1. Introduction
In the recent past, several users of fuel oils have been experiencing problems such as blockages in fuel oil filters and injector nozzles, increased wear and failures of pumps and, in some cases, decreased calorific efficiency of combustion. These problems appear to be linked to the chemical and physical properties of the components of the fuel oil. The most prominent problematic part of the composition appears to be the presence of asphaltenes [
1].
Asphaltenes are components of heavy fuel oil with complex aromatic structures containing heteroatoms (N, O, S) and metals (V, Fe, Ni) [
2,
3]. They are major precursors of sludge or sediments and are known to contribute to coke and sludge formation in refineries in addition to catalyst deactivation during processing [
4], ([
5] p. 1). Precipitation and deposition of asphaltenes occur during normal refining operations which means that it is hardly avoidable (e.g., during the production of heavy fuel oil, lighter hydrocarbons or paraffinic oils are used as diluent to reduce the viscosity of crude oil). The use of lighter hydrocarbons or paraffinic oils can trigger asphaltene precipitation in tubes, pipelines and on exposed surfaces, as asphaltenes dissolve in light aromatic hydrocarbons and precipitate in low carbon number n-alkane solvents [
4,
6]. These problems and their causes need to be confirmed and addressed in a way that will help to improve current fuel oil requirements for industrial use.
The aim of this investigation was to determine the lubricating properties of fuel oils by focusing on their friction and wear capabilities taking into account the presence of particulate matter and of asphaltenes. The objective of this investigation was therefore to perform friction and wear tests on the filtered and unfiltered fuel oils at different temperatures using a HFRR tribometer, which has become an important industry standard test for diesel fuel lubricity ([
7] p. 914), [
8].
Severely loaded systems such as nozzles, injectors or pumps require lubricants with high lubricity to avoid wear problems [
9]. Poor lubrication is indicated by a high coefficient of friction (COF) with significant metal-to-metal contact taking place in the boundary lubrication regime resulting in wear. Significant friction occurs when both contacting surfaces are without chemical films and adsorbed components [
8]. Poor lubricants have a potential for causing premature wear which leads to high repair costs and failure of pumps [
10]. Friction reduction and wear control are therefore of particular importance for economic reasons and long-term reliability of equipment [
11] (p. 4).
Fuel oils comprise a range of liquid combustibles, mainly hydrocarbons, which are obtained either as distillates in the working up of crude oil or as residues after the lighter fractions have been removed [
12] (p. 120). Fuel oils are used for industrial and marine operations because fuel oils are cheaper than diesel [
13]. Burner shut-downs can be caused by contamination of fuel, fuel degradation and poor fuel quality. Fuel stability, thermal stability, viscosity and fuel cleanliness affect flow and the nozzle and filter clogging tendencies of the fuel in use [
14]. The formation of particulate matter introduces problems related to blockages and wear resulting in expensive maintenance costs. Moreover, the solid particles associated with the heavy fuel oil’s exhaust gases result in harmful effects on the environment and human health [
13]. Abrasive contaminants in fuel and active chemical compounds intensify wear processes [
15]. The maximum particulate contamination in middle distillate fuel oils is 25 (mg) m
−3 by filtration according to the ASTM D6217-18 standard test method [
16].
