Pyrolysis of macroalgae residue from the agar industry for silica-rich biochar and other sustainable chemicals: Process performances, product applications, and simple business scenario

The macroalgae residue from the industrial agar extraction process contains a significant amount of carbon and has potential as a renewable feedstock. Unfortunately, it is often overlooked and is poorly utilized. This study aims to valorize this macroalgae residue through pyrolysis to produce silica-rich biochar and other value-added products in the form of biocrude oil (BCO) and biopyrolysis gas. The macroalgae residue was pyrolyzed at 300–700 °C with a heating rate of 20–40 °C/min. Yields of biochar, BCO, and gas of 62%, 25%, and 13% were obtained at a temperature of 700 °C and a heating rate of 20 °C/min. Biochar has a porous structure, a surface area exceeding 15 m²/g, and is dominated by amorphous silica of up to 13%. This silica-rich biochar also contains Na and K, which hold potential benefits in agriculture, serving as soil ameliorants and playing a crucial role in enhancing soil fertility and promoting plant growth. In the meantime, BCO contains 29.3% carboxylic acid group as the most important chemical component. Other than that, the biopyrolysis gas contains mainly CH₄ and H₂ (up to 24–32%), which can act as chemical building blocks. Finally, a simple business scenario of silica-rich biochar production reveals that it has a specific cost of 0.37 US$/kg. It could be economically viable as a soil ameliorant or fertilizer. However, challenges persist in scaling up production to an industrial scale.