Algae for biofuels
Research is examining microalgae—20 to 80 percent oil by dry weight biomass—as a biofuel energy crop. There are several different production systems for algae production, yields, costs and challenges to this exciting new feedstock for biodiesel production.
Algae are organisms that grow in aquatic environments and use light and carbon dioxide (CO2) to create biomass. There are two classifications of algae: macroalgae and microalgae.
Macroalgae, which are measured in inches, are the large, multi-cellular algae often seen growing in ponds. The largest multicellular alga is the giant kelp plant, which can be more than 100 feet long. Microalgae, on the other hand, are measured in micrometers and are tiny, unicellular algae that normally grow in suspension within a body of water.
Microalgae grow quickly and contain high oil content compared with terrestrial crops, which take a season to grow and only contain a maximum of about 5 percent dry weight of oil. They commonly double in size every 24 hours. During the peak growth phase, some microalgae can double every three and one-half hours.
Oil content of microalgae is usually between 20 percent and 50 percent (dry weight), while some strains can reach as high as 80 percent. Thus, use of microalgae as an alternative biodiesel biofuel feedstock is gaining increasing interest from researchers, entrepreneurs, and the general public.
Algal biomass contains three main components: carbohydrates, proteins, and lipids/natural oils. Because the bulk of the natural oil made by microalgae is in the form of tricylglycerol, which is the right kind of oil for producing biodiesel, microalgae are the exclusive focus in the algae-to-biodiesel arena.
In addition to biodiesel, microalgae can also be used to generate energy in several other ways. Some algal species can produce hydrogen gas under specialized growth conditions. The biomass from algae can also be burned similar to wood or anaerobically digested to produce methane biogas to generate heat and electricity. Algal biomass can also be treated by pyrolysis to generate crude bio-oil.
There has also been some study on the value of algaebiofuel byproducts being fed to cattle. So far the results have been promising.
Most microalgae are strictly photosynthetic— that is, they need a light and carbon dioxide as energy and carbon sources. This culture mode is usually called photoautotrophic. The practice of algal mass culture can be performed on non-arable lands using nonpotable saline water and waste water.
Some algae species, however, are capable of growing in darkness and using organic carbons such as glucose or acetate as energy and carbon sources. This culture mode is termed heterotrophic. Due to high capital and operational costs, heterotrophic algal culture is hard to justify for biodiesel production. In order to minimize costs, algal biofuel production usually relies on photoautotrophic culture that uses sunlight as a free source of light.
A variety of photoautotrophic-based microalgal culture systems are available.
For example, the algae can be grown in suspension or attached on solid surface. Each system has its own advantages and disadvantages. Currently, the suspension-based open ponds and enclosed photobioreactors are commonly used for algal biofuel production. In general, an open pond is simply a series of raceways outside, while a photobioreactor is a sophisticated reactor design which can be placed indoors in a greenhouse, or outdoors.
The photosynthesis process generates oxygen. In an open-raceway system, this is not a problem as the oxygen is simply returned to the atmosphere. However, in the closed photobioreactor, the oxygen levels will build up until they inhibit and poison the algae. The culture must periodically be returned to a degassing zone, an area where the algal broth is bubbled with air to remove the excess oxygen. Also, the algae use carbon dioxide, which can cause carbon starvation and an increase in pH. Therefore, carbon dioxide must be fed into the system in order to successfully cultivate the microalgae on a large scale.
The biomass productivity of photobioreactors can be 13 times greater than that of a traditional raceway pond, on average. Harvesting of biomass from photobioreactors is less expensive than that from a raceway pond, since the typical algal biomass is about 30 times as concentrated as the biomass found in raceways.
However, enclosed photobioreactors also have some disadvantages. For example, the reactors are more expensive and difficult to scale up. Moreover, light limitation cannot be entirely overcome since light penetration is inversely proportional to the cell concentration. Attachment of cells to the tube walls may also prevent light penetration. Although enclosed systems can enhance the biomass concentration, the growth of microalgae is still suboptimal due to variations in temperature and light intensity.
