Optimized fermentation of algae biomass through modeling and simulation

Over the next two decades, the worldwide demand for electrical energy is expected to increase by about 70 percent. Additional power plant resources are required to cover the growing demand for electricity. Moreover, the challenges of global climate protection can only be met with a sustainable energy supply. To achieve these goals, the increased and efficient use of biomass for the production of electricity and heat is indispensable. The coupling of a thermal gasifier or a biogas reactor with a microturbine is a good way to achieve this. Depending on the process, algae, wood, sludge, peat, waste, pomace and other organic residues can be used as feedstock.

The DeDeBio research project develops mathematical models for the design of decentralised biomass power plant concepts. For the production of biogas, both a thermal wood gasification process (DLR) and a biological process using algae as the starting material are to be considered. Besides the development of tools for CFD-based combustion chamber design (computational fluid dynamics), the focus is on the modelling of the biogas reactor and the wood gasifier. These numerical models will then be used to simulate the product gas composition and, in combination with microturbine models, to design and evaluate different plant and operating concepts.

In a biogas reactor, the substrates used are converted in several reaction steps to biogas, consisting of the main components CH₄ and CO₂. The gas yields and gas compositions achieved in this process depend on various factors such as process control, substrate preparation and substrate composition. The biogas yield of plants is usually limited by the more or less large proportion of lignocellulose which is difficult to utilise. In contrast, the use of microalgae poor in lignocellulose, such as Chlorella vulgaris, Phaeodactylum tricornutum and Spirulina platensis, enables an almost complete conversion of the organic substance. In the real process, after prior extraction of valuable substances from the algae, the residual substances can thus be converted into biogas under mesophilic conditions in a continuous two-stage gas-lift loop reactor (see graph on the right). The types of algae used could be fermented with varying degrees of success: Both the composition of the biogas and the yield varied depending on the cell contents, the cell wall components and the cell wall stability. In particular, the protein content of the cell plays a major role. The biogas yield was between 280 and 400 L / kg organic dry residue (oTR) depending on the type of algae.

For the biogas reactor, Fraunhofer IGB has created a black box model using stoichiometric estimation according to Buswell and Boyle, which shows the composition of the biogas as a function of the feedstock using stoichiometric compositions. If one takes into account the incomplete stoichiometric conversion of the supplied biomass into biogas in the real process (a part of the supplied biomass goes into the growth of the bacteria) as well as the limited availability of the organic materials (unresolved cell components) via a correction factor, the model can be used to estimate both the product gas yield and the product gas composition at the reactor outlet. In this way, changes in the process parameters or the plant configuration can be simulated and the iteration steps in the design of the plants for product gas generation can be reduced in the future.

The resulting mathematical models for the simulation of biogas plants operated with algae residues can be used accordingly for the further development and optimisation of biological reactors. The generated model can thus support the scaling of the plants. The knowledge gained and models developed within the project will for the first time allow comprehensive modelling of these plant concepts and plant components - and help decentralised, gas turbine-based biomass power plants to reach market maturity more quickly.

[1] Becker, E. W. (2004) Microalgae in human and animal nutrition. In: Richmond, A. (ed.) Handbook of microalgal culture. Oxford: Blackwell Publishing: 312-351

[2] Samson, R.; Leduy, A. (1982) Biogas production from anaerobic digestion of Spirulina maxima algal biomass, Biotechnol. Bioeng. 24: 1919-1924.

We would like to thank the Baden-Württemberg Energy Research Foundation for funding the project "Development and validation of design tools for the design of decentralised biomass power plant concepts for combined heat and power generation - DeDeBio".

• German Aerospace Center (DLR), Stuttgart