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UTA researchers demonstrate one-step solar process to convert CO2 and H2O directly into renewable liquid hydrocarbon fuels

Researchers at the University of Texas at Arlington have demonstrated a new solar process for the one-step, gas-phase conversion of CO2 and H2O to C5+ liquid hydrocarbons and O2 by operating the photocatalytic reaction at elevated temperatures and pressures.

The photothermocatalytic process for the synthesis of hydrocarbons—including liquid alkanes, aromatics, and oxygenates, with carbon numbers (Cn) up to C13—ran in a flow photoreactor operating at elevated temperatures (180–200 °C) and pressures (1–6 bar) using a 5% cobalt on TiO2 catalyst and under UV irradiation. A paper describing the process is published in Proceedings of the National Academy of Sciences (PNAS).

We are the first to use both light and heat to synthesize liquid hydrocarbons in a single stage reactor from carbon dioxide and water. Concentrated light drives the photochemical reaction, which generates high-energy intermediates and heat to drive thermochemical carbon-chain-forming reactions, thus producing hydrocarbons in a single-step process.

—Brian Dennis, UTA professor of mechanical and aerospace engineering and co-principal investigator of the project

The process uses cheap and earth-abundant catalytic materials. Further, the unusual operating conditions expand the range of materials that can be developed as photocatalysts. Although the efficiency of the current demonstration system is not commercially viable, they team noted, it is also far from optimized and it opens a promising new path by which such solar processes might be realized.

This simple and inexpensive new sustainable fuels technology could potentially help limit global warming by removing carbon dioxide from the atmosphere to make fuel. The process also reverts oxygen back into the system as a byproduct of the reaction, with a clear positive environmental impact, researchers said.

The eventual replacement of oil with fuels generated from sustainable and carbon-neutral sources is necessary if we are to avoid harmful climate change due to the buildup of greenhouse gases in the atmosphere. Advances in solar-based technologies are the most promising; however, these technologies generally produce either electricity or hydrogen, neither of which is an ideal replacement for liquid hydrocarbons. The least disruptive technology would replace oil-derived hydrocarbons with liquid hydrocarbon fuels derived from CO2, water, and a clean energy source, such as the sun, leading to a carbon-neutral fuel cycle.

Currently, there are a number of promising strategies to harness solar energy to generate high-energy molecules (fuels) from water and/or carbon dioxide, including (i) high-temperature thermochemical cycles, (ii) coupling photovoltaics to water electrolysis (PV-EC), (iii) developing single or tandem photoelectrochemical cells (PEC), or (iv) direct photochemical methods (PC) using semiconductor materials, often modified by added cocatalysts or nanostructuring techniques. Hydrogen, carbon monoxide, C1 hydrocarbons, and syngas are the most commonly produced fuels and are derived from water or water and CO2. Hydrogen produced via the water-splitting reaction (WSR) … is arguably the easiest to produce and stores the most energy on a mass basis (kJ/kg); however, it is not a particularly attractive replacement fuel for transportation, due to technological issues with low-volume energy density, safe storage, and transportation.

One commonly proposed solution to this dilemma is to use the H2 generated via the WSR, reaction 1, in combination with CO2 to synthesize liquid hydrocarbon fuels, using the reverse water–gas shift (RWGS), reaction 2, and Fischer–Tropsch synthesis (FTS), reaction 3. … We report here a photo-thermochemical process for driving the alkane reverse combustion (ARC) reaction (reaction 4) to produce C1 to C13 hydrocarbons in a single operation unit.

—Chanmanee et al.

H2O → H2+ ½O2 ΔG˚= 237.3kJ/mol WSR [1]
CO2 + H2 ⇌ CO + H2O ΔG˚= 25.2kJ/mol RWGS [2]
(2n +1)H2 + nCO → CnH(2n+2) + nH2O ΔG˚~ -99 n kJ/mol FTS [3]
(n+1)H2O + nCO2 → CnH2n+2 + (3/2n + ½)O2 ΔG˚~ 665 n kJ/mol ARC [4]
The solar photothermochemical alkane reverse combustion (SPARC) uses the sun process to provide both photons and heat. To examine the SPARC reaction, the team built a fixed-bed, tubular flow reactor in which the catalyst bed could be both heated and irradiated.

