Greenhouse Gas Emissions of Corn Ethanol Production

AgMRC Renewable Energy Newsletter
August 2009

Don HofstrandDon Hofstrand
Professor Emeritus
Iowa State University

(Second in a Series of Two)

In last month’s article, we discussed recent University of Nebraska research showing the Net Energy Balance, Ethanol-to-Petroleum ratio and Greenhouse Gas (GHG) emissions from the corn ethanol industry. Variations in these coefficients under alternative ethanol production systems were described. In addition, emerging technologies for producing corn and ethanol were discussed and their potential impact on these factors.

In this article, we will continue to focus on GHG emissions including a detailed analysis of the sources of these emissions from corn and ethanol production. We will also show how GHG emissions from corn production vary, based on region of the country. The sale value of GHG credits will be discussed briefly. We will conclude with a summary of the research focus areas for improving corn ethanol production technologies.

Detailed GHG emissions from a natural gas ethanol biorefinery located in Iowa are presented in Table 1. Emissions are divided into those from corn production, ethanol production and the emissions credit from the co-product. 

Crop production accounts for 50 percent of the total emissions from ethanol production. One-half of these emissions are in the form of nitrous oxide (N2O.) Nitrous oxide is a very powerful GHG (289 times as powerful as carbon dioxide.) So a small amount of nitrous oxide has a powerful GHG impact. Nitrous oxide is produced naturally in soils through the microbial processes of nitrification and de-nitrification. During nitrification, ammonium (NH4) produces nitrates (NO3.) During de-nitrification, nitrates (NO3,) are reduced to nitrogen gas (N2). An intermediate step in both of these processes is the creation of nitrous oxide (N20). The large increase in the use of nitrogen fertilizer for producing high nitrogen consuming crops like corn has increased nitrous oxide emissions. 

The manufacture of fertilizer and lime is the second largest source of emissions. A large portion of the emissions is from the production of nitrogen fertilizer (natural gas for producing ammonia.) Fertilizer, lime and nitrous oxide combined make up 80 percent of the GHG emissions from corn production. The remaining 20 percent represents seed, pesticides, fuel, etc. 

Although nitrogen fertilizer is essential for profitable crop production, the development of practices for the more efficient use of nitrogen fertilizer has the potential to significantly reduce nitrous oxide emissions while also reducing production costs and mitigating the nitrogen contamination of surface and ground waters. Over time, as the allowable GHG emissions are anticipated to gradually be reduced, these technological changes will reduce corn’s GHG emissions.

Table 1.  Greenhouse Gas Emissions from Ethanol Production (Iowa natural gas biorefinery) (emissions expressed as CO2 equivalents)
  Emissions per Unit of Energy Produced * Percent
Corn Production    
   Fertilizer and Lime 8.6 15%
   N2O Emissions 14.1 25%
   Seed and Pesticides 1.7 3%
   Fuel 2.1 4%
   LP Gas and Electricity 1.6 3%
   Depreciation Capital 0.3 0%
Total 28.3 50%
   Natural Gas 19.7 34%
   Electricity 6.5 11%
   Depreciation Capital 0.5 1%
   Grain Transport 2.1 4%
Total 28.8 50%
Grand Total 57.1 100%
Co-product Credit -16.5 -29%
Ethanol Transportation 1.4  
Net Emissions 42.0  
Gasoline 92  
Reduction Relative to Gasoline 50 54%
* Grams of CO2 equivalent emissions per megajoule of energy produced.
Source: University of Nebraska.

The ethanol biorefinery accounts for the other half of the emissions. Natural gas contributes about two-thirds of these emissions. Natural gas and electricity together account for about 90 percent of the biorefinery emissions. 

The distillers grains co-product provides a 29 percent GHG emission credit. Most of the credit is due to the reduction in emissions resulting from substituting distillers grains for other feeds (that emit GHG in their production) in cattle rations.  

So, the net emissions from corn ethanol production are 42 units of emissions per unit of energy produced (grams of CO2 emissions per megajoule of energy produced). This compares to 92 units for gasoline and represents a 54 percent reduction in GHG emissions as compared to gasoline.

Regional Variations in Crop Production

Geography impacts the GHG emissions from corn production. This is due to a variety of regional factors. Factors impacting this variability include:

  • Yield – Areas that produce high yields per acre generally have lower per bushel fossil fuel energy requirements and GHG emissions. This occurs because the per acre energy needs for many of the growing and harvesting operations are spread over more bushels. Hence, the energy requirements per bushel are reduced. States like Iowa with high corn yields have less GHG emissions than lower yielding states like Texas.
  • Soil Properties – Soils high in organic matter produce higher yields and require less fertilizer to be applied (for a given amount of corn production). Once again this favors states like Iowa over Texas.
  • Climate – The combination of adequate rainfall, optimum temperatures and productive soils combine to provide a high-energy yield with limited GHG emissions.
  • Access to Irrigation – Irrigation adds an additional fossil fuel energy component to corn production that is not needed in dryland farming. So states like Nebraska and other Great Plains states tend to have higher emissions. However, this is partially offset by higher yields and more efficient fertilizer usage under irrigation. Also, Great Plains states tend to have higher cattle numbers that can utilize wet distillers grains. 
Table 2.  GHG Emissions and Net Energy Yield of Corn Production by State
State GHG Emissions Reduction versus Gasoline Net Energy Yield per Acre of Corn *
   Iowa 51% 18.3
   Illinois 52% 16.9
   Minnesota 53% 16.8
   Nebraska 48% 14.4
   Texas 40% 8.3
* Gigajoules of energy per acre.
Source:  University of Nebraska

Ethanol GHG emissions for a sampling of states is shown in Table 2. The analysis assumes that all of these states use a modern natural gas-powered biorefinery. Greenhouse gas emission reductions of ethanol versus gasoline range from 40 percent in Texas to over 50 percent in the Corn Belt. The table also shows the net amount of energy produced from an acre of corn for each state. It shows the amount of ethanol energy produced less the amount of fossil fuel energy required to produce the corn and power the biorefinery. States like Iowa with relatively high corn yields, good soils that require less fertilizer and adequate rainfall have substantially higher net energy yields than states like Texas where soils are not as fertile and the crop requires irrigation.

