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Microwave-assisted fast pyrolysis of hazardous waste engine oil into green fuels 121

as well as the warming presentation of microwave pyrolysis can be dramatically pro-
moted by adding catalysts.
The addition of proper susceptors in the microwave method can increase the yield
value of the liquid oil. Silicon carbide activated carbon and fly ash have been used as
susceptors in the microwave pyrolysis of polypropylene, and the liquid yield percent-
age are 31.89%, 29.80%, and 21.07%, respectively [13]. Moreover, the heating rate
for silicon carbide as a susceptor is higher compared to the other two. Municipal solid
waste is used in the pyrolysis process, and different susceptors have been used for
increasing the efficiency of the microwave pyrolysis process. For municipal solid
waste activated carbon, solid beads, fly ash, and aluminum are the suitable susceptors.
The temperature rise for an activated carbon susceptor is more than flyash. However,
the liquid oil yield percentage for silicon beads as the susceptor is higher than other
susceptors [14]. So, suitable susceptors need to be chosen depending upon the feed for
pyrolysis process.

5.2 Microwave heating mechanism

As mentioned in the international agreement, the preferred frequencies of microwave
heating are 915MHz (λ¼ 33cm) and 2.45GHz (λ¼ 12cm) [15]. The electromag-
netic radiation generated by the magnetron causes the dipolar molecules to try and
rotate in phase with the alternating electric field. At the molecular level, resistance
to this rotation leads to the friction between the molecules and causes the heating
effect [16].
In conventional thermal heating, the process is regulated by the temperature of the
surface and also by some physical properties of the material being heated, such as heat
capacity, density, and thermal diffusivity. Whereas, in microwave heating, the heating
effect is due to the interaction of dipoles with electromagnetic radiation. In microwave
heating, the material gets high temperatures and heating rates [17]. The efficiency of
electrical energy to heat energy conversion is high (80%–85%) in microwave
heating [18].
The microwave heating technique is a volumetric heating method that includes
other heating procedures such as heating due to conduction in the operating frequency
range of 0–6Hz and heating due to induction in the operating frequency range of
50Hz–30kHz. Ohmic heating occurs in the frequency range between the induction
and conduction. Radio frequency heating in the frequency range of 1–100MHz is used
for workloads with high resistivity when placed between electrodes [15]. Meredith
[19] gives a typical electromagnetic spectrum with examples of applications per-
formed at different frequency ranges.
There are many principles and theories available in textbooks that explain the
microwave heating mechanism. In general, there are three mechanisms by which
the heating effect is achieved in microwave heating methods, which are summarized
as follows.
Dipole reorientation or polarization: This mechanism explains how the heating
effect is achieved in polar compounds. When a polar compound is subjected to