Bioethanol : Micro-Algae
in this article, Amir focuses on micro-algae potential for bioethanol production and illustrates technical challenges that hinder economic feasibility
Micro-Algae are considered biodiesel source since 2004 with companies raising billions of dollars to develop process versions of this technology. However, most of these companies either went out of business or had to pivot to byproducts and applications in nutriceuticals, food additives, cosmetics, specialty oils, and recently bio-chemical carbon dioxide capture. Solazyme, an american algae-based biodiesel producer abandoned its business and became biotechnology company under different brand names. Algenol, another american algae-based biodiesel producer completely turned into nutriceuticals.
There was so much hype in the first place, as biodiel is derived from triglycerides contained in micro-algae and studies have suggested that 4000 square meters micro-algae farm can produce up to 90 liters of biodiesel per day which is way higher than other biodiesel alternatives, this is enough to get excited about it, not to mention fast cultivation, adaptability to different environments, it doesn’t compete with food crops, it can be fed with wastewater, and it is carbon neutral.
Ask anybody with a swimming pool, actually it's interesting to think about how much money people spend trying to keep micro-algae from growing !!
Consequently, micro-algae biodiesel companies failed because like most applicable technologies, they have technical ability to do this but not economic feasibility, as algae biofuels over the years has ranged from highly optimistic less than dollar per liter up to 4 dollars per liter but more recent estimates have put it somewhere between 2.7 and 4 dollars per liter while regular diesel price has been around 0.7 per liter in the United States and recently surged to 1.4 per liter upon russian invasion of Ukraine.
Micro-Algae are mostly utilized for biodiesel production due to high lipid content, while ignoring significant amounts of polysaccharides embedded, thus microalgae can be utilized for bioethanol production directly or by processing residue after oil extraction as bioethanol production is based on fermentation of polysaccharides which are starch, sugar, and cellulose.
Considering micro-algae for bioethanol production seems promising in reference to advantages mentiond above besides that algae-based ethanol finds niche applications in perfumes, sanitizers, and fancy liquors. It is worthy to mention that an american start-up based in new york called air company is utilizing -on pilot scale- captured carbon dioxide from industrial points of emission to produce impurity-free alcohols for niche consumer goods, thus algae-based bioethanol has similar competitive features, specially if carbon neutral process is established through capturing carbon dioxide emissions in fermentation to be utilized in algae farming. Process for producing algae-based bioethanol is illustrated in phases, available data and challenges are listed below : farming phase >> pretreatment phase >> fermentation phase
Based on available research papers, micro-algae with high carbohydrate content are chlamydomonas (60%) and chlorella (54%). Furthermore, carbohydrate content can be increased through nutrient stress cultivation as nitrogen limitation increases carbohydrate content, this approach is beyond selective breeding and genetic modification. Regarding cellular structure, micro-algae cell is composed of outer cell wall composed of certain polysaccharides such as pectin, agar and alginate as composition varies from species to species. In addition, inner cell walls of micro-algae constitute cellulose and hemicellulose.
Generally, micro-algae is cultivated using open ponds or closed-loop reactors. Mainly, open ponds are less expensive, less maintenance expenses, but highly susceptible to contamination. On the other hand, closed-loop reactors are more expensive, required maintenance, but highly resistant to contamination, and optimum for bioethanol production especially that chlamydomonas and chlorella cultivation technology is well matured using closed-loop reactors such as vertical tubular photobioreactors.
challenges : no challenges as farming phase is feasible and cultivation technology is matured
Here you will find information to further illustrate design aspect and operation of photobioreactors for micro-algae cultivation. Almost simple compared to conventional chemical reactors as it based on providing carbon dioxide source, traces of nitrogen nutritients, light supply, temperature, pH and salinity.
This phase is critical as it aims to release carbohydrate content within micro-algae cell wall through induced rupture to provide fermentable content by hydrolysis, prevent degradation, and limit fermentation inhibitors. Thus most challenges encountered in micro-algae bioethaol production are during pre-treatment phase.
Although micro-algae pre-treatment for biodiesel production requires thickening through flocculation, filtration, and drying. Thickening is not essential for bioethanol production as pretreatment aims to enhance hydrolysis mainly through chemical methods, including treatment with dilute acid, dilute alkaline, and organic solvents to make carbohydrate content more accessible. Pretreament methods that utilize microwave ovens, super-heated steam, hot water, and enzymes are not considered due to high operational expenses.
Dilute Acid Method
Dilute sulphuric, hydrochloric, nitric and phosphoric acids can be used as using concentrated acids is less preferable due to inhibitors formation besides equipment corrosion. Usually, dilute sulfuric acid is applied within moderate temperatures to hydrolyze hemicelluloses and facilitate hydrolysis.
Based on available research papers, highest hydrolysis yield was obeserved upon using 0.9 mol per liter, also it was obeserved that hydrolysis yield decreases upon increasing acid concentration not to mention formic acid and furfural formation which inhibit bioethanol formation.
challenges : conventional methods for separation of inhibitors are energy-intensive
Dilute Alkaline Method
Dilute sodium hydroxide, potassium hydroxide, calcium hydroxide and aqueous ammonia can be used regarding that using concentrated alkalines is less preferable due to inhibitors formation besides equipment corrosion. Also solubility of hemicelluloses and cellulose are less in alkaline pre-treatment compared to acid pre-treatment. Furthermore, dilute alkaline pre-treatment forms traces of furfural, hydroxymethylfurfural, formic acid, and unrecoverable salts.
challenges : separation of inhibitors , solubility , unrecoverable salts
Organic Solvent Method
Solvents such as methanol, ethanol, acetone, and ethylene glycol are used. Catalysts are also added along with solvents such as hydrochloric acid, sulphuric acid, sodium hydroxide and ammonia. This method has multiple disadvantages like oxidation, volatilization and safety concerns. Also solvents must be recovered due to formation of significant amounts of furfural and phenols besides higher operational expenses compared to previous methods.
Fermentation of hydrolyzed carbohydrate content is normally carried out in stainless steel stirred tank reactors fitted with manholes, inspection sight glass, thermometer, carbon dioxide vent, and temperature control system. Less challenges encountered in fermentation of hydrolyzed micro-algae content. Practically , bio-ethanol mass yield approx. 5% - 13% of micro-algae biomass.
challenges : determine optimum method for hydrolyzed content fermentation
A. yeast vs. bacteria fermentation B. free vs. immobilized systems
Here you will find yeast species, bacteria species, catabolism, glycolysis scheme, kintetic parameters, and immobilized systems to further illustrate bioethanol production through fermentation.
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