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Separating Fact From Fiction in the Newest U.S. Federal Ethanol Study

Debates about the merits of biofuels have been going on for at least a generation. My favorite clip from the early, oil-crisis era ethanol push was Nicholas Wade’s article, “Oil pinch stirs dreams of moonshine travel,” published by Science in June 1979. Save for one topic, the terms of the debate — the costs of producing biofuels, whether ethanol took more energy to make than it delivered, the extent to which it really helps energy security, the hope for cellulosic biofuels and the food-versus-fuel dilemma — were the same nearly forty years ago as they are today.

Global warming is the topic not on the table then that is so important now. The effect of biofuels on greenhouse gas (GHG) emissions is the focus of many recent studies. To compare fuels according to their GHG impact, policymakers have adopted a form of computer modeling known as lifecycle analysis (LCA). A new report from the U.S. Department of Agriculture (USDA) is the latest LCA study to claim significant GHG reductions from the use of corn-based ethanol, concluding that it has net GHG emissions 43 percent lower than those of petroleum gasoline. Those results are similar to the findings of lifecycle modeling from Argonne National Laboratory (ANL), on which this latest USDA study heavily relies.

My own work has long come to an opposite conclusion. It shows that the use of biofuels (both ethanol and biodiesel) makes GHG emissions worse that they would otherwise be. This finding is not based on computer modeling, but relies instead on field data to assess the real-world CO2 flows involved when substituting biofuel for fossil fuel.

A key issue is that LCA is a scientifically illegitimate way to evaluate the situation. Indeed, the original developer of the lifecycle methods embraced by policymakers now concludes that “Using attributional lifecycle assessment to estimate climate-change mitigation benefits misleads policy makers,” as stated by the title of a paper he co-authored. There are several serious problems with LCA but the most telling is its use of an accounting shortcut that zeros out the CO2 emitted when biofuels are burned.

Carbon is the backbone of any practical liquid fuel, whether a simple, two-carbon molecule such as ethanol or a more complex organic mixture such as gasoline, diesel and biodiesel. Burning carbon-based fuels releases CO2 into the atmosphere regardless of their origin. For the most common fuels, these direct exhaust emissions average 73 (±2) grams of CO2 per megajoule (gCO2/MJ) of lower heating value (see adjoining chart).

Carbon dioxide emissions from combustion per unit of useful fuel energy

Carbon dioxide emissions from combustion per unit of useful fuel energy


Like many studies sponsored by the U.S. Department of Energy (DOE), which underwrites ANL’s lifecycle model, the USDA report sidesteps this basic fact of chemistry. They claim that the carbon in a biofuel — called biogenic carbon, coming from contemporary biomass — need not be counted because it was recently absorbed from the air by crops, trees or other vegetation that can soon grow back. This logic is used to justify the omission of tailpipe CO2 from biofuel LCA tallies. As seen in the USDA chart below, the lifecycle results for gasoline are dominated by the large red bar for tailpipe emissions. Only a tiny sliver of red (for non-CO2 GHG emissions) is shown for the ethanol results, even though the actual amount of CO2 emitted when burning ethanol is nearly as large as the amount emitted when burning gasoline.

Lifecycle GHG results from USDA corn ethanol report of January 2017

Lifecycle GHG results from USDA corn ethanol report of January 2017


Although the reasoning behind this accounting shortcut might seem simple, it is unfortunately simplistic and incomplete. It does not reflect the fact that productive farmland removes carbon from the atmosphere regardless of how the harvest is used. The photosynthesis of growing crops does not absorb more CO2 from the air just because the corn is used for fuel instead of food. The LCA models used by USDA erroneously credit cropland CO2 uptake as a full offset of biogenic emissions, including the CO2 from fermentation as well as that from tailpipes, even though only a portion of that uptake can be properly credited as an offset. The correct principles for such analysis were spelled out by my Biofuel’s Carbon Balance paper in 2013, but were not addressed by the USDA study.

The extent to which carbon uptake offsets biogenic CO2 emissions is an empirical question, one that can be evaluated using field data. Evaluating a state-of-the-art corn ethanol facility and the farmland serving it, we found no significant difference in net GHG emissions compared to gasoline. This contradicts LCA modeling that claimed a 40 percent GHG reduction for the same ethanol. Even this result reflects a one-time gain in carbon uptake from crop rotation; as shown in the next figure, GHG emissions from corn ethanol would be worse (up to nearly 70 percent higher) than those from gasoline under more realistic assumptions about prior crop production. Moreover, that’s before considering the commodity market and land-use changes that inevitably push the net CO2 emissions due to biofuel production even higher.

