Pyrolytic degradation of spent coffee ground: A thermokinetic analysis through the dependence of activation energy on conversion and temperature

This work focuses on the thermal degradation of spent coffee ground (SCG) under a non-oxidizing atmosphere using thermoanalytical techniques (TGA-DTG/DSC). The study describes the thermokinetic behavior of SCG through the dependence of activation energy (E_α) as a function of the conversion degree (α) and temperature (T), through the combined use of different model-free kinetic algorithms. The dependencies of E_α on α and T exhibit different steps and critical stages, which define the control mechanism and the yield in the desired products. The obtained trends reflect the complexity of the process and highlight the limitations of traditional isoconversional approaches in describing the behavior of processes that involve several phases and multi-components. The present study discloses the direct dependence of the rate constant, k, on the temperature, T, and predicts the influence of the saturation pressure (P) of the gaseous products on both the control mechanism and the products yield in the pyrolysis process, as a function of the heating program (β). Furthermore, the study shows that the degradation rate (dα/dt)_p of each pseudocomponent (hemicellulose, cellulose and lignin) at the maximum rate temperature, T_p, satisfies that, f(α)_p is independent of β, and, hence, f’(α), is independent of α at that instant. Therefore, kinetic behavior at maximum conversion rate, can be unequivocally described through a simple first-order kinetic law (f(α) = 1-α). Finally, the findings this study, provide recommendations of wide relevance and practical interest, to understand, control and modulate the thermochemical conversion of lignocellulosic wastes into value-added products.