What Is Biodiesel? Overview and Impacts

Biodiesel is diesel fuel made from biomass rather than petroleum (petrodiesel). It’s used primarily in transportation, but it can also be used for home heating, electricity generation, and other uses. Common sources of biodiesel are organic materials high in lipids (fats) such as cooking oil, animal fat, and algae.

Biodiesel is often promoted as a more sustainable way to meet the world’s rising demand for transportation fuels while limiting the environmental impact of fossil fuels. In 2018, for example, as part of its attempt to meet its commitments to the Paris Climate Agreement, the European Union identified renewable biodiesel as a suitable fuel for diesel engines while reducing emissions. But biodiesel’s status as a sustainable fuel is subject to debate.

The Problem with Petrodiesel

Ever since the 1997 Kyoto Protocol to reduce greenhouse gas (GHG) emissions, petrodiesel has been seen as a more environmentally friendly transportation fuel than gasoline.

Petroleum-based diesel has a higher energy density than gasoline, and the way it is burned in a diesel engine (through compression rather than a spark plug) makes it more energy efficient, meaning it produces fewer GHG emissions per mile than gasoline. Compression-initiated ignition also provides diesel vehicles with greater torque, which is why so many heavy-duty and long-distance transportation vehicles (ships, trains, trucks) and electricity generators use it, among other industrial uses. While diesel costs more at the pump, it is around 30% less expensive to use and reduces wear and tear on an engine.

Yet burning diesel has serious health and environmental consequences. “Less worse than gasoline” is not a great selling point when the transportation sector generates 27% of total greenhouse gas emissions in the United States and 14% around the world.

GHG emissions aren’t the only problem with diesel. While “cleaner” diesel engines do exist, diesel still produces levels of particulate matter and nitrogen oxides known to increase mortality rates from respiratory diseases, which disproportionately impact low-income communities and communities of color.

Transportation also makes up 90% of the U.S.'s black carbon (soot) emissions, and diesel exhaust is responsible for most of that.

How Is Biodiesel Made?

Biodiesel is made by separating glycerin (found in soaps, toothpaste, and other domestic products) from its biomass source, leaving behind methyl esters, the chemical name for biodiesel. Pure biodiesel can only be used in its pure form if diesel engines are modified, so biodiesel is usually mixed with petrodiesel for transportation use.

Common blends of biodiesel and petrodiesel contain from 2% to 20% biodiesel, with blends marked by their biodiesel percentages, such as B2 or B20. Most diesel engines in the United States can use blends up to B20, usually with B20. Biodiesel can also be blended with heating oil for home heating.

Biodiesel production is often divided into three generations, depending on its source:

• The first generation uses industrial food crops;
• the second generation produces biodiesel from biomass residues (such as stubble), non-edible crops, and wastes such as restaurant grease;
• the third generation largely refers to algae-derived biodiesel.
  

Renewable but Not Necessarily Sustainable

Biodiesel can be produced from renewable sources, but most biodiesel produced today is not sustainable and often not even renewable. First-generation biodiesel is usually made from monocropped sources such as canola seed, corn oil, soybeans, sugar cane, and palm oil. While these feedstock crops are by definition renewable, their production comes at a high environmental cost such as deforestation, soil degradation, and nutrient pollution.

Given that biodiesel currently supplies less than 1% of the demand for transport fuels, dramatically increasing its production from first-generation sources to replace petrodiesel would be environmentally devastating. It also comes at the cost of replacing valuable farmland needed to feed a growing world population.

While not yet available on a commercial scale, microalgae are increasingly seen as both a renewable and sustainable source of biodiesel. Microscopic algae have a lower environmental footprint, are not grown on arable land, do not lead to soil degradation, and do not compete with the production of edible oils and fats.

Microalgae require far less real estate to produce the same amount of energy as crops. Microalgae are carbon-negative organisms, absorbing nearly twice as much carbon dioxide from the atmosphere as they produce as biomass.

The production of algae-based diesel also has side benefits, including creating valuable byproducts such as biofertilizers and absorbing the nutrient pollution in wastewater that creates algal blooms.

Benefits of Biodiesel

Multiple studies show the environmental superiority of biodiesel over petrodiesel. In general, the higher the percentage of biodiesel in a diesel blend, the cleaner the fuel.

According to the U.S. Department of Energy, a B20 blend can reduce greenhouse gas emissions from diesel engines by up to 15%. A life-cycle analysis of biodiesel emissions—one which takes into account all aspects of the production and consumption of the fuel—finds that biodiesel can reduce GHG emissions from 40% to 69% compared to petrodiesel, depending on the biomass source of the biodiesel. Deriving biodiesel from recycled sources such as used cooking oil achieved even higher reductions of up to 86%.

Drawbacks of Biodiesel

Biodiesel is generally less energy efficient than other forms of diesel, producing lower engine performance—one of the key attractions of diesel over gasoline. In algae-derived biodiesel, as the percentage of biodiesel increases in a diesel blend, the fuel becomes less ignitable, creating an “ignition delay” and decreasing the engine’s torque.

When considering the energy needed to produce biodiesel, the energy return on investment (EROI) is low; when using some biomass sources, it can even be of minimal energy benefit. Producing biodiesel from rapeseed/canola can have an EROI hardly above one, meaning it takes nearly an equivalent amount of energy to produce the energy needed for the process. In some regions where more fertilizers are needed to grow crops or where farming practices are less efficient, rapeseed/canola’s EROI can even be negative.

A number of technologies do exist to increase the energy efficiency of the production and consumption of biodiesel, but they still fall short of the efficiency of petrodiesel and make the product more expensive.

Outlook of Biodiesel

Electric vehicles present a superior alternative to diesel (and gasoline) engines in terms of their life-cycle emissions. However, the shift to zero-emissions vehicles will not happen overnight, as cars and light-duty trucks stay on the road for an average of 12.1 years. Adding biodiesel to petrodiesel may offer a short-term, lower-emissions option for vehicles still in service, but "lower emissions" does not mean "zero emissions," and the window of time for expanding the use of short-term “bridge fuels” is closing fast.

Biodiesel is currently more expensive to produce than petrodiesel and requires private and public financing to make it a viable alternative transportation fuel. Amid the urgent need to reduce emissions, the European Union and numerous American states have banned the sale of new gasoline and diesel cars by 2035, while environmental groups like Greenpeace have called for the bans to begin sooner. Faced with the increasingly limited market for biodiesel in transportation, investment in developing the technology is likely to dry up.