Fuels from algae or organic waste can be produced using similar processing pathways. Therefore, they are discussed together in this chapter. We first explain the specific characteristics of both feedstocks and present biomethane production through anaerobic digestion. Then, we explain hydrothermal liquefaction as a second promising option for processing biofuels from algae or waste biomass. Finally, the production of biohydrogen from algae is described.

Organic waste

Organic (wet) wastes cannot easily be used in thermochemical convertion processes with subsequent Fischer-Tropsch synthesis like described in the previous chapter (2.3). Reasons are high water contents, low heating values, pollutants, high salt, ash and heavy metal concentrations. These properties are causing problems like slagging, incomplete conversion and complications with ash removal. Therefore, usually organic wastes are converted in gasous fuels (e.g. bio-methane) via anaerobic treatment. Bio-methane can then also be liquefied (liquefied gas). A further promissing option for fuel production from wet organic waste is hydrothermal liquefaction.

Algal biomass

After extraction and concentration of the respective raw materials from the algae biomass they can be processed like described in the previous chapters. The following processing routs are possible.

For biogas production, macroalgae are typically used because, unlike microalgae, they do not accumulate significant amounts of lipids (including fats and oils). However, they do accumulate considerable amounts of carbohydrates like glucose or galactose, which can be used for biogas production or converted into bioethanol or biobutanol through hydrolysis followed by fermentation.

Microalgae, on the other hand, are more suitable for biodiesel production due to their ability to accumulate lipids. For example, fatty acid methyl esters (FAME) biodiesel or renewable hydrocarbon-based diesel can be derived from extracted algal lipids (see the previous lesson on biodiesel).

Alternatively microalgae can be biochemically converted into biogas through anaerobic digestion (AD). Microalgae have a high energy content and low ash contents (<10%) which is advantagous for AD. Just their often low C:N ratio may make the process challenging. Subsequently, the biogas can be converted into liquid fuel via FT-synthesis(see the previous lesson on Fischer-Tropsch-synthasis).

 Fuel from algae by Honeywell (CC BY-SA)