Hydrotreated vegetable oil. Hydrotreated vegetable oil (HVO) is a biofuel made by the hydrocracking or hydrogenation of vegetable oil. Hydrocracking breaks big molecules into smaller ones using hydrogen while hydrogenation adds hydrogen to molecules. These methods can be used to create substitutes for gasoline, diesel, propane, kerosene and other chemical feedstock. Diesel fuel produced from these sources is known as green diesel or renewable diesel.

Diesel fuel created by hydrotreating is distinct from the biodiesel made through esterification.

Feedstock
The majority of plant and animal oils are triglycerides, suitable for refining. Refinery feedstock includes canola, algae, jatropha, salicornia, palm oil, tallow and soybeans. One type of algae, Botryococcus braunii produces a different type of oil, known as a triterpene, which is transformed into alkanes by a different process.

Chemical analysis

Synthesis 
The production of hydrotreated vegetable oils is based on introducing hydrogen molecules into the raw fat or oil molecule. This process is associated with the reduction of the carbon compound. When hydrogen is used to react with triglycerides, different types of reactions can occur, and different resultant products are combined. The second step of the process involves converting the triglycerides/fatty acids to hydrocarbons by hydrodeoxygenation (removing oxygen as water) and/or decarboxylation (removing oxygen as carbon dioxide).

A formulaic example of this is  + 12  ⟶  + 3  + 6

Chemical composition 
The chemical formula for HVO Diesel is CnH2n+2

Chemical properties 
Hydrotreated oils are characterized by very good low temperature properties. The cloud point also occurs below −40 °C. Therefore, these fuels are suitable for the preparation of premium fuel with a high cetane number and excellent low temperature properties. The cold filter plugging point (CFPP) virtually corresponds to the cloud point value, which is why the value of the cloud point is significant in the case of hydrotreated oils.

Comparison to biodiesel 
Both HVO diesel (green diesel) and biodiesel are made from the same vegetable oil feedstock. However the processing technologies and chemical makeup of the two fuels differ. The chemical reaction commonly used to produce biodiesel is known as transesterification.

The production of biodiesel also makes glycerol, but the production of HVO does not.

Neste has published the differences between biodiesel and renewable diesel (HVO) which are summarized in the table below.

Commercialization
Various stages of converting renewable hydrocarbon fuels produced by hydrotreating is done throughout energy industry. Some commercial examples of vegetable oil refining are:

 Neste NExBTL
 Topsoe HydroFlex technology
 Axens Vegan technology
 H-Bio, the ConocoPhilips process
 UOP/Eni Ecofining process. 

Neste is the largest manufacturer, producing ca. 3.3 million tonnes annually (2023). Neste completed their first NExBTL plant in the summer 2007 and the second one in 2009. Petrobras planned to use  of vegetable oils in the production of H-Bio fuel in 2007. ConocoPhilips is processing  of vegetable oil. Other companies working on the commercialization and industrialization of renewable hydrocarbons and biofuels include Neste, REG Synthetic Fuels, LLC, ENI, UPM Biofuels, Diamond Green Diesel partnered with countries across the globe. Manufacturers of these renewable diesels report greenhouse gas emissions reductions of 40-90% compared to fossil diesel, as well as better cold-flow properties to work in colder climates. In addition, all of these green diesels can be introduced into any diesel engine or infrastructure without many mechanical modifications at any ratio with petroleum-based diesels.

Renewable diesel from vegetable oil is a growing substitute for petroleum. California fleets used over  of renewable diesel in 2017. The California Air Resources Board predicts that over  of fuel will be consumed in the state under its Low Carbon Fuel Standard requirements in the next ten years. Fleets operating on Renewable Diesel from various refiners and feedstocks are reported to see lower emissions, reduced maintenance costs, and nearly identical experience when driving with this fuel.

Sustainability concerns 

A number of issues have been raised about the sustainability of HVO, primarily concerning the sourcing of its lipid feedstocks. Waste oils such as used cooking oil are a limited resource and their use cannot be scaled up beyond a certain point. Further demand for HVO would have to be met with crop-based virgin vegetable oils, but the diversion of vegetable oils from the food market into the biofuels sector has been linked to increased global food prices, and to global agricultural expansion and intensification. This is associated with a variety of ecological and environmental implications;  moreover, greenhouse gas emissions from land use change may in some circumstances negate or exceed any benefit from the displacement of fossil fuels.

