Biofuels from algae

In my opinion, the production of biofuels and biobased chemicals from algae is one of the most fascinating topics in the sector of the biorefineries and it will be one of the niches of opportunity with highest potential in the medium and the long terms. This post pretend to be a very basic approach to the topic Algae-to-Biofuels through three general key points.

Algae ^1,2,3,4,5

Algae include a wide variety of photosynthetic organisms capable of transforming light, water and specific nutrients into products that can be used with commercial and industrial purposes. We can say that algae are sunlight-driven factories with potential to convert CO₂ into biofuels, high-value biochemical, food and feed.

There are four algae cultivation technologies currently in use for commercial algae cultivation and proposed for algal biofuel production: extensive or open ponds, intensive or raceway ponds, closed photobioreactors in many designs and closed fermenter systems.

Algal biofuel conversion technologies

Algae can be processed in different ways to obtain a wide spectrum of products. For some time now, their use as an alternative to the current biomass feedstocks has been generating interest among the scientific community, companies and the general public. In fact, algae are recognized as a potential source for the biodiesel production due to their high oil content and fast growth. However, this is not the only application. Next, a brief description of the main pathways to produce renewable fuels from algae. The classification is based on the three general categories exposed in the “National Algal Biofuels Technology Roadmap” of the DoE.

Figure 1. Algenol facility for direct production of biofuels from algae (extracted from Algenol web page)

1. Category: Direct production of biofuels from algae

This category encompasses the direct algal production of recoverable fuel molecules from algae without the need for extraction.

1.1 Pathway: Heterotrophic fermentation. / Product: Alcohols. ^1,6

Algae are capable of producing ethanol and other alcohols through heterotrophic fermentation of starch. This can be accomplished through the production and storage of starch via photosynthesis within the algae, or by feeding sugar to the algae directly, and subsequent anaerobic fermentation of these carbon sources to produce ethanol under dark conditions. Moreover, it is possible to enhance this natural ability found in strains of cyanobacteria to produce ethanol by over expressing fermentation pathway enzymes channeling the majority of photosynthetically fixed carbon into ethanol production rather than routine cell maintenance.

1.2 Pathway: Heterotrophic fermentation. / Product: Alkanes. ^1,7

In the same way as alcohols, alkanes can be produced directly by heterotrophic metabolic pathways using algae. Engineered photosynthetic bacteria can produce and secrete targeted types of alkanes in a continuous, single-step conversion process.

1.3 Pathway: Biophotolysis. / Product: Hydrogen. ^8,9

Biophotolysis is the action of light on biological systems that results in dissociation of water into molecular hydrogen and oxygen. Light intensity ad nutrient availability are important environmental factors to induce the production of hydrogen in the case of green microalgae. They can produce biohydrogen through both direct (under light irradiation) and indirect (dark fermentation on endogenous carbohydrates) biophotolysis.

2. Category: Processing of whole algae

It covers those pathways that process whole algal biomass to yield fuel molecules.

2.1 Pathway: Pyrolysis. / Product: Bio-oil (liquid fuels can be obtained through subsequent upgrading). ^1,10

Biomass pyrolysis is its thermal decomposition in the absence of oxygen to produce liquid, char and gas. The bio-oil or pyrolysis oil is the liquid fraction. The optimal conditions to produce algal bio-oils from different feedstocks need to be carefully studied. According to first studies, algal bio-oil can exhibit a higher carbon and hydrogen content, lower oxygen content and a greater heating value than wood bio-oil.

2.2 Pathway: Gasification. / Product: SynGas (liquid fuels can be obtained through Fischer-Tropsch or mixed alcohol synthesis). ¹

Syngas is a mixture of mainly carbon monoxide and hydrogen. It is produced by subjecting biomass to thermal degradation in the presence of an externally supplied oxidizing agent (air, steam or oxygen) in a process known as gasification. It is necessary to determine the optimum conditions for gasification of algae.

2.3 Pathway: Liquefaction. / Product: Bio-crude (liquid fuels can be obtained through subsequent upgrading). ¹

Direct hydrothermal liquefaction in subcritical water is a technology that can be employed to convert wet algal biomass to a range of liquid fuels. Water in subcritical environments is capable of decomposing the algal biomass into smaller molecules of higher energy density or more valuable chemicals. The main product of this liquefaction process is a bio-crude that can be upgraded further.

2.4 Pathway: Supercritical Processing. / Product: Biodiesel. ^1,11

The supercritical extraction process can be coupled with a transesterification reaction scheme to enable Single-Step Conversion Process to biofuel production. Using water in wet algae as a tunable co-solvent in supercritical methanol process not only accelerates the conversion of fats and algal oils to fatty acid methyl esters (FAMEs), but also increases solubility and acidity.

2.5 Pathway: Anaerobic digestion. / Product: Biomethane. ^1,12,13

The production of biogas can be interesting for two approaches: whole-cell or algal residue. On the one hand, from a whole-cell perspective, microalgae cultivated for wastewater treatment and environmental protection purposes, are typically low in lipid content. Anaerobic digestion is a simple way to valorize them. On the other hand, current production methods for liquid biofuel production from microalgae produce approximately 60–70% residual biomass that is currently a byproduct. Anaerobic digestion of this subproduct can produce biomethane and essential nutrients.

