Microwave pyrolysis of Laminaria digitata to produce unique seaweed-derived bio-oils

Highlights: • Microwave pyrolysis of Laminaria digitata and its extraction residue were investigated. • Microwave pyrolysis was achieved without use of microwave susceptors. • Energy requirements of 1.84–2.83 kJ g⁻¹ pyrolysed 55–70% of both feedstocks. • Maximum pyrolysis processing time of 200 s required. • Bio-oils contained no phenolic compounds but an abundance of nitrogen based compounds. -- Abstract: Microwave pyrolysis has become an attractive form of processing technology to generate bio-oil, bio-char and syngas from different biomass feedstocks. In this study, microwave pyrolysis was performed on the UK native seaweed Laminaria digitata and its extract residue from a bio-refinery process. Pyrolysis of these two feedstocks was successfully achieved without the requirement of microwave susceptors, as pelletizing the biomass was sufficient to allow microwave pyrolysis to occur. It was found that average energy requirements as low as 1.84–2.83 kJ g⁻¹ were required to pyrolyse 55–70% of both feedstocks and bio-oil yields of 5–8% and 10–14% for native and extraction residue L. digitata were produced, respectively. Maximum microwave pyrolysis processing times were in the order of 200 s. The bio-oil generated from both feedstocks contained no phenolic based compounds, but a greater number of nitrogen-containing compounds and compounds derived from macroalgal polysaccharides. Yields of certain compounds differed in bio-oils generated from the two L. digitata feedstocks, however it was observed that specific energy did not have a direct influence on bio-oil compound yield. Furthermore, the identification of a particular nitrogen-containing compound L-Proline, 1-methyl-5-oxo-, methylester is thought to be a unique product of microwave pyrolysis when carbon-based additives are avoided.