The biofuel story can be a confusing one, since there are so many different types from so many different sources that are used in so many different ways. Let’s see if we can make some kind of sense of all this while gaining some understanding of where they come from and how they are produced.
Biofuel producers often speak in terms of generations, which are really more of a historical classification than a functional one. First generation biofuels are the ones that have been around for a while. They generally come from the starches and sugars found in food crops, along with animal fats and vegetable oils. Like most biofuels, they can be divided into two types: biologically derived material that can fermented to produce alcohol (starch), and natural oils that can be processed to create biodiesel (lipids).
Alcohol-based biofuels are produced in much the same way as liquor, though, of course with different emphasis. In fact, ethyl alcohol, or ethanol, which is the most commonly used bio-alcohol, is the same type of alcohol that is found in whiskey, vodka, gin, etc. All of these are based on the fermentation of starches and sugars. The ingredients are gathered, ground up, mixed with water and left to ferment with the help of yeast or other organisms for some period of time (three years for Scotch whiskey!) before it is finally distilled into the final product. Whiskey is made from barley. Vodka was traditionally made from potatoes, though various grains are often used now. Bourbon contains 51 percent or more corn. Tequila is made from agave. Rum is made from molasses, and gin comes from juniper berries. Brandy and cognac both come from grapes. All of these are fermented to produce alcohol. In the case of biofuel, which primarily uses corn, because of the quantities required, the emphasis is on productivity, which is why enzymes are often used to accelerate the breakdown of starches into sugars.
Biodiesel, on the other hand, is derived from plants that produce vegetable oils or from animal fat. The most common oil-rich plants used include palm and soybeans, though many other feedstocks can be used including recycled grease and vegetable oil from restaurant deep fryers. In some cases, cooking oil can be burned directly in diesel engines, though it doesn’t flow well at cold temperatures, which is why it is generally converted into diesel first.
These fats can be converted into fuel through a process called transesterification. This is a long word to describe a chemical process in which glycerine is removed from vegetable oil (or grease) to produce biodiesel, which is otherwise known as methyl ester. The process consists of mixing the oil with methanol and sodium hydroxide and then removing the glycerine. It is really a fairly straightforward process which is why people are able to do it at home. Generally speaking, fairly pure feedstocks must be used to ensure a usable product, but the range of inputs can be expanded through the use of enzymes to allow for lower purity and lower cost inputs.
Other first generation biofuels include biogas, which is made by anaerobic digestion of vegetable matter and syngas, which is made from wood. Biogas generally takes the form of methane. It is being produced successfully in conjunction with both landfills and farming operations. Syngas is produced in a process called pyrolysis, in which wood or other biomass is heated in an oxygen-deprived environment. The byproduct of this process is called biochar, which is being hailed as a potential climate change solution, since, when produced as described above and added to the soil, it has a carbon negative impact, pulling net carbon out of the atmosphere and sequestering it in the ground, where it will remain for centuries.
Second generation biofuels utilize dense cellulosic inputs such as wheat straw, wood chips, corn stover, and municipal solid waste. Among them are Fischer-Tropsch diesel, bio-DME, DMF, biobutanol, biomethanol, biohydrogen, and wood diesel. Ethanol is another well-known second generation biofuel.
The Fischer-Tropsch process is a series of chemical reactions that convert carbon monoxide and hydrogen into liquid hydrocarbons. Feedstocks can come from coal, natural gas or biomass. It was developed in Germany in the 1920s, and was used by the Germans during WWII to fuel their military machinery. Oil companies are currently pursuing it as a way of converting natural gas into liquid fuel (GtL), but it also holds promise as a means to produce biodiesel. Biomass–to–liquids, or BtL, based on this same technology, might be seen as a green alternative to GtL.
Bio-DME refers to di-methylether, which is derived from methanol. It is currently used as an aerosol propellant, but it also has an application as a diesel substitute. It can be produced through the collection of pyrolysis gases from garbage or waste.
DMF, or Dymethylfuran, is another new biofuel that can be made from sugar which shows promise as a possible direct replacement for gasoline. Biomethanol and biobutanol are both types of alcohol that can be used like ethanol. Biomethanol can be synthesized from the glycerine by-product of biodiesel. Biobutanol, which is produced like ethanol, is considered by many to be another excellent candidate to directly replace gasoline. Biohydrogen uses microbes to produce hydrogen from sunlight using photosynthesis. Wood diesel can be made from forest products, the long time source of methanol,which also known as wood alcohol. This could either be made directly from certain plant gums, or from synthesis gas through the Fischer-Tropsch process.
Another high-tech biofuel input is algae. Algae-based biofuel is being hailed as a possible substitute for either gasoline or diesel. Still highly experimental, much of the production has moved indoors, away from shallow ponds and into bio-reactors. It is touted as having the potential to produce far more energy per unit area than any other biofuel. The airlines have shown great interest and a number of experimental flights have been successfully conducted using algae-based fuel blended with conventional jet fuel at ratios as high as forty percent. The National Renewable Energy Laboratory, after testing 3000 types, concluded that algae could potentially replace fossil fuels for both home heating and transportation purposes. Though there are still many unanswered questions, the big cause for excitement is the fact that the land required, at 100,000 gallons per acre per year, compares very favorably with the less than 500 gallons per acre of corn or other crops.
The carbon balance looks very good with all these biofuels, provided the amount of fertilizer required to grow them is not too great. Excessive use of fertilizers could tip the balance away from these fuels, so developing a methodology that provides for recycling of nutrients is vital to the ultimate success of any of these fuels. Opponents have also raised concerns about the use of water to grow biofuels. Some studies have shown that unless biofuel crops can be grown without irrigation, they will be more water-intensive than fossil fuels, which could be a problem in a water-constrained world. However, these constraints will need to be balanced with the environmental challenges of other fuel sources. In the future, when we talk about miles per gallon, we might need to specify if we’re talking about fuel or water.
[Image credit: Luc V. de Zeeuw: Flickr Creative Commons]
RP Siegel, PE, is an inventor, consultant and author. He co-wrote the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water in an exciting and entertaining format. Now available on Kindle.
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