The carbonate looping (CaL) process is an efficient post-combustion carbon capture technology to reduce the amount of CO2 released by fossil fuelled power and industrial plants. The presented work discusses the results obtained in long-term pilot tests performed in a 1 MWth CaL pilot plant at Technische Universitat Darmstadt. These long-term pilot tests aimed to improve the process and gain reliable experimental data to scale up the CaL process to industrial size. The type of fuel, sorbent, flue gas composition, reactor design, and operating conditions were varied to investigate long-term effects on the performance during pilot operation. Steady-state conditions were achieved while parameters were not changed during periods up to 60 h. The stability of the CaL process in semi-industrial size was proven by steady-state CO2 absorption for more than 1200 h with interconnected circulating fluidized bed reactors with absorption rates in the carbonator higher than 90% and overall capture rates higher than 95%. A long-term pilot test is presented and the stability of hydrodynamics and temperature profiles in carbonator and calciner reactor are shown. The enrichment of inactive sorbent impurities like ash or calcium sulphate was observed and is dependent on the fuel type and fuel particle size in the calciner.