CO2 and steam were flowed at 40 standard cubic centimeters per minute (sccm) over the 5% cobalt on a TiO2 catalyst bed, which was heated via an internal electric heater and irradiated with four surrounding 250-W Hg lamps.

Arc
Schematic diagram of photothermal flow reactor with cartoon picture of a single Co/TiO2 particle undergoing catalysis and TEM picture of cobalt on P25 TiO2 catalyst. Chanmanee et al. Click to enlarge.

The products were collected by passing the hot effluent gas through a condenser unit at 0 °C to capture condensable products; through a back-pressure regulator to drop the pressure to 1.0 bar; and then through a sampling loop of an automated online gas chromatograph.

A parametric study of temperature, pressure, and partial pressure ratio showed that temperatures in excess of 160 °C were needed to obtain the higher Cn products in quantity. The product distribution also shifted toward higher Cn products with increasing pressure. In the best run, more than 13% by mass of the products were C5+ hydrocarbons. Some of those, such as octane, are drop-in replacements for existing liquid hydrocarbon fuels.

At present, this gas-phase SPARC reaction is far from optimized and simply shows proof of principle. Higher productivities and better product distributions are likely to be realized as pressure, temperature, reactant ratio, space velocity, and catalyst are optimized.

… Although the current SPARC technology is currently im- practical on a commercial scale, it does offer a conceptually new and commercially promising solar fuels technology that would be simple and inexpensive relative to most PV-EC and PEC systems. The direct production of the value-added hydrocarbons liquid fuel minimizes the number of unit operations involved and the associated efficiency losses and capital expenses of each. In a field operation, it is easy to imagine the use of parabolic mirrors to focus and concentrate sunlight onto a catalyst bed, providing both the photons required for photoexcitation and the thermal energy needed to run the reaction. Assuming such a system may require active cooling, the excess thermal energy could be used for downstream product separations or other applications in which relatively low-grade heat can be applied. In this respect, an SPARC process can realize greater efficiencies than process requiring ambient or near-ambient temperatures in that the low-energy photons are used to help heat the SPARC reaction and to heat a working fluid to a more useful temperature (i.e., 200 °C).

—Chanmanee et al.

One of the next steps for the team is to develop a photo-catalyst better matched to the solar spectrum, said Frederick MacDonnell, UTA interim chair of chemistry and biochemistry and co-principal investigator of the project.

The research was supported by grants from the National Science Foundation and the Robert A. Welch Foundation.

Resources

  • Wilaiwan Chanmanee, Mohammad Fakrul Islam, Brian H. Dennis, and Frederick M. MacDonnell (2016) “Solar photothermochemical alkane reverse combustion” PNAS doi: 10.1073/pnas.1516945113

Comments

Davemart

~200C is not an excessively high temperature.

If they can step this up, bang goes much of the rationale for battery electric and hydrogen fuel cell cars.

High temperature fuel cells would still be good to have though, as not only are they more efficient than combusting the fuel,but they produce tiny amounts of pollutants.

You just can't tell what is going to come out of left field.

ds

Very promising indeed. There was a similar approach with an electrical current to get syngas from co2 and h2o. Still I think BEV are unbeatable for simplicity of construction and rumourless operation. In any case two thirds of the population will live in walkable neighbourhoods with streetcars and bikes ...

gorr

Do synthetic gasoline and put it on the market right away at a lesser price than petroleum polluting gasoline.

It's easy to see that all that is just a gimmick to cash-in some subsidies and it don't really work and it is also the 50 th time that i read the same thing since the last 5 years.

sd

More magic with chemistry. I am just about convinced you can make any chemical starting with any other chemical if the atomic constituents add up (right number of H, C, O atoms, etc -- still can not make gold from lead) if you have the right conditions and ADD ENOUGH ENERGY. That last phrase is generally the killer. This was probably a good academic project but now what? I think that if you have solar energy, the most effective thing is probably to add it to the grid and avoid using coal and/or natural gas to generate electric power and now you do not have the CO2 to worry about. Of course, I would still advocate using nuclear power for most of our base load electrical power generation.

Davemart

@sd:

Since the temperatures and pressures are pretty modest, this hardly sounds like an energy hog.

