NOVELTY - Closed-loop system comprises: a bioreactor (28) for growing algae cells in an aqueous solution having an initial sodium bicarbonate concentration to supply at least 50% of carbon for algae growth from the sodium bicarbonate; a separator (36) for receiving algae and solution from the bioreactor and producing an algae depleted media; and a regenerator for receiving the algae depleted media, introducing carbon dioxide into the algae depleted media to regenerate sodium bicarbonate from the sodium carbonate and delivering the regenerated sodium bicarbonate to the bioreactor for algae growth.    USE - The closed-loop system is useful for growing algae and producing biofuel from algae (all claimed), where the biofuel includes biodiesel and hydrocarbon fuel.    ADVANTAGE - The closed-loop system allows simple and economical production of biofuel with less effort and without pollutants in an eco-friendly manner.    DETAILED DESCRIPTION - Closed-loop system comprises: a bioreactor (28) for growing algae cells in an aqueous solution having an initial sodium bicarbonate concentration sufficient to supply at least 50% of carbon for algae growth from the sodium bicarbonate, where the sodium bicarbonate is converted to sodium carbonate in a bioreactor during algae growth; a separator (36) for receiving algae and solution from the bioreactor and producing an algae depleted media; and a regenerator for receiving the algae depleted media, introducing carbon dioxide into the algae depleted media to regenerate sodium bicarbonate from the sodium carbonate, and delivering the regenerated sodium bicarbonate to the bioreactor for algae growth. An INDEPENDENT CLAIM is included for producing biofuel from algae, comprising growing algae in an aqueous solution in the bioreactor, concentrating algae to produce an algae depleted media, introducing carbon dioxide into the algae depleted media to regenerate sodium bicarbonate from the sodium carbonate, delivering regenerated sodium bicarbonate to the bioreactor for further algae growth, and processing algae to form biofuel.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a system for producing biofuel.    Scrubber (12)    Gas outlet (24)    Bioreactor (28)    Chemostat (30)    Separator. (36)