There's only one reason to calculate how much ethanol you can extract from a bushel of corn. It's a handy measure of how efficiently your plant converts starch or fermentable sugars into fuel. A high yield gives you bragging rights to high performance. 
 	
 Given the importance of making a claim with a concrete number attached, it shouldn't be surprising that ethanol yield is one of the most abused performance parameters in the industry. There are many games people can play to exaggerate the result. Things aren't always what they seem. So let's take a look at seven ways inaccuracies can creep into the calculations and skew the numbers. Your own. And your competitors'.
 

 You can be hot or cold in determining yield.
 Yield is traditionally stated in terms of gallons of ethanol per bushel of grain. The first problem is to define "what is a gallon of ethanol". A "gallon" is a volumetric measure equal to 0.1337 cubic feet (3.785 liters). 
 	
 How much ethanol will a gallon container hold? Well it depends on the temperature of the ethanol. If you fill the container to the brim with ethanol at 60 degrees Fahrenheit and then heat it to 100 degrees, it will overflow and 2.5% of your ethanol will spill. Why? Because liquids expand when they are heated and contract when cooled. So a one-gallon container holds 2.5% less ethanol at 100°F than it does at 60°F. 
 	
 Our first problem then is to establish the temperature to use as the basis for our one-gallon measurement. That's not so difficult, because the decision has already been made for you. It is an accepted practice that ethanol for commerce is measured at 60°F (15°C), and most quality flow meters are temperature compensated to correct for temperatures higher or lower than 60°F. However, the ethanol that flows to your storage tank is rarely as low as 60°F. It's normally more like 100°F, so if you just "stick" your tank or rely on a tank gauge, you will think you have more ethanol than you really have. You must always adjust (shrink) the volume from that measured-at-tank-temperature back to 60°F. A 2.5% difference may seem small, but that "little" error will affect all your calculated efficiencies (capacity, yield, energy consumption), and besides that, your accountant will wonder where the rest of the ethanol went. A 40 mmgpy plant will show a discrepancy of one million gallons annually between your actual production and your tank measurement.
 	
 You can rest assured the market will buy either by weight or through a temperature-compensated meter.
 
 What does a gallon weigh?
 If we instead measure our production by weight, we need to know how many pounds a gallon of ethanol weighs at our reference temperature, and we can just weigh it to determine how much ethanol we produced, right? Well there's another problem. When I left the aerospace industry in the late 70's and became an "alcoholic," I was very frustrated because all the reference books used in the chemical engineering field listed different values for the density of ethanol. If you check "Perry's Chemical Engineer's Handbook", the "CRC Handbook of Chemistry and Physics", the "International Critical Tables", "Lange's Handbook of Chemistry," and other chemical references, you will find conflicting data ranging from a density of 6.57 to 6.62 pounds per gallon. Depending on which value you use to measure your ethanol, it can overstate or understate your yield by up to 1%. It doesn't sound like much, but for a 40 million GPY plant, it can cause an error of 400,000 gallons per year.
 
 The other barriers to accuracy.
 Problem one: most technical data is compiled for pure anhydrous ethanol. Your fuel ethanol plant doesn't produce pure ethanol. It will contain variable amounts of fusel oils, heads and water, each of which affect the ethanol weight. For example, if your ethanol contains 1% water (an amount allowed by ASTM Specification D 4806-01 for fuel-grade ethanol), it will increase your ethanol density by 0.4%. 

 	
 And what about the grain? A bushel is also a volumetric measure, but it's accepted for commercial purposes that a bushel of corn weighs exactly 56 pounds. But what about trash? Corn can contain from 1% to 3% trash (I've seen some as high as 8%). It is not fair to hold your ethanol plant responsible for not converting the trash to ethanol, so we normally include only the cleaned corn that entered the hammermill in calculating yield.
 	
 Moisture also varies from one load to another, and it generally declines the longer the corn remains in storage. The difference between 13% and 15% moisture corn doesn't sound like much, but a plant that yields 2.60 gallons per bushel with 15% moisture corn will produce 2.66 gal/bu with 13% corn.
 	
