Comparative analysis of Microwave and Thermal Pyrolysis of used Tyres to produce Liquid Fuel

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.