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PSI team demonstrates direct hydrocarbon fuel production from water and CO2 by solar-driven thermochemical cycles

Solar-driven thermochemical cycles offer a direct means of storing solar energy in the chemical bonds of energy-rich molecules. By utilizing a redox material such as ceria (CeO2) as a reactive medium, STCs can produce hydrogen and carbon monoxide—i.e., syngas—from water and CO2. The syngas can subsequently be upgraded to hydrocarbon fuels by the Fischer-Tropsch process.

Now, a team from the Paul Scherrer Institute (PSI) in Switzerland has demonstrated the direct production of hydrocarbon fuel—specifically methane—from water and CO2 by incorporating a catalytic process into STCs. A paper on their work is published in the RSC journal Energy & Environmental Science.

Lin1
Schematic illustration of direct hydrocarbon (CxHyOz) formation from water and carbon dioxide during the reoxidation of reduced ceria doped with a catalyst (Cat.). Lin et al. Click to enlarge.

Most research efforts in designing redox materials for STCs are devoted to improving syngas production. This can be achieved by doping ceria with heterocations, or by fabricating porous structures to facilitate mass/heat transfer and improve the redox kinetics. A very attractive route is the direct production of hydrocarbon fuels from water and carbon dioxide by realistic STCs. This new concept, if high selectivity for hydrocarbons is achieved, inherently bypasses a second stage conversion, such as methanation or Fischer-Tropsch processes. This could potentially make the solar fuel production chain much more economical. In addition, storage and transportation of syngas would not be required.

… With the aim to produce hydrocarbon fuels directly from water and carbon dioxide, we propose a strategy of incorporating a catalytic process into STCs by adding a catalyst to ceria. This study shows that direct methane production in STCs is possible. The primary role of the catalyst is to drive the formation of hydrocarbon molecules. The formation of these hydrocarbon fuels can be either from the conversion of the syngas generated by water and carbon dioxide splitting, or directly from water and carbon dioxide without the intermediate formation of syngas, or from a combination of both.

—Lin et al.

In the study, the team worked with nickel-doped ceria and rhodium-doped ceria. Both materials, after being reduced by hydrogen at 600 ˚C, are active in producing methane during their reoxidation by water and carbon dioxide at 500 ˚C.

The researchers then further evaluated both materials in realistic thermochemical cycles, in which they were activated by thermal reduction.

Generally, they found that the nickel-doped ceria exhibited poor dynamic redox capacity; they further concluded that nickel-doped ceria is not chemically stable at the extreme temperatures required for realistic STCs.

However, with rhodium on ceria, they were able to achieve direct and sustained production of methane from water and CO2. The material exhibited methane, hydrogen and carbon monoxide formation activity during 58 cycles with the activation of the material carried out at 1400 ˚C.

Encouragingly, the material exhibits steady increase of activity for methane production when activated at 1500 ˚C. X-ray diffraction reveals the presence of metallic rhodium in the materials after cycling at 1400|500 ˚C and after additional cycling at 1500|500 ˚C, indicating metallic rhodium as the active catalyst for methane formation. This proof-of-principle study leaves significant room for improvement and may stimulate a new research area of solar thermochemical fuel production. Future research efforts shall be directed towards improving the product selectivity to methane and potentially to other hydrocarbons, preferably liquid hydrocarbons like oxygenates.

—Lin et al.

Resources

  • Fangjian Lin, Matthäus Rothensteiner, Ivo Alxneit, Jeroen A. van Bokhoven and Alexander Wokaun (2016) “First demonstration of direct hydrocarbon fuel production from water and carbon dioxide by solar-driven thermochemical cycles using rhodium-ceria” Energy Environ. Sci. doi: 10.1039/C6EE00862C

Comments

Engineer-Poet

It requires very clear skies to achieve even 600°C with concentrating solar.  This is well within the range of molten-salt reactors, though.  Combined with an atmospheric capture system, these catalysts could achieve direct conversion of heat to carbon-neutral fuel.

Almost 1% of thermal-neutron fissions in U-235 yield rhodium; the catalyst might be produced in part from high-level nuclear waste!

ai_vin

There is nothing exceptional in getting 600°C with concentrating solar. The temperature at the focal point of a solar furnace may reach 3,500°C. The reason solar thermal power plants operate at lower temperatures has more to do with economics than capability: Lower operating temperatures mean cheaper construction materials can be used.

