Improving bio-oil quality and stability based on capping reactions


With the rapid development of global economy, there is an ever-growing demand for energy. The excessive use of traditional fossil fuel exposes the human to multiple environmental issues. As a promising technology, thermochemical conversion of biomass is able to provide not only environmental-friendly substitute fuel, but also other value-added chemicals. However, the complex composition and poor quality of conversion products hinders industrial application of biomass in large scale. This study focuses on pyrolysis conversion of biomass and explores possible ways to improve bio-oil quality and stability. First, sodium formate was selected as hydrogen donating agent and co-pyrolyzed with lignin in a micropyrolyzer. It was found that the presence of sodium formate promotes the production of simple and/or stable phenols such as phenol, syringol and ethylphenol, while reducing the yields of reactive vinylphenols. Among the pyrolysis products, acetic acid was eliminated by neutralization. As a result, the pyrolysis oil produced from co-pyrolysis of lignin and sodium formate contained an increased amount of phenolic monomers, and also had an improved thermal stability during aging tests compared to pyrolysis–oil of lignin. Deuterated sodium formate was also employed in the present study to investigate the mechanism of hydrogen transfer during lignin pyrolysis. The presence of hydrogen mainly affected depolymerization of lignin polymer through a series of reactions that involving both primary and secondary reactions to form alkylated phenols. Electrophilic substitution of hydrogen atoms in phenolic aromatic rings was observed. Next, the effect of hydroquinone (HQ) on bio-oil storage stability was investigated as HQ is a well-known free radical scavenger. The addition of HQ in previously condensed bio-oil had no effect on bio-oil aging. In comparison, quenching pyrolysis vapors in HQ containing solvent preserved more monomers after aging by suppressing bio-oil polymerization. The electro paramagnetic resonance (EPR) was used to analyze free radicals in the bio-oils condensed with or without HQ addition. The comparison of the EPR spectra of fresh and aged bio-oil samples showed that addition of HQ in the vapor quenching solvent effectively reduced the concentration of the free radicals in bio-oil. The study suggests that reactive free radicals present in both pyrolysis vapors and freshly condensed bio-oil. Eliminating these free radicals using capping reaction improves bio-oil stability

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