Reaction engineering and kinetics of algae conversion to biofuels and chemicals via pyrolysis and hydrothermal liquefaction

Algae are becoming increasingly popular as feedstocks for various fine chemicals and fuel intermediates. Cultivation of algae can be environment-friendly as they can fix high amounts of carbon dioxide in the environment, and also remove hazardous pollutants from wastewater. Moreover, algae cultivation presents a high per-hectare oil yield and faster growth rates than terrestrial biomasses. Owing to these advantages, algae are attractive candidates for harnessing energy. Thermochemical conversion techniques are promising as they offer a single-step conversion of algae species into valuable chemicals. In this article, pyrolysis and hydrothermal liquefaction technologies for the conversion of a variety of microalgae and macroalgae to bio-oil and biochemicals are discussed comprehensively. Different pyrolysis strategies such as fast pyrolysis, co-pyrolysis, microwave-assisted pyrolysis, and hydropyrolysis for the production of bio-oil of varying properties are outlined. The effect of catalysts in upgrading the quality of the bio-oil is evaluated. On the hydrothermal liquefaction front, the effects of operating parameters such as temperature, time, pressure, and solvent on the yields of products and the quality of bio-crude are covered in detail. Due emphasis is given to the kinetics of pyrolysis and hydrothermal liquefaction of algae, and different types of models, viz., apparent kinetics and lumped semi-detailed models, are discussed. The complex conversion pathways involved in these two processes are unraveled by presenting plausible reaction mechanisms and discussing the fate of nitrogen present in the algae. Furthermore, this review throws light on various aspects ranging from algae cultivation, the effect of culture conditions on the biochemical composition of algae species to techno-economic and lifecycle assessment of biofuels and chemicals derived from algae via thermochemical technologies. Finally, the challenges involved in the scale-up of thermochemical technologies and the development of detailed kinetic models are presented.