Combination of Hydrothermal Carbonization and Anaerobic Digestion for Solid Fuel and Biogas Production

Biogas from anaerobic digestion (AD) is a significant renewable energy resource. The sharply developing of AD produces numerous digestate. However, the ever-increasing production of digestate brings other crucial problems: greenhouse gas emission during storage if not covered, high nitrogen content, and transportation costs, which restrict its application as fertilizer. Hydrothermal carbonization (HTC) can convert the water-rich digestate to a lignite-like solid product (hydrochar). The liquid phase (process water) is rich in organic substances and has the potential to produce methane by AD. This dissertation aimed to understand the effects of digestate origin, HTC temperature, and residence time on the properties of digestate hydrochar, the influence of HTC residence time on the combustion characteristics of energy crop digestate hydrochar and methane production the process water, and to investigate a novel concept of coupling HTC and two-stage AD. The results revealed that increased HTC severity only showed a positive effect on the carbon content (dry basis) and higher heating values (HHV, dry basis) of low ash-containing digestate (cow manure digestate). Followed up study on the energy crop digestate indicated that slagging and fouling could not be avoided during the combustion of hydrochars produced at 210 °C with different residence times. The HTC residence time also showed no significant effect on the specific methane yield of the process water. The co-digestion test of process water and hydrolysate proved the feasibility of combining HTC and two-stage AD, while further optimization is needed. This dissertation’s findings provide helpful knowledge for treating different digestates by HTC in a more effective way to improve digestate management.