Logical utilization of CO2? LZN researchers made significant findings pertinent to sustainable fuels and chemicals

In the fight against rapidly rising carbon dioxide levels in the atmosphere, new methods of reducing CO2 emissions are being increasingly developed. The transport sector is particularly important as it contributes a significant amount of CO2 to the atmosphere through the burning of fossil fuels. Prof. Dr. Ingo Krossing from the Institut für Anorganische und Analytische Chemie of the University of Freiburg Freiburg and Dr. Achim Schaadt from Fraunhofer ISE want to play their part in finding a solution to this problem and collaboratively conduct research on the topic of “Power-to-Liquid” (PtL).

The PtL concept is based upon the process of converting power from renewable energies first in hydrogen and thereafter, in combination with isolated carbon dioxide, into liquid energy carriers and chemicals. The liquid products can then be implemented in various sectors such as mobility, chemistry and energy for the production of power, heat or cooling energy.

The synthesis of methanol from hydrogen via water electrolysis with carbon dioxide is a key part of the above-described process chain. An interesting aspect thereof is that methanol can easily be converted into many other fuels and chemicals such as dimethylethers (DME) and oxymethelenethers (OME). OMEs, if sustainably produced, could replace current diesel fuels and thereby significantly reduce the CO2, NOx and particulate matter emissions of diesel vehicles. The required carbon dioxide could be extracted from industrial processes such as steel works, from biomass (biogas plants) or directly from the air. The latter would prospectively be the best option to close the global carbon dioxide cycle.

The current dependency on coal, crude oil and natural gas could thereby be reduced if an efficient process for the conversion of power from renewable energies into liquid fuels or chemicals were established.
The goal of the collaborative project HyCO2 was, among others, to promote the previously established relationship between researchers of the University and Fraunhofer. Ingo Krossing developed the research topics around the expertise of his colleagues. Concretely, he joined fellow researchers from the Fraunhofer-Institute for Solar Energy Systems (ISE) and the Fraunhofer Institute for Mechanics of Materials (IWM) in the pilot project “HyCO2: Hydrogenation of CO2 to Liquid Fuels” of the Sustainability Center Freiburg. The long-term goal of the project was the conversion of carbon dioxide with hydrogen to liquid fuels as a basic technology for the chemical storage of fluctuating renewable energies. “We had worked with Prof. Krossing before and were confident that we would reach interesting conclusions in this new collaboration,” reports Dr. Achim Schaadt from Fraunhofer ISE. The natural scientists and engineers focused not only on the examination of the mechanism of methanol-, DME- and OME-synthesis but also on the development of new, more active catalysts. The social scientists in the group examined the market acceptance of such new products.

OMEs do not contain any C-C bonds with the result that they burn clean and soot free, are not poisonous and are a clean alternative to fossil fuels. However, an economic yet ecological production thereof remains a significant hurdle: a few Chinese manufacturers produce OMEs already, but their processes are relatively expensive and inefficient. The researchers of the HyCO2 project therefore decided to build their own laboratory set-up within which to examine and optimize the methanol-, OME- and DME-synthesis. As Franz Mantei, doctoral candidate at ISE, explains, “thermodynamic analyses and simulations helped us immensely in our creation of a process concept and in the realization of a good set-up.”

OMEs were exemplarily produced from methanol and formaldehyde in an aqueous solution. The ensuing mixture is made of OMEs with varying chain lengths, from 1 to 10. However, only OMEs of a certain chain length (ideally 3 to 5) can be used as fuels since their characteristics match those of diesel fuels best. The separation of these OMEs from the others can be done via a so-called “batch-distillation”. As part of the distillation, the mixture is specifically heated and mixed with the result, that the various molecules can be separated from one another based on their different boiling points. Through this thermal separation, the researchers are able to collect a few grams of the desired product per hour. “These pre-tests are important for the understanding of this process. Once we fully understand it, we can build a similar plant on a much larger scale to then answer the demand for large amounts of OMEs,” explains Schaadt.

A so-called “mini-plant” for methanol synthesis, also found in a laboratory at Fraunhofer ISE, serves as an exemplary set-up for a later OME-manufacturing plant. This set-up was created as part of the project Carbon2Chem®, led by thyssenkrupp. This plant is completely automated and can be controlled by a computer. Researchers are able to manufacture about one liter of raw methanol per hour which, once cleaned, can be used as a reactant in the OME-synthesis. The end goal is to create a mini-plant for continual OME-production at Fraunhofer ISE which is powered by green power such as from PV or water electrolysis. The end goal for the future is to build an industrial-sized plant for the production of OMEs with help from industrial partners. To meet this goal, the team has to continue to develop the catalysts and processes in experimental examinations and simulations.

To what extent society would accept OMEs as an alternative fuel was examined in the form of a master’s thesis at the University of Freiburg. Given that OMEs offer many benefits as compared to other fuels, it is to be expected, that the acceptance would be quite high. The examination revealed that people are generally willing to accept OMEs but the supply of green energies is still seen as a significant obstacle. The production of the hydrogen needs to be done with “green” power meaning from PV, wind and biomass plants. Ideally, most of the power would stem from inland sources which could be inspected for their sustainability. However, this would require the defossilization of the fuel and chemistry sectors which is a great hurdle. Furthermore, in certain regions such as Norway or northern Africa, the power could be produced more ecologically (smaller CO2- footprint) and economically (high sun hours, more usable hours) than in Germany.

“Since we also wanted to examine the economics of this product, we also completed a Life Cycle Analysis (LCA) and techno-ecological analysis (TEA),” reports Dr. Robin J. White, a fellow Fraunhofer ISE researcher. With such analyses, it is possible to describe which resources are required during the individual phases of a product’s lifecycle and what kinds of ecological and/or economical costs ensue. The analysis was conducted in terms of CO2, water and power for the entire process from methanol to the end product OME. The PtL concept could help significantly reduce CO2 and pollutant emissions. “Since there is such a large amount of interdependency and other influencing factors, we have to conduct further analyses,” says White. An important point is the analysis of the CO2-footprint of the used power. Further LCA/TEA analyses also need to examine how the side products i.e. the OMEs with chain lengths of less than three and more than five could be used or reused in any following processes. This could also further increase the environmental balance of the product.

The successful collaboration between the different Fraunhofer institutes and the university came to an end in the summer. “The competencies from all sides harmonized perfectly and we had a very productive exchange of knowledge and ideas. Numerous publications were released and many final papers were also written during the study period. The relationship between the two research bodies was further strengthened. “We are excitedly awaiting the next opportunity to work on a collaborative project with our colleagues,” summarizes Achim Schaadt.