The hydrothermal liquefaction as a promising procedure for microalgae-to-biofuel conversion: A general review and some thermophysical problems to be solved

At present, microalgae are industrially produced mainly for the extraction of high-value products for food additives. At the same time, the microalgae possess also environmental advantages as it can be used for wastewater treatment, mitigation of industrial CO2 emissions as well as for oxygen production and atmospheric CO2 capturing. Due to increasing the environmental problems, it is reasonable to expand the “green” applications of microalgae and increase significantly their output. From this point of view, the problem of utilization of the microalgal biomass becomes more important and one of the most reliable ways to do it is a conversion of the biomass to a biofuel. It is expected that such a conversion can be implemented into the existing infrastructure for traditional hydrocarbons. In the case of microalgae, the hydrothermal liquefaction (HTL) with the production of bio-oil as a target product has attracted more attention in recent years because the bio-oil can be used in the existing refinery industry. The paper is also concerned with the use of microalgae to solve the environmental issues on the basis of HTL as a convenient and efficient method for the biomass-to-biofuel conversion. The known advantages of the HTL are the possible use of fresh microalgae just after harvesting, the processing of the whole biomass and high thermodynamic efficiency. In the paper it is shown that the latter is achieved due to the high HTL pressure that keeps the high-temperature potential of aqueous media after hydrothermal treatment and so creates the opportunity of more efficient heat recovery. The fundamental aspects of the process thermodynamics are discovered in the paper. It is shown that one of the main advantages of the process is provided by a combination of thermodynamic parameters. The problem of solar radiative transfer in photobioreactors with suspended microalgae and the desired thermophysical properties of the refined biofuels are also briefly discussed in the paper.