Mild upgrading of biomass pyrolysis vapors via ex-situ catalytic pyrolysis over an iron-montmorillonite catalyst

Abstract

Pyrolysis vapor upgrading can improve the stability of pyrolysis liquids to facilitate storage, transport, and drop in use as fuels and chemical feedstocks in existing refining infrastructure. This study investigates a low cost iron based catalyst for mild in-situ upgrading of fast pyrolysis vapors to increase the stability of pyrolysis liquid. Iron-doped montmorillonite K10 catalysts were characterized by ICP-OES, BET porosimetry, XRD, XPS, and NH3-TPD. The effect of three upgrading process parameters are evaluated by Box-Behnken experimental design using a fixed catalyst bed. Predictive model equations are developed based on response surface methodology to describe the effect of iron loading, catalyst to biomass ratio, and catalyst bed temperature on yields of pyrolysis liquids, non-condensable gas, and coke on the catalyst. Compared to non-catalytic pyrolysis, pyrolysis vapor upgrading over iron catalyst reduce pyrolysis liquid yields and increase gas yields as pyrolysis vapors are catalytically cracked into lighter molecules. Liquid yield decreases with C:B ratio while gas yields increase. By increasing Fe loading from 0% to 10%, catalyst coking decreases from 11.7% to 3.5% yield. Iron loading of 10% is the least susceptible to coking, but results in greater amounts of polyaromatics in the pyrolysis liquid product. According to GCMS analysis of the pyrolysis liquids, upgraded pyrolysis liquids have lower concentrations of unstable carboxylic acids and aldehydes. In addition, formation of phenols shift towards a greater fraction of alkyl phenols and lower fraction of methoxy phenols leading to a more stable liquid product as a result of upgrading. Overall, iron-doped montmorillonite catalyst is demonstrated to be a viable, low cost catalyst for mild upgrading of fast pyrolysis vapors.