The ASTM D396 standard specification for fuel oils covers grades of fuel oil intended for use in fuel oil burning equipment. The specification classifies fuel oils into six grades numbered in order of increasing density and viscosity, with number six being the heaviest fuel oil and number one being the lightest fuel oil [
14]. The fuel oils labelled no. 1, 2 and 3 are referred to as distillate fuels oils, whereas no. 4, 5 and 6 are referred to as residual fuel oils [
17] (p. 336). Distillate fuel oil is lighter, thinner and better for a cold start when compared to residual fuel oil [
18]. Straight run kerosene is a light distillate fuel oil, without further processing, which consists primarily of hydrocarbons in the C
11–C
20 range [
14]. Kerosene is intended for use in vaporising type burners due to its high volatility [
18]. Fluid catalytic cracking (FCC) reactor units in a refinery convert heavy fuel oils into lighter products. Light cycle oil (LCO), a distillate fuel oil, is a by-product of the FCC reactor. LCO can be used as a blending component in heavy fuel oils for viscosity adjustments in order to reduce the final viscosity [
19,
20]. Diesel fuel oil (No. 2 heating oil) is generally a blend of straight-run product, LCO and hydrocracked products. Diesel fuel oil, a heavier distillate fuel oil than kerosene, contains approximately 64% aliphatic hydrocarbons, 35% aromatic hydrocarbons and 1–2% olefinic hydrocarbons [
21]. Diesel fuel oil, used in commercial-industrial burners and in domestic burners, is produced for atomizing type burners [
22]. The distinction between diesel fuel used in internal combustion engines (IC) for vehicles and diesel fuel used as heating fuel oil is its sulphur content. Diesel for vehicles, must have a cetane number higher than diesel fuel oil (No. 2 heating oil), a key characteristic for diesel engines, which affects how smoothly the fuel burns, however, it is irrelevant for heating fuel oils [
14,
23]. Residual fuel oil is the residue of crude oil distillate that is still flowable under standard temperature and pressure (25 °C and 101.325 kPa). Waste oils from other sources are often added to residual fuel oils. A distinct composition cannot be determined for residual fuel oils, because of their complexity and variability. Residual fuel oils have complex composition and impurities compared with distillate fuel oils [
13,
21].
Properties of heavy (residual) fuel oils show that the polar fractions, including asphaltenes, are likely to be irreversibly adsorbed onto a metal surface and are less resistant to oxidation as compared to paraffins because of their polarity [
3,
4], ([
24] p. 38).
Asphaltenes play a major role in the high viscosity behaviour of the heavy fuel oils [
3]. Properties of heavy fuel oils show that the polar fractions, more specifically asphaltenes, have been identified as the cause of their unique rheological behaviour [
25]. Asphaltenes reduce the economic value of crude oil. As asphaltene content increases from 0% to 40%, the crude oil’s viscosity and density increase drastically and the crude oil changes colour from clear to dark brown [
26]. Viscosity depends directly and exponentially on asphaltene content. Experimental results from a study, where asphaltenes were removed from heavy fuel oils and then added back to the maltenes (saturates, aromatics and resins), indicate that viscosity of the reconstituted heavy fuel oil samples increases exponentially as the asphaltene content increases at constant temperature [
3]. Distillate fuel oils can be used as a blending component in heavy fuel oil for viscosity adjustments in order to reduce the final viscosity of heavy fuel oil [
20].
This investigation focused on fuel oil used in industrial burners in order to obtain a fuel oil benchmark to quantify lubricity of fuel oils in a similar way to other liquid products, for which standardised lubricity specifications exist. According to ASTM D396-18a, the maximum wear scar diameter (WSD) for distillate fuel oils is 520 μm, however the standard does not report on WSD specifications for residual fuel oils [
27]. As mentioned earlier, the aim of this investigation was thus to determine the lubricating properties of fuel oils used in power generation by focusing on their friction and wear capabilities, whilst also taking into consideration the effect of the presence of asphaltenes in fuel oils over a range of temperatures for filtered and unfiltered fuel oil samples.
The three representative fuel oil samples used in this study were provided by Sasol (a local refiner of crude oils and producer of synthetic fuels) namely:
- (i)
A light cycle oil (LFO), which is a distillate fuel oil not containing asphaltenes;
- (ii)
A medium wax-blend fuel oil (MFO) containing high molecular weight paraffin (wax), low concentration of asphaltenes and solid particles;
- (iii)
A crude-derived heavy fuel oil (HFO) containing high concentrations of asphaltenes and solid particles.
2. Materials and Methods
Experiments were designed to measure physical and chemical properties, followed by friction and wear testing under various temperatures. Triplicate test runs were performed for most experiments to ensure consistent results.
