2.3.1. Higher Heating Value (HHV)

An IKA C 200 bomb calorimeter (Staufen, BW, Germany) was used to determine higher heating values (HHV) of the raw and hydrochar pellets by following the ASTM D240 method. Energy yield (EY) was calculated by Equation (2). Triplicates were performed for each sample to report reproducibility.

$$\text{EY}(\%) = \text{MY}(\text{wt. }\%) \times \frac{\text{HHV of dried hydrogen} \left(\frac{\text{MI}}{\text{kg}}\right)}{\text{HHV of untreated dry feedstock} \left(\frac{\text{MI}}{\text{kg}}\right)} \tag{2}$$

#### 2.3.2. Ash

The ASTM D1102 method was followed to determine the ash content of the dry solid samples by using a muffle furnace (Thermo Scientific, Model # FB1415M, Waltham, MA) at 575 ◦C for 5 h 30 min. The ash yield (AY) was calculated by using Equation (2). Triplicates were performed for each sample to report reproducibility.

$$\text{AY (wt. \%)} = \text{MY(wt. \%)} \times \frac{\text{Ash in hydrochar (wt. \%)}}{\text{Ash in raw feedstock (wt. \%)}} \tag{3}$$

#### 2.3.3. Thermogravimetric Analysis (TGA)

The volatile matter (VM) and fixed carbon (FC) of samples were determined by TGA using a TGA Q5000 (TA instruments, New Castle, DE, USA). The experimental procedure was taken from the literature [33]. Experiments were carried out under inert atmosphere using a constant flowrate (10 mL/min) of nitrogen to avoid any possible oxidation and to continuously purge the VM. The sample was first heated to 105 ◦C and kept isothermal for 5 min. The mass loss at this temperature accounts for the moisture content (MC). The sample was then heated to 900 ◦C at a ramp rate of 20 ◦C/min and kept isothermal for 5 min. The mass loss from 105 ◦C to 900 ◦C accounts for VM. Finally, the FC (wt. %) was then calculated from Equation (3):

$$\text{FC (wt. }\%) = 100 \text{ wt. }\%-\text{MC (wt. }\%) - \text{VM (wt. }\%) - \text{ash (wt. }\%) \tag{4}$$

#### 2.3.4. CHNS/O Analysis

A FLASH EA 1112 Series (Thermo Scientific, Waltham, MA, USA) elemental analyzer was used to quantify the elemental carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) content in the sample using the method described in the literature [34]. For the analysis, 2, 5-Bis (5-tert-butyl-benzoxazol-2-yl) thiophene (BBOT) was used as a calibration standard

and vanadium oxide (V2O5) as a conditioner for the samples, which were combusted around 950 ◦C in ultra-high purity oxygen with helium carrier gas and passed over copper oxide pellets and then electrolytic copper. The produced gases were then analyzed by a thermal conductivity detector (TCD), with the peak areas of detection being compared to those of BBOT standards. The oxygen (O) content was found by subtraction method. The following equation was used to find the oxygen wt. %.

O (wt. %) = 100 wt. % − C (wt. %) − H (wt. %) − N (wt. %) − S (wt. %) − ash (wt. %) (5)
