Abstract:
Used tyres are non-biodegradable, and the current methods of disposal pose a threat to
environment. They can be valorized through decomposition to produce alternative fuel
using thermal pyrolysis technique. Microwave pyrolysis is an alternate method which
uses microwave irradiation, saves energy, and is environmentally friendly. The main
objective of this study was to perform microwave pyrolysis of used tyres (Triangle
1000/R 20/10.00X20) to produce liquid fuel and compare with thermal pyrolysis. The
specific objectives were to study the effects of pyrolysis operating variables,
optimization of liquid fuel yield for microwave pyrolysis and compare with thermal
pyrolysis and characterization of liquid fuel. Microwave pyrolysis variables were
microwave power, reaction time, and particle size while thermal pyrolysis variables
were reaction temperature, reaction time, and particle size. Process variable levels for
microwave were power levels of 20, 30, 40, 50, 60, 80, and 100%; reaction time as 8,
13, 18, 23, and 28 minutes. Thermal pyrolysis reaction temperatures were 200, 300,
400, 500, 600 and 700oC; reaction time 10, 20, 30, 40, 50, 60, 70, 80, 100 and 120
minutes. Particle sizes for both processes were 25, 50, 60, 100, 125 and 200mm2
. A
domestic 900W microwave oven was modified to be used for microwave pyrolysis,
while the thermal pyrolysis was carried out in a furnace, fabricated using furnace clay,
rated 600W. In both processes, 500 ml round bottomed flask was used as a reactor.
Design Expert 13 was used for data analysis and optimization, Gas chromatography–
mass spectrometry (GC-MS) was used for chemical composition analysis, while the
physiochemical properties of liquid were tested using standard methods. The yield of
the liquid products for microwave and thermal pyrolysis was correlated as a quadratic
function of the variables. Response surface methodology (RSM) was used to study the
effects of variables and identify optimal points. For both pyrolysis, yield decreased as
particle size increased. The yield increased with increase in microwave power for
microwave pyrolysis optimal being 50% microwave power and similar trend was
observed with temperature for thermal pyrolysis optimal being 500oC. Yield increased
with increase in reaction time for both processes. For microwave pyrolysis, the highest
liquid yield of 39.1 wt % was at 50% power, 18 minutes reaction time, and particle size
of 25 mm2
; optimal yield of 40.4 wt. % in thermal pyrolysis corresponded to
temperature of 500oC, time of 80 min for 60 mm2
size. RSM gave conditions for optima
close to that of experimental results. The calorific value for liquid fuel from microwave
and thermal pyrolysis were 48.99 and 47.31 MJ/kg respectively. GC-MS analysis
showed that oil comprised of complex mixtures of organic compounds with Limonene,
Toluene and Xylene as major components. Both processes gave similar maximum yield
but microwave process was superior due to 77.5% and 71.6% reduction in time and
energy respectively. The fuel properties meet requirements for heavy fuel. Microwave
pyrolysis was recommended because of savings in time and energy requirements.
Refining of liquid fuel and up scaling of microwave pyrolysis is recommended for
further research.