus air capture is such that with 2 cent power its $2 a gallon this is economical right now with nuclear power. forget wind mills with a 30% rated capacity those are pipe dreams nuclear plants have a 95% capacity factor some reach 98% thats 2-3 cent power 24/7 over a multiyear period. locate a synfuel plant next to a nuke plant and take advantage of cheap power.
Posted by: TXGeologist | 31 July 2010 at 02:36 PM