*2.1. Raw Material*

The exhausted olive pomace (EOP) used in the present study was recuperated from an extraction factory at Sfax city in Tunisia. The tested EOP sample is a mixture of residues of olive stones and olive pulp extracted from both traditional olive pressing and 3-phase centrifugation systems, followed by solvent extraction using n-hexane for olive pomace oil recovery.

The EOP was sieved using a 10–20 mesh (2.00 ± 0.85 mm) particle size. The sample of EOP was shipped to Galbraith Analytical Laboratory, (Knoxville, TN, USA), where it was analyzed to determine its elemental compositions and to estimate the high and low heating values. For the determination of lignin, cellulose, and hemicellulose, the first removal of soluble extractives was performed according to the standard TAPPI T-264 cm-97 [50]. Then, lignin and cellulose were determined according to the standard TAPPI T 222om-83 and TAPPI T 203 os-74, respectively [50], and holocellulose was determined according to Browning [51]. Hemicellulose concentration was calculated as the difference between holocellulose and cellulose. Ash content was determined according to the ASTM D 482. A proximate analysis (moisture and volatile matter) was performed using a Thermogravimetric Analyzer TGA Labsys (Setaram Instruments, Paris, France). About 20 mg of the sample was loaded into a platinum pan and heated at 20 ◦C min−<sup>1</sup> until 600 ◦C under a nitrogen atmosphere (nitrogen flow rate of 40 mL·min−<sup>1</sup> ). The weight loss from 30 to 150 ◦C was due to the moisture content; from 150 to 600 ◦C, it was due to the volatile matter content. The residual mass was assigned to a mixture of fixed carbon and ash. Fixed carbon was determined by the difference. The analytical results are summarized in Table 1.

The high heating value (HHV) of the EOP biomass feedstock is determined using the modified Dulong's formula based on the following equation [52]:

$$\text{HHV(M).kg}^{-1}) = (33.5 \times \text{C} + 142.3 \times \text{H} - 15.04 \times \text{O} - 15 \times \text{N})/100 \tag{11}$$

where C, H, O, and N are the carbon, hydrogen, oxygen, and nitrogen contents (wt%), respectively.

The low heating value (LHV) is calculated as a function of the HHV and the hydrogen content [52]:

$$\text{LHV (MJ.kg}^{-1}) = \text{HHV} - 2.442 \times 8.936 \times (\text{H}/100) \tag{12}$$


**Table 1.** Proximate and elemental analysis of the EOP raw material and its derived biochar.

\* Dry basic. \*\* By difference.