3. Category: Conversion of algal extracts

Those pathways that process algal extracts (e.g., lipids, carbohydrates) to yield fuel molecules are covered by this category.

3.1 Pathway: Oil transesterification. / Product: Biodiesel. ^1,3,5

Depending on species, microalgae produce many different kinds of lipids, hydrocarbons and other complex oils. Not all algal oils are satisfactory for making biodiesel, but suitable oils occur commonly. This triacylglycerols are reacted with methanol in the presence of a catalyst to produce FAME (biodiesel) and glycerol as a co-product. One variation of the process implies the use of biocatalyst (lipases) in a biochemical conversion.

3.2 Pathway: Oil hydroprocessing. / Products: Renewable diesel, jet biofuel, bionaphta and biopropane. ^3,5

Hydroprocessing is an alternative process to esterification to produce diesel from biomass. Hydrogen is used to remove the oxygen from the triglyceride producing a mix of linear paraffins, CO₂ and water. Then, the product of the first stage is isomerized, always in presence of hydrogen, in order to branch the linear chains for improving the cold flow properties of the final products. The alkane mixture can be fractionated to produce renewable diesel, synthetic kerosene jet fuel, bionaphta and biopropane.

3.3 Pathway: Fermentation. / Product: Alcohols. ^14,15

Algae capable of accumulating starch and cellulose can serve as an alternative to food crops for bioethanol production. Carbohydrate-rich microalgae can be used as feedstock for bioethanol production via hydrolysis strategies and fermentation processes.

Advantages ^1,4

These are some of the advantages of using microalgae in biofuels production instead of agricultural crops:

• High productivity per area unit. Unlike other oil crops, algae grow rapidly and many are exceedingly rich in lipid oil (oil levels of 20 percent to 50 percent are quite common).
• Non-food resource. Using algae to produce feedstocks for biofuels production does not compromise the production of food derived from terrestrial crops.
• Use of otherwise non-productive land. Unlike terrestrial energy crops, the cultivation of algae will not need to compete with farmland for food production.
• Utilization of a wide variety of water sources. The water used to grow algae can include waste water and non-potable saline water that cannot be used by conventional agriculture or for domestic use.
• Mitigation of GHG release into the atmosphere. Algae have a tremendous technical potential for GHG abatement through the use of CO2-rich flue gases from coal burning power plants as well as from natural gas recovery operations.

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REFERENCES

1 U.S. DOE 2010: “National Algal Biofuels Technology Roadmap”. U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Biomass Program.

2 Muhammad Syukri Abd Rahaman Li-Hua Chengb, Xin-Hua Xub, Lin Zhanga, Huan-Lin Chena: “A review of carbon dioxide capture and utilization by membrane integrated microalgal cultivation processes”. Renewable and Sustainable Energy Reviews 15 (2011) 4002– 4012.

3 Y. Chisti: “Biodiesel from microalgae”. Biotechnology Advances 25 (2007) 294–306.

4 I. Priyadarshani , B. Rath: “Commercial and industrial applications of micro algae – A  review”. J. Algal Biomass Utln. 2012, 3 (4): 89–100.

5 A. Darzins, P. Pienkos, L. Edye: “Current Status and Potential for Algal Biofuels Production”. A Report to IEA Bioenergy Task 39, Report T39-T2, 6 August 2010.

6 http://www.algenol.com/direct-to-ethanol/the-technology.

7 http://www.jouleunlimited.com/joule-expands-intellectual-property-direct-conversion-co2-hydrocarbons.

8 J. Yu, P. Takahashi: “Biophotolysis-based Hydrogen Production by Cyanobacteria and Green Microalgae”. Communicating Current Research and Educational Topics and Trends in Applied Microbiology. A. Méndez-Vilas (Ed.). Formatex 2007.

9 K. Skjånes: “Potential for use of green microalgae to produce hydrogen from solar energy, with subsequent use of algal biomass for pharmaceutical or industrial products”. International workshop on use of solar energy for CO2 capture, algae technology and hydrogen production, and subsequent use of algal biomass for commercial purposes. 17-18 2011, Kolkata, India.

10 D. Mohan, C.U. Pittman, P.H. Steele: “Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review”. Energy & Fuels 2006, 20, 848-889.

11 P.D. Patil et al: “Optimization of direct conversion of wet algae to biodiesel under supercritical methanol conditions”. Bioresource Technology, 102 (2011), 118–122.

12 A.J. Warda, D.M. Lewisa, F.B. Green: “Anaerobic digestion of algae biomass: A review”. Algal Research . http://dx.doi.org/10.1016/j.algal.2014.02.001.

13 B. Zhao, J. Ma, Q. Zhao, C. Frear: “Anaerobic Digestion of Algal Biomass Residues with Nutrient Recycle”. Final Report, Washington State University Subcontract on DoE Project 22902.

14 S. Nahak, G. Nahak, I. Pradhan, R.K. Sahu: “Bioethanol from Marine Algae: A Solution to Global Warming Problem”. J. Appl. Environ. Biol. Sci., 1(4) 74-80, 2011.

15 S.H. Ho, S.W. Huang, C.Y. Chen, T . Hasunuma, T . Kondo, J.S. Chang: “Bioethanol production using carbohydrate-rich microalgae biomass as feedstock”. Bioresour Technol. 2013 May;135:191-8. doi: 10.1016/j.biortech.2012.10.015. Epub 2012 Oct 16.