Toward a Low-Carbon Transportation Future: Part I

by Tomas Endicott, TerViva Processing & Markets Manager


It’s the 21st century. We have amazing technology! Certainly we can reduce greenhouse gas (GHG) emissions from the transportation sector in the U.S., but how? We’re doing it! Did you know that already we have good systems for tracking and measuring our progress?

Fossil fuels are the mainstay for transportation energy in the U.S., and there is no question that they power our economy, but renewable fuels like ethanol, biodiesel and renewable natural gas are also making significant contributions. In the U.S., biofuels are derived from primarily corn and soybeans, as their production systems are well-established and their production volumes are extremely large. But just wait! In time, perennial crops like seeds from pongamia trees will contribute even lower-carbon fuels to the transportation mix.

Carbon efficiency is cool. Carbon intensity is not.

Carbon efficiency is cool because it reduces greenhouse gases and mitigates climate change. Hydrocarbon-based fuels have carbon embedded in their molecular structures, but they also generate carbon emissions in the form of the heat, power and transportation that are consumed to produce them and to get them to market. The less carbon emissions generated to produce a particular fuel, the more carbon efficient it is. In comparison, fuels that generate larger carbon emissions have a higher carbon intensity (CI). Not surprisingly, fuels that are more carbon efficient are generally more energy efficient as well. The less energy we consume as a society, the less greenhouse gases we generate.

We consume a lot of transportation fuel in the U.S. and that generates a lot of greenhouse gas (GHG) emissions.

In the U.S., the transportation sector generates about 27% of all greenhouse gas (GHG) emissions annually—second only to electricity generation, which accounts for 29% of GHG emissions. GHG emissions from transportation are the result of burning liquid fuels—primarily gasoline and diesel fuel.

According to the U.S. Energy Information Administration (EIA), in 2016 U.S. drivers consumed about 143 billion gallons of finished motor gasoline (that’s a BILLION with a “B”), a daily average of about 392 million gallons. U.S. drivers consumed about 44 billion gallons of diesel fuel in 2016, a daily average of about 122 million gallons.

TE part 1

Plants use sunlight, water and carbon dioxide to produce biofuels for transportation

Did you know that renewable fuels are required by law?

In the U.S., the federal government requires fuel companies to blend a minimum volume of renewable fuels, like ethanol and biodiesel, into the total fuel volume consumed every year. This program, the Renewable Fuel Standard (RFS), began in 2005 with the passage of the Energy Policy Act. Congress expanded the law in 2007 with the Energy Independence and Security Act (EISA). How much renewable fuel does the RFS require? More than 19 billion gallons of renewable fuel, primarily ethanol and biodiesel, in 2017! Of the 188 billion gallons of total transportation fuel Americans will consume this year, renewable fuel will make up a little more than 10%. In other words, renewable fuels could supply the U.S. transportation fuel markets for almost 38 days. Not bad really, but we can do even better!

All fuels are not created (carbon) equal.

All liquid fuels -whether bio-based or fossil-based- are composed of hydrocarbons: long chains of carbon and hydrogen atoms bonded together in a variety of ways. All of these molecular bonds contain energy that is released when the bonds are broken -when the fuel is burned. The result: the fuels are converted into energy and their molecules are converted into primarily carbon dioxide (CO2) and water (H20).

This is where biofuels and fossil fuels are different. Fossil fuels mine ancient hydrocarbons from beneath the earth’s surface and add new carbon to the atmosphere, but biofuels recycle the same contemporary carbon in the atmosphere on an annual basis. The CO2 released by burning biofuels today is re-captured by living plants to create more biofuel during the next harvest cycle—a net zero effect for the carbon embedded in the biofuel.

Carbon attributed to different fuels includes more than just embedded carbon, though. To create fossil fuels requires exploration, extraction, refining and transport -all of which generate carbon emissions. To create biofuels from recycled materials (i.e. food waste, used cooking oil) requires collection, refining and transport. To create biofuels from agricultural crops requires cultivation, fertilization, harvesting, processing, refining and transport.

Accounting for carbon is the first step.

As scientists and policymakers acknowledge the effects of increasing greenhouse gas (GHG) emissions and the need to reduce them, they create systems for measuring the GHG emissions generated by different activities. Carbon accounting tracks the amount of carbon emissions required to produce a particular fuel.

The federal RFS tracks the carbon intensity of transportation fuels. Carbon intensity is the measure of lifecycle greenhouse gas emissions attributed to all activities required to produce a transportation fuel, expressed in grams of carbon dioxide equivalents per megajoule of energy or gCO2e/MJ. Wow! That’s a mouthful, but what it means, simply, is that making different transportation fuels generates different amounts of carbon emissions. The lower the carbon emissions generated to produce any particular transportation fuel, the lower the carbon intensity.

Who accounts for GHG emissions in transportation fuels?  How do they make sure that comparing all fuels is comparing apples to apples?

In 1999, the U.S. Department of Energy’s Argonne National Laboratory developed the GREET model.  The Greenhouse Gases, Regulated Emissions and Energy Use in Transportation (GREET) model is a “well to wheel” or “farm to wheel” life-cycle model that is used to establish a specific carbon intensity (CI) value for every type of fossil fuel and renewable fuel that is consumed in the U.S. transportation sector. A fuel pathway describes the feedstock and the process for how each fuel is made, so each fuel pathway has a unique CI value.

This chart shows the carbon intensity (CI) values for different renewable fuels compared with gasoline and diesel fuel. Gasoline has a CI value of 95.86, whereas the CI for ethanol ranges from 77.44 to 120.99 -depending on where the ethanol is made and what type of energy is used for the heat and power required in the production process. Ethanol from Brazilian sugarcane has a CI value of 73.4. Petroleum diesel has a CI value of 94.71, whereas the CI for soy-based biodiesel is 83.85. Biodiesel from recycled cooking oil has a CI value of 11.76 or 15.84. Wow! That’s a reduction. Almost 90%!

What specific factors contribute to higher or lower carbon intensity (CI) values of transportation fuels?

Different feedstocks -both bio-based and fossil-based- and different fuel production pathways generate different amount of CO2 emissions.  What are the factors that affect the carbon intensities for different feedstocks and for different fuel production pathways?

Stay tuned.  We will pick it up there next week.

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