The Effect of Residence Time and Heterogeneous Catalysis on Continuous Hydrothermal Liquefaction of Food Waste

The United States releases a significant amount of methane through the decomposition of waste in landfills; methane is a greenhouse gas with a global warming potential 25 times greater than carbon dioxide [1]. According to the EPA, landfilling accounts for approximately 17% of the total US methane emissions [1], and food waste is the largest non-recyclable component of municipal waste [2]. Although food waste currently harms the environment, it also presents an opportunity to solve another looming threat: the US dependency on non-renewable fossil fuels. Mainstream Engineering, in collaboration with Worcester Polytechnic Institute, is developing technology to convert food waste to energy through hydrothermal liquefaction (HTL) that simultaneously addresses our reliance on fossil fuels and the environmental impact of food waste.

HTL converts food waste, and other wet wastes, into an energy dense biocrude by reacting the food mixture of carbohydrates, lipids, and proteins in near supercritical water. Traditionally, HTL reactions have been uncatalyzed or used a homogeneous catalyst (i.e., Na₂CO₃). We have shown previously that using a solid oxide, heterogeneous catalyst, CeZrO_x, in a batch reactor improves biocrude yield and energy recovery [3]. We have subsequently studied the effects HTL residence time and catalyst addition in a continuous stirred tank reactor (CSTR). A model food waste slurry was fed to the CSTR via a dual high pressure syringe pump system. The pressure of the system was maintained using a back pressure regulator rated to 300 ºC and 5,000 PSI. Commercially available CeZrO_x powder was compressed into tablets and immobilized in the CSTR using a catalyst cage. The catalyst tablets were able to withstand the hydrothermal environment without physically disintegrating. The stability of the catalyst tablets was characterized using XRD and BET before and after HTL. The energy recovery, product yields, and biocrude properties (HHV, TAN, and CHNO) were determined to characterize the effectiveness of CeZrO_x in a continuous HTL reactor and evaluate the impact of residence time on continuous HTL of food waste.

References

[1] EPA. Greenhouse Gas Emissions. [cited 2021; Available from: https://www.epa.gov/ghgemissions/overview-greenhouse-gases.

[2] USDA. Why should we care about food waste? [cited 2021; Available from: https://www.usda.gov/foodlossandwaste/why.

[3] Maag, A.R., et al., Catalytic hydrothermal liquefaction of food waste using CeZrOx. Energies, 2018. 11(3): p. 564.