Fuel for the Future
Trucks and some cars need it to get from point A to point B. Railcars and ships can’t move without it. And in many places, it helps keep the lights on. We’re talking, of course, about petroleum-derived fuels. And though diesel fuel may be one of the most common types, unless you’re involved in the industry, you may not be aware of ongoing efforts to improve the environmental profile of a product that is especially vital to land, sea, and air transportation.
Biobased diesel is the broad term for fuels derived from biological feedstocks such as vegetable oils and animal fats. Underneath the biobased umbrella you’ll find two variations that sound similar but are distinguished by their unique chemical structures: renewable diesel and biodiesel.
ASTM International test methods, specifications, and other standards have been an important part of the diesel fuel ecosystem for decades. The standards continue to both reflect and support the evolution of greener alternatives. Here we look at new developments in vehicle and aircraft fuels, and how standards are evolving to meet the needs of an industry on the move. But first, a chemistry lesson.
Similar but Different
Renewable diesel sounds like a generic catch-all phrase that could cover a range of different, greener diesel fuel options, including biodiesel. However, it is actually a type of biobased diesel with a very specific chemical signature.
READ MORE: Driving the Biobased Economy
“’Renewable diesel’ is not an official definition at ASTM yet, but there’s a common understanding in the industry,” says David Slade, Ph.D., chief technologist at Renewable Energy Group and a long-time member of several subcommittees in the committee on petroleum products, liquid fuels, and lubricants (D02). “Renewable diesel is a hydrocarbon fuel, biodiesel is an ester fuel.”
This distinction means that, while both types of fuel are derived from biological feedstocks, their ultimate chemical composition is very different. Biodiesel is always an oxygenated fatty acid methyl ester (FAME), whereas renewable diesel is a hydrocarbon (as is petroleum diesel, interestingly).
Producing renewable diesel requires deoxygenation of the lipids (fats and oils) used as feedstocks. “You’re removing the oxygen atoms and leaving behind just the carbon and hydrogen. That’s why we say it’s a hydrocarbon,” Slade says. “One thing to keep in mind, though, is that there are multiple ‘classes’ of hydrocarbon molecules in diesel fuel, and the renewable diesel produced today only includes one of those: paraffins.”
Biodiesel and renewable diesel differ in the ways they mitigate environmental impacts. “From a production standpoint, biodiesel production has a little less carbon intensity than renewable diesel production,” Slade explains. “From an engine emissions standpoint, biodiesel is very good for reducing particulate matter and hydrocarbon emissions, and renewable diesel is very good for reducing nitrogen oxide emissions.”
While both categories of biobased diesel compare favorably to petroleum diesel in terms of carbon intensity and emissions performance, renewable diesel, because it’s a hydrocarbon product, can be derived from a wider variety of alternate inputs. However, these biological feedstocks — including starch, sugar, cellulosic biomass (organic matter like agricultural crops, wood and wood waste, and grasses), and lignocellulosic biomass (plant-based material not used for food, such as wood, wood waste, and forestry residues), among others — are not yet available on a commercial scale. “So both biodiesel and renewable diesel are currently made from the same set of feedstocks, which are fats and oils,” Slade says.
The National Biodiesel Board estimates that the United States consumed about 3 billion gallons (11.3 billion liters) of biobased diesel fuel in 2020, and it believes that amount could double over the next eight years. This total includes both biodiesel and renewable diesel.
Looking more closely at renewable diesel, in July 2021, the U.S. Energy Information Administration (EIA) pegged U.S. production capacity at just over half a billion gallons per year (1.9 billion liters), or about 38,000 barrels per day. The EIA also noted that if all the proposed renewable diesel projects come to fruition and are added to those under construction, the total could increase nearly tenfold to 330,000 barrels per day, or about 5.1 billion gallons (19.3 billion liters) per year, by the end of 2024. This would represent approximately 8% of total U.S. diesel production capacity.
On the international level, consulting firm FutureBridge pegs current global renewable diesel production at about 1.4 billion gallons (5.5 billion liters) and forecasts strong growth, up to 3.4 billion gallons (13 billion liters), by 2024.
