SOIL CARBON AND CLIMATE CHANGE NEWS
Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS)
Charles W. Rice, K-State Department of Agronomy, National CASMGS Director
(785) 532-7217 email@example.com
Scott Staggenborg, K-State Department of Agronomy (785) 532-7214 firstname.lastname@example.org
Steve Watson, CASMGS Communications (785) 532-7105 email@example.com
renewable energy and
greenhouse gas emissionS
One of the ways agriculture is helping reduced greenhouse gas emissions is through the production of renewable fuels such as ethanol -- especially cellulosic ethanol. Corn-based fuel ethanol can achieve moderate reductions in greenhouse gas (GHG) emissions compared to gasoline, according to a report from Argonne National Laboratory. But cellulosic ethanol can achieve much greater reductions in GHG emissions compared to gasoline. See page 35 of the presentation at: http://www.transportation.anl.gov/pdfs/TA/347.pdf
The most detailed analysis of GHG emissions from ethanol is in a report titled “Effects of Fuel Ethanol Use on Fuel-Cycle Energy and Greenhouse Gas Emissions” from Argonne National Laboratory. This report can be found at: http://www.ipd.anl.gov/anlpubs/1999/02/31961.pdf
The authors of this report calculated the effect of various blends of ethanol-gasoline on GHG emissions, on the basis of “Per Vehicle Mile.”
Current technology, corn-based ethanol:
* Use of E10 results in a 1 percent reduction in GHG emissions
* Use of E85 results in a 14-19 percent reduction in GHG emissions
Near-future technology, biomass-based (cellulosic) ethanol:
* Use of E10 results in a 6-9 percent reduction in GHG emissions
* Use of E85 results in a 68-102 percent reduction in GHG emissions
E10 is a blend of 10 percent ethanol and 90 percent gasoline. E85 is a blend of 85 percent ethanol and 15 percent gasoline.
The wide range of emissions reductions for biomass-based ethanol reflects the different possible feedstocks that could be used. Cellulosic ethanol made from herbaceous biomass achieves smaller reductions in GHG emissions than does cellulosic ethanol from woody biomass.
How can ethanol result in greater than a 100 percent reduction in emissions (e.g. E85 ethanol from biomass)? This is attributable to the elimination of emissions from electric power plants. Electricity generated in cellulosic ethanol plants, because of the plant design, exceeds the internal needs of the plant. The excess is exported to the electric grid, reducing emissions from other generators of electricity.
The bottom line is that cellulosic ethanol can potentially achieve significant energy and GHG emissions reduction benefits.
The production and use of ethanol has a fundamentally different impact on the CO2 cycle than the production and use of petroleum fuel. The impact is different for grain-based than biomass-based ethanol production. The production of grain-based ethanol requires more inputs than biomass-based ethanol, which affects the overall GHG emissions balance. And grain-based ethanol production has a more uncertain effect on soil carbon sequestration and sustainability than cellulosic ethanol production.
The basic carbon/ethanol cycle is:
There are four primary reasons that the production and use of ethanol reduces GHG gas emissions.
1. Less CO2 is emitted from the tailpipe when burning ethanol instead of gasoline. Burning ethanol (from any source) releases less CO2 into the atmosphere than gasoline, according to a report from the Energy Information Administration (EIA), U.S. Department of Energy. See: http://www.eia.doe.gov/cneaf/alternate/page/environment/chap3.html
Ethanol releases 325.5 grams of CO2 per vehicle mile traveled, compared to 347.3 grams for gasoline. Ethanol does release much more water vapor into the atmosphere than gasoline per vehicle mile traveled. Currently, however, water vapor from fuel combustion is not believed to have a significant impact on atmospheric water vapor concentrations, according to the EIA report.
2. The carbon-based energy captured in grain and plant materials is used directly as a fuel source, completing the carbon cycle. Plants take CO2 from the atmosphere and convert it to carbohydrates using the sun as an energy source. The carbon is then converted within an ethanol plant into combustible fuel, which enters the atmosphere when it is burned by vehicles. These carbon emissions are then used by next year’s crop and the cycle is complete.
When plant materials are not used for ethanol, part of the carbon is eventually sequestered in the soil and part is emitted into the atmosphere as the plant material decomposes on the soil surface. When plant materials are used for ethanol, the portion of the carbon that would normally be decomposed and released into the atmosphere is instead used to replace fossil fuel combustion.
3. With perennial biomass-based ethanol crops, there is an increase in the amount of root mass and an elimination of tillage. This results in an increase in the amount of carbon sequestered in the soil. Also, the lignin by-product of cellulosic ethanol production can be used to at least partially fuel the operation of the ethanol production plant itself, meaning less reliance on coal, natural gas, or other fuels. The CO2 emissions released by the combustion of lignin to fuel the ethanol plant are treated as zero because they are taken up again by the corn or biomass plants during photosynthesis.
4. Cellulosic ethanol plants can potentially generate excess electricity from the combustion of lignin through co-generation facilities, and that electricity is exported to the power grid. This direct offset reduces the emissions that would have come from the normal process used by the power plant.
There is some variability in the amount of GHG emissions reductions from renewable energy because of many factors, such as the:
* Different types of fuel used to operate ethanol production plants. Many ethanol production plants currently use coal or natural gas as the primary energy source in their operations. Plants that use biomass combustion to fuel the plants have the most favorable GHG emissions balance.
* Amount of inputs needed to raise the ethanol feedstocks. With corn-based ethanol, considerable nitrogen fertilizer is used in the production of corn. The potential nitrous oxide emissions from the increased fertilizer use offsets some of the benefits of reduced CO2 emissions.
* Fuel use in feedstock production. The amount of equipment time used in the production of the feedstock will have an impact on the overall GHG emission budget.
* Soil management and tillage practices. If soils are subject to increased erosion or tillage intensity during the production of biofuels, this can cause an increase in carbon emissions from land use changes and offset some of the benefits.
* Yield of the feedstocks compared to the inputs used. Corn producers are getting higher yields per unit of input now than in years past, which improves the overall energy and GHG emissions balance.
* Method and distance of transportation needed to take the ethanol from the production plant to the end user.
The estimates on GHG emissions reductions from the production and use of ethanol are based on the GREET model (Greenhouse gases, Related Emissions, and Energy use in Transportation) from the Center for Transportation Research at Argonne National Laboratory. For details of the GREET model, see:
-- Steve Watson, CASMGS Communications