Renewable diesel is the trade name for hydrotreated vegetable oil, which may also go by the monikers biomass diesel, biomass-based diesel, hydrogenated vegetable oil, and HVO. Renewable diesel is produced from the same feedstocks as biodiesel, any combination of vegetable oils and animal fats, but the two fuels are fundamentally distinct from one another due to the unique chemical compositions that result from their respective production processes.
At the molecular level HVO is a hydrocarbon in the same manner that petroleum diesel fuel is. In fact, HVO is comprised of a very similar assortment of hydrocarbon chains as petroleum diesel. It is produced from a hydrotreating process, which requires hydrogen gas to be introduced and absorbed by the feedstock. Biodiesel is comprised of fatty methyl esters (FAME), not hydrocarbons. It is produced from a chemical reaction called transesterification, where a catalyst and an alcohol are added to the feedstock to separate the fatty acid chains and glycerol molecules from a triglyceride. The glycerol and impurities are then removed, leaving the biodiesel.
While both renewable diesel and biodiesel can be used to replace or supplement (as a blending agent) petroleum based diesel fuel, they have unique properties. Whereas biodiesel is riddled with significant challenges in its pure, unblended form, renewable diesel is a suitable drop-in replacement for petroleum diesel in any concentration. Renewable diesel does not face the same hardships with regard to storage and fuel system compatibility, thus it is significantly more suitable to use in its pure form.
Renewable diesel can be blended with petroleum diesel fuel at any ratio, but it is most commonly available in blends that are 95% or more renewable diesel. The correct nomenclature for renewable diesel blends is the letter "R" followed by the percentage of HVO in the fuel blend by volume. "R95" refers to a 95% HVO, 5% petroleum diesel blend, "R99" refers to a 99% HVO, 1% petroleum diesel blend, and so on. The term R100 is used to denote pure, unblended renewable diesel.
California is unsurprisingly the largest consumer of renewable diesel, although it is becoming increasingly common amongst the Western chain of States. California's Low Carbon Fuel Standard provides incentives for fuel producers and large consumers to use renewable diesel. California also recognizes it as a fuel that results in lower particulate matter, NOx, and CO emissions. Within the State, it is commonly available in R95 and R99 blends or as R100. An R80, B20 blend is also becoming more prominent, but biodiesel and renewable diesel are not excellent companions in fuel blends.
How Renewable Diesel is Made
HVO can be produced from various bio-chemical and thermo-chemical strategies, but hydrotreating is the most common method employed by commercial producers. It requires reacting the feedstock with hydrogen gas. Hydrotreating is a well established technology that refiners also use to remove sulfur and other impurities from petroleum diesel fuel.
Noting that certain steps involved in the production of renewable diesel may be proprietary, it begins with a pretreatment of the feedstock to remove impurities and contaminants that could impede the chemical reaction. Next, a catalyst is added and hydrogen gas is infused into the feedstock at elevated pressure and temperature. As the reaction takes place, oxygen is removed from the triglyceride and it is separated into three hydrocarbon chains.
Simultaneously, the glycerol component is hydrogenated into C3 hydrocarbons (propane). Additional byproducts include water, carbon dioxide, bio-mass jet fuel, and bio-mass gasoline, but feedstock choice, catalyst selection, and processing conditions play a critical role in the reaction products. Note that in this process the glycerol component of the oil/fat molecule is not treated as discard as it is in the production of FAME.
The resulting C8 to C25 hydrocarbons are very similar to those found in petroleum diesel. As such, HVO must adhere to the same fuel standards (ASTM D975) and specifications as petroleum diesel. Because FAME is chemically different than these hydrocarbon fuels, it has its own set of standards (ASTM D6751), which define separate performance expectations and impurity limits.
Feedstocks
Like FAME, HVO can be produced from any combination of vegetable oils and animal fats, but soybean oil remains the predominant constituent. These are all triglycerides - a glycerol molecule with three fatty acid chain branches. Converting the feedstock into a usable fuel requires modifying the molecular structure. In biodiesel, the glycerol is removed and discarded. In renewable diesel, the glycerol component is converted into propane.
Feedstock selection in the production of renewable diesel is of a lesser concern than it is in the production of biodiesel because there is significantly more control in the hydrotreating process than there is in transesterification. The resulting HVO can undergo additional treatments, as necessary, to better control the properties of the final fuel products. Feedstock blends are therefore more likely to change the processing conditions than the properties of the finished fuel.
