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Microwave-assisted fast pyrolysis of hazardous waste engine oil into green fuels 137

5.11.2 Noncondensable gases yield

In electrical pyrolysis with minimum nitrogen flow rate at 20 rpm stirrer speed has
increased the noncondensable gas yield fraction because of the secondary cracking
due to lower kinetic energy and retention time. However, in the case of 10rpm stirrer
speed and low nitrogen flow rates, as the temperature increased slightly, this decreased
the noncondensable gas yield fraction due to optimum kinetic energy to reduce the
secondary cracking of the pyrolysis gases. Stirrer speed did not affect much more
on the gas yields obtained due to the constant heating rate of the electrical pyrolysis
process. The slight difference was only observed in electrical pyrolysis. However, at
300°C and 10rpm stirrer speed, as the nitrogen flow rate increased, there was a
decrease in the percentage of gas yield due to the reduction of secondary cracking
reactions because of its optimum kinetic energy condition.
At low temperatures, an increased nitrogen flow rate improved the noncondensable
gas yield because of low retention time roots in the condenser. On the other hand, at a
low nitrogen flow rate, an increase in temperature decreased the gas yield due to the
reduced amount of retention time as well as secondary cracking at 10rpm stirrer speed.
However, at 20rpm stirrer speed and low nitrogen flow rates, an increase in temper-
ature increased the amount of noncondensable gases when compared with gas yield
obtained at 10rpm stirrer speed because of secondary cracking. Secondary cracking
increased due to stirrer speed, and it splashed out the feed, which came in contact with
the surface of the reactor vessel and caused secondary cracking.
In microwave pyrolysis, an increase in temperature as well as nitrogen flow rate
improved the amount of noncondensable gas yield due to very high localized heat gen-
eration and low retention time (i.e.) from 20% to 25% at 10rpm stirrer speed. More-
over, there was an improvement in the gas yield from 20% to 30% with 20rpm stirrer
speed as temperature and nitrogen flow rate increased. At low temperatures, increased
nitrogen flow rate improved the gas yield because of the low retention times and
instantly carried away the gases that were produced. From Fig. 5.8, it can be concluded
that MAP improved the amount of noncondensable gases obtained when compared
with electrical pyrolysis for any stirrer speed.

5.11.3 Char yield
The percentage of char formed in electrical pyrolysis decreased with increasing tem-
perature and nitrogen flow rate. However, the percentage of char yield was high with a
reduction in the temperature. This is due to partial evaporation of feed at low temper-
atures and implying high elimination volatile components. The latent heat of evapo-
ration was not reached for the whole of the species, which involved a weaker cracking
of the hydrocarbon chain, which improved the percentage of char in the residue. The
higher rate of evaporation caused by increased temperature led to an increase in
kinetic energy, which reduced the amount of char formation. In electrical pyrolysis,
the trend of change of char formation was the same for both stirrer speeds (i.e., 10 and
20rpm). At low temperatures, an increase in nitrogen flow rate decreased the char
yield for both stirrer speeds. An increase in stirrer speed resulted in a decrease of char