DP1: Conversion of aquatic biomass

Photosynthetic algae (including macro- and micro-algae) and photosynthetic cyanobacteria have the potential to produce considerably greater amounts of biomass per hectare than terrestrial crops and some species could even directly produce fuel (H2, ethanol or hydrocarbons). Aquatic biomass can be cultivated using industrial carbon dioxide as carbon source and wastewater as nutrient input (nitrogen and phosphorus), thereby not competing with food crops for land or other resources. However, due to the large volumes of water involved in the cultivation, nutrient balancing and recovery, as well as contaminant control are essential for economic and environmental feasibility.

Large scale cultivation of microalgae in onshore, outdoor open pond systems and raceways is well established but limited to a few algal species which can tolerate extreme environmental conditions such as high salinity (Dunaliella), high pH (Spirulina (Arthrospira)) or undergo extremely high specific growth rates (Chlorella). Closed cultivation systems for microalgae, usually onshore, utilize photobioreactors made of transparent tubes, plates, bags or domes, which permit culture of single species at higher productivity than in open systems. However, inhibition due to the oxygen formed is a scale-up issue and furthermore, the prevention of intrusion of other competing species is also a challenge. Macroalgae (seaweeds) are usually cultivated in offshore farms but their productivity is much lower than that of microalgae and may also have a seasonal growth pattern.

The typical microalgae concentration in cultivation broths is as low as 0.1 - 1 % of total dry suspended solids, which requires at least two-step recovery by thickening and dewatering to reach a concentrate of 15 25 % dry matter. To avoid degradation, the concentrate must be dried. The harvesting and drying are very energy demanding and account for a large part of total energy consumption, and methods for using e.g. waste or solar heat are pursued.

Algae biomass composition consists of carbohydrates, proteins, lipids and other products such as pigments, vitamins, etc., for use in food, cosmetics, biostimulants and other niche markets. The lipids, up to 70% on a dry basis, have been the most interesting fraction for conversion into biofuels. However, the extraction of the lipids requires energy-consuming methods for breaking the cell walls. The lipids, once separated from the other components, can then be converted to FAME and HVO, see EVC1: Transesterification to biodiesel and EVC2: Hydrotreatment to HVO.

In some cases, in particular for macroalgae, the cell walls are composed of carbohydrates that can be hydrolysed and the sugars fermented to ethanol.

In addition, hydrothermal liquefaction (HTL), see Hydrothermal liquefaction (HTL) to bioliquid intermediates, of aquatic biomass is being developed as this process can avoid the need for extensive dewatering and drying. The entire algae biomass may also be used without or with limited dewatering via anaerobic digestion to produce bio-methane, see EVC4: Anaerobic digestion to biogas. These two technologies are also considered for processing the algal biomass residues, once lipids and sugars have been recovered.

Algal-derived biofuels would be advanced biofuels in the EU. Nevertheless, at present algal conversion routes would have a problem of meeting the GHG reduction requirements, with the possible exception of HTL and bio-methane pathways.

During the last decade, there has been a number of developments at TRL 5 pilot scale to cultivate algae for production of bio-methane, lipids for FAME and HVO, as well as sugars for ethanol, as described in the SGAB report[1]. However, the decrease in the energy prices in 2014 have for most developers meant that the interest has shifted from fuels to high-value chemical specialities, and the energy is more seen as a way of valorising biomass as biomethane.

Fact Sheet: Aquatic Biomass

Acknowledgement: Large parts of the texts were taken from Lars Waldheim´s contribution to the report “The Contribution of Advanced Renewable Transport Fuels to Transport Decarbonisation in 2030 and beyond”

[1] SGAB Technology status and reliability of the value chains: 2018 Update. 28 December 2018. Ed. I Landälv, L Waldheim, K Maniatis. artfuelsforum.eu/news-articles/updated-sgab-report-technology-status-and-reliability-of-the-value-chains/