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New catalyst opens door to CO2 capture in coal-to-liquids process

World energy consumption projections expect coal to stay one of the world’s main energy sources in the coming decades, and a growing share of it will be used in CT—the conversion of coal to liquid fuels (CTL). Researchers from the National Institute of Clean-and-Low-Carbon Energy in Beijing and Eindhoven University of Technology have developed iron-based catalysts that substantially reduce operating costs and open the door to capturing the large amounts of CO2 that are generated by CTL. An open-access paper on their work is published in the journal Science Advances.

The conversion of coal to liquid fuels is especially relevant in coal-rich countries that have to import oil for their supply of liquid fuels—such as China.

China represents the largest coal market in the world and is expanding its coal-to-liquids (CTL) technology by ~2% per year. By 2020, CTL is expected to account for 15% of the coal use in China. Accordingly, there is a large incentive to improve current CTL technology, which can convert coal into liquid fuels and valuable chemicals.

—Wang et al.

The first stage in CTL is the conversion of coal to syngas—a mixture of carbon monoxide (CO) and hydrogen (H2). Using the Fischer-Tropsch process, these components are converted to liquid fuels. But before that can be done, the composition of the syngas has to be changed to make sure the right products come out in the end. Some of the CO is taken out of the syngas (rejected) by converting it to CO2, in a process called the water-gas shift.

In this chain the researchers tackled a key problem in the Fischer-Tropsch reactor. As in most chemical processing, catalysts are required to enable the reactions. CTL catalysts are mainly iron-based. Unfortunately, they convert some 30% of the CO to unwanted CO2, a byproduct that in this stage is hard to capture and thereby often released in large volumes, consuming a lot of energy without benefit.

The Beijing and Eindhoven researchers discovered that the CO2 release is caused by the fact that the iron-based catalysts are not pure, but comprise several components. They were able to produce a pure form of a specific iron carbide—epsilon iron carbide—that has a very low CO2 selectivity.

The existence of ε-iron carbide was already known but until now it had not been stable enough for the harsh Fischer-Tropsch process. The Sino-Dutch research team has now shown that this instability is caused by impurities in the catalyst. The phase-pure epsilon iron carbide they developed is, by contrast, stable and remains functional, even under typical industrial processing conditions of 23 bar and 250 ˚C.

The new catalyst eliminates nearly all CO2 generation in the Fischer-Tropsch reactor. This can reduce the energy needed and the operating costs by roughly €25 million per year for a typical CTL plant. The CO2 that was previously released in this stage can now be removed in the preceding water-gas shift stage.

We are aware that our new technology facilitates the use of coal-derived fossil fuels. However, it is very likely that coal-rich countries will keep on exploiting their coal reserves in the decades ahead. We want to help them do this in the most sustainable way.

—lead researcher Professor Emiel Hensen of Eindhoven University of Technology

The research results are likely to reduce the efforts to develop CTL catalysts based on cobalt. Cobalt-based catalysts do not have the CO2 problem, but they are expensive and quickly becoming a scarce resource due to cobalt use in batteries, which account for half of the total cobalt consumption.

Hensen expects that the newly developed catalysts will also play an import role in the future energy and basic chemicals industry. The feedstock will not be coal or gas, but waste and biomass. Syngas will continue to be the central element, as it is also the intermediate product in the conversion of these new feedstocks.

Resources

  • Peng Wang, Wei Chen, Fu-Kuo Chiang, A. Iulian Dugulan, Yuanjun Song, Robert Pestman, Kui Zhang, Jinsong Yao, Bo Feng, Ping Miao, Wayne Xu, Emiel J. M. Hensen (2018) “Synthesis of stable and low-CO2 selective ε-iron carbide Fischer-Tropsch catalysts” Science Advances Vol. 4, no. 10, DOI: 10.1126/sciadv.aau2947

Comments

Engineer-Poet

Interesting that I don't read about CO2 formation in MeOH catalysts.  It would be inconsequential anyway, because the typical copper-oxide MeOH catalyst can digest CO2 as well as CO; if something reacts CO + H2O -> CO2 + H2, they both get consumed a short time later as CO2 + 3H2 -> CH3OH + H2O.  The net reaction remains CO + 2 H2 -> CH3OH

Lad

Regardless of how you use coal, as a liquid fuel, solid fuel or as feed stock, it is a damaging carbon pollutant. The gas, fly ash and mountains of residue are loaded with toxins. The last thing we need is liquid fossil fuels derived from coal; the whole idea behind cleaning up the World is to replace fossil chemicals with clean energy where possible, including coal.

mahonj

It is hard to know if this is good or bad news.
(Technically, it is good news.)

If the use it to create liquids that they would have created anyway, but with less CO2, it is good news. If it is used to create liquids from biomass, it is good news.
However, if it postpones the switch to electric vehicles, it is bad news.
+ coal is the "worst" fossil fuel due to its high carbon content and all the other bad things in it.

Engineer-Poet

Note that this catalyst works with syngas in general, and coal does not have to be the source.  The syngas can come from biomass or biogas, and would likely require less cleanup from these sources.

I am given to wonder if the CO2 is coming from the 2 CO -> C + CO2 reaction, which is favored at low temperatures.  This deposits coke on the catalyst and degrades it.  This seems likely to be the problem, because CO + H2O -> CO2 + H2 only requires a change in the input ratio of CO to H2 to compensate for it and the CO2 can even be recycled to the gasifier.

SJC

If you have lots of CO, you water gas shift it to more H2 to make more fuel.

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