Improving gaseous biofuel yield from seaweed through a cascading circular bioenergy system integrating anaerobic digestion and pyrolysis

Abstract

Advanced biofuels include biomass sources free from land use such as seaweed. Seaweed biomethane may contribute significantly to a climate-neutral transport future; however, seaweed has limited biodegradability via anaerobic digestion (AD). To address this issue, the authors proposed a cascading circular bioenergy system incorporating pyrolysis (Py) for production of biochar, syngas and bio-oil, with the primary use of biochar in AD to promote biomethane production through direct interspecies electron transfer. The feasibility of the proposed AD-Py system was demonstrated by integrating a seaweed-based AD and a residue-based Py system to enhance advanced biofuels production. The AD results showed biochar achieved comparable performances to high-cost graphene in terms of enhancing biomethane production from seaweed. When digesting Laminaria digitata (common kelp), optimal biochar addition at 1/4 (biochar mass: volatile solid of seaweed) increased biomethane yield by 17% and peak production rate by 29% with accelerated volatile fatty acids conversion during AD. When digesting Saccharina latissima (sugar kelp), biomethane yield increased by 16% with optimal biochar addition. A mass and energy balance analysis indicated that processing 1.000 t of Laminaria digitata in AD, combustion of syngas and surplus biochar (in excess of biochar added in AD) from Py of 1.254 t forest residue and 0.078 t dried digestate could fulfil all the heat demand for the integrated AD-Py system. The process integration increased biomethane yield by 17% and bio-oil yield by 10%. Furthermore, a 26% decrease in digestate mass flow could be achieved, thereby reducing the demand for agricultural land for digestate application.