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Subitec GmbH – binding carbon dioxide, generating energy

Algae have become the beacons of hope in terms of energy generation and carbon dioxide fixation. Stuttgart–based Subitec GmbH has developed a unique reactor system to improve the cultivation of algae. The establishment of further pilot plants gives the company access to the constantly growing energy market.

Haematococcus pluvialis microalgae produce the red dye astaxanthin. © Subitec GmbH

In 1996, a research report by the US Department of Energy concluded that algae would not become effective energy suppliers in the future. However, Subitec GmbH have proved otherwise. Scientists led by Prof. Dr. Walter Trösch from the Fraunhofer Institute for Interfacial Engineering and Biotechnology have been focusing on the topic of microalgae since 1995. Subitec GmbH, whose CEO is Dr. Peter Ripplinger, was founded in 2000 and started full-scale operation in 2007.

The DECHEMA “Algal Biotechnology” workgroup believes that if long-term development work is carried out in this field, algae have huge future potential. The ability of algae to bind carbon dioxide (CO2) contained in industrial emissions has attracted the interest of industry which currently uses algae to produce expensive materials as well as to generate energy.

Subitec GmbH focuses on the production and commercialisation of microalgae and has developed a globally patented flat-panel airlift photobioreactor (FPA reactor). The FPA reactor enables algal biomass to be produced with a net gain in energy, something that has not been possible with traditional production systems.

Light for any algal cell

FPA reactors developed by Subitec GmbH © Thomas Ernsting

The FPA reactor enables algae to be cultivated in a closed bioreactor with optimal process control. Subitec offers the system in different sizes (5 l, 30 l and 180 l). In the FPA reactor, phototrophic algae produce important substances such as fatty acids, proteins, dyes, oils and polysaccharides in a process known as photosynthesis. The FDA reactor functions according to the principle of the airlift loop reactor. However, there is one particular problem: different zones of different light intensity develop in the reactor. Algae in the outer layer do not carry out photosynthesis as they are exposed to very high light intensity; those in the inner layer live in their own shadow and are only exposed to limited light. Therefore, only the bacteria in the thin middle layer are exposed to optimal light intensity. To improve this situation, Subitec GmbH has integrated static mixers into the reactor that ensure that the layers are properly mixed and transported in a specific direction. The rising gas bubbles are diverted to the static mixers and start the culture medium flowing in a stationary, circular current. The algae are therefore transported to the light and then back into the shaded zone, and are thus exposed to optimal light intensity. This enables very dense algal cultures to be used. In addition, the FPA reactor is suitable for sterile production, which is a specific requirement of the pharmaceutical and cosmetics industries. The FPA reactor is manufactured from plastic film by means of a patented deep-drawing technology in the shape of two half-shells that are then fused together.

Complete use of the algae

The algae are grown with the sun’s energy and industrial CO2. The algal biomass can be used for biogas production once valuable materials have been extracted from the algae. The CO2 produced in the biogas plant is integrated into the production of algae, thereby closing the cycle. The concept of biomass refinery thus enables algal biomass to be used in its entirety.

In 2008, Subitec, in cooperation with its project partner EnBW AG, set up a pilot plant with three 180 l FPA reactors in the city of Eutlingen-Weiting. Another pilot plant was set up in Hamburg-Reitbrook in cooperation with E.ON Hanse AG. Since early 2010, the company has been constructing an algal production plant on behalf of Stadtwerke Reutlingen and FairEnergie GmbH in the city of Reutlingen that will eventually be coupled to a gas-operated block heat and power plant. Since June 2010, Subitec operates a plant in the city of Senftenberg, a project that is being carried out in cooperation with Gesellschaft für Montan- und Bautechnik GmbH. This plant is coupled to a brown coal co-generation power station. In the next years, Subitec GmbH also plans to set up and operate further pilot plants, in the Mediterranean area for example, in order to further reinforce its technological lead.

Website address: https://www.biooekonomie-bw.de/en/articles/news/subitec-gmbh-binding-carbon-dioxide-generating-energy