Thermophilic anaerobic digestion of cattail and hydrothermal carbonization of the digestate for co-production of biomethane and hydrochar

Thermophilic anaerobic digestion (AD) of cattail followed by hydrothermal carbonization (HTC) was studied. The intent of the research was to develop agricultural waste-based biorefining technologies for bioenergy production along with value-added products. Cattail was anaerobically digested at 55 °C for 14 days and protein and cellulose components were partially degraded. The average methane yield was 230–280 mL/g volatile solids and the total solids decreased by 33–55%. When the particle size of cattail was reduced from 1 in. to 1 mm, the lag phase was shortened from 1.48 to 0 d. Following the AD process of cattail, the AD digestate was hydrothermally carbonized at 250 °C for 4 h, yielding approximately 6.7–7.5 wt % gaseous products, 64 wt % liquid products and 28 wt % hydrochar. The gaseous products contained >5000 ppm H2S and liquid products possessed fewer chemicals and higher ratio of phenolic compounds compared to the liquid products from HTC of original cattail. The hydrochar had a higher carbon content (76.8–79.8%) and a higher specific surface area (∼10 m2/g) than those of the feedstock. Hydrochar was further activated by using Na2CO3, NaHCO3 and NaCl. The activation process increased the carbon content and specific surface area to 84–93% and 250–630 m2/g, respectively.