Ethanol Fuel – Is it worth it???

Now that we have recipe for starch based ethanol we should ask the age old engineering question, “We can do it, but is it worth it?”  If it takes more fuel to make the ethanol than the heating value it produces we should think about using our limited fuel in a better manner.  Here is my attempt to answer the question.

There are three main energies we should weigh against each other.  First there is the energy we used to hydrolyze the starch.  Secondly, there is the energy we used in distillation.  Lastly there is the energy we get back from burning the product.  Let’s take a look at each one:

Hydrolysis:

The energy used in hydrolysis is the energy to heat the water from 50F, which is my ground water temperature, to 212F plus the energy to heat the corn meal from room temperature to 212F.:

Energy used = (mass of water)*(specific heat of water)*(temperature rise)+(mass of corn meal)*(specific heat of corn)*(temperature rise)

Energy used = (volume)*(density)*(specific heat)*(temperature rise))+(mass of corn meal)*(specific heat of corn)*(temperature rise)

Energy used = (4 liters)*(1kg/liter)*(4.184kJ/(kgK))*(373K-283k)+(.44kg)*(3.62kJ/(kg*K))*(373K-300K)

Energy used = 1506 kJ + 116 kJ = 1.622MJ

Rough Distillation :

Energy = energy to heat the mash + Energy to vaporize the alcohol + the balance to evaporate 20% of the initial mash volume.

Energy = (volume)*(density)*(specific heat)*(temperature rise))+(mass of corn meal)*(specific heat of corn)*(temperature rise)+(ethanol mass) *(heat of evaporation) + (water mass)*(heat of evaporation)

Energy =(4 liters)*(1kg/liter)*(4.184kJ/(kgK))*(373K-283k)+(.44kg)*(3.62kJ/(kg*K))*(373K-300K) + (4 liters)*(.05)*(.789 kg/l)*(846 kJ/kg)+(.40 liters)*(1 kg/liter)*2260 kJ/kg)

Energy = 1622 kJ + 1037.5 kJ = 2.660 MJ

Fine Distillation  (ignoring the energy to bring the products up to temperature) :

Energy = (ethanol mass) *(heat of evaporation)

Energy = (Mash volume)*(alcohol concentration)*(alcohol density)*(heat of evaporation)+

Energy = (4 liters)*(.05)*(.789 kg/l)*(846 kJ/kg)

Energy = 133.5 kJ = .1335 MJ

Energy produced from burning the ethanol:

Energy = mass * low heat of combustion(no condensing the exhaust steam)

Energy = (volume)*(density)*(heat of combustion)

Energy = (.20 liters)*(.789 kg/liter)*(28.865 MJ/kg)

Energy = 4.555 MJ

Net Energy:

Energy = Energy from combustion – Hydrolysis energy – Rough Distillation – Fine Distillation

Energy = 4.555 MJ – 1.622MJ – 2.660 MJ –  .1335 MJ

Energy = .14 MJ

Conclusions:

Starch based ethanol is not a fuel source at the small producer level.  Making ethanol you will barely break even with the energy you have to put in even if all your processes are perfect and there is no heat loss.  This can’t happen.

Commercially if we had a continuous process it would be a different story.  Through the liberal use of heat exchangers we could eliminate most of the energy requirement for hydrolysis and most of the alcohol’s evaporation energy could be recycled as boiler combustion air preheat.  The vast majority of the plant’s fuel usage would be used to make up process losses and could be somewhat efficient.

At the homestead/farm level ethanol should be used where no less processed fuel will do.  The big example that comes to mind is a chainsaw.  Wood gas can not be used because lubrication needs to come from the fuel.  It would also be very exhausting to carry a saw, gasifer, fuel, condenser, and filter around the woods!