Finding a Use for Fly Ash

Coal-fired energy plants previously managed their coal ash by mixing the fly ash with bottom ash before conveying it through a pug mill mixer, where the mixture was blended with a small amount of water to control dust. The mixture was then disposed of at the landfill. By volume, fly ash is the second-largest waste material in the US, behind only household trash. To better manage their fly ash, rather than disposing of the coal by-product as waste, many coal-fired energy plants upcycle it by supplying it to the cement industry.

Today, approximately 43% of the fly ash generated at US coal-fired energy plants is upcycled by the cement industry as a pozzolan. Pozzolans are siliceous and/or aluminous materials which, in a finely divided form and in the presence of water, react with calcium hydroxide at ordinary temperatures to produce cementitious compounds.

This clever use of by-products has many benefits, including:

• Improved workability in plastic concrete.
• Added strength and durability in hardened concrete.
• An additional revenue stream for coal-fired energy plants.
• A cost reduction for cement processors. When fly ash is added to cement blends, the amount of Portland cement used can be reduced. This also allows suppliers to offer cement at lower costs with similar or improved profit margins.
• Reduction in waste at the landfill, therefore improving the environment.

Note: Fly ash upcycling is only economically feasible if coal-fired energy plants and cement processors are located in close proximity to one another, for ease of purchase and transport.

Case study: coal-fired energy plant handling fly ash

Process

A coal-fired energy plant in West Virginia is conveying fly ash through pressure pots and into storage bins via a dense phase system. The plant was using disc valves to shut off flow at the inlet of each pressure pot.

Complications of the disc valves included:

• The sliding disc was subject to wear and abrasion from the material handled.
• Fly ash would pack between the disc and the seals upon valve closure, creating sealing and actuation issues.
• The disc valves required frequent maintenance, which resulted in expensive system shutdown.

For these reasons, conveyance through the pressure pots was an issue.

Solution

After analysing the application parameters, the Vortex Titan Pressure Valve was recommended. The valve was originally designed for conveying cement in dense phase pneumatic systems. It is also a perfect fit for this application; the reasons being:

• Abrasion resistance – The blade and seats are constructed from 440 C stainless steel (Rockwell hardness 60 C).
• Temperature resistance – In this application, fly ash is being handled at temperatures of 400 – 450°F (205 – 230°C). The Vortex Titan Pressure Valve is rated to handle material temperatures up to 660°F (350°C).
• High pressure conveying – In this application, fly ash is being conveyed at 25 – 35 psi (1.7 – 2.4 bar). The Vortex Titan Pressure Valve is rated to handle pressures up to 100 psi (6.8 bar).
• Gate closure/sealing design – Spring-loaded blade guides keep the blade in constant contact with the seat throughout the stroke of the blade. Upon gate closure, the leading edge of the blade lifts into an end seat to ensure a positive seal of pressure and material across the gate. The end seat features a displacement pocket to minimise material jamming and packing upon gate closure. Keeping the end seat clear of materials ensures positive gate closure.
• Air purge assembly – As standard, the Vortex Titan Pressure Valve is equipped with an air purge assembly. The air purge pressurises the bonnet area to minimise fine material build-up in the valve body. Because fly ash is an especially fine material, this feature is critical to preventing actuation issues.

Results

This coal-fired energy plant sourced one Vortex Titan Pressure Valve to test its performance in the unique application. That initial valve performed very well and over the past three years, the plant has continued to source Titan Pressure Valves as replacements to their disc valves. Over time, the plant will replace the balance of their disc valves with Vortex Titan Pressure Valves.

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