Renewables are great for electricity, but that does not help much without chemical storage in places where they have winters.

Nuclear is also great.
The problem is that they are not building it in the West.

electric-car-insider.com

Exciting breakthrough. I wish the UTA team all the best.

You have a good point about seasonal supply/demand mismatch, Davemart. There are places solar won't work well. Every region needs to optimize its natural RE resources. Denmark is doing well with wind: 43% of national consumption, expected to be 85% by 2035. Pacific Northwest US uses 60-80% hydro depending on snowpack.

Solar is making advances to achieve 24 hour baseload:

Crescent Dunes 24-Hour Solar Tower Is Online
http://cleantechnica.com/2016/02/22/crescent-dunes-24-hour-solar-tower-online/

No one solution is going to solve the entire planet's energy problem.

Davemart

Denmark happens to have Norway handy with its fjords and hydro, which enable their high renewables proportion.

Maybe every country should have at least one Norway, but they haven't, and so need chemical energy storage, as Germany are setting up.

D

@ electric-car-insider.com
Crescent Dunes 24-Hour Solar Tower Is Online
What went wrong with the one in California? Ivanpah Solar I think. That one is not working out at all. It requires constant use of natural gas to keep the salt hot.

Davemart

BTW:

Germany would need around 10-20 Norways with all its mountains and fjords to run renewables the same way as Denmark do.

Oddly they don't fancy trying to build that, and the real estate is not available, so they are going for chemical storage.

sd

@Davemart

The temperature and pressures are not the problem. The problem is in the required solar energy to drive the reactions

H2O → H2+ ½O2 ΔG˚= 237.3kJ/mol WSR [1]
CO2 + H2 ⇌ CO + H2O ΔG˚= 25.2kJ/mol RWGS [2]
(2n +1)H2 + nCO → CnH(2n+2) + nH2O ΔG˚~ -99 n kJ/mol FTS [3]
(n+1)H2O + nCO2 → CnH2n+2 + (3/2n + ½)O2 ΔG˚~ 665 n kJ/mol ARC [4]

See those KJ/mol. What they do not tell you is the KJ/mol they have created but if I did the basic math and chemistry correct (It has been 50+ years since I had college chemistry), I think that heptane, C7H16 (molecular weight of 100) which is a major constituent of gasoline has 47 MJ/KG or 4.7 MJ/mol. The last conversion takes 665 n KJ/mol where n is 7 or 4.65 MJ/mol, about the same was the energy you put in plus you have 8 H2 at 237.3 + 25.2 KJ/mol or another 2.4 MJ/mol plus the heat and pressure and other assorted losses.

Anyway, the academics could have done the math for us and given us an efficiency but I am fairly sure that this is not the break-thru that will save the planet.

electric-car-insider.com

@D, Ivanpah got a slow start in 2014.. My understanding is that the steam technology used did not provide sufficient continuity to meet capacity factor targets. 2015 performance was substantially better. I expect they'll get it dialed in.

Crescent Dunes uses molten salts, allowing continuity through both transient weather disruptions and overnight.

Davemart

sd:

Since I have not taken chemistry, you will have to translate that some more for it to mean much to me.

What overall efficiencies are you getting?

My ( limited ) understanding is that the more complex molecules take more energy to form, but I would have thought that methanol, DME or ammonia should do the job fine, although to be sure some alterations of the supply chain would be needed.

SJC

"Ivanpah generation was up 170 percent over the same quarter in 2014 – 108 gigawatt-hours compared to 40 GWh..."
https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility

Roger K. Brown

In order for this process to be carbon neutral it has to use CO2 from the atmosphere rather than CO2 obtained from fossil fuel plants or from cement processing. So in addition to improving the economics of the synthesis process, the economics of atmospheric CO2 extraction must also be improved. The second requirement may prove far more difficult than the first.

Davemart

Roger:

At the moment CO2 from industrial processes just goes straight into the atmosphere.

Lets not make the perfect the enemy of the good.

Technologies don't start off fully developed.

SJC

RKB,
Use CO2 twice, cut emissions in half.

Juan Valdez

There is plenty of energy in solar from mirrors, so the heat and pressure are free. The CO2 comes out of the atmosphere, so what am I missing?

