Their work was published as an open access paper in the ACS journal Environmental Science & Technology on 22 Oct.

This choice of ranges is somewhat arbitrary but has been made based on the likelihood that 0.5% is too dilute for economical recovery and that 5% would have a high likelihood of economical recovery, in that the separation process could employ standard column distillation.

—Luo et al.

Algenol algae. Click to enlarge.

The process that was modeled envisioned cyanobacteria grown in flexible-film, polyethylene-based photobioreactors containing seawater or brackish water as the culture medium. To provide sufficient carbon dioxide to support efficient algal growth, the production facility is located near a fossil-fuel power plant or industrial source of carbon dioxide. The study’s calculation was based on use of industrial CO₂, such as the byproduct CO₂ from ethylene oxide production.

In the model, CO₂ was injected in the headspace of the photobioreactor; ti could also be dissolved in the seawater growth medium. Nitrogen and phosphorus fertilizers were introduced into the photobioreactors to support the initial cyanobacterial growth.

The cyanobacteria remain in the photobioreactors producing ethanol and, unlike other algae-to-fuel processes, are not harvested for fuel or other purposes. The researchers anticipated that the photobioreactors will be emptied no more than once per year to replace the seawater, growth media, and cyanobacteria.

Algenol aims to produce about 56,000 L (about 15,000 gallons US) of ethanol per hectare per year using about 430 polyethylene photobioreactors per hectare, each with about 4500 L of culture medium containing about 0.5 g/L of cyanobacterial biomass.

This production target is within achieved photosynthetic yields and corresponds to 1.8% solar energy conversion efficiency for average incident sunlight energy levels in the United States.

The energy required for ethanol separation increases rapidly for low initial concentrations of ethanol, and, unlike other biofuel systems, there is little waste biomass available to provide process heat and electricity to offset those energy requirements. The ethanol purification process is a major consumer of energy and a significant contributor to the carbon footprint. With a lead scenario based on a natural-gas-fueled combined heat and power system to provide process electricity and extra heat and conservative assumptions around the ethanol separation process, the net life cycle energy consumption, excluding photosynthesis, ranges from 0.55 MJ/MJEtOH down to 0.20 MJ/MJEtOH, and the net life cycle greenhouse gas emissions range from 29.8 g CO₂e/MJEtOH down to 12.3 g CO₂e/MJEtOH for initial ethanol concentrations from 0.5 wt % to 5 wt %. In comparison to gasoline, these predicted values represent 67% and 87% reductions in the carbon footprint for this ethanol fuel on a energy equivalent basis.

—Luo et al.

Further reductions in energy consumption and greenhouse gas emissions can be achieved via employment of higher efficiency heat exchangers in ethanol purification and/or with use of solar thermal for some of the process heat, they found.

Resources

• Dexin Luo, Zushou Hu, Dong Gu Choi, Valerie M. Thomas, Matthew J. Realff, and Ronald R. Chance (2010) Life Cycle Energy and Greenhouse Gas Emissions for an Ethanol Production Process Based on Blue-Green Algae. Environ. Sci. Technol., Article ASAP doi: 10.1021/es1007577