Efficiency and Environmental Improvements of Corn Ethanol Production
AgMRC Renewable Energy Newsletter
Agricultural Marketing Resource Center
(First in a Series of Two)
Much has been written about the net energy balance and greenhouse gas (GHG) emissions of corn ethanol production. In this series of articles we will explore the issues of energy usage and greenhouse gas emissions from corn ethanol production. We will cite recent research from the University of Nebraska that shows improvements in these areas. We will conclude with a summary of new investigations into technologies that hold the potential to improve the net energy balance and GHG emissions from corn ethanol.
Net Energy Balance
The Net Energy Balance focuses on whether it takes more energy to produce ethanol than is contained in the ethanol. The answer to this question is yes. If it took less, ethanol production would be breaking the laws of physics and creating energy where there was none before. Most of the energy to produce ethanol comes from sunlight and produces energy that is stored in the corn plant through the process of photosynthesis. The stored energy is then converted into ethanol. The relevant question is, “how much fossil fuel energy is required to produce ethanol”? Fossil fuels are used to grow and harvest the corn and process it into ethanol. So the relevant question becomes, “does it take more fossil fuel energy to produce ethanol than contained in the ethanol”?
Ethanol to Petroleum Ratio
The Ethanol to Petroleum issue focuses on the amount of petroleum (primarily gasoline and diesel fuel) used in ethanol production. This issue is important in assessing our ability to wean ourselves from imported crude oil. It is a different issue than the net energy balance because a large portion of the fossil fuels used in producing ethanol is in the form of non-petroleum fossil fuels such as natural gas and coal.
Greenhouse Gas Emissions
The Greenhouse Gas (GHG) emissions issue focuses on the “carbon footprint” of ethanol. How do the GHG emissions of ethanol compare to those of gasoline. This includes the most common greenhouse gas of carbon dioxide (CO2), but also includes methanol (CH4) and Nitrous oxide (N2O) that are common agricultural emissions.
Ethanol versus Gasoline
There are two important differences between ethanol and gasoline that should be recognized. The first difference is readily understood – gasoline comes from crude oil which has a fixed supply whereas ethanol comes from crops and biomass that are a renewable supply, so there is no limit to the amount that can be produced (although there are some non-renewable concerns about ethanol).
The second difference is that crude oil for gasoline is mined (pumped from inside the earth) and refined into gasoline. So there are limited alternative feedstocks. Conversely, ethanol is manufactured from a feedstock. The major feedstock is currently corn, but ethanol has the potential to be a manufactured from a wide range of feedstocks.
Ethanol is an emerging manufacturing industry. So, we can expect significant technology improvements to occur in the efficiency of the industry. Also, being it is a manufacturing industry, the production of existing feedstocks and the use of alternative feedstocks is expected to significantly improve the efficiency of the industry. Conversely, the petroleum industry is a mature industry, so most of the easily developed technological advancements have already occurred. Also, because it is a mining industry, the source material will be more difficult to recover and the quality of the source material will require more refining (e.g. Canadian tar sands). So, the net energy balance of gasoline will continue to erode while the greenhouse gas emissions will continue to increase.
While various studies have yielded a range of outcomes for these issues, most analysts point to the following information. If you are driving up to a gasoline pump to fill your car’s fuel tank, every Btu of gasoline you pump into your tank requires about 1.23 Btus of fossil fuels to produce the gasoline. This is because, in addition to crude oil being a fossil fuel, it takes fossil fuels to pump the crude oil, transport and refine it. Conversely, every Btu of ethanol you pump into your fuel tank requires about .74 Btus of fossil fuel to produce it. Although this is not a great reduction, it is substantially better than gasoline.
Likewise, corn ethanol GHG emissions are about 80% (20% reduction) of those of gasoline. The emissions reduction is expected to be significantly larger for cellulosic ethanol, but firm estimates are not available because large commercial scale cellulosic ethanol systems have not yet been built. In addition to the importance of reducing greenhouse gas emissions to mitigate the impact of climate change, the biofuels mandates in the Energy Independence and Security Act include greenhouse gas reduction requirements to meet the mandate. To qualify, corn-starch ethanol must show a 20% greenhouse gas reduction from gasoline. Advanced Biofuels and Cellulosic Ethanol must show 50% and 60% reductions respectively. So, greenhouse gas emissions are an important factor in the future of the ethanol industry.
While the traditional estimates are an important starting point in making these assessments, they are expected to improve over time. Research from the University of Nebraska confirms this improvement. Below are five ethanol production systems. The first three represent technologies currently used in the ethanol industry. The remaining two represent improved technologies for corn and ethanol production.
|Table 1. Energy Use and Greenhouse Gas Emissions from Alternative Ethanol Production Systems|
|Current Production Systems||Improved|
|Modified Wet (%)||32%||0%||0%||32%||0%|
|Net Energy Balance*||1.50||1.79||1.29||1.60||2.23|
|Ethanol to Petroleum Ratio **||10.10||10.90||10.30||18.80||9.30|
|GHG Emissions Reduction ***||48%||59%||17%||52%||67%|
|*Btus of ethanol produced per Btu of fossil fuel
** Btus of ethanol produced per Btu of petroleum
***Reduction in GHG emissions versus gasoline
Source: University of Nebraska
1) Current Production Systems
Below are corn-starch ethanol production systems currently being used in the ethanol industry. Although they exist in several Midwestern states, these systems are assumed to be located in Nebraska. The systems vary by the type of energy source (natural gas and coal) and by the amount of distillers grains that is dried. All of the systems use electricity which is produced from a variety of sources. About half of the electricity produced in the U.S. comes from coal fired plants. A significant portion of the remainder comes from natural gas fired plants, with a small portion coming from renewable sources like wind.
