The increased use of green diesel to combat climate change and the depletion of fossil fuels, is creating more intensive investigation into cost-effective catalysts such as nickel-alumina playing a key part in the process.

The vast majority of renewable fuels today are either ethanol (corn-based in the US and sugar-based in Brazil) or biodiesel, which is made by transesterification of vegetable oils. Both ethanol and biodiesel suffer from blending constraints limiting their use as well as lower energy content compared to either gasoline or petroleum-derived diesel. Renewable diesel (RD) is made by hydroprocessing vegetable oil, and waste fats and greases, in a process very similar to those used to make diesel in a conventional refinery. Renewable diesel’s close chemical relative, sustainable aviation fuel (SAF), can be made by different processes, including hydrotreating vegetable oils, if the oil feedstock has the appropriate chemical composition. Renewable diesel meets all the conventional specifications for diesel fuels, so it can be used without blending, unlike biodiesel.

Lower carbon intensity

Different oils are used in making renewable diesel, most commonly rapeseed, soybean, and palm oil. Beef tallow and waste greases can also be used. Due to concerns over replacement of food sources for fuel, non-traditional oils like camelina and jatropha are also considered. Depending on the source and the exact processing methods, its carbon intensity varies, but renewable diesels are considered to have about 30% of the carbon intensity of petroleum-based diesel.

While the main driver for renewable diesel is to lower CO2 emissions, there are other benefits.

Renewable diesel typically has only about 1 ppm sulphur, compared to 10–15 ppm (depending on the more common regional fuel standards), so it can be used to lower the sulphur content of petroleum diesel by blending. The cetane number of renewable diesel ranges from 70–90, compared to a minimum cetane value of 40 in the US and 49 in Europe. Higher cetane fuels burn cleaner, making regeneration of soot filters easier and less frequent, saving fuel and reducing maintenance costs. Renewable diesel is made by first pre-treating oils to remove metallic contaminants that may be present and acid from rancidification of oils. The oil is then sent to a hydrotreater (similar to petroleum) to break the triglyceride and remove oxygen. Hydrogenation of unsaturated bonds also occurs as well as some cracking of larger molecules to smaller molecules. Finally, the hydrotreated oil is isomerised to improve the cold flow properties of the diesel to meet fuel specifications (Figure 1). The main products from this process are renewable diesel and propane, with heavier fractions becoming sustainable aviation fuel (SAF) and lighter fractions converted to renewable gasoline (Figure 2).

Nickel catalysts

The catalysts used for hydrotreating vegetable oil are Ni or NiMo supported on alumina. Ni provides good functionality for saturating double bonds as well as promoting the activity for the removal of oxygen. Ni impregnated in zeolites and other molecular sieves are also used in the isomerisation step, replacing more expensive catalysts based on platinum or palladium.

Dynamic growth

According to IEA, global renewable diesel production in 2021 was only 170,000 barrels/day but is forecasted to grow to between 420,000 and 600,000 barrels/day by 2027. Similarly, SAF production was only 2,500 barrels/day in 2021 but expected to grow to 1–2% of global demand by 2027, equivalent to 75,000–150,000 barrels/day. Governments have mandated the use of renewable fuels, so as the demand for renewable fuels increases so will the demand for nickel catalysts, which are an indispensable part of renewable fuel technologies.