article · ACS Omega
Waste coal fines pose an environmental challenge, but producing pyrolysis oil from coal and algae mixtures offers a potential route to alternative fuels. Phase separation and chemical aging severely undermine the stability, handling, and quality of these oils. An investigation into the mechanisms driving this instability evaluated phase separation, gas purging, and adsorptive treatments over time. Bubbling separated oil phases with nitrogen gas suppressed autoxidation and interphase reactions, which lowered water content by 22.7%. In contrast, hydrogen purging accelerated condensation reactions, causing higher viscosity and gum formation. Spectroscopic analysis confirmed that oxygenated compounds, particularly phenols and carbonyls, drive the aging process. Treating the oil with an in-house adsorbent successfully removed these reactive oxygenates, halting phase separation entirely and decreasing water content by 62.4%. These findings show that targeting reactive oxygenates is an effective method for stabilising coal-based pyrolysis oils.
Accumulated waste coal fines from mining create serious environmental hazards. Converting these fines into liquid pyrolysis fuels offers a sustainable alternative to fossil resources, but rapid chemical degradation and phase separation make the resulting oils difficult to handle and refine. Demonstrating that targeted adsorbent treatments and inert gas purging can halt degradation provides practical methods to stabilise these oils for reliable energy use.
This research is relevant to alternative fuel producers and coal mining companies aiming to beneficiate waste coal fines into usable fuel feedstocks. The findings represent applied laboratory-stage research. Real-world adoption will require developing scalable regeneration methods for the adsorbent, validating continuous flow processing, and testing the treated oil within existing commercial fuel upgrading systems.
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Abstract South Africa greatly suffers from the accumulation of waste coal fines due to its heavy reliance on coal mining. The processing of pyrolysis oil ensures the beneficiation of waste coal fines to provide alternative sustainable fuels. However, phase separation and aging reactions affect the stability of the pyrolysis oils negatively, affecting their processing and the product yields and quality. Understanding and managing the stability of Coalgae pyrolysis oil remain gaps to resolving this issue, and as such, systematic time-course monitoring of Coalgae pyrolysis oils was considered. Phase separation by decanting, nitrogen purging to eliminate the oxidative environment, hydrogen purging to investigate a reducing protic environment, and adsorptive treatment to eliminate coordinating species were used to distinguish possible reactions. The rate and extent of aging reactions were reduced by bubbling the separated phases of pyrolysis oil with nitrogen gas, thereby limiting the interphase and autoxidation reactions. Furthermore, a 22.7% reduction in water content and over 9 MPa·s in P2 viscosity were observed. The use of hydrogen in pyrolysis oils resulted in gum formation and increased viscosity via promotion of condensation reactions. FT-IR, GC-MS, and NMR analyses hinted that oxygenates (phenols and carbonyls) play a major role in phase separation and aging of pyrolysis oil. The use of the in-house adsorbent prevented phase separation by removing oxygenates and resulted in a 62.4% decrease in the water content. The study revealed that removing reactive compounds, such as oxygenates, has a greater effect on managing phase separation and aging in coal-based pyrolysis oil, allowing better handling for further processing.
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DOI: 10.1021/acsomega.6c00546
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