Solar treatment of cohesive particles in a rotary kiln and role of mixing

Kurzfassung

This thesis deals with the use of concentrated solar radiation in the thermal process of cement production and the analysis of the heat input into the material in powder form. This was tackled in this study in two parts: the first dealing with the solar calcination and the second with the heat transfer analysis into cohesive particle beds together with improving measures. Several studies have been performed up to now on the solar calcination which is the main thermal step of the process. However, only one of the studies up to now considered the calcination of cement raw meal. Due to the reactor concept its applicability to industrial size cement raw meal is limited and new concepts were needed to address the emission reduction in the cement industry. First, the calcination of the cement raw meal, the first step of the thermal treatment, was considered in a solar heated rotary kiln. For this purpose, built-ins for a better mixing of the particle bed were investigated and strip-shaped built-ins were selected. These were used in the crucible of the solar rotary kiln, which was used to investigate the calcination of the industrial cement raw meal. The tests were performed in a solar simulator, which has similar properties to sunlight. A total of 22 tests were performed of which 5 were performed with inert bauxite particles and 17 with the cement raw meal. Operation with bauxite was possible in a closed configuration with transparent quartz glass and efficiencies of over 60% were achieved. Once the powder was used, the tests in the closed configuration could not be completed because of depositions on the glass. Removing the glass and using suctions at the aperture allowed an open operation of the reactor. Material flows in the range of 4–12 kg/h were successfully treated and calcination rates of 24–99 % were achieved. The overall efficiency, heating including reaction, was in the range of 19–39 %. Although temperatures above the reaction temperature were achieved in all tests, the degree of calcination remained below expectations. A correlation with the mixing of the bed was shown, which is particularly relevant for higher mass flows. The experimental calcination rate for all tests was between that of a bed that was not mixed and one that was ideally mixed. Concluding estimates for increasing the efficiency showed that overall efficiencies in the range of 60-80 % are possible in open operation. The analysis of the heat input into the particle bed as a function of mixing was carried out in a further setup. It was shown that the heat transfer from the wall to the reference material sand has the value predicted by the literature models. However, if the cement raw meal is used, the measured values are far below those of the models. Mixing the material with identical fixtures from the solar tests doubled this heat transfer and resulted in better agreement with the models. Since temperature homogeneity is decisive for a homogeneous reaction of the bed, the effective thermal conductivity was investigated in mixed beds. It was shown that the thermal conductivity could be increased by 6–24 as soon as mixing took place. This corresponds to a value of 3.36 W/(m K) compared to 0.14 W/(m K).