After the 2008 incident involving the failure of a large surface impoundment containing wet coal ash, the EPA began investigating all coal-fired power plants employing this wet coal ash management method. Now a new dry ash management technology offers coal-fired power plants an environmentally suitable alternative for handling coal ash that also increases energy efficiency.
Existing coal ash management regulations are undergoing close scrutiny by the U.S. Environmental Protection Agency (EPA). This action was initiated in the wake of the 2008 incident in which a containment dike at a Tennessee Valley Authority coal ash disposal pond failed, spilling approximately a billion gallons of wet coal ash over about 300 acres and into a nearby river. Consequently, many coal-fired electric power producers are now investigating alternative methods of handling coal ash that do not require using water.
Bottom ash is agglomerated ash particles formed in pulverized coal furnaces that are too large to be carried in the flue gases and that impinge on the furnace walls or fall through open grates to an ash hopper at the bottom of the furnace. Physically, bottom ash is typically grey to black in color, is quite angular, and has a porous surface structure.
Typically, bottom ash as a byproduct at coal-fired power plants is conveyed in a hydraulic system in which the ash is entrained in a high-flow, circulating water system and delivered to either an ash pond or dewatering storage bins. In the alternative, bottom ash can be conveyed by a mechanical drag system to dewatering storage bins.
Now a new bottom ash management technology has been developed that does not require the use of water and thereby avoids the creation of wet ash that has to be stored in surface impoundments or dewatering storage bins. DRYCON is a mechanical conveying system that handles hot bottom ash being discharged directly below the boiler throat. Ron Grabowski, vice president of sales at Clyde Bergemann Delta Ducon, discussed the technology with POWER in May.
How the Dry Ash Conversion Technology Works
Hot bottom ash falls onto the DRYCON conveyor, where it is simultaneously conveyed and cooled, Grabowski explained. The DRYCON technology uses the negative draft of a pulverized coal-fired boiler to draw ambient air through the conveyor. This airflow cools the hot ash while supplying about 1% of the combustion air into the boiler. The dry bottom ash can then be stored in a silo and off-loaded into trucks for disposal or resale (Figure 1).
|1. High and dry. The new dry bottom ash–handling system promotes higher efficiency by returning heat energy to the boiler. In addition, with this new system in place, plants can also avoid using large amounts of water for ash cooling and conveying. Courtesy: Clyde Bergemann Delta Ducon|
“DRYCON is the next generation of dry bottom ash systems,” he said. “One of the unique design features of DRYCON is that it does not use belt technology; rather, it conveys the bottom ash via a series of high-temperature alloy pans.”
With the new system, ash is conveyed on top of the pans as they move gently via externally mounted rollers. This design allows the new system’s conveyor to incline at a 40-degree angle from grade. This feature can reduce the footprint of the conveyor and provide additional flexibility not found in other conveyors, according to Grabowski.
Another innovation is the use of “jaw crushers” above the inlet of the DRYCON conveyor, he pointed out. The jaw crushers crush large clinkers before they enter the conveyor (Figure 2).
|2. Breaking up is easy to do. “Jaw crushers” are shown above the inlet of the DRYCON conveyor. This equipment breaks up and crushes large clinkers before they enter the new system’s conveyor. Courtesy: Clyde Bergemann Delta Ducon|
Grabowski acknowledged that “there can be many obstacles when converting a wet bottom ash system to a dry system.” By using standard components, however, a dry ash conversion system can be customized to meet the capacity, distance, particle size, and loss on ignition (LOI) requirements of a plant.
“With any wet-to-dry bottom ash conversion, the main challenge is finding a direct path for the conveyor to exit the bottom of the boiler without interfering with existing structural steel or surrounding equipment,” he said.
Because the wet-to-dry conversion requires demolition of the existing wet bottom ash equipment directly under the boiler, a boiler outage must occur. However, proper planning and preassembly of equipment can reduce the duration of the outage to less than 25 days.
Advantages of the Dry Conversion System
Advantages of using dry bottom ash systems at coal-fired facilities instead of the traditional wet ash-handling systems include:
- Economical performance: The elimination of water pumps, water treatment, and related equipment reduces maintenance and creates an overall cost savings benefit.
- Efficiency: By removing the existing water-impounded bottom ash hopper, the plant will see a reduction of thermal energy losses that come from water evaporation and reductions in unburned carbon or LOI.
- Revenue: A dry bottom ash system can increase the plant’s revenue through the benefits of boiler efficiency and the sale of dry bottom ash.
- Environmental benefits: With a dry bottom ash system, the need for an ash pond, water slurry pumps, and water treatment is completely eliminated. This eliminates many environmental issues associated with these component parts (Figure 3).
|3. Beating the competition. The dry ash system offers many advantages and cost savings over the traditional wet ash-handling system. Courtesy: Clyde Bergemann Delta Ducon|
As for the impact on a coal-fired plant’s water usage, “When a plant converts to a dry bottom ash system, it can eliminate the ash slurry pumps that use large amounts of water that can range from 2,800 gallons per minute (gpm) to 4,000 gpm during operation,” he said.
With typical water-impounded bottom ash hoppers, water evaporation plays a part in reducing boiler efficiency. When hot bottom ash falls into a wet bottom ash hopper, the ash is immediately quenched. In contrast, with a dry ash conversion system, the bottom ash continues to burn as it falls onto the conveyor pans. This reduces LOI in the bottom ash and thereby increases the boiler’s overall efficiency (Figure 4).
|4. Burn, baby, burn. With a dry ash-handling system, bottom ash continues to burn as it falls onto the conveyor pans. This reduces loss on ignition in the bottom ash and thereby increases the boiler’s overall efficiency. Courtesy: Clyde Bergemann Delta Ducon|
There are no notable disadvantages of using a dry bottom ash system, according to Grabowski. He also pointed out that the life span of a dry bottom ash system meets the typical 30-plus-year requirements of the power industry.
Recently, Clyde Bergemann Power Group Americas was awarded a significant contract to convert an existing wet bottom ash removal system on two 650-MW coal-fired units for a utility plant in Florida to a dry system utilizing its DRYCON technology. “This award marks the first utilization of the DRYCON technology in North America and is a milestone for the Clyde Bergemann Power Group,” Grabowski said. “It is also the first conversion of a wet to a completely dry bottom ash system in almost 20 years in North America.”
From Ash to Cash: Turning Bottom Ash into Marketable Products
Plant personnel can dispose of dry bottom ash much as they have handled dry fly ash for many years, Grabowski explained. Several grades of ash are outlined in ASTM standards. Ash with low amounts of carbon or LOI is the most desirable. It can be used in building products such as concrete and cinder blocks. “A DRYCON system has the ability to reduce unburned carbon and provide an ash quality that is marketable,” he said.
According to the EPA, bottom ash applications can include:
- Filler material for structural applications and embankments
- Aggregate in road bases, sub-bases, and pavement
- Feedstock in the production of cement
- Aggregate in lightweight concrete products
- Snow and ice traction control material
Grabowski discussed important trends that he thinks will promote the increased use of dry bottom ash systems at coal-fired power plants.
“As we all wait for regulations to be finalized, the general opinion of the industry is that ash pond storage will be eventually be eliminated by direct or indirect regulations,” he said.
He noted another important trend that is seldom mentioned: the possible need for existing plants to consider replacement of their wet ash systems, even if they do not have ash ponds. “Because a large percentage of wet bottom ash systems are near the end of their projected life span, plants are faced with the decision of rebuilding or replacing them,” he said. “A dry bottom ash system may be the best economical solution to this situation.”
— Angela Neville, JD, is POWER’s senior editor.