Thermal pyrolysis of used tyres to produce liquid fuel: process optimization and how it compares to microwave pyrolysis

Abstract

Used tyres are not biodegradable, and the current methods of disposal pose a threat to the environment. Such tyres can be valorised through decomposition to produce liquid fuel, an alternative diesel fuel, using thermal pyrolysis technique. Microwave pyrolysis is an alternate method which uses microwave irradiation, saves energy, and is better environmentally. The main objective of this study was to perform microwave pyrolysis of used tyres to produce liquid fuel and compare with thermal pyrolysis. The specific objectives were to study the effects of pyrolysis operating variables and optimization of liquid fuel yield for thermal pyrolysis, compare with microwave pyrolysis, and characterize the liquid fuel. Thermal pyrolysis variables were reaction temperature, reaction time, and particle size. Thermal pyrolysis reaction temperatures were 200, 300, 400, 500, 600, and 700°C; reaction time 10, 20, 30, 40, 50, 60, 70, 80, 100, and 120 minutes. Particle sizes were 25, 50, 60, 100, 125, and 200mm2. Thermal pyrolysis was carried out in furnace fabricated using furnace clay rated 600 W. A 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 physiochemical properties were tested using standard methods. The yield of the liquid product was correlated as a quadratic function of the reaction variables. Response surface methodology (RSM) was used to study the effects of operating variables and identify points of optimal yields. The yield decreased as particle size increased. Yield increased with increase in temperature optima being 500°C. Yield increased with increase in reaction time, and the pyrolysis time was 80 minutes. The highest liquid yield of 40.4 wt. % corresponded to temperature of 500°C, time of 80 min for 60 mm2 size. The calorific value for liquid fuel was 47.31 MJ/kg and GC-MS analysis showed that the oil comprised of complex mixtures of organic compounds with limonene, toluene, and xylene as major components. When compared to the published literature on microwave pyrolysis, both processes gave similar maximum yield but microwave process was superior due to a 77.5% reduction in reaction time, resulting in a 73.02% saving in energy requirement.