Role of Biodiesel in the Enhancement of GTL Diesel Fuel Properties
Authors: Elbashir Nimir , Ahmed Wajdi Jamal , Alrachid Ahmad , Mohammed Nasr , Elwahsh Marwan 
Published: July 2019

The Conversion of natural gas into synthetic liquid fuel carried out through a catalytic conversion process, known as the gas-to-liquid (GTL) technology, had a great impact since its discovery. The process utilizes the Fischer-Tropsch synthesis (FTS) chemistry that was first developed in 1925. Gas-to-liquid (GTL) fuels are synthetic fuels that are intrinsically cleaner than conventional fuels.[1] They are environmentally benign due to the absence of sulfur content and the extremely low aromatic content. Gas-to-liquid (GTL) diesel fuel, being one of the most important GTL products, is therefore classified as ultra-clean fuel with lower emissions of carbon monoxide and particulates upon combustion. Nevertheless, the lack of sulfur and aromatic in the diesel fuels negatively impacts certain important physical characteristics such as lubricity and density. This fact makes these fuels improper for use in existing diesel engines. The lubricity issue is significant for GTL diesel fuels, as it not match the standards required by regulations in the United States, Europe and elsewhere. Lubricity is defined as “a qualitative term describing the ability of a fluid to affect friction between, and wear to, surfaces in relative motion under load.” [2]

The aim of this research project is to design new generation ultra clean diesel fuels, from the GTL process, that are environmentally friendly, have less emissions upon combustion but also practicable and compliant with global standards. The focus of this study is to develop a comprehensive knowledge and data to observe the possibility of stretching and tailoring the GTL diesel properties such that they meet the standards needed to be used in existing engines. The enhancement of lubricity, which serves as the major disadvantage, is to be carried out while maintaining all other physical properties within the range needed for existing engines and ASTM D975 and D7467 standards.

Fatty acid methyl and ethyl esters are biodiesels that can provide significant improvements in the lubricity of diesel fuel. Previous studies on the effects of fatty acid methyl esters on diesel fuel lubricity have shown an increase in lubricity associated with the addition of these esters [3]. Addition of these esters in GTL diesel was found to be the best fit for lubricity enhancement. Our campaign, unlike other studies on diesel lubricity, does not only focus on certain physical properties of diesel but provide a comprehensive study showing all changes in other properties that accompany the change of the targeted property, being diesel in this case.

For this study, four lubricity enhancement additives were selected. The selection covers a wide range of biodiesel characteristics and would thus allow deeper understanding of other factors such as saturation and chain length of esters on diesel lubricity and properties enhancement. The four esters that proved to have significant impact are Methyl Oleate (C19H36O2), Methyl Stearate (C19H38O2), Ethyl Oleate (C20H38O2) and Ethyl Stearate (C20H40O2). Both methyl and ethyl esters were found to affect the lubricity. However, other studies showed variation on which serves as a better lubricity enhancement additive. Our research will validate these findings and widen the study to experiment the effect of mixing different additives on all other diesel fuel properties. The concentrations of additives were selected to be in the range of 5%–20% by volume. For this purpose 28 blends were prepared for conducting the research.

Subsequently blends were tested for fuel characteristics and physical properties. The tests nclude vapor pressure, viscosity, flash point, pour point, cloud point, distillation performance, heat content and lubricity. Current results show an increase in density, viscosity, flash point and vapor pressure with increasing additive concentrations of biodiesel. In contrast, the increase in additive concentration decreased the pour point, cloud point, recovery by distillation and calculated cetane number. All the changes lie well within American Society of Testing and Materials (ASTM) standards.

The work will be further continued to validate experimental results with industry partners, testing of lubricity characteristics using advanced microscopy and conducting a thorough result analysis and representation statistical methods and visualization techniques.

This work will suggest ideal mixtures of GTL diesel and biodiesel additives that will best enhance the GTL diesel fuel. Such findings can be used in blending commercial GTL diesel fuel with suggested biodiesel to produce practicable, engine compliant and ASTM standard compliant GTL diesel. Since Qatar has the largest GTL plant in the world, this research is essentially significant and relevant. It also enriches Qatar's scientific research culture and serves the world by creating cleaner fuels for a cleaner environment.

References

[1] Elmalik, E.E., Raza, B., Warrag, S., Ramadhan, H., Alborzi, E., Elbashir, N.O. (2014). Role of Hydrocarbon Building Blocks on Gas-to-Liquid Derived Synthetic Jet Fuel Characteristics. Industrial & Engineering Chemistry Research, 53, 1856–1865.

[2] Crown, L., Warfield, L., Hockert, C. (2013). Handbook 130 (2013)- Uniform Laws and Regulations in the areas of legal metrology and engine fuel quality.

[3] Geller, D. P., & Goodrum, J. W. (2004). Effects of specific fatty acid methyl esters on diesel fuel lubricity. Fuel, 83(17-18), 2351–2356.