Description: Biofuels

The origin of all fuel and biofuel compounds is ultimately the sun, as solar energy is captured and stored as organic compounds through photosynthetic processes. Certain biofuels, such as oils produced by plants and algae, are direct products of photosynthesis. These oils can be used directly as fuel or chemically transesterified to biodiesel. Other biofuels such as ethanol and methane are produced as organic substrates are fermented by microbes under anaerobic conditions. Hydrogen gas can be produced by both routes, that is, by photosynthetic algae and cyanobacteria under certain nutrient- or oxygen-depleted conditions, and by bacteria and archae utilizing organic substrates under anaerobic conditions. Electrical energy produced by microbial fuel cells—specialized biological reactors that intercept electron flow from microbial metabolism—can fall into either category, depending on whether electron harvest occurs from organic substrates oxidized by organotrophic cultures or from photosynthetic cultures.

A comparison of biofuel energy contents reveals that hydrogen gas has the highest energy density of common fuels expressed on a mass basis. For liquid fuels, biodiesel, gasoline, and diesel have energy densities in the 40 to 46 kJ/g range. Biodiesel fuel contains 13 percent lower energy density than petroleum diesel fuel, but combusts more completely and has greater lubricity.7 The infrastructure for transportation, storage, and distribution of hydrogen is lacking, which is a significant advantage for adoption of biodiesel.

Another measure of energy content is energy yield (YE), the energy produced per unit of fossil fuel energy consumed. YE for biodiesel from soybean oil is 3.2 compared to 1.5 for ethanol from
corn and 0.84 and 0.81 for petroleum diesel and gasoline, respectively.8 Even greater YE values are achievable for biodiesel created from algal sources or for ethanol from cellulosic sources.9 The high net energy gain for biofuels is attributed to the solar energy captured compared to an overall net energy loss for fossil fuels.