Chemical Changes in Fossil and Biogenic Heating Oils on Long-Term Storage

The formation of sediments in biogenic and fossil heating oils as well as in their blends is a well-known problem. These deposits can plug filters and nozzles in heating systems and, consequently, cause economic losses. Polymerization and the formation of corrosive acids are possible explanations for these incidents. To study the influence of long-term storage on different heating oils (biogenic, fossil, and a 10% blend) and to investigate the changes in their composition, the oils were stored for a period of 12–24 months at nearly ambient (40 °C) and analyzed with different techniques every 6 weeks. The formation of several kinds of oxidation products was demonstrated, including ketones, epoxides, aldehydes, carboxylic acids, and furans. Size-exclusion chromatography was used to demonstrate the formation of oligomeric products of the fatty acid methyl esters (FAMEs) (up to pentamers). Short-chain (C₁–C₆) carboxylic acids were quantified with ion chromatography, and larger carboxylic acids were indicated by mass spectrometry. The first recorded experimental evidence for a coupling reaction between a FAME and components of the fossil oil, namely, such containing the nitrogen heterocycle indols, is described. Cross-coupling products between biogenic and fossil compounds were detected using Orbitrap ultrahigh-resolution electrospray ionization mass spectrometry

Influence of Different Compound Classes on the Formation of Sediments in Fossil Fuels During Aging

The formation of sediments is a serious instability problem in the storage of fossil fuels. Reactions that lead to sediment formation can be linked to the oxidation of certain fuel components that contain oxygen, nitrogen, or sulfur. To study the oxidation reactions that occur during aging of fuels, we doped a model fuel with several representatives of such compound types. The compounds used were 2,6-dimethyl­phenol, 2-naphthol, 2,5-dimethyl­pyrrole, 2-methylindole, dibenzo­thiophene, and penta­methylene sulfide. After an artificial aging of the samples according to the DGMK-714 protocol, the formed sediments were analyzed by electrospray ionization mass spectrometry (Orbitrap, ESI-MS), elemental analysis, infrared measurements, and mass analysis. Mass spectrometry indicated monomeric and dimeric oxidation products with two to nine oxygen atoms as well as products with different hydrocarbon structures (different C/H ratios) from 2,6-dimethyl­phenol. 2-Naphthol led to oligomers consisting of up to six monomer units and showing different degrees of oxidation. The first ever recorded cross-coupling between 2,6-dimethylphenol and 2-methylindole and between 2-naphthol and 2,5-dimethyl­pyrrole is also shown. In general, the tested nitrogen compounds and especially the phenols tended to form oxidized oligomers, whereas the sulfur compounds led to sulfoxides and sulfones