Hydrothermal conversion of beef cattle manure can enhance energy recovery in confined feedlots

Abstract

Conversion of manure into energy products via hydrothermal liquefaction was studied. Hydrothermal reactions, conducted at different temperatures (200–300 °C), were compared on the basis of conversion yields, carbon and energy recoveries and product quality. The results demonstrate how reaction temperatures affect the relative production of biocrude oil and hydrochar. Comprehensive analyses of both biocrude oil and hydrochar suggest that decarboxylation was the dominant mechanism involved in the reaction and was enhanced with temperature. With a relatively high carbon (67–74%) and low oxygen (13–18%) content, biocrude oil seems to provide a better platform to deliver recovered energy, with higher heating values of 30–35 MJ kg⁻¹. An overall energy balance, supported by a sensitivity analysis, demonstrated how the higher temperature enhanced the overall energy return. Twelve scenarios were used to estimate the daily net energy gained in feedlots of different sizes. All scenarios provided a positive energy balance with returned energy found to be 2 to 3 times higher than invested energy. The estimation presented herein highlights the feasibility of hydrothermal technology to return energy by multiple products and, hence, to provide a sustainable waste management practice in confined feedlots.