life-cycle analysis of greenhouse gas emissions

We already know that most of our energy comes from fossil fuels AND fossil fuels spew tons of greenhouse gases (GHG) that contribute to global warming, but are biofuels actually much better?? The answer is...it depends.

Energy analysts and economists are now looking at the entire life-cycle of producing fuels (which includes extraction of fuel, or planting to processing, transport, and the burning of fuels as emissions). Gasoline is usually used as the reference, and ideally, other forms of fuels emit LESS greenhouse gases than gasoline when the entire life cycle is taken into account.

In this 2009 life-cycle analysis of different types of fuels (http://www.pnas.org/content/106/6/2077.full), Hill et al. found that corn based ethanol produced as much or even more greenhouse gas emissions (including CO2, N2O, and CH4), than gasoline, a surprising result at first. But the GHG emissions differed in production of corn ethanol depending on the source of heat at the biorefinery (whether it was fueled by natural gas, coal, or corn stover--the left-over "leaf" part of a corn plant after the fruits are harvested).

As a bright point, cellulosic ethanol has significantly reduced levels of GHG emissions relative to either corn ethanol or gasoline, when the life-cycle of the fuel production is accounted for. Of note, however, is that the data used to estimate emissions for cellulosic biofuels is relatively limited as this form of ethanol production has not been done to scale.

Costs of GHG (A) and particulate matter: PM2.5 (B) emissions. Per liter and per gallon estimates are shown alongside total costs arising from production of an additional billion gallons of ethanol or an energy-equivalent volume of gasoline. (C) Combined costs of GHG and PM2.5. From PNAS article by Hill et al. 2009. http://www.pnas.org/content/106/6/2077.full

Why is corn-based ethanol not an improvement over gasoline with respect to fossil fuel emissions? The answer is that corn is a fairly intensive crop, it needs a lot of nitrogen fertilizers , and has higher fossil fuel input, all of which contribute to increased GHG emissions. Cellulosic ethanol production is better, when the life-cycle is accounted for, because it requires little to no fertilizer and lignin combustion from the cellulosic crops provides excess heat and power at the biorefinery, which displaces fossil fuel and electricity consumption.

While cellulosic biofuels are a large improvement over conventional forms of corn-ethanol and gasoline in terms of GHG emissions, there is rarely such thing as a golden ticket. That is, there are trade-offs to GHG reductions in terms of other forms of air pollution. From studies (http://dancingflames.org/dancingflames/EnvSci/Articles/EnvScipdffiles/EthanolPublicHealth.pdf, http://www.afdc.energy.gov/afdc/pdfs/technical_paper_feb09.pdf) looking at E85 blend fuels (85% ethanol blended with 15% gasoline) compared to straight gasoline, emissions are reduced for several greenhouse gases, but emissions INCREASE for other pollutants (such as ethanol, formaldehyde, and acetaldehyde). Both formaldehyde and acetaldehyde are nasty carcinogens, as classified by the U.S. EPA. Unburned ethanol can also oxidize to acetaledehyde, so these air pollutants (while not classified as greenhouse gases) have potential human health risks.

Like most things, there is no simple single solution. And it would be good to see ways to reduce both GHG emissions AND other forms of air pollution if we want to move forward with renewable fuel sources. Sorry, but the grass is not always cleaner.