New catalyst paves the way for capturing carbon dioxide when converting coal into liquid fuels

The world's energy consumption forecasts predict that coal will remain one of the world's leading energy sources over the next decades, and an increasing share of it will be used in the CTL, the conversion of coal into liquid fuels. Researchers from the National Institute of Clean and Low-Carbon Beijing and the Eindhoven University of Technology have developed iron-based catalysts that significantly reduce the costs of And open the door to capturing large quantities of CO₂ which are generated by CTL. Their results are published in the journal Progress of science.

To understand the importance of this realization, it is necessary to know the CTL process. The first step is the conversion of coal into syngas, a mixture of carbon monoxide (CO) and hydrogen (H₂). By using the so-called Fischer-Tropsch process, these components are converted into liquid fuels. But before that can be done, the composition of the synthesis gas must be modified to ensure that the process results in liquid fuels. So some of the CO is removed from the synthesis gas by converting it into CO₂ in a process called water-gas transfer.

The researchers tackled a key problem in Fischer-Tropsch reactors. As in most chemical treatments, catalysts are needed to allow reactions. CTL catalysts are mainly iron-based. Unfortunately, they convert about 30% of CO to undesirable CO.₂, a by-product that is difficult to capture at this stage and, as a result, often discarded in large quantities, consuming a lot of energy without benefit.

Researchers in Beijing and Eindhoven have discovered that CO₂ the release occurs because the iron-based catalysts are not pure, but consist of several components. They were able to produce a pure form of a specific iron carbide, called epsilon iron carbide, which has a very low CO₂ selectivity. In other words, it generates almost no CO₂ at all. Existence was already known, but so far it was not stable enough for the hard Fischer-Tropsch process. The Sino-Dutch research team has now shown that this instability is caused by impurities in the catalyst. The pure phase of epsilon iron carbide that they have developed is, on the contrary, stable and remains functional even under typical industrial treatment conditions of 23 bar and 250 ° C.

New catalyst removes almost all CO₂ generation in the Fischer-Tropsch reactor. This can reduce the necessary energy and operating costs of about 25 million euros per year for a typical CTL plant. Commander₂ those previously released during this stage can now be eliminated in the previous phase of passage to the water. This is good news because it is much easier to capture at this point. The technology to achieve this is called CCUS (Capture, Use and Storage of Carbon). It has been developed by other parties and is already applied in several pilot plants.

The conversion of coal into liquid fuels is particularly relevant in coal-rich countries that need to import oil in order to supply liquid fuels, such as China and the United States. "We are aware that our new technology is facilitating the use of fossil fuels derived from coal, but it is very likely that coal-rich countries will continue to exploit their coal reserves in the coming decades. want to help them do it in the future "most sustainable way," said Emiel Hensen, senior professor at the Eindhoven University of Technology.

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

Hensen expects that recently developed catalysts will also play an important role in the future energy and basic chemicals industry. The raw material will not be coal or gas, but waste and biomass. Synthesis gas will continue to be the central element, it is also the intermediate product in the conversion of these new raw materials.

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More information:
"Synthesis of Fischer-Tropsch catalysts of iron carbide ε stable and selective in CO2," Progress of science (2018). DOI: 10.1126 / sciadv.aau2947 http://advances.sciencemag.org/content/4/10/eaau2947