Without a doubt, the world is now dependent on a limited supply of fossil fuels. But what if, instead of drilling into wells, we harvested plants for fuel? Many plants can be converted into energy, but algae are a particularly promising source of biofuels. Algae grow quickly and easily, and their abundance makes them a promising source of renewable energy. As Dr. Julie Zimmerman of the Yale School of Engineering and Applied Sciences and the Yale School of Forestry and Environmental Studies describes it, “Algal biodiesel’s advantage over other biofuels such as corn biofuel is that it doesn’t compete with food demand, algae grow on marginal land, and it produces more oil per area of crop.”
At the moment, algal biodiesels are not commercially produced and no economically viable production processes exist. To determine the most beneficial pathway for algal biodiesel production based on current technology, Zimmerman has conducted a life-cycle assessment (LCA) of algal biodiesel. This LCA follows algal biodiesel production through five stages: cultivation, harvesting, lipid extraction, conversion to biodiesel, also known as transesterification, and the disposal and reuse of the leftover biomass. For each step, Zimmerman analyzed the advantages and disadvantages of different methods, taking into account factors such as the energy required, the energy produced, and the environmental impact. Through her analysis, she found a pathway for biodiesel production with net energy savings 85% greater than the average biodiesel pathway used today.
One Process, Many Options
A full LCA requires the evaluation and systematic exclusion of the many options and pathways in the harvesting, extraction, and processing of the algae. On a large commercial scale, the types of bioreactors in which the algae are cultivated vary widely. Zimmerman’s LCA considered four bioreactor models: an open raceway pond (ORP), an annular photobioreactor (PBR), a flat panel PBR, and a tubular PBR. Twenty-four factors relating to the bioreactor were investigated, ranging from productivity to the materials of and the lifetime of the bioreactor.
Different mechanisms and properties of centrifugation, chamber press filtration, and flocculation, necessary for harvesting the algae, were also investigated by Zimmerman’s LCA. Centrifugation spins the algae sample at a high speed in order to separate the denser algal cells from the less dense growth media while chamber press filtration separates the solid algal cells from the surrounding liquid. Flocculation occurs when a chemical, known as a flocculant, is added to the algal sample and causes the algal cells to separate from the liquid. Parameters examined for this stage were the cell recovery efficiency, electricity use of the method, and the material use of the equipment.
After harvesting the algae, lipids must be extracted from the algae. Several methods for this extraction were compared. The first method follows a modified version of soybean oil processing in which the algae cells are broken by a drill press, followed by lipid extraction through by hexanes solvent. A second method extracts lipids using supercritical CO2, which is obtained by subjecting CO2 to extremely high pressure and temperature so that it’s neither liquid nor gas. Supercritical CO2 behaves as an organic solvent, making it a strong candidate for lipid extraction. A third method, in which methanol and sulfuric acid are added to dried cells, has been shown to result in direct transesterification. Finally, a last potential mechanism relies on supercritical methanol, which behaves like supercritical CO2, combining lipid extraction and transesterification into one step. Zimmerman’s LCA analyzed these options based on eight parameters, including extraction and conversion efficiency, and heat use.
After each of these processes, algal biodiesels are ready for commercial use. The product is 95% biodiesel and 5% glycerol, but the leftover algal biomass must be disposed or recycled. Because so much of this process is energy- and material-intensive, this final step is crucial to making algal biodiesels a commercially feasible industry. The LCA considered two options for disposal, landfilling and bacterial anaerobic digestion of the residual algal biomass. Given all of these potential methods for each stage of biodiesel production, Zimmerman created a matrix with 160 possible pathways and whittled down the choices to the most environmental-friendly and commercially viable option.
When choosing the type of bioreactor for cultivation, Zimmerman considered algal growth factors, such as CO2 and light requirements, and the restraints of each type of bioreactor design. In the case of ORPs, disadvantages such as low volumetric productivity and low light utilization outweigh advantages such as larger unit capacity. This left the annular PBR, flat panel PBR, and the tubular PBR. In the end, the flat panel PBR was chosen because it required the least energy: 544 kWh versus the 43900 kWh required by tubular PBRs and the 13700 kWh of annular PBRs. Furthermore, the flat panel PBR had the lowest environmental impact.
For the next step in the process, flocculation was chosen for harvesting because it required 90% less electricity than cen-trifugation and 89% less electricity than filtration. The types of flocculant considered were lime, aluminum sulfate, and chitosan. Chitosan has the lowest energy burden of the three flocculants. Ideally however, self-flocculant algae might one day be developed, completely eliminating the need for chitosan.
