Proof of Concept Calcination Kinetic in TGA and Fluidised Bed Reactor

Calcination of limestone releases large amounts of CO₂, 60 % by the limestone itself. In order to capture CO₂, the indirect fluidised-bed calcination is a suitable option where CO₂ is the fluidisation medium. However, there are also challenges as the fluidisation with CO₂ encourages sintering and requires temperatures above 900 °C in order to achieve complete decomposition. An alternative is steam, which lowers the CO₂ partial pressure and has a catalytic effect on the calcination reaction.

Steam contents up to 60 % are well studied in the lap scale by thermogravimetric analysis (TGA). The study is intended to close the gap between TGA experiments and the steam calcination in an up-scaled fluidised bed under consideration of a 100 % steam influence. Experiments are carried out to determine the kinetics of calcination (0-100 % H₂O/N₂ in CO₂, T = 700- 950 °C) by TGA and the fluidised bed reactor. The fluidised bed is interspersed with electrically heated tubes, which provide the required heat (20 kW) for the 18 kg reactor content.

For two particle size distributions (100-315/300-700 µm) it can be shown that the start of the decomposition temperature sinks from 900 °C at 100 % CO­₂ to 740 °C at 100 % H₂O. As the CO₂ partial pressure increases over the height of the fluidised bed due to the CO₂-release there is not a 100 % steam atmosphere in the reactor even with 100 % steam fluidisation in the inlet. Contrary to expectation the residence times (40-60 min, >95 %, at 900 °C) is the same for both scales.

Finally, the samples are analysed for reactivity using the T-60 test, which is well-known in the lime industry, in order to create a link to the product-quality and the reactivity of lime.