ALCOHOLS / ETHANOL
Question #1: Can ethanol from corn or other grain replace gasoline?
Answer: Certainly not, for several reasons.
There isn’t enough grain. The best process we have makes about 2.66 gallons of ethanol from a bushel of corn (maize). The 2004 maize harvest was about 11.8 billion bushels; if all of it was used for ethanol, it could make a maximum of 31.4 billion gallons of ethanol (with energy equivalent to about 22 billion gallons of gasoline). US gasoline consumption in 2003 was roughly 134 billion gallons, or more than 6 times the amount which can be replaced by ethanol production from corn. Total US motor fuel consumption (gasoline and diesel fuel) is approximately 200 billion gallons per year.
Ethanol requires too much other fuel to produce it. A gallon of ethanol (84,200 BTU) consumes about 33,000 BTU of heat in the distillation process alone. Some of this heat comes from coal or cogenerators, but most distillers burn natural gas or LPG. LPG is a petroleum byproduct, and natural gas supplies are tight and getting tighter. Ethanol producers are competing with people who need to heat their homes. The energy losses of the ethanol process make it more efficient to burn the grain for heat, and use the LPG or natural gas as motor fuel (source).
Question #2: Someone sent me an e-mail about bio-butanol as a replacement for gasoline. Could we get rid of oil this way?
Environmental Energy, Inc (www.butanol.com) claims a process which yields 2.5 gallons of butanol per bushel of corn (maize). They further claim 105,000 BTU/gallon of butanol vs. 84,200 BTU/gallon of ethanol (~25% more energy) which makes it a superior fuel. This is true so far as it goes, but this also runs into limits of raw materials; 11.8 billion bushels of corn would make 29.5 billion gallons of butanol. This would displace less than 1/5 of US gasoline consumption, with nothing extra to replace diesel fuel. The major advantage of butanol over ethanol is that it would require far less energy to separate it from water; it would be worthwhile to promote butanol rather than ethanol for energy-security reasons.
Source: Environmental Energy, Inc (www.butanol.com)
Question #3: Could ethanol from crop wastes replace gasoline?
Answer: Probably not; there almost certainly isn’t enough biomass available.
The surplus biomass of corn stalks and such (corn stover) is the largest single biomass source in the US; it yields about 2.5 tons/acre (source) at the average yield of 146 bu/ac. The surplus biomass over the entire 80.7 million acres planted to corn is roughly 200 million dry tons per year. Even if the entire dry mass was converted to ethanol with the same efficiency as grain (2.66 gallons per 56-lb bushel, or 31.3% by weight), it would only produce 62.6 million tons (19.0 billion gallons) of ethanol, equivalent to about 13.3 billion gallons of gasoline. In practice only 30% to 60% of this biomass could be made available for fuel production, and the energy requirements for distillation come on top of this.
Source (corn stover yield)
Source (2004 corn harvest)
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