*2.2. Hydrothermal Processing*

Hydrothermal processing of the swine manure was performed in an unstirred 600-mL Parr reactor (Parr, Moline, IL, USA). For each experiment, the reactor was filled with 24 g of swine manure and either 220 mL of de-ionised water (pH ≈ 6) or solutions of 0.1 M NaOH, 0.1 M H2SO4 (pH 13 and 1, respectively), or 0.1 M organic acid (CH3COOH and HCOOH) (pH 2.88 and 2.38, respectively) to form a slurry. Hydrothermal processing was performed at 120 ◦C, 170 ◦C, 200 ◦C, and 250 ◦C for 1 h. The heating rate was approximately 10 ◦C·min−1, and the residence time was taken from when the reactor reached the desired temperature. After one hour, the reactor was removed from the heating jacket and then allowed to cool to room temperature before the products were separated. Bio-coal samples were dried in an oven at 60 ◦C for a minimum of 24 h, and yields were taken as dry bio-coal mass compared with the original dry mass of unprocessed manure.
