Posts Tagged ‘Fischer-Tropsch’

Carbon dioxide – aka CO2 – has a lousy reputation in today’s world. Most of us regard it as a significant greenhouse gas contributor to atmospheric heating and all the bad climate stuff that comes with it: heat domes, wildfires, droughts, flash floods, mudslides, loss of land, loss of property, loss of life. The best available science warns us not to keep adding more CO2 to what’s already up in the air, so it’s understandable that our basic instinct is to capture it before emission, pump it into caverns underground, and leave it there.

Carbon Capture and Sequestration (CCS) is already being implemented in subterranean spaces such as depleted oilfields. The CO2 is captured from power stations and industrial facilities such as cement works, and either directly piped or shipped to the disposal point.

As a business model it is capital intensive and relies on the costs of sequestration being sufficiently lower than the costs of emission to make the undertaking economically viable. Also CCS intrinsically demonstrates a preference for permanent disposal over carbon re-utilization and implicit in that choice is a value assessment of the carbon contained in CO2.

Such an assessment is not easily made without familiarity with carbon’s capacity to form bonds with itself and other atoms, a knowledge of what that means in terms of carbon’s oxidation state range, and an understanding of how that range makes carbon a suitable vehicle for energy storage and release, as well as feedstock for a broad spectrum of industrially useful molecules.

It is asking a lot of those educated in political science rather than physical science to make reasoned judgements on the desirability of permanent disposal or carbon re-utilization. And it is rare indeed for heads of government to have skills in both disciplines, although it has been known.

Fortunately however, state-backed investments in CCS need not carry the risk of being the wrong choice long term because it has been shown possible to re-utilize sequestered CO2. Here’s how.

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The Sabatier-Senderens reaction

In 1897, two French chemists discovered that carbon dioxide could be reduced by hydrogen over a nickel catalyst at elevated temperature and pressure to form methane and water vapor. This is the same carbon transformation that occurs in the Carbon Cycle via photosynthesis and the gradual degradation of biomass to coal, oil and natural gas – a process that takes Nature millions of years to accomplish. Sabatier and Senderens found a way to do it in real time.

Now consider the conditions in the underground caverns where carbon dioxide gas is sequestered. The temperature is elevated and so is the pressure. And in the pores of the geological formations holding the gas there are microbes capable of catalyzing the Sabatier-Senderens reaction. All it needs is to add hydrogen, which can be produced electrolytically from water using solar or wind.

To get technical for a moment, it is instructive to apply Hess’s Law (G.H. Hess, 1840) to the electrolysis of water and the Sabatier-Senderens reaction

In effect the energy released by the combustion of hydrogen is being used to reverse the combustion of methane, the energy being stored in the stable C-H bonds of the methane molecule. Note that the oxygen is formed above ground during electrolysis and is either stored for commercial use or vented to the atmosphere.

The pressurized subterranean gas can be piped up and passed through a separator where methane is extracted and carbon dioxide and hydrogen are returned underground to continue the reaction. Now if the sequestered CO2 was captured before emission, it would defeat the object to use the methane in the energy supply infrastructure since combustion would simply release CO2 to the atmosphere. So what can be done with this methane? The next section supplies the answer.

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Tri-Reforming Methane

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An LNG tanker

Methane (CH4) has certain characteristics which make it attractive as an energy carrier. It is not difficult to liquefy at 1 atmosphere pressure, it is energy dense and relatively efficient to transport as LNG (Liquid Natural Gas). So it is a practical proposition for methane gas produced in underground caverns to be transported to plants where CO2 capture is taking place.

Again to get technical for a moment, we notice that the oxidation states of carbon in the two molecules are at opposite ends of the scale. Methane has the most reduced form of carbon (-4) while carbon dioxide has the most oxidized form of carbon (+4). A redox reaction between the two looks possible and indeed is possible, albeit at elevated temperatures:

This catalyzed process, by which two greenhouse gases are converted into two non-greenhouse gases, is called dry reforming of methane (DRM) and was first introduced by Franz Fischer and Hans Tropsch in Germany in the 1920s. The 1:1 mixture of carbon monoxide and hydrogen is called syngas (synthesis gas), a key intermediate in the production of industrially useful molecules.

Germany’s dynamic duo: Franz Fischer and Hans Tropsch

Because of the high process temperature, DRM also results in thermal decomposition of both methane and carbon dioxide and the deactivating deposition of carbon on the catalyst. This problem can however be mitigated in a very neat way by combining DRM with another methane reforming process, namely steam reforming (SRM). This not only re-utilizes the deposited carbon but also adds another syngas product with a 3:1 ratio:

All these reactions are endothermic (requiring heat). This heat can be supplied by adding oxygen to the reactant stream, which allows partial oxidation of methane (POM) and catalytic combustion of methane (CCM) to take place, both of which are exothermic reactions (producing heat):

Putting three reforming agents – carbon dioxide, water and oxygen – together in the reactant stream with methane feedstock creates a sufficiently energy-efficient overall process known as Tri-reforming.

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Bringing it all together

We have seen that the decision to avoid CO2 emissions to the atmosphere by employing capital intensive Carbon Capture and Sequestration (CCS) in subterranean locations such as depleted oilfields does not preclude the subsequent or concurrent addition of hydrogen to facilitate a gas phase redox reaction in which carbon dioxide is converted into energy-rich methane.

Methane feedstock can efficiently be transported to plants where CO2 capture is taking place and fed into a tri-reforming reactor together with carbon dioxide, oxygen and steam to create commercially valuable syngas and obtain a return on CCS investment through the production of industrially useful carbon-containing molecules that do not pose a greenhouse gas risk.

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Suggested further reading

A mini-review on CO2 reforming of methane
Published in June 2018 this is a useful and easily readable grounder covering the thermodynamic, kinetic, catalysis and commercial aspects of the subject.

