LUBRICITY CHALLENGES OF RENEWABLE DIESEL FUELS


The development of renewable sources of fuel has been pivotal in the reduction of greenhouse gas emissions, as the demand for cleaner fuels continues to grow. With the founding and creation of renewable diesel, processes performed using diesel now have a suitable substitute that not only exceeds the performance of traditional petrol diesel but is also cost-effi cient. One highly effective and reproducible method of producing renewable diesel involves hydrotreating biomass-derived materials such as vegetable oils [1]. The process of hydrotreating involves the removal of oxygen and other heteroatoms, such as sulfur, by selectively reacting these less desirable materials with hydrogen in a reactor at relatively high temperatures and pressures. [2] A schematic of the hydrotreatment process is shown in Figure 1.


globally. A strict regulation was placed to keep sulfur content at a low 15 ppm, according to EPA regulations [7]. While sulfur is a pertinent lubricating agent in petroleum products, regulations have prompted the removal of most of the sulfur in refi nery processes, resulting in a loss of fuel lubricity. Due to these lubricity challenges, there is a need for continued research on how renewable diesel can be improved to replace the traditional petroleum diesel.


Figure 1. Hydrotreating Process in the Production of a Renewable Diesel [6]


One drawback of this process is the poor lubricity of newly composed renewable diesel. Considering that sulfur acts as a lubricant in fuel, the low sulfur content in renewable diesel will lead to low lubricity. Additionally, the oxygen-containing components removed during hydrotreating have been studied and proven to signifi cantly reduce wear and improve lubricity to acceptable levels [3]. An effective and reproducible way of measuring the lubricity of diesel fuel is described in the test


method ASTM D6079 [4]. By using a High Frequency Reciprocating Rig (HFRR) we can ensure a diesel fuel’s lubricity is within the requirements as per the Standard Specifi cation for Diesel Fuel Oils in ASTM D975 (<520 µm) [5]. The HFRR instrument and the ASTM D6079 test method involves rubbing a metal ball in an oscillating motion against a platform metal disk under known conditions while fully immersed in the sample heated to 60o


C. The output


value from HFRR testing is the wear scar diameter, measured in microns. The wear scar diameter is observed and measured after a test by looking at the ball under a microscope or digital camera and averaging the width and the length of the small blemish formed during testing. An unadditized renewable diesel sample typically has an HFRR wear scar diameter over 700 µm, which is far above the permissible level in any of the diesel fuel specifi cations, typically 450 to 520 µm. Therefore, lubricity improver additives are commonly used with renewable diesel [1].


The HFRR instrument (Figure 2) and the ASTM D6079 test method involves rubbing a metal ball in an oscillating motion against a platform metal disk under known conditions while fully immersed in the sample heated to 60 degrees C.


The lubricity of a fl uid is often defi ned as the fl uid’s ability to reduce friction between that fl uid and the solid surface during motion. Lubricity is a key fuel property due to the potential to increase the longevity of a part as well as ensuring maximum performance of the system. When a fuel’s lubricity value does not conform to regulations, metal parts are likely exposed to each other, resulting in wear or scarring. In the late 2000s, the lubricity of fuels became a controversial topic due to the increased gas emissions. The high sulfur content in petroleum fuels has been identifi ed as a cause for harmful exhaust emissions, which has led to strict regulations on the allotted sulfur content in diesel fuels


The growth in production and usage of renewable diesel shouldn’t be a surprise as renewable diesel’s composition is shockingly similar to traditional crude oil-derived diesel. When atoms such as sulfur, nitrogen, and oxygen are removed during the hydrotreating process, the triglyceride molecules from the base oil are converted into paraffi nic hydrocarbons (alkanes) [8]. Traditional petrodiesel contains a combination of hydrocarbons (predominately paraffi ns) with fewer cycloalkanes and aromatic hydrocarbons. The n-paraffi n molecular chain is the base of both fuel types. As shown in Figure 3, renewable diesel maintains and, in some cases, exceeds the performance of traditional diesel fuel.


However, the lubricity of renewable diesel is one property that is negatively affected during its production, which is often less than the ASTM diesel fuel specifi cation of 520 µm max wsd. One paper from 2014 illustrates the differences in lubricity between renewable diesel in the form of hydrotreated vegetable oil (HVO)


Figure 3. Properties of Traditional Diesel and Renewable Diesel [9]


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