Laser pyrolysis of biomass: Comparing cellulose-rich and lignin-rich feedstocks for hydrogen rich-gas production

Abstract

The increasing global demand for sustainable energy solutions necessitates innovative approaches to hydrogen production. This study investigates the use of laser pyrolysis as a rapid and efficient method for converting biomass into hydrogen-rich gas, biochar, and bio-oil. Olive stones and onion post-harvest waste were chosen as feedstocks due to their abundance, low cost, and distinct chemical compositions. Experiments were conducted under varying laser power levels (25 %, 50 %, and 100 %), from a continuous wave CO₂ laser to achieve high heating rates and rapid decomposition. Comprehensive analyses, including FTIR, GC-MS, and Raman spectroscopy, were employed to characterize the products. The results revealed that hydrogen yield increased with increasing laser power for both biomass samples. Notably, the hydrogen yield was significantly higher for onion waste compared to olive stones, primarily due to the lower cellulose and hemicellulose content in olive stones. The general trend observed for this yield was: Biomass_{100 % power} > Biomass_{50 % power} > Biomass_{25 % power}. Specifically, Hydrogen (H₂) was the dominant component for onion waste, with its concentration increasing from 31.50 % at 25 % power to 36.18 % at 100 % power, and olive stones exhibited lower hydrogen production, with H₂ concentrations increasing modestly from 14.31 % at 25 % power to 15.78 % at 100 % power. Moreover, the hydrogen yield for onion waste at 100 % power reached 177.72 L/kg, substantially higher than that of olive stones at 100 % power, which yielded 49.78 L/kg. Furthermore, onion waste produced biochar with a more porous structure compared to olive stones. The gaseous products, predominantly hydrogen and methane, showed significant tunability based on feedstock and laser power, highlighting the flexibility of this technique for targeted product optimization. This study reveals the potential of laser pyrolysis as a transformative technology for biomass valorization, offering a sustainable pathway for hydrogen rich-gas production and co-generation of value-added products.