Algae blooms occur when algae grow rapidly and form visible patches that can sometimes cover large areas of water. While they can have detrimental effects, such as reducing oxygen levels when they decay, algae are also potentially valuable sources of renewable bioenergy. There are several ways that the biomass from algae blooms could be converted into useful energy forms.
Using Algae for Biofuel Production
Algae are potentially very efficient producers of biofuels. Under the right conditions, certain algal species can double their biomass multiple times per day. The oils and carbohydrates in algal cells can be converted into various transportation fuels.
Biodiesel from Algal Oil
Many species of algae produce substantial quantities of oil, which is composed largely of triglycerides. Algal oil can be converted into biodiesel – a renewable alternative to petroleum diesel fuel – through a process called transesterification. This involves reacting the triglycerides with an alcohol, usually methanol, to produce fatty acid methyl esters and glycerol.
Biodiesel derived from algae has some advantages over biodiesel made from terrestrial crops:
- Algae can produce much more oil per unit of cultivated land area compared to oilseed crops like soy or canola.
- Algae cultivation avoids competing with food production for arable land and freshwater. Algae can be grown in brackish or saltwater.
- Certain algal species have very high oil contents, over 50% of their dry biomass.
Bioethanol from Algal Carbohydrates
The carbohydrates in algal cells, such as starch and cellulose, can also be broken down into fermentable sugars and converted into bioethanol – a gasoline substitute. There are several ways this can be achieved:
- Acid hydrolysis using sulfuric acid or other acids liberates fermentable sugars from algal carbohydrates.
- Enzymatic hydrolysis using combinations of enzymes like amylases, cellulases, and hemicellulases also breaks down algal carbohydrates into fermentable sugars.
- The sugars are then fermented into ethanol using yeast or other microbes.
As with biodiesel production, algae can produce far more fermentable sugars per hectare than terrestrial biofuel crops.
Other Algal Biofuels
In addition to biodiesel and bioethanol, algal oils can also be converted into renewable diesel, aviation fuel, and other hydrocarbons through thermal processing methods like hydrotreating. Algae may also be a source of biohydrogen through biochemical or thermochemical conversion processes.
Generating Electricity from Algal Biomass
In addition to liquid transportation fuels, the biomass from algal blooms can also be used to generate electricity through several processes:
Algal Biomass Combustion
After drying, algal biomass can be burned directly to produce heat and generate electricity. This is similar to burning terrestrial biomass fuels. The high moisture content of fresh algal biomass needs to be reduced through drying before combustion.
Anaerobic Digestion of Algal Biomass
Algal biomass can be digested by bacteria in anaerobic digesters in the absence of oxygen. This produces a mixture of gases called biogas, composed primarily of methane and carbon dioxide. The methane can then be burned in a generator to produce electricity.
Some potential advantages of anaerobic digestion of algal biomass include:
- Ability to utilize wet biomass.
- Production of nutrient-rich digestate as a byproduct.
- Lower emissions compared to combustion.
Direct Conversion of Algal Oils into Electricity
Algae oils may also be directly converted into electricity in microbial fuel cells. These use microorganisms to catalyze the oxidation of oils and other organic matter, generating an electrical current. While yields are currently low, microbial fuel cell technology has potential for further development.
Integrated Algal Bioenergy Systems
To maximize energy yields and overall sustainability, algal bioenergy systems are ideally integrated with wastewater treatment, CO2 capture from industrial sources, and production of other co-products like proteins and nutrients from the algal residual biomass after oil extraction.
Key Challenges in Scaling Up Algal Bioenergy
While algae have great potential as a renewable feedstock, there are several technological and economic challenges that still need to be solved for large-scale implementation:
- Achieving very high biomass productivities sustainably and cost-effectively.
- Developing algal strains with very high oil/carbohydrate contents and fast growth.
- Efficiently harvesting dilute algal cultures and extracting/drying the biomass.
- Reducing energy inputs for cultivation, harvesting, and processing.
- Integrating with wastewater treatment and industrial CO2 sources.
- Competing economically with low petroleum prices.
Intensive R&D efforts are underway to try and address these issues and realize the promise of algal bioenergy on a commercial scale. With sufficient advancement of technologies for cultivating, harvesting, and converting algal biomass, algae blooms could become a valuable source of renewable fuels and electricity rather than just an environmental nuisance.