The effect of temperature, residence time, and water-sludge ratio on hydrothermal carbonization of DAF dairy sludge

Batch hydrothermal carbonization (HTC) experiments were carried out using dissolved air flotation (DAF) dairy sludge in order to investigate the effects of changing temperature, residence time and water-sludge ratio on the yield and quality of the products. The highest solid hydrochar (HC) yield of 84 % (dry basis) and highest HC energy yield of 96 %, were achieved at a temperature of 250°C, a residence time of 4h and a water content of 96 wt.%. The wt.% of carbon and corresponding energy yield of HC increased with process severity while the oxygen and volatile matter contents decreased. Similarly, HC ash content and ash elemental composition increased and the resulting solid became more stable and hydrophobic. The majority of the compounds detected in the liquid product were acids, with carbon chain ranging between C1 and C10. The total acid-phenol concentrations increased with the severity of the HTC operating conditions, but remained around 2500 ppm. The hydrocarbon content of the gaseous product was low and a high CO2 concentration was observed, while H2S concentration increased significantly with the increase in temperature, residence time and water content. A degradation mechanism for proteins and fats was developed and a full elemental mass balance was performed.