Waste-to-Resources: Ash Properties by Thermodynamic Modelling

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E. Nagels, S. Arnout, L. Arnout, L. Machiels, Y. Pontikes, D. Geysen

Advanced technological waste solutions are increasingly being investigated and implemented, even for low-value fractions such as refuse derived fuel (RDF). This paper presents a methodology to investigate the feasibility and guide engineering, based on process and flowsheet modelling using thermodynamic calculations. The model functions as a framework for decisions on investment and design. An important aspect is the behaviour of slag or ash, both in the process and in the afterlife or the by-products, preferably as high-value construction materials. This work focuses on a case-study (Group Machiels Closing the Circle), in which state-of-the-art technology will be applied to landfill-derived waste fractions. The recovered RDF fraction is planned to be used for gasification and conversion to syngas. During plasma conversion, the dustladen gas is turned into a clean syngas and a molten slag. This creates an opportunity to create a valuable building material rather than a waste. It has been shown that versatile inorganic polymer cements and geopolymer mortars can be produced using this by-product. From the models, the expected metal recovery and fuming during the melting step can be estimated, based on metallurgical experience and thermodynamics. The behaviour of the impurities is very relevant to the leaching properties of the final slag. Also, the oxidation state of iron oxide and the mineralogy can be calculated. These directly link to the hydraulic, pozzolanic, or other binding properties of the material. On the process side, slag overheat and viscosity are important values to allow for easy handling and low installation maintenance. All of these properties can be influenced by fluxing or by the addition of different waste materials. Furthermore, any additions or operating temperature changes have an energetic effect and may influence syngas quality. Given the complexity of the system, a global predictive model is essential in order to guarantee proper strategic decisions on the waste fractions the installation should focus on, and may also serve as a guide to interpret and leverage industrial data.

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