Why is alge considered to be a third generation biofuel and not a first generation biofuel?

It’s considered the third generation because it’s not considered a good solution…

Algae can be grown to produce biohydrogen. In 1939 a German researcher named Hans Gaffron, while working at the University of Chicago, observed that the algae he was studying, Chlamydomonas reinhardtii (a green-alga), would sometimes switch from the production of oxygen to the production of hydrogen. Gaffron never discovered the cause for this change and for many years other scientists failed to repeat his findings. In the late 1990s professor Anastasios Melis, a researcher at the University of California at Berkeley, discovered that if the algae culture medium is deprived of sulfur it will switch from the production of oxygen (normal photosynthesis), to the production of hydrogen. He found that the enzyme responsible for this reaction is hydrogenase, but that the hydrogenase lost this function in the presence of oxygen. Melis found that depleting the amount of sulfur available to the algae interrupted its internal oxygen flow, allowing the hydrogenase an environment in which it can react, causing the algae to produce hydrogen. Chlamydomonas moeweesi is also a good strain for the production of hydrogen. Scientists at the U.S. Department of Energy’s Argonne National Laboratory are currently trying to find a way to take the part of the hydrogenase enzyme that creates the hydrogen gas and introduce it into the photosynthesis process. The result would be a large amount of hydrogen gas, possibly on par with the amount of oxygen created.

Algae can be used to make biodiesel (algaculture), bioethanol and biobutanol and by some estimates can produce vastly superior amounts of vegetable oil, compared to terrestrial crops grown for the same purpose.

Algae can be used in oil production which could replace the petrol and other gas products in the near future.

Algae can be grown to produce biomass, which can be burned to produce heat and electricity.