May 23, 2010
30 Year CO2 Payback For Miscanthus Energy Source
The carbon dioxide released to create farm fields to grow a perennial grass as a biomass energy source won't get paid back in avoided CO2 emissions from fossil fuels until the field has been used to grow miscanthus as an energy source for 30 years. That seems like a long time.
Champaign, Il – May 3, 2010 - An article in the current issue of Global Change Biology Bioenergy reveals that Miscanthus x giganteus, a perennial grass, could effectively reduce our dependence on fossil fuels, while lowering atmospheric CO2.
Using a simulation tool that models the future global climate, researchers predict that the carbon that is released into the atmosphere from the loss of natural vegetation will be paid back by Miscanthus within 30 years. Previous estimates for other liquid biofuels, such as corn ethanol, were estimated to take 167-420 years to pay back their carbon debt.
Miscanthus is better than corn ethanol only because corn ethanol is such a bad idea in the first place. Corn ethanol economic viability was made possible by government subsidies put in place at the behest of the farm and agricultural industry lobbies.
Miscanthus isn't economically viable yet and it is not clear when it might become economically viable as an energy source. But if it takes off then the result will be a spike in CO2 emissions as more land gets cleared for its growth.
But growing Miscanthus instead of corn wouldn't do that.
It's heresy to say that in Iowa. You can't spit without hitting an ethanol plant.
> Using a simulation tool that models the future global climate
Having been burned so often lately by outrageous claims obtained via computer modeling, I have to wonder why we should trust any of them without persuasive evidence of their reliability. How has this model been validated?
All climate models are "validated" by tuning them until they match the past.
None of them have ever successfully predicted future climate.
The 30 years is an interesting number- especially when compared to corn. What type of ground cover does the model assume? Grass versus trees? However it is only useful if you compare it to the break even point versus burning oil. The "less carbon than oil" break even point is probably years ahead of the "carbon just from this plot of land" point.
I wonder if the fossil fuels used to harvest and process the grass is included in the calculation. That would increase the payback time.
Also what is the price point that this is economically viable without subsidy?
I don't understand how growing anything as a fuel can reduce CO2 emissions. It seems to me that the cycle is perfectly balanced: Solar energy plus CO2 produce O2 and stored energy in the plant. When that energy is used by the plant via respiration, by an animal that eats the plant, or when the plant is burned, the exact same amount of O2 gets turned back to CO2, precisely reversing the process.
How could it be otherwise?
Here's the idea:
1) The ground is cleared for miscanthus farming and the existing vegetation breaks down and releases some large chunk CO2.
2) The miscanthus bushes are raised up, processed into ethanol, and sold. This supposedly displaces fossil fuels from being burned as the ethanol supposedly gets used in place of gasoline from oil.
3) But since it takes some fossil fuels to operate the farm tractors, haul the miscanthus to a processing plant, run the processig planet, etc the amount of ethanol produced only partially displaces fossil fuels.
4) The ethanol gets burned, thereby releasing CO2 back into the atmosphere. But it gets sucked up into miscanthus again in the next growing season. Now the same carbon keeps cycling between atmosphere and plants rather than adding more carbon from burning fossil fuels.
I see the reasoning that leads to a 30 year payback as excessively optimistic for a few reasons:
- We can't even cost effectively convert miscanthus into ethanol now. If that's the case the chances are it costs more energy to do the conversion than you get out. Maybe that'll improve with time. But how much of an improvement?
- The displacement of fossil fuels is probably a mirage. If we burn less oil in the US the price of oil will be lower in the world market which will just increase demand elsewhere. Much of the demand reduction will just get canceled out by demand increase elsewhere.
- There's a limited amount of oil left in the ground which will cause oil extraction to slow even without getting miscanthus involved. At best miscanthus will delay the extraction of some of the oil. But I doubt the delay will be great.
"Now the same carbon keeps cycling between atmosphere and plants rather than adding more carbon from burning fossil fuels."
OK, they're saying "reduces" in the sense of "doesn't increase (as much)". There will continue to be the same amount of CO2.
Here's a thought: Why don't we grow some plants, then bury them to sequester lots of CO2? Oh, that's right, I'm not ALLOWED to put yard waste in my trash, because they don't want that in the landfill.
The only way I can see this resulting in lower CO2 over all is after buring most of the Carbon will be release as CO2 back in the atmosphere. There will be some carbon leftover in the Ash however. Sequestering this ash should result in a slight net drop in CO2 in the air as we continue this cycle over and over again.