Sensitivity of GHG emissions increasing when using corn ethanol instead of gasoline to varying baseline assumptions

Sensitivity of GHG emissions increases when using corn ethanol instead of gasoline to varying assumptions about prior (baseline) crop production


It is fairly straightforward to analyze the data available since the Renewable Fuel Standard (RFS) was passed in 2005. A retrospective analysis based on nationwide crop production statistics reveals that the gains in CO2 uptake during crop growth were only enough to offset 37 percent of the biogenic CO2 emissions associated with biofuel use. Once one factors in other impacts, including those from the land-use changes that have been directly observed with satellite data and the indirect land-use changes projected globally, the implication is that corn ethanol makes matters worse as far as the climate is concerned.

Although USDA’s carbon accounting is wrong, the parts of the analysis that address GHG emissions on farms and at biorefineries are probably fine. Ethanol production efficiency has improved over the years and so new facilities built after the RFS was put in place use less energy than older ones. Thus, recent production is more efficient than it was a decade ago, and the USDA analysis reasonably projects that processing gains and other improvements will continue through 2022.

On the other hand, it is difficult to trust the sections of the USDA report that address the effects on agricultural (including livestock) sectors and land use in the United States and globally. Little of the computer modeling cited is scientifically verifiable and aspects of it are mathematically incoherent. Largely performed by ANL and other entities funded by bioenergy advocates at DOE and elsewhere, these elaborate calculations were done in response to critical studies by independent academics. But even granting USDA its disputable results about agricultural system optimizations and land use, the study’s carbon accounting errors still undermine the claims that corn ethanol reduces net GHG emissions.

In short, the latest USDA analysis mixes facts about gains in corn ethanol processing efficiency with the fictions that LCA is a legitimate way to analyze GHG impacts and that the CO2 released when biofuels are burned does not count. Those errors are egregious and fundamental. Once one faces the facts about carbon uptake, it is clear that ethanol use worsens rather than lessens GHG emissions.

Content Discussion

John Miller's picture
John Miller on January 23, 2017

John, the full-lifecycle carbon impacts of ethanol biofuels could possibly be worse than what you have found. A few years ago I completed a detailed analysis of the full-lifecycle of petroleum gasoline based on my refining industrial experience and Solomon Associates (a major Refining Engineering Co.) data. Re. a past TEC article: Is Ethanol a Cost Effective Solution to Climate Change. As shown in this article, the GREET model significantly over projects the full lifecycle (well-to-wheel) carbon emissions of petroleum gasoline.

Another EPA example of over predicting the value of ethanol has been their recent certification of sugarcane ethanol as an ‘advanced’ biofuel. Re. a more recent TEC article on the subject.

Hops Gegangen's picture
Hops Gegangen on January 24, 2017

It seems like something is going to grow on arable land no matter what, and it will eventually by decomposed by fungi and bacteria, releasing the CO2. Or if it goes to food, people will eat it and release CO2, or it goes to landfills to become methane. So it just seems intuitive that capturing the carbs for transportation fuel rather than for growing fungi makes sense, unless moving the biomass and distilling it takes more energy than you get out of it.

Anyway, don’t wee need a certain amount of ethanol in our gasoline?

Engineer- Poet's picture
Engineer- Poet on January 24, 2017

Gasoline is easier to handle without EtOH.  It does not have nearly so many problems with water uptake and emulsification, and the vapor pressure and evaporative emissions are lower.

The point about arable land is a good one.  If we’re going to grow particular crops for low-carbon energy (e.g. not using byproducts of food production) we should use whatever grabs the most carbon.  If this is coppicing of poplar instead of specific maize cultivars, so be it.  The fertilizer and pesticide demands of poplar will be vastly lower than maize.

Every time this topic comes up now I find myself flashing back to the molten-salt “supertorrefaction” work of Frank Shu.  This is a very simple thermal cracking process which, due to the passing of the pyrolysis products through hot salt, appears to crack essentially all the tars and other troublesome products and produces a clean syngas.  Clean syngas can in turn be converted to methanol.

