Biochar and hydrochar in the context of anaerobic digestion for a circular approach: An overview

• Published in: The Science of the total environment Vol. 822; p. 153614
• Main Authors: Cavali, Matheus, Libardi Junior, Nelson, Mohedano, Rodrigo de Almeida, Belli Filho, Paulo, da Costa, Rejane Helena Ribeiro, de Castilhos Junior, Armando Borges
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 20-05-2022
• Subjects: Anaerobiosis; Biofuels; Biogas purification; Charcoal - chemistry; Gasification; Hydrothermal carbonization; Methane; Pyrolysis; Siloxane; Pyrolysis; AC; AD; GHG; Biogas purification; VFAs; CHP; LCA; HRT; Biofuels; IIET; TEA; DIET; OLR; Hydrothermal carbonization; HTC; Siloxane; HHV; Gasification; SOFC
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2022.153614

Biochar and hydrochar are carbonaceous materials with valuable applications. They can be synthesized from a wide range of organic wastes, including digestate. Digestate is the byproduct of anaerobic digestion (AD), which is performed for bioenergy (biogas) production from organic residues. Through a thermochemical process, such as pyrolysis, gasification, and hydrothermal carbonization – HTC, digestate can be converted into biochar or hydrochar. The addition of either biochar or hydrochar in AD has been reported to improve biochemical reactions and microbial growth, increasing the buffer capacity, and facilitating direct interspecies electrons transfer (DIET), resulting in higher methane (CH4) yields. Both biochar and hydrochar can adsorb undesired compounds present in biogas, such as carbon dioxide (CO2), hydrogen sulfide (H2S), ammonia (NH3), and even siloxanes. However, an integrated understanding of biochar and hydrochar produced from digestate through their return to the AD process, as additives or as adsorbents for biogas purification, is yet to be attained to close the material flow loop in a circular economy model. Therefore, this overview aimed at addressing the integration of biochar and hydrochar production from digestate, their utilization as additives and effects on AD, and their potential to adsorb biogas contaminants. This integration is supported by life cycle assessment (LCA) studies, showing positive results when combining AD and the aforementioned thermochemical processes, although more LCA is still necessary. Techno-economic assessment (TEA) studies of the processes considered are also presented, and despite an expanding market of biochar and hydrochar, further TEA is required to verify the profitability of the proposed integration, given the specificities of each process design. Overall, the synthesis of biochar and hydrochar from digestate can contribute to improving the AD process, establishing a cyclic process that is in agreement with the circular economy concept.
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•Digestate can be treated by pyrolysis, gasification, or hydrothermal carbonization.•The addition of biochar and hydrochar in anaerobic digestion (AD) is advantageous.•Biochar and hydrochar can be used to remove CO2, H2S, and NH3 from biogas.•Biochar and hydrochar are potential adsorbents of siloxanes in biogas.•LCA of AD coupled with thermochemical processes have shown positive results.