Algae are one of the most promising sustainable biofuel sources. As photosynthetic organisms, algae can produce lipids and hydrocarbons that can be refined into various biofuels. With their fast growth rates and high oil yields, algae are an ideal candidate for scalable and economically viable biofuel production.
Why Algae for Biofuels?
Algae have several key advantages that make them well-suited for biofuel production:
High Oil Content
The oil content of algae can reach up to 70% of their dry cell weight. This is much higher than traditional oilseed crops like soybean, palm and canola which have oil content ranging from 10% to 50%. The high oil productivity per acre of algae cultivation can produce over 15,000 gallons of biofuel feedstock, compared to only 50 gallons from soybeans.
Fast Growth Rates
Algae are capable of extremely rapid growth under optimal conditions. Their doubling time during exponential growth can be as little as 3.5 hours. Algal biomass can double several times per day. This allows for much higher yields of biofuel feedstock per acre compared to conventional crops.
Use Marginal Lands and Water
Algae can grow in salty or brackish water which is not suitable for food crop cultivation. This reduces competition with conventional agriculture over scarce freshwater resources. Algae can utilize otherwise non-productive marginal lands for biofuel production.
Recycle Waste CO2 and Nutrients
Algal biofuel production can recycle waste carbon dioxide from power plants and industrial exhaust gases as a feedstock for growth. The algal biomass also consumes nutrients like nitrogen and phosphorus from agricultural runoffs and wastewaters. This provides environmental benefits beyond fossil fuel displacement.
Microalgae vs Macroalgae
The two main groups of algae that can be used for biofuels are:
Microalgae are unicellular photosynthetic organisms. Common microalgal species used for biofuels include Chlorella, Dunaliella, Nannochloropsis and Botryococcus. They have extremely fast doubling times of 3-8 hours under optimal conditions. Most microalgae have oil contents between 20-50%. Some species like Botryococcus braunii can reach 75% oil content by dry weight.
Macroalgae are multicellular seaweeds like kelp, sargassum and gracilaria. They display slower growth than microalgae, with doubling times of 1-7 days. Their advantage lies in ease of harvesting from natural marine environments. Macroalgae typically have lower oil content of 1-5%, but can be processed into biogas via anaerobic digestion.
Key Steps for Microalgal Biofuel Production
The basic process for producing biofuels from microalgae involves:
Algae Strain Selection
Select algal strains with high growth rates, lipid productivity and tolerance to environmental conditions. Strains like Nannochloropsis and Chlorella are commonly used for their 20-30% oil content and fast doubling times. Genetic engineering can also enhance algal oil yields.
Algae are cultivated in open raceway ponds or closed photobioreactors. Optimal light, CO2, temperature, nutrients and pH are provided to maximize algal biomass and oil production. Algal culture is continuously circulated from the inoculation pond to the production ponds during scale up.
Algal cultures are harvested periodically during the growth cycle by centrifugation, membrane filtration or chemical flocculation. This concentrates the algal biomass to 15-25% solids content.
Oil extraction is done using organic solvents like hexane or supercritical fluids to disrupt the algal cells and extract the lipids. The solvents are then recovered, leaving behind the extracted algal oil.
The algal oil undergoes transesterification with an alcohol like methanol to produce biodiesel fuel. Glycerol is produced as the major by-product. Unextracted algal biomass can also be converted into bio-oil via hydrothermal liquefaction.
Challenges for Economically Viable Algal Biofuels
While algae biofuels offer great promise, several technological and economic hurdles must be crossed for their commercial viability:
Achieving very high biomass and oil productivities in large scale cultivation with economical use of water and nutrients.
Developing low energy intensive harvesting techniques. Centrifugation and filtration have high energy demands at industrial scales.
Improving oil extraction efficiency and reducing use of expensive organic solvents.
Identifying valuable co-products from algal biomass to improve the overall economics.
Engineering highly productive algal strains that can thrive in outdoor conditions.
Reducing contamination risks in large open ponds from invasive species.
Optimizing biorefinery technologies like hydrothermal liquefaction to directly process wet algal biomass.
The Future of Algal Biofuels
With sustained R&D efforts to tackle the above challenges, algae can become a major sustainable source for renewable fuels. Their high yields per acre can replace a substantial portion of fossil fuels without competing for food production. Algal biofuel commercialization will likely take off after 2020-2025, with increasing production if oil prices rise. Various startup companies are pursuing algae to biofuel projects. Government funding and private investments will help bring promising technologies to commercial scale in the coming decade.
Algae biofuel is poised to become a major pillar of the emerging bioeconomy. Their multitude of high-value applications can improve the economics and viability of algal biofuels. With rising concerns over energy security and climate change, algal biofuels can provide sustainable, scalable and eco-friendly solutions. Harnessing algae for fuels and other products will be an important step towards a greener future.