Explanation of GHG Emission Calculations and References

1.0  Introduction

This document describes the calculations done on this website – data sources, emissions included, numerical calculations, etc. For the purposes of this website, greenhouse gas emissions (GHG) include CO₂, CH₄, N₂O and HFCs (from leaky air conditioners). The gases are combined into one CO₂-equivalent emission value. This analysis uses full life-cycle emissions for fuels, not just in-vehicle emissions. That is, the emissions associated with the production, transportation and use of the fuel are included in this analysis. Two things should also be noted about this analysis. First, the equations given below may not seem to pencil out at first glance; this is only due to rounding errors. Second, these emission calculations are a work in progress and an attempt to use the best values available at this time. As better data becomes available, these calculations will be adjusted appropriately.

2.0  Calculating Emissions

2.1  Gasoline & Petro-diesel

Emissions (lb/gal) = Well-to-tank (WTT, aka upstream) emissions + in-vehicle emissions

Gasoline:          5.6 lb/gal + {(19.4 lb/gal)*(100/95)}  =  26.1 lb/gal

Petro-diesel:     5.9 lb/gal + {(22.2 lb/gal)*(100/95)}  =  29.2 lb/gal

Well-to-tank (WTT) or upstream emissions are from Argonne National Laboratory’s (ANL’s) report on well-to-wheels emissions from various fuels and vehicles[1]. This report uses ANL’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model[2] to determine life-cycle emissions. In-vehicle emissions are from EPA’s emission fact sheets[3]. These emissions include the CO₂, produced from fuel combustion as well as EPA’s estimations of CH₄, N₂O and HFC emissions from a typical vehicle. EPA estimates that these non- CO₂ GHG emissions represent 5% of the overall GHG emission profile for a vehicle, hence the 100/95 factor in the equation.

ANL’s report on the GREET model also includes the emissions of the tank-to-wheels (TTW) portion of a fuel’s use in a vehicle, but EPA’s numbers are used in this analysis instead of ANL’s TTW numbers. This is done for two primary reasons. First, ANL’s TTW analysis uses a particular vehicle type and mileage. In some cases users of this site’s calculator do not input information on vehicle type and mileage so a default value would have to be assumed to use the TTW approach. which introduces uncertainty. More importantly, the TTW analysis is more useful when it comes to comparing different types of fuel and vehicle technologies rather than calculating baseline emissions. The primary purpose of this calculator is to help users calculate a baseline emission level, not to compare different combinations of fuels and vehicles. Thus, using EPA’s method of calculating in-vehicle emissions, which is based on the chemistry of the fuel and associated vehicular GHG emissions, along with the upstream (WTT) emissions from ANL’s report provides an appropriate means of estimating a fleet’s GHG emissions. This analysis obviously does not take into account the type of vehicle in which the fuel is used or the distance that can be traveled with a gallon of fuel, but it provides a useful baseline number that can be used with other data; e.g., vehicle miles traveled, to assess a fleet’s environmental impact.

2.2  Biodiesel

In order to estimate the GHG reductions from substituting biodiesel for regular diesel, we used results from a comprehensive study conducted jointly by US Department of Agriculture and the US Department of Energy compared the overall CO₂ emissions of a bus using pure biodiesel to one using regular diesel.

The analysis included all aspects of the life cycle of the fuel, from the extraction of raw materials from the environment to the final end-use. The authors found the bus using pure biodiesel reduced CO₂ emissions by 78.45% when compared with petro-diesel.  Similarly, buses using B20 reduce their overall CO₂ emissions by 16 percent.[4] We recognize that particulate matter (PM) reductions from biodiesel may also offer climate protection benefits, as PM has been shown to act as a GHG and may contribute to global warming, but climate forcing effects of PM are not included in our calculations.[5]

GHG reduction percentages are assumed to vary linearly with biodiesel content and are as follows:

B99:     77% reduction  =  6.65 lb/gal

B50:     39% reduction  =  17.8 lb/gal

B20:     16% reduction  =  24.6 lb/gal

2.3  Ethanol

There are a variety of sources available for GHG reductions from ethanol compared to gasoline. For ethanol derived from corn kernels (grain), the GHG reductions are estimated at between 11 and 28% on a per gallon basis. For the purposes of this analysis, a reduction of 11% for corn grain-based E85 is used. Similarly, sources show a range of estimated GHG reductions expected from cellulosic-based ethanol. This range is 58 to 130%. For this calculator a reduction of 67% from cellulosic-based E85 is used. The 11% and 67% values were chosen because these are the values that were given in the recent peer-reviewed article in Science that compared a variety of previous ethanol studies and because they are conservative numbers given the wide ranges of values.