2.1. Elemental Analysis
In order to characterize the elemental composition of the fuel oil, analysis was performed using ICP-OES (Inductively coupled plasma-optical emission spectrometry) using a Spectro Arcos machine supplied by the Materials Division of Ametek Inc., Berwyn, PA, USA. The elemental analysis was performed on the unfiltered and filtered fuel oil samples. The preparation method used for the analysis was the wet-ashing procedure [
28]. A mass of 2 g of fuel oil sample was weighed in a platinum crucible. 2 mL of sulphuric acid (98%) was slowly added into the crucible and mixed with the fuel oil. The sample was incinerated at 550 °C for 2 h. Once the crucible had cooled down, the ash was dissolved in 1.5 mL of nitric acid and then diluted with 40 g of distilled water [
28].
2.2. Precipitation of Asphaltenes
Asphaltene fractions and solid particles were removed by precipitation from the three fuel oils by treatment with a non-polar hydrocarbon solvent (n-heptane) [
2]. A sample of 10 mL of fuel oil was mixed with 400 mL of 99% purity n-heptane. The mixture (fuel oil + n-heptane) was stirred for 24 h at 25 °C with a stirrer speed of 250 rpm and then allowed to settle for approximately 3 h at 25 °C [
5] (p. 1). It was ensured that the beaker is covered using a watch glass throughout the stirring and settling period to reduce the rate of evaporation of n-heptane. Although LFO does not contain asphaltenes (because LFO is a distillate fuel oil), the solid particles in LFO were removed as the asphaltenes filtration procedure does not only remove asphaltenes but also removes solid particles.
The precipitated asphaltenes (if any) as well as solid particles were separated from the remaining fuel oil and solvent mixture by vacuum filtration through two 0.2-micron nylon membrane filters. For the vacuum filtration setup (used to separate the solid particles and/or precipitated asphaltenes from the maltenes and n-heptane mixture) the 0.2-micron nylon membrane filters were placed in an oven at 110 °C for 10 min to eliminate moisture. The filter papers were then weighed individually and placed back in the filtration apparatus. The pump was connected to the Büchner flask and switched on to start the filtration process, as shown in
Figure 1 below.
Once the fuel oil and n-heptane mixture was poured into the long tube, the asphaltenes and solid particles were subjected to vacuum filtration at room temperature for approximately 24 h for LFO and HFO, and for approximately 36 h for MFO (MFO has waxy precipitants which take longer to separate from the asphaltenes). It was ensured that the vacuum filtration remained on for 24 h for LFO and HFO and 36 hrs for MFO even though the filtration may have been completed earlier—this aids in the separation of the maltenes and the n-heptane (evaporation of n-heptane). After filtration, the filter papers were dried in an oven at 110 °C (higher than the boiling point of n-heptane) for 15 min to make sure that the asphaltenes and filter papers were completely dry. The filter papers were removed from the oven to cool off and then weighed. The filtered mixture was poured into a container and left in a desiccator for 24 h, ensuring that the mixture container was open to allow for evaporation of the remaining n-heptane. After 24 h, the container was closed for storage until required for the HFRR friction and wear testing.
While establishing the vacuum filtration procedure, the filters used were 0.8 micron nylon membrane filters. It was found that this filter size was too big since visual inspection of “filtered fuel oil” showed that particles were still present. This is an indication that most of the asphaltenes particles are below 0.8 micron and therefore a smaller filter size was required. The 0.8 micron nylon membrane filters were replaced with 0.2 micron nylon membrane filters. It was also found that, after filtration of the fuel oil, the Büchner flask should be heated to 50 °C in order to decrease the viscosity and therefore remove most of the filtered fuel oil for storage and sebsequent use in friction and wear testing. Heating of the filtered fuel oils resulted in increased viscosity of the filtered MFO, which contains waxes, once the sample had cooled down. The filtrates were removed from the Büchner flask without heating to ensure that the physical properties would remain the same.
2.3. Lubricity Tests
The HFRR produces results via measurements of the continuous friction force, the contact resistance (film) and the test fluid temperature values. The film measurement results are a qualitative indicator of film formation and cannot be used to deduce the real area of contact. Therefore, the results are not as significant as the COF and inconsistency of results are expected [
29]. The components of the machinery comprise a mechanical unit, control unit and a computer. The control unit is connected to a computer for data logging, graphical representation of test parameters and results storage [
8].
