Internal circulating fluidized bed boiler of multi-hearth structure

The invention discloses an internal circulating fluidized bed boiler of a multi-hearth structure. The internal circulating fluidized bed boiler mainly comprises high-speed beds, low-speed beds, an air distribution device, soaking heating surface pipes, multi-layer negative pressure feeders and hearths. The multiple hearths are designed, each hearth is internally provided with stepped bed surfaces at a high position and a low position, the high-speed beds are at the high positions, and the low-speed beds are at the low positions. The air distribution device comprises high-speed bed air chambers and low-speed bed air chambers. A plane air distribution plate structure is adopted for the high-speed beds, and the lower portions of the high-speed beds are connected with the high-speed bed air chambers. Each low-speed bed is provided with the corresponding soaking heating face pipe and a soaking superheater. Fuel enters the high-speed beds from the multi-layer negative pressure feeders. The fuel and inertia material