Mass, Carbon and Energy Balances of Thermochemical Processes for Digestate Valorization

Abstract

The aim of this study was to evaluate the valorization of digestate collected from a collective biogas plant. Raw digestate was converted by a hydrothermal process by using a fast-heating induction autoclave or alternatively dewatered by a screw press (from 96.3 to 84 wt%raw basis of water content) and dried prior to be valorized through fast pyrolysis, steam gasification, and combustion fluidized bed reactors (at 500-900 °C) and slow pyrolysis tubular reactor (at 400-800 °C). The reactor products were thoroughly analyzed, and results were modeled with Aspen Plus to compare the mass, carbon and energy balances. Most of carbon (79 wt%) and organic matter present in raw digestate remained in the liquid phase after screw press separation. Thus, the hydrothermal conversion enabled to valorize a much higher yield of organic matter because it did not require a mechanical dewatering of digestate. The energy efficiency for dry thermochemical processes was evaluated between 49 and 93% without considering the screw press, dryer and pelletizing steps, depending on the process and temperature. Contrarily, if the whole valorization chain is considered this efficiency falls between -19 and 6% due to low mass recovery obtained after screw press separation and the high heat demand for drying. It was also shown that hydrothermal conversion and dry thermochemical processes could be combined to promote digestate conversion of both liquid and solid phase. In a global unit, the biogas produced by the anaerobic digestion could be partly valorized into energy to dry the solid fraction of the digestate prior to its dry thermochemical conversion.

Keywords: digestate, steam gasification, Combustion, fast and slow pyrolysis, hydrothermal liquefaction, Aspen Plus