In a retrofit system for hot solids combustion and gasification, a chemical looping system includes an endothermic reducer reactor 12 having at least one materials inlet 22 for introducing carbonaceous fuel and CaCO3 therein and a CaS/gas outlet 26. A first CaS inlet 40 and a first CaSO4 inlet 64 are also defined by the reducer reactor 12. An oxidizer reactor 14 is provided and includes an air inlet 68, a CaSO4/gas outlet 46, a second CaS inlet 44, and a second CaSO4 inlet 66. A first separator 30 is in fluid communication with the CaS/gas outlet 26 and includes a product gas and a CaS/gas outlet 32 and 34 from which CaS is introduced into said first and second CaS inlets. A second separator 50 is in fluid communication with the CaSO4/gas outlet 46 and has an outlet 52 for discharging gas therefrom, and a CaSO4 outlet from which CaSO4 is introduced into the first and second CaSO4 inlets 62, 66. The chemical looping system is in fluid communication with at least a portion of an existing power generation system.