Power plant ash handling systems pump coal ash (fly ash and bottom ash) as water slurry from boilers to disposal areas or dewatering systems. These slurries are highly abrasive (silica, alumina) and often alkaline (lime, calcium), requiring robust slurry pumps. This guide covers slurry pump selection for power plant ash handling.
Ash Handling Overview
- Fly ash: Captured by ESP/baghouse from flue gas (fine, <100μm)
- Bottom ash: Collected from boiler hopper (coarse, up to 50mm)
- Economizer ash: Collected from economizer hoppers
- Slurry transport: Ash mixed with water, pumped to ash pond or dewatering
- Disposal: Ash pond, landfill, or beneficial use (cement, concrete)
- High-pressure systems: Long-distance pipeline to ash pond (5-20km)
- Low-pressure systems: Short transport to nearby sump or dewatering
Ash Slurry Characteristics
| Parameter | Fly Ash | Bottom Ash | Mixed Ash |
|---|---|---|---|
| Particle size | <100μm (fine) | 1-50mm (coarse) | Mixed |
| Solids % | 20-50% | 10-40% | 20-50% |
| SG | 1.8-2.4 | 1.8-2.5 | 1.8-2.4 |
| Abrasion | High (silica) | Very high (coarse) | Very high |
| pH | 8-12 (alkaline) | 8-12 | 8-12 |
| Hardness | Mohs 6-7 (silica) | Mohs 6-7 | Mohs 6-7 |
| Temp | 20-60°C | 50-90°C (hot water) | 20-90°C |
Pump Type Selection
Horizontal Centrifugal Slurry Pump
- Most common for ash slurry transport (high abrasion, large flow)
- Flow: 100-10,000 m³/h, head: 30-200 m
- Large passages, thick volute, hard metal or rubber-lined
- Material: high-chrome iron (HCCI), Ni-hard, or rubber-lined
- Advantages: high flow, handles large solids, replaceable wear parts
- For high-pressure long-distance: multistage or multiple boosters
Vertical Slurry Pump
- For ash sumps, hoppers, wet pits
- Flow: 50-5,000 m³/h, head: 20-100 m
- Submerged or semi-submerged, no priming
- Material: HCCI or rubber-lined
- Advantages: handles variable level, no priming, compact
- For bottom ash hoppers: vertical pump common (submerged in ash water)
Positive Displacement (PD) Pump
- For high-pressure, high-solids ash paste (dewatered ash transport)
- Types: piston, peristaltic, progressive cavity
- Flow: 10-200 m³/h, pressure: 20-150 bar
- For ash paste (60-80% solids): piston pump
- Advantages: handles high viscosity, high pressure, consistent flow
- Limitations: lower flow, higher maintenance
Water Pump (for Ash Water)
- For clean ash water (after dewatering, recirculation)
- Flow: 100-5,000 m³/h, head: 20-100 m
- Material: cast iron, 316, or bronze
- For recirculation: centrifugal clean water pump
Material Selection
Wetted Parts
| Material | Abrasion | Alkali Resistance | Best For |
|---|---|---|---|
| High-chrome iron (HCCI) | Excellent | Fair | Fly ash, bottom ash, high abrasion |
| Ni-hard | Excellent | Fair | General ash slurry |
| Rubber lining (natural) | Excellent (fine) | Good | Fly ash, fine particles |
| Polyurethane | Excellent | Good | Abrasive, fine solids |
| 316 stainless | Fair | Good | Low abrasion, alkaline water |
| Cast iron | Poor | Fair | Clean ash water only |
Shaft and Seals
- Shaft: 4140 or 316 with replaceable sleeve (HCCI, ceramic, 316)
- Seal: expeller seal (most common for ash slurry, no flush)
- Packing: with water flush (for coarse bottom ash)
- For alkaline ash water: verify seal material (EPDM or Viton)
- Shaft sleeve: ceramic or HCCI (abrasion resistant)
Design Considerations
Abrasion Resistance
- Ash is highly abrasive (silica, alumina, Mohs 6-7)
- Bottom ash: coarse particles (up to 50mm), very high wear
- Fly ash: fine particles, still abrasive (high velocity impact)
- Wear rate ∝ velocity³ — keep velocity low (1.5-3 m/s)
- Material: HCCI for coarse bottom ash, rubber for fine fly ash
- Replaceable wear parts: volute liner, throatbush, impeller, wear plate
- Monitor wear: performance curve shift, vibration, visual inspection
Alkaline Corrosion
- Ash slurry is alkaline (pH 8-12) from lime and calcium compounds
- Alkaline water can cause corrosion in some metals
- Cast iron: generally OK in alkaline (passivation layer)
- Aluminum/zinc: not compatible (alkaline attack)
