Innovative cascade processes for CO₂ conversion into fuels and chemicals

Initial situation and goal

Multiple national and international agreements (Der Klimaschutzplan 2050, UN Sustainable Development Goals, UNFCCC Paris Agreement) aim to increase the share of clean, affordable renewable energy and to limit CO₂ emissions. Motivated by these agreements, Fraunhofer IGB Biocat focuses its work on the development of new combined chemical-biotechnological technologies for the conversion of CO₂ and energy into fuels and chemicals.

In general, the technology development comprises CO₂ conversion into C1 intermediates, combined with fermentation of the C1 compounds to chemicals such as lactic acid, isoprene, polyhydroxybutyric acid and long-chain terpenes. The market for all these chemicals is growing substantially in accordance with the growth rate of their end uses. Conventional end uses of these compounds include the production of rubbers, plastics, food and feed additives. A new use of isoprene is opened up by oligomerization to C10-C15 compounds as precursors of drop-in synthetic fuels.

Because of the geographical distribution of available regenerative energy and CO₂, the development of small-scale, decentralized processes is of particular interest. The build-up of small-scale chemical plants, however, is frequently limited by economic boundary conditions.

Combined electrochemical and biotechnical CO₂ conversion

A prominent example of an integrated technology is the recently patented process for combined electrochemical and biotechnical CO₂ conversion. The process comprises CO₂-based methanol synthesis which was adapted for the combination with C1 fermentation using Methylobacterium ssp in an integrated reactor plant [1]. In this way, methanol synthesis is carried out using a CO₂ and H₂ mixture over a conventional Cu-based catalyst in a plug-flow heterogeneous catalytic reactor. Subsequently, methanol and water formed in the reactor is condensed from the gas phase in a specially designed liquid/gas separator and dosed into a fermenter containing a suitable medium and preculture at given time intervals. The microorganisms use methanol as the sole carbon source for their own growth and to generate the product.

Advantages

This new combined chemical-biotechnological process enables the production of value-added chemicals in only two steps, methanol being the only intermediate. These products are typically not achievable by conventional chemical catalytic processes. Two features of the patented process ensure low capital and operational costs. The methanol-water mixture can be fed directly, without any separation step, into the fermenter and the fermentation runs at room temperature. Integration of captured CO₂ and regenerative energy for the generation of the H₂ ensures the synthesis of regenerative products.

Another advantage of this process is of an economic nature. Estimations showed that the cost of regenerative methanol synthesis is only economically competitive with petrochemical methanol market prices (170 – 390 EUR/ton) when the plant is operated for a long time (over 4000 – 5000 h/year) and has access to cheap regenerative energy (≤ 10 ct/kWh). Typical small-scale decentralized CO₂ emitters (biogas plants, breweries, etc.) usually do not have access to regenerative energy at this rate; a methanol synthesis plant would therefore not be economically feasible. The combined chemical-biotechnological process presented here enables the further conversion of methanol to compounds with a much higher market price compared to methanol. Therefore, this process has the potential to turn small-scale CO₂ valorization into a profitable business [3].

Literature

[1] L.-I. Csepei, F. Steffler, T. Gärtner, V. Sieber, DE102016203889, WO2017153396A.

[2] A. M. Bazzanella, F. Ausfelder, Low carbon energy and feedstock for the european chemical industry, DECHEMA e.V., 2016, 66-67

[3] L. I. Csepei, 36th Annual World Methanol Conference, IHS Markit, Vienna, 5 Oct 2018, Presentation at the Panel Discussion: Are There limits to the Size of Methanol Units? How Can Small-Scale Plants be Profitable?