Collaborative Research: Combustion Behavior of Hydrochars from Wet Biomass

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Carbon-based waste from everyday sources, like landfills, agricultural processes, wastewater treatment, and more, are typically “wet” wastes, which makes them difficult to convert to biofuels using traditional methods. This work focuses on a process called “hydrothermal carbonization” (HTC), which converts these already wet bio-materials into high-grade fuels, which is both more efficient and cost effective than typical methods. The resultant fuel is known as “hydrochar,” a carbon-condensed solid often referred to as bio-coal for its “coal-like” properties. Compared to coal combustion, hydrochars emit fewer hazardous air pollutants and have a near net-zero CO2 impact. During the formation of these hydrochars, however, a tar-like substance forms on the surface, which initial research suggests could significantly change their combustion behavior. The goal of this work is two-fold: first, understand the formation pathways and chemical composition of the tar-like substance and second, understand the impact that the tar-like substance has on important combustion behaviors like ignition and particle burn-out. The work will include a close collaboration between different fields, bringing the fields of fuel synthesis and combustion closer through carefully-designed experiments. Significant outreach to both technical communities and the industries they support will support the adoption of hydrochars as a replacement for coal in energy production. The ability to use such renewable solid fuels for energy generation can help mitigate climate change and transform the U.S. into a green energy exporter and job opportunity creator. To fill such high-tech positions, we need a diverse workforce; the PIs – tenure-track female professors at Cornell and Penn State – both have strong track records training students from under-represented groups. At least two graduate and two undergraduate students will be trained on this project.Hydrothermal carbonization is widely touted for its ability to transform moist biomass streams into renewable solid fuels that could be used as replacements for coal in energy generation. Despite the potential benefits of HTC for waste management, a reactive amorphous secondary char often forms on the surface of the solid hydrochar during carbonization. This secondary char may hamper the ability of these “bio-coals” to be used as drop-in fuels for combustion due to the drastically different reactivity of the secondary char. To date, the renewable fuels literature characterizes hydrochars only through thermogravimetric analysis of rapid oxidation, not true combustion behavior. As such, we know little about their combustion behavior, relying only on basic fuel characterizations to gauge this renewable solid fuel’s potential. The goal of the work – the first study of its kind – is to understand the fundamental combustion behavior of biomass-based hydrochars, a solid fuel with unique burning characteristics. The work will improve our fundamental understanding of (1) the mechanisms of hydrothermal carbonization and (2) the combustion behavior of renewable solid fuels with condensed tarry phases. A key broader impact of the proposed work is to rigorously connect two (surprisingly) disparate fields – Renewable Fuel Science and Combustion Science. The closely-coupled work between the PIs will provide not only a scientific foundation for linking these two communities, but the PIs will actively participate in each other’s research communities to bridge the gap. The PIs will work with an HTC technology development firm to disseminate information to the designers of large-scale HTC processing equipment and potential customers interested in firing hydrochars for energy generation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.