Disclosed is an apparatus and process for an energy producing plant utilizing a circulating fluidized bed system having an integral fluidized bed heat exchanger, the fluidized bed reactor and integral heat exchanger having a common wall with at least one perforation for fluid communication therebetween. In the system, solids are separated from the reactor flue gas, and in dependence upon a process variable, are directed in varying amounts either directly back into the reactor or into the integral heat exchanger, the latter being operated as a classical and/or bubbling bed at a higher pressure than the reactor. A heat exchange medium passes through tube bundles immersed in the exchanger fluid bed and the bed solids and fluidizing gas in the heat exchanger are passed through the perforation to control the reactor operation. Because of its structural integrity this system allows for greater flexibility in control and aids in turndown operations as well as controlling variations experienced under otherwise constant load conditions. The system and arrangement find use both in utility plants and process operations.