Lubricating properties of fuel oils : quantification of lubricity

Abstract

Asphaltenes are components of heavy fuel oils with complex aromatic structures containing heteroatoms (N, O, S) and metals (V, Fe, Ni) and contribute to the high viscosity of heavy fuel oils. Fuel oils are passed through pumps, filters, nozzles and other equipment before such fuel oils reach the burner section and therefore require good lubricating properties. In the recent past, several users of fuel oils have been experiencing problems such as blockages in the fuel oil filters and injector nozzles, increased wear and failures of pumps and, in some cases, decreased calorific efficiency of combustion. The high frequency reciprocating rig (HFRR) lubricity tester (method ISO 12156-1) was used to perform lubricity tests on fuel oil samples at different temperatures and atmospheric conditions to obtain a better understanding of the friction and wear behaviour of fuel oils. Three fuel oil samples with different asphaltene concentrations were selected for this purpose. These fuel oils were namely: a light cycle oil (LFO), a medium wax-blend oil (MFO) and a crude-derived heavy fuel oil (HFO). In an attempt to understand the role of oxygen as a contributing cause of the problems experienced with fuel oils at different temperatures, two atmospheres (i.e. oxygen-rich (atmospheric air) and inert (nitrogen)) were used to perform the lubricity tests. Results indicate that the presence of asphaltenes changes the viscosity behaviour of fuel oils, which, in turn affects the lubricity behaviour. LFO with no asphaltenes (solid particles only) has little impact on the coefficient of friction and wear from 60 to 115 °C. MFO containing high molecular weight paraffin (wax), low concentrations of asphaltenes and solid particles results in a more stable fuel oil, resulting in less friction and wear and good high temperature performance. HFO containing high concentrations of asphaltenes and solid particles results in very high coefficient of friction (COF) and severe abrasive wear at high temperatures. At low and moderate temperatures, unfiltered HFO performs comparable to filtered HFO without asphaltenes and solid particles. Key performance indicators like the wear scar diameter on the ball as well as the wear track on the test ball, showed unexpected results indicating that temperature and atmospheric conditions contribute to the friction and wear behaviour of the fuel oils, but that composition of each of these fuel oils played a significant role. For MFO, at high temperatures, wear increases drastically under nitrogen atmosphere when compared with oxygen-rich atmosphere. For HFO, the high concentration of asphaltenes content results in minimal change in the COF and wear with a change in environmental atmosphere. The poor lubricity of HFO is predominantly due to the asphaltenes and solid particles in HFO.