Figure 2 shows the schematic of the HFRR machine.
Friction and wear tests were performed according to ISO 12156-1 on the HFRR under atmospheric air for filtered and unfiltered fuel oil samples. In order to report on the effect of temperature, the HFRR tests were performed at different temperatures (25, 60, 100 and 115 °C). Triplicate test runs were performed to ensure consistent results. A summary of the HFRR test conditions is shown in
Table 1.
The lubricity tests were chosen to be performed on filtered and unfiltered fuel oils performed at 25, 60, 100 and 115 °C. The 115 °C HFRR runs were performed simply to confirm that the investigation should not go beyond the normal boiling point of water. Expected evaporation takes place at 115 °C because the fuel oils contain certain amounts of volatiles. The results at 115 °C do not contribute to understanding of the temperature effects on fuel oils, which is seen by the change in trend at 115 °C in the HFRR results below. The fuel oils were filtered to investigate how the fuel oils would be performing without the presence of solid particles and/or asphaltenes. This helps to determine the impact of the solid particles and/or asphaltenes on the lubricity of fuel oils. Comparison of the filtered and unfiltered fuel oil lubricity results will give an indication of the effect of the solid particles and/or asphaltenes on the lubricity of the different fuel oils.
The 50% relative humidity (RH) choice under air atmosphere was selected because the humidity is within the acceptable laboratory air conditions for a standard lubricity test according to ISO 12156-1.
A humidifier was used to control the relative humidity of the HFRR chamber to 50% (RH) under atmospheric air. The humidity was monitored throughout the HFRR test using a Sensirion SHT2x humidity sensor since the chamber is not a completely closed system as some gaseous emissions leave the chamber throughout the test.
Air supply passes through moisture and particle filters. The wet air from the bubbler and dry air from the dryer mix to give the required relative humidity in the HFRR chamber. The flow rates of the bubbler and dryer were controlled by adjusting the needle valves throughout the test as shown in
Figure 3.
5. Conclusions and Recommendations
Filtration of HFO should be performed before use for better performance because filtered HFO results in better lubrication due to removal of abrasive particles resulting in reduced friction and wear and to avoid sediment deposition. Filtration of LFO and MFO is not required before use. Unfiltered LFO and MFO have good lubricating properties and much less particles when compared with HFO. Filtering LFO results in minimal change in lubrication and filtered MFO results in worse lubrication when compared with unfiltered MFO due to the removal of waxes which have good lubricating properties. Fuel oils at lower temperatures protect equipment from premature wear and breakdown of equipment because there is less concentration of heat on the metal surfaces in comparison with what happens at higher temperatures.
Two perfectly stable fuel oils can be incompatible resulting in atmospheric sludge precipitation when mixed. In addition, two fuels may be compatible at some mixing ratios and not compatible at other mixing ratios, or two fuel oils can be compatible or incompatible over the entire mixing range. LFO could possibly be mixed with either MFO or HFO to reduce the viscosity because the main use of LCO is as a blending component in heavy fuel oils for viscosity adjustments in order to reduce the final viscosity of heavy fuel oils. This should be confirmed. Dispersants can be used to control the compatibility problems which could possibly arise [
17] (p. 347).
Fuel oils at lower temperatures protect the equipment from premature wear and breakdown of equipment because there is less concentration of heat on the metal surfaces than at higher temperatures. Pumping unfiltered HFO and unfiltered MFO at lower temperatures may result in a much longer pumping period and require more power for the fuel oil to reach the burner section due to high viscosity of HFO and MFO as a result of the presence of asphaltenes. LFO can be pumped to reach the burner section without any problems due to low viscosity, low concentration of particles and the absence of asphaltenes. The fuel oils should not be preheated to high temperatures because more energy is required to preheat the fuel oils to a higher temperature than a lower temperature and better lubrication at lower temperatures.
In conclusion, it is recommended that unfiltered LFO be used at lower temperatures (25–60 °C) for best performance and HFO should be filtered before use at lower temperatures (25–60 °C) for best performance. Unfiltered MFO should be used at 60 °C because MFO requires to be preheated before use to avoid wax solidifying in the equipment.