Extractive-Transesterification of Algal Biomass into Biodiesel under Supercritical Methanol and Microwave Conditions

摘要

Biodiesel can be produced from a variety of feedstocks, including edible oils (corn, peanut, soybean named as first-generation biodiesel feedstock); non-edible oils (jatropha, karanja, animal fats and waste cooking oils: second generation); and algae (numerous species: third generation). Among these feedstocks, biodiesel production from algae has drawn special attention for the following reasons: (a) high lipid content (20–50%) and high growth rates (1–3 doublings per day); (b) tolerant to severe environmental conditions; (c) sequester carbon dioxide from the flue gases; (d) harvesting and transportation are economical compared to other crops (due to small size and the use of arid and marginal lands); (e) very high actual photosynthetic yields (3–8%) compared to other terrestrial plants (0.5%). Algal biodiesel production consists, primarily of five steps. They are: (a) algae production; (b) algae harvesting; (c) oil extraction; (d) transesterification or chemical treatment; and (e) separation and purification9. All the steps involving from harvesting to purification are both energy and cost intensive. Currently, major hurdles for the algal biodiesel production are: dewatering the algae; oil extraction and transesterification. The algal culture is usually concentrated to 15–20% from its original concentration of 0.02–0.05% through various techniques and contains major portion as water, which poses serious problems in the chemical treatment step, i.e., transesterification. Apart from this, extraction of algal oil is not as simple as that would be from other crop seeds, which are usually done by mechanical pressing and solvent extraction methods, due to their rigid cell wall structure. As such, these three steps add significantly to the cost of the algal biodiesel product. Hence, it is critical to develop fast and easy methods that would reduce the chemical and energy consumption and processing time of the overall biodiesel production process.