Engineer-Poet

You need almost all direct radiation to get such temperatures with solar, which precludes operation in areas with significant cloudiness (even haze).

ai_vin

Maintaining power output and designed temperatures under less than very clear skies is easily handled by bringing more heliostats to bear on the central tower. A proper design will have enough of these in reserve.

Engineer-Poet

So if weather conditions result in 50% of the clear-sky direct radiation coming from further off-axis than the angular width of the target, the plant will be designed with 100% excess heliostats (which go unused much of the time) to generate full thermal power anyway?

Give me a break.

ai_vin

No. The reserve has to be there anyway to handle variations in demand, just like any other power plant.

ai_vin

Also, this article is discussing using solar-driven thermochemical cycles as a "direct means of storing solar energy in the chemical bonds of energy-rich molecules" so the main job of the power plant would be electricity production and we'd be talking about the "excess" capacity going into methane production whatever type of power plant you'd want to use - be it solar or nuclear.

Engineer-Poet
The reserve has to be there anyway to handle variations in demand

None of the descriptions of CSP plants I've seen mention reserve heliostats.  They have empty acreage so that heliostats do not shadow each other, but none appear to have excess mirrors; they can use every ray they can capture.

just like any other power plant.

No other type of power plant is in the position of suddenly requiring a great deal more of its basic input to maintain the same output.  Ivanpah doesn't even try; it lights up its natural-gas burners when the sun falls short.

If Ivanpah is the example, its pathetic thermal efficiency (<29%) despite its high steam temperature does not make it something to emulate.  The typical nuclear plant has a thermal efficiency around 33% despite steam temperatures in the neighborhood of 300°C.

this article is discussing using solar-driven thermochemical cycles

The catalyst requires reduction with hydrogen.  Something else has to produce that hydrogen.  You will have to find your panacea elsewhere.

ai_vin

Something else has to produce that hydrogen.

Read the first 2 paragraphs of this article.

ai_vin

No other type of power plant is in the position of suddenly requiring a great deal more of its basic input to maintain the same output.

That's not what I'm saying and I think you bloody well know it. I'm saying that because demand is variable some of a power plant's capacity has to be held in reserve to handle sudden increases in demand. And yes, Ivanpah is a bad example - on many fronts - which is why you used it. But then nuclear has had its bad examples too so lets stay out of that mud-pit ok?

ai_vin

None of the descriptions of CSP plants I've seen mention reserve heliostats. They have empty acreage so that heliostats do not shadow each other, but none appear to have excess mirrors; they can use every ray they can capture.

Don't get so hung up on the current practice. CSP plants (and nuclear) are still outnumbered by fossil fuel plants so using every ray they can capture keeps hydrocarbons from being burnt in another plant elsewhere. For now, but in the future when fossil fuel plants have been put out of our misery...

Engineer-Poet
Read the first 2 paragraphs of this article.

I did better than that, I went to the paper.  What it actually says is this:

After being thermally reduced at extreme temperatures of 1400 oC and 1500 oC, metallic rhodium is formed in rhodium-doped ceria. The activated rhodium-ceria produces methane directly from water and carbon dioxide during reoxidation.

So you need temperatures over 1400°C to thermally reduce the ceria, otherwise you need some other reductant (e.g. hydrogen).  The sulfur-iodine cycle requires far less extreme conditions.

No other type of power plant is in the position of suddenly requiring a great deal more of its basic input to maintain the same output.
That's not what I'm saying and I think you bloody well know it.

It doesn't matter what you're saying, you're confused.  The fact is that if you lose half the direct radiation which can be focused on your target, you need to double the capture area (if that's even possible).  I'm not sure what drives the cost of CSP systems, but heliostats are certainly a very big part of it.  Idle heliostats would be a waste of money in what is already a very costly system.

I'm saying that because demand is variable some of a power plant's capacity has to be held in reserve to handle sudden increases in demand.