A 2022 study published by the International Council on Clean Transportation found that the anticipated scale-up of renewable diesel capacity in the U.S. would quickly exhaust the available supply of waste and residual oils, and increasingly rely on domestic and imported soy oil. The report also noted that increased U.S. renewable diesel production risked indirectly driving the expansion of palm oil cultivation in Southeast Asia, where the palm oil industry is still endemically associated with deforestation and peat destruction.

Challenges in Producing Fuels from Bio-Derived Feedstocks with HVO 
Refinery hydrotreaters are used for processing HVO. Introducing even minor amounts of biomaterial into a diesel hydrotreater has implications and potential risk factors. The main issues are corrosion, high hydrogen consumption, and catalyst deactivation.

According to Haldor Topsoe's experience with their licensed units, HVO production poses certain challenges for hydrotreaters including:

Corrosion - There are several corrosion mechanisms from hydrotreating vegetable oils and animal fats. Most are acidic though this is tempered by being bound into tri and di-glycerides. However, difficult feedstocks like Distillers Corn Oil can contain 10-15% free fatty acids. These acids can attack non-stainless steels in the preheat train, fired heater, piping, valves and reactors. In addition, chlorides that contaminate feeds can be converted to hydrogen chloride in the reactor which can then cause accelerated corrosion in the effluent lines and for sour water. The presence of chlorides in a wet environment is also problematic for the common stainless steel grades 304 and 316 due to the potential of intergranular stress chloride cracking. In addition, the formation of carbon dioxide from decarboxylation reactions during hydrotreating can form carbonic acid when contacted with water. 

Hydrogen Consumption - removing oxygen, cracking long-chain molecules, and saturating olefinic bonds will chemically consume two to four times the hydrogen of a conventional ULSD hydrotreater. ULSD hydrotreating chemical hydrogen consumption is typically 300-600 scf/bbl of feed depending on the aromatic saturation required for cycle oils and other cracked feedstocks. Chemical consumption for HVO approaches 2,500 scf/bbl depending on the level of saturation of feedstock and the length of the carbon chains. Delivering hydrogen for consumption, in addition to quench and additional excess circulating hydrogen can pose significant challenges to unit revamp design and operation with hydraulics, distribution, and compressor power being critical. 

Fouling - alkali metals and especially phosphorus must be kept low in HVO feedstocks in order to minimize pressure drop from fouling and general catalyst deactivation. Phosphatidic glassing is an aggressive catalyst poisoning mechanism that will not only plug off a reactor's pore spaces thus causing rapid pressure drop, but will also interfere with the catalysts acid sites by coating the outside of the catalyst and begin adhering to other catalyst particles.

Operational history 
HVO processing is a young technology relative to most other refining processes. The first commercial scale unit started up in Louisiana in 2010 with a capacity of  per year.  

A newbuild plant was constructed in 2010 Geismar, LA by the Syntroleum Corporation and its joint-venture partner Tyson Foods. The plant initiated startup in the 3rd quarter with a target of  per year.  Feedstock for the plant was vegetable oil and pretreated rendered poultry fat. The site achieved 87% of its design capacity in 2011. Corrosion, including chloride-linked stress corrosion cracking shut the plant down in 2012 for more than a year.  Tyson sold its 50% ownership to Renewable Energy Group (now Chevron) and Syntroleum's stock was announced by the same buyer in 2013 with closing in 2014. In 2015, two fires caused damage to the plant with major damage being incurred.

Operable capacity

See also 
 Biodiesel
 Indirect land use change impacts of biofuels
 Algae fuel
 Renewable hydrocarbon fuels via decarboxylation/decarbonylation
 Sustainable oils
 Vegetable oil fuel

References

External links
 University Of Wisconsin / College Of Engineering (June 6, 2005). Green Diesel: New Process Makes Liquid Transportation Fuel From Plants. ScienceDaily. Retrieved August 10, 2010
 Renewable Diesel Primer. Retrieved August 10, 2010

Oil refining
Synthetic fuel technologies
Renewable energy technology
Biofuels
Renewable fuels