Alcohol Stabilization of Low Water Content Pyrolysis Oil during High Temperature Treatment

Abstract

The addition of alcohols is a promising method to pretreat and stabilize pyrolysis oil by converting carboxylic acids and reactive carbonyl compounds into esters, ethers, and acetals. In this study a series of alcohols - methanol, ethanol, 1-propanol, 2-propanol, or 1-octanol - was added to crude pyrolysis oil, and these mixtures were stored at 200 °C for different periods from 6 to 50 h to investigate the impact of heat treatment on the oils' physicochemical properties. All oil/alcohol mixtures were characterized by Karl Fisher titration, viscometer/rheometer, differential scanning calorimetry (DSC), gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and gas chromatography-mass (GC-MS) spectrometry. Phase separation is observed for all aged oil/alcohol mixtures. The time-dependent rheologies of heat treated oil/1-propanol, oil/2-propanol, and oil/1-octanol mixtures are found to be well fitted by the Herschel-Bulkley model. Isothermal DSC traces directly confirm that low molecular mass (LMM) alcohols (methanol, ethanol, 1-propanol, and 2-propanol) improve the stability of pyrolysis oil. Although 1-octanol is less efficient in slowing the aging reactions, it significantly reduces the increase rate of viscosity and molecular weight of pyrolysis oil compared with LMM alcohols. FTIR spectra suggest reactive carbonyl and aldehyde groups are captured by the added monofunctional alcohols. GC-MS results indicate esterifications contribute significantly to mitigate aging reactions. The introduction of LMM alcohols or a combination of LMM alcohols and HMM alcohols is a promising pretreatment method before the further catalytic upgrading procedure on the crude pyrolysis oil.