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.
Highlights
IntroductionAging changes in conventional liquid fuels occurring during their storage (especially prolonged storage) in depots are well known
Aging changes in conventional liquid fuels occurring during their storage in depots are well known
The recorded impedance spectra of aged biodiesel samples are presented in the form of Nyquist of a resistor in parallel with a capacitor
Summary
Aging changes in conventional liquid fuels occurring during their storage (especially prolonged storage) in depots are well known. The evaluation of changes occurring in the fuels becomes more and more important as use of liquid fuels systematically increase. This means that the amount of fuel stored for longer periods of time increases as well. National Reference Goals requires the addition of biocomponents to conventional fuels. In the case of biodiesel, methyl (or ethyl) esters from fatty acids of plant origin or their mixtures at concentrations from 7% (v:v) are routinely added. Biofuels are produced from a variety of plant oils or Energies 2019, 12, 738; doi:10.3390/en12040738 www.mdpi.com/journal/energies
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