BIOSCOPE

Bioengineers Current Journal and Paper in Review

Vol. 9

Algae: Biofuel but Better!

A Review by the Journalism Division of SBE UISC 2022

Based on the article Algal biofuels: Technological perspective on cultivation, fuel extraction and engineering genetic pathway for enhancing productivity by P.R. Yaashikaa, M. Keerthana Devi, and P. Senthil Kumar.

Published in Fuel on 16th of March 2022

(https://doi.org/10.1016/j.fuel.2022.123814)

Introduction

Biofuels are renewable energy sources derived from biomass that have great potential towards being a sustainable alternative energy source. They are efficient as fuels, renewable, and can even lower the level of greenhouse gasses. The majority of feedstock for biofuel comes from oil that originated from agricultural plants. However, this presents us with problems like the lack of space for feedstock farming, water consumption, and food crisis. As an alternative source of biomass, microorganisms have been considered as potential feedstocks for producing biofuels and other value-added products. Commonly used microorganisms such as bacteria, cyanobacteria, and microalgae for biofuel production seem to be promising to replace the feedstocks of biofuels.

 

Algal biofuel is biofuels derived from the biomass that is obtained from algae and microalgae. Algae and microalgae are composed of lipids, proteins, and carbohydrates that can be used to make biofuels. Those compositions are produced by algal cells from carbon dioxide and sunlight. Several studies have proven the abilities of algae and microalgae to produce a wide range of fuels such as bioethanol from algae-extracted biomolecules, biohydrogen production, syngas, methane, hydrogen, and electricity.

Highlighted Topics

Third Generation Biofuel

  • Third generation biofuels refer to biofuels derived from algae, microalgae, and cyanobacteria. 

  • The advantages of using algae as biofuel include the ability to solve the food and fuel crises because algae is a non-consumable feedstock, higher biomass productivity, using water as an electron donor for photosynthesis, the ability to produce a variety of products, and algae growth 30 times faster than food crop growth.

Algae Cultivation and Condition

  • Open Ponds 
    Open ponds are the simplest systems in which algae species grow in a pond in the open air. Open ponds are the most cost-effective method of cultivating algae. However, this system has a number of limitations, including contamination by bacterial or fungal growth and invasion by other algae species.​

  • Photobioreactors
    Photobioreactors are the most advanced and most difficult systems to implement. The photobioreactors are made up of glass or acrylic materials that are transparent and help in photosynthesis and algal growth by allowing sunlight to pass through the reactor. Photobioreactors are expensive compared to open-culture systems. However, there are numerous advantages, including reduced contamination, reduced loss due to the evaporation process, continuous production even at night by providing an artificial light source, and the ability to control and monitor nutrient content during the process.

Biovalorization of Algae Biomass to Biofuels

1. Biodiesel

  • Algae is considered as a potential feedstock to produce biodiesel. Biodiesel production can be facilitated by transesterification, in which triglycerides are converted into fatty acid methyl ester and glycerol.

  • The study showed that lipid yield in the range of 62% was obtained from microalgae biomass. 

  • There are several factors that affect the yield of methyl ester, such as the quality of oil, catalyst, temperature, selection of alcohol, and impurities present in oil. Algae lipids are rich in oleic acid, palmitic acid, and stearic acid. The catalysts mostly used for the transesterification process are bases, acids, and enzymes. Among the catalysts, base is the most efficient, with maximum yield achieved in a short time. Bacillus sp. can produce biodiesel at a constant temperature.

2. Bioethanol

  • Bioethanol is a biodegradable and eco-friendly fuel produced from various feedstock such as cellulosic biomass, agricultural, and other lignocellulosic wastes. 

  • Algae involve three production stages to produce bioethanol including pretreatment, enzymatic hydrolysis, and fermentation. 

  • A pretreatment process is used to make algae more susceptible to further breakdown by separating the cellulose and hemicellulose fractions. Pretreatment is mainly done by making cellulose into ethanol. Cellulose is converted to sugar molecules through enzymatic hydrolysis. Sugar molecules, especially simple monosaccharide sugar molecules, are recovered from algae biomass for the production of bioethanol through the fermentation process.

