Exploring the Biomethane Potential of Different Industrial Hemp (Cannabis sativa L.) Biomass Residues

Industrial hemp stands out as a promising candidate for clean and sustainable biomass-to-bioenergy systems due to its multipurpose, high biomass yield and resource efficiency features. In this study, different hemp biomass residues (HBRs) were evaluated as a potential feedstock for renewable biomethane production through anaerobic digestion (AD). The biochemical methane potential (BMP) of the raw and pretreated fibers, stalks, hurds, leaves and inflorescences was investigated by means of batch anaerobic tests. The highest BMP was obtained with the raw fibers (i.e., 422 ± 20 mL CH4·g VS−1), while hemp hurds (unretted), making up more than half of the whole hemp plant dry weight, showed a lower BMP value of 239 ± 10 mL CH4·g VS−1. The alkali pretreatment of unretted hurds and mechanical grinding of retted hurds effectively enhanced the BMP of both substrates by 15.9%. The mix of leaves and inflorescences and inflorescences alone showed low BMP values (i.e., 118 ± 8 and 26 ± 5 mL CH4·g VS−1, respectively) and a prolonged inhibition of methanogenesis. The latter could be overcome through NaOH pretreatment in the mix of leaves and inflorescences (+28.5% methane production).