Co-production of hydrochar, levulinic acid and value-added chemicals by microwave-assisted hydrothermal carbonization of seaweed

Seaweed is considered a third-generation biomass for transport biofuel production with great potential for bioenergy and biorefinery applications. Microwave-assisted hydrothermal carbonization (HTC) offers a conversion route for seaweed valorization to multiple value-added products. However, the HTC products (such as hydrochar and process liquor) are highly dependent on feedstock properties and HTC conditions. This study investigated the valorization of Laminaria digitata to a range of products (primarily including hydrochar, levulinic acid and 5-hydroxymethylfurfural) through microwave-assisted HTC at a range of low temperatures (160, 180 and 200 degrees C) and levels of dilute acid addition (1, 2 and 4% H2SO4 concentration). The work suggests an optimized process at a temperature of 200 degrees C with 4% H2SO4, with levulinic acid and 5-hydroxymethylfurfural in the process liquor achieved the highest yields of 12.5% (12.5 g per 100 g dry L. digitata) and 5.8%, respectively. The obtained hydrochar had a higher heating value of 25.5 MJ/kg volatile solids, comparable to lignite coal. At such a condition the total carbon recovery in the produced hydrochar and process liquor accounted for 84% of carbon in the original seaweed. The 200 degrees C derived hydrochar presented the highest carbon content of 65.4%, but the lowest abundance in oxygen-containing functional groups compared to the hydrochars obtained under other conditions (160 and 180 degrees C). The economic benefits of the process might be significantly increased if hydrochar was upgraded into activated carbon instead of being used as a solid fuel.