“Renewable diesel is a new product,” Slade points out. “It hasn’t been on the market in large volumes until maybe the last five years, whereas biodiesel has been out there for about 15 years in large volumes.”
So, who’s buying? Slade’s company sold a lot of biodiesel to petroleum majors during those first 10 years. They blended it with petroleum diesel and sold it to their downstream customers and to truck stops. In recent years, companies with large fleets have increased their biodiesel purchases.
“Package delivery services, big trucking and mining companies — anyone that uses a lot of fuel, and therefore has their own tankage and infrastructure,” Slade elaborates. “And then, because biodiesel at least has been normalized by the truck stops, a lot of it has been going into the smaller retail market as well. Not just the big truck stops, but the local retail chains.”
Fuels derived from biological feedstocks are becoming increasingly popular.
Incentivizing Biobased Diesel: Tax Incentives
Financial incentives may well be the most important factor driving the growth of biobased diesel fuels over the last 15 years. Three have been particularly impactful in the United States.
The first is the $1 USD/gallon biodiesel tax credit (BTC). Rolled out as part of the American Jobs Creation Act of 2004, the BTC has been amended periodically since then and currently extends through the end of this year. Credits are earned by companies that buy biodiesel or renewable diesel and blend it with petroleum diesel, helping to encourage the use of such blends. “That led to a lot of biodiesel usage back in the mid-2000s, the really early days,” Slade says.
The second major incentive is the renewable fuel standard (RFS). Unveiled in 2005 and expanded two years later, this U.S. Environmental Protection Agency (EPA) program applies to so-called “obligated parties” — refiners or importers of diesel fuel (and gasoline) — and is based on transactional credits called renewable identification numbers (RINs).
RINs are generated when a biobased fuel is produced. They can then be bought and sold through a system similar to carbon credits. “Obligated parties have to acquire enough RINs to cover their renewable volume obligation, or RVO,” Slade explains. “EPA formulas, recalculated every year, tell them their RVO based on the volume they produced or imported, and they have to get enough RINs to cover that obligation.” The RFS has fostered a robust marketplace for these credits.
A third key factor in increased biobased diesel production and use is California’s low carbon fuel standard, which was first implemented in 2011 and has been modified several times. According to the state’s Air Resources Board, the standard is designed to “decrease the carbon intensity of California’s transportation fuel pool and provide an increasing range of low-carbon and renewable alternatives.” The program’s structure is similar to the federal RFS — simply substitute “deficits” for RVOs.
Countries around the world are also incentivizing the use of biobased diesel. The European Union’s “Fit for 55” package includes a proposed doubling of the renewable energy target for transportation, from 14 to 28%, as well as a proposed blending mandate for aviation fuels. China has announced that it plans to “vigorously promote” biofuels, while India has an ambitious goal of reaching a 20% blending mandate in five years.
“B” is for “Blend”
ASTM standards are especially valuable to new product categories as they build their resumes, helping them gain the confidence of the marketplace. For biodiesel, two standards have been particularly important: the specification for biodiesel fuel blend stock (B100) for middle distillate fuels (D6751), which addresses the use of 100% biodiesel (referred to as B100) as a blend stock, and the specification for diesel fuel oil, biodiesel blend (B6 to B20) (D7467), which covers biodiesel blended with diesel fuel oil in amounts from 6 to 20%.
As technical director of the National Biodiesel Board and a longtime D02 member, Scott Fenwick is uniquely qualified to provide some history. “The first revision of ASTM D6751 for biodiesel as a blend stock with diesel fuel began as provisional standard PS121-99, the last provisional standard that ASTM published. Since then, D6751 has been revised 28 times over the last 22 years,” he says.
This specification establishes a number of properties that must be present, and limits that must not be exceeded, for a fuel to qualify as biodiesel. “It’s not, by definition at least, a fuel standard,” says Slade. “It’s a blendstock standard, and that means it defines this material so that if someone wants to blend it with petroleum fuel, they can.” He adds that people who use straight biodiesel often refer to D6751 for the quality properties, although it was not developed as a standalone fuel standard.