After harvesting through flocculation, the algal cells still retain some water, which affects the methods of lipid extraction and transesterification. The modified version of soybean oil processing and the direct addition of methanol were not chosen because they require the energy-intensive drying of algal cells, leaving the options of supercritical methanol and supercritical CO2. Ultimately, supercritical methanol was chosen because it had the advantage of combining the stages of lipid extraction and transesterification into one step.
Upon harvesting algal cells, supercritical methanol was applied to the cells to directly produce biodiesel. Although the pressure and temperature must be high to produce supercritical methanol, the energy demand is outweighed by its benefits. The high biodiesel efficiency of 98% has a trickle-down effect on each method: less algal biomass would be required to start the synthesis, meaning fewer nutrients and less water and electricity would be needed throughout the entire process. Furthermore, supercritical methanol more selectively extracts the lipids needed for biodiesel, which results in purer fuel. This purity means that less nitrogen remains in the algal biodiesel, reducing the amount of nitrous oxide (N2O) produced when the biodiesel is burned.
Finally, anaerobic digestion of the residual algal biomass was chosen over landfilling as the waste disposal method. Anaerobic digestion produces methane gas, which can be burned to produce energy and nutrients such as phosphorous and nitrogen. Theoretically, these nutrients can be funneled back into the cultivation of algae, increasing the sustainability of biodiesel production.
Based on the methods considered in the LCA, Zimmerman concluded that the current best pathway for algal biodiesel production uses a flat plate PBR for cultivation, flocculation with chitosan for harvesting, combining lipid extraction and transesterification with supercritical methanol, and finally anaerobic digestion to recycle the residual algal biomass. When the energy cost and benefit of this set of methods were quantified for the production of 10 GJ of biodiesel, the net energy sum was over 65 GJ greater than that of the average pathway used today.
Economic and Industrial Implications
The best-case scenario pathway chosen by Zimmerman has a net energy savings of 85% compared to the current pathway, but even this pathway results in a slight energy loss. More refined methods for harvesting, lipid extraction, and transesterification, and advances in bioreactor technology would surely improve the energy balance in biodiesel production and make it commercially viable.
The last stage, the recycling and disposal of residual biomass, provides the greatest opportunity for the largest gain in energy. This stage allows for creativity in research because the residual algal biomass has many useful applications. For instance, Zimmerman says, “Tuning the organic solvent allows us to extract different compounds from the biomass. Omega-3 and omega-6 fatty acids can be extracted for vitamin supplements that are potentially more valuable than biodiesel.” The profit from selling these extra compounds could be used to help offset the energy cost of biodiesel production.
Environment-Friendly Nature of Algal Biodiesels
The benefits of algae in biofuels are matched by the environmental savings and benefits of their use. Because algae is a plant, it easily decomposes in landfills. The nutrients produced through anaerobic digestion of the residual biomass have the potential to be funneled directly back to the first step of cultivation. This feedback loop would make algal biodiesel production partially self-sustainable and is something under investigation in Zimmerman’s lab.
Further environmental benefits come from the carbon-neutral nature of the biodiesel production process. During cultivation, CO2 from the waste gas of coal-fired powerplants and ammonia plants is funneled into the bioreactors to provide algae with the CO2 needed for growth. This reduces the carbon footprint of algal biodiesel production. But it must be noted that this LCA does not consider the carbon cost of stages outside of production, such as transportation and disbursement.
The Future of Algal Biodiesels
At the moment, the lack of infrastructure for large-scale production of algal biodiesel prevents its use as a commercially feasible fuel alternative. However, ongoing research to more efficiently produce environmentally friendly algal biodiesel is promising. Zimmerman predicts, “Future research will be done to investigate how to optimize algal growth conditions for biodiesel production. For instance, where in the growth curve is best to harvest algae?” Her LCA brings the world a step closer to a day when we can fill up our gas tanks, not with gasoline from precious oil, but with biodiesel from tanks of common green algae.
About the Author
JENNIFER KY is a sophomore Molecular, Cellular, and Developmental Biology major in Silliman College. She is a writer for Yale Scientific and works in Dr. Shawn Ferguson’s lab studying defects in the function of lysosomes.
The author would like to thank Professor Zimmerman for her time and eagerness to discuss her research.
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