Biorefinery based on starch-rich algal biomass

At present, biofuels such as ethanol and biodiesel are produced primarily from organic raw materials such as cereal starches or rapeseed oil. This means that arable land is no longer available for the production of food. Such competition for use can be circumvented by the cultivation of microalgae. These organisms also offer a wide range of other advantages compared to higher plants, including greater yields per unit area and lower water demand. A number of microalgae are able to accumulate starch as a storage product when nutrients are limited. The German Federal Ministry of Food and Agriculture (BMEL) has funded a project in which algal biomass is produced in closed photobioreactors and starch, the main component of these algae, is used for the production of ethanol.
 
The second major component of these starch-rich algae are proteins. To increase the value of the production chain, we are also investigating whether the algal proteins can be used as a constituent of culture media for the production of ethanol from cereal starches or as a feed ingredient. For closing loops between ethanol fermentation and algae production, two waste streams are utilizable: Fermentation off-gas can be used as a cheap, high-quality source of CO₂ for photosynthetic algae production. The second loop is to use the liquid digestate of digested fermentation mash and algae residues as ammonium- and phosphate-rich nutrient source for algae production. This biorefinery concept enables utilization of the entire biomass and increases the value of the algal biomass.

The first step involved screening the starch production capacity of various algae strains. The microalga Chlorella sorokiniana (SAG 211-8k) proved to be the most promising due to its high growth rate and its ability to accumulate large quantities of starch. This alga was then used to establish a two-step process in which first biomass is produced and then, under nitrogen limitation, starch is accumulated in the algal cells. Starch production was optimized in terms of light availability (light on the reactor surface per gram of biomass in the reactor over a defined period).

In order to produce algal biomass suitable for biofuel production, the process has to be transferred to outdoor cultivation conditions using natural sunlight. The challenge is to establish a process that will operate in a stable manner and produce starch-rich biomass even under the variable light and temperature conditions of the natural day-night cycle.

We utilized a test facility with five south-facing 28-liter flat panel airlift reactors to characterize the starch production process using the microalga Chlorella sorokiniana.

Laboratory and outdoor cultivation experiments have shown that depriving the culture of nitrogen results in an increase in starch content in microalgal cells. In lab experiments, a starch content of 50 percent was reached within two days of nutrient depleted cultivation of the algae. Light availability per gram biomass was hereby a key variable influencing the rate of starch production. This variable can be easily adjusted in the laboratory, but to investigate different levels of light availability in an outdoor setting three reactors with differing cell concentrations are required. Hence, each reactor operates under a different quantity of light per gram of biomass. In this way, the starch content of Chlorella sorokiniana increased to 50 percent of dry weight in the outdoor setting as well, but the process required 7 to 8 days due to recurrent day-night rhythm and changing weather conditions resulting in variable process temperatures and light intensities compared to laboratory tests.

A pilot algae facility with a volume of 4.3 m3 was constructed on the site of a bioethanol plant at CropEnergies in Zeitz in cooperation with project partner Subitec. The system involves 24 reactors each with 180 liters and will be used for a pilot scale starch production process. Industry partner Südzucker is responsible for the preparation of the biomass and fermentation of the algal starch to ethanol, as well as extraction of the protein fraction. In the upcoming year, we plan to use the liquid effluent of digested fermentation mash and algae residues as ammonium- and phosphate-rich nutrient source for cultivating the algae.

Funding

We would like to thank the German Federal Ministry of Food and Agriculture (BMEL) and the Agency for Renewable Resources (FNR) for funding the project “Bioraffinerie auf Basis kohlenhydratreicher Algenbiomasse, Nutzung von Stärke und Protein”, promotional references 22403211 and 11EKF032 resp.

Project partners

• Südzucker AG, Mannheim, Germany
• Subitec GmbH, Stuttgart, Germany