Conversion of biodigestate into activated carbon for electrochemical application: Process performance and life cycle assessment

Abstract

Carbons derived from pyrolysis and activation of waste biomass are attracting much attention as components of energy technologies, such as batteries, supercapacitors and fuel cells. This experimental study focuses on the production of a high-surface-area biochar (LAC) obtained from the treatment of lignin-rich waste of a biodigester plant, by applying KHCO3 as an activating agent. The pyrolysis-activation conditions were set by following the process by thermogravimetric analysis and by checking the purity and porosity of the resulting carbon by several analytical techniques. The best pyrolysis condition provided a microporous carbon featuring up to 1840 m2 g−1, which was demonstrated at a 25 g biodigestate batch-scale. Moreover, the production process was critically analyzed by means of life cycle assessment to identify environmental hotspots and thus derive recommendations for process optimization. The impact of substance- and energy-recovery and the use of renewable energy sources on the sustainability of the product was demonstrated in several scenarios, complemented by benchmarking and an outlook regarding further optimization needs. In the best case scenario, the global warming potential of the proposed biochar could be reduced to 15.9 kg CO2-eq per kg of LAC. The activating agent KHCO3 was dominant in almost all environmental impact categories, hence, a theoretical recovery process for this substance was suggested and evaluated via life cycle assessment.