Lab-scale pyrolysis and hydrothermal carbonization of biomass digestate: Characterization of solid products and compliance with biochar standards

Carbonization of anaerobic digestion digestate from an Italian plant fed with local feedstock was investigated by slow pyrolysis (SP) and hydrothermal carbonization (HTC); the properties of pyrochar and hydrochar were compared and framed into the International guidelines for soil improvers/amendments in agriculture. Experiments were carried out in newly designed thermochemical conversion systems, with the aim of generating the original data necessary for future scale-up and for evaluating the characteristics of the product versus existing biochar market standards. SP was carried out in a paddle pyrolysis reactor at 500 °C for 1 h, while several HTC experiments were performed in a custom-made test bench, investigating the influence of temperature (200–250 °C) and time (0.5–3 h) on hydrochar properties. Both processes were run in batch mode. SP showed lower char yield (33.1%w/w db), while the maximum hydrochar yield was obtained at 200°C-0.5 h (72.6%w/w db). Most char properties fall within the thresholds set by biochar standards. The Carbon content was slightly higher for pyrochar (64.3%w/w db) than hydrochar (62.9%w/w db at 250°C-3 h). The specific area was rather low for both chars; however, pyrochar area (23.10 m2 g−1), was one order of magnitude higher than hydrochar. The ash content of pyrochar was nearly twice the hydrochar, while O/C and H/C ratio of the former were lower. The HTC aqueous phase was also characterized in terms of inorganics concentration and organic composition. In average, the concentration of the former increased with reaction severity: more than 50 compounds were identified and 20 quantified. Acetic acid was the most abundant (up to 3.26 g l−1).