Experimental investigation of the calcination reactor in a tail-end calcium looping configuration for CO₂ capture from cement plants

The calcium looping process is a high temperature post-combustion CO₂ capture technology using limestone or a calcium containing sorbent to separate CO₂ from flue gases. Hence, it is expected to be a particularly suitable CO₂ capture technology for the decarbonisation of cement plants. Sharing a common feedstock, CaCO₃, enables the reutilisation of purged calcium looping sorbent (i.e. CaO) in the cement clinker manufacturing process. Thus, allowing the operation with large amounts of sorbent make-up and setting new beneficial boundary conditions for the calcium looping process. So far, calcium looping CO₂ capture has been demonstrated at semi industrial scale for fossil fuelled power plants but not yet for cement plants. In this work, results obtained from University of Stuttgart's 200 kW calcium looping pilot plant investigating the so called tail-end calcium looping cement plant integration are presented focusing on the operation of the oxy-fuel calciner. Different integration levels between the cement plant and the calcium looping process have been assessed with make-up ratios up to 0.9 mol mol⁻¹ and CO₂ flue gas concentrations up to 0.35 m³ m⁻³. The calcination performance covering a broad range of sorbent make-up rates was assessed using the active space time approach adapted to the calcium looping calciner. The calcination performance was minorly influenced by the calcination temperature in the investigated temperature range (895 °C–932 °C). An active space time of approx. 37 s was sufficient to achieve a regeneration efficiency of 0.9 mol mol⁻¹. Uniform temperature profiles without major hotspots were obtained throughout the experiments operating with recirculation rates as low as 20% and oxygen inlet concentrations as high as 0.56 m³ m⁻³ on a wet basis. The calciner's flue gas showed an increased formation of NO_x associated to the combustion in a lime bed ranging from 500 × 10⁻⁶ m³ m⁻³ to 750 × 10⁻⁶ m³ m⁻³ increasing linearly with increasing excess oxygen concentration. Contrarily, SO₂ emissions were significantly decreased averaging around 10 × 10⁻⁶ m³ m⁻³, due to beneficial desulphurisation conditions (i.e. high Ca/S ratio and optimal oxy-fuel sulphation temperature). Based on the obtained results CO₂ purities around 0.95 m³ m⁻³ can be anticipated for the calciner's flue gas.