Algae: Future Feedstock for Biofuels

Algae as Biofuel Feedstock

The production of 1^st generation biofuels is mainly dependent on the biomass of plants such as corn, soybean, sugarcane, and oil-palm, etc. The major issue with first-generation feedstock is its competition with food. Cost-effectiveness, environmental sustainability, and waste generation during the operation are some of the other major concerns for 1^st generation biofuels. Considering the problems, 2^nd and 3^rd generations have been explored as alternative options.

Third generation biofuels are based on bioenergy generated from microorganisms such as microalgae or macroalgae and cyanobacteria. Algae are unicellular microorganisms, capable of photosynthesis. They are considered as a potential oleo-feedstock, as they produce lipids through photosynthesis using only carbon, water, sunlight, phosphates, nitrates, and other (oligo) elements. Microorganisms such as cyanobacteria, bacteria, and microalgae can be applied as a potential feedstock for the production of biofuels and biomaterials cost-effectively and sustainably. They can produce biomass that is mainly composed of lipids, carbohydrates, and proteins using sunlight and CO₂. Apart from lipids, algae also produce proteins, isoprenoids, and polysaccharides. Some strains of algae produce fermentable sugars that can produce alcohol, under the right growing conditions. The biomass can be processed to different sorts of chemicals and polymers (Polysaccharides, enzymes, pigments, and minerals), biofuels (e.g. biodiesel, alkanes, and alcohols), food and animal feed (PUFA, vitamins, etc.) as well as bioactive compounds (antibiotics, antioxidants, and metabolites) through down-processing technology such as transesterification, pyrolysis and continuous catalysis using microspheres. One of the most important advantages of using algae as the source is that it can be grown very easily. The growth rate of algae is 20–30 times faster than other conventional crops like Jatropha.

Algae Biofuels Conversion Routes

Biofuel production from algal biomass is mainly governed by effective extraction methods. It should be more specific towards the extraction of particular bioproducts and simultaneously minimize impurities.

• Lipid or oil extraction from algal biomass
  • Several methods can be applied to extract oils or lipids from algal biomass, such as physical, chemical, mechanical, and enzymatic (biological) methods.
• Transesterification of lipids or oil
  • The process of transesterification of lipids or oil is performed in the presence of a chemical catalyst such as acid and alkali or a biological catalyst such as lipase along with alcohol. The end products are biodiesel and glycerol.
• Pretreatment and saccharification of algal biomass
  • The production of biofuels from these saccharide based feedstocks via the process of microbial fermentation requires prior pretreatment and saccharification. By applying potential microbial strains that can produce amylase enzyme and induce the process of saccharification.
• Microbial fermentation of algal biomass
  • The production of bioethanol using yeast fermentation technology is well recognized at the commercial level. To enhance the production of bioethanol, various parameters have also been considered, such as the screening of robust strains, genetic manipulation, substrate selection, and modification, etc.
• Anaerobic digestion of microalgal biomass
  • The production of biogas from microalgal biomass using the anaerobic digestion process has become an attractive and sustainable approach. The production of biogas, control of greenhouse gas emissions, and production of organic manures are the various advantages of anaerobic digestion.
• Hydrothermal liquefaction (HTL) of algal biomass
  • In this method, organic materials such as biomass and biowaste are subjected to direct liquefaction to bio-oil at temperatures below 400°C in the presence of catalysts and water. To achieve better product yield though HTL, several crucial parameters must be considered, such as the oleaginous microalgal strains, biomass concentration, and reaction type, type of catalyst, temperature, and heating rate, among others.
• Gasification of algal biomass
  • The process of gasification involves the conversion of carbon-rich feedstocks into syngas through partial oxidation in the presence of a limited supply of oxygen or air or steam at temperatures ranging from approximately 100–1000°C.
• Pyrolysis of algal biomass
  • Pyrolysis is a well-known process for the production of the carbon-rich solid-phase and volatile organic phase. The feedstock is subjected to temperatures ranging from 400–800ºC. The vapor phase obtained in this process is finally converted to bio-oil and acid extract after condensation.

Bio-valorization of Algal Biomass

Algae can be grouped into macro and microalgae-based on their morphological appearance. Green seaweed, brown algae, and red algae belong to the macroalgae, while green algae, Chlorella, and spirulina belong to the group of microalgae. Compared to macroalgae, microalgae are preferable feedstocks for biofuels and bioenergy industries due to their oleaginous nature, higher biomass generation, and simple cellular structure.

