Products distribution during in situ and ex situ catalytic fast pyrolysis of Chinese herb residues

Catalytic fast pyrolysis (CFP) for biomass treatment is a research hotspot but there is little information about the difference between the in situ and ex situ methods. In present work, the Ni-Fe/CaO-Al₂O₃ catalysts with different Ni/Fe ratios have been synthesized by coprecipitation method, and the product distribution about the Chinese herb residue (CHR) catalytic fast pyrolysis by in situ and ex situ methods in a quartz tube reactor system has been investigated. The results show that the CFP pyrolysis would upgrade the quality of bio-oil but decrease the yields, no matter in situ or ex situ CFP process. During the in situ CFP process, heteroatoms may be absorbed by the catalyst support and cannot be transferred to the bio-oil, but the results of ex situ CFP are the opposite. In addition, the ex situ CFP reaction significantly increases the content of aromatic hydrocarbons. As to the gas products' distribution, the effect of Fe in catalysts to promote CH₄ formation is reflected in in situ CFP process, while the promotion effect of H₂ generation for Ni added in catalyst is mainly reflected in ex situ CFP process. However, due to the high reaction temperature (800 °C), the adsorption of CO₂ by CaO support is not particularly significant. The possible mechanism of CHR in CFP process has also been summarized for understanding the process of in situ and ex situ CFP, and this study may provide a choice or reference for CHR treatment.