Grape pomace is a multiphasic and hierarchical biomass waste material that offers challenges and opportunities for pyrolytic conversion to chemicals and biochars. Specifically, grape cuticle waxes and seed oils are solvent-soluble lipids of aliphatic composition, seed endosperm comprises starches, while the seed coat and rachis are woody and lignocellulosic. This study used nuclear magnetic resonance spectroscopy (NMR) with electron microscopy and N2 and Hg porosimetry to gain a primary understanding of the thermochemical controls on the physical and molecular properties of biochars from grape pomace under conventional pyrolysis conditions. Biochars formed at 350 oC had a substantial proportion of aliphatic C (33 mol %) with a high degree of molecular mobility. Increasing the duration of heating at 350oC decreased alkyl chain length and mobility. Exhaustive solvent extractions did not alter the biochar’s alkyl C content or mobility, suggesting that native lipids and starches in the pomace feedstock underwent pyrolytic conversion to insoluble rubbery domains with covalent bonds to the aromatic C. The extraction of lipids from pomace before pyrolysis produced less aliphatic biochar with a higher proportion of micropore volume, suggesting that rubbery domains reside within micropores. Increasing the pyrolysis temperature from 350 to 500 oC increased aromatic condensation from 2 fused rings to ≥ 4, with a greater abundance of stable organic radicals and fewer oxygen atoms. Biochar surface wettability remained poor, regardless of pyrolysis temperature, duration, or lipid extractions, indicating that the pomace biochars are inherently hydrophobic. Macropore volume and structure exhibited a temperature-dependent evolution relating to the formation and loss of the mobile alkyl domains and the differential swelling and shrinkage of the seed coat and endosperm tissues. The data in this study provides new evidence that differential thermal evolution of discrete biological tissues in grape pomace feedstocks allows significant control over the physical, chemical, and functional properties of biochar.

Renewable biochar from winery grape pomace: The influence of pyrolysis temperature, duration, and mobile-free lipids

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ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.