Power plants that burn blended coal need the pulverized product to reach the burners with a consistent composition, otherwise combustion stability and emissions suffer. A plate type coal powder mixer homogenizes the coal powder stream as it flows between the mill outlet and the burner piping, smoothing out the variations that come from feeding different coals in sequence. The mixer is a passive device with no moving parts in most designs.
Why Coal Powder Needs Mixing
Raw coal quality varies from one delivery to the next and even within a single delivery, so the pulverized coal leaving the mill reflects that variation. When the plant switches between coal sources to manage cost or availability, the powder composition at the burners changes in steps rather than smoothly. These step changes affect flame stability, slagging behavior, and the performance of the combustion control system.
Mixing the powder after pulverization smooths these variations before the coal reaches the burners. The mixer redistributes the powder across the pipe cross-section so that every burner receives a similar mixture regardless of where in the stream the variation occurred. The result is steadier combustion, better mill performance, and more predictable furnace conditions for the operators.
How the Plate Type Mixer Works
The mixer consists of a short duct section containing a set of staggered plates or vanes arranged across the flow path. The powder stream impinges on the plates, which deflect the flow and create turbulence that mixes material from different parts of the pipe cross-section. The staggered arrangement forces every portion of the stream to pass through several deflections before exiting the mixer, achieving thorough redistribution in a short length.
Because the device has no moving parts, it requires no drive, no lubrication, and no control system. The mixing action comes entirely from the flow energy of the coal powder stream. The pressure drop across the plates is modest and is accounted for in the system design, and the plates are arranged so that they do not create dead zones where powder could accumulate or bridge.
Construction for Abrasive Coal Service
Coal powder is highly abrasive, so the mixer plates and housing are protected by wear-resistant materials in the areas of direct impingement. Replaceable wear liners or hard-faced plates extend the service life of the mixer between overhauls. The housing is fabricated to match the connecting piping, with flanged ends that allow removal of the complete unit for inspection or replacement of the wear parts.
Access openings allow the plant to inspect the condition of the plates without removing the mixer from the line. The design avoids internal fasteners and crevices that could catch powder and create accumulation points. For applications where the powder temperature is elevated or the gas contains corrosive constituents, the material selection is upgraded accordingly to match the service conditions.
Application and System Integration
The mixer is installed in the pulverized coal piping downstream of the mill outlet and upstream of the burner distribution, where there is sufficient straight run for the mixing action to take effect. The exact location depends on the piping layout and the mixing requirements of the application. In some plants the mixer is installed at the mill outlet riser where multiple coal sources are combined before distribution to the burners.
Integration with the rest of the system is simple because the mixer has no electrical or control connections. The plant needs only to provide the pipe supports and access space around the unit. Working with a manufacturer that produces the full range of pulverized coal piping components, from feeders to burners, ensures that the mixer dimensions and connections match the rest of the system and that the supplier understands the coal handling application.
References
American Society of Mechanical Engineers. (2022). ASME B31.1, Power Piping. ASME.
American Society for Testing and Materials. (2022). ASTM A36, Standard Specification for Carbon Structural Steel. ASTM.
National Fire Protection Association. (2023). NFPA 85, Boiler and Combustion Systems Hazards Code. NFPA.
American Society of Mechanical Engineers. (2020). ASME PTC 4, Fired Steam Generators Performance Test Code. ASME.
