Biomass direct-combustion speed variation mixed circulation fluidized bed boiler

The utility model discloses a biomass direct-combustion speed variation mixed circulation fluidized bed boiler. The biomass direct-combustion speed variation mixed circulation fluidized bed boiler comprises a biomass material inlet, a hearth, a first return stroke, a second return stroke, a U-shaped separator, a third return stroke, a middle-temperature cyclone separator, a heater, a coal economizer and an air preheater. Biomass is adopted for the boiler; air cooling and negative-pressure type feeding is adopted for the biomass fuel inlet, tempering and smoking are not generated, and feeding is smooth; the hearth is low in combustion temperature; mixing is full; the combustion efficiency is high; coking is difficult to occur; the smoke temperature is low in the middle-temperature separator, and coking is difficult to occur; horizontal high-torque and sequential arrangement is adopted for a heated surface pipe at the tail part; the flue gas is moderate in speed; ash is difficult to accumulate, and abrasion is difficult; the ash residue after combustion is high in activity, and is high in comprehensive utilization value.