How does a sludge circulating fluidized bed boiler improve sludge incineration efficiency and enhance heat transfer?
Publish Time: 2026-01-05
The sludge circulating fluidized bed boiler (CFB boiler), an advanced incineration equipment designed specifically for treating municipal sludge with high moisture content and low calorific value, boasts core advantages not only in meeting environmental standards but also in significantly improving incineration efficiency and heat transfer performance through a unique fluidized bed combustion mechanism. Facing challenges such as low calorific value, high viscosity, and easy agglomeration of sludge, the CFB boiler utilizes high-intensity material circulation, a uniform temperature field, efficient gas-solid contact, and multi-stage waste heat utilization technologies to transform "difficult-to-burn" sludge into a stable heat source and efficiently recover energy, achieving a balance between volume reduction, harmlessness, and resource recovery.1. High-intensity internal material circulation: Extending fuel residence time and promoting complete combustionThe core feature of the CFB boiler is the high-speed turbulent movement of the gas-solid two phases within the furnace, with unburned particles continuously returned via a cyclone separator. After the sludge enters the furnace, it is subjected to a low-temperature combustion environment of 850–900℃. Although its calorific value is low, it is enveloped and dispersed by a large amount of high-temperature bed material, forming a "fuel-bed material" mixed fluidized state. Incompletely burned sludge particles are efficiently captured in a cyclone separator along with the flue gas, and then returned to the furnace via a return feeder, achieving multiple cycles of combustion. This process extends the solid residence time to several seconds or even tens of seconds, far exceeding that of traditional grate furnaces, ensuring that even low-calorific-value sludge can be fully burned, with a carbon burnout rate exceeding 98%.2. Uniform Temperature Field and Strong Turbulent Mixing: Eliminating Local Oxygen Deficiency and Cold ZonesTraditional incinerators often suffer from localized oxygen deficiency or uneven temperature due to sludge accumulation, resulting in black slag or incomplete combustion products. In CFB boilers, high-speed fluidization causes intense mixing of the bed material and sludge particles, creating a near-isothermal combustion environment. This highly uniform temperature distribution avoids localized overheating and coking or low-temperature flameout, while simultaneously enhancing the diffusion rate of oxygen to the fuel surface. Furthermore, adding limestone into the furnace allows for simultaneous desulfurization, and the reaction product CaSO₄ also participates in the circulation, further improving the efficiency of synergistic pollutant control.3. High-Density Suspended Bed: Significantly Enhances Convective and Radiative Heat TransferThe high concentration of solid particles in the dense phase zone of the CFB furnace acts like a "heat carrier cloud," continuously scouring the water-cooled walls and screen-type heating surfaces during its ascent, greatly enhancing the convective heat transfer coefficient. Simultaneously, the incandescent particles themselves have strong radiative capacity, improving radiative heat transfer efficiency. This "gas-solid-wall" three-phase heat transfer mechanism allows the boiler to maintain high evaporation rates and stable steam parameters even under low-calorific-value fuel conditions.4. Staged Air Distribution and Combustion Optimization: Precisely Controlling the Combustion ProcessThe CFB boiler employs multi-stage segmented air supply: primary air is introduced through the air distribution plate to maintain fluidization; secondary air is injected from above the dense phase zone, providing the oxygen needed for combustion and agitating the flue gas. Considering the high volatile matter and low fixed carbon content of sludge, the ratio of primary to secondary air can be adjusted to avoid initial deflagration or later oxygen deficiency. Some systems also incorporate flue gas recirculation to reduce oxygen concentration, suppress NOx formation, and regulate furnace temperature, further optimizing combustion efficiency.5. Deep Waste Heat Recovery: Improving Overall Energy Conversion RateThe high-temperature flue gas generated from incineration passes through a cyclone separator and then sequentially through superheaters, economizers, and air preheaters, efficiently converting heat into superheated steam for power generation or heating. Some advanced systems also incorporate external fluidized bed heat exchangers, precisely controlling the main steam temperature by adjusting ash temperature, thus improving the flexibility and efficiency of the thermal system. The overall system thermal efficiency can reach over 70%, far exceeding that of ordinary sludge drying and incineration combined processes.The sludge circulating fluidized bed boiler transforms low-grade waste sludge into a reliable heat source through a dual mechanism of "circulation-enhanced combustion and fluidized bed enhanced heat transfer." It is not only an environmental treatment terminal but also an energy recovery hub. Driven by the "dual carbon" goal, this efficient, clean, and flexible sludge disposal technology is becoming a key support for wastewater treatment plants to achieve energy self-sufficiency and green transformation.