Shu’s process yields syngas with an excess of hydrogen beyond what’s needed to convert all the CO and CO2, but more of the charcoal product could be gasified to use this hydrogen.  What’s left after this is char (both separable chunks and fines mixed with the salt), water, MeOH, some H2 or CO depending which is in excess, and methane.

Methane plus a bit of hydrogen (“Hythane®” is one brand) makes a pretty good motor fuel.  It’s not a bad heating fuel either.

Char is a fairly stable form of carbon which is readily sequestered by mixing with soil.  It increases soil fertility if properly made (see “terra preta do indio”).  This suggests a multi-product farm and carbon sink; when the maize is harvested the grain goes to feed people and livestock and the cobs and excess stover are shredded and pyro-processed over the winter.  Methanol is sold for motor fuel, a hydrogen-methane stream goes as a CNG substitute, heating fuel and generator fuel, and the washed char is drilled back into the soil the following spring.  Perhaps half the carbon which winds up in crop byproducts never leaves the farm again.

Sean OM's picture
Sean OM on January 25, 2017

For the most common fuels, these direct exhaust emissions average 73 (±2) grams of CO2 per megajoule (gCO2/MJ) of lower heating value (see adjoining chart).

Most vehicles don’t use heating values to drive their wheels. In fact the only one I can think of is the Stanley Steamer. The Gas engine is only like 20-30% efficient. You have to tune the engine and you can get good results with ethanol. The E85 vehicles nowadays, almost all adjust ignition based on volume of ethanol which results in better mileage.

The photosynthesis of growing crops does not absorb more CO2 from the air just because the corn is used for fuel instead of food.

You are making an invalid assumption that the corn can be used as food. We are used the subsidy money for price support and fallowing and invested in the ethanol industry, because our cereal grains can’t be sold at a fair price. 180 countries limit, tariff or outright ban importation of our cereal grains. It also replaces MTBE which was imported from Qatar and brought jobs back as well as price stability and reduces smog.

Ethanol can also easily be used a precursor chemical for many industrial chemical processes.

In reality if they are getting more energy out then they put in, it is merely a bonus. The MTBE they were using cost more, and this saves significant amounts of taxpayer money. Plus the protein that is left from the process gets around some of the import laws as a feed supplement.

I personally don’t want to pay for both Qatar chemicals and ag subsidies which is what we were doing.

But even granting USDA its disputable results about agricultural system optimizations and land use, the study’s carbon accounting errors still undermine the claims that corn ethanol reduces net GHG emissions.

You have also not made any reference to the plant that is left on the field including the leaves and root system. I am finding both your mathematical and research skills dubious at best.

Roger Arnold's picture
Roger Arnold on January 26, 2017

I’m having a hard time figuring what to say about this. It’s an important issue for anyone interested in sustainability, but I can’t see a clear field to set down in.

I’m prepared to believe that the LCA study from the USDA is flawed in making a claim that the carbon in a biofuel need not be counted (as a tailpipe emission) “because it was recently absorbed from the air by crops, trees or other vegetation that can soon grow back.” That’s too simplistic, I agree.

OTOH, the author’s logic in arguing against the USDA position seems a bit muddled. And perhaps equally simplistic? The statement that the photosynthesis of growing crops “does not absorb more CO2 from the air just because the corn is used for fuel” is off kilter. Of course it doesn’t! That’s obvious. But it has nothing to do with the issue at hand. If the author doesn’t see that, he’s not thinking straight. If he sees it but chose to throw the statement in anyway because it sounded good, then I question his intellectual integrity.

Of course it’s also possible that I’m simply missing something. Given the slowly eroding state of my own mental faculties, “missing something” may top the list in the order of likelihood.

I think Hops has it at least partially right. Whatever grows on arable land will eventually be burned, metabolized, or decomposed (all ultimately, in a sense, the same thing) back to CO2 and / or methane. So why not burn it as biofuel and take advantage of its potential energy? That would at least be better than burning fossil carbon, would it not? But that’s also too simplistic — as I suspect Hops would agree. It fails to account for the element of time, and the effect of residence times on the sizes of different carbon reservoirs.

