The Calcium Looping (CaL) technology has recently emerged as a viable option for efficiently decarbonizing power plant flue gases. The process is based on the sequential calcination and carbonation of a calcium-based sorbent, usually limestone. Although CaO-based sorbents offer many advantages, they typically suffer from a rapid decline in CO2 capture during cyclic operation. This latter aspect has remained an urgent issue to be addressed for CaL power plant application. Besides, coal-fired power stations are expected to operate in a load-following mode due to the growing share of renewable energy. Moreover, the usage of alternative fuels in existing coal-fired power units is envisaged to ensure sustainable energy generation and to avoid an overshoot of CO2 emissions into the atmosphere. This thesis aims at addressing each of the previously anticipated challenges individually. A range of experimental investigations studying the cycling conversion of three originally distinct limestones – Rheinkalk, Riyadh, and Saabar – were conducted by thermogravimetric analysis (TGA). To this end, several carbonation routines were employed, including SO2 and steam. The Saabar metamorphosed limestone showed to be negatively influenced by the presence of steam, while limited sulfation positively affected its CO2 capture performance. This unusual behavior can be ascribed to pore blocking during carbonation. Besides, Rheinkalk and Riyadh behaved similarly, resembling the typical behavior of common unmetamorphosed limestones. The decay in Rheinkalk conversion upon cycling was further explored at a 20 kWth CaL facility setting different carbonation conditions. A mathematical expression is proposed to compare the results obtained in both facilities, correlating them with adequate accuracy. In the following, an alternative reactor concept based on a bubbling fluidized bed (BFB) carbonator was employed for flexible load operation. Within the first phase of the tests, a parametric study was conducted at the 20 kWth CaL facility to evaluate the influence of temperature, CO2 loading, and steam concentration upon the BFB carbonator performance. Hereafter, investigations at a 200 kWth semi-industrial CaL plant were conducted to evaluate the flexible behavior of the suggested carbonator setting. It was demonstrated that the BFB carbonator can be operated stably with gas superficial velocities ranging from 0.8 to 2.0 m/s without affecting the solid circulation between reactors. The latter range corresponds to a maximum reduction in the flue gas load of 60 % with respect to the nominal operation case. A simple carbon material balance was applied for preliminary validation of the carbonator performance. In addition, a carbonator model approach based on the active space time (𝜏active) was proposed for a more detailed result interpretation. According to the results, an active space time value of 41 s was identified as sufficient to achieve an equilibrium normalized capture efficiency (Enorm) of 90 %. Moreover, the circulating fluidized bed (CFB) calciner operation appeared independent from the flue gas load set in the carbonator. The reactor could be successfully operated with recirculation rates as low as 27 %, reaching inlet dry oxygen concentrations as high as 0.55 m3/m3. Within the next phase of the experiments, the impact of fuel selection in the calciner was evaluated. Oxy-combustion of hard coal, wheat straw, and solid recovered fuel (SRF) was demonstrated during more than 43 h of continuous operation. A range of experiments was conducted to address the influence of fuel blending and inlet oxygen concentration on pollutant formation (i.e., NOx, SO2, HCl) and hydrodynamic behavior. The calciner inlet O2 concentration appeared to barely affect the pollutant formation process. In contrast, biomass substitution influenced gaseous emissions by modifying the fuel mixture's nitrogen and chlorine content. Concurrently, specific HCl emissions were significantly reduced by the presence of Ca-species in the calciner solid inventory, yielding chlorine retention rates above 0.90 mol/mol with at least 30 % biomass substitution. Besides, ash accumulation led to elevated pressure drops over the CFB riser when operating with alternative fuels. Although this was not a limiting aspect in this work, results anticipate that ash accumulation might constitute a key challenge to be addressed in fluidized beds employing combustion of low-grade quality fuels.