First generation biofuel comes from edible plants like corn. But this is problematic, because there isn’t enough available land and it can raise the price of food.
Second-generation biofuels made from cellulosic material—colloquially, “grassoline”—can avoid these pitfalls. Grassoline can be made from dozens, if not hundreds, of sources: from wood residues such as sawdust and construction debris, to agricultural residues such as cornstalks and wheat straw, to “energy crops”—fast-growing grasses and woody materials that are grown expressly to serve as feedstocks for grassoline.
There are huge amounts of such feedstock available. And scientists have been making major breakthroughs in turning the cellulose into biofuel.
But it’s not just from plant sources
[A pork processing plant in Oklahoma] is expected to turn 30 million pounds of lard into 30 million gallons of biodiesel a year.
Scientific American has a detailed article.