Application of the Impedance Spectroscopy as a New Tool for Studying Biodiesel Fuel Aging Processes

Abstract

Fatty acid methyl esters (FAME), which are presently the main component of biodiesel fuels, undergo relatively fast oxidation processes. This behavior prevents long term storage of this fuel. From laboratory practices, it transpires that even after a very short period of storage, the oxidative stability of the biodiesel exceeds the values required by European regulations. Therefore, the goal of this work was to devise a parameter (marker) allowing for fast and convenient identification of the chemical stability of biodiesel. Moreover, we were aiming to devise a marker which can also be used for the evaluation of the chemical stability of other hydrocarbon fuels containing biocomponents. To this end, in the presented study, selected biodiesel samples were subjected to controlled aging processes in laboratory conditions at 95 °C and oxygen flow according to the norm. Then, physico-chemical parameters were selected that are critical from the point of view of the fuel practical application. Those included density, refractive index, oxidative stability and resistance to oxidation. The appropriate physico-chemical properties were measured before and after an aging process conducted for various times. Simultaneously, electrochemical impedance spectroscopy (EIS) studies were performed for all the studied samples yielding the electrical parameters of the sample, including resistance, relaxation time and capacitance. Subsequently, a correlation between the results of the EIS studies and the selected critical parameters has been established. The obtained results indicate that the resistance, relaxation time and capacitance of the studied biodiesel fuel increase with aging time. This indicates the formation of long chain compounds with increased polarity. Interestingly, the electrical parameter changes are faster at the early stages of the aging process. This suggests a change of the oxidation mechanism during prolonged aging. The devised methodology of impedimetric biodiesel testing can be proposed as a fast and inexpensive method of fuel chemical stability evaluation, allowing for estimating the useful storage time of biodiesel in real conditions.

Fatty Acids Methyl Esters (FAME) autoxidation: New insights on insoluble deposit formation process in biofuels

“…On the other hand, biodiesel oil-oil complex system is unstable, the polarity between aggregates and solvent seems to be remarkable. This is in agreement with the work of Biernat et al [27] that showed the increase of the polarity of oxidized FAME (at 95 °C, oxygen flow rate of 3 L/h) by using Electrochemical Impedance Spectroscopy (EIS). It is noteworthy that since B0 formulation contains only paraffins, naphtenes and aromatics, so, it does not contain any oxygenated specie prior to oxidation, whereas biodiesel samples have a great amount of methyl esters as well as unsaturated molecules, both of which are known to favor the oxidation process.…”

Miniaturized 3D-Printed Cell Enables Water/Ethanol Quantification Using Electrochemical Impedance Spectroscopy

“…Its popularity arises from its ability to conduct non-destructive analyses. In the context of biofuel analysis, EIS has emerged as a promising analytical tool for monitoring and evaluating the biodiesel and diesel-biodiesel blends [ 20 , 21 ] and aging processes of biodiesel fuel [ 22 ], and for assessing corrosion of metals in bioethanol-gasoline mixtures [ 23 ]. Most of the electrochemical cells used for this purpose are based on a capacitive system, which must have a large surface area and a short distance between the parallel plates of the capacitor to provide conditions for analyzing the dielectric constant of these mixtures.…”

Characterization of Biodiesel Obtained from Vegetable Oil; Conventional and Residual

“…In other hand, the literature about the research made in biodiesel is wide, and involves several aspects such as production, potential sources, oil extraction techniques, effects of fatty acid composition, characterization, diesel engine performance (7,8). The characterization of biodiesel involves many aspects, including the aging process (9), water content (10), dielectric properties (11) or the determination of antioxidants (12). These aspects define the quality of a biodiesel and there are different characterization techniques such as impedance spectroscopy (11,9) and voltammetry (13,14).…”

“…The characterization of biodiesel involves many aspects, including the aging process (9), water content (10), dielectric properties (11) or the determination of antioxidants (12). These aspects define the quality of a biodiesel and there are different characterization techniques such as impedance spectroscopy (11,9) and voltammetry (13,14). The characterization of the behavior of biodiesel in the engine is also widely studied, especially in the part of gas emissions (15,16) and performance (17,18,19).…”