Washing walnut shells with the aqueous part of pyrolysis liquids: effect on biomass and pyrolysis product quality

Biomass, as a renewable and clean energy source, can be utilized for partial fuel substitution through the process of fast pyrolysis, which converts it into bio-oil. However, the presence of naturally occurring metallic elements in biomass has an adverse effect on its pyrolysis process. In this study, the aqueous fraction separated from biomass pyrolysis liquid, characterized by its acidity and high water content, was used for washing pretreatment of walnut shells. The analysis of the treated walnut shells was meticulously conducted employing techniques, including inductively coupled plasma mass spectrometry, thermogravimetric analyzer, and pyrolyzer-gas chromatography/mass spectrometer. The results indicate that this method has a positive influence on the composition and structure of walnut shells, with removal rates of 75% for Na and 45% for K. The thermal decomposition peak on the thermogravimetric curve became more distinct and shifted to higher temperatures. In additionally, the maximum weight loss rate increases to 0.83 °C/%, and the final residue decreased to 17%. The yields of acids and ketones in the pyrolysis products decreased by 49.43% and 53.69%, respectively. Meanwhile, the yields of phenols and sugars increased by 17.43% and 80%, respectively. Furthermore, the influence of pyrolysis conditions (temperature and time) on the pyrolysis products was investigated, and the optimal pyrolysis conditions (500 °C and 20 s) were determined. Therefore, washing pretreatment of the aqueous part of pyrolysis liquid can effectively enhance the quality of biomass and pyrolysis products. It not only contributes to improving the utilization value of pyrolysis liquid by-product but also holds practical significance for the sustainable use of resources and energy production.