Biofuels and Ethanol: Liquid Fuels from Living Matter

Throughout human history, the primary source of energy for transportation has been biological: the metabolic energy of horses, oxen, camels, and other animals converted grass and grain into the motion that moved people and goods. The twentieth century's replacement of animal power with liquid fuels derived from petroleum seemed to sever the ancient link between biology and transport energy — but biofuels have re-emerged as a significant component of the global transportation fuel supply, driven by concerns about fossil fuel dependence and greenhouse gas emissions.

Biofuels are liquid or gaseous fuels derived from biological materials — primarily crops and crop residues, but also waste oils, animal fats, and in the future potentially algae and wood — that can substitute for or blend with fossil fuels in existing engines and infrastructure. The oldest and simplest biofuel is ethanol (grain alcohol, or ethyl alcohol) produced by fermenting sugar-containing plant materials with yeast, the same process that produces beer and wine. The oldest liquid fuel burned in engines was likely vegetable oil: Rudolf Diesel, the German engineer who invented the compression-ignition engine in the 1890s, demonstrated his engine running on peanut oil at the 1900 World Exhibition in Paris, and explicitly envisioned vegetable oils as potential fuels for his engines.

Modern biofuels encompass a diverse family of technologies and feedstocks: sugar- and starch-based ethanol (from sugarcane, corn, wheat, and other crops); cellulosic ethanol (from wood, straw, and other lignocellulosic materials); biodiesel (fatty acid methyl esters produced from vegetable oils and animal fats by transesterification); hydrotreated vegetable oil (HVO, a drop-in diesel substitute produced by hydrocracking vegetable oils); and advanced biofuels including synthetic aviation fuel from biomass gasification and Fischer-Tropsch synthesis.

The global biofuels industry is enormous in scale: the United States, Brazil, the European Union, and China together account for the majority of global biofuel production, with the US and Brazil alone producing approximately seventy-five percent of the world's fuel ethanol. Total global biofuel production exceeded two hundred billion liters per year by the mid-2020s, providing approximately four to five percent of global transportation fuel on an energy basis. Biofuels have been mandated, subsidized, and promoted in most major economies as components of energy security and renewable energy strategies, generating both significant economic activity and sustained controversy about their environmental and social impacts.

The Chemistry and History of Fermentation

The production of ethanol by fermentation — the anaerobic conversion of sugars to ethanol and carbon dioxide by yeast — is one of the most ancient biotechnological processes known to humanity, practiced for at least nine thousand years for the production of alcoholic beverages.

The chemistry of fermentation is simple in outline: glucose (C6H12O6) is converted by yeast enzymes to two molecules of ethanol (C2H5OH) and two molecules of carbon dioxide (CO2) in the absence of oxygen. The pathway — the Embden-Meyerhof glycolytic pathway plus alcohol dehydrogenase — is one of the most studied metabolic sequences in biochemistry and operates in Saccharomyces cerevisiae (baker's and brewer's yeast) with remarkable efficiency, converting approximately ninety to ninety-five percent of the theoretical maximum ethanol yield from pure glucose.