Pulverized coal systems must discharge powder from cyclones and hoppers without letting conveying air escape through the same opening. A gravity type air lock performs this sealing function by maintaining a material column between the vessel outlet and the atmosphere. The device depends on the weight of the coal powder itself to form a seal, which makes it simple, reliable, and free of external power requirements.
Why Air Locks Are Needed in Powder Systems
Cyclones separate coal powder from the conveying air stream, but the collected powder must leave the cyclone while the air continues to the fan or filter. If the discharge opening simply stayed open, conveying air would short-circuit through the outlet, carrying powder back into the system and destroying the separation efficiency. The air lock breaks this connection by keeping a dense column of powder in the discharge path at all times.
The same problem appears at hopper outlets, dust collector hoppers, and any point where powder crosses a pressure boundary. Losing sealing at these points wastes conveying energy, pollutes the work area, and creates a dust explosion hazard in coal service. A gravity air lock provides a passive solution with no moving parts in the sealing zone, which explains its popularity in older plants and its continued use in new designs.
Operating Principle and Sealing Mechanism
The device consists of a vertical or inclined housing connected below the cyclone or hopper outlet, with the discharge opening positioned above the downstream equipment or conveying line. Powder entering the housing fills the internal volume and forms a continuous column. The column height and the internal friction of the coal powder resist the air pressure difference between the vessel interior and the outside, so air cannot flow upward through the material.
Discharge occurs when the powder level in the housing exceeds a certain height, allowing material to flow out through the bottom opening. The housing geometry is arranged so that a fresh column of powder immediately replaces the discharged material, maintaining the seal continuously.
Design Features for Coal Powder Service
Coal powder is abrasive and can pack or bridge inside poorly shaped housings, so the internal geometry avoids sharp corners and dead zones where material could accumulate. Wear-resistant liners protect the housing walls and the outlet area from erosion by the flowing powder. The inlet connection includes a flange or flexible joint that accommodates thermal movement and vibration between the cyclone and the air lock.
The outlet arrangement must match the receiving equipment, whether that is a screw conveyor, a pressure vessel, or a pneumatic conveying line. Inspection openings allow operators to check the internal condition and clear any bridging without disassembling the unit. A factory that supplies air locks as part of a complete pulverized coal piping product line can match the device precisely to the plant layout and operating conditions.
Selection Criteria and Operational Care
Selection depends on the required throughput, the pressure difference to be sealed, the physical properties of the powder, and the space available below the vessel outlet. Moisture content is a critical factor, because damp powder flows poorly and may not form the free-flowing column the seal depends on. For systems handling wet or sticky material, a rotary air lock or other positive displacement device may be a better choice than the gravity type.
Operational care focuses on keeping the powder flowing freely: avoiding storage of wet coal, maintaining adequate hopper aeration, and inspecting the wear liners during scheduled outages. The device has no seals to replace and no motor to maintain, so its maintenance burden is low compared with rotary feeders. Selecting a reputable manufacturer with field experience in coal powder applications ensures the design details are proven in service.
References
American Society of Mechanical Engineers. (2022). ASME B31.1, Power Piping. ASME.
National Fire Protection Association. (2023). NFPA 85, Boiler and Combustion Systems Hazards Code. NFPA.
American Society for Testing and Materials. (2022). ASTM D3174, Standard Test Method for Ash in the Analysis Sample of Coal and Coke. ASTM.
International Organization for Standardization. (2020). ISO 5370, Solid biofuels -- Determination of fines content. ISO.
