EFFECT OF A POLYCYCLIC AROMATIC HYDROCARBON–RICH FRACTION ON THE LUBRICITY PROPERTIES OF GTL DIESEL FUEL

ABSTRACT

GTL (gas-to-liquids) diesel fuels are characterized by high cetane numbers and low emission levels; however, their lubricity is low due to the almost complete absence of aromatic compounds. In this study, a new lubricity additive based on an aromatic–PAH (polycyclic aromatic hydrocarbons)–rich fraction was investigated for GTL diesel fuel. GTL diesel samples containing 0.5–1.0 vol% of the additive were prepared. Lubricity was evaluated using the HFRR method according to ISO 12156. The results showed that the addition of the additive significantly reduced the HFRR wear scar and achieved compliance with EN 590 requirements.

АННОТАЦИЯ

Дизельные топлива GTL (gas-to-liquids) характеризуются высоким цетановым числом и низким уровнем выбросов; однако их смазочные свойства низкие из-за практически полного отсутствия ароматических соединений. В этом исследовании был изучен новый присадочный компонент для улучшения смазочности, основанный на фракции, богатой ароматическими полициклическими углеводородами (PAH), для дизельного топлива GTL. Были подготовлены образцы GTL-дизеля с содержанием присадки 0,5–1,0 об.%. Смазочные свойства оценивались методом HFRR согласно ISO 12156. Результаты показали, что добавление присадки значительно уменьшало износ по HFRR и обеспечивало соответствие требованиям EN 590.

Keywords: GTL diesel; lubricity additive; aromatic hydrocarbons; polycyclic aromatic hydrocarbons; HFRR; EN 590

Ключевые слова: GTL-дизель; смазывающая присадка; ароматические углеводороды; полициклические ароматические углеводороды; HFRR; EN 590

Introduction

GTL diesel fuels are produced from natural gas via the Fischer–Tropsch synthesis and are characterized by a high cetane number, nearly zero sulfur content, and a very low fraction of aromatic compounds. These features make GTL diesel fuel environmentally friendly and a promising option for modern diesel engines [1]. However, the lack of aromatic and polar components represents a major drawback of GTL diesel fuels, resulting in low lubricity.

In modern high-pressure fuel systems (common-rail), sufficient lubricity of diesel fuel is crucial for reducing friction, preventing component wear, and extending the service life of the system. Therefore, strict lubricity requirements for diesel fuels are established in the EN 590 standard, and they are evaluated using the HFRR method according to ISO 12156 [2].

In practice, biodiesel (FAME) and ester-based additives are used to improve the lubricity of GTL diesel fuels. However, these components can reduce fuel oxidation stability, negatively affect cold-flow properties, and, when used in high concentrations, decrease the high cetane number of GTL fuel [3]. Therefore, the search for alternative and functional additives remains a relevant task.

Recent studies have shown that certain aromatic hydrocarbons and PAHs, when used at low and controlled concentrations, can activate the boundary lubrication mechanism and form a protective layer on metal surfaces [4]. These properties make them promising candidates as potential lubricity additives.

The aim of this study was to evaluate an aromatic- and PAH-rich fraction as a new lubricity additive for GTL synthetic diesel fuel. During the study, mixtures of GTL diesel with varying proportions of the additive were prepared, and their lubricity properties were determined using the HFRR method according to ISO 12156. The obtained results serve to assess the effectiveness of the aromatic–PAH–based additive in GTL diesel fuel.

Materials and methods

Additive preparation. The aromatic–PAH-rich fraction used as the additive was obtained from the processing of waste “Yellow Oil” [5–6] and was used without any additional chemical modification.

Preparation of test samples. The additive was added to GTL diesel in the range of 0.5–1.0 vol% (Table 1). The samples were prepared as follows: first, the components were brought to a temperature of 20–25 °C, measured with a precision of ±0.01 vol%, and then placed into glass containers.

Table 1.

Types of samples prepared from GTL diesel and the additive

The samples were mixed using a mechanical stirrer for 15 minutes. Subsequently, a filtration process was carried out. All samples were filtered through a new filter individually, maintaining a flow rate of 20 ml/min. The filtered samples were stored in glass containers at 20–25 °C in a dark place. Pure GTL diesel was taken as the control sample, and all prepared samples were used for subsequent physicochemical and lubricity tests.

Evaluation of sample properties. The lubricity of the samples was evaluated using the HFRR method according to ISO 12156. Additionally, density was determined according to EN ISO 3675/12185, cetane number according to EN ISO 5165, cetane index according to ISO 4264, and aromatics and PAHs were analyzed by the GC-MS method. The obtained results are presented in Table 2.

Table 2.

Effect of additive concentration on the properties of GTL diesel fuel

Results and discussions

In Table 3, the effect of additive concentration (from S-0.5 to N–1.0) on the properties of GTL diesel fuel was investigated. The results showed that increasing the additive content led to significant changes in the main operational and environmental characteristics of the fuel.

Although the cetane number decreased from 74.0 to 70.0 with increasing additive concentration, all samples remained well above the EN 590 standard requirement (≥51.0). Similarly, the cetane index showed a decreasing trend, dropping from 78.0 to 73.0, yet fully complied with the regulatory requirements. This indicates that the addition of the additive has a certain effect on the combustion properties, but does not negatively impact engine performance.

The fuel density at 15 °C increased from 778 kg/m³ to 796 kg/m³ with increasing additive content. The results indicate that the density approaches the EN 590 standard range (820–845 kg/m³), which contributes to higher energy content and improved combustion efficiency of the fuel.

Lubricity (HFRR), one of the important operational parameters, significantly improved with increasing additive content: the value of 650 µm for pure GTL diesel reached the standard level starting from sample S–0.7 and decreased to 390 µm at S–1.0. This indicates that the addition of the additive effectively compensates for the low lubricity, which is one of the main drawbacks of GTL diesel.

Conclusion

In this study, the effect of additive concentration on the quality characteristics of GTL diesel fuel was investigated comprehensively. The results showed that increasing the additive content led to consistent and systematic changes in the physicochemical and operational properties of the fuel.

During the study, a slight decrease in cetane number and cetane index was observed; however, in all samples, these values remained well above the EN 590 standard requirements. Increasing the additive concentration also led to an improvement in fuel density within the standard limits.

One of the key findings is that the addition of the additive effectively addressed the low lubricity issue of GTL diesel fuel. At concentrations of S–0.7 and above, the HFRR values fully met the EN 590 standard requirements, significantly reducing the impact of the fuel on engine components.

Overall, the results of the study confirm that selecting an additive concentration in the range of 0.7–0.9 % optimally balances the operational, environmental, and technical properties of GTL diesel fuel, thereby expanding its practical applicability as a diesel fuel for diesel-engine vehicles.

References:

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