In recognition of the limitations facing us with fossil fuels, many alternative fuel production methods are being tried. To date, they have focused on products available from terrestrial plants.
Sugars are the medium-term energetic intermediates favored by terrestrial plants. While sugars contain less energy than lipids, the technology to ferment them into ethanol has been around for thousands of years.
Corn is an unlikely candidate for sugar production; its low sugar content as a ratio of total biomass makes it an inherently inefficient crop. Nonetheless, massive subsidies of corn production in the US have rendered sugars from corn a relatively cheap input, and because of the negative effects of subsidies on the per-bushel price of corn, ethanol production has been seen as a way to increase demand. Additional subsidies and grants have been provided for the production of ethanol from high-fructose corn syrup.
In addition to its low sugar content, corn has an extraordinarily high nitrogen requirement, amounting to an average of 120 kg/hectare for cultivation in the United States. This nitrogen requirement is met through the Haber-Bosch process invented in 1918 by Fritz Haber and translated into the mass scale by Carl Bosch. This process, requiring hundreds of atmospheres of pressure and temperatures around 600ºC, joins the hydrogens from fossil methane to atmospheric dinitrogen to form ammonia. The carbon from the methane is oxidized to CO2 which is sometimes retained for additional industrial processes but generally vented into the atmosphere. In the most modern of plants, every kilogram of ammonia produced involves the emission of 1.5 kg CO2 and the consumption of 30 megajoules of energy.
This alone renders corn sugar as an inappropriate source of fuel; the other costs that go into its growth and conversion mean that 130% of the energetic value of the ethanol end product has already been consumed, largely in the form of fossil fuels.
Sugar cane is one of the only crops with a positive energy and carbon balance in the production of ethanol. It requires half the nitrogen of corn, and the practice of burning non-sugar elements of the plant (begasse) to provide the energy required to extract the sugar means that the yield of ethanol contains some net energetic yield. The energy released from burning begasse is greater than the requirements for sugar extraction, so many sugar processing facilities perform cogeneration and have an electrical output in addition to their sugar output, further reducing the total cost of ethanol production.
Its extensive use in Brazil has reduced CO2 emissions and dramatically reduced nitrous oxide and other pollutants characteristic of fossil fuels. Sugar cane prefers a semi-tropical climate for cultivation, and the ecosystems it displaces tend to have had a high biodiversity. In the United States, conversion of large portions of the Everglades wetlands in Florida to sugar cane production has caused severe environmental stress to the remaining wetlands due to water diversion for irrigation, eutrophication from nutrient runoff, and changes in salinity levels.
Poor cultivation practices, including crop burning before harvest, over fertilization, and haphazard soil management have meant that historically sugar cane cultivation has caused many negative ecological effects. Using a modern approach to its cultivation allows sugar cane to be no more destructive than other agricultural crops.
The severity of the global warming problem and sugar cane’s status a net-positive biofuel source means that expansion of sugar cane production is likely. Even if we were willing to countenance monocropping sugar across half the arable land of earth, the 79.5 liters of ethanol per ton of sugarcane and ethanol’s lower energetic value mean that we would produce about 56% of our 2005 fuel consumption.
65 tons/ha x 79.5 l/ton = 5167.5 l/ha.
5167.5 l x 0.0063 barrels/liter = 32.55 barrels ethanol
32.55 barrels ethanol x 66% energy value of crude = 21.5 barrels crude equivalent
21.5 x (1.65 x 109 hectares arable land / 2) = 17.738 x 109 barrels crude equivalent/year
31.426 x 109 barrels oil consumed in 2005
Cellulose has been picked as a likely molecular candidate for processing into ethanol because of its terrestrial status as “most common molecule”. It has attained this status by being extremely strong, yet ductile, and by being quite resistant to chemical decomposition.
Eukaryotes form cellulose into fibers that make it even more difficult to break apart. The cellulose is first sandwiched together in sheets. In these sheets, the cellulose forms hydrogen bonds to its adjacent neighbors, gaining strength and minimizing the water-friendly area it exposes (fig 1, 2). These strips are associated into a diamond formation which are glued together with lignin and hemicellulose (fig 1, 3-4).
In order to get at the sugars inside this fibril, a substantial amount of energy must be expended. First the cellulose source needs to be mechanically shredded to expose the sugars to enzymatic action. Then the shredded cellulose is placed in a tank with steam, acid, and cellulase enzymes. The mechanical grinding, pressurized steam and acid are necessary to pry apart the sheets of cellulose so that the cellulase enzymes can access to the bonds between the sugars. From this point on the processing resembles that of sugar-based processes.
In all, this process consumes 45% more energy than produced when using switchgrass as a feed stock and 57% more energy than produced when using wood as a feed stock.
Having given up on exterminating animals for oil, efforts have been made to produce fuel from oily plants, with soybeans being the leading contender. The association of soybeans with nitrogen-fixing bacteria makes them a much more energetically efficient crop than either corn or sugar cane, but their oil content is only around 18%. The process for processing oils into diesel has been calculated to create about an 8% energetic deficit14, but there is a good deal of innovation in this field.
A new biodiesel production process of Neste Oil called NExBL17, whose pilot plant enters production in 2007, claims extremely low CO2 emissions and results in a fuel that is superior in all respects to existing petroleum diesel. If this proves as viable at large scale as their existing data, this process will make oils from many feedstocks viable sources of fuel oil.
Approximately 436 liters of biodiesel can be produced per hectare of soybeans. Combined with the high energy density of diesel, this is the equivalent of 786 liters of ethanol per hectare, well below the 5100 liters produced by sugar cane. This suggests that while investment in biodiesel production and particularly the synthesis of biodiesel from various fats makes energetic sense, additional supplies of oils will be necessary to provide for our planetary fuel requirements.