How Refractories Can Help Power Plants Be Environmentally Sound

Due to environmental concerns and economic factors, power plants that use slurry containment systems are transitioning from traditional ash handling systems to more environmentally sound alternatives.

This movement has created an opportunity for refractory solution providers to help with the transition, demolition, and installation of the updated ash handling system.

Power Plants Take Notice

About 10 years ago, power plants, specifically power plants that use coal, took notice of the risk of containment failure and started making ash system changes.

In 2008, a containment failure in the ash pond of a power plant in the Southeast U.S. released 1.1 billion gallons of coal fly ash slurry into 300 acres of the land surrounding the pond. The slurry ended up flowing into several local rivers.

Cleanup from this devastating failure cost approximately $1.1 billion and eventually the lives of more than thirty people who participated in the cleanup effort.

This incident, as well as other economic factors, have resulted in the transition to better ash handling systems. These better systems eliminate the need for ash slurry ponds.

The transition to better systems has created an opportunity for experienced refractory solution providers.

Specifically, each new slurry system is fed by multiple boilers that need to be adjusted, ash hoppers need to be relined with new refractories, and the demolition and installation of key refractory system components must accommodate the upgraded system.

Wet Ash Hopper System

One of the two systems that have been designed to handle ash without a slurry pond is the wet ash hopper system. This type of system has been used for decades and offers more environmentally sound practices than slurry ponds.

The wet system dries out the ash as it moves to the ash containment building or a dump truck for removal.

This process involves four steps:

1. The ash falls through a transition chute into a water-filled ash hopper from the superheater and economizer tubes.
2. When the ash hits the water, it is shattered from thermal shock.
3. The ash then settles to the bottom of the ash hopper and into its submerged scraper.
4. With the scraper, the ash is transported by conveyor up an incline, where dewatering takes place. Final water content of 15 to 20% is the norm.

There are several advantages to this system:

• Less water is required
• Less energy is required to move the ash
• Water treatment is reduced
• The dewatered ash can be transported more easily and quickly.

Another advantage of the wet ash hopper system is the retractable ash hopper, which can be detached and rolled away from the boiler on a rail system. This allows easy access to the interior of the boiler (economizer and superheater tubes), from the floor. It also creates a much sturdier base for repairs and better preserves the life of the refractory.

Dry Ash Hopper System

An alternative system is the dry ash hopper system. Like the wet system, the dry system is lined with refractory and is attached to the bottom of the boiler with an airtight seal at the top of the hopper.

There are three steps:

1. The fly ash drops from the boiler into the bottom of the dry hopper.
2. The ash is then cooled by ambient air and the negative pressure created by the system.
3. The negative air pressure also activates a reburn process in the ash and leads to total combustion of the ash.

As a bonus, the ash reburn process also creates greater thermal efficiency within the boiler.

Advantages of the dry ash hopper system over the current slurry pond system include these:

• The lack of water required for processing the ash
• The absence of chemical treatments for the ash-infused water
• The reduction of energy that is required to move the ash slurry.

Dry ash hoppers, like those in wet ash systems, can optionally include a retractable bottom to expose the boiler. This has advantages similar to those for the wet hopper system: easy access to the interior of the boiler, a sturdier base for repairs, and preservation of refractory life.

Another appealing advantage is the monetization of fly ash. Changes in the cement and concrete industry have now created a market for low-sulfur coal, which is found in fly ash.

Refractory Experience Rules the Day

Power plants rely on experienced refractory solutions providers for the success of both types of ash hopper systems. Conversion to either system requires downtime and is considered a capital project costing $8 to $12 million, depending on size and control modifications.

Only a professional refractory company with access to the correct specialty equipment, pump operators, and nozzlemen, coupled with strong and abrasion-resistant refractory material, will be able to handle the scope of the job and time pressures.

Regardless of the hopper system chosen, five distinct services must be completed:

1. Demolition

The demolition of the existing refractory can be performed via cut or flame/torch.

• Cut: A hole is cut into the side of the system, large enough to accommodate an automatic tear-out machine. Depending on hopper style, this may be difficult, especially if the ash hopper is of a cricket design. This process works best on straight vertical side halls.
• Flame/Torch: Using a torch, vertical windows are cut on the outside of the steel hopper for access to the inside. The refractory is then broken into large chunks and removed. The refractory can be broken from the inside to remove the refractory with anchors attached to the steel casing. Or an automated tear-out machine could be used to break the refractory from the outside after the steel casing has been removed.