By Lindsay Morris, Associate Editor
When the ash storage pond at Tennessee Valley Authority’s (TVA’s) Kingston Fossil Plant in Harriman, Tenn. overflowed into the surrounding areas on Dec. 21, 2008, ash handling processes met the inevitability of change. Ash handling systems have experienced gradual changes since the 1970s, but the Kingston event triggered the U.S. Environmental Protection (EPA) to create new regulations for ash handling that will have a significant impact on coal-fired power plants.
The EPA has proposed two options that will determine the regulatory proceedings for coal ash handling. Under the first proposed option, coal ash would be categorized as a special waste regulated by the Resource Conservation and Recovery Act (RCRA) Subtitle C hazardous waste provisions. If adopted, this option would create federally enforceable requirements for ash management, including waste generation, transportation and disposal.
Under the second proposed option, coal ash disposal would be regulated under RCRA’s Subtitle D non-hazardous waste provisions. In either scenario, EPA regulations will be more stringent with the elimination of wet ash handling and the phasing out of surface impoundments (ponds) for all coal-fired power plants. While a decision on which option will be adopted was expected to be made in 2010, proceedings were delayed. 2011 may bring added pressure from the new Congress to bring about a rulemaking.
Wet bottom ash material handling systems and surface impoundments are currently regarded as the industry standard and are the most commonly used method in the coal-fired power industry around the world. However, due to the Kingston event and resulting EPA investigation of all existing facilities using this technology, alternatives that will meet the proposed EPA regulations specifications are being considered. Many power generators are already evaluating converting to dry bottom ash systems, assuming that EPA will rule coal ash non-hazardous. Plants still using ponds to store ash by-product have important choices to make as they look to replace wet systems with dry alternatives.
Thirty years ago, almost all ash from coal-fired power plants was conveyed as slurry and collected in ash ponds and many of these ponds are still in use today. In the late 1970s, dry fly ash collection systems began growing in popularity, especially for new coal-fired installations. Over the past two decades, existing plants have been phasing out pond-based systems for fly ash disposal. In fact, since the 1980s, plants have ceased designing ponds for new projects, said Gary Mooney, sales engineer for Delta Ducon. Instead, most power plants prefer continuous removal systems like a submerged scraper conveyor (SSC), which will be discussed in more detail later.
Today, dry fly ash conveying is more common than dry bottom ash conveying. Many utilities converted wet fly ash to dry systems in conjunction with Powder River Basin fuel switching initiatives. Others converted to dry as local markets emerged, enabling the reuse and sale of fly ash to concrete suppliers. Still others have removed fly ash from capacity constrained ponds as a means to extend pond life for bottom ash storage.
Today, nearly two-thirds of plants with ash ponds contain dry fly ash systems, while over 90 percent of bottom ash systems remain wet. The goal for most operators is to remove the pond, not necessarily eliminate the use of water from the bottom ash conveying process. However, some plants in addition to removing the pond, also want to eliminate water from the conveying process. These conditions frame the choices available today, which range from continued use of water to 100 percent water elimination.
Delta Ducon’s solutions
When the Kingston Pond spill occurred, TVA began looking for an ash handling solution that would prevent future mishaps and enable operations to resume. Delta Ducon, owned by Clyde Bergemann Power Group Inc., bid its Ashcon wet solution that accommodated TVA’s special requirements for the plant. At print time, TVA had not yet awarded a contract to any bidder.
The Ashcon operates continuously, much like a regular SSC. Slurry exiting the incoming sluice pipes enters a contained area that directs the water and waste down toward the base of the upper submerged trough. All of the water exiting the sluice pipes must pass under the submerged underflow baffle of the contained area much like the underflow baffle of a dewatering bin. After the water and fines reach the surface of the conveyor outside the perimeter of the contained area, the movement of the water in the conveyor is nominally directed towards the dewatering incline. The use of serrated weirs, decanting screens and plate separators are all available means of addressing various types of ash.
With the Ashcon, overflow troughs run down both sides of the conveyor to provide as much or more overflow weir length as normally found in a dewatering bin for the same water flow rate. The ash that reaches the base of the submerged upper trough of the conveyor is scraped along an abrasion resistant replaceable liner on the floor by the flight bars, which are spaced four feet apart. At the end of the conveyor’s horizontal section, flight bars turn through the incline angle (nominally 35 degrees) and begin to rise out of the water. As the flight bars rotate around the main drive shaft, they drop the final dewatered product downward and return the rear of the conveyor in the lower dry return section.
