工程科学与技术 (Jan 2025)
CPFD Simulation of Operation Characteristics of a 10t/d Sludge and Biomass Co-incineration Fluidized Bed Reactor
Abstract
ObjectiveThis research aims to comprehensively investigate the operational dynamics of a 10t/d sludge and biomass co-incineration fluidized bed reactor through Computational Particle Fluid Dynamics (CPFD) modeling, addressing critical gaps in industrial-scale waste valorization. The study establishes four primary objectives: 1) Quantify hydrodynamic transitions (bubbling/turbulent regimes) under variable fluidizing air conditions; 2) Decipher fuel property impacts (sludge/biomass particle size, feedstock type) on nitrogen/sulfur pollutant transformation pathways (HCN→CNO→NO, H₂S→SO₂); 3) Evaluate bed material efficacy (dolomite, olivine, quartz sand) for in-situ emission control; and 4) Optimize reactor geometry (height-diameter ratios, feed configurations) to maximize combustion efficiency while minimizing environmental footprints. The work provides foundational insights for scaling sustainable co-processing systems that address sludge disposal challenges and biomass utilization.MethodsNumerical simulations employed Barracuda™ VR 17.3.0's Multi-Phase Particle-in-Cell (MP-PIC) algorithm, resolving gas-solid reactive flows in a 3D reactor comprising a lower cylindrical section (Ø0.7 m×H1.7 m), upper expansion zone (Ø1.3 m× H3.8 m), and conical transition (H0.5m). An adaptively refined Cartesian grid (88,500 cells) prioritized resolution in high-activity regions. The physicochemical framework integrated 16 kinetic reactions: devolatilization (sludge/biomass→H2O+volatiles+char), homogeneous combustion (CₓHᵧ/CO/H₂oxidation), heterogeneous reactions (char gasification), and pollutant transformations (HCN→CNO→NO; H2S→SO2; CaO-SO2 sulfation). Fuel properties derived from proximate/ultimate analyses of wastewater sludge (80% moisture, 1-5 mm particles), woody biomass (1-5cm cylinders), and rice husk were implemented. Bed materials included quartz sand (75-413μm), dolomite (140-250μm), and olivine (140-250μm) with density-adjusted compositions. Orthogonal parametric studies covered 36 operational scenarios: fluidizing air (primary: 595-805 Nm³/h; secondary: 0-345 Nm³/h; temperature: 38-52℃), fuel parameters (sludge size: 1-5 mm; biomass size: 1-5 cm; feed rate: 52-68 kg/h; type: wood/rice husk), bed attributes (material type, size, height: 400-600 mm; temperature: 750-850℃), and reactor geometry (height: 5.1-6.9 m; diameters: lower 0.595-0.805 m/upper 1.105-1.495 m; feed: single/opposing inlets). Simulations executed 15s physical time using NVIDIA GTX4060 GPU acceleration with adaptive timestepping (10-7-10-4s), incorporating Gidaspow drag and LES turbulence models. Validation against turbulent fluidization literature confirmed hydrodynamic reliability.Results and Discussions Increasing primary air flow from 595 to 805 Nm³/h transitioned bed regimes from bubbling (voidage 0.75-0.85) to turbulent fluidization (voidage 0.55-0.65), enhancing particle entrainment into the freeboard. Secondary air injection (255-345 Nm³/h) improved burnout (CO2↑14.06→14.33%; H2O↑38.06→41.05%) but elevated CNO intermediates by 115% due to thermal quenching of oxidation kinetics, while total air increases diluted pollutants but accelerated CNO→NO conversion (CNO↓0.47→0.13%; NO↑1.81→1.96%). Biomass particle enlargement (1→5 cm) impaired diffusion-limited conversion (O2↑48.75→56.79%; CO2↓13.67→10.66%) and suppressed HCN oxidation (CNO↓0.29→0.14%), whereas sludge sizing variations exerted negligible S/N effects (ΔH2S<0.05%). Rice husk outperformed wood in emission control, reducing CNO by 73% (0.06% vs. 0.22%) and H2S by 15% (0.17% vs. 0.20%) attributable to lower nitrogen (0.61% vs. 3.84% daf) and sulfur content (0.16% vs. 0.18% daf). Dolomite demonstrated limited in-situ desulfurization (SO2↓0.01% vs. quartz's 0.02%) through CaO sulfation, though reaction efficiency was hindered by poor gas-solid contact—SO2 concentrated near the outlet where bed material presence diminished. Olivine exhibited no catalytic benefits. Bed material enlargement (225-413μm) degraded fluidization quality (bed height↓40%), exacerbating CNO accumulation (↑330%) and NO reduction (↓20%) via inhibited gas-solid contact. Higher bed temperatures (750→850℃) enhanced fuel conversion (CO2↑12.03→13.46%) and H2S oxidation (↓0.20→0.16%), while increased bed height (400→600 mm) paradoxically raised CNO by 57% despite improved residence time, suggesting channeling issues. Reactor geometry adjustments induced <2% compositional changes, but opposing feed inlets revolutionized mixing dynamics: fuel dispersion uniformity eliminated localized rich zones, intensifying turbulence to boost CO2 yield by 24% (15.74% vs. 12.66%), enhance CNO→NO conversion (NO↑14%; CNO↓32%), and homogenize temperatures—though a persistent flow dead zone in the upper-right corner induced a 1,217 K hotspot.ConclusionsThis study establishes definitive operational guidelines for sludge-biomass co-incineration FBRs: 1) Maintain primary air >805 Nm³/h to ensure turbulent fluidization while limiting secondary air to ≤30% total flow to balance burnout enhancement against CNO accumulation; 2) Restrict biomass particle size ≤3 cm (equivalent spherical diameter) and prioritize rice husk over wood for high-S/N sludge blends to minimize CNO/H2S emissions; 3) Employ dolomite bed material (150-275μm) at 500 mm initial height and 800-850℃ for moderate SO2 control while avoiding particle sizes >225μm that degrade fluidization; 4) Implement opposing feed inlets as a low-cost retrofit to intensify turbulence, elevating NO output by 14% and reducing CNO by 32%—though complementary design modifications (e.g., baffle installation, outlet repositioning) are recommended to eliminate identified thermal hotspots. The CPFD framework demonstrates exceptional utility in predicting scaling effects, with the optimized operational envelope (primary air 700-805 Nm³/h, biomass ≤3 cm, dolomite 150-275μm, opposing inlets) reducing theoretical CNO emissions by 38-72% versus conventional configurations. Future work should integrate ash agglomeration models and validate transient CNO profiles at pilot scale.
