By April Hirsch / Published April 2020
A distinction without a difference? No. Biodiesel and renewable diesel are distinct and different entities. They differ in composition, and they differ in the way they are made. What they have in common is that neither one originates with a fossil fuel source.
Let’s start with the differences as delineated by the U.S. Department of Energy (DOE):
Biodiesel is produced exclusively from lipids, which are fats and oils. Vegetable oils, animal fats, grease, and algae may all be in the final mix of biodiesel. It is a mono-alkyl ester produced by transesterification—the process by which nonedible oil is allowed to chemically react with alcohol.
In the strictest sense, biodiesel is made from excess products or waste and not from biomass grown specifically to be converted. It is not intended to be used as a replacement for diesel but instead as a fuel to blend (at 5 to 20 percent) with diesel.
Renewable diesel is produced from lipids and cellulose-structured biomass (e.g., crop residues, wood, crops) grown specifically to be converted to the fuel. It is chemically similar to petroleum diesel. It may be produced by hydrotreating, gasification, pyrolysis, and other means (both thermochemical and biochemical).
Like other renewable hydrocarbon biofuels, renewable diesel is made from renewable sources of plant material that are typically grown specifically to be converted to renewable diesel. Renewable diesel, renewable gasoline, and renewable jet fuel all belong to the category of renewable hydrocarbon fuels. The renewables are meant to serve as fuels all on their own (i.e., no mix with fossil fuel-based gasoline, diesel, or jet fuel).
Other names are used to refer to the category of renewable hydrocarbon fuels. They include “green” hydrocarbons, biohydrocarbons, drop-in biofuels, and sustainable or advanced hydrocarbon biofuels. The many names for the category can lead to confusion.
For more on the DOE classification and description of fuels see afdc.energy.gov/fuels/emerging_hydrocarbon.html. AFDC stands for the Alternative Fuels Data Center at DOE, and it is replete with useful information.
The DOE lists four major benefits of renewable hydrocarbon biofuels. One is engine and infrastructure compatibility. The others are increased energy security, fewer emissions, and more flexibility.
In anticipation of good results, some states are already mandating use of renewable diesel and biodiesel. For example, Oregon mandates that all diesel must contain at least five percent biodiesel or renewable diesel.
There are concerns across all industries about the actual level of engine and infrastructure compatibility, and they will have to be addressed. “We are concerned and have experienced problems in the early stages of using these new fuel types—including ethanol gas and biodiesel types—in small engines and pressure washer applications,” says Dennis Black, president of McHenry Pressure Cleaning Systems Inc. in Frederick, MD.
“It appears to us that the root source of the problems is that these fuels are being created to be used in larger combustion gas and diesel engines,” explains Black. He cites ethanol as an example.
“Ethanol types have appeared to create problems in smaller gas engines that may not occur in car and truck engines,” says Black. “These problems include shorter fuel life and storage concerns, as well as changes in engine performance when using these fuels.”
True interchangeability is ideal. “Biodiesel was created for diesel engines, not necessarily to be used in atomizing the type of burners that are used on pressure washers,” explains Black.
“If a customer uses biodiesel for their equipment, then naturally they want to use it in everything,” says Black. “It is difficult to tell the customer that he has to use another fuel type just for his pressure washer. Biodiesel storage has also been shown to be problematic because of the potential for it to actually grow bacteria.”
Although there are problems with the alternative fuels, Black expects refinement to eventually counter them. “Hopefully, if these fuels are mandated for the future, their use in different types of equipment may improve.”
The benefits to be accrued from the use of biodiesel and renewable diesel may not be fully realized yet. One could argue with some of the perceived benefits, such as the reduction of carbon emissions based on the idea of carbon in and carbon out, with plants grown specifically to become renewable diesel. After all, more trees could be planted to counter fossil fuel use, thereby achieving the same balance. There is also concern about taking arable land out of food production.
The best expectation, however, is that alternative fuels are here to stay, and that mandates for use will be spreading across states, if not ultimately introduced at the federal level. So, our industry must be ready.
