Hydrothermal carbonisation of solid recovered fuel – a model material approach

In order to facilitate gasification of heterogeneous waste such as solid recovered fuel (SRF), a pre-treatment by hydrothermal carbonisation (HTC) is considered. This study uses a model material approach to represent SRF and study its behaviour during HTC, and in particular the hydrochar properties (yield, ash content, LHV, apparent density, carbon concentration, fixed carbon, inorganic content). HTC experiments were conducted at temperatures between 200 and 280 °C on one SRF and seven model materials (waste wood, cardboard, polyethylene, PET, polyamide, PVC and waste tyres) selected to represent the variety of materials and compositions. When comparing HTC behaviours of the resources, various profiles appear: biomass-based, reactive plastics (PET and PVC) and non-reactive plastics (PA, PE, waste tyres), which however show some physical transformations (melting, formation of powder). Results show that HTC of SRF is mainly controlled by the degradation of lignocellulosic molecules (in particular cellulose and hemicellulose), but that degradation is itself enhanced by the increase in acidity induced by the degradation of plastics (PET and PVC). The LHV and C content of SRF hydrochar are higher than those of the initial SRF (increase by 48 and 36 percentage points after HTC at 240 °C, respectively), while fixed carbon is not significantly affected in these conditions. The apparent density is improved (doubled after HTC at 240 °C) due to an increase of the apparent density of most of the model materials, including non-reactive ones. All these changes could be positive for an easier gasification of the hydrochar compared to the initial SRF. Finally, N content is unchanged (it leaves the solid in the same proportions as C), and Cl and S contents decrease, as for model materials.