Developing Farm Power makes us spend a lot of time with numbers, both in our projections and in the performance of other systems. When I compare anaerobic digesters to other types of land-dependent renewable energy, I continue to find that our numbers look far more sustainable than the rest. The food-versus-fuel debate has been driven mainly by critics of corn ethanol, but emerging ag energy sources could actually end up being worse. In comparison, manure digesters create energy while actually strengthening food production--I'll show that in numbers below.
Corn Ethanol=500 gallons per acre (net 35 MMbtus of final energy)
I have little love for ethanol, but in fairness I must bring up two points. First, ethanol production leaves behind at least a quarter of the nutrient value of corn. One acre of Midwestern corn yields about 150 bushels, which modern plants can convert to ethanol at rate of 2.75 gallons per bushel. We now have 400 gallons of low-density motor fuel, but we also have over a ton of high-density animal feed, replacing a quarter-acre of forage that would otherwise be planted. So ethanol gets a 25% bonus for leaving some food value.
Second, many ethanol plants are owned by the communities that supply them with corn. The same interest in self-reliance that sparked the formation of ethanol co-ops now drives efforts to replace natural gas with biomass for creating process heat. For that reason, I don't subtract process energy from the net final energy; I do, however, subtract a couple million btus per acre for fertilizer production, as corn is so nitrogen-hungry that even the best crop rotations still require extra N.
Energy Crops to Digesters=7.5MWh per acre (65 MMbtus intermediate, net 25 MMbtus of final energy)
This is quite common in Europe; taking the animal out of the loop allow digesters to get the full value of corn silage rather than just the leftovers. The digested liquid fertilizes the next season's crops, making a neat closed loop; unfortunately, food production is completely displaced.
Schmack Bioenergy's Pliening biomethane plant in Germany cleans up biogas and injects it into the natural gas grid for use at distant power plants. The plant consumes about 40,000 tons of silage per year, requiring at least 2,000 acres of corn and other forage crops. Germany pays such high rates for renewable energy that this sort of scheme makes sense even though the linear process yields no other benefits. For certain countries, Food versus Liquified Natural Gas from the Middle East to Keep Warm in Winter might be a debate that food occasionally loses.
Cellulosic Ethanol/Biomass Gasification=?
Range Fuels may be closer to finding the energy holy grail George Bush introduced to the country two years ago, but until the technology actually becomes commercialized we won't know much about how efficient it is. Everyone seems to agree that the final energy yield of a ton of biomass will be considerably lower than a ton of corn, so the process had better be much more efficient. No one has developed such a process to date.
Crops to Dairy Cows, Manure to Anaerobic Digesters=1.5MWh per acre plus six tons of milk (5MMbtus of final energy, to be enjoyed with plenty of cheese and butter)
The main difference between feeding corn to ethanol plants and feeding it to dairy cows supplying a manure digester is that the byproduct of the latter process is edible to most humans. To this point in history, ruminant animals have been our most efficient way of turning cellulosic biomass (or grass) into usable energy (or fats, proteins, and carbohydrates). Dairy production falls short of soybeans in raw protein yield per acre, but having animals closes the fertilizer loop and--after adding anaerobic digesters--produces energy as well. Here's the basic calculation: the 5-7 dry tons of forage from one acre, when fed to a comfortable cow, turns into milk and at least twenty wet tons of manure. Wet manure, when fed to an anaerobic digester, yields a half-million btus of biogas per ton, which is combusted for green power.
The number of acres of corn planted for ethanol and the number of acres planted to feed dairy cows will be roughly the same in 2008 (almost 20 million). Wouldn't you rather have a cold glass of milk with your renewable electricity rather than some moonshine with your commute?