To sustainably meet rising global food demands, we must improve the efficiency of water and nutrient use by converting agricultural waste to renewable bioproducts. Hydrothermal carbonization (HTC) and pyrolysis are thermochemical processes that convert carbonaceous wastes to hydrochar and biochar, respectively. However, neither alone is a perfect solution to biomass waste management. HTC, which utilizes water as both a solvent and catalyst, is well-suited to treating wet wastes, but the resulting hydrochar can be unstructured and unstable. Pyrolysis can produce highly structured biochars, but it is not suitable to deal with wet biomass streams due to the required drying required as a pre-treatment. This work examines two potential pathways for co-processing agricultural wastes by cascading HTC and pyrolysis. In the first pathway, a wet waste (apple pomace or dairy whey) is co-carbonized with a dry waste (barley straw or cherry pits) at ratios of 90:10 and 80:20 (wet waste : dry waste). The resulting hydrochar is then pyrolyzed, extracting additional biofuels and densifying the solid product. In the second pathway, wet waste is first carbonized and a dry feedstock is combined with the hydrochar for pyrolysis at ratios of 80:20 and 50:50 (dry waste : hydrochar). Ratios were chosen to represent industrially relevant mixtures; due to the water requirements of HTC, adding more than 20% dry waste would require additional water input. The resulting heterogeneous solids were characterized via FTIR, pH, EC, BET surface area analysis, proximate analysis, and ultimate analysis to gauge their potential use as soil amendments or for upgraded carbon applications. Biochars with hydrochars incorporated as feedstocks were found to have higher surface areas and fixed carbon, suggesting that these treatment pathways could be used to produce stable and structured soil amendments from heterogeneous feedstocks.

Cascaded thermochemical processing of wet and dry agricultural biomass waste for upgraded fuels and solid products

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ACS Fall 2022

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