Robust automation concept for the outdoor production of algal biomass

The production of biofuels on the basis of food crops or feed crops (e.g. biodiesel from rapeseed oil or palm oil) is in direct competition with food and feedstuff production. Producing second-generation biofuels with plants not used as food or feedstuffs, for example Jatropha, results in competition in terms of water consumption and cropland. Oil from microalgae is a potential alternative to plant biofuels and belongs to the third generation of biofuels. Compared with the cultivation of higher plants, the cultivation of microalgae offers numerous advantages. These include a higher yield per area, a reduced requirement for water and the possibility of growing microalgae on land that can not be used for agriculture. Oils produced by algae can be used as a biofuel, resulting waste gases fed back into the process and the residual biomass that remains is fermented to produce biogas. In order to convert the process to an industrial scale, we have developed a standardized process automation concept for the cultivation of microalgae.

For the commercial production of microalgae and their use as a sustainable energy source, outdoor cultivation using solar energy is essential. Here, special challenges for the process control are the changeable weather conditions and the inherent day-night-rhythm. To deal with these circumstances, it is important to establish as robust a biomass production process as possible, comprising a high degree of automation and simple measurement technique. The process control should therefore be based exclusively on the measurement of the pH value and the reactor temperature.

The starting point for all experiments was the biomass production process with the microalga Chlorella vulgaris in a 30-liter flat panel airlift (FPA) reactor. In order to achieve a stable production process, it is vital to supply the microalgae culture continually with carbon dioxide, to make required nutrients such as ammonium available and to maintain the pH value and temperature within an optimum range. The higher the CO₂ concentration in the supply air, the more becomes dissolved as carbon dioxide in the culture medium. This lowers the pH value. This is counteracted by the ammonium dissolved in the medium: the higher the ammonium concentration, the higher the pH value in the culture medium. In addition, the solubility of CO₂ in the medium is influenced by its composition and by the temperature. If, in such a system, the pH value is constantly regulated by means of the carbon dioxide concentration in the supply air, this allows conclusions to be drawn about the ammonium concentration in the reactor. This link was used to determine the consumption of nutrients in the reactor. On the basis of these calculations, we were able to successfully control feeding cycles and exclude nutrient and carbon dioxide limitation.

The automation concept was achieved – in line with the current industry standard – with the aid of a programmable logic controller (SIMATIC S7-1200, Siemens) and set up outdoors on a test rig with 30-liter flat panel airlift reactors. When setting up the control software, it was ensured that it was very user and operator-friendly.

The overall process was visualized on a display screen and all online data continuously recorded.The control software is constructed in a modular way and hence can be easily used for new production facilities. Individual program modules can be recombined and may also contribute to controlling other production processes in algal biotechnology.

With the aid of automation we were able to establish a stable growth process outdoors. Biomass was produced over a period of 113 days with an average productivity of 0.50 g/(L*d), with the average biomass concentration at the point of harvesting being 8.5 g/L. Process monitoring and control were performed exclusively on the basis of reactor temperature and pH value. The established process does not depend on constant productivity and is therefore suitable for outdoor production with changing light and temperature conditions. On the basis of these trials, the production process now needs to be transferred to an industrial scale and production costs reduced further.

Funding

We would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the project “EtaMax – More biogas from low-lignocellulosic waste and microalgal residues through a combined bio/hydrothermal gasification”, promotional reference 03SF0350A.

Project partners

• Daimler AG
• EnBW Baden-Württemberg AG
• FairEnergie GmbH
• Fraunhofer IVV
  
• Karlsruhe Institute of Technology (KIT)
• Netzsch Mohnopumpen GmbH
• Paul Scherrer Institute PSI
• City of Stuttgart
• Stulz Wasser- und Prozesstechnik GmbH