The only practical way to do that with CSP is to use heat storage.

nuclear has had its bad examples too

There are several existence proofs of almost completely decarbonized industrial-scale electric grids using a foundation of nuclear power, and zero using wind, solar or any combination.  When you can show me one, you will have a point.

None of the descriptions of CSP plants I've seen mention reserve heliostats.

Don't get so hung up on the current practice.

So you admit that the reserve heliostats you mentioned don't exist.  Making Shit Up is not a way to have a productive discussion.

Engineer-Poet

(bah, moderated... reposting)

Read the first 2 paragraphs of this article.

I did better than that, I went to the paper.  What it actually says is this:

After being thermally reduced at extreme temperatures of 1400 oC and 1500 oC, metallic rhodium is formed in rhodium-doped ceria. The activated rhodium-ceria produces methane directly from water and carbon dioxide during reoxidation.

So you need temperatures over 1400°C to thermally reduce the ceria, otherwise you need some other reductant (e.g. hydrogen).  The sulfur-iodine cycle requires far less extreme conditions.

No other type of power plant is in the position of suddenly requiring a great deal more of its basic input to maintain the same output.
That's not what I'm saying and I think you bloody well know it.

It doesn't matter what you're saying, you're confused.  The fact is that if you lose half the direct radiation which can be focused on your target, you need to double the capture area (if that's even possible).  I'm not sure what drives the cost of CSP systems, but heliostats are certainly a very big part of it.  Idle heliostats would be a waste of money in what is already a very costly system.

I'm saying that because demand is variable some of a power plant's capacity has to be held in reserve to handle sudden increases in demand.

The only practical way to do that with CSP is to use heat storage.

nuclear has had its bad examples too

There are several existence proofs of almost completely decarbonized industrial-scale electric grids using a foundation of nuclear power, and zero using wind, solar or any combination.  When you can show me one, you will have a point.

None of the descriptions of CSP plants I've seen mention reserve heliostats.

Don't get so hung up on the current practice.

So you admit that the reserve heliostats you mentioned don't exist.  BS is not a way to have a productive discussion.

ai_vin

So you need temperatures over 1400°C to thermally reduce the ceria

Which are temperatures that can be generated by solar so it is still "using solar-driven thermochemical cycles."

Back in the day I owned a portable solar cooker and soon needed to buy new cookware because I exceeded the melting point of the copper. If I could do that with something that could be backpacked into a campsite...

Engineer-Poet
So you need temperatures over 1400°C to thermally reduce the ceria
Which are temperatures that can be generated by solar

But not with anything resembling current CSP systems, only specialized solar furnaces with very specific requirements and relatively low wattages.

Back in the day I owned a portable solar cooker and soon needed to buy new cookware because I exceeded the melting point of the copper.

Elemental copper melts at a mere 1084 C, and alloys no doubt much sooner.  Besides, what you could focus on a poorly-cooled spot on an ideal day has nothing to do with what even a well-engineered collector can achieve with what Nature provides on a string of bad days.

If I could do that with something that could be backpacked into a campsite...

... you'd be the average geek buying Fresnel lenses from Edmund Scientific.

gorr

O.K it work so now what i want is that they start selling synthetic gasoline instead of methane in my area at a cheaper price than actual gasoline made of conventional petroleum, go go go, i will be the first customer, im so exited

ai_vin

But not with anything resembling current CSP systems

This is true, but then it was you who started this by saying "this is well within the range of molten-salt reactors." So, how many of these molten-salt reactors are currently in commercial operation? And while your at it tell us if any molten-salt reactors ever built actually operated at the 1400°C you now say we need to use this rhodium-doped ceria.

In any case we could still use hydrogen to reduce the catalyst, as all we need to get that is an electrolyzer plugged into the power plant of your choice.

Engineer-Poet

I must also correct myself:  no MSR, historical or proposed, is going to reach the 1400°C temperatures required for thermal reduction of this rhodium-ceria catalyst either.

ai_vin

Fair enough.
BTW, I found something on the larger topic of using solar heat for industry. It covers the good, the bad, and the ugly;
http://www.lowtechmagazine.com/2011/07/solar-powered-factories.html

One of the goods? Something that should be real obvious: If you're using heat to produce electricity you lose 2/3 of the energy - but the flip side of that is if you can use the heat directly you get 3 times the work.

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