Mass, carbon and energy balances of thermochemical processes for digestate valorization

• Digestates valorized by dry and aqueous phase thermochemical processes. • Experiments modeled under Aspen Plus. • High resolution mass spectrometry used to define surrogate molecules. • Important carbon loss (79 wt%) during screw press separation. • 78 % of carbon transferred to liquid phase by hydrothermal liquefaction. 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. Raw digestate was alternatively dewatered by a screw press (from 96.3 to 84 wt% raw basis of water content), dried and then converted by fast pyrolysis, steam gasification, and combustion (in a fluidized bed reactor at 500–900 °C) and slow pyrolysis (at 400–800 °C). The products (gas, liquid and char) were thoroughly analyzed. Surrogate molecules were defined based on high resolution mass spectrometry analysis in order to better describe the complex liquid oils. The experimental results were modeled under Aspen Plus to assess the mass, carbon and energy balances of all these processes. Most of carbon (79 wt%) and organic matter present in raw digestate remained in the liquid phase after screw press separation. Therefore, the hydrothermal conversion converts a much higher yield of organic matter because it does not require a mechanical dewatering of digestate. The energy efficiencies for all the thermochemical processes were quantified as a function of the dewatering step and the thermochemical conditions (temperature, residence time, etc.). The heat required to dry the solid fraction of the digestate (prior to its dry thermochemical conversion) can be produced by the biogas generated by the anaerobic digestion.