Residue or Residon’t: Untapped fuel yields from the US corn production system

Why not use the materials we don’t need, to produce the fuels we do need?

The Digest looks at the hard data.

Wherever you look, there are concerns about energy production — cost, infrastructure, environment, emissions, and water impact, just to name a few. No matter what energy system you develop, it seems like someone will toss out at least one of these concerns as a reason to stay with the status quo.

Which brings us to the primary mission: reduce, re-use, recycle. Use what’s already there. And there’s nothing more powerful in the Imperial Department of Re-Use than sustainable agricultural residues.

In the US corn production system — which will produce some 14 billion bushels this year for feed, fuel and food products — there are two primary residues that are available and just starting to be utilized. There’s corn stover (including stalks and cobs — the residue left in the field after harvest), and there’s CO2 (produced by yeast fermentation of corn starch, in the same yeast-starch reaction that makes bread rise when it is baked).

How much fuel is left in the residues?

The Corn Stover story

First, let’s look at the theoretical. For corn stover, there were 91.6 million acres planted last year, with 183.2 million tons of stover that can be sustainably lifted according to conservationists, yielding around 16.49 billion gallons of ethanol-equivalent fuels at yields of 90 gallons/ton.

Now, let’s look at the practical. According to the experts at Pacific Ag, “The key question is the grower adoption rate within any supply shed or biomass draw area.  If we are looking at a longer time frame where the bell curve on adoption rates of sustainable stover removal have had the chance to be established in an area with anticipated positive agronomic results, then a 30% adoption rate is quite realistic within a 50 mile radius.

“Our research and experience tells us at least 75 – 100 million sustainably harvestable tons across the 9 states,” said Pacific Ag Vice President Harrison Pettit.

Though bullish, Pettit cautions against thinking about corn stover as a no-brainer. “In many areas of the corn belt harvesting corn stover is a new practice that requires a sophisticated and relentless outreach program coupled with a very strong harvest service ethic and performance.  Growers have to be shown the benefits and the perceived risks addressed.”

Back to the numbers: this translates to 6.75-9.00 billion gallons of ethanol-equivalent fuel at 90/gallons per ton.

According to this infographic from DuPont, the numbers could go even higher — they see as much as 375,000 tons per year in the “feasible” belt.

The CO2 story

In this case, the feedstock is already aggregated and generally speaking, as long as the economics work, biorefineries jumpt at the chance to make money and be more sustainable. In the US, there are 14.5 billion gallons of fuel produced at corn dry mill biorefinery — which produce ethanol, corn oil and distillers grains (used as animal feed). The yields of CO2 liquefaction projects at ethanol plants bring in around 3.67 pounds of CO2 per ethanol gallon produced. Another calculator run shows:

14.5 billion gallons of ethanol

26.58 million tons of CO2 (53.16 billion pounds)

3.828 billion gallons at 144 gallons/ton

(We’ve taken the yields from Algenol’s system, which yields 144 gallons per ton of CO2 in the following ratio: 87% ethanol, 6% diesel, 4% gasoline, 3% jet fuel).

Going to the tote board.

We’ve found 10.6-12.8 billion gallons of fuel. For Renewable Fuel Standard purposes, we need to convert the gasoline, diesel and jet gallonages (they have higher energy densities). That’s 10.9-13.2 billion gallons of ethanol-equivalent fuel.

Now, stir in some Municipal Solid Waste

Think about the organic fraction of muncipal solid waste (around 61 percent, according to the EPA, of the 164 million tons in landfills) — enough to produce another 7.5 billion gallons of ethanol-equivalent fuel at 75 gallons/ton.

Add that to today’s US Renewable Fuel production (in RINs) of 17.297 billion gallons (the data is here, via the EPA’s EMTS tracking system).

Presto, you have 35.7-38.0 billion gallons — that’s Congressionally-set target of 36 billion gallons by 2022. Not a square foot of additional crop planted or other residues or biomass tapped in the process.

Now, the limitations

Distribution. Nor does that address the limitations on ethanol distribution imposed by growing but still emerging numbers of flex-fuel cars, and blender pumps that can distribute higejr ethanol blends.

Cost. Cellulosic fuels are not yet cost-competitive, in these days of low gas prices brought on by the war over market share between oil producers.

Policy. The US government has been wavering on enforcing renewable fuel mandated targets. And hasn’t yet fully embraced carbon capture and use — vs carbon capture and storage.

The options

Solving cost but not distribution. According to Vertimass’ mass-balance calculations, it’s process can generate 1 gallon of jet fuel from 1.56 kilos of ethanol fuel. Meaning that, if you solve the cost problem but not the distribution problem, fuels can be distributed in the form of jet fuel, where blends are up to 50% renewables.

Collection volumes. Keep in mind that there are lots of other feedstocks — dedicated energy crops, municipal solid waste, woody biomass, animal residues, even carbon monoxide — that will compete successfully with marinal corn stover acres (that are too far from a biomass plant).

The Brazil example. Worth noting that Brazil uses 27% minimum ethanol blends — it was just a matter of enforcing changes in the vehicles. Brazil also has a lively market for even higher ethanol blends — but that’s a product of building a blender pump infrastructure, which has proven a daunting challenge based on US fuel economics and consumer attitudes.

The bottom line

The good news — though cost, infrastructure and logistics concerns are everyday issues in the biofuels business, there really isn’t a feedstock shortage. Just collecting all the carbon produced by the US corn production system — without adding an acre of corn or any other feedstock, would march the US very close to its Renewable Fuel Standard targets, if not meeting them altogether.

The bad news — something has to move on infrastructure. The energy security, local economic and emissions benefits are there to be had.