Pyrolysis kinetics and determination of organic components and N-alkanes yields of Karamay transformer oil using TG, FTIR and Py-GC/MS analyses

In this study, the pyrolysis characteristics of Karamay transformer oil are studied using Thermogravimetry (TG), Fourier Transform Infrared (FTIR), and Pyrolysis Gas Chromatography and Mass Spectrometry (Py-GC/MS) analysis. Four model-free methods of Friedman, Starink, KAS, FWO and one model-fitting method of Multivar-NLR are applied to determine the pyrolysis kinetics. The results show that the apparent activation energy varies less than 10% from 0.05 to 0.95 conversion rate regardless of the calculation method, and the average value is 40.24 kJ/mol, which is close to that of diesel, 35.82 kJ/mol. The best-fit kinetic model for characterizing the pyrolysis of transformer oil is the order-based kinetic model of f(alpha) = (1-alpha)(0.46) where the pyrolysis reaction rate is a function of the concentration of remaining reactants. The results of FTIR indicate that there is a large amount of alkanes with a higher number of carbon atoms and a small amount of aromatic hydrocarbons in the original transformer oil, while no unsaturated carbon has been identified. Consequently, the pyrolysis of transformer oil produces organic products including the n-alkanes, branched-chain alkanes, olefins, alcohols and phenols. Through Py-GC/MS analysis, n-alkanes are identified as the major group of volatile products by mass. The yields of n-alkanes are quantified using the external standard method in Py-GC/MS analysis where the results show that Karamay transformer oil presents the characteristic of "single peak with odd carbon predominance", consistent with the characteristic of the recent sediments in China. The total yield of n-alkanes is 0.55 +/- 0.06 g/g with n-heptadecane (C17) as the primary volatile product. The n-alkanes yield of transformer oil is higher than that of gasoline, 0.41 g/g and similar to that of diesel, 0.52 g/g. The dominating heavy components of Karamay transformer oil have higher carbon numbers than those of crude oils in other areas of the world. The current research provides theoretical and experimental basis for either direct use or further refinement of transformer oil into environmentally friendlier alternatives.