Welcome to European Tribune. It's gone a bit quiet around here these days, but it's still going.
OK I got curious about the energy math of those ethanol plants burning coal in the US.  As usual the hardest part of this kind of thing is pulling together the numbers from umpteen different sources.  Watch me closely here as it is late and I'm sleepy and may drop a decimal point if I'm careless... so look over my shoulder, all you lovers of big numbers :-)

Energy Density Reference Table

tce -- ton of coal equivalent (1000 kg):  MJ
                29,308  International Energy Agency
                29,310  Center for Energy Efficiency
                29,290  Carbon Dioxide Information Analysis Center

If we look up densities of some common substances, we can find that ethyl alcohol, or ethanol, has a density of 790 kg/m3.

So the energy in a m3 of ethanol is 790 times density 29.7 MJ/kg or 23,463 MJ

and the energy in a ton of coal equivalent is about 29,300 MJ per the table above.  not so different are they :-)

So if we burn 300 tons of coal per day to make ethanol it would be nice if we ended up with as much energy stored in energy as coal energy expended, at the end of the day.  It would be much, much nicer if we ended up with at least 2 times as much.  In fact... well, what do we really end up with?

MJ potential of coal burned in a day:   300 * 29300 =  8,790,000 MJ (300 ton of coal equivalents)

which would be 8,790,000/23,463.0 or 374.63 cubic meters of ethanol or (for those of us who still think in nonmetric metrics:

        1 m3 is 1000 litre
        1 gal is 3.785 litre

374.63/3.785 * 1000 = 98,977.54 gallons per diem)

The article I quoted says airily, in passing, "With 97 ethanol refineries pumping out some 4 billion gallons of ethanol [per annum]..."

So we can guesstimate that one refinery on average pumps out 1/100th of 4 billion or

        4e9 / 1e2 = 4e7 gallons per annum,

so therefore each day same plant pumps out 1/365th of this or

        109,589.04  gal per diem

in other words, very close to the break even point for the coal burned, and this is before we even considered the lossiness of the corn production, transport costs, etc.

We are "making" energy here by spending a BTU to create a BTU, even without considering the energy spent to create the feedstock (the corn) or the opportunity cost of using that acreage for corn rather than for some other cultivar, such as a staple food or fibre crop.

One more thing I want to point out before I shut up for a while, and that is that to my ear, this talk of "burning the vegetable waste to fuel the process" is still robbing Peter to pay Paul.  

Why?  Well, in a sustainable agrarian system, waste plant matter is composted and returned to the soil (or vice versa depending on your methods) -- thus recycling nutrients, increasing water absorption rates and retentivity, etc.  If we pull that matter out of the cycle and burn it, we have no seasonal nutrient flow to return to the soil... and we then have to import [the word "import" should by now appear in Blinking Red Boldface whenever you read it, because "import" means "transport" and "transporting" tons of material means Fossil Fuel Expenditure Galore] soil amendments to compensate for having "thrown away" our local soil-amending nutrients and fibre by burning them.

Or, looked at another way:  You can use plant matter to feed humans;  you may be able to feed animals or soil or both with the leftovers, and if you compost your animal (including human) waste and use it for soil amendment (replenishment is more like it), you have closed the loop.  Or you can use plant matter exclusively to feed animals which indirectly feed, clothe, or carry humans, and return the leftovers and the animal waste as before.  You can use plant matter just to feed soil (cover cropping, nitrogen fixing) which then grows more plant matter to feed humans or animals.  And you can burn plant matter to produce heat.  It is true that "what you don't eat you can burn," but it is also true that what you burn is not available to eat -- not for animals or humans or soil organisms, not for birds or beneficial insects or fungi...  so if you burn all the stalks and shucks and "byproducts" that would otherwise be mulch or compost, then you have to make up that deficit from somewhere else;  and where are we making it up from?  Why, by refined fossil products (artificial fertilisers) or by transporting natural fertilisers vast distances (guano, feedlot manure, etc).

Repeat after me:  There Ain't No Such Thing As A Free Lunch.

The nutrient cycle of living systems, be they prairie or forest or rainforest or any other variant, is complex, intricate, and exquisitely balanced.  Stuff goes round and round and entropy is very slow (leaching of nutrient value out of the system -- absolute loss -- is very slight, so such systems can run for millennia absent an epochal event like meteorite strike or volcanic eruption);  they can even build up local surpluses, resulting in increasing complexity, diversity, and depth of the system.  Once we start subtracting (stealing) nutrients from this cycle beyond a certain sustainable level it is like taxing the peasantry into immiseration and mass suicide;  the virtuous cycle of nutrients breaks down and more and more outlandish improvisations have to be made to replace what has been stolen/lost;  and the fabric of the nutrient cycle gets simpler, thinner, poorer, and more precarious.  Or to return to an earlier theme, lower- and lower-grade substitutions must be made as the high-value nutrients are stripmined and not replaced.

Much of what we (industrial civilisation) do seems perversely dedicated to stealing from the nutrient cycle and refusing to replace:  for example, our cultural obsession with diverting our own bodily wastes away from the soil we have depleted, which would benefit from this percentage return of what was removed, and dumping them into another system (waterways and oceans) which has no need for them and in fact suffers damage from this "overnutrition" (not to mention the toxic chemicals we "enlightened" folks use to make this waste "safe").  We even embalm our dead in a stew of toxic chemicals to ensure that the substance of our bodies is withheld from the nutrient cycle after death.  It's a strange development for any species to try to separate itself fastidiously from the natural economy.  [Strange, and in my gloomier moments I suspect ultimately suicidal.]

The difference between theory and practise in practise ...

by DeAnander (de_at_daclarke_dot_org) on Fri Mar 31st, 2006 at 01:37:50 AM EST

Others have rated this comment as follows:


Occasional Series