Thermal decomposition characteristics and study of the reaction kinetics of tea-waste

This study aimed to investigate waste-tea’s pyrolysis kinetics and thermodynamics to assess its potential for thermochemical processes. In this study, three primary samples of tea-waste are prepared for investigation: raw, torrefied at 200 °C, and torrefied at 600 °C. Under a nitrogen environment, thermogravimetric analyses (TGA) were conducted at seven heating rates (10, 15, 20, 25, 30, 35, and 40 °C/min) to investigate the effect of heating rates on the kinetic parameters at temperatures ranging from 27 to 1000 °C. Using seven heating rates was beneficial to take advantage of multiple heating rates techniques alongside single heating rate techniques. These heating rates were combined, forming four heating rate groups (HRG). The pyrolysis kinetic parameters are determined using two model-fit-methods, direct Arrhenius and Coats-Redfern methods, and two model-free methods, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods. Thermodynamic data comprising ΔH, ΔG, and ΔS are addressed. The X-ray fluorescence (XRF) and Fourier transform infrared (FTIR) spectrum are used to assess the presence of natural minerals in tea-waste. The results indicated that the tea-waste material has the potential to produce syngas. The torrefaction process at 600 °C shows a 53% increase in the energy content compared to the raw biomass. The Coats-Redfern is shown to be more reliable than the direct Arrhenius method. The activation energy (E_a) witnesses rising with the heating rate (β) from E_a = 55.27 kJ mol⁻¹ at β = 10 °C min⁻¹ to 60.04 kJ mol⁻¹ at β = 40 °C min⁻¹ for raw tea-waste using Coats-Redfern method. For model-free approaches, the minimum activation energy values of the raw tea-waste samples are 82 kJ/mol for FWO and 78 kJ/mol for KAS, whereas the peak values are 420 kJ/mol for KAS and 411 kJ/mol for FWO. A comparison of the effect of heating rate groups for FWO method in raw material case indicated that HRG1 has the maximum activation energy average value. The resulting values of HRG1, HRG2, HRG3, and HRG4 were 269 kJ/mol, 145 kJ/mol, 174 kJ/mol, and 202 kJ/mol, respectively.