A comparative review on photo and mixotrophic mode of algae cultivation: Thermochemical processing of biomass, necessity of bio-oil upgrading, challenges and future roadmaps

Availability of renewable biomass for the production of bioenergy is need of hour as fossil-based energy con-sumption has led to both environmental and health problems. Algae biomass gains advantages over other bio-masses (agriculture, sewage, forest, sludge, municipal waste) due to their availability, biomass generation in wastewater and negative carbon dioxide emission. In this review paper, we have explored the algae biomass cultivation routes, thermochemical conversion and upgradation of algal biomass. Photo bioreactors (tubular, bag, flat panel, etc) are preferred for microalgae cultivation in association with raceway open ponds for liquid hydrocarbons production. Algae biomass are processed through hydrothermal liquefaction (Temperature range of 220-320 ?, time of 60 min, pressure of 5-20 bar), pyrolysis (Temperature range of 320-420 ?) and co -liquefaction. The commonly upgradation techniques are catalytic cracking, hydrodeoxygenation, etc. Hydro-deoxygenation was performed to nullify the oxygen content of bio-oil, since in adverse case the bio-oil with higher content causes corrosion and coke formation in combustion engines. The algae biofuel production cost depends upon the conversion techniques and upgrading processes. Further in future this review provides strong base for exploring the aqueous phase obtained from thermochemical process for bio-hydrogen production via photo-catalytic reforming processes.