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Ethanol worse than gas? Not so fast

Author: Kristy

Cellulosic ethanol from corn residue worse than Gasoline? NOT…SO…FAST… 

A new academic paper on biofuels was released this week, and some media outlets have reached sweeping conclusions. “Fuels from corn waste not better than gas ,” claimed the AP. The Daily Caller one-upped them with “Study: Corn Ethanol is Nature’s Enemy”.  A careful reading of the paper doesn’t in any way support these conclusions.

In fact the academic study in question should be seen as reinforcing what many advocates for cellulosic biofuels have known all along: cellulosic biofuel production systems that systematically deplete soil organic carbon are unsustainable. The author does not argue that cellulosic biofuels are inherently unsustainable, or even that systems relying on crop residues are inherently unsustainable. This isn’t a general indictment of cellulosic biofuels, though it’s certainly being played that way in the media. Simply put, there are good and bad ways to produce biofuels. This study points out the risks of unsustainable crop residue management. It also makes a few puzzling analytical decisions on the way, but we’ll get to that.

The study, “Biofuels from crop residue can reduce soil carbon and increase CO2 emissions” by lead author Dr. Adam Liska from University of Nebraska, appears in the April 2014 issue of Nature Climate Change.  I know and like Dr. Liska, and respect his work.  He has participated in several GPI projects and events.

The important thing to note about this paper is that it assumes bad agronomic practices – it assumes that crop residues are being removed in a way that depletes soil organic carbon (SOC).The paper is based around a simulation exercise, wherein the authors run a large model (“geospatial model and supercomputer simulations”) to estimate soil organic carbon loss across a wide geographic area due to residue removal. This is based on the assumption that if you simply leave the residue on the ground, it degrades slowly (around 20% is left after 10 years, mostly the lignin part). But if you remove it, the carbon is released quickly during the process of making biofuels, or by burning the biofuel in an engine. Liska, et al., conclude that impact of SOC loss adds 50-70 grams of CO2 per megajoule (gCO2/MJ) to the lifecycle greenhouse gas (GHG) intensity of stover-based ethanol (around 30 gCO2e/MJ before SOC loss – or lower according to many studies). For context, the GHG intensity of gasoline is around 94 gCO2e/MJ, so this SOC loss is enough to push it well above the 60% GHG reduction threshold required by EPA for a fuel to be considered a cellulosic biofuel in the RFS.

There are many reasons not to take the high level conclusions at face value.  Others have pointed out that the authors use unrealistic assumptions for the level of stover removed by assuming that 75% is removed when typically only around 25% is actually removed. For example see USDA research leader Doug Karlen’s comments in this story from E&E’s Climate Wire. The paper fails to note that extensive research has been dedicated – with considerable success – to harvesting stover while avoiding soil carbon loss.

But now for the puzzling part – the authors invalidate the “big headline” conclusions in the discussion section of the study, but fail to follow through by changing their results. First, the authors acknowledge that they do not calculate a co-product credit in their lifecycle emissions assessment (LCA) and thus include no benefit from burning lignin to provide process heat in the cellulosic conversion plant.  However, every cellulosic plant I’m aware of does this. For LCA wonks – this is a bit like failing to account for DDGs in a corn ethanol LCA. The authors state that:

The lignin fraction of residue can also potentially be burned to produce electricity, offsetting coal generated electricity and saving emissions of up to 55g CO2e/MJ

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