Tri-reforming: a new process for reducing CO2 emissions
A bedrock paper (January 2001) from the legendary Chunshan Song at Penn State.

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P Mander February 2022

Credit: Extinction Rebellion

On Tuesday 2nd April 2019, environmental activists from the Extinction Rebellion group staged a parliamentary protest in London by glueing their bottoms to the windows of the viewing gallery of the House of Commons during a Brexit debate.

They used this cheeky tactic to call on British politicians to act on the ‘Climate and Ecological Crisis’. On its website, Extinction Rebellion wrote that “Government must act now to halt biodiversity loss and reduce greenhouse gas emissions to net zero by 2025”.

There is an irony here in that the call to immediate action was delivered at the precise moment when the UK government was in a state of complete paralysis over Brexit. Then again, the demonstrators’ demands weren’t meant to be taken entirely at face value. Even the most optimistic environmental biologists and chemical engineers would shake their heads at what was proposed to be accomplished in the space of just six years.

What the demonstration did achieve was to focus attention on the task in hand and the current state of progress. And it prompted CarnotCycle to pen this post on one promising technology for carbon capture and conversion (CCC) in which UK researchers are playing a key role.

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It is a well-known fact, not to mention a subject of serious environmental concern, that cows burp methane as a result of anaerobic fermentation of the grass they eat. And being aerobic respirers they also exhale carbon dioxide. Both these substances are potent greenhouse gases associated with human activity whose atmospheric levels, according to the best available science, must at least be stabilized in order to stand a chance of keeping global warming within manageable limits.

The cow in our picture is asking us an interesting question. What if it were possible to react carbon dioxide and methane together to form products that are not greenhouse gases? Even better, what if the reaction products could be put to useful purposes? Just think how cute that would be!

Putting our physical chemistry hats on for a moment and looking at the above equation, we notice that the oxidation states of carbon in the two molecules are at opposite ends of the scale. Methane has the most reduced form of carbon (-4) while carbon dioxide has the most oxidized form of carbon (+4). A redox reaction between the two looks possible and indeed is possible, albeit at elevated temperatures:

This process – called dry reforming of methane or DRM – was first introduced by Germany’s dynamic duo, Franz Fischer and Hans Tropsch, in 1928 but extensive investigation only started in the 1990s when increasing concerns about the greenhouse effect were raised by the international scientific community.

Notice how DRM simultaneously converts two greenhouse gases into two non-greenhouse gases which together make valuable syngas (1:1), a key industrial intermediate in the production of chemicals and clean fuels.

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Well, sort of. The big difficulty in making DRM viable relates to side reactions. The DRM reaction proceeds above 918K but then so does the thermal decomposition of both methane and carbon monoxide which results in carbon being deposited on the catalyst, clogging up the pores and thereby deactivating it.

This problem can however be mitigated in a very neat way by combining DRM with another methane-reforming process, namely steam reforming (SRM). Coupling reactions in this way not only reutilizes the deposited carbon but also adds a product stream with an H2/CO ratio of 3:1 which enables the syngas ratio to be adjusted for the synthesis of methanol, ethanoic acid or dimethyl ether (DME), which has promise as a sulfur-free diesel fuel, or towards the (2n+1):n H2/CO ratio required for Fischer-Tropsch synthesis of alkane fuels

The other problem is that all these reactions are endothermic (heat requiring). This energy has to be obtained from somewhere, and now here comes the next neat idea. Adding oxygen to the reactant stream allows partial oxidation of methane (POM) and catalytic combustion of methane (CCM) to take place, which are exothermic reactions that can supply the necessary heat

Putting three reforming agents – carbon dioxide, water and oxygen – together in the reactant stream with a methane feedstock creates a sufficiently energy-efficient overall process known as ‘tri-reforming’.

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To complete the conceptual scheme, the tri-reforming process is integrated into a cycle where the syngas output is utilized in the tri-generation of fuels, industrial chemicals and electricity, with the flue gases from these processes being fed back after nitrogen purging to the tri-reforming reactor. Carbon dioxide can also be fed into the cycle from external sources such as power plants and cement works.

Note that in principle, carbon dioxide can be fed into the cycle from established carbon capture and storage (CCS) processes. In this way underground reservoirs of anthropogenic carbon dioxide can be utilized as a feedstock for additional tri-generation.

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Almost. I mentioned at the start that the UK was involved in CCC research so I should say a few words about that. Scientists at Oxford and Cambridge are working with the King Abdulaziz City for Science and Technology in Saudi Arabia and the National Natural Science Foundation of China on the tri-reforming/tri-generation technology detailed above.

Since China is the world’s largest CO2 emitter and Saudi Arabia is the world’s largest oil producer, the Anglo-Sino-Saudi initiative seems a sensible geoscientific cluster. The news was announced on 28 January 2018, more than a year before the Extinction Rebellion demonstrations in London.

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Suggested further reading

A mini-review on CO2 reforming of methane

Published in June 2018 this is a useful and easily readable grounder covering the thermodynamic, kinetic, catalysis and commercial aspects of the subject.

Turning carbon dioxide into fuel

A paper co-written by the Oxbridge scientists involved in the Anglo-Sino-Saudi initiative. It was published in 2010, which shows that these guys have been on the case a while. Climate activists take note, and read their stuff.

Turning carbon dioxide into fuel – a new UK-China-Saudi Arabia initiative

The January 2018 press release referred to above. I reckon my poly-alliterative Anglo-Sino-Saudi sounds better.

Tri-reforming: a new process for reducing CO2 emissions

A bedrock paper (January 2001) from Chunshan Song at Penn State. The process diagram featured above is taken from this paper. If you don’t read anything else, read this one.

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P Mander May 2019