Chemical pathways in thermal decomposition of citrus waste via slow pyrolysis

Pyrolysis provides a facile alternative route for the upgrading of agro-wastes to energy and chemicals. However, a widespread diffusion of the technology is currently hampered by lack of knowledge of the thermochemical pathways during conversion. As a result, consistent quality of the final products of pyrolysis is still a goal to be achieved at industrial level. The present study aims at investigating the evolution of solid products of pyrolysis of citrus waste. Chars derived from slow pyrolysis (50 K/min, 200–650 °C peak temperature) of orange (OP) and lemon (LP) pulp in a horizontal batch reactor were characterized by means of Differential Scanning Calorimetry (DSC), Electron Paramagnetic Resonance (EPR) and Raman spectroscopy. In a previous study, the present authors highlighted that significant changes in char structure, occurred between 300 °C and 500 °C peak temperatures. In this work, previous findings are further developed to show how the onset of breaking of C-H covalent bonds in matrix is triggered by reaching process peak temperatures of 330-350 °C. Around that temperature, the population of free-radicals significantly increases on solids surface (Fig.1 - a) and chars become more reactive, thereby favoring retrogressive, recombination and secondary solid-vapors reactions. Results also show that the higher presence of lignin on LP, may facilitate faster aromatization above 500 °C. This trend is also confirmed by DSC patterns (Fig. 1 - b) in which, above 500 °C, significantly more endothermic reactions occur in LP as a comparison to OP. This conclusion is further corroborated by a more pronounced G-band Raman shifts shown for LP (Fig.1 - c) as a comparison to OP (Fig.1 - d). The present results shed new light on the thermochemical breakdown of solid agro-wastes and provide insights for development of slow pyrolysis technology.