Hydrothermal carbonization of anaerobically digested maize silage

Hydrochars were prepared by hydrothermal carbonization (HTC) of maize silage previously treated at 55 °C in a two-stage solid-state reactor system. The HTC was carried out in a 1-L stirred pressure reactor with pH regulation by citric acid. The treated silage carbonized at relatively mild conditions (190 °C, 2 h), and the hydrochars showed mainly amorphous macro-size features with a carbon content of 59–79% (ash-free, dry) and a higher heating value of 25–36 MJ kg−1. Temperature was the main influencing factor. The surface area according to Brunauer–Emmett–Teller (BET) analysis was highest at 190 °C (12.3 m2 g−1). Based on these results, the hydrochars are potentially interesting for applications such as an alternative fuel or a soil conditioner.