Several species of cyanobacteria, including Chlamydomonnas reinhardtii, Oscillatoria limosa, Microcystis PCC7806, Cyanothece PCC7822, Microcystis aeruginosa PCC7806 and Spirulina pla — tensis produce ethanol via an intracellular photosynthetic process. After selecting strains for ethanol, salt and pH tolerance, ethanol production can be enhanced through genetic modifi­cation [33]. These strains are long-lived and can be grown in closed photobioreactors to pro­duce an ethanol containing algae slurry. This process for ethanol production from algae is currently being demonstrated by Algenol Biofuels [34—36]. The cyanobacteria are grown in flexible-film, polyethylene-based closed photobioreactors containing seawater or brackish water as medium. Industrial (or other waste) CO2 is sparged into the bags to enhance growth of the microorganisms. Nutrients (primarily nitrogen and phosphorus) are supplied to sustain growth. At maturity, the microorganisms produce ethanol. The ethanol in the liq­uid phase will maintain an equilibrium with the ethanol-water in the vapor phase. The etha­nol-water in vapor phase condenses along the walls of headspace which is collected by gravity for ethanol recovery. Algenol aims to produce 56,000 L of ethanol per hectare per year using 430 polyethylene bags established over a one hectare footprint each containing 4500 L of culture medium with a cyanobacteria concentration of 0.5 g/L. Unlike other algae derived biofuel processes, the algae are retained in the bags while the ethanol water conden­sate is removed for ethanol recovery. It is expected that the photobioreactors will be emptied once a year to replace the seawater, growth media and cyanobacteria.

The ethanol concentration in the algal cultures is expected to range between 0.5 and 5 % (w/w) depending on the ethanol tolerance levels of the strain and that of the condensate be­tween 0.5 and 2% [36]. Since the maximum ethanol concentration is expected to be only 2 %, conventional distillation for ethanol recovery will not be energy efficient. A vapor compres­sion steam stripping (VCSS) process is being developed to concentrate the ethanol to 5-30 % (w/w) range. VCSS is a highly heat integrated process that offers the potential for energy ef­ficient separation even at low ethanol concentrations. This is then followed by a vapor com­pression distillation process to concentrate ethanol to an azeotropic 94% concentration. Life cycle energy requirements and greenhouse gas emissions for the process are dependent on the ethanol content of the condensate from the photobioreactors. Detailed analysis using process simulation software have shown that net life cycle energy consumption (excluding photosynthesis) is 0.55 down to 0.2 MJ/MJethanol and net life cycle greenhouse gas emissions is 29.8 down to 12.3 g CO2e/MJelhanol for ethanol concentrations ranging from 0.5 to 5% by weight [36]. Compared to gasoline these values represent a 67% and 87% reduction in the carbon footprint on an energy equivalent basis [36].

One of the technological challenges for this approach appears to be developing genetically engineered cyanobacterial strains that can tolerate high concentrations of ethanol. The etha­nol concentration in the growth medium will affect the vapor phase ethanol content which in turn will affect the content of the condensate recovered from the photobioreactor. There is a dramatic increase in energy consumption in a conventional distillation process as ethanol content decreases below 7.5% (by volume). Energy required almost doubles when ethanol content decreases from 12% down to 5% (by volume).

Another challenge would be the economical disposal of spent algal cultures. Sterilization and inactivation of large volumes of biomass can involve extremely energy intensive unit operations like heating, or expensive processes like ultra violet treatment or chlorination.