Slow pyrolysis of biomass: effects of effective hydrogen-to-carbon atomic ratio of biomass and reaction atmospheres

In this study, slow pyrolysis of eight biomass feedstocks was examined at 350°C for 60 min. The effects of the biomass effective hydrogen-to-carbon atomic ratio (H/C_eff) and reaction atmosphere (N₂, H₂, CO₂, and CH₄) on the product yields and properties of the pyrolysis oil were studied. Basically, biomass with a higher H/C_eff ratio produced a higher pyrolysis oil yield and a lower solid yield, which varied between 4.66 and 45.57 wt.% and 22.10 and 54.17 wt.%, respectively. The pyrolysis oil yield is proportional to the protein+lipid content, while the solid yield is proportional to the cellulose +hemicellulose lignin content of the biomass feedstock. Lignin is the most difficult component of the biomass feedstock to convert. The effect of the reaction atmosphere on the pyrolysis product distribution depends on the biomass H/C_eff. The pyrolysis oil+gas yield decreased in the order H₂> N₂> CO₂> CH₄ with biomass H/C_eff ≤ 0.61, CH₄> H₂> CO₂> N₂ with biomass H/C_eff between 0.61 and 0.96, and N₂> CO₂> H₂> CH₄ with biomass H/C_eff ≥0.97. The gaseous products mainly consisted of CO₂, CH₄, CO, H₂, C₂H₆, and C₃H₈, and CO₂ always had the highest yield. All the pyrolysis oils had higher C and H content and lower O and S content compared with that of the biomass feedstock, and biomass with a higher H/C_eff ratio produced a pyrolysis oil with a higher H/C_eff ratio. Furthermore, biomass with a higher H/C_eff ratio produced a pyrolysis oil with a higher saturated hydrocarbon content, while biomass with a lower H/C_eff ratio produced a pyrolysis oil with a higher content of ketones and phenolic compounds.