Circulating fluidized bed
A circulating fluidized bed (CFB) is a fluidized bed reactor system in which solid particles, typically inert bed material like sand mixed with fuel, are suspended and rapidly circulated by an upward-flowing gas stream through a tall riser column, with entrained particles captured by cyclones and recycled back to the furnace base to sustain high solids inventory and vigorous mixing for combustion or gasification processes.[1][2] This design achieves fluid-like behavior at velocities exceeding those of bubbling beds, typically 4-10 m/s, enabling dense-phase operation with solids concentrations of 5-15% by volume throughout the riser.[3][4] CFB technology emerged in the mid-1970s as an advancement over bubbling fluidized beds, with the foundational patent filed in 1976, driven by needs for cleaner coal combustion amid oil crises and environmental regulations.[5] It gained traction in the 1980s for utility-scale boilers, now dominating installations for fuels like low-grade coal, biomass, and waste, with over 5,000 units worldwide by the 2010s due to operational scales exceeding 600 MW per unit.[6][7] Key defining characteristics include combustion at 800-900°C to minimize NOx formation via staged air and low excess air ratios, alongside in-bed limestone injection for >90% SOx capture without external scrubbers, yielding emissions compliant with stringent standards like those under the U.S. Clean Air Act.[8][3] The technology's advantages stem from superior gas-solid contact and heat transfer, supporting fuel flexibility across calorific values from 7-30 MJ/kg and high sulfur contents up to 10%, while achieving boiler efficiencies of 85-90% and rapid load response suitable for grid integration.[9][10] Drawbacks include higher capital costs from complex solids handling and potential erosion in risers, though mitigated by advanced materials and designs in modern supercritical units operating above 540°C for enhanced cycle efficiency.[11][12] CFBs also extend to gasification, where they process solids into syngas under reducing conditions, facilitating applications in chemical production and integrated gasification combined cycles.[2]Fundamentals
Fluidization Regimes and Classification
Fluidization regimes describe the distinct hydrodynamic states in gas-solid systems as superficial gas velocity increases from below the minimum fluidization velocity (Umf), where particles remain packed, to higher velocities enabling suspension and transport.[13] These regimes transition progressively: fixed bed (stationary particles with gas percolating through voids), bubbling (gas voids or bubbles rise through the bed, causing expansion), slugging (in confined beds, bubbles coalesce into large slugs spanning the vessel diameter), turbulent (chaotic motion with reduced bubble coherence), fast fluidization (high entrainment with particle recirculation), and pneumatic conveying (dilute-phase transport).[14] The transitions depend on particle size, density, gas properties, and bed dimensions, with Umf marking the onset where drag force balances particle weight.[13] In circulating fluidized beds (CFBs), operation centers on the fast fluidization regime, where gas velocities surpass single-particle terminal velocity (Ut), entraining fines while coarser particles circulate via external loops, yielding a dense bottom zone (voidage ~0.6) and dilute riser top (voidage >0.9).[15] This regime sustains solids flux through cyclone separation and return, distinguishing CFBs from bubbling beds by enabling higher throughput and uniformity, though it involves core-annular flow patterns with solids clustering near walls.[16] Dense-phase conveying may coexist in lower sections, bridging turbulent and fast regimes.[15] Powder classification, notably Geldart's 1973 framework, categorizes particles by mean diameter (dp) and density difference (ρs - ρg), predicting regime transitions and fluidization quality; CFBs favor Groups A and B for their entrainment and circulation efficacy.[17]| Geldart Group | Typical dp Range (μm) | Density Characteristics | Fluidization Behavior |
|---|---|---|---|
| A (Aeratable) | 20–100 | Low ρs - ρg | Particulate expansion before bubbling; smooth at low U, suitable for CFB entrainment.[13] |
| B (Sand-like) | 100–1,000 | Moderate ρs - ρg | Immediate bubbling; good circulation in fast regime but higher Umf.[13] |
| C (Cohesive) | <20 | High cohesion | Difficult fluidization with channeling; unsuitable for CFB due to agglomeration.[17] |
| D (Spoutable) | >1,000 | High ρs | Spouting over bubbling; limited to niche CFB variants, prone to defluidization.[17] |