At the rear section itself, chain and flight bars turn vertically upwards in a dry area where they can be inspected on-line. They then turn over idlers connected to adjustable shafts for tensioning and return to the submerged section to pick up more ash. The two chains in continuous loops with associated flight bars carry all of the ash though the submerged section and up the incline, discharge the ash, return along the dry bottom, rise through the rear tail section, turn over the tensioning shafts and return the submerged section.
The Ashcon, however, is not Delta Ducon’s main product. The first option Delta Ducon recommends to most companies is the Drycon, a Dry Continuous Conveyor that operates under a boiler.
In Drycon operations, air travels along the surface of the ash which rests on the steel plate conveyor. The air travels in counter flow direction along the ash activating a re-burning process of the glowing ash. This reduces the unburned carbon level and frees up additional thermal energy. Before entering the combustion chamber, air is heated and adds additional thermal energy to the steam generating process within the boiler. The combustion process and exhaust gas composition are not adversely affected (no more than 1 percent of the combustion air is added to the boiler).
Delta Ducon’s Gary Mooney said the advantages of using a Drycon include increased boiler efficiency, elimination of water, profitable bottom ash quality, increased ash capacity, the removal of simple fines, enhanced ash cooling, the handling of large lumps, high resistance to temperature, easy maintenance and low overall height.
United Conveyor’s solutions
United Conveyor specializes in a number of vibration-based systems for eliminating the storage of bottom ash in ponds. A recirculation system converts a wet sluice system into a dry ash system with a minimal outage time. Recirculation systems use separating tanks to clarify ash sluice water usually sent to the ponds with the ash. The first tank, called the dewatering bin, collects and dewaters bottom ash solids to approximately 15 to 18 percent moisture. The clarified water is stored in a surge tank and reused during the conveying cycle. The ash is then unloaded into trucks.
|The Vooner FloGard liquid ring pump scrubs any excess fly ash so that the air coming out of the liquid ring pump is free of solids carryover. Courtesy of Vooner FloGard|
United Conveyor also produces a submerged flight conveyor (SFC). SFC systems replace existing bottom ash equipment under the boiler with a submerged mechanical chain and flight conveyor. Ash falls from the boiler and accumulates in the upper trough, which is filled with water to quench and cool the ash. Horizontal flights move the accumulated ash along the trough and up a dewatering ramp. At the top of the ramp, the ash falls through a discharge chute to a truck or bunker. The bottom ash in the bunker is picked up once or twice a day with a front-end loader and put into trucks.
|The United Conveyor recirculation system converts a wet sluice system into a dry ash system with a minimal outage time. Courtesy of United Conveyor.|
United Conveyor’s continuous dewatering and recirculation (CDR) system is a new product for 2011. The CDR system combines the benefits of a traditional recirculation system with the proven dewatering technology of the SFC. The CDR system is designed to be added to existing wet bottom ash systems, requiring no changes to existing hoppers and minimal outage time. The ash dewatering and removal conveyor is located outside of the boiler area.
Pneumatic conveying of dry bottom ash is United Conveyor’s 100 percent dry solution, and it was the method used in the 1920s through 1940s. This system uses a vacuum design to convey ash in a dry system. No water is needed, resulting in reduced cost and time. Bottom ash is stored dry in a refractory-lined hopper under the boiler. Percolating air cools the ash, helps complete combustion of unburned material and protects ancillary equipment. Large pieces are crushed small enough to feed into a pneumatic conveying line and conveyed to a dry storage silo. The ash is unloaded through a damp ash conditioner, then belt-conveyed or gravity-unloaded to a truck.
Vibrating ash conveying (VAX) is United Conveyor’s dry solution technology, released in 2010. Vibrating conveying provides continuous removal of ash under the boiler, similar to SFC or moving belt systems, except the VAX system has no moving mechanical parts under the boiler. The catch and throw motion of the vibrating deck moves ash from under the boiler to a crusher, where it is fed to a secondary conveyor. Cooling air is forced up from under the vibrating deck, combusting any unburned bottom ash.
