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A Student's Guide to Alternative Fuel Vehicles


Gasoline

How much plant material does it take to make a gallon of gasoline? Well, according to an expert - it takes 98 tons of prehistoric, buried plant material - that's 196,000 pounds - is required to produce each gallon of gasoline we burn in our cars, SUVs, trucks and other vehicles, according to a study conducted at the University of Utah.

"Can you imagine loading 40 acres worth of wheat - stalks, roots and all - into the tank of your car or SUV every 20 miles?" asks ecologist Jeff Dukes, whose study was published in the November 2003 of the journal Climatic Change.

But that's how much ancient plant matter had to be buried millions of years ago and converted by pressure, heat and time into oil to produce one gallon of gas, Dukes concluded.

Dukes also calculated that the amount of fossil fuel burned in a single year - 1997 was used in the study - totals 97 million billion pounds of carbon, which is equivalent to more than 400 times "all the plant matter that grows in the world in a year," including vast amounts of microscopic plant life in the oceans.

"Every day, people are using the fossil fuel equivalent of all the plant matter that grows on land and in the oceans over the course of a whole year," he adds.

In another calcultation, Dukes determined that "the amount of plants that went into the fossil fuels we burned since the Industrial Revolution began [in 1751] is equal to all the plants grown on Earth over 13,300 years."

Explaining why he conducted the study, Dukes wrote: "Fossil fuel consumption is widely recognized as unsustainable. However, there has been no attempt to calculate the amount of energy that was required to generate fossil fuels, (one way to quantify the 'unsustainability' of societal energy use)."

The study is titled "Burning Buried Sunshine: Human Consumption of Ancient Solar Energy." In it, Dukes conducted numerous calculations to determine how much plant matter buried millions of years ago was required to produce the oil, natural gas and coal consumed by modern society, which obtains 83 percent of its energy needs from fossil fuels.

"Fossil fuels developed from ancient deposits of organic material, and thus can be thought of as a vast store of solar energy" that was converted into plant matter by photosynthesis, he explains. "Using published biological, geochemical and industrial data, I estimated the amount of photosynthetically fixed and stored [by ancient plants] carbon that was required to form the coal, oil and gas that we are burning today."

Dukes conducted the study while working as a postdoctoral fellow in biology at the University of Utah. He now works for the Carnegie Institution of Washington's Department of Global Ecology on the campus of Stanford University in California.


How the calculations were done

To determine how much ancient plant matter it took to eventually produce modern fossil fuels, Dukes calculated how much of the carbon in the original vegetation was lost during each stage of the multiple-step processes that create oil, gas and coal.

He looked at the proportion of fossil fuel reserves derived from different ancient environments: coal that formed when ancient plants rotted in peat swamps; oil from tiny floating plants called phytoplankton that were deposited on ancient seafloors, river deltas and lakebeds; and natural gas from those and other prehistoric environments. Then he examined the efficiency at which prehistoric plants were converted by heat, pressure and time into peat or other carbon-rich sediments.

Next, Dukes analyzed the efficiency with which carbon-rich sediments were converted to coal, oil and natural gas. Then he studied the efficiency of extracting such deposits. During each of the above steps, he based his calculations on previously published studies.

The calculations showed that roughly one-eleventh of the carbon in the plants deposited in peat bogs ends up as coal, and that only one-10,750th of the carbon in plants deposited on ancient seafloors, deltas and lakebeds ends up as oil and natural gas.

Dukes then used these "recovery factors" to estimate how much ancient plant matter was needed to produce a given amount of fossil fuel. Dukes considers his calculations good estimates based on available data, but says that because fossil fuels were formed under a wide range of environmental conditions, each estimate is subject to a wide range of uncertainty.


Plants in your tank?

Dukes calculated ancient plant matter needed for a gallon of gasoline in metric units:

  • One gallon of oil weighs 3.26 kilograms. A gallon of oil produces up to 0.67 gallons of gasoline. So 3.26 kilograms for a gallon of oil divided by 0.67 gallons means that at least 4.87 kilograms of oil are needed to make a gallon of gasoline.

  • Oil is 85 percent carbon, so 0.85 times 4.87 kilograms equals 4.14 kilograms of carbon in the oil used to make a gallon of gasoline.

  • Since only about one-10,750th of the original carbon in ancient plant material actually ends up as oil, multiply 4.14 kilograms by 10,750 to get roughly 44,500 kilograms of carbon in ancient plant matter to make a gallon of gas.

  • About half of plant matter is carbon, so double the 44,500 kilograms to get 89,000 kilograms - or 89 metric tons - of ancient plant matter to make a gallon of gas. In U.S. units, that is equal to a bit more than 196,000 pounds or 98 tons.

Dukes made similar calculations for oil, natural gas and coal to determine that it took 44 million billion kilograms (97 million billion pounds) of carbon in ancient plant matter to produce all the fossil fuel used in 1997. That includes 29 million billion kilograms of prehistoric plants to produce a year's worth of oil (including gasoline), almost 15 million billion kilograms of buried plant matter to make all the natural gas used in 1997, and 27,000 billion kilograms of dead plants to produce all the coal used in the same year.

"It took an incredible amount of plant matter to generate the fossil fuels we are using today," says Dukes. "The new contribution of this research is to enable us to picture just how inefficient and unsustainable fossil fuels are - inefficient in terms of the conversion of the original solar energy to fossil fuels. Fortunately, it is much more efficient to use modern energy sources like wind and solar. As the reasons keep piling up to switch away from fossil fuels, it is important that we develop these modern power sources as quickly as possible."

Reprinted from the:

University of Utah Public Relations
201 S Presidents Circle, Room 308
Salt Lake City, Utah 84112-9017
(801) 581-6773 fax: 585-3350
www.utah.edu/unews

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