Continuing previous thoughts, I decided that it would be a good thing if someone analyzed the merits of converting corn to ethanol vs. burning it directly for heat.
The heating value of shelled corn has several different values published on-line; my first two results were 314,000 BTU/bushel
and 381,000 BTU/bushel
. (Unfortunately, the graph presented in the latter is not easily examined to determine if the two calculations are actually using very similar figures and the latter is merely a character-swapped typo.) Assuming the lower figure is relatively safe (favors the status quo), so here goes.
Converting corn to ethanol at a rate of 2.66 gallons per bushel and using 33,000 BTU/gal of gas for distillation yields 2.66 gallons (224,000 BTU) of ethanol, at a cost of 87780 BTU of natural gas.
Burning shelled corn (314,000 BTU/bu) at an efficiency of 75% yields 235500 BTU of heat at zero cost in natural gas. The natural gas freed up (87,780 BTU not used in distillation + 235500 BTU not used for heat) totals 323,280 BTU/bushel, or 32% more than the heating value of the ethanol the corn would otherwise produce. The first 235500 BTU of natural gas could be used to power NGV's, and the rest would be surplus over the ethanol scenario. (This comparison would be far more lopsided in favor of burning corn if the 381,000 BTU/bushel figure was used.)
Conclusion: Not considering other value-added products, it is energetically more efficient to burn shelled corn for heating fuel and use natural gas for motor fuel than it is to use the corn and gas to make ethanol for motor fuel.
UPDATE 2005-Sep-08: temposter offers the figure of 392,000 BTU/bushel (citing an Ontario source
) in the comments. Given that maize is a natural product and the fuel value is likely to vary based on oil content (which in turn depends on the exact strain and growing conditions), the value of 381,000 BTU/bu seems realistic. At the 381,000 BTU value, each bushel burned for heat would produce 285,750 BTU of useful heat and displace 373,530 BTU of natural gas or LPG. The ethanol which could have been produced from the maize would have produced 220,400 BTU at most
, the net benefit from burning the corn as heating fuel is at least 153,000 BTU/bushel.
Labels: ethanol, ethanol mirage