How Does a Circulating Fluidized Bed Boiler "Fuel Up"—What's the Secret to Efficiently Combusting Low-Calorific-Value, High-Ash Fuels?
Publish Time: 2025-12-19
In the context of diversified energy structures and increasingly stringent environmental requirements, traditional boilers often struggle with "coarse grain" fuels such as low-quality coal, coal gangue, oil shale, biomass, and even industrial waste. However, the circulating fluidized bed boiler (CFB), due to its unique combustion mechanism, is hailed as an "omnipotent expert" among industrial boilers—it can not only stably burn low-quality fuels with calorific values as low as 3000 kcal/kg and ash content exceeding 60%, but also achieve efficient combustion and low pollutant emissions. Its secret to "fueling" low-quality fuels without "indigestion" stems from three core technological advantages: strong gas-solid mixing, a wide combustion temperature window, and an efficient material circulation system.1. Vigorous Disturbance and Thorough Mixing: Ensuring Thorough Combustion of Low-Quality FuelsThe core of the circulating fluidized bed boiler lies in the "fluidization" state. High-speed air is introduced into the bottom of the furnace through an air distribution plate, suspending solid particles and causing them to move in a boiling motion, forming a highly turbulent gas-solid two-phase flow. This intense disturbance greatly enhances the contact area and reaction rate between oxygen and fuel particles, allowing for rapid ignition and burnout even with low calorific value and low volatile matter in the fuel. Especially for high-ash fuels, unburned carbon is trapped in an ash shell and difficult to oxidize. However, in a CFB furnace, continuous collisions and friction between particles effectively peel off the ash layer, exposing fresh carbon surfaces and significantly improving burnout rate.2. Low-Temperature Combustion and In-Furnace Desulfurization: Balancing Efficiency and Environmental ProtectionCFB boilers typically operate in a "low-temperature" range of 850–920℃. This temperature is much lower than that of pulverized coal boilers, but it falls precisely within the optimal reaction temperature range for limestone desulfurization. When limestone is incorporated as a desulfurizing agent, SO₂ fixation can be completed simultaneously during combustion, achieving a desulfurization efficiency of over 90%, significantly reducing the burden on subsequent flue gas treatment. Simultaneously, this temperature also effectively inhibits the formation of thermal NOx. More importantly, low-temperature combustion avoids the melting of alkali metals or minerals in high-ash fuels, significantly reducing the risk of coking and fouling—a key bottleneck preventing many boilers from burning low-quality fuels.3. Material Circulation System: Extending Fuel Residence Time and Improving Combustion EfficiencyCFB boilers are equipped with efficient gas-solid separation devices and a return system that captures unburned fine particles from the flue gas and returns them to the furnace, forming a "multiple circulation, repeated combustion" mechanism. This design greatly extends the effective residence time of fuel in the furnace, which is especially crucial for slow-burning, difficult-to-burn low-calorific-value fuels. Data shows that the carbon burnout rate of CFB boilers can reach over 98%, far exceeding that of ordinary chain grate boilers or reciprocating boilers. Furthermore, the circulating ash also acts as a "heat carrier," helping newly added fuel to heat up and ignite quickly, maintaining a stable bed temperature.4. Wide Fuel Adaptability: Flexible Switching from Coal Gangue to BiomassThanks to the aforementioned mechanism, CFB boilers can handle almost any type of solid fuel: coal gangue and middlings from coal mines, petroleum coke from the chemical industry, agricultural waste such as rice husks and straw, and even urban sludge and waste-derived fuels. Stable combustion of different fuels can be achieved simply by adjusting the feeding method, air volume ratio, and bed material proportion. This flexibility makes it play a key role in comprehensive resource utilization and the circular economy, reducing fuel costs and contributing to the harmless treatment of solid waste.The circulating fluidized bed boiler's ability to efficiently handle even low-quality fuels does not rely on expensive additives or complex post-treatment, but rather on ingenious physical process design—achieving thorough mixing through fluidization, environmentally friendly low-temperature combustion, and improved burnout through material circulation. It transforms the "disadvantages" of low-quality fuels into "advantages" for system operation, ensuring energy supply while promoting green transformation. In today's world, where both "dual carbon" goals and resource conservation are equally important, the "coarse grain philosophy" of CFB boilers is providing a pragmatic and efficient technical path for clean combustion in industry.
