Power Plants

Induced Draft (ID) Fan: Creates negative pressure (suction) pulling flue gas from boiler through air preheater, ESP/bag filter, FGD (if present), and exhausting to stack. Handles hot (180-280°C), dirty (with fly ash) flue gas. Largest and most critical fan—failure stops unit. 500 MW unit: 2,000,000 m³/hr at 250 mmWC, 2,500 kW motor. Forced Draft (FD) Fan: Pushes combustion air into boiler furnace at 150-250 mmWC pressure overcoming windbox resistance. Handles ambient air (simple duty). 500 MW: 1,500,000 m³/hr, 1,800 kW. Primary Air (PA) Fan: Supplies high-pressure (600-1200 mmWC) air to coal mills for pulverizing and conveying coal to burners. Handles heated air (250-350°C) after air preheater. 500 MW: 400,000 m³/hr, 1,200 kW. Sizing logic: ID fan larger than FD due to: (1) Thermal expansion—flue gas at 200°C vs ambient air occupies 1.7× volume for same mass. (2) Air leakage—boiler operates at negative pressure, air leaks in adding 15-25% to gas volume. (3) FGD addition—wet scrubber adds water vapor increasing volume. Control: ID and FD fans modulate together maintaining furnace draft at -5 to -10 mmWC (slight negative preventing flame puffback). PA fan follows coal feed rate. Operational note: Never lose ID fan during operation—would cause furnace over-pressure and potential explosion. Redundancy critical.

Erosion from fly ash: #1 cause. Coal ash (SiO2, Al2O3 hardness 6-7 Mohs) at 30-50 g/Nm³ concentration impacts impeller at 50-100 m/sec wearing blades 5-15mm/year. Tip erosion worst (highest velocity). Poor-quality coal (high ash %) accelerates wear. Corrosion: Sulfur in coal forms SO2/SO3 → H2SO4 below ~150°C acid dew point. Eats metal, especially at startups/shutdowns when temperatures cycle through dew point. High-sulfur coal (>2% S) requires stainless or acid-resistant coatings. Thermal stress: Repeated heating/cooling during startups/shutdowns causes metal fatigue cracks. Rotor thermal growth/contraction (150-250mm for large fans) requires proper expansion joints—inadequate provision causes rubbing or misalignment. Bearing failures: Oil contamination (water, dust ingress), improper lubrication, high vibration from imbalance/misalignment. Bearing failure leads to catastrophic rotor drop. Vibration: Imbalance from non-uniform erosion, ash buildup on blades, thermal distortion, loosened components. Vibration >11 mm/sec (rms) causes bearing damage, foundation cracking, seal failure. Material buildup: Fly ash accumulation on impeller (especially forward-curved blades) causing imbalance. Mitigation: AR400/AR500 abrasion-resistant steel (3-5× wear life), replaceable wear liners, oversized fans reducing velocity, regular online/offline cleaning, vibration monitoring (shutdown if >18 mm/sec alarm), thermal expansion provisions, acid-resistant materials for high-sulfur fuels. Preventive replacement: Many plants replace ID fan rotors/impellers every 5-8 years proactively vs waiting for failure (sudden failure causes ₹5-10 crore lost generation).

Capital cost examples: 500 MW unit ID fan (2,000,000 m³/hr, 250 mmWC): ₹10-18 crore including motor, VFD, installation. FD fan (1,500,000 m³/hr, 200 mmWC): ₹6-12 crore. PA fans (400,000 m³/hr, 1000 mmWC, 2 units): ₹8-14 crore total. Total per 500 MW unit: ₹25-45 crore for all draft fans. Operating cost: ID fan 2,500 kW running 8,000 hrs/year: 2,500 × 8,000 × ₹7/kWh = ₹14 crore/year electricity. Over 25-year plant life, operating cost = ₹350 crore (25× capital cost!). Maintenance cost: Annual routine = ₹50-80 lakh (inspections, lubrication, minor parts). Major overhaul every 5 years = ₹2-4 crore (rotor refurbishment, bearing replacement, alignment). Unplanned failure = ₹5-15 crore (emergency rotor replacement + ₹2-5 crore lost generation during 5-10 day outage). Lifecycle cost: 25-year total = ₹15 crore capital + ₹350 crore energy + ₹25 crore maintenance = ₹390 crore. Energy dominates lifecycle cost—every 1% efficiency gain saves ₹3.5 crore over plant life. Efficiency focus: Utilities willing to pay 30-50% premium for high-efficiency designs (backward-curved or airfoil impellers, wide impellers, precision-manufactured housings) that save 3-8% energy. Payback <2 years on energy savings alone.

Fly ash is incombustible mineral residue from coal combustion. Composition: SiO2 (silicon dioxide) 40-60%, Al2O3 (aluminum oxide) 20-40%, Fe2O3, CaO, MgO, carbon (unburned coal 1-10%). Hardness: SiO2 and Al2O3 have Mohs hardness 6-7 (between feldspar and quartz), making them highly abrasive. Size distribution: 80% by mass <100 micron, 50% d50 ~10-50 micron, with significant fraction <10 micron penetrating deep into lungs (respirable dust hazard). Concentration: Raw flue gas 30-80 g/Nm³ at boiler exit (Indian coal 25-45% ash vs imported 10-20%). After ESP/bag filter: <50 mg/Nm³ meeting emission norms. Erosion mechanism: Fly ash particles entrained in flue gas at 50-100 m/sec velocity impact impeller blades and housing, mechanically abrading metal. Wear rate: Proportional to V³ (velocity cubed!), particle hardness, concentration, and impact angle. Impact on fans: (1) Blade erosion: Leading edge and tip wear worst (highest velocity + impact angle). Mild steel: 8-15mm/year wear. AR400: 2-4mm/year. (2) Housing wear: Scroll cutwater and high-velocity zones erode through, requiring liner replacement or housing renewal. (3) Imbalance: Non-uniform erosion creates rotor imbalance causing vibration → bearing failure. (4) Buildup: Sticky ash (high carbon content, condensed sulfates) cakes on blades increasing weight, reducing airflow, and causing imbalance. Mitigation: Pre-separation (cyclone removing 50-70% of coarse ash before fan), abrasion-resistant materials, oversizing fan (lower velocity = V³ effect dramatically reduces wear), radial blade design (self-cleaning), online water wash or air blast cleaning.

