# Study of the Applicability of Thermochemical Processes for Solid Recovered Fuel
###### Abstract  
Within the context of the new circular model for wastewater treatment aimed at achieving zero waste, this research seeks an alternative to landfill disposal of waste screenings. It examines the feasibility of thermochemical processes--combustion and gasification--for the valorisation of solid recovered fuel (SRF) derived from screening wastes, which are the only waste in wastewater treatment plants (WWTPs) that typically have an absence of existing recycling or valorisation processes. Laboratory-scale experiments assessed the technical viability of gasification, and energy balances were calculated for both combustion and the syngas obtained from gasification experiments. Results indicate that both processes are feasible for SRF valorisation. Combustion demonstrated the highest energy efficiency, yielding up to 1.6 MJ per kg of raw SRF, compared to gasification's maximum of 1.4 MJ. The moisture content in SRF feedstock influences both processes, underscoring the need to optimise moisture levels. Additionally, combustion showed a higher conversion efficiency due to the complete oxidation of the feedstock, whereas gasification produced valuable syngas that can be further utilised for energy production or as a chemical feedstock. The study concludes that, from a purely energetic perspective, combustion is the most efficient process for SRF valorisation. However, gasification offers significant environmental and sustainability advantages, including lower greenhouse gas emissions and the potential for integrating with renewable energy systems, making it a more attractive option for long-term sustainability goals. of other municipal solid wastes (MSWs) as rejects from mechanical biological treatment (MBT) plants [10]. Similar to screening waste, these rejects consist primarily of fractions such as paper and cardboard, plastics, and organic matter [11]. These rejects already have established energy recovery treatments, such as the production of solid recovered fuel (SRF) to be used in thermochemical processes [12]. In addition, the latest update of the ISO 21640:2021 [13] considers solid waste from urban wastewater treatment as a possible origin for SRF production [13]. The technical feasibility of this process was demonstrated in a laboratory-scale project in Granada, Spain [14]: both a non-densified and a densified SRF [15] were produced from the screening waste from a WWTP (called \"Biofactoria Sur\") in Granada (Spain). The properties of the produced biofuel met the requirements of ISO 21640:2021 for lower heating value (LHV), chlorine content, and mercury content. According to Directive 2018/850, the waste hierarchy consists of five steps: prevention, preparation for reuse, recycling, other recovery (including energy recovery), and disposal [16]. At this point, the management of the produced SRF falls within the fourth step of the hierarchy, which pertains to \"other recovery, e.g., energy recovery\". In this context, thermal routes such as combustion and gasification, which aim at the energy recovery of SRF, could be considered [17]. Combustion, also known as controlled incineration, is a process that involves the burning of some feedstock, including waste, in the presence of oxygen at temperatures between 850 and 1100 \({}^{\circ}\)C [18]. The primary purpose of this process is to reduce the volume and weight of the waste while recovering some of the energy contained in the waste to generate electricity, steam, or heat [19]. After incineration, atmospheric emissions and the resulting ash must be treated appropriately [20]. In combustion, controlling different operational parameters such as excess oxygen, minimum combustion temperature, and minimum residence time at minimum combustion temperature after the last air injection [21] is a key issue in the proper development of the process [22].