It’s really the sizes of different carbon reservoirs and the movement of carbon among them that we’re talking about. Four reservoirs in particular: standing biomass, soil carbon, atmospheric carbon, and of course fossil carbon. The carbon in standing biomass and in the atmosphere were estimated to be about equal until fairly recently. Around 600 gigatonnes (or petagrams). In the last few decades, atmospheric carbon has surged due to massive injections from the fossil carbon reservoir. It’s now around 750 gigatonnes. Standing biomass is now estimated a bit lower than before, a recent source putting it at 560 gigatonnes. The soil carbon reservoir is estimated at 1500 gigatonnes, twice the atmospheric carbon.

Cutting down a forest and burning it for energy promptly moves a lot of carbon from standing biomass into the atmosphere. No question about that. But does it enable an equal movement of carbon from the atmosphere back into standing biomass? That’s been the theory. It assumes that a mature forest has reached an equilibrium in which average tree size is as large as it’s going to get and photosynthetic creation of new biomass is matched by decomposition of fallen trees and forest litter. In that case, harvesting is supposed to clear the way for resumed growth of standing biomass, at the expense of decomposition.

That sounds plausible, but I think I’ve read that recent studies have disproven it. Even when a forest has reached the point where the average tree size has topped out, creation of new biomass is not fully matched by loss to decomposition and metabolism. Instead, soil carbon builds up.

If that’s so, then harvesting and burning is not carbon neutral.

That’s forests, however. This discussion is about agricultural crops, and particularly corn ethanol. But perhaps there’s something similar going on with regard to soil carbon?

Modern corn is a very productive crop for photosynthetic production of fermentable starch. But repeated harvesting does appear to deplete soil carbon. It’s not that soil carbon is “mined” by plants and carried away by harvesting. But what’s carried away by harvesting is lost to the soil fauna and microfauna that contribute to (and are a part of) soil carbon. There’s a natural slow turnover of soil carbon. When the input of new material is slowed by harvesting, the result is a slow loss.

I guess that means I’m coming down on the side of the author: burning biofuel is not carbon neutral.

I just wish he had explained it better.

Roger Arnold's picture
Roger Arnold on January 26, 2017

I need to correct / amplify on something I said above. I concluded that burning biofuel is not carbon neutral. What I should have said is that it’s not automatically carbon neutral. That’s how the author characterizes the USDA position, and if it is, then I’m agreeing with the author (that the position is wrong).

OTOH, what I think I’ve figured out as being the key point in all this is that loss of soil carbon to the atmosphere is decoupled from plant growth. Soil carbon “self-consumes” itself as the bacteria and fungi that constitute a good part of it die and are themselves metabolized. It’s a slow process, but soil carbon has to be constantly renewed for the overall reservoir size to remain stable. But there’s no fixed relationship between the rate of plant growth and the replenishment of soil carbon. Replenishment iw related, in part, to the amount of dead plant matter left on the ground, but that could be a smaller fraction of a fast-growing crop like corn, or a larger fraction of a slow-growing crop.

The end result is that production of biofuel, overall, can be carbon positive, carbon negative, or carbon neutral, all depending on what’s happening with the balance of soil carbon on the lands where the fuel crop is grown. But to the extent that crop growth and loss and replenishment of soil carbon are decoupled, biofuel production, per se, is carbon neutral. I.e., the USDA position is correct. I think I’ve got that right.

The reason I think it’s important to understand the decoupling is that it suggests avenues toward boosting sequestration of carbon in soils that may not have been explored. In addition to the recognized options of amending with biochar and switching from annual crops to perrenials (with their larger, deeper root systems), perhaps it’s possible to increase replenishment rates by engineering less efficient decomposers. I.e.,organisms that consume dead biomass but metabolize less of it to CO2 while producing more recalcitrant waste hummus.

I believe something similar has already been done with fermentation yeasts. They metabolize less of the sugar they feed on to CO2 and produce more ethanol “waste”. Less efficient as biological organisms but more efficient as chemical converters.

Simon Friedrich's picture
Simon Friedrich on January 26, 2017

Countries may have policy reasons for using liquid biofuels but the greenhouse gas emissions resulting from such use need to be counted in country total emissions. These emissions only occur because of mankind’s need to divert forests and grasslands to agricultural use for both food and energy production. These generally reduce the land’s ability to store carbon, for example decarburization resulting from deforestation. Unfortunately governments ignore the emissions because the United Nations Intergovernmental Panel on Climate Change (IPCC) has chosen not to include these emissions in country totals. This promotes the use of bioenergy at the expense of increased greenhouse gas emissions since there is no need to account for these emissions in meeting greenhouse gas reductions goals.