Pyrolysis of biofuels of the future: Sewage sludge and microalgae – Thermogravimetric analysis and modelling of the pyrolysis under different temperature conditions

Highlights: • Pyrolysis of microalgae and sewage sludge is studied by thermogravimetric analysis. • Accurate values of the kinetic parameters of the pyrolysis reaction are reported. • Pyrolysis is modeled for parabolic and exponential temperature increases. • Estimations of the model are compared with experimental measurements in TGA. • Excellent agreement is reached between the model estimations and the experiments. - Abstract: The pyrolysis process of both microalgae and sewage sludge was investigated separately, by means of non-isothermal thermogravimetric analysis. The Distributed Activation Energy Model (DAEM) was employed to obtain the pyrolysis kinetic parameters of the samples, i.e. the activation energy E_a and the pre-exponential factor k₀. Nine different pyrolysis tests at different constant heating rates were conducted for each sample in a thermogravimetric analyzer (TGA) to obtain accurate values of the pyrolysis kinetic parameters when applying DAEM. The accurate values of the activation energy and the pre-exponential factor that characterize the pyrolysis reaction of Chlorella vulgaris and sewage sludge were reported, together with their associated uncertainties. The activation energy and pre-exponential factor for the C. vulgaris vary between 150–250 kJ/mol and 10¹⁰–10¹⁵ s⁻¹ respectively, whereas values ranging from 200 to 400 kJ/mol were obtained for the sewage sludge activation energy, and from 10¹⁵ to 10²⁵ s⁻¹ for its pre-exponential factor. These values of E_a and k₀ were employed to estimate the evolution of the reacted fraction with temperature during the pyrolysis of the samples under exponential and parabolic temperature increases, more typical for the pyrolysis reaction of fuel particles in industrial reactors. The estimations of the relation between the reacted fraction and the temperature for exponential and parabolic temperature increases were found to be in good agreement with the experimental values measured in the TGA for both the microalgae and the sludge samples. Therefore, the values reported in this work for the activation energy and the pre-exponential factor of the C. vulgaris can be employed as reference values in numerical studies of the pyrolysis process of this biofuel since its chemical composition is quite homogeneous. In the case of sewage sludge, due to the heterogeneity of its composition, the results reported for the kinetic parameters of the pyrolysis process can be employed to describe the pyrolysis of sludge with similar composition.