Apr 21 2014
I have yet to be convinced that biofuels will be a significant benefit in our attempts to achieve sustainable energy production. Ideally we would run our civilization on energy that does not burn a limited resource or contribute CO2 or other compounds into the atmosphere. Any limited resource will eventually run out, by definition. Further, no matter what you think about the current effects of climate change, it’s hard to deny that if we continue to pump CO2 into the atmosphere this is likely to be a problem.
Biofuels sound like a good idea at first. Plants get their energy from the sun and fix CO2 from the atmosphere. Because we can grow plants, this is a renewable resource. When we burn fuel made from plants we are releasing the previously captured CO2 back into the atmosphere, and so the process is carbon neutral. Sound pretty good.
However, experts argue that we have to consider in this equation every aspect of the production of biofuels. The equation will change depending on the source of the fuel, and it is possible that if we use the right plant source and have a sufficiently efficient process, then we might have a biofuel with a net benefit. I think we will get there, but even still how much of a benefit is an important question.
In the US, corn stover is a major source of ethanol for biofuel. Corn stover is the parts of the plant left over after the corn itself is harvested – the stalks, leaves, and cobs. A new computer simulation analyses the net carbon effects of either leaving the corn stover in the field vs harvesting the stover, making it into biofuel, and then burning that fuel. Here is a summary of the results:
The researchers, led by assistant professor Adam Liska, used a supercomputer model at UNL’s Holland Computing Center to estimate the effect of residue removal on 128 million acres across 12 Corn Belt states. The team found that removing crop residue from cornfields generates an additional 50 to 70 grams of carbon dioxide per megajoule of biofuel energy produced (a joule is a measure of energy and is roughly equivalent to 1 BTU). Total annual production emissions, averaged over five years, would equal about 100 grams of carbon dioxide per megajoule — which is 7 percent greater than gasoline emissions and 62 grams above the 60 percent reduction in greenhouse gas emissions as required by the 2007 Energy Independence and Security Act.
In other words, making biofuel from the corn stover releases more CO2 into the atmosphere than burning an equivalent (by energy production) amount of gasoline.
The primary negative consequence of making biofuel from corn stover is from removing the stover from the field. If the stover is left in the field then some of the captured carbon will enrich the soil. Messing with the soil tends to release CO2 into the atmosphere. This is also a major problem for tilling soil in the farming process, and why no or low til farming is becoming more popular.
It seems that so far this study has held up to peer review, but of course one study is unlikely to be the final word. Still, it is a serious challenge to the current practice of making ethanol from corn stover.
One potential downside to biofuels (not a factor in the current study) is land use. If we need more farmland to grow crops not for food but for biofuel, then that might also have a net negative effect on CO2 and the environment. That is precisely why corn stover seems like an attractive option – it is using the left-overs of corn growing, and does not require land for itself.
Researchers are looking into other biofuel feedstocks that have optimal features for a net benefit from biofuel production. Any plant that contains cellulose is potentially a feedstock. Wood, for example, could be used (if we had a sufficiently efficient process that could be performed on a large scale and cost effectively). Algae grown in ponds is another option. High energy density perennial crops may also be an option.
Biofuels may ultimately play a role in our sustainable energy future, but I still don’t think it will be a major role. Storing solar energy as cellulose (or any carbohydrate), then making that cellulose into burnable fuel, may simply be too inefficient a process to ever be very useful.
There are other ways to capture solar energy that are likely to be more efficient. Photovoltaic cells capture electricity from sunlight. We can also capture heat from sunshine.
Perhaps the most efficient process may be to use sunlight to cleave hydrogen from water. While hydrogen is not a source of energy, because there is little free hydrogen on the earth, hydrogen is potentially an excellent way to store energy. If we made hydrogen from sunlight and water, then burned the hydrogen for fuel, resulting in water as a waste product, this could be an entirely sustainable and environmentally neutral energy infrastructure.
The hydrogen economy may still be coming (just like Winter in Westeros).
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