Honeycomb Reactors for the Energy Transition

Foil-wrapped vessels, brightly colored rotary  valves, and glass gas-washing bottles intertwined with silverly pipelines - this is the latest process technology in a laboratory at KIT's Engler-Bunte Institute (EBI), which looks more like a wild art installation. This is where Dominik Schollenberger conducts his research. Carefully, he removes a short metal cylinder from the complex experimental setup. When he holds is up to the light, a dense metal structure becomes visible inside, reminiscent of a honeycomb and shimmering in a matte gray. 

"The honeycomb consits of alternating layers of smooth and corrugated sheet metal coated with a special catalyst - a patented process that we developed here at the Engler-Bunte Institute," explains Schollenberger. He is wearing gloves to protects the surface, as the coating is very senstitive. "The honeycomb structure increases the reactive surface area in the coating. It also helps to dissipate the heat generated by the reaction efficientyl, allowing us to make good use of it." Installed in a suitable sythesis reactor, the honeycomb structure helps convert hydrogen and Co2 into methane, which is easy to store and highly versatile and is also the main component of convetional natural gas. "The fact that we are becoming increasingly proficient at this conversion is good news for climazte protection," say Schollenberger. He is part of a growing group of scientists who see chemical energy carriers such as hydrogen and methane as key components of the energy system of the future, which will rely primarily on renewable energy. 

As the energy transition progesses, the requirements placed on infrastructure are changing. The greater the share of renewable energy in the grid, the more important storage and transportation options for the volatile electricity generated by solar and wind power become. Power-to-gas technologies could provide the missing piece of the puzzle. By first convertign electricity into hydrogen though electrolysis and then converting the hydrogen together with Co or Co2 into methane, climate-friendly energy can be used in a much wider range of applications. These include sectors that cannot easily be electrified, its use as a raw material in the chemical industry, as a fuel for passenger cars, trucks, and ships, or as a fuel dor generating heat. In addition, electrical energy can be efficiently stored and transported via the PtG process. Germany's existing gas network, for example, transports around 1,000 billion kWh of energy per year through transmission and distribution networks with combined lenght of approximately 500,000 kilometers. 

PtG is already a reality today. At Uniper's WindGas demonstratin plant in Falkenhagen, Brandenburg, for example, electricity generated by wind turbines has been successfully converted into hydrogen, oxygen, and heat through water electrolysis since 2013. In May 2018, a demonstration-scale methanation stage using KIT's new honeycomb reactor was comissioned there. In this process, "green" hydrogen is converted into Synthetic Natural Gas (SNG), or synthetic methane. Since January 2019, the SNG produced there has been fed into the natural gas grid operated by gas network operator Ontras.  The methanation reactor incorporating KIT technology was built as a part of the EU research project STORE&GO. At two other European sites, the project also brought different PtG technologies from the experimental stage into practical application. in addition to KIT and Uniper, a total of 27 European research insitutions and companies are involved in STORE&GO. Overall coordination is provided by the DVGW Research Center at the KIT's Engler-Bunte Institute, wich will also conduct a comparative assessment of the different technologies. "Many PtG processes are already ready for practical use today," says Dr. Siegfried Bajohr, whoe research group at the EBI developed the honeycomb reactor. "We now want to demonstrate the performance of our technology over a period of two years under continuous operation and dynamic load changes." 

From a historical perspective, PtG processes do not actually involve entirely new concepts. The basic principle of methanation, for example, was first described in 1902 by French chemist Paul Sabatier. In this process, hydrogen and carbon dioxide - or carbon monoxide - are converted into water and methane with the aid of catalysts. However. the process has so far mainly been used industrially to remove harmful carbon monoxide residues when purifying hydrogen, for example before ammonia synthesis using the Haber-Bosch process. Methanation is also still used on an industrial scale to produce SNG from coal. In these applications, however, little attention is paid to the efficiency of the overall process or to the Cos emissions associated with the use of coal.

In the 1990's, Japanese scientist Koji Hashimoto was the first to propose using PtG processes to produce SNG from renewable electricity in a climate friendly way. "With the honeycomb reactor, we have finally brought this process into the 21st century, "says Bahjor. Methanation using the honeycomb reactor not only enables a technically straighforward conversion on a single reaction stage, without complex recirculation or product purification. The honeycomb reactor is also particularly easy to manufacture and, thanks to its good scalability and flexibility under changing loads, is suitable both for small-scale, decentralized applications and for series production in large-scale plants. 

"Only climate-friendly SNG will enable us to achieve a rapid energy transition in the mobility and heating sectors as well. I see the honeycomb reactor as something like the chemcial transformer in the energy grid of the future," says Professor Thomas Kolb of the Engler-Bunte Institute. However, one fundamental question remains to be answered if methanation is to be deployed on a large scale: Where will the Co2 required for the conversion come from? "For the demonstration plant in Falkenhagen, we use carbon dioxide generated as a by-product of a bioethanol plant," explains Dominik Schollenberger. However, this would not be suficient for lage scale deployment in Germany, as cautious estimated suggest that millions of tons would be required. 

"Of course, we must not make the mistake of simply capturing fossil Co2, for example from a coal-fired power generation, and using it here," says the young scientist. "In the long term, I believe that extracting Co2 directly from the atmosphere could become an attractive option, even though the effort and cost involved are still very high." For the forreseeable future, however, none of the current PtG technologies will be able to compete with fossil energy sources in an open maket as long as the impact of Co2 emissions on the global climate is not reflected in prices. Regardless of the technology used, significant political measures will therefore be necessary to fully harness the sustainability potential of PtG.

Contact: dominik schollenberger ∂does-not-exist.kit edu

siegfried bajohr ∂does-not-exist.kit edu