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

Abstract

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.

Keywords:

• Biogas
• methane
• particle size
• HTC
• biochar
• micropore
• molten salt activation

Additional information

Funding

The publication was made possible by the United States Department of Agriculture (USDA National Institute of Food and Agriculture (NIFA) award: NC.X-314-5-18-130-1). This work was partially performed at the Joint School of Nanoscience and Nanoengineering, a member of Southeastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542174).