An apparatus for cooling hot ash particles discharged from a pressurized fluidized bed reactor, comprises a vessel, a solids inlet, a plurality of cooling pipes housed inside the vessel, and a solids outlet, wherein the cooling pipes are connected to a header located outside the vessel and are arranged such that the solids can flow between the cooling pipes under gravity without being blocked by the header, and wherein cooling liquid flows from the header via a flow path through the pipes, and exchanges heat with the hot ash particles through walls of the cooling pipes. The apparatus may further comprises a computerized evaporative cooling device comprising water nozzles and a thermocouple that measures the temperature inside the vessel. Also provided is a fluidized bed reactor comprising the cooling device.