Mechanistic exploration of activated carbon synergistic solar drying through theoretical analysis coupled with experimental validation

The rapid increase of sludge has brought significant environmental problems, the solar drying is an effective and economical method for sludge reduction. However, single solar drying has key issues such as low efficiency and long time. Thus, an interesting activated carbon synergistic solar (ACSS) technology which dried municipal sludge with was innovatively proposed in this paper. The changes of sludge pore structure were characterized by fractal dimension and scanning electron microscope, and surface elements were detected and analyzed. The reflectance and light transmittance were determined by UV–VIS-NIR to obtain the light absorptivity. The effect of key parameters including sludge thickness, light intensity and activated carbon content on drying rate (D_r) and moisture content (M_c) were comprehensively quantified to reveal the coupling mechanism. The results showed that when activated carbon was added in the range of 0–20 %, with the thickness (0.5–2 cm) thinning, the light intensity (0.5–2 kW/m²) and the D_r increased, and the M_c decreased rapidly. When sludge cake thickness was 0.5 cm, light intensity was 1 kW/m² and drying for 120 min, the M_c dried by single solar energy can be reduced from 83 % to 60.5 %, while adding 2 % activated carbon can be reduced to 29.96 %, showed better drying effect. At the same time, the fractal dimension increased from 1.603 to 1.741. The favorable photothermal conversion efficiency was contributed to the more complex pore structure and better full spectrum absorbance. This study is helpful to understand the mechanism of the ACSS to dry sludge, and provide useful guidance for solar drying technology.