• Biomethane
  • The production of biomethane (CH4) via anaerobic digestion of residual algal biomass is well known as it produces a mixture of gases in which the CO2 proportion ranges from 30–50% and CH4 contributes 50–70%. The production yield of biomethane from algal is governed by various parameters, such as temperature, biomass loading rate, and volume, duration, bacterial strains, and algal cell wall composition, etc.
• Biodiesel
  • Biodiesel is produced by the process of transesterification. The fatty acid composition of algal lipids is dominated by stearic acid, palmitic acid, and oleic acid, which is similar to the biodiesel standard.
• Bio-jet fuel
  • The oil of microalgae can be converted into jet fuel by hydrotreatment (hydrotreated fatty acids and esters, HEFA). The resulting fuel can be used commercially in blends containing a minimum of 50% conventional jet fuel. This fuel is also referred to as Hydrotreated Vegetable Oil (HVO), Hydrotreated Renewable Jet (HRJ), or Bio-derived Synthetic Paraffinic Kerosene.
• Bioethanol
  • Bioethanol is considered a carbon-neutral fuel, which is mostly produced from plant waste material. Algal biomass can also be applied for the production of bioethanol using several groups of microbes, such as yeast, bacteria, and fungi, under anaerobic fermentation conditions. The algal biomass can be considered a promising feedstock for the production of bioethanol, although its commercialization is still challenging.
• Biochar (BC)
  • BC is a carbonaceous material produced by the thermal treatment of biomass at a moderate temperature under a limited supply of oxygen. Compared to lignocellulosic BC, BC produced from algal biomass shows a low carbon content. The simultaneous production of biofuels and BC using biorefinery approaches makes this process interesting for the future expansion and exploration of algal biomass as feedstock.
• Pigments, Nutraceuticals, and Functional Foods
  • The algal biomass emerges as a potential feedstock for the production of other multiple products such as pigments, and nutraceuticals, having several applications in the fields of healthcare, pharmaceuticals, and cosmetics.

 Algae Biorefinery Overview

The term biorefinery describes the production of a wide range of chemicals and biofuels from biomass by the integration of bio-processing and appropriate low environmental impact chemical technologies in a cost-effective and environmentally sustainable. The concept of biorefining is similar to the petroleum refineries in which multiple fuels and chemicals are derived using crude oil as the starting material. Similarly, biorefining is biomass processing to obtain energy, biofuels, and high-value products through processes and equipment for biomass transformation.

There are four main types of biorefineries:

• Biosyngas-based refinery
• Pyrolysis-based refinery
• Hydrothermal upgrading-based refinery
• Fermentation-based refinery

The main goal of the biorefinery is to integrate the production of higher-value chemicals and commodities, as well as fuels and energy, and to optimize the use of resources, maximize profitability and benefits and minimize wastes.

Algal biodiesel production is currently 2.5 times as energy-intensive as conventional diesel, but co-production and decarburization of the electricity utilized in the production process will make algal biodiesel a financially and environmentally viable option for future transport energy infrastructure. The economic feasibility of algal biofuel production will depend on, lowering the costs and/or increasing the efficiency by controlling the following parameters;

• Culturing systems, water, nutrient and CO₂ requirements
• Modification of the photosynthetic capability and productivity
• The method of cell rupture and/or subsequent lipid extraction
• Cost of the bio/green diesel production from the crude lipid fraction

Sustainable development of an algal biorefinery requires,

• Selection of robust algal strains having enhanced growth characteristics and lipid productivity
• Cultivation strategy with an optimized photosynthetic and nutrients use efficiency
• Use of the co-location strategy (advantage of the industrial wastes of energy, CO2, water, and nutrients)
• Extraction of valuable chemicals from the biomass that can increase the whole value of microalgae biomass
• Adopting greenways to pretreat, extract, and process the valuable metabolites for its applications in various energy and non-energy sectors 

Challenges for Algae-based feedstock and fuels

• Algae growth may create quality variations during the refinement process
• Algae biofuel doesn’t always meet its energy efficiency targets
• Algae growth creates regional sustainability problems
• Algae might grow quickly, but it still needs time to produce viable oils
• Algae growth requires high levels of fertilizer to maximize production
• Algae require significant water resources to produce oil for collection
• High production costs for growing, harvesting, collection, transportation, storage and pre-treatment and low-cost efficiency with high initial capital investment
• Limited practical experience in biofuel production
• Technological problems during algae production such as missing technological infrastructures and well-managed practices, complications with the algae cultivation and harvesting

Conclusion

Algae-based fuels are considered to be sustainable, renewable, effective, and environment friendly as well as the renewable energy resource that can meet the global demand for fuels in the long-term. However, the indefinite availability of sustainable algae resources for the production of biofuels is a very important obstacle because the conversion of algae sources from natural ecosystems into energy resources may lead to serious environmental problems. The potential favorable algae resources for biofuel production should be focused preferably on aquatic phytomass such as non-edible natural algae but grown only in existing extra population and eutrophication zones of rivers, lakes, seas, and oceans and specifically cultivated non-edible algae plantations grown in freshwater, seawater, and wastewater. Algae to biofuel conversion technologies have been already proved that algae can be used as feedstock, but it is too expensive to create biofuel using today’s technology. The industry should continue to look for new ways to maximize the refinement process while minimizing the time investments and capital costs.