VALORIZATION OF SOLID DIGESTATE THROUGH AUTOTHERMAL HYDROTHERMAL LIQUEFACTION

This project explores new ways to address the growing problem of wet wastes ranging from animal manure produced on farms, to wastewater from processing fruits and vegetables in the food industry. Wet wastes usually contain more water than solid material wastes, preventing the normal transport and storage at landfills or treatment by burning. Instead, biological processes are often employed to treat wet wastes, but these processes are slow and often leave a significant solid residue requiring special disposal.

This project explores the hydrothermal liquefaction (HTL) process, a high-temperate, high-pressure process that dissolves oxygen into the wet wastes, to convert this waste into liquid fuel without requiring external energy sources to drive the process. By turning waste into liquid fuel, the process both eliminates the waste and generates a valuable product.

If successful, this project will provide new ways to scale up HTL processes to make them available for more common use, thereby strengthening the Nation’s energy security while reducing the amount of waste from agriculture and other food industries. This project will also train postdoctoral researchers graduate students to effectively mentor their own students in future engineering efforts through the use of a tiered mentoring approach guided by the lead investigator.

This goal of this project is to intensify hydrothermal liquefaction (HTL) through directly coupled autothermal operation. This operation will overcome the heat transfer bottleneck of providing energy to the process and allow for scale-up to commercial use. Previous research at Iowa State University demonstrated autothermal operation of a fast pyrolyzer by admitting a small amount of air to the reactor.

This increase in oxygen achieved a three-fold increase in throughput compared to conventional (non-oxidative) pyrolysis with minimal loss in bio-oil production. This study proposes a similar autothermal operation of an HTL reactor, providing the enthalpy for the process through partial oxidation of some of the low value aqueous phase products of liquefaction.

It is hypothesized that oxygen will preferentially dissolve in the aqueous fraction of HTL products where it will readily react with dissolved organics. In contrast, the more valuable biocrude, an emulsion suspended in the aqueous fraction with far less exposure to the dissolved oxygen, will not substantially oxidize. The approach to this project is to develop a calorimetric HTL reactor that will allow energy flows as well as product composition to be determined during HTL of solid digestate from anaerobic digestion of animal manure.

Experiments under both inert and oxidative environments will determine the extent that the enthalpy for liquefaction can be provided by partial oxidation of the less valuable products of liquefaction. The oxidation of organic compounds in high temperature, high pressure aqueous environments has been little studied outside wet oxidation of dissolved solids at atmospheric conditions.

If successful, this research will help remediate the approximately 565 teragrams of wet waste generated annually in the United States. Additionally, the scientific and engineering principles implemented in this work will serve as foundational research required for the process intensification of HTL for a range of wet wastes including agricultural residue, food wastes, municipality solid wastes, sewage sludge, and animal manure.

A formal postdoctoral researcher and graduate student mentoring structure, combined with effective outreach to kindergarten through twelfth grade students, will help inspire new and train future researchers in these engineering practices.

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.