The Ergosphere
Monday, September 05, 2005

Ethanol Mirage II

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.

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There is a parallel path being blazed in the development of ethanol fermentation processes. This path does indeed involve "burning corn" but also involves burning corn stover, other agricultural crops, agricultural wastes, forestry wastes, and urban wastes (including landfills and sewage). In fact, feedstock can include anything containing C-H-O (even fossil fuels). I invite you to visit

How you burn is important from an environmental standpoint. Combustion is out of the question for obvious reasons. Gasification and pyrolysis are two methods that can reduce the feedstock to its components while generating enough heat to co-generate electricity. The resulting syngas can be converted into ethanol using bacteria as the converting agent. The process is amazingly efficient (7 minutes vs. 36-48 hours for sugar fermentation) and results in net energy gain in electricity and liquid fuel while reducing the societal pariahs of landfills, waste, and fossil fuel dependency.

Ethanol is the lingua franca of liquid renewable energy. It can blended with gasoline or replace gasoline in already available Flexible Fuel Vehicles of many available models. Distribution nfrastructure (thanks to sugar-fermentation ethanol development) already exists in many parts of the U.S. It is environmentally cleaner than gasoline. It is the transport medium for hydrogen should hydrogen fuel cells ever development into a renewable fuel alternative.

The future of ethanol is no mirage.
But that's the future; in the present, use of ethanol as vehicle fuel means we have less vehicle fuel AND less heating fuel than we would otherwise.

Subsidizing a system which yields less benefit than a rather simple alternative is idiotic, and the impending crisis of heating for this winter makes it imperative that all gas- and LPG-fired distilleries be shut down immediately.  The difference is 4 million homes receiving heat this winter, or not.

I'll be the last to object to ethanol being produced from crop wastes or MSW (unless there are better things to do with them and ethanol subsidies prevent it from happening), but ethanol from grain is more of a problem than a solution.
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Please watch your duplicates.
Indeed, and do a combined heat and power scheme like this

and it gets even more efficient.

The trouble is investment costs, to burn the corn you need to install stoves capable of burning it efficiently, and to use the nat gas for vehicles, you need to spend extra there as well.
Probably better to take the congressional subsidies, turn them into cash and burn the money.
The Energy Blog posted a link to a set of ethanol studies:

One very interesting quote:

We are looking at a very interesting integrated distillery approach being developed by the Brazilians, where instead of going for the large 300,000 litres per day plants, a fully integrated approach is taken with a 1,500 ha area, farmed by small growers, and feeding sugarcane and sweet sorghum into a 20,000 litres per day plant, with cattle feedlots at the distillery, the manure going into [biogas] digesters with the stillage, producing enough energy for the distillery, leaving the bulk of the bagasse to be used for power generation to supply the surrounding areas." (Energy projects in Africa.)

I have to agree totally that the idea of a single family operator being able to take advantage of all the energy pathways is remote. There are too many processes, and the whole situation is too complex. An integrated, managed situation is necessary to make biofuels (more) energy positive.
Again, the question should be "Would alternate paths yield more useful output?"

I'm not quite clear on the pathways in that example you quoted; it sounds like the "stillage" is going for biogas, which leaves what to feed the livestock?  Further, straight cellulose with a bit of nitrogen suffices for cattle feed; I read of an experiment which fed cows on pelletized newsprint with a touch of urea, and that was done in the sixties.  Bagasse, corn stover or any variety of straw could probably be turned into cattle feed by mechanical preparation to reduce the fiber size.  Then all the manure goes for biogas production, and all the biogas gets cleaned up for CNG motor fuel.

That might yield too much meat and not enough energy, but you could instead convert bagasse and whatnot to electricity rather than fermenting and distilling it, and using the electricity to run vehicles.  If the conversion efficiency from carbohydrates to ethanol is 75% (energy), you use 40% of the ethanol's energy as gas in distillation and the vehicles are 17% efficient, the overall efficiency of the biomass to wheels pathway is 0.17/(1.33+0.40) = 9.8%.  You can do at least twice as good as 9.8% by burning the biomass in a steam-cycle plant to make electricity, and many of the alternatives allow for cogeneration of other useful products.  And that's not even considering what you could do if you pyrolized the biomass to get carbon for a thermochemical zinc process and burned the pyrolysis off-gas for electricity....

Ethanol is a dead end.  It's immediately useful in current vehicles, but concentration on it to the exclusion of more efficient and more useful pathways is crippling.
I don't disagree -- methane is a more useful hydrocarbon than ethanol. Moreover the impact of biomass is small in the short run so it may as well be used efficiently, i.e. for heat.
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