Impact of Co-Hydrothermal carbonization of animal and agricultural waste on hydrochars’ soil amendment and solid fuel properties

Hydrothermal carbonization (HTC) is a promising strategy to upcycle wet biomass wastes to carbon-condensed solid hydrochars. While cow manure is an ideal substrate for HTC in terms of its moisture content, resulting hydrochars lack any degree of aromatization and easily disintegrate under pressure. Such properties render manure hydrochars suboptimal soil amendments or solid fuels. We explore the co-hydrothermal carbonization of cow manure with two dry agricultural biomasses, grape marc and corn stover, to determine the degree to which mixing these biomasses improves the hydrochars' soil amendment or solid fuel properties. Contrary to some prior literature, the co-HTC did not synergistically enhance solid yield or higher heating value, though it did increase the electrical conductivity of the hydrochars beyond an additive prediction. Overall, hydrochars have higher fixed carbon and lower H/C and O/C atomic ratios than their parent feedstocks, reducing their susceptibility to thermal degradation and decreasing the bioavailability of heavy metals. Given the potential to reduce phosphorous run-off via co-HTC, the land application of hydrochars could reduce eutrophication versus land application of raw manure. Despite these improvements over the raw feedstock, the surface areas and elemental carbon contents are still lower than most pyrolysis-based biochars. Higher heating values, relatively low ash content and reduced oxidative reactivity suggest that the blended hydrochars are likely better suited for utilization as a solid fuel than as a soil amendment.