After growing in open ponds or photobioreactors, the microalgae biomass needs to be harvested for further processing. The commonly used harvest method is through gravity settlement, or centrifuge. The oil from the biomass will be removed through solvent extraction and further processed into biodiesel.
Depending on the culture systems used (opens ponds vs. enclosed photobioreactors), microalgae production yield is expressed as the amount of biomass per unit of surface area (for open ponds), or per unit of reactor volume (for enclosed photobioreactors). A typical open pond can produce 5 to 10 grams (g) of biomass (dry basis) per meter squared (m2) of surface area per day, which translates to 7.4 to 14.8 tons (dry biomass) per acre per year. Some researchers reported that biomass yield can be as high as 50 g/m2 per day, i.e., 74 ton biomass/m2 per year in an open pond.
For enclosed photobioreactors, the biomass yield can be approximately 2 to 3 g/L per day, i.e., 0.73-1.05 ton (dry biomass)/m3 per year. The oil content of the dry biomass is a highly variable parameter, while some strains can reach as high as 80 percent.
The U.S. Department of Energy (DOE) has performed a significant effort to pursue the commercial production of algal biofuel through its ASP program from the 1980s to 1990s. After 16 years of research, DOE concluded that the algal biofuel production was still too expensive to be commercialized in the near future.
Three major factors limiting commercial algal production exist: the difficulty of maintaining desirable species in the culture system, the low yield of algal oil, and the high cost of harvesting the algal biomass. DOE concluded that there was a significant amount of land, water, and CO2 to support the algal biofuel technology.
Despite that, algal biofuel production has gained renewed interest in recent years. Both university research groups and start-up businesses are researching and developing new methods to improve the algal process efficiency with a final goal of commercial algal biofuel production. The research and development efforts can be categorized into several areas:
• Increasing oil content of existing strains or selecting new strains with high oil content.
• Increasing growth rate of algae.
• Developing robust algalgrowing systems in either an open-air environment or an enclosed environment.
• Co-product development other than the oil.
• Using algae in bioremediation.
• Developing an efficient oil-extraction method.
One way to achieve these goals is to genetically and metabolically alter algal species. The other is to develop new or improve existing growth technologies so that the same goals listed above are met. However, it should be noted that this new wave of interest has yet to result in a significant breakthrough.
The production cost of the algal oil depends on many factors such as the yield of biomass from the culture system, the oil content, the scale of production systems, and the cost of recovering oil from algal biomass.
Currently, algal oil production is still far more expensive than petroleum diesel fuels. For example, it is estimated that the production cost of algae oil from a photobioreactor with an annual production capacity of 10,000 tons per year, assuming the oil content of the algae to be around 30 percent, would incur a cost of $2.80/L ($10.50/gallon) of algal oil.
This estimation did not include the costs of converting algal oil to biodiesel, or the distribution and marketing cost for biodiesel and taxes. At the same time, the petroleum diesel price was $2 to $3 per gallon. Whether algal oil can be an economic source for biofuel in the future is still highly dependent on the petroleum oil price.
In addition to producing biofuel, algae can also be explored for a variety of other uses, such as fertilizer and pollution control.
Certain species of algae can be land-applied for use as an organic fertilizer, either in its raw or semi-decomposed form. Algae can be grown in ponds to collect fertilizer runoff from farms; the nutrient-rich algae can then be collected and reapplied as fertilizer, potentially reducing crop-production costs. In wastewater-treatment facilities, microalgae can be used to reduce the amount of chemicals needed to clean and purify water.
In addition, algae can also be used for reducing the emissions of CO2 from power plants. Coal is, by far, the largest fossil energy resource available in the world. About one-fourth of the world’s coal reserves reside in the U.S. Consumption of coal will continue to grow over the coming decades, both in the U.S. and the world.
Through photosynthetic metabolism, microalgae absorb CO2 and release oxygen. If an algae farm is built close to a power plant, CO2 produced by the power plant could be utilized as a carbon source for algal growth, and the carbon emissions would be reduced by recycling waste CO2 from power plants into clean-burning biodiesel. — Zhiyou Wen, Biological Systems Engineering Department, Virginia Tech