Membrane reactors suitable for power stations

ECN is developing a new technology that will soon allow power stations to capture CO₂ more cheaply. This technology requires new catalysts, something which ECN is conducting research into. “We recently submitted a patent application for one of those catalysts,” says Jean-Pierre Pieterse, a researcher from the ECN unit Hydrogen & Clean Fossil Fuels.

Pieterse is carrying out research on catalysts that enable CO₂ to be captured prior to the combustion of natural gas and coal. “This can be done, for example, by converting natural gas with steam into CO₂ and hydrogen. The hydrogen is then separated from the CO₂-rich gas mixture and added as fuel to a gas turbine in order to generate electricity. And the CO₂ is stored in an empty gas field, as the idea goes.”
That is fairly expensive with existing technology: electricity from a power station that captures CO₂ is 30 to 40 percent more expensive since CO₂ capturing requires additional energy and equipment. “There are also the costs of CO₂ transport and storage. Our challenge is to develop processes and materials that will make this considerably cheaper,” explains Pieterse.
ECN is working on two options in this regard: separating hydrogen and CO₂ with the help of a membrane through which only hydrogen can pass, and absorbents that capture CO₂ and allow hydrogen to pass through. “Both processes use catalysts to convert the fossil fuel into hydrogen and CO₂. They are far more efficient than existing processes for capturing CO₂ from gas flows,” reveals Pieterse.

Configuration of a power station running on natural gas where CO₂ is captured beforehand with an ECN membrane reactor.

Membrane and catalyst integrated in reactor
ECN has big test installations at its disposal that allow large-scale testing of these technologies. Pieterse talks about the catalysts that are used and refers to membrane separation as an example. Producers of hydrogen for oil refineries and ammonia producers also use methane (natural gas) but as a raw material, and also convert this with steam into hydrogen and carbon monoxide (steam reforming) with a nickel catalyst at temperatures of around 850°C. Pieterse: “We can do that between 500 to 600°C because we use a membrane that is integrated in a reactor with a nickel-containing catalyst.”
The hydrogen that forms on the catalyst surface is immediately removed via the membrane. The reaction from natural gas to hydrogen continues due to the removal of the hydrogen. Without the removal of hydrogen, the entire conversion of natural gas to hydrogen can only be achieved at a far higher temperature. Pieterse: “We can save energy since we do not need to heat the feed gas up until 850°C but only until approximately 600°C. With the membrane, you can also immediately separate hydrogen from CO₂ at that temperature. There is no need to cool the gas mixture to 35°C first to this end (as is the case with conventional CO₂ capturing) in order to heat up the hydrogen again afterwards before it enters the gas turbine. This also allows you to save energy. We could obtain an annual energy saving of an estimated 24 petajoules if we were to use these hydrogen membrane reactors in the Netherlands on a large scale for electricity production and in the process industry. This corresponds to the energy content of 575,000 tonnes of oil, which is roughly the capacity of a supertanker.”

Promoters adjust catalysts
The process looks simple on paper but is tricky in practice. Under prevailing conditions, part of the carbon monoxide molecules in the membrane reactor react with one another to form soot particles that reduce the catalyst’s performance. Since hydrogen is extracted at the same time, that ‘sooting’ process occurs even faster. “We can tackle this by adding specific promoters to the nickel catalyst, substances that help the catalyst steer the reactions in the right direction,” says Pieterse. He does not wish to divulge further information given that ECN is busy with a patent application.
Catalyst manufacturers often use promoters. These include alkali metals and rare earth metals, such as scandium, yttrium and lanthanides. The exact ones they use and in which combination is a closely guarded secret. ECN also conducts research in this field in cooperation with TU Eindhoven.
The high partial pressures of the substances in the membrane reactor also cause another problem, namely sintering. The nickel particles that are spread finely over the support surface of the catalyst start coagulating, making the catalyst less active. Pieterse: “You can overcome this by anchoring the active metal particles in the porous structure of the support material of the catalyst. This reduces their freedom of movement and makes coagulation more difficult.”
In addition to steam reforming with membrane reactors, ECN is also looking at membranes and water-gas-shift catalysts for the reaction of carbon monoxide with water (steam) to produce CO₂ and hydrogen. “We are examining whether we can use commercial catalysts with iron oxide and chrome oxide and membranes. We are also searching for new materials with a higher activity that work at a lower temperature. The gas flow originating from coal gasification also contains sulphur compounds, incidentally. These catalysts must therefore be able to withstand them.” 

Two reactions in a single reactor plus a suitable catalyst for separating CO₂ from H₂ fuel; that is the membrane reactor of ECN.

Numerous investments and demonstration projects
ECN participates in Dutch and European research programmes such as CATO2 and CACHET 2. The aim of these is to test technologies for capturing CO₂ on a large scale in order to facilitate the following step towards an industrial scale. There are already companies, incidentally, that offer installations with membranes for the production of hydrogen, such as Ida Tech in the USA and Tokyo Gas in Japan. ECN also provides complete membrane units under the name HYSEP® for benchmarking and evaluating hydrogen production.
Pieterse has been involved in ECN’s catalyst research since 2000. “It originally revolved around catalysts for creating pure hydrogen for fuel cells and also catalysts for breaking down nitrogen oxides that acidify the environment. We have also helped DSM and other companies to eliminate the emission of nitrous oxide during nitric acid production. The greenhouse effect of this gas is 310 times greater than that of CO₂, so this is of some use.”
Pieterse believes that it may still take ten years before the energy sector and industry can use membrane technology for capturing CO₂ on a large scale. “We definitely envisage a future for capturing CO₂ with this new technology, but the development thereof still requires considerable investment and demonstration projects during the years ahead. The implementation speed of CO₂ capturing also depends on political decisions of course. As companies pay ever higher taxes for CO₂ emissions , CO₂ capture will become more appealing,” concludes Pieterse.

Contact
Jean-Pierre Pieterse
ECN Hydrogen & Clean Fossil Fuels
Phone: +31 (0)22 456 8154
E-mail: Jean-Pierre Pieterse

Text: Erik te Roller

Info
On the potential of Ni-based catalysts for steam reforming of methane in membrane reactors, or as a scientific article in Catalysis Today.

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