Ash content: Indian coal typically 25-45% ash vs imported coal 10-20%. Impact: Higher ash = more fly ash erosion on fans/ductwork, larger ESP/bag filter needed (+30-50% capital cost), higher ash handling costs, lower plant efficiency (ash doesn't burn, just thermal ballast). Moisture content: Indian coal 8-15% inherent moisture, some lignites 30-50%. High moisture reduces calorific value (energy wasted evaporating water), increases stack losses (wet flue gas larger volume), can cause coal handling issues (sticking, freezing in winter). Sulfur content: 0.3-1.5% typical, some coals >2%. Sulfur forms SO2/SO3 → acid corrosion below 150°C dew point. High sulfur requires FGD (flue gas desulfurization) adding ₹200-400 crore capital cost for 500 MW unit. Volatile matter: Bituminous 20-35%, lignite 45-55%. High volatiles = easier ignition,lower excess air needed, but more soot formation. Calorific value: Indian coal 2,500-5,000 kcal/kg vs imported 6,000-7,000 kcal/kg. Low calorific value requires burning more coal per kWh (higher mining/transport costs), larger bunkers/conveyors, bigger pulverizers and burners. Sizing: Lump coal (>50mm) vs fine coal (<6mm). Excessive fines cause classifier bypass in mills (unburned carbon loss), dust handling issues. Economic impact: Switching from imported coal (6,500 kcal/kg, ₹9,000/ton) to domestic (3,500 kcal/kg, ₹4,000/ton): Domestic requires 1.86× mass for same energy = ₹7,440/ton effective cost + higher ash disposal + more wear. Sometimes imported coal cheaper despite higher price! Blending strategy: Mix domestic + imported optimizing cost vs performance avoiding worst consequences of either extreme.

India CPCB 2015 norms (Dec 2024 deadline extended): Particulate matter: <30 mg/Nm³ (older plants <50 mg allowed till Dec 2024, then <30mg). Requires ESP or bag filter achieving 99.5-99.9% efficiency. SOx: <200 mg/Nm³ for coal plants >500 MW in critically polluted areas, <600 mg elsewhere. Compliance via low-sulfur coal blending or FGD (Flue Gas Desulfurization) limestone scrubber removing 90-95% SO2. FGD capital cost ₹35-60 crore per 100 MW. NOx: <300 mg/Nm³ (from 2024). Requires Low-NOx burners (20-30% reduction) + SNCR (Selective Non-Catalytic Reduction, urea/ammonia injection,40-60% reduction) or SCR (Selective Catalytic Reduction, 80-90% reduction but ₹100-200 crore per 100MW). Mercury: <0.03 mg/Nm³ (proposed, not enforced). Requires activated carbon injection. Water consumption: <3.5 liters/kWh specific consumption. Drives dry cooling adoption despite efficiency penalty. Ash utilization: 100% utilization mandated (use in cement, bricks, roads vs landfilling). Stack height: Minimum calculated based on capacity ensuring adequate dispersion. Non-compliance penalties: Older plants ₹10 lakh/month + potential closure orders. New plants must meet from day one. Compliance cost: Retrofitting existing 500 MW plant for full compliance: ESP upgrade ₹50 crore, FGD ₹250 crore, SNCR ₹30 crore, total ₹330 crore = 30% of original plant cost. Many older plants uneconomic to upgrade, facing early retirement. New plant designs incorporate all controls from start.

Energy savings: Fan power proportional to speed³ (affinity laws). Reducing fan speed 20% (from damper/vane control to VFD) cuts power 49% [ (0.8)³ = 0.512 ]. Typical ID fan cycling between 80-100% load: VFD saves 15-30% average power vs damper control. 500 MW ID fan 2,500 kW: savings 375-750 kW = ₹20-40 lakh/year at ₹7/kWh, 8,000 hrs. Payback: VFD capital ₹50-80 lakh for 3 kV, 2,500 kW drive = 1.5-3 years payback from energy alone. Precise control: VFD maintains exact pressure/flow setpoint ±1% vs ±5-10% damper control. Critical for furnace draft control (target -8 mmWC maintained precisely preventing puffback or implosion). Soft start: VFD ramps motor smoothly eliminating 6-8× inrush current that stresses electrical system. Mechanical shock reduced extending coupling/bearing life 30-50%. Grid support: During grid frequency drops, VFD can temporarily reduce fan load shedding 10-20% power in <1 second helping grid stability (ancillary service revenue potential). Reduced maintenance: Eliminates damper/vane linkages (wear points), reduces motor starts (thermal cycling), smoother operation (lower vibration). Challenges: Harmonic distortion (requires filters), bearing currents from high dv/dt (requires insulated bearings), higher capital cost, requires skilled maintenance. Modern trend: Every new power plant specifies VFD on major fans (ID, FD, PA) for efficiency and control benefits. Retrofit ROI compelling even for existing plants.