Univ. of Oklahoma study suggests that biocorrosion associated with ULSD not related to level of sulfur in the fuel
Findings from a study by a team at the University of Oklahoma suggest that the stability of diesel fuel—i.e., either the biodegradation of diesel hydrocarbons or, by inference, the degree of biocorrosion—is independent of the concentration of organosulfur species in the fuel. In other words, accelerated biocorrosion associated with the use of ultra-low sulfur diesel (ULSD) likely does not result from the lower level of sulfur in the fuel.
The study is published in the ACS journal Environmental Science & Technology.
The global shift toward reducing atmospheric particulate emissions led many governments to reduce the sulfur content of transportation fuels. The production of ULSD requires extensive processing of crude oil to reduce the amount of sulfur to a maximum of 15 ppm S. This refining tends to alter the properties of ULSD such that additives are typically required to improve lubricity and corrosion resistance characteristics. Nevertheless, there are many reports of increased corrosion in the fuel infrastructure associated with the introduction and use of ULSD.
We hypothesized that these reports might be due to either the removal of organosulfur compounds that would normally inhibit biodegradation and biocorrosion activities or to the acceleration of these activities through the differential microbial metabolism of fuel components formed or added during oil refining. One report suggests that small molecular weight components can be formed as a consequence of the refining process used to produce ULSD [catalytic hydrodesulfurization of hydrotreating]. Fuel components that are more labile than hydrocarbons might accelerate the deleterious processes associated with the use of ULSD.—Lyles et al.
Organosulfur compounds are known antimicrobial agents and have been detected in more than 200 structurally diverse forms in diesel fuels and other petroleum products including thiols, thiophenes, and sulfide derivatives, the authors noted. Concentrations can range from 0.05% to 14% by weight in crude oils and natural bitumens.
To test these hypotheses, the researchers took fuels from a refinery at various stages in the desulfurization process—ULSD (4ppm S), low- (LSD, 249 ppm S), high-sulfur diesel (HSD, 441 ppm S) and Navy Ship diesel ( 1,528 ppm S). They also tested the influence of a soy-based biodiesel. They explored the connection between organosulfur content and the susceptibility of fuels to anaerobic biodegradation and, by inference, their propensity to exacerbate biocorrosion in marine systems.
Gas chromatographs of the ULSD, LSD, HSD, and Navy shipboard diesel revealed that the fuels were qualitatively similar in terms of the range and nature of the component hydrocarbons. A separate analysis of the organosulfur content of the fuels confirmed that they varied widely (4 - >1500 ppm S). Therefore, if the organosulfur content of fuels was a major factor influencing susceptibility to biodegradation and biocorrosion, it should have been evident in the experiments, they noted.
The amended an inoculum from a seawater-compensated ballast tank with fuel from the same ship or with the refinery fractions of ULSD, low- (LSD), and high sulfur diesel (HSD) and monitored for sulfate depletion.
Our findings suggest that the organosulfur status of diesel fuels does not significantly influence anaerobic hydrocarbon biodegradation with any of the tested inocula. Moreover, the enrichment of microbial communities, while different for various inocula, was independent of the fuel type used in the incubations.
...The major implication of this work is that ULSD, LSD, HSD, and even a traditional diesel fuel formulation can undergo biodegradation to a similar extent and stimulate sulfate reduction when oxygen becomes limiting in seawater compensated ballast tanks. This process would produce higher levels of hydrogen sulfide (H2S) and thus an elevated biocorrosion rate. However, it is unlikely that either the biodegradation of diesel hydrocarbons or, by inference, the degree of biocorrosion is likely to be influenced by the concentration of organosulfur species in the fuel.—Lyles et al.
Christopher N. Lyles, Deniz F. Aktas, Kathleen E. Duncan, Amy V. Callaghan, Bradley S. Stevenson, and Joseph M. Suflita (2013) Impact of Organosulfur Content on Diesel Fuel Stability and Implications for Carbon Steel Corrosion. Environmental Science & Technology doi: 10.1021/es4006702