Turning to D7467, the main requirement is that the two components of a blended fuel — biodiesel and hydrocarbon fuel it is blended with — meet their respective individual standards prior to blending. “What that means is that anything you use to blend B6 to B20, under this standard it has to have met D6751, the biodiesel standard, prior to blending,” Slade notes. “And any hydrocarbon fuel that you use as part of the mixture has to have met the specification for diesel fuel, D975.”
Petroleum diesel is the hydrocarbon fuel most frequently blended with biodiesel. It’s interesting to note that small amounts of biodiesel (up to 5%) can be incorporated into diesel fuel without affecting its designation as a D975 fuel. When the portion of biodiesel rises above 5%, however, the relevant standard becomes D7467, the B6 to B20 specification.
So what about B21 and up? “Right now the only official ASTM standards other than D6751, which is a blendstock standard, go up to B20,” Slade states. “The next discussions are examining the best way to get standards out there for higher blends, and examining what that next blend level should be. Should it just go all the way to a B100 finished fuel spec? Should it be B50? Do you do it as part of D7467, which is currently the B6 to B20 spec, adding a new category for B21 to B100, or B21 to B50? Or does it need its own new standard entirely? I think, generally speaking, people would prefer to put things in existing standards as long as they fit.”
However this debate is resolved, flexibility will be a key element of any new or modified standard. “Everyone is trying to be very cognizant of the fact that in the future things could change, and we’re trying not to be limiting in the language as we create or adapt specifications,” Slade concludes.
Sustainable Aviation Fuel
Conventional jet fuel is a lighter distillate of petroleum; the fuel type is kerosene, rather than diesel. But the use of blending to introduce ingredients with lower carbon footprints — described earlier in the context of biobased diesel used for ground transport — is also growing in the aviation industry.
FOR YOU: Making Electric Aircraft a Reality
“An alternative to conventional petroleum-based fuel, sustainable aviation fuel is a low-carbon fuel that is derived from renewable sources or waste byproducts,” explains George Zombanakis, member of the subcommittee on aviation (D02.J0) and a master engineer with United Airlines. “SAF is a ‘drop-in’ fuel, meaning it can be safely used in existing aircraft and airport systems without changes to infrastructure. United’s SAF available today has approximately 80% lower greenhouse gas emissions on a life- cycle basis.”
United has bought approximately 1 million gallons (3.8 million liters) of SAF per year since 2016, according to Zombanakis. This figure represents a fraction of the airline’s typical annual consumption of 4 billion gallons (15.1 billion liters), and reflects limited supplies of sustainable fuel. However, more producers are expected to come online in the future, and the Sustainable Aviation Fuel Grand Challenge announced by the U.S. government last fall hopes to increase that supply to at least 3 billion gallons (11.3 billion liters) per year by 2030.
The specification for aviation turbine fuel containing synthesized hydrocarbons (D7566) has been vital to the emergence of sustainable aviation fuels as a viable option for airlines. It includes seven approved pathways, or annexes, that specify properties for synthetic fuels produced from biomass, fermentable sugars, and organic oils derived from algae (to name a few), which are ultimately blended with traditional petroleum-based fuels.
The specification for aviation turbine fuels (D1655) and the practice for evaluation of new aviation turbine fuels and fuel additives (D4054) are also important resources for airlines that are trying to implement greener alternatives. Fenwick points out that D1655 encompasses parameters and limits for traditional aviation fuels, blended fuels, and even those mixtures in which the biomass feedstocks are processed simultaneously with petroleum during production.
Zombanakis applauds subcommittee D02.J0 for its work on D4054. “The ‘Fast Track’ process in D4054 will help the industry and the production companies tremendously,” he says, noting that additional SAF fuel blends are already in the pipeline for evaluation. He is also confident that existing specification standards for SAF production will be amended as necessary to reflect new technological developments.
Scott Fenwick sums up. “While the criteria for the ‘sustainability’ of fuels is primarily left to other bodies, ASTM continues to research and develop the standards necessary on new and emerging liquid fuels. All of these specifications help to ensure performance in their intended applications and the ability to reduce greenhouse gas emissions to address climate change.” ■