What is Fly ash?
Fly ash is the residue left from burned pulverized coal. Since there isn’t complete combustion, there’s always something left after fuel burns. This happens during the coal burning process: some mineral impurities in the coal (clay, feldspar, quartz, and shale) fuse in suspension and fly out of the combustion chamber as they’re carried in the air by the hot exhaust gases generated.
The ashes ascend in the hot air stream and this fused material rises. While it gets further away from the burning center, it cools down and solidifies into particles with a spherical shape and a glassy look. It’s normally used to generate energy at power plants and industrial facilities. Fly ash is produced at a rate of approximately 140 million tons/year. If it’s not properly used or disposed, then it can become a serious environmental problem – hence, the importance to handle and transport fly ash appropriately.
Fly ash is usually collected from the exhaust gases with electrostatic precipitators or bag filters. After it’s collected, it looks like a very fine powder that resembles Portland cement but is chemically different.
All fly ashes have some cementitious properties that vary in type and degrees, depending on the chemical composition result from the burnt coal and/or the physical properties of both the fly ash (including burning temperature) and cement. The fly ash reacts with calcium hydroxide released by a chemical reaction when cement and water are mixed to create many desirable properties in the resulting concrete.
The chemical reaction between fly ash and calcium hydroxide typically is slower than the one between cement and water, which results in a delay for the concrete to harden. Sometimes some properties change in desirable or undesirable ways but, when done in a controlled environment, the result is always positive.
The characteristics change from hard to porous, water-absorbent, etc. All controllable changes prove that more than 50% of the concrete in the U.S. contains fly ash. Amounts vary from case to case, but they are normally between 15 to 25% mass of the total cement material reaching up to 40% for some specific types and uses.
The two most common fly ash types are:
- Class C: from lignite and sub-bituminous
- Class F: from bituminous and anthracite
Transportation methods for fly ash and difficulties
The descriptions and concepts above show that transporting fly ash is similar to transporting cement, but has particularities to consider when defining transport methods (i.e.: the hygroscopic behavior and storage system).
After it’s captured in an electrostatic precipitator or in filter bags, the ashes fall into multiple hoppers placed at the bottom of the equipment. Through gravity, the ashes are transferred into a silo storage system that usually has the capacity to hold a truckload in each silo. The load is then transferred to one truck at a time. From silo to silo, or even to load trucks, it is common to have pneumatic conveyor systems with vacuum / pressure combined.
As the particle is too fine and somehow hygroscopic, the flowability of the fly ash is bad. It clogs the silo outlets by bridging (also known as arching) or ratholing, it makes the flow of material more difficult, and limits the flow rate. This hard-to-flow characteristic is also experienced during loading and unloading the fly ashes from and to tank trucks or containers.
Proposed solutions to handle Fly Ash
Some possible solutions to improve the flow rate and avoid blockages in the silos’ outlets, tank trucks or containers when unloading hard-to-flow bulk solids are available.
- Vibrators attached to the hopper-shaped outlet region on silos or tank trucks, or to the bottom of the container is a very popular solution, but it has limitations regarding how and where to fix it.
- Another method is to have an airflow opposite to the bulk solids’ flow so to avoid the blockages. This backflow of air can be in pulses with steady application using a fluidizing bed on the sloped region of the hopers, silos, tank trucks or in front of the outlet, or a fluidizing device inside the liners that’s installed inside the container to transport fly ash into other containers.
For containers, the fluidizing liner is still more important than for silos or tank-trucks after the unloading is completed from a tilted container. For hard-to-flow product like transport fly ash, there’s another possible phenomenon with very bad results: flooding.
Flooding occurs when the bottom plane where a bulk solid is standing still must be tilted far beyond the repose angle just to start the flow. If this is the case, after the flow starts, the flow of material is almost uncontrollable once as it discharges all at once towards the bulkhead, bending the bars or even breaking them with the possibility of creating a disaster.
The BULK-FLOW solutions
Bulk-Flow developed two solutions that are suitable for the problem presented above: how to move the fly ashes from where they were generated to where they’re used.
The first solution is the Fluidizing Liner System
A fluidizing liner has a device on the bottom that, when fed with compressed air, creates a flowing bed of material, improving the movement characteristics of the bulk solids in it. When a fluidizing liner is used, the hard-to-flow bulk solid behaves like a fluid and none of the above-mentioned blockage problems happen. The tilted angle to unload is lower and there is no risk of flooding. The flow rate is also dramatically improved.
A second solution is the Tilt-less Liner System
The Tilt-Less system is the newest technology developed by Bulk-Flow to overcome such difficulties with hard-to-flow products. It consists of a custom liner that allows the bulk solids to flow out using unloading equipment but without tilting the container. As a result, flooding or blockage problems aren’t possible.
Another advantage of the Tilt-Less system is the possibility of having the container on the field (i.e.: at construction sites, road work, etc.), where there is no unloading equipment, transfer equipment, any sort of transfer belts, buckets or pneumatic conveying systems available most of the time.
Even more so, the Tilt-Less liner container can also be used as a storage unit when the whole load doesn’t need be unloading all at once.