It is acknowledged that ethanol has less energy content than gasoline, and in today’s flex-fuel engines a loss of fuel economy often occurs when using E85. Thus, a per gallon comparison may not be the best way to compare E85 to gasoline. If a user only inputs information on the gallons of fuel used or money spent on fuel, however, a per gallon comparison is the only way to estimate GHG reductions from fuel switching. Once a user inputs more data on his or her fleet, a more detailed comparison of GHG emissions on a per mile basis would be preferable.

Corn grain-based E85: 11% reduction  =  23.2 lb/gal

Cellulosic-based E85:  67% reduction  =  8.60 lb/gal

 
            2.4  Summary Table of Fuel GHG Emissions for Fuel Use Input Scenarios

3.0  Default Fleet Emissions Growth Rate

If the growth rate of the fleet is not known, we recommend that the user assume the fleet emissions will grow at a rate that is similar to employment and population growth patterns.  The Puget Sound Regional Council estimates that employment growth will average approximately 1.1% per year over the next forty years, and that employment growth will average approximately 1.2% per year over the next forty years.  (The actual forecasts are on the PSRC web site, at http://www.psrc.org/data/forecasts/2006_small_area_forecasts.xls) Given that fleet emissions are expected to be a combination of the services provided to populations and the number of employees of the organization, we averaged the two projected growth rates for a default value of 1.15% per year.

 4.0  References

Argonne National Laboratory. Effects on Fuel Ethanol Use on Fuel-Cycle Energy and Greenhouse Gas Emissions. Jan. 1999. (M. Wang, C. Saricks, and D. Santini) ANL/ESD-38. http://www.transportation.anl.gov/pdfs/TA/13.pdf.

Argonne National Laboratory. Fuel-Cycle Assessment of Selected Bioethanol Production Pathways in the United States. ANL/ESD/06-7. Nov. 2006. http://www.transportation.anl.gov/pdfs/TA/377.pdf.

Brinkman, Norman, et al. Well-to-Wheels Analysis of Advanced Fuel/Vehicle Systems – A North American Study of Energy Use, Greenhouse Gas Emissions, and Criteria Pollutant Emissions. May 2005. http://www.transportation.anl.gov/pdfs/TA/339.pdf.

Emission Facts: Greenhouse Gas Emissions from a Typical Passenger Vehicle. US EPA. Office of Transportation and Air Quality. Feb. 2005. EPA420-F-05-004. http://www.epa.gov/otaq/climate/420f05004.pdf. (See also EPA420-F-05-001: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel, http://www.epa.gov/otaq/climate/420f05001.pdf; EPA420-F-05-002: Metric for Expressing Greenhouse Gas Emissions: Carbon Equivalents and Carbon Dioxide Equivalents, http://www.epa.gov/otaq/climate/420f05002.pdf; and EPA 420-F-05-003: Calculating Emissions of Greenhouse Gases: Key Facts and Figures, http://www.epa.gov/otaq/climate/420f05003.pdf).

Farrell, Alexander, et al. Ethanol Can Contribute to Energy and Environmental Goals. Science. v. 311, 27 Jan 2006, pp. 506-508. http://rael.berkeley.edu/EBAMM/FarrellEthanolScience012706.pdf.

Hansen, J., et al.  Efficacy of Climate Forcings.  J Geophysical Research.  Vol. 110, D18104. 2005.

National Renewable Energy Laboratory. Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus: Final Report. May 1998. NREL/SR-580-24089 UC Category 1503. http://www.nrel.gov/vehiclesandfuels/npbf/pdfs/24089.pdf.

US Department of Energy. Ethanol: The Complete Energy Lifecycle Picture. Fact Sheet. Mar. 2007. http://www1.eere.energy.gov/vehiclesandfuels/pdfs/program/ethanol_brochure_color.pdf.

Wang, Michael. Argonne National Laboratory. Updated Energy and Greenhouse Gas Emissions Results of Fuel Ethanol. Paper and presentation to the 15^th International Symposium on Alcohol Fuels. Sep. 2005. http://www.transportation.anl.gov/pdfs/TA/354.pdf