10 - Thermal depolymerization of biogas digestate as a viable digestate processing and resource recovery strategy

Abstract

The proliferation of biogas technologies has resulted in the increased challenge of handing the large mass of digestate generated as a biogas byproduct. This is because current practice requires that the biogas digestate is processed using complex and expensive operations to sterilize the digestate, recover nutrients, and recover a purified liquid phase. Due to the complexity of the existing digestate processing methods, hydrothermal liquefaction (HTL) is proposed as a digestate processing technological alternative because it will facilitate the sterilization of the biogas digestate byproduct while also enabling further resource recovery. In this chapter, it was demonstrated that up to 0.220kg of hydrophobic biocrude and 1.34kg of insoluble solid biochar can be obtained from 100kg of high moisture biogas digestate containing solids 3.02wt% total solids. Masses of other hydrothermal liquefaction products, namely soluble solids in the post-HTL water phase and gaseous products, were measured to be 0.31kg and 1.15kg, respectively. Also, a comparative economic assessment, albeit largely approximate, was undertaken with the unit digestate processing cost of the HTL-based one-step processing pathway shown to have the potential of being considerably lower than the unit digestate processing cost of existing digestate treatment and processing strategies.