SOIL
CARBON AND CLIMATE CHANGE NEWS
From
Consortium for Agricultural Soils
Mitigation of Greenhouse Gases (CASMGS)
http://soilcarboncenter.k-state.edu
Charles W. Rice, K-State Department of
Agronomy, National CASMGS Director
(785) 532-7217 cwrice@ksu.edu
Scott Staggenborg, K-State Department of
Agronomy (785) 532-7214 sstaggen@ksu.edu
Steve Watson, CASMGS Communications (785)
532-7105 swatson@ksu.edu
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:
http://www.transportation.anl.gov/modeling_simulation/GREET/index.html
-- Steve Watson, CASMGS Communications