Effects of dry and hydrothermal carbonisation on the properties of solid recovered fuels from construction and municipal solid wastes

The growing production of non-recyclable urban wastes generates environmental problems that impose new alternatives for their energetic valorisation. Nonetheless, some of these wastes do not have adequate properties to be directly used in waste-to-energy technologies, thus requiring adequate pre-treatments to improve their fuel properties. This work aimed to evaluate dry and hydrothermal carbonisation (DC and HTC) as technologies to convert solid recovered fuel (SRF) from construction and municipal solid wastes to biochars or hydrochars with improved fuel quality. The operational parameters evaluated were temperature, mass ratio of SRF:water, and incorporation of a liquid additive (used cooking oil, UCO). The chars were characterised for chemical composition, calorific value, TGA profiles and surface functional groups. The chemical oxygen demand (COD), concentration of total phenols, pH, conductivity and the main components in process waters were determined. Results showed an improvement of fuel characteristics in terms of hydrophobicity and calorific value, enabling the use of chars for waste-to-energy technologies. HTC produced hydrochars with better fuel characteristics, presenting calorific values of 28–33 MJ/kg db, and lower average ash and chlorine contents (2.8 wt% db and 3.1 wt% db, respectively). The addition of UCO improved these fuel characteristics. However, the generation of an effluent that needs further decontamination and the lower amount of moisture present in SRF possibly made DC more attractive in terms of energy and costs requirements. A treatment at 350 °C during 30 min was recommended for a good compromise among process costs and char properties.