Hydrothermal processing of algal biomass for the production of biofuels and chemicals

The area of processing algal biomass under hydrothermal conditions has received increasing interest over the last decade. The process has been identified to be especially suited for high moisture aquatic biomass such as microalgae and macroalgae, as the biomass is processed wet as a slurry in hot-compressed water. At lower temperatures, hydrothermal processing leads to carbonization reactions of the biomass and the primary product is a biochar - so-called hydrothermal carbonization. At intermediate temperatures, below the critical point of water, the process is referred to as hydrothermal liquefaction and the primary product is a biocrude. Above the critical point, gasification reactions predominate and the primary product is a syngas. Microalgae are regarded as a promising source of third-generation biofuels due to their high lipid content, fast growth rates and the absence of competition for food production. Macroalgae have high carbohydrate content and represent a largely untapped source of biomass for bioenergy and chemicals. The scope of this review is to summarize the current state of research on the hydrothermal carbonization, liquefaction and gasification of algal biomass. The effect of heterogeneous and homogeneous catalysis, operating conditions, energy balances and nutrient recycling are discussed.