Temperature-dependent effects of carbonization and washing on fuel quality of refuse-derived fuel

In Portugal, the production of refuse-derived fuel (RDF) from mixed municipal solid waste (MSW) is limited by low density, high chlorine content, and insufficient heating value, reducing its suitability as an alternative fuel. This study explores carbonization as a pretreatment to enhance RDF fuel properties. Carbonization was conducted between 300 and 550 °C in 25 °C increments, producing 11 char samples characterized regarding their physical, chemical, and fuel properties. All chars were washed to assess chlorine reduction. Results showed that increasing temperature reduced mass yield, particularly above 425 °C, while producing more homogeneous chars. The highest heating value (32.5 MJ/kg) was observed for chars produced at 375 °C, although significant particle agglomeration was observed. Chars produced at 450 °C and higher, showed a sharp increase in ash content, reducing their energy value. The medium temperature range (400–425 °C) achieved a balance of high heating value (24.0-30.7 MJ/kg), structural integrity, and low agglomeration. Washing reduced chlorine content in all chars to below 1% (0.29% to 0.67%), highlighting its importance. Overall, RDF carbonization followed by char washing presents a promising alternative to enhance heating value while also reducing RDF’s chlorine content to levels that are acceptable for use as alternative fuel (e.g., cement manufacturing process), while enhancing resource efficiency in line with circular economy principles. This study examines the carbonization of refuse-derived fuel (RDF) as a pretreatment to enhance their physical, chemical, and combustible properties. In several countries, including Portugal, the RDF produced is of low quality for use as an alternative fuel in energy-intensive industries. As a result, much of this waste ends up in landfills, leading to a significant waste of resources and energy. While some research has indicated that carbonization can improve the fuel properties of RDF, there is still limited knowledge regarding the optimal carbonization conditions and the resulting characteristics of the chars produced under different conditions. The main innovation of this work lies in investigating the RDF carbonization process and char washing to produce chars with suitable qualities for use as alternative fuels. Determining the properties of the produced chars is essential due to the high moisture, chlorine, and ash content, as well as the heterogeneity of the material. Although RDF holds considerable potential to replace fossil fuels in industrial processes, various logistical and environmental challenges must be addressed to realize this potential effectively. This paper introduces three key innovative aspects: (i) a comprehensive temperature range analysis (300–550 °C) with fine 25 °C increments, enabling a limited, granular mapping of process-performance relationships and identification of an optimal operational window (400–425 °C) balancing HHV, yield, and homogeneity; (ii) the integration of post-carbonization char washing, achieving chlorine reduction to < 1%, a critical threshold for industrial acceptability; and (iii) a tailored solution to Portugal’s RDF management challenge, where 96% of RDF is landfilled, demonstrating a pathway to valorize low-quality domestic RDF for industrial use. Based on the experimental results, this approach is discussed as a potential strategy to upgrade RDF into a denser and higher-calorific solid fuel (up to 32.5 MJ/kg) and to improve handling-related properties. Its broader implications in terms of logistics, environmental performance, and contribution to circular economy and decarbonization goals are therefore considered as prospective benefits, which require further dedicated technical, economic, and environmental assessment. This work addresses the lack of systematic studies evaluating the combined effect of carbonization temperature (with fine thermal resolution) and post-carbonization washing on the fuel-relevant properties of low-quality RDF.