(314a) Hydrothermal Carbonization of Wastes for Simultaneous Nutrient Recovery and Energy Capture

Hydrothermal carbonization (HTC) is a process that converts wet biomass into carbon-rich, energy dense solid fuel, called hydrochar, and an aqueous product. The process is done in hot, compressed water, typically in a temperature range of 180 °C - 260 °C. The thermodynamic properties of water change greatly in the subcritical region, and as a result, subcritical water behaves as a non-polar solvent with high ionic activity. When subjected to HTC, biomass undergoes rapid hydrolysis and produces solid hydrochar, as well as an aqueous product containing organic acids, sugars, and sugar derivatives [1].

Industrialized societies produce large waste streams with high moisture content, such as wastewater solids, animal manure, food processing wastes, and some industrial wastes. HTC can readily produce a carbon-rich, hydrophobic solid hydrochar from these waste streams. Most nitrogen compounds (derived from protein) are found in the aqueous stream, while much of the phosphorous is tightly bound in the hydrochar matrix [2]. Ideally, the nutrients can be captured in the aqueous stream, while the carbon (energy) is retained in the hydrochar. Recently, it has been shown that the presence of Ca, Mg, and Fe, as well as pH and reaction temperature, all affect the extraction of P and N from swine manures [3].

In this study, we evaluate the effects of reaction temperature, reaction time, pH, and especially addition of specific metal oxides on HTC of cow manure. We will show that the energy can be most effectively concentrated in the hydrochar at high reaction temperature with short reaction time. Under most reaction conditions, the nitrogen is readily partitioned to the aqueous phase. However, phosphorous capture requires longer residence times and addition of MgO, CaO, or Fe₂O₃.

References

[1] J. G. Lynam, M. T. Reza, W. Yan, V. R. Vásquez, and C. J. Coronella, “Hydrothermal carbonization of various lignocellulosic biomass,” Biomass Convers. Biorefinery, vol. 5, no. 2, pp. 173–181, 2015.

[2] M. T. Reza, A. Freitas, X. Yang, S. Hiibel, H. Lin, and C. J. Coronella, “Hydrothermal carbonization (HTC) of cow manure: Carbon and nitrogen distributions in HTC products,” Environ. Prog. Sustain. Energy, vol. 35, no. 4, pp. 1002–1011, Jul. 2016.

[3] U. Ekpo, A. B. Ross, M. A. Camargo-Valero, and L. A. Fletcher, “Influence of pH on hydrothermal treatment of swine manure: Impact on extraction of nitrogen and phosphorus in process water,” Bioresour. Technol., vol. 214, pp. 637–644, Aug. 2016.