Effects of nascent char on ex-situ catalytic fast pyrolysis of wheat straw

Abstract

This study aims to investigate the effects of nascent char in pyrolysis process on the product distribution from ex-situ catalytic fast pyrolysis (CFP) of biomass. Fast pyrolysis of wheat straw without HZSM-5 as well as with insufficient/sufficient HZSM-5 were performed with controlled extents of volatile-char interaction in a two-stage fluidized-bed/fixed-bed combination reactor. The results show that nascent char has the catalytic ability to promote the secondary cracking of pyrolysis vapors and the secondary formation of solid organics. The carbon yield of olefins + aromatics increased from 12.33% to 14.70% and then decreased to 12.68% with intensified volatile-char interaction when HZSM-5 was insufficient. The initial increase can be explained by the higher conversion efficiency of residual light compounds for HZSM-5 catalysis after volatile-char interaction. Nascent char was observed to have negative effects on the production of aromatics and olefins when HZSM-5 was sufficient. However, with a slight decrease (by 2.04%) in the carbon yield of olefins + aromatics at the beginning of char accumulation, coke carbon yield significantly decreased by 15.88%. CFP of raw and acid-washed wheat straw over insufficient HZSM-5 with controlled extents of volatile-char interaction were further compared. It is demonstrated that the inorganic minerals in raw biomass play as the active components of nascent char in volatile-char interaction.