Fly ash handling is an essential process in any coal-fired or biomass power plant due to stringent environmental regulations – not to mention the need to maintain general plant efficiency.
Designing an appropriate fly ash handling system can be difficult without experience and qualified engineers. There are several challenges that often arise with these systems, and several points of failure that can interfere with – or outright prevent – proper ash handling.
Problems with Proper Flow
To function correctly, the system has to allow for smooth, uninterrupted flow from the boiler to the storage unit (e.g. a silo). Failure to ensure proper flow means a system can become inhibited to the point where it no longer works at all – and where system components themselves can become damaged which will then cause the boiler to be shut down.
One example of a common flow issue is when there’s reduced or nonexistent flow from the hopper or silo outlet. This no-flow condition means that fine dry fly ash isn’t making it to the silo or through the silo to the outlet (where it’s loaded on trucks for transport).
There are two main reasons why no-flow conditions happen: arching and ratholing. Due to the cohesive strength of fine dry fly ash, the material is a great source of friction. In arching, the material forms an arch (or a bridge) above the outlet that prevents further flow. In ratholing, flow resistance increases as the fly ash level in the silo drops, meaning it’ll drop to the point where no more material flows through the outlet – even though the silo is still loaded.
Of course, on the other side of the equation, you can have too much flow, which is called flooding. Flooding happens when there’s enough air present to make the fly ash behave like a liquid. Often, air in the silo can’t escape because the handling rate is too high, which can make the fly ash “flood” through the outlet and overwhelm the feeder. This is where a properly sized and right application feeder is needed.
If you’re not careful, the contents of a silo can flood out in minutes, even seconds.
The Impact of Improper Flow on Equipment
The structural integrity of a fly ash handling system can be compromised by any of the above flow problems. This is because the equipment is subjected to sudden dynamic forces that can buckle the silo and collapse the walls.
Also, since fly ash is quite abrasive, thanks to the presence of silicon dioxide, improper flow can cause more wear and tear on equipment than anticipated. This can result in a constant need to patch up holes or replace sections (or entire parts) of equipment.
Silos aren’t the only components that can be subjected to damage or wear and tear. Conveying lines from the hopper or ESP to the silo can become plugged or worn down, for example.
Creating a Better Fly Ash Handling System
To overcome these challenges, the system has to be engineered in such a way as to mitigate or prevent flow issues.
The first consideration is the flow pattern that the material will take in the silo. There are three types:
- Funnel flow
- Mass flow
- Expanded flow
With funnel flow, only a portion of the material is moving at any given time (in a funnel shape, hence the name). The remaining material on the sides of the hopper remains stationary. With mass flow, by contrast, all of the material moves at once, in a first-in, first-out sequence to create a steady flow.
Expanded flow actually combines both by placing the mass flow hopper under the funnel flow hopper. Benefits include less headroom and less capital cost.
For fine dry fly ash, funnel flow is insufficient and not recommended. A mass flow system can prevent the material from caking over time, as well as ratholing or arching (as long as the hopper is designed properly). Expanded flow can work if there’s not enough headroom or if you have a larger volume of storage.
Engineers should conduct wall friction tests to evaluate how well the hopper walls will move the material through without causing excess friction that can lead to improper flow. The outlet has to be properly sized as well, as to prevent arching. Finally, air flow must be optimized to create a sufficiently fluid state that can nevertheless be controlled so that flooding doesn’t occur.
When correctly engineered, fly ash handling systems can avoid the flow problems that often plague incorrectly designed or constructed equipment.