- 316: good in alkaline
- Rubber: good in alkaline
- Seal elastomers: EPDM (good in alkaline) or Viton
Hot Water (Bottom Ash)
- Bottom ash quenched with water → hot slurry (50-90°C)
- High temp increases wear and corrosion rates
- For >80°C: use high-temp materials, seal cooling
- For bottom ash: verify pump temp rating
- Bearing: water-cooled or finned housing (heat from slurry)
Large Solids (Bottom Ash)
- Bottom ash: up to 50mm particles (clinkers)
- Use large passage slurry pump (no small clearances)
- Open or semi-open impeller (handles large solids)
- Piping: large diameter, long radius elbows, no dead legs
- Maintain velocity above deposition (2-3 m/s for coarse ash)
- Grizzly or screen upstream: remove oversize (prevent blockage)
System Design
High-Pressure Ash Slurry System
- Long-distance transport (5-20km) to ash pond
- Multiple booster pumps in series (maintain pressure)
- Flow: 500-5,000 m³/h, head: 200-500m total
- Pump: horizontal slurry pump (each booster)
- Pipe: rubber-lined steel or HDPE (abrasion resistant)
- Velocity: 2-3 m/s (above deposition, below wear threshold)
- Key: reliability, abrasion resistance, easy maintenance
Low-Pressure Ash Sump System
- Short transport to nearby sump or dewatering
- Vertical or horizontal slurry pump
- Flow: 100-2,000 m³/h, head: 20-80 m
- Key: handle variable level, no priming, reliable
Ash Dewatering and Water Recirculation
- Thickener: separate ash from water (recover water)
- Filter press or vacuum belt: dewater ash to <30% moisture
- Recirculation pump: return water to ash handling system
- For recirculation: horizontal split case or centrifugal
- Key: water recovery (reduce makeup water, environmental)
- For ash pond: water recovery system (decant, recycle)
Bottom Ash Handling
- Boiler hopper: water-filled, collects bottom ash
- Submerged conveyor or jet pump: removes ash from hopper
- Slurry pump: transports to dewatering or disposal
- Key: handle hot water, coarse particles, reliable
- Water seal: prevent air ingress (boiler draft)
Common Issues
- Excessive wear: Silica ash, high velocity. Use HCCI/rubber, lower velocity, replaceable parts.
- Bottom ash blockage: Coarse clinkers, large particles. Use large passages, screen upstream, maintain velocity.
- Hot water: Bottom ash quenching, high temp. Use high-temp materials, bearing cooling, verify rating.
- Alkaline corrosion: Ash water pH 8-12. Use compatible materials (CI, 316, rubber), EPDM seals.
- Pipe wear: High velocity, abrasive ash. Use rubber-lined/HDPE, lower velocity, rotate pipe.
- Seal failure: Abrasive slurry, no flush. Use expeller seal, proper flush, monitor.
- Settling/blockage: Low velocity, high solids. Maintain velocity, agitate sumps, flush.
- Ash pond water quality: Heavy metals, alkaline. Use lined pond, water recovery, environmental compliance.
Procurement Recommendations
- Provide: ash type (fly/bottom), particle size, solids %, SG, pH, temp, flow, head, distance
- For fly ash: specify horizontal slurry pump, rubber-lined or HCCI
- For bottom ash: specify HCCI slurry pump, large passages, high-temp rating
- For sumps/hoppers: specify vertical slurry pump
- For high-pressure long-distance: specify multiple booster pumps, rubber-lined pipe
- For ash paste: specify PD pump (piston/peristaltic)
- For water recirculation: specify horizontal split case or centrifugal
- Specify material per abrasion/alkali (HCCI, rubber, 316)
- Include expeller seal (no flush water needed)
- Include replaceable wear parts (impeller, volute, throatbush, wear plate)
- For hot bottom ash: specify high-temp materials and bearing cooling
- Include instrumentation: flow, pressure, vibration, temp
- Verify supplier has power plant ash handling references
Related Industries
Power plant ash slurry pump principles apply to mining (tailings, ore), cement (raw meal, kiln dust), wastewater (sludge, grit), and general industrial (abrasive slurry). The core principle — reliable handling of highly abrasive alkaline solids-laden fluid with proper materials, wear parts, and seal — is consistent across ash handling applications.