A promising way to reduce CO2 emissions of fossil-fired power plants is the carbonate looping process with the reversible carbonation-calcination reaction of limestone. The focus is on the gas release in an indirectly heat pipe-heated calciner. Experiments in an electrically heated lab-scale batch calciner demonstrated that transition from a fixed bed to a bubbling fluidized bed is possible without an external fluidization agent only by heating up limestone. Several indirectly heated calciner concepts are presented.