From: Steven M Smith (smsmith@usgs.gov)
Date: Thu Nov 13 2003 - 12:48:38 EST
After skimming several posts on this ASA thread I ran across the following
web article from SpaceDaily.com which addresses several of the issues under
discussion. I'll just include a few 'teasers' here. You can read this
popular account of the study at
<http://www.spacedaily.com/news/energy-tech-03zp.html>
<<Ninety Eight Tons Of Primordial Plant Matter Per Gallon
Salt Lake City - 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.>>
[Snip]
<<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.>>
[Snip]
<<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.>>
[Snip]
<<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....>>
Nothing was said in the SpaceDaily.com article about the energy costs
needed to grow and harvest 22% of all land plants. The article concludes:
<<"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.">>
_____________
Steven M. Smith, Geologist, U.S. Geological Survey
Box 25046, M.S. 973, DFC, Denver, CO 80225
Office: (303)236-1192, Fax: (303)236-3200
Email: smsmith@usgs.gov
-USGS Nat'l Geochem. Database NURE HSSR Web Site-
http://pubs.usgs.gov/of/1997/ofr-97-0492/
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