*2.3. Physico-Chemical Characterization of the EOP Biochar*

Determining the elemental composition is a crucial analysis to be carried out for a biochar, which could be used, thereafter, as a fuel, to predict its heating capacity, combustion performance, and exhaust emissions. The percentages of C, H, N, and S (CHNS) of the biochar were determined using the Vario Micro Elementar CHNS system. The oxygen content was calculated by the difference.

The HHV of the biochar was calculated using Equation (11). The volatile matter, moisture, and ash content of the EOP biochar were determined using a thermogravimetric analysis and the ASTM D 482, respectively. XRF was carried out using the S4 Pioneer system in order to determine the chemical composition of the EOP biochar ash. X-ray diffraction (XRD) analysis was performed in order to identify the crystal structure of the EOP biochar, using a Philips X'pert Pro super Diffractometer with monochromatic Co Kα radiation in the 2θ range from 10◦ to 80◦ at a scanning rate of 2◦ ·min−<sup>1</sup> . The layer dimension perpendicular to the basal plane, Lc, of the EOP biochar was obtained from the (002) reflection angle following the Debye–Scherrer equation, whereas the d-spacing was determined according to the Bragg formula, with n being equal to 1 and θ representing the (002) reflection angle [18].

The porous morphology of a biochar is an important property to investigate, considering its relationship to the surface area, the distribution of the pore's diameter, its porosity, and the effect of these parameters on gaseous species absorption.

Representative SEM showing the primary particle sizes of the EOP biochar was recorded using a Hitachi S-4800 scanning electron microscope. In addition, an Autopore IV 9500 mercury porosimeter (Micrometrics, York, PA, USA) was used to measure the pore volume distribution, the surface area, and the porosity of the EOP biochar by measuring the amount of mercury penetrating into the sample pores [53].

The temperature-programmed oxidation analysis (TPO) of the EOP biochar sample was carried out using a thermobalance (NETZSCH STA, 449F3) with an air flow of about 80 mL·min−<sup>1</sup> at the standard conditions of temperature and pressure (STP) and at a heating rate of 10 ◦C·min−<sup>1</sup> . The temperature ranged from room temperature to 1000 ◦C.

The EOP biochar's chemical functional groups were identified by FTIR analysis using a Nicolet 380 spectrometer (Thermo-Scientific, Waltham, MA, USA) in the range of 4000–400 cm−<sup>1</sup> . The standard IR spectra of hydrocarbons were used to identify the functional groups of the biochar components.
