Page 156 - Advances in Eco-Fuels for a Sustainable Environment

Microwave-assisted fast pyrolysis of hazardous waste engine oil into green fuels 127

5.5 Designation of microwave heating on the materials

The efficiency of microwave pyrolysis is a function of the dielectric nature of the
material. Apart from the conventional classification of materials according to their
nature of interaction with the microwaves, as a conductor, insulator, and an absorber,
there exists a fourth type of interaction called mixed absorption. Mixed absorption
refers to the selective absorption of the microwaves by a multiphase compound, where
the material having a high dielectric loss acts as an absorber and the material with the
low dielectric loss as a conductor. Due to this phenomena, new chemical reactions
may be initiated, which is otherwise not possible with convention heat addition that
will be added uniformly [40, 41].
Owing to their poor dielectric properties, the waste materials are unable to generate
sufficient heat in the microwave pyrolysis process. The presence of water makes it
more difficult, with the quantity of heat absorbed being barely sufficient to dry the
mixture. This necessitates the use of dopants that are sensitive to the microwaves
and readily absorb them. The activated carbon, coal, char and graphite are used like
receptors in the microwave field because of their polar nature. It is thus evident that the
effect of microwave assisted pyrolysis will depend mainly on nature on the receptors
[42–44].
The temperature variation of the material can be explained in four stages. The ini-
tial step is the dielectric relaxation of the water molecules, which leads to initial
heating. Dielectric relaxation means the adjustment of dielectric displacement or
polarization to the time-dependent electrical field. The second stage involves attaining
constancy in temperature, which mainly depends upon the nature of the receptor. In
the third stage, the temperature rises rapidly with a sharp drop in mass, and this is
followed by the last stage, which is the attainment of thermal equilibrium. The final
pyrolysis temperature is a function of the heat absorbed by the dopant as well as the
heat absorbed by the solid residue. Sometimes, due to the concentration of heat within
receptors, the rate of absorption increases drastically. This phenomenon is known as
thermal runaway [45].
Initially, the receptor absorbs microwaves and then heats up the remaining mole-
cules. This removes the volatile component, leaving behind char, which absorbs
microwaves further and hence the process of pyrolysis is sustained. Depending upon
the dispersion of the receptors, the first, second, and third generation of receptors may
be found. The lower the generation, the better the dispersal of the receptor. There have
been instances where the pyrolysis process has been sustained without the addition of
receptors. In some instances, the pyrolysis of wood has been achieved by heating
water alone. Some biomass derivatives also do not require the addition of microwave
absorbers. Apart from microwave absorption, the receptors can also alter the pyrolysis
products. Dominguez et al. [46] showed with experiments that the oil yield obtained
by using char as a receptor was higher than that obtained by using graphite as a recep-
tor. Use of graphite, however, favored the cracking of large aliphatic chains. The
microwave-assisted pyrolysis of polystyrene was affected by the variation of size
and shape of the iron mesh used as an antenna. From this study, the cylindrical antenna
gave a better result than others.