Pyrolysis Characteristics and Hydrogen Production Mechanism of Biomass Impregnated with Transition Metals

Abstract

Impregnation with transition metals changes the product distribution of biomass after three-stage pyrolysis, which can be analyzed to reveal the transfer path of H atoms and the formation mechanism of H2. The effects of impregnation pretreatment with three different transition metals (iron, cobalt, and nickel) on the pyrolysis characteristics of corn straw were evaluated. Three transition metal solutions with concentrations of 0.5 M, 1 M, and 2 M were used to explore the influence of impregnation concentration on the yield and composition of the pyrolysis products. The results showed that iron promoted the formation of liquid products, whose proportion increased by 37.29% upon increasing the impregnation solution concentration. Cobalt and nickel tended to produce gases, and when the impregnation solution concentration was greater than 0.5 M, the gas production rate exceeded 45 wt.%. Nickel tended to form H2 and CH4, whose yields were increased by 63% and 20% respectively compared with raw materials. Py-GC/MS analysis showed that the main component of tar was alcohol. Metal impregnation decreased the content of aromatic compounds in tar, which increased the stretching vibrations of C-O and C-H bonds in char. Ring-opening rearrangements of polycyclic aromatic hydrocarbons were promoted, which formed more -H. Nickel catalyzed the removal of –COOH and –CH3 groups, which formed more H2 and CH4. These findings provide an avenue to optimize the metal impregnation conditions to produce hydrogen via biomass pyrolysis.

Keywords: Pyrolysis, corn straw, transition metal impregnation, hydrogen production, mechanism