Barriers that are currently preventing the use of greater percentages of biomass-based diesel fuel blended into petroleum diesel have been identified, along with strategies to overcome them, according to researchers at the National Renewable Energy Laboratory (NREL).

The bio-derived diesel fuel in use today is blended into petroleum diesel at a relatively low percentage, typically from 5% to 20%. An NREL team investigated the performance of much higher blends of biodiesel into both renewable diesel and petroleum diesel. They specifically examined blends of 20%, 40%, 60%, and 80%.

A switch to using higher percentages of biomass-based diesel fuels would reduce the amount of greenhouse gases emitted by the transportation sector. Biodiesel is an oxygenate made from fats, oils, and greases. Renewable diesel is made from the same feedstocks but processed to be a hydrocarbon chemically similar to petroleum diesel.

"It's amazing to me, but there are thousands of papers published every year on biodiesel, and almost nobody looks at blends over 20%," said NREL Senior Research Fellow Robert McCormick, corresponding author of the newly published research paper titled "Properties That Potentially Limit High-Level Blends of Biomass-Based Diesel Fuel," which is published in the journal Energy & Fuels.

"This research addresses a major data gap regarding biodiesel blends, both because it looks at high-level blends and because it looks at blends with renewable diesel as well as petroleum diesel. Biodiesel blends with renewable diesel are 100% renewable."

The paper was coauthored by researchers Gina Fioroni, Nimal Naser, and Jon Luecke, all also from NREL.

The researchers examined biodiesel produced from soybean oil, which is the most common feedstock used in the United States to make fuel. They pointed out that a detailed understanding of the properties of biodiesel blended at levels above 20% is lacking.

Heavy-duty long-haul trucks and off-road equipment, marine shipping, and commercial aircraft are expected to continue requiring liquid fuels even as electrification of smaller vehicles ramps up. These fuels will need to yield low-net greenhouse gas emissions—such as biodiesel and renewable diesel—and be compatible with existing engines. The use of biodiesel and renewable diesel is forecast to reduce transportation-related greenhouse gas emissions from 40% to 86% compared to petroleum diesel, depending upon the feedstock used.

McCormick said with a biodiesel blend greater than about 50%, "you start to have property differences with petroleum that could be problematic." At less than 50%, the differences do not pose much of a challenge.

Challenges with biodiesel blends greater than 50% can be mitigated, however. For example, diesel fuels must be reformulated in winter months to ensure that the cloud point—the temperature where wax begins to form—is below the expected ambient temperature.

Wax can cause fuel filter clogging such that the engine cannot operate. Biodiesel cloud point can be as low as 20°F, but for soy, the biodiesel cloud point is around 32°F, making the use of 100% biodiesel problematic in areas with colder winters.

"This issue can be managed by reducing the blend level or by blending the biodiesel into different hydrocarbon blendstocks with a lower cloud point during the winter months—as is commonly done today for B20 [a blend of 20% biodiesel and 80% petroleum diesel]," McCormick said. "A similar strategy could be used to mitigate the high boiling point of biodiesel, which is near the top of the diesel boiling range."

A hydrocarbon blendstock with a lower boiling point, such as kerosene, could be used for biodiesel blends over 50%, alleviating challenges in cold starting the engine, accumulation of fuel in the engine lubricant, and potentially failure of emission control catalysts to "light-off" or achieve high enough temperature.

The research also examined other fuel properties such as density, oxidation stability, and water content to determine whether these might limit the blending of biodiesel. Oxidation stability, for example, may be reduced as more biodiesel is blended, but the problem can be overcome by using higher levels of antioxidant additives.

Significant future research is needed to address the challenges of high level biodiesel blends, particularly on how they impact diesel engine emission control systems. The NREL paper serves as a research road map for addressing these challenges.