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NREL/TP-580-24772
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, Colorado 80401-3393
A national laboratory of the U.S. Department of Energy
Operated by Midwest Research Institute
Under Contract No. DE-AC02-83CH10093
Prepared under Task No. BF886002
May 1998
| This overview is extracted from a detailed, comprehensive report entitled Life Cycle Inventories of Biodiesel and Petroleum Diesel for Use in an Urban Bus., NREL/SR-580-24089 UC Category 1503, National Renewable Energy Laboratory, Golden, CO. That report contains the detail engineering analysis, assumptions, and other technical material that supports this overview. For a copy of that report, contact the same addresses provided on the previous page. |
Biodiesel is a renewable diesel fuel substitute that can be made by chemically combining any natural oil or fat with an alcohol such as methanol or ethanol. Methanol has been the most commonly used alcohol in the commercial production of biodiesel. In Europe, biodiesel is widely available in both its neat form (100% biodiesel, also known as B100) and in blends with petroleum diesel. Most European biodiesel is made from rapeseed oil (a cousin of canola oil). In the United States, initial interest in producing and using biodiesel has focused on the use of soybean oil as the primary feedstock, mainly because this country is the world's largest producer of soybean oil.
Proponents of biodiesel as a substitute for diesel fuel (neat or in blends) point to its advantages:
Life cycle analyses look at the whole picture of how a fuel is made, from "cradle to grave." The life cycles begin with the extraction of all raw materials to make petroleum diesel and biodiesel, and end with using the fuels in an urban bus. Examining global issues, such as CO2 emissions, requires a comprehensive life cycle analysis. Understanding the benefits of biodiesel requires us to compare its life cycle emissions to those of petroleum diesel. This study examines biodiesel energy's balance, its effect on greenhouse gas emissions, and its effects on the generation of air, water, and solid waste pollutants for every operation needed to made biodiesel and diesel fuel. We made no attempt to quantify its domestic economic benefits.
| This study provides a life cycle inventory of environmental and energy flows to and from the environment for both petroleum diesel and biodiesel, as well as for blends of biodiesel with petroleum diesel. |
Life cycle analysis is a complex science. One characteristic of the process is that you have to choose very specific technologies and assumptions to represent very complex and diverse industries and systems. So life cycles can oversimplify "reality." Also, you cannot model a system or process without data. So the availability of published data often determines which technologies and assumptions are modeled in a life cycle analysis. For example, a great deal of information is available from bus engine tests and bus demonstrations of soybean-derived biodiesel, so we chose to model soybean oil production and conversion to biodiesel, and based the end use on bus applications.
Life cycle analyses all have similar limitations. Incomplete data are the rule rather than the exception. We have varying degrees of confidence in the results, but the most reliable conclusions are for overall energy balance and CO2 emissions. For these two measures, our data are the most complete. More importantly, our sensitivity studies show that the estimates of CO2 emissions and energy requirements are very robust: they show little change in response to changes in key assumptions.
Biodiesel offers tremendous potential as one component of a strategy for reducing petroleum oil dependence and minimizing fossil fuel consumption.
| The benefit of using biodiesel is proportionate to the blend level of biodiesel used. Substituting B100 for petroleum diesel in buses reduces the life cycle consumption of petroleum by 95%. A 20% blend of biodiesel and petroleum diesel (B20) causes the life cycle consumption of petroleum to drop 19%. |
Biodiesel and petroleum diesel production processes are almost equally efficient at converting raw energy resources (in this case, petroleum or soybean oil) into fuels. Biodiesel's advantage is that its largest raw resource (soy oil) is renewable. So biodiesel requires less fossil energy (only 0.31 units) to make a 1 unit of fuel.
| Biodiesel yields 3.2 units of fuel product energy for every unit of fossil energy consumed in its life cycle. The production of B20 yields 0.98 units of fuel product energy for every unit of fossil energy consumed. |
By contrast, society uses 1.2 units of fossil resources to produce 1 unit of petroleum diesel. Such measures confirm the "renewable" nature of biodiesel.
Because biodiesel production requires such small amounts of fossil fuel, its CO2 life cycle emissions are, not surprisingly, much lower than those of petroleum diesel. Displacing petroleum diesel with biodiesel in urban buses is an extremely effective strategy for reducing CO2 emissions.
| Biodiesel reduces net CO2 emissions by 78.45% compared to petroleum diesel. For B20, CO2 emissions from urban buses drop 15.66%. |
The effect of biodiesel on air quality is complex and requires an understanding of the chemical interactions of air pollutants. To begin such an analysis, you need to know the amounts and type of air pollutants each fuel releases into the environment. Biodiesel, as it is available today, substantially reduces some air pollutants; it leads to increases in others.
| Using B100 in urban buses substantially reduces life cycle emissions of total particulate matter (32%), CO (35%), and SOx (8%), relative to petroleum diesel's life cycle. |
Biodiesel reduces particulate, carbon monoxide, and sulfur dioxide emissions compared to diesel fuel. The EPA targets these three emissions because they pose public health risks, especially in urban areas where they can affect more people. Because transportation emissions contribute significantly to urban concentrations of these pollutants, reducing tailpipe emissions is a powerful tool for improving air quality. Using biodiesel in buses operating in urban areas significantly reduces these pollutants.
| Tailpipe emissions of particulates smaller than 10 microns are 68% lower for buses that run on biodiesel (compared to petroleum diesel). Tailpipe CO emissions are 46% lower. Biodiesel completely eliminates tailpipe SOx emissions. |
The reductions in air emissions reported here are proportional to the amount of biodiesel in the fuel. Thus, for B20, users can expect to see 20% of the reductions reported for B100.
NOx is one of three pollutants implicated in the formation of ground-level ozone and smog in urban areas (NOx, CO, and HCs). Biodiesel increases tailpipe NOx emissions, and these emission sources dominate its life cycle NOx emission levels.
| The use of B100 in urban buses increases NOx life cycle emissions by 13.35%. Blending biodiesel with petroleum proportionately lowers NOx emissions. B20 exhibits a 2.67% increase in life NOx cycle emissions. Most of this increase is directly attributable to increases in NOx tailpipe emissions. B100, for example, increases NOx tailpipe levels by 8.89%. |
Our results are based on the performance of current fuel and engine technologies. Our study points out the need for research on improving engine design and biodiesel fuel formulation to address this problem.
The biodiesel life cycle also produces more hydrocarbon (HC) emissions compared to the diesel fuel life cycle. Most of the biodiesel life cycle emissions are produced during farming and soybean processing operations. Tailpipe HC emissions are actually lower for biodiesel than for diesel fuel.
| Total life cycle emissions of HCs are 35% higher for B100 than for petroleum diesel. However, HC emissions at the bus's tailpipe are 37% lower. |
These results point out opportunities for improving the life cycle of biodiesel. Future agricultural research should focus on ways of reducing HC releases from today's agricultural systems.
We designed this study to identify and quantify the advantages of biodiesel as a substitute for petroleum diesel. These advantages are substantial, especially in the areas of energy security and control of greenhouse gases. We have also identified weaknesses or areas of concern for biodiesel—such as its emissions of NOx and HCs. We see these as opportunities for further research to resolve these concerns. We hope our findings will be used to focus research on these critical issues.
Much can be done to build on and improve the work we have done here. Next steps for this work include:
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NOTICE: This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.
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