If the efficiency can be scaled up, this seems like a potentially great solution.

sd

Davemart:

This process requires H2 and CO (Carbon monoxide). There are using energy to split H2O into H2 and O (237.3kJ/mol). A mol or mole is just a mass measurement where you take the molecular weight of a given substance and make it into grams. A mol of H2 weights 2 grams, a mol of H20 weights 18 grams, and a mol of heptane or C7H16 weights 7 x 12 grams for the carbon and 16 x 1 grams for the hydrogen or 100 grams. Then they use more energy and some of the hydrogen to strip an oxygen from the CO2 to get CO.

Anyway, there is no free lunch. You use more energy to get the liquid hydrocarbon than there is in the liquid hydrocarbon. What I was saying is that if you have solar power, just put it in the grid and if you need liquid hydrocarbons, it is easier to make them from the natural gas you just saved.

This was a nice research project -- end of story.

Lad

If we can move 90% of worldwide transportation, power generation and industry off fossil fuels, that would do it for GHGs and would clean up at least 70% of the air in the U.S. alone. Can you imagine actually being able to again view the San Gabriel Mountains from downtown L.A. the whole year?

The idea is to only use oil for feedstock to produce petrochemical products and not fuel that is burned in the air.

If the above described process will produce carbon feedstock for products, other than fuel, by using CO2 from the air and Sun energy, I'm for it because it reduces the need to damage the air, land, water and human health by mining for fossil feedstock.

Davemart

sd:

Sure, everything is lossy.
The question is, how lossy.

I can't read from what you have said anything about the efficiencies.

It seems from reading the article that a lot of the losses are taken into account by upping the temperature to 200 C and the pressure - that is pretty much what the sunlight is doing.

This is also an unoptimised process, as they say it is using a relatively limited light bandwidth.

So what are you saying that I am missing, other than that it is early stage, which the authors make clear anyway?

SJC

They can get the CO2 from several renewable sources.
Ethanol plants, landfills and water treatment are just a few.

Seer

Efficiencies please.

sd

Davemart et al

The easier way to think about this is that when you burn a hydrocarbon or react it with oxygen, you end up with H20 and CO2 along with energy. These guys are basically reversing the reaction. They adding sufficient energy to split the H20 and strip an O of the CO2 and then combining the CO with H2 with enough extra H2 to go to CnHn+2 + H20. They even refer to it as reverse combustion. It always takes more energy to reverse a reaction. There is no free lunch. You can not build a perpetual motion machine. Their only real claim is that they could use solar energy to drive the reverse reaction. My question to all of these different schemes is if you have the energy and especially if you have electric power why not send it to the grid and replace the coal or gas combustion and not generate the CO2 to start with. If we had excess base load nuclear power, then you could worry about all of these crazy schemes.

Anyway, the good thing is that some grad students are probably using this as a research project to get degrees and hopefully will go on to do something useful.

Cautious9

My concern about this process is the use of ultraviolet light. UV photons can drive useful reactions such as splitting water to produce H2. But in the sunlight striking earth, not much of the energy is in the form of UV photons. So if the process is limited by the amount of UV photons, then the energy efficiency cannot be very good. The review of the process doesn't discuss the efficiency in any detail, but it seems likely that most of the energy in the sunlight goes into heating the sample, producing a good environment for catalysts that produce higher hydrocarbons. But that energy probably is not increasing the energy of the fuel, just helping the catalyst reform it. It seems likely that most of the net energy is coming from the UV - producing H2 which can then react with the CO2 and water to form hydrocarbons. This means the process will be highly inefficient unless one gets very lucky and finds a catalyst that can produce high energy chemicals from low energy visible photons. The efficiency matters because this process will have all the capital cost of a solar thermal electricity generating system. If the efficiency is small compared to the solar thermal system (or a photovoltaic system), then it would be much better to use the solar to make electricity. If liquid fuels are needed, the electricity can be used to make H2, and normal catalysis can produce hydrocarbons.

ai_vin

C5 hydrocarbon? That's pentane. Pentane is generally used as a solvent and only rarely as a fuel because it is a colorless liquid at room temperature which can be easily evaporated away. In fact, pentane boils at 37 degrees Celsius (98.6 Fahrenheit), which is body temperature.

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