For this reason, animal fats are more readily transformed into HVO than they are FAME. Biodiesel that is derived from animal fats or feedstock blends that have a high ratio of animal fats yield a fuel with a very high cloud point, rendering it unusable at cooler temperatures. HVO, on the other hand, can undergo addition chemical transformations to lower the cloud point of the final fuel and make it suitable for use in cold weather.
Properties of Renewable Diesel
Renewable diesel is clear and transparent with little to no aroma and near zero aromatic content. It has an excellent cetane rating, generally in excess of 70. It resists oxidation and is suitable for long term storage provided the conditions are satisfactory and there is no FAME content. Research suggests that renewable diesel tailpipe emissions have lower NOx, carbon monoxide, gaseous hydrocarbon, and particulate matter concentrations. The actual reductions in emissions products can vary depending on feedstock and processing conditions.
HVO has a lower energy content by volume but a higher energy content by weight than petroleum diesel. The U.S. Department of Energy claims that (on average) one gallon of R100 contains the same energy as 0.96 gallons of petroleum diesel fuel, or a 4% lower energy density by volume. Its energy content, however, is significantly closer to that of petroleum diesel than biodiesel, which is on the order of 7% less energy dense in its pure form.
Renewable diesel displays very poor lubricity in its pure, "as refined" form and an additive package is required to bring the fuel up to ASTM standards. In fairness, ultra low sulfur diesel fuel does not generally meet ASTM criteria in this category and must also be treated with additives in order to meet minimum lubricity requirements. This is what makes biodiesel, an excellent lubricant, an attractive, low cost substitute to commercial fuel treatments. The lower energy content of HVO fuels generally results in negligible reductions in performance and fuel economy.
Advantages
- Excellent oxidation stability, can be stored for long periods of time without degradation
- Extremely high cetane rating, significantly greater than conventional diesel or biodiesel
- Similar material compatibility as conventional petroleum diesel fuel
- No special storage and transportation considerations
- Drop-in replacement for petroleum diesel fuel, compatible with all modern emissions equipment
- Low risk of water contamination, does not dissolve water like FAME, water separates similarly to petroleum diesel fuel
- Low NOx, hydrocarbon, particulate matter, and carbon monoxide emissions
Disadvantages
- Higher cost than petroleum diesel fuel; the production and refining process is resource intensive
- Probable compatibility concerns when blended with biodiesel
- Possible effects of feedstock production and land-use-changes on agricultural markets
Labeling Requirements & Confusion
Renewable diesel must be labeled at the pumping station if the fuel contains 5% or more HVO by volume. The labeling requirements overlap with those governing biodiesel. Federal law requires that the blending volume be identified and the label contains the statement "contains biomass-based diesel or biodiesel in quantities between 5 percent and 20 percent" or "contains more than 20 percent biomass-based diesel or biodiesel", depending on the blend.
These labeling requirements fall short of distinguishing between renewable diesel and biodiesel. HVO and FAME both fall into the category of "biomass-based" fuels because they are derivatives of the same feedstocks, but they also have very different properties. In practice, labels could hide actual biodiesel content due to the "biomass-based" umbrella. Mislabeling and labeling confusion is common in California where renewable diesel is widely used.
Controversies
Most of the challenges and controversies renewable diesel fuel are related to feedstock production and acquisition. These controversies are no different for biodiesel, or any other "biofuel" for that matter. Biofuel production has the opportunity, if not the propensity to displace ecosystems and disrupt agricultural markets. Large scale adoption of biofuels, such as that we're seeing in the State of California, could contribute to deforestation, increased food prices, and a disruption in industries that rely on petroleum production.
The "renewable" moniker in HVO fuels is also the subject of controversy as it may be categorized as misleading in a similar manner that the phrases "carbon neutral", "environmentally friendly", and "zero emissions" are often challenged. Furthermore, the models used in life-cycle greenhouse gas emission calculations may make generalized assumptions with regard to otherwise uncontrollable or immeasurable variables and take an opportunistic approach intended to support agenda initiatives.
While renewable diesel is required to be labeled in a similar manner as biodiesel, it creates a potential umbrella that allows retailers to hide the FAME content of renewable diesel blends. Producers may utilize biodiesel as a blending component to increase the lubricity of HVO fuels, but even small concentrations of biodiesel may carry serious concerns as there are complications with the blending of FAME and HVO.