Value of Greenhouse Gas Emissions Reduction

With the specter of a price being put on GHG emissions through a cap and trade system, the sale value of GHG emissions reductions in the form of credits is a potential income source for corn ethanol production. Based on the data in Table 1, a 100 million gallon ethanol biorefinery will reduce GHG emissions by approximately 400,000 metric tons of carbon dioxide equivalents per year. 

Table 3.  Value of GHG Emissions Reduction under Various GHG Prices (carbon dioxide equivalents)
GHG Price Total Emissions Ethanol Portion Corn Portion
($/metric ton) (ethanol & corn) (biorefinery) (total) (per acre)
$5 $2,000,000 $1,000,000 $1,000,000 $5.00
$10 $4,000,000 $2,000,000 $2,000,000 $10.00
$20 $8,000,000 $4,000,000 $4,000,000 $20.00
$30 $12,000,000 $6,000,000 $6,000,000 $30.00
$40 $16,000,000 $8,000,000 $8,000,000 $40.00
$50 $20,000,000 $10,000,000 $10,000,000 $50.00
Source:  University of Nebraska

The value of the GHG emissions reduction is shown in Table 3 under various GHG prices. For a GHG price of $20 per metric ton (2,204 lbs.), the value of the annual reduction is $8 million dollars. If the reduction is equally divided between the biorefinery and the corn producers, each group is responsible for about 200,000 metric tons of reduction per year. The value at $20 per metric ton is $4 million for each group. In northern Iowa it takes about 200,000 acres of corn (180 bu. per acre) to supply a 100-million-gallon ethanol biorefinery. If the corn producer’s portion of the payment is spread equally across all acres, the reduction is one ton of GHG emissions per acre and results in a payment of $20 per acre at a sale price of $20 per ton. These payments are substantially smaller if emissions from indirect land use are included with these direct emissions.

Current Trends for Improving GHG Emissions

The corn ethanol industry is a new and emerging industry.  So new technologies that improve the efficiency, lower the cost and reduce the GHG emissions can be expected over coming years.
1)    Corn Production

  • Higher Yields – Higher yields mean that the fixed energy usage per acre (e.g., tillage, etc.) is spread over more bushels, subsequently reducing the energy usage per bushel. Higher yields occur through improved seed genetics and improved cultural practices. Seed corn companies claim that substantial corn yield increases are forthcoming.
  • Conservation and Minimum Tillage – Less tillage means less diesel fuel and gasoline are consumed. Also it potentially sequesters more carbon in the soil and reduces the amount released as CO2 emissions into the atmosphere.
  • Improved Fertilizer Efficiency – This means that more of the fertilizer applied to the crop is actually utilized by the crop. It leads to less runoff and volatization. Improved fertilization efficiency results in reduced application rates and less environmental damage.
  • Reduced Corn Drying – Improvements in corn drying would lead to less LP gas and electricity usage. These improvements may come in the form of improved drying technologies and improved corn genetics.
  • Environmentally Friendly Energy Sources – Gasoline and diesel fuel are the major energy sources for operating machinery and equipment on farms. However, as the availability of renewable fuels expands, these new fuels can substitute for traditional diesel fuel and gasoline in growing corn for ethanol production. 
  • Renewable Ammonia – Ammonia is the feedstock for nitrogen fertilizers. Ammonia requires a large amount of fossil fuels in the form of natural gas for its production. Renewable feedstocks such as wind, biomass and other feedstocks are being explored for producing ammonia. 
2)    Ethanol Production
  • Increased Energy Efficiency – Various ways of reducing the amount of thermal heat for ethanol production are being researched. In addition to reduced energy usage, some of these areas include the potential for reduced water use and the development of higher value co-products. 
  • Environmentally Friendly Energy Sources – The major energy source for ethanol production is currently fossil fuels (natural gas and coal). However, research is focusing on using renewable sources such as biomass. This could be used in combination with fossil fuels or as a complete substitute for fossil fuels. Electricity production is heavily dependent on coal. However, renewable electricity from sources such as wind is growing. The opportunity even exists for on-site generation from sources such as wind.
  •  Integrated Systems – More feeding of wet distillers grains to local livestock operations has the potential to significantly reduce energy usage. When combined in a “closed loop” system, energy usage is reduced even more.
  • High Value Co-products – The potential for developing various high-value co-products from an ethanol biorefinery is being investigated by researchers. If these products are developed, fossil fuel energy usage would be spread over a wider variety and value of products, thereby reducing the amount allocated to ethanol production.

As we noted in an article last month on Efficiency and Environmental Improvements of Corn Ethanol Production, ethanol’s GHG emissions challenges will increase over time if California’s proposed GHG emissions standards go into effect for that state as well as the 13 others that are considering following its lead. The proposed California standards assume that ethanol’s emissions will remain constant over time, but that almost certainly will not be the case.  As the corn ethanol industry matures, significant improvements in its production efficiency and carbon footprint can be expected. Thus, over time it is almost certain that corn-ethanol emissions will decline as GHG emission regulations tighten.

More information on the 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.