To achieve climate goals, renewable energy sources will play an increasingly important role, including in the heating market. These include paraffinic liquid fuels (electricity-based e-fuels and hydrogenated vegetable oils, HVO) as well as fatty acid methyl esters (FAME). As drop-in components, these alternative fuels are blended with conventional heating oils (H1 and H2). The goal is to produce liquid fuel blends designed to replace mineral oil-based variants on the market. These drop-in components can be used without any modifications to the existing infrastructure. In light of the increasing use of renewable liquid fuels in the heating market, it should be investigated whether aging effects—including the formation of deposits—are to be expected in fuel blends, which could become relevant in practical operation compared to the heating oil grades used to date. Furthermore, it must be determined which technical or process-related measures, if any, are suitable for counteracting such effects.
In this project, a test bench (ATES) was used that incorporated technical components similar to those found in conventional heating oil systems. The aging of the liquid fuels was carried out in a closed-loop system through repeated exposure to temperature, pressure, atomization, light, and catalytically active metals such as copper. Due to the design of the test rig used, the fuels were subjected to increased stress compared to real-world conditions; while this allows the aging effects to be observed within the timeframe of a project, it also necessitates a careful evaluation of the results in terms of their practical applicability. An NMR-MOUSE (nuclear magnetic resonance-mobile universal surface explorer) was also integrated into this test rig, enabling in-situ monitoring of aging-induced changes over a period of 1,000 hours. A particular focus was on identifying conditions that could promote the formation of deposits.
Furthermore, as part of this project, the effect of various additives on the aging behavior of pure heating oil (H1), pure GTL, and their 1:1 mixture (H1P50) was initially investigated using a “no-harm” test bench over an aging period of 300 hours. The aim was to use a shorter aging period under more severe aging conditions to enable time-efficient aging screening of the samples, both in their pure form and when blended with various additives. The additives investigated included three different antioxidants, a detergent, and a combination of a detergent and a stabilizer. For the different fuels H1 and GtL, as well as their 1:1 blend H1P50, varying effects of the additives on the aging-related changes in the fuels were observed in some cases. In addition, additives identified as promising for the H1P50 fuel mixture were aged for over 1,000 hours using the ATES test bench. This demonstrated a clear and effective slowing of the aging processes through the addition of additives.
Samples were taken at specific intervals from the two test benches and characterized using standardized analytical methods. These include measurements of density, viscosity, water content, acid number, oxidation stability, and total contamination. In addition, modern analytical methods—including 1H and 13C high-field (HF) and low-field (LF) nuclear magnetic resonance (NMR) spectroscopy, LF-NMR relaxometry and diffusometry, as well as Fourier-transform infrared spectroscopy (FTIR), were used to obtain comprehensive information on the chemical and physical changes induced by aging. Due to the very small amount of collected deposits, their chemical compositions were primarily analyzed using FTIR and, in a few cases, solid-state NMR. A key finding is that, even under the increased stresses placed on the fuel mixtures in the test rigs used, the fuel mixtures containing paraffinic components showed virtually no deposit formation, and only isolated test rig failures occurred within the aging period under investigation.