Waste-to-Heat: Thermophysical and Rheological Properties of Fly Ash Dispersions in Deep Eutectic Solvents

Deep eutectic solvents (DESs) have emerged as promising heat-transfer fluids. Incorporating near-zero-cost waste-derived particles into DESs may enhance their thermophysical performance while supporting circular economy principles. To the best of our knowledge, this is the first report on the use of municipal solid waste incineration fly ash (FA) in DES-based heat-transfer fluids. This study investigates the thermophysical, rheological, morphological, and stability properties of dispersions prepared using choline chloride/ethylene glycol (ChCl/EG) DESs with molar ratios of 1:5 and 1:10. Dispersions containing 2-20 wt% FA were prepared by ultrasonication without particle milling or surfactants and characterized using SEM-EDS, laser diffraction, multiple light scattering, transient hot-wire method, and rotational rheometry at 25 °C. Increasing FA loading resulted in a linear increase in thermal conductivity, reaching maximum enhancements of 19% for ChCl/EG (1:10) and 13% for ChCl/EG (1:5) at 20 wt%. Particle size analysis indicated effective deagglomeration after ultrasonication at low FA concentrations (2-5 wt%), whereas higher loadings promoted gradual re-agglomeration. All dispersions exhibited Newtonian liquid-like behavior, with viscosity increasing with FA concentration while remaining below 90 mPa·s. During the initial 3 h observation period, increasing FA loading reduced dispersion stability, whereas a higher ChCl content improved stability. Overall, the results demonstrate the potential of waste-derived FA in DES-based heat-transfer fluids and support its use within the waste-to-heat concept, linking waste valorization with heat-transfer applications.