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Energy transition

Biogas from unicellular green algae

Microalgae are among the most promising sources of sustainable, carbon-neutral biofuels for the future. They are already being used as feedstock for producing biogas, biodiesel, bioethanol and kerosene, but the associated production methods consume a great deal of energy and are rather costly. Dr. Nikolaos Boukis from the Karlsruhe Institute of Technology (KIT) is working on the development of a sophisticated, thermochemical process with an energy balance that promises to improve the situation.

The microalgae-powered BIQ (Bio Intelligence Quotient) House is in the Wilhelmsburg district in Hamburg. Its 200 sqm façade is covered in flatbed reactors. © Sherif Elsayed

Hamburg is home to the world's first house with a living façade. 129 transparent louvres cover the front of the so-called Bio Intelligence Quotient (BIQ) House. Inside the louvres are microalgae that provide power to the building. The idea behind the algae-powered building is fascinating: microalgal organisms carry out photosynthesis, just like land plants. When the sun is out and CO2 and nutrients are available, the algae multiply inside the bioreactors and form biomass, which can be harvested and used to power the building.

Nikolaos Boukis and Sherif Elsayed from the KIT are involved in the PHYKON research project and work together with Dr. Martin Kerner on the efficient use of algal biomass. Kerner is CEO of SSC Strategic Science Consult and driving force behind the development of the BIQ House.

From biomass to biogas

The researchers are using a thermochemical process known as hydrothermal gasification to power the BIQ House. The algal (Acutodesmus obliquus) biomass is broken down in something known as supercritical water, i.e. water at temperatures between 400 and 600°C and a pressure of 300 bar. "Algal biomass consists of organic matter, small quantities of inorganic components and a lot of water," says Boukis. "This mixture is broken down into its constituents at high temperature and high pressure, resulting mainly in hydrogen, carbon dioxide (CO2) and methane – a gas mixture that can be directly used to produce heat and electricity."

Green algae are all-rounders

Microalgae, which include green algae and cyanobacteria (formerly classified as blue-green algae), appear to be the perfect choice for sustainable energy production: production of energy from renewable resources is, in contrast to fossil fuels, essentially carbon neutral. When algal biogas is burned to generate electricity, the amount of CO2 emitted is essentially the same as the algae have previously absorbed as food. The combustion of fossil fuels, however, emits bound carbon. Additionally, algae do not require valuable farmland for growth, unlike energy crops such as rapeseed, soybeans and corn, and are therefore not in direct competition with food production. The algae-based system can easily be installed in the building's basement. Another advantage of algae is that they multiply rapidly and therefore produce more biomass per area unit than land plants. Algae produce up to 100 t of dry matter while wheat produces 3.5 t of dry matter per hectare per year.

Dr. Nikolaos Boukis © private

What makes the process so special is its high energy efficiency: on the one hand, the researchers always use algal biomass as an aqueous solution, i.e. without pre-drying. With other methods, biomass usually needs to be dried in order to access the valuable oils inside the algae. It goes without saying that drying is high in energy consumption.

On the other hand, Boukis and Elsayed use a counter-flow heat exchanger. The researchers feed algal biomass into a concentrically shaped tubular reactor. Cold algal biomass flows through one pipe, and the hot reaction mixture through another right next to it. "This allows us to recover more than 80 percent of the energy that we use for heating the algal biomass," says Boukis.

How good the energy balance is depends on different factors: plant size, insulation, and algal biomass concentration. As a slightly increased algae concentration (10 percent) leads to better results, the researchers moderately enrich the algal biomass by centrifugation. "We are able to convert almost all biomass, and we turn up to 70 percent of energy contained in the biomass into fuel," says Boukis. In contrast, biotechnological processes convert only about 50 percent of the biomass, and their energy balance is poorer by a factor of 2 or 3. Another plus of the new method is its speed: whereas the fermentation of biomass may take several weeks to complete, the thermochemical process only takes a few minutes.

An almost closed circuit

The VERENA (“Test facility for the energetic use of agricultural substances") pilot plant has been designed for a working pressure of up to 35 MPa and a maximum temperature of 700 °C. VERENA is the world's first completely equipped, continuously operated plant for biomass gasification in supercritical water. © KIT

The KIT researchers are developing the plant as self-sustaining, closed system: the goal is to recover nutrients in the decomposed algal biomass, recycle the water in which the algae were grown and separate the CO2, which algae require for growth, from the gas mixture. Nutrients, water, CO2 and heat are then recycled to be used for cultivating algae, so no additional fertiliser and no external CO2 source is required.

However, although the technology is highly promising, it still has one serious drawback: the production of energy from algae is not yet financially viable with any of the methods used. There are two main reasons for this: on the one hand, the technologies are still rather young and need to be optimised, and on the other, energy is still a cheap product. "Put it like this: as long as oil in Saudi Arabia keeps gushing out of the ground and the crude oil price remains so low, no other technical process can compete," says Boukis, who nevertheless has a clear vision of his research mandate: "We have no choice but to carry out research for the post-oil era. And we urgently need to reduce our CO2 emissions. In order to achieve this, we must be able to work on technologies that are not financially viable as of tomorrow."

However, algae already generate good profits: they are used to produce valuable materials such as pigments, oils and proteins for the cosmetics and food industries. The bioenergy sector might indirectly benefit from this by coupling the processes so that remaining algal biomass can be used for energy production once the valuable materials have been extracted. 

Sources:

  • Nikolaos Boukis (IKFT)

  • https://www.energieundklimaschutzbw.de/wp-content/uploads/2015/02/Sherif-Elsayed_Algal-Cultivation-and-Hydrothermal-Gasification-Biomass-and-Energy-Production.pdf

Website address: https://www.biooekonomie-bw.de/en/articles/news/biogas-from-unicellular-green-algae