(300d) Rotary Kilns: A Powerful and Effective Tool for Running Particle-Based Solar-Chemical Processes

Considering that for heat storage applications a high mass flow rate and large amounts of active material are important, particle receivers are an attractive option. Rotary kilns combine the advantages of an effective homogenisation of the materials, effective heat transfer between the gas and solid phase, a high mass flow rate, and a high reactive surface area. Beyond this, the particles motion should decrease their sintering. Rotary kilns are a common and established device for diverse applications dealing with the thermal treatment of particulate material, with advantages of easy control of operating parameters, such as residence time or temperature, together with a simple design. They are well established devices for numerous thermochemical applications, e.g. the high temperature treatment of wastes, calcination processes, and the recovery of other non-ferrous metals.

The suitability of solar heated rotary kilns for thermochemical application was shown in several cases. For example, solar heated rotary kilns were tested for treatment of inert material, they were used to effect the endothermic calcination reactions (Moumin et al., 2019; Meier et al., 2006) or were used for the reduction of volatile metal oxides such as ZnO (Charvin et al., 2008), and for aluminum remelting (Glasmacher-Remberg et al., 2001).

Based on these experiences, the present study analyses the use of directly solar irradiated rotary kilns exemplarily for thermochemical heat storage, through the reduction and oxidation of non-volatile metal oxides, and for solar calcination processes. The main tasks and issues of such devices, i.e. effective mixing and heat transfer, managing particle flows and avoiding particles exiting the reactor, and ensuring the transparency of a window, are addressed based on experimental investigations in lab and pilot scale and numerical models.

In the first example the reactant is a metal oxide, precisely a mix of manganese and iron oxide, which undergoes consecutive reduction (endothermic) and oxidation (exothermic) reaction and thus serves as a thermochemical storage material. The metal oxide is shaped in mm-size granules. The reduction takes place in a solar rotary kiln which directly transfers the concentrated solar power to the reactive particles, which get heated and react. The system can treat several tenths of kg per hour at temperatures up to 1100°C, allowing to store a power of about 3kW.

The second example addresses the solar calcination of limestone. The experimental part addresses the testing of rotary kiln pilot calciner (up to 30 kg capacity and 5-10 kW solar power) in a solar simulator. A numerical model was developed to identify and verify the heat losses measured in the experiments. The parameters of the model were changed to best approach the experimental temperature distribution.

It became evident from experiments and models that beds exhibiting small size particles, like cement raw meal, show very little mixing in a cylinder without built-ins and the measured heat transfer coefficient was much lower than the model predictions. Using longitudinal strips as built-ins along the wall caused the material bed to collapse regularly, increasing its heat transfer by a factor of 2–3, up to the range of predictions from theoretical models.

Finally, an electrostatic precipitation system that can work at high temperature has been developed and adapted to the rotary kiln to keep the window free of dust. Significant improvement of window performance could be reached by this measure.

References:

Moumin, G., Tescari, S., Sundarraj, P., de Oliveira, L., Roeb, M., Sattler, C. 2019. Solar treatment of cohesive particles in a directly irradiated rotary kiln, Solar Energy 182, 480-490.

Meier, A., Bonaldi, E., Cella, G.M., Lipinski, W., Wuillemin, D., 2006. Solar chemical reactor technology for industrial production of lime. Solar Energy 80, 1355–1362.

Charvin, P., Abanades, S., Neveu, P., Lemont, F., Flamant, G., 2008. Dynamic modeling of a volumetric solar reactor for volatile metal oxide reduction. Chemical Engineering Research and Design 86, 1216–1222.

Glasmacher-Remberg, C., Roeb, M., Dersch, J., Schäfer, R., Funken, K.-H., 2001. Solar thermal recycling of aluminium. AL Aluminium and Its Alloys 135, 73–77.