 Depending on the corn variety, growing season, climatic conditions and numerous other factors, the starch content of corn typically ranges from 68% of the bone-dry weight of the corn to over 74%. Starch is the only component that your plant can convert to ethanol. Well, to be more accurate, some grains such as wheat also contain small amounts of sugar that will convert to ethanol. If present in significant amounts, any C6 sugars should also be included as fermentable substrate.
 	
 Another big error can be introduced by calculating your performance in terms of denatured ethanol. Some simply call it "fuel ethanol". Unless you have a petroleum refinery as part of your ethanol plant, you did not produce the denaturant, and you should not include it in your performance calculations unless you make it very clear to your board (and to the other ethanol plant managers you're bragging to) that you have done so. Calculating your performance based on denatured ethanol will overstate all your performance parameters by 4.76%.
 So what is your bottom line?
 Let's look at the effect all these factors can have on calculated yield. Let's assume we have two plants that are each well engineered, maintained and operated, so that each one is able to convert one pound of starch into 0.52 pounds of ethanol. One plant calculates yield based on absolute (200 proof) undenatured ethanol adjusted to 60°F at 6.60 pounds per gallon, and uses corn with 15% moisture and 69% starch. The yield is then calculated as: 
 
 (1.00 - 0.15) x 56 x 0.69 x 0.52 / 6.60 
 = 2.59 gallons per bushel
 
 Not bad, but not spectacular either. And it's probably below the yield guaranteed by your plant builder. But look at your guarantee language, and you will probably find that the calculation basis is more optimistic (or not stated at all). 
 	

 If the other plant assumes as a basis 6.58 pounds per gallon of denatured ethanol with 1% moisture (sometimes just called "fuel ethanol"), and corn with 13% moisture and 74% starch, the calculated yield would be:
 
 (1.00 - 0.13) x 56 x 0.74 x 0.52 x 1.0476 / (0.99*6.58) = 3.01 gallon per bushel
 
 Quite a difference…. and all with the plant yielding exactly the same 0.52 pounds of ethanol per pound of starch. 
 	
 Let's put some dollar signs in this equation. We've already established for the purpose of this example that the two plants had identical actual yields, so at the same production rate they would use the same amount of feedstock. But what if the two plants actually did have different yields of 3.01 and 2.59 based on the exact same yield calculation assumptions? At 40 million gallons per year, the plant with a yield of 2.59 would use 2,154,979 more bushels to produce the same amount of ethanol as the plant achieving a comparable yield of 3.01. Assuming $2.00 corn, this amounts to an additional $4.3 million in corn purchases per year for the lower yield plant. Allowing for approximately one third of this cost to come back in the form of extra DDGS revenue for the plant with the lower ethanol yield, this equates to a net difference of approximately $2.8 million to the bottom line with all other factors being equal. 
 	
 Let's look at this from a sensitivity analysis standpoint. This is a linear equation, so maintaining the 40 mmgpy plant and $2.00 corn assumption, your bottom line would be affected about $100,000 per year for every .01 distortion in yield. You can see that as you buy a new plant with yield guarantees or benchmark your existing plant against the industry, it certainly pays to become expert at how to define and/or calculate yield.
 	
 Remember that all plant performance calculations will be distorted by these same factors. 
 	
 There are other games you can play to enhance plant performance, but usually with a negative impact on plant profits. For example, if you use an overdose of enzymes or yeast to boost yield, you will produce slightly more ethanol, but the additional yield will probably not be worth the extra cost. 
 	
 So the next time one of your friends tells you his plant yields over 3 gallons per bushel, ask him, as Paul Harvey would say, to tell you "the rest of the story." EP

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Upcoming Events

2024 International Fuel Ethanol Workshop & Expo

Biodiesel Summit: Sustainable Aviation Fuel & Renewable Diesel

Carbon Capture & Storage Summit

North American SAF Conference & Expo

2025 International Biomass Conference & Expo

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