3. Biogas

  • The ability of microalgae towards the efficient photosynthetic conversion of sunlight into chemical energy provides the focus on biogas production through anaerobic digestion.

  • Methane/biogas is produced by anaerobic digestion involving four steps: hydrolysis (breaking down molecules into simple monomers), acidogenesis (converting monomers to fatty acids), acetogenesis (converting fatty acids to intermediate products such as CO2, hydrogen, and acetic acid), and the last step, methanogenesis (converting intermediate products to methane and CO2).

  • The factors that affect the effectiveness of biogas production include algae cell walls, temperature, time, and biomass volume. 

 

4. Biohydrogen

  • Hydrogen from microalgae is considered as an alternative source to replace fossil fuels because it is clean and non-toxic.

  • Algae produce biohydrogen through three methods: dark fermentation, photo fermentation, and biophotolysis.  

  • Clostridium sp. and Thermatoga sp. are involved in dark fermentation that converts organic matter to hydrogen in the absence of sunlight.

  • Photo fermentation involves sunlight for producing hydrogen by conversion of organic matter.

  • Biophotolysis can be done by utilizing environmental H2O and direct sunlight. Sunlight helps H2O split into H2 and O in the atmosphere and H2 can be collected and utilized for energy generation.
     

Limitations

  • Biomass produced using a raceway pond and photobioreactor was complex and challenging due to algal contamination, attack of insects, less growth rate and yield of biomass, low carbohydrates and lipids content, maintenance and harvesting cost, and utilization of chemicals.

  • Pretreatment process would result in production of toxic chemical compounds which need adsorbent materials to remove.

  • Biofuel can’t be produced from algae in a continuous process and need a higher cost for transport, storage, and production of biofuel.

Economic and Ecological Aspects

  • The costs required for cultivation of algal species is thrice more than their initial capital investment.

  • Depending on the cost required, the reactors used for algal cultivation can be summarized from high to low as: Raceway pond > horizontal tubular > vertical stacked > flat panels.

  • The harvesting and separation of algal biomass is another major challenge in algal cultivation.

  • The cost can be minimized by utilizing various harvesting methods such as centrifugation and coagulation followed by gravity sedimentation.

  • Closed photobioreactor system helps in reducing the harvest cost compared to the open system.

  • Microalgae as feedstock capture carbon in the atmosphere and support carbon mitigation.

  • The transformation of microalgae to valuable products is complex since the process requires it to be changed throughout the value chain.

  • Algal biorefineries is a carbon sequestration process that minimizes the overall emissions.

Future Outlooks

  • Biofuel production from algae species has several merits such as having the ability to grow at a higher rate than other sources, more productivity, feasibility to be grown in water, minimization of greenhouse gasses emission, production of high energy content, etc.

  • However, they also have demerits such as sustainability issues, time required to produce viable oils, higher production cost, high chance of contamination, the need for high amounts of water for cultivation, etc.

  • In order to combat some of the drawbacks, scientists use genetic approaches such as engineering the strain to over-express some genes so they could produce more starch and lipid, or modify algae to possess enhanced photosynthetic efficacy so they can produce more biomass, etc.

  • The field of genetic engineering, system biology, and molecular biology can be associated with life cycle assessment for optimizing the production of algal biofuels.

  • Biorefinery approach where total biomass of algal species has been utilized for producing various biofuels and other value added products resulted in successful outcome from a techno-economic perspective.

Additional readings:

  • Ganesan, R., Manigandan, S., Samuel, M. S., Shanmuganathan, R., Brindhadevi, K., Lan Chi, N. T., Duc, P. A., & Pugazhendhi, A. 2020. ‘A review on prospective production of biofuel from microalgae’. Biotechnology reports, 27. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7396912/?msclkid=e09722eac55911eca59edd9480085f42

  • Sivaramakrishnan, R., Suresh, S., Kanwal, S., Ramadoss, G., Ramprakash, B. & Incharoensakdi, A. 2022. 'Microalgal Biorefinery Concepts Developments for Biofuel and Bioproducts: Current Perspective and Bottlenecks’. International Journal of Molecular Sciences, 23(5), p.2623. Available at: https://www.mdpi.com/1422-0067/23/5/2623