An additional implementation some producers may want to incorporate to help adjust to upcoming fly ash handling regulations is a liquid ring vacuum pump. The purpose of a liquid ring vacuum pump is to draw fly ash from the stack to the baghouse and the water used for the seal ring, makes a natural scrubber. Vooner FloGard is a manufacturer of liquid ring vacuum pumps and has worked with United Conveyor and other fly ash conveying OEMs. Vince Visconti, sales and marketing manager, said that a liquid ring pump scrubs any excess fly ash so that the air coming out of the liquid ring pump is free of solids carryover. The pumps have an expected lifetime of 20 to 30 years.
|United Conveyor’s Submerged Flight Conveyor replaces existing bottom ash equipment under the boiler with a submerged mechanical chain and flight conveyor. Courtesy of United Conveyor.|
The downfall of a traditional liquid ring vacuum pump is that water is sent to drain, requiring about 20 gallons a minute or more. The water sent to drain is contaminated with fly ash and acidic. However, Vooner FloGard is releasing a fix in 2011 that recirculates the seal water, allowing over 90 percent of the seal water to be re-used.
Allen-Sherman-Hoff, a division of Diamond Power International Inc., also offers alternatives to ash storage ponds. One bottom ash system conversion is the Hydrobin Dewatering Bin system, which separates and dewaters bottom ash from the conveying supply water. Bottom ash disposal with a dewatering bin involves pumping the bottom ash as a slurry from the ash hopper to an outside the dewatering bin for ash water separation and loading of the dewatered ash into trucks for hauling to the disposal site. The addition of a water recovery system allows recirculation and re-use of conveying water. The dewatering bins may be advantageous to power plants because they have a low initial cost, do not require an outage for conversion, possess a short lead time and eliminate the need for ash storage ponds. The disadvantages of dewatering bins include a high lifecycle cost, the use of large amounts of water and a low environmental risk mitigation.
|The Allen-Sherman-Hoff Dewatering Bin system separates and dewaters bottom ash from the conveying supply water. Courtesy of Diamond Power International Inc.|
Allen-Sherman-Hoff also offers an SCC system. These systems were introduced in Europe 30 years ago and have become popular in the U.S. over the last two decades. The SCC collects ash that is continuously falling from the boiler throat into a water-filled trough. Ash is conveyed along the trough by steel flight bars, and is then elevated up an inclined section to allow the water to drain back into the trough.
The dewatered ash is typically continuously discharged to a three-sided concrete bunker located below the SCC discharge outside the powerhouse building wall. The ash can then be hauled away. Water must be continually added to maintain temperatures below 140 F within the conveyor. Closed-loop cooling and recirculation systems can be used to minimize water consumption.
SCCs are advantageous in that they require lower water usage than traditional hydraulic systems when a water recirculation system is used. Implementation of a SCC also results in a reduction of power consumption, reduced maintenance with fewer components and moderate initial cost.
Allen-Sherman-Hoff’s third alternative to a bottom ash storage pond is the MAC Magaldi ash cooler system. Ash is cooled by a controlled amount of ambient air. The negative draft of the boiler is used to draw in ambient air for cooling, so no fans are required. As the air cools the ash, the heat energy is transferred to the air, which is returned to the boiler as pre-heated air for combustion.
The Allen-Sherman-Hoff MAC Magaldi ash cooler system is one of the company’s alternatives to a bottom ash storage pond. Courtesy of Diamond Power International Inc.
The main component of the MAC system is the MAC extractor, which is designed to operate in harsh conditions including exposure to high temperatures and impact loads caused by the fall of large clinkers. The MAC extractor is connected to the boiler throat through a refractory-lined hopper or a transition chute, which provides a volume for temporary ash storage. The hopper is available with bottom doors which can be closed to isolate the MAC extractor from the boiler and allow downstream intervention, if necessary. The ash hopper can be also used for storage.
From the MAC extractor, the cooled ash is discharged into a crusher, which reduces the large ash clinkers to a size suitable for conveying to a silo. MAC system advantages include elimination of all water in the bottom ash process, as well as associated corrosion problems, increased bottom ash system reliability: no forced boiler outages, lowest lifecycle costs and recovery of most of the heat energy from bottom ash with potential for ash recycling.
In 2010, Allen-Sherman-Hoff added a material handling valve, which is designed to be a solid material flow regulating valve used when the ash is completely dry and the transporting medium is air-operated below atmospheric pressure. The valve can be used in either a dry fly ash system or the MAC dry bottom ash system retrofit, if it includes vacuum pneumatic ash conveying to a silo. It also can be used for direct replacement of some competitor’s valves, often resulting in improved lifecycle performance.
With regulatory changes on the horizon, coal-fired power plant operators are preparing for modifications in ash handling procedures. A number of operating options are available, such as solutions for pond removal and eliminating the water from the conveying process. For many coal-fired plants, an ash handling retrofit promises to become inevitable in the years to come.More Power Engineering Issue Articles
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