Shaft kiln for calcining raw meal for clinker production in the cement industry - an efficient technology for concentrating CO₂ in the flue gas

The capture of CO₂ and its subsequent use and storage are essential elements of the cement industry's decarbonisation strategy. The capture of CO₂ from the flue gas is energy-intensive and complex. It can be carried out more easily and efficiently by maximising the CO₂ concentration in the flue gas.

The pre-heating and calcination of briquetted raw meal (Fig. 1) in a shaft kiln that is operated with the oxyfuel technology with CO₂ recirculation offer a promising possibility to significantly increase the CO₂ concentration in the flue gas compared to classical rotary kiln systems.

Raw meal, which can contain up to 5 % water, is compacted at 50 - 375 N/mm² and formed into briquettes.

 
A modern Parallel Flow Regenerative-shaft kiln (PFR kiln) consists of two connected shafts which operate alternately as burning shaft or as regenerative shaft (Fig. 2). They are connected with a duct. In the burning shaft, process gases and kiln feed move downward in co-current flow. In the regenerative shaft the kiln feed moves downward and the process gases rise in counter-current flow. In the shaft kiln the limestone (CaCO₃) in the raw meal is decomposed into gaseous carbon dioxide (CO₂) and calcium oxide (CaO) at temperatures between 800 and 1200 °C. Both shafts change their flow direction in regular periods of about 10 - 15 minutes. As a result, an extremely high thermal efficiency of over 80% can be achieved. For the use in the shaft kiln the kiln feed material (raw meal) is briquetted. This ensures the flow of process and cooling gases through the material.

In oxyfuel operation, almost pure oxygen is used instead of air as oxidant in the PFR kiln. This significantly increases the CO₂ concentration in the kiln's flue gases. The elimination of the nitrogen component of the air significantly reduces the gas volume. In conventional rotary kiln operation with air, the CO₂ concentration is around 20 % by volume. The oxyfuel-operated shaft kiln with CO₂ recirculation can achieve concentrations of over 90 to 95 % by volume. The shaft kiln operates with overpressure. This way, the entry of false air and, as a consequence, undesirable amounts of nitrogen in the system can be avoided. This is a clear advantage over conventional preheater/precalciner kilns in terms of the required sealing due to the slight underpressure.

Furthermore, a CO₂ barrier layer is formed during operation of the PFR kiln. It prevents the mixing of cooling gases with combustion gases and thus supports the gas separation in the oxyfuel process.

The clinker phases are then produced in a separate burning process with the material that was already largely calcined in the PFR kiln. The separate burning process can be carried out in a standard preheater kiln with electrical energy or with hydrogen which results in significantly reduced CO₂ emissions.