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The present study examines the effect of both the pressure and peak temperature on the potential stability of the biochar produced from the slow pyrolysis of two-phase olive mill waste. On the basis of the available studies in the literature, the following properties were taken as rough indicators of the potential stability of biochars in soils: the fixed-carbon yield, the fraction of aromatic C, and the molar H:C and O:C ratios. Pyrolysis experiments were performed in a laboratory-scale fixed-bed device and planned following a central composite design. The product gas yield and composition values at the outlet of the secondary cracking reactor (a fixed-bed of activated alumina particles at 700 °C) were also analyzed as a function of pressure and peak temperature. The results from the statistical analyses indicate that both the analyzed factors have a strong influence on the distribution of the pyrolysis products, as well as on the properties of the produced biochar. The most potentially stable biochars were obtained at the highest values of pressure and peak temperature (1.1 MPa and 600 °C). A positive effect of the pressure on the pyrolysis gas yield (at the expense of the total liquid fraction) was also observed.

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The present study analyzes the effect of several operating factors (initial sample mass, heating rate, conditions of the crucible, and inorganic matter content) on the pyrolysis of two-phase olive mill waste. Simultaneous TGA−DSC measurements were performed according to a one-half fractional factorial design to obtain statistically significant conclusions. The results from this screening design pointed out that the behavior of the TPOMW pyrolysis process is statistically affected to a greater or lesser extent by the four factors analyzed. The factors related to the promotion of secondary charring reactions (the initial sample mass and the conditions of the crucible) had the main influence on both the apparent heat and charcoal yield. The enhancement of the charring reactions implies a dramatic decrease in the apparent heat of pyrolysis from highly endothermic (2559 ± 100 kJ kg −1 ) to slightly exothermic (−44.2 ± 49 kJ kg −1 ). On the other hand, a new approach for modeling the apparent kinetics of TPOMW pyrolysis (both primary decomposition and secondary charcoal production) is proposed. The performance of this approach is good enough to provide satisfactory estimations of a large number of unknown kinetic parameters in very short computational times.

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