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Microwave-assisted fast pyrolysis of hazardous waste engine oil into green fuels 123

5.3 Influence of microwave heating on the pyrolysis

In the application of microwave heating for pyrolysis, refreshing circumstances
related to temperature distribution, the heating rate, and the residence time of volatiles
have been noticed when equated with the conditions obtained from conventional
heating. Thus, different pyrolysis products outputs are authorized to form a different
chemical profile for each heating system [24]. The conventional electrical heating
method describing the heat energy is transferred from the outermost layer to the inner-
most layer by conduction, convection, and radiation [25, 26]. On microwave pyroly-
sis, the conversion of electromagnetic energy represents the formation of heat and
thermal energy. The electromagnetic waves (microwaves) can penetrate through
the pyrolysis feed and generate the heat energy. Microwaves can penetrate materials
and deposit energy due to the formation of a hotspot on the pyrolysis feed, not in the
mode of heat transfer. Therefore, both heating systems have different thermal gradi-
ents. The heat generation occurs in the entire volume of the material. Microwave
heating recognizes the complex reactions due to volumetric heating. Therefore,
microwave heating is energy conversion contrary to heat transfer [27].
The higher heating rate can be achieved in case of microwave heating because heat
canbedirectlytransferredthroughmolecularinteractionwiththeelectromagneticfield,
andthereisnonecessitytoheatthesurroundings.Moreover,duetohigherheating rates,
residence times of volatiles can be reduced because they travel faster from the inner hot
regions to the outside cold regions, hence avoiding secondary vapor phase reactions.
Reactions of devolatilization and conventional heating is ameliorated in the gas-phase
homogeneous reactions, in the same manner with the reactions that take place
extraterrestrially. Moreover, the lesser temperatures in the microwave cavity help to
condense the terminal vapors of pyrolysis and to overturn inappropriate reactions [28].
Men endez et al. [29] observed that microwave pyrolysis used to obtain higher
pyrolysis evolved a gas yield and less char residue. Compared to conventional tech-
nology, the microwaves in heterogeneous processes are to eliminate the heat transfer
resistance in the conventional chemical process. Thus, the main advantages of micro-
wave pyrolysis concerning conventional pyrolysis would be higher efficiency and a
higher heating rate, therefore a more significant saving of time. Solid Char acts like
a reactive catalyst because of the active metal centres located at the surface. Moreover,
some of the heterogeneous reactions observed in a microwave pyrolysis compared to
conventional pyrolysis. With certain specific reactions such as methane decomposi-
tion reaction, solid metal char will act as a catalyst to obtain better conversion rates in
heating by microwaves [30].
If a material has a significant moisture content, microwave pyrolysis shows a
unique pattern in the heating process of the particle. Water molecules have good har-
mony with microwaves. The steam generated by the vaporization of moisture in the
core of the particle is swiftly exited into the adjacent area. It sweeps not only the
vapours but also creates favourable channels in the solid, which increases the perme-
ability and supports the vapour release at low temperature [31]. Microwave pyrolysis
offers an alternate worldview in molecule warming wherever the electromagnetic
field penetrates the strong and collaborates straightforwardly with dipoles in