Influence of the initial substrate concentration on the products obtained during hydrothermal carbonisation of kitchen waste

The aim of this study was to determine the influence of the initial concentration of kitchen waste (KW) on the hydrothermal carbonization (HTC) process. Increasing the initial concentration of KW resulted in an increase in hydrochar yield (from 24.5 to 52.3 % w/w), whereas the higher heating value (HHV) remained at a similar level (25.7 MJ/kg). Lower substrate concentrations led to stronger leaching of inorganic compounds from the hydrochar. Hydrochar produced at lower substrate concentrations exhibits higher thermal stability during combustion in a thermobalance. Higher KW concentrations caused an increase in the carbon, nitrogen and acetic acid content in the liquid. At the highest substrate concentration, the concentrations of acetic acid, carbon and nitrogen in the liquid phase were equal to 13.1, 32.0 and 1.6 g/L, respectively. Analysis of volatile organic compounds in the liquid fraction revealed that increasing the substrate concentration decreased the number of phenolic compounds (from 8 to 3) and aldehydes (from 3 to 0). The liquid produced after HTC with a high initial KW content can be used for biogas production because it contains a high load of organic carbon and few substances that inhibit microorganisms. In the gas phase, a higher initial KW content increased the amount of CO₂ (from 95.1 to 98.8 % vol.) and a decreased the amount of CO produced (from 4.9 to 1.2 % vol.).