The Weekly Reader

Link: http://www.eurekalert.org/pub_releases/2003-10/uou-bm9102603.php

October 28, 2003
98 Tons Of Buried Plant Material To Produce Each Gallon Of Gasoline

It took a lot of ancient plant matter to make fossil fuels.

Oct. 27, 2003 – A staggering 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.

Follow up:

"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 will be published in the November issue 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.

Keep in mind that the ratio between the amount of grown plant carbon matter to actual amount of carbon that gets deposited and eventually turned into fossil fuels is an extremely high number as stated below. Therefore if grown plant matter is used directly as an energy source the pounds of plant matter to make energy equivalent to a gallon of gasoline would be orders of magnitude less than the amount that had to be grown and deposited to eventually become oil.

"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 calculation, 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.

Very little of ancient plant material ever ended up getting incorporated into oil, gas, and coal deposits.

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.

If these calculations by Dukes are accurate then biomass from conventional plants can at best provide for a very small portion of our energy usage.

Unlike the inefficiency of converting ancient plants to oil, natural gas and coal, modern plant "biomass" can provide energy more efficiently, either by burning it or converting into fuels like ethanol. So Dukes analyzed how much modern plant matter it would take to replace society's current consumption of fossil fuels.

He began with a United Nations estimate that the total energy content of all coal, oil and natural gas used worldwide in 1997 equaled 315,271 million billion joules (a unit of energy). He divided that by the typical value of heat produced when wood is burned: 20,000 joules per gram of dry wood. The result is that fossil fuel consumption in 1997 equaled the energy in 15.8 trillion kilograms of wood. Dukes multiplied that by 45 percent – the proportion of carbon in plant material – to calculate that fossil fuel consumption in 1997 equaled the energy in 7.1 trillion kilograms of carbon in plant matter.

Studies have estimated that all land plants today contain 56.4 trillion kilograms of carbon, but only 56 percent of that is above ground and could be harvested. So excluding roots, land plants thus contain 56 percent times 56.4, or 31.6 trillion kilograms of carbon.

Dukes then divided the 1997 fossil fuel use equivalent of 7.1 trillion kilograms of carbon in plant matter by 31.6 trillion kilograms now available in plants. He found we would need to harvest 22 percent of all land plants just to equal the fossil fuel energy used in 1997 – about a 50 percent increase over the amount of plants now removed or paved over each year.

"Relying totally on biomass for our power – using crop residues and quick-growing forests as fuel sources – would force us to dedicate a huge part of the landscape to growing these fuels," Dukes says. "It would have major environmental consequences. We would have to choose between our rain forests and our vehicles and appliances. Biomass burning can be part of the solution if we use agricultural wastes, but other technologies have to be a major part of the solution as well – things like wind and solar power."

World energy usage is rising as the total world economy grows. So the amount of biomass needed to serve as a replacement would have to grow to an even higher level.