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Discussion of ethanol production assumptions

So, lets look into the assumptions, as analyzed by MathPro, a small, independent consulting firm specializing in energy economics with clients in industry, government and law firms:

The assumptions:  P&P researchers assume modern processing plants produce nitrogen fertilizer but otherwise assume average production technology produces corn.  The USDA assumes high-yield corn figures whereas P&P assume average-yield.  Both the USDA and P&P assume:

  • modern processing plants convert corn to ethanol
  • modern processing plants produce nitrogen fertilizer
  • the government allocates energy credits to ethanol co products.  What this means is the government subsidizes ethanol production for the energy contained in replaced animal feed because farmers substitute left-over ethanol-production food (namely, distiller's dry grains and solubles) for other animal feed.

However, the USDA uses a 40% credit; whereas, P&P uses only 9% to "reflect the actual nutrients in the animal feed," says Pimentel.  A 40% credit makes ethanol energy costs look better than a 9% credit.

Moreover, the 2002 USDA and the P&P estimates in the table below used energy-credit allocation, which although complicated, is well defined and in common use.  The 2004 USDA estimate did not, using instead, according to Dave Hirshfeld of MathPro, an "unexplained" calculation having nothing to do with feed substitution, but producing a "more optimistic result than any other such estimate ever produced."

The P&P estimate includes, as energy inputs to corn production, the farmer's labor, farm machinery and irrigation.  In addition, they include, as energy inputs to ethanol production, processing water and equipment.  This is a similar approach agricultural economists use in crop-production assessments.  The USDA estimates do not include any of these line items, which provides "a positive energy return desired by the pro-ethanol lobby, but it is not accurate," says Pimentel.

Here's the entire table, with all the input energy line items, and the resulting totals.

Side-by-side Comparison of (Adjusted) Published Estimates of Corn Ethanol's Energy Efficiency
from MathPro
 
Energy (BTU/gal ethanol) USDA
 2002
USDA
2004
Pimentel-
Patzek
Energy Use by Category
Corn Production 19,525 18,713 37,890
  Seed 83 227 2,428
  Fertilizer 7,366 10,177 15,333
  Energy 9,367 6,474 6,733
  Chemicals 1,286 1,106 4,202
  Custom work/labor 1,190 594 2,157
  Input hauling 234 76 789
  Farm machinery

4,753
  Irrigation

51 1,494
Corn Transport 2,121 2,120 4,837
Ethanol Production 46,662 49,733 56,441
  Thermal energy 42,546

38,383
  Electricity 4,303

15,188
  Process water

2,389
  Equipment

481
Co-product energy credit -12,995 -26,251 -6,685
Ethanol Distribution 1,495 1,487

Totals

     
Energy input to ethanol production 57,005 45,802 92,483
Ethanol energy content 76,330 76,330 76,330
Ratio of (energy input to ethanol production)  to  (ethanol energy content) 0.75 0.60 1.21
       Please note:  Pimentel and Patzek published a ratio of 1.29 instead of 1.21.  Likewise, the USDA in 2004 published a 0.75 ratio instead of the table's of 0.60.  The table reflects adjustments in energy units (BTU/gallon of ethanol) and energy content (expressed as lower heating values) to enable comparisons on a consistent basis.

(Answered Aug. 1, 2006)

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