In the context of achieving readiness for mandated conversion, if it comes, there is opportunity. AFDC at DOE offers a useful compendium of financial incentives for those contributing to the transition. They include direct grants and loan guarantees for activities including research and development, manufacturing changes, and education. Every manufacturer reading these words ought to take time to assess the opportunities by visiting afdc.energy.gov/fuels/laws/BIOD?state=US.
“In 2019, the biodiesel and renewable diesel market was estimated at 2.8 billion gallons, or a little more than five percent of our on-road diesel fuel used nationwide last year,” says Kaleb Little, a National Biodiesel Board spokesperson. The headquarters for the board is located in Jefferson City, MO.
“There is significant room in the marketplace for both biodiesel and renewable diesel fuels to grow, and we expect to see that as more markets demand lower-carbon, renewable alternatives to petroleum diesel,” says Little.
“The two fuels have some tradeoffs but are essentially drop-in fuels for diesel users,” explains Little. “Both regularly have a much higher cetane number than diesel fuel, meaning better combustion and lower emissions, and are very low-sulfur fuels.”
Biodiesel has some unique positive attributes as well. “Biodiesel has lubricity benefits compared to ULSD [ultra-low-sulfur diesel] and renewable diesel,” explains Little. “Bio-diesel is typically more readily available as it has been in commercial production in the United States for longer and in higher volumes than renewable diesel.”
The future, though, will welcome renewable diesel, too. “There is certainly excitement and growth in renewable diesel as more companies look to produce it domestically to help meet low-carbon fuel demand,” says Little.
“Leading markets for biodiesel and renewable diesel use—like the West Coast—are increasingly seeing the two products used together in varying blend levels to take advantage of the different performance benefits of each,” says Little. “Some usage is as high as 100 percent biodiesel/renewable diesel, completely cutting out petroleum diesel.”
In the first section, we emphasized, as does the DOE, the differences between biodiesel and renewable diesel. Little offers a broader, more inclusive view. “While biodiesel and renewable diesel are made through a different production process, they have more similarities than differences,” explains Little. “Both fuels are made from an increasingly diverse mix of resources such as recycled cooking oil, soybean oil, and animal fats; are renewable, clean-burning diesel replacements that can be used in existing diesel engines without modification; are advanced biofuels commercially produced here in America; and are low carbon transportation and heating options that are available today.”
As another resource for under-standing the production processes that give us biodiesel and renewable diesel, Little points us to the standards from ASTM International. “Biodiesel is made through a process called transesterification, which results in a product meeting its own fuel standard, ASTM D6751. Renewable diesel is typically created by hydro-treating vegetable oil, turning it into a product that meets the specification for petroleum diesel fuel, ASTM D975.”
Philosophically, all things are in a transition every second of every day. Yet with respect specifically to alternative fuels, the year 2020 seems likely to be particularly pivotal for renewable hydrocarbon biofuels.
Many states (and other jurisdictions) are considering mandates for use of biodiesel and renewable diesel. There are more and more stand-alone production plants and plants co-located with petroleum refineries. Interest in capturing waste (e.g., animal fats) and using it to the fullest extent possible—in the spirit of conserving by recycling and reusing—is acute.
Even so, do not place an entire wager regarding future fuel sources on renewable hydrocarbon biofuels alone. Hydrogen energy might fulfill the promise it has long held for many.
For all of us who fret that exploiting solar and wind for renewable energy has limitations (distribution, storage, or calm or cloudy days), there is some reassurance in the push being made to use excess electricity to create hydrogen. The hydrogen can then be stored and used to create electricity.
It all works thanks to a gas turbine that is turned by a high-pressure gas mixture of hydrogen and compressed air: electricity to hydrogen for storage, hydrogen from storage to high-pressure gas to move a turbine and generate electricity anew. The scaling possibilities (modular, different size generators) makes hydrogen storage and electricity generation exciting. (See an excellent article about process and possibilities, authored by Peter Fairley and titled “The H2 Solution”, in the February 2020 issue of Scientific American.)
What will be the predominant fuel source in 10 years? It’s an open question.