What are the technical characteristics of bubble fluidized bed boilers compared to circulating fluidized bed boilers?

(1) the fluidization speed of bubble fluidized bed boiler is generally 1.5~3.5m/s, which is much lower than 4~6m/s of circulating fluidized bed boiler. When the primary air volume of the bubble fluidized bed boiler exceeds the critical fluidization air volume, the airflow will flow through the bed material in the form of bubbles, and the material layer shows a low fluidization state of boiling; the design fluidization speed of the circulating fluidized bed boiler is always greater than the minimum fast bed starting speed of the material in the furnace, and it will only show the characteristics close to that of the bubble fluidized bed boiler when operating at a low load of 30%~60% (circulating fluidized bed boiler at different (when operating at different loads, the material in the furnace will be bubbling bed, low rate bubbling bed, turbulent bed and fast bed several fluidization states respectively).

(2) bubbling fluidized bed boiler there is an obvious dense phase area of the material layer interface, the concentration of fine particles in the suspension section is much lower than that of circulating fluidized bed boiler, the pressure decay of the material layer is very fast, there are abrupt inflection points in the flow field and temperature field distributed along the height of the furnace, and the temperature difference of the material layer is large. The average particle size of the material in the suspended rising section of circulating fluidized bed boiler is generally around 200μm, which is much smaller than the average particle size of 1~3mm of the material on the surface of the dense phase area of the bubbling fluidized bed boiler. Circulating fluidized bed material concentration in operation is exponentially distributed along the bed height, its concentration depends on the bed inventory of fine materials in the combustion chamber, no longer have a clear interface as in the bubble bed, solid particles composed of materials filled with the entire rising section of space, it is difficult to distinguish between the dense phase area and the thin phase area of the dividing line;.

(3) the vast majority of combustion share in the bubble fluidized bed boiler is concentrated in the material layer, because the regional heat load has been unable to fully rely on the primary air and the upper water-cooled wall to fully cool, so the vast majority of bubble fluidized bed boilers are equipped with buried tubes immersed in the material layer to balance the temperature of the material layer, because the share of combustion in the suspension section is very low, does not produce a high intensity of secondary suspension combustion. Circulating fluidized bed boiler gas and particles or particles between the slip velocity is large, the mass transfer process is very strong, through the control of the lower part of the furnace combustion share, circulating ash heat absorption or exothermic, can obtain the ideal bed temperature control effect. When burning the same fuel, the share of combustion in the lower dense phase area of circulating fluidized bed boilers is much lower than that of bubbling fluidized bed boilers.

(4) bubble fluidized bed boiler furnace internal upper and lower temperature difference, the upper part of the circulating fluidized bed along the bed height there is a strong material re-mixing, particle clusters are constantly formed and disintegrated. Strong particle heat and mass transfer process will form a uniform and good mixing of gas-solid two-phase flow, making the flue gas temperature distribution relatively uniform throughout the rising section, and the overall furnace chamber has a uniform temperature field.

(5) Although bubbling fluidized bed boiler can also set up gas-solid separator at the exit of the combustion chamber to separate the flue gas entrained fly ash and send it back to the combustion chamber for re-combustion, the overall circulation multiplier is low and not enough to change the heat balance in the combustion chamber. Circulating fluidized bed boilers extend the residence time of solid materials in the furnace, fine particles are easy to burn out, so its fly ash carbon content is much lower than that of bubbling fluidized bed boilers, boiler combustion efficiency and higher utilization of limestone.

(6) drum bubble fluidized bed boiler is difficult to achieve large-scale, under the same conditions of evaporation, its air distribution plate area is larger than circulating fluidized bed boiler. And circulating fluidized bed boiler furnace has a strong ash circulation process, a large amount of heat can be absorbed and utilized in the furnace chamber and the external circulation circuit, arranged in the furnace chamber and the external heat exchanger in the superheater, reheater and water-cooled wall, can ensure that the main steam and reheat steam temperature is stable and easy to manipulate.

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