The systems also vary by the amount of distillers grains that is dried. Drying distillers grains consumes a large portion of the energy required to operate an ethanol plant. So the amount of distillers grains sold wet or partially dried will make a significant improvement in energy consumption. Wet distillers grains need to be fed locally while dried distillers grains can be shipped to other regions of the country or exported.
Natural Gas Powered with 32% Dry Distillers Grains – This is a typical type of ethanol production system currently used in the Midwest. Most modern ethanol facilities are powered by natural gas. Roughly equal portions of the distillers grains are sold wet, partially dried or fully dried.
The net energy balance is 1.5, meaning that 1.5 Btu’s of energy are produced for every Btu of fossil fuels used in the production process. For every Btu of petroleum used, 10 Btu’s of ethanol are produced. Also, this system produces only about half of the GHG emissions of gasoline.
Natural Gas Powered with 100% Wet Distillers Grains – This system is similar to the system above except that none of the distillers grains are dried (100% is sold wet), which significantly reduces the energy consumption of the ethanol facility. This is only possible if there are large numbers of cattle feedlots or dairies nearby that can utilize the wet distillers grains.
In this system, the net energy balance improves to 1.79 Btu’s of energy produced for every Btu of fossil fuel. The ethanol to petroleum ratio improves slightly to almost 11 to 1. This ratio did not improve significantly because the energy savings from not drying the distillers grains is in the form of natural gas and electricity.
Coal Powered with 100% Dry Distillers Grains – The coal fired plant with the drying of all of the distillers grains is the least efficient of all of the systems studied. It is not typical of Midwest ethanol biorefineries but a few exist. The net energy balance is 1.29 and GHG emissions are reduced by only 17 percent. The ethanol to petroleum ratio does not change significantly from the other systems.
2) Improved Technologies
Below are corn-starch ethanol production systems that reflect improved technologies over current systems. The first system shows improved corn production technologies combined with a traditional ethanol production system. The second shows an improved ethanol production system combined with a traditional corn production system. Combining these two systems could provide an even greater improvement in net energy production and greenhouse gas emissions than the two individually.
Progressive Crop and Soil Management, Natural Gas Powered with 32% Dry Distillers Grains – This system reflects potential advanced crop production technologies combined with a traditional ethanol production facility. Specifically, it represents an eastern Nebraska irrigated production system that utilizes innovative crop and soil management practices to achieve high yields with improved efficiencies for both irrigation and nutrient management.
Although this system shows only slight improvement in net energy balance and GHG emissions over conventional corn production systems, a significant improvement occurs in the ethanol to petroleum ratio where almost 20 gallons of ethanol are produced per gallon of petroleum. This is a result of the significant increase in yields without a comparable increase in diesel fuel and gasoline required to produce corn.
Closed Loop Ethanol Production System with 100% Wet Distillers Grains – In this system the distillers grains is fed to cattle in a feedlot located on site. The manure from the feedlot is used in an anaerobic digestion system to produce methane that is used to power the ethanol plant. So, the ethanol process is referred to as a “closed loop” system.
The distillers grains is fed wet to the cattle to save drying energy. The energy and emissions savings (credits) from feeding the wet distillers grains are computed from the energy usage and GHG emissions saving from not producing the feed that the distillers grains is substituted for.
Over half of the natural gas normally used in the ethanol plant is replaced with methane from the anaerobic digestion process. If the methane had not been captured with the anaerobic digestion process for usage in the ethanol plant, most of it would have escaped into the atmosphere. Also, nitrogen is captured by the anaerobic digestion process and used as nitrogen fertilizer.
With this system, the net energy balance and GHG emissions are improved substantially over standard ethanol production system. Well over two Btus of ethanol energy are produced for every Btu of fossil fuel energy. Also, GHG emissions are reduced by two-thirds from those of gasoline. The ethanol to petroleum ratio is reduced slightly.
Next month we will examine greenhouse gas emissions in detail and how the level of emissions is impacted by corn produced under alternative conditions and locations.
More information on this research is available at:
Improvements in Life Cycle Energy Efficiency and Greenhouse Gas Emissions of Corn-Ethanol, Journal of Industrial Ecology, Adam J. Liska, Haishun S. Yang, Virgil R. Bremer, Terry J. Klopfenstein, Daniel T. Walters, Galen E. Erickson and Kenneth G. Cassman, University of Nebraska, 2008