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An aerial view of the Ghent CCR facility shows the scale of the project. |
By Richard Munson
Kentucky Utilities’ Ghent power station recently commissioned a coal combustion residue (CCR) waste conveying system to move gypsum, conditioned fly ash and bottom ash to a new landfill. The project scope included new dewatering and storage plant conveyors to move the CCR from production locations to a pipe conveyor. A 400-mm pipe conveyor transports the CCR waste from the plant to the landfill load-out tower. The tower is used to load trucks that move the CCR waste to the desired location within an expansive landfill.
CCRs are generated from burning coal. The percentages of the total waste mix vary based on coal grade, however, fly ash is usually the largest component by volume.
Many projects are contemplating the design of dry CCR handling systems to comply with environmental regulations and/or to relieve pressure on existing ash ponds. For each of these projects, one critical aspect of the feasibility study is to determine how to store the CCR at the point of production and how to get it to the landfill. One problem is the high capital and operation cost of intermediate storage systems to handle the different kinds of CCRs due to their difficult handling characteristics.
Each CCR waste component has unique handling challenges. Fly ash needs to be conditioned with water to be transportable. Otherwise, it is too dusty. Gypsum needs to be dewatered from its slurry state of production. Bottom ash also needs to be dewatered, as it is usually pumped to the dewatering location using jet pumps.
The design of a CCR handling system is complex, requiring significant capital outlay to house the CCR during production and to move it to the transport mechanism no matter how it is transported.
Transport Options
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Trucking
Troughed Conveyor
Pipe Conveyor
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There are generally three different transport options for long distances to new landfills: trucking, troughed conveyors and pipe conveyors.
Trucking is the most obvious choice for any power station. Trucking offers flexibility in place of low cost. Many economic evaluations have been performed for trucking versus conveying. Trucking costs range from $2.25/ton to more than $4/ton. If the volume exceeds 1 million tons per year (tpy) of CCR waste then it becomes impractical to move the CCR by truck. Another disadvantage is simply the room available for the number of trucks required on the roads for higher volumes.
A troughed conveyor can move large capacities of material at low power consumption, but many plants are reluctant to use this type of conveyor because it does not totally enclose the material. If the route is relatively simple without sharp curves, this option may be viable.
The pipe conveyor can negotiate shorter vertical and horizontal curves, and it encloses the material in both directions of travel. The dirty side of the belt always faces inward. So it is an ideal choice for most long routes to new landfills.
The two graphs on the next page summarize the two critical questions for any CCR disposal system. The key variable is the volume that needs to be transported to the landfill. Volumes range tremendously from several hundred-thousand tpy to more than 3 million tpy. For higher volume plants where the landfill is some distance from the plant, conveying makes more sense for a variety of reasons.
The power plant operator has to determine its expected capital cost of conveying. The main variables are distance and topography. Most landfill sites are in the range of one to three miles from the plants.
For numerous projects, the economic incentive to install conveying is very high if sufficient volume exists. There is no hard and fast rule because there are too many variables to consider, such as trucking distance and infrastructure costs (civil costs) and operating and maintenance costs the plant wishes to apportion to the conveying system. But the power cost to convey 1 ton of material by conveyor is very low, perhaps $0.10.
For numerous projects, the economic incentive to install conveying is very high if sufficient volume exists. There is no hard and fast rule because there are too many variables to consider, such as trucking distance and infrastructure costs (civil costs) and operating and maintenance costs the plant wishes to apportion to the conveying system. But the power cost to convey 1 ton of material by conveyor is very low, perhaps $0.10.
The cost of CCR storage is expensive because each product requires its own storage and handling method. Two products have to be dewatered and the third has to be conditioned before transport. In general it can be said that the CCR material is easy to transport but hard to store and reclaim.
The Ghent Project
The Ghent power station houses systems for gypsum dewatering and storage, fly ash storage and conditioning, and bottom ash collection and dewatering areas. After each waste product is readied for transport, the material is put on conventional conveyors to move it to the tail end of the overland pipe conveyor for comingled transport to the landfill.
The footprint required for this type of system is quite large because the gypsum must be dewatered in a dewatering building and then transported to a separate storage building housing an overhead stacking system and a portal reclaimer. The fly ash is stored in two concrete silos with conditioners located under each silo. The footprint of these silos is relatively small compared to the gypsum handling facilities. The bottom ash design has a separate building housing the submerged flight conveyors, which feed another enclosed area where the bottom ash is placed on concrete pads for further dewatering after coming off the SFCs. The fly ash and bottom ash are moved to the loading end of the pipe conveyor on the same conveyors. The gypsum is moved to the loading end of the pipe conveyor on a separate reclaim conveyor. The system also has an emergency truck load out building and another truck unloading feeder to move trucked material onto the pipe conveyor.
The pipe conveyor was selected as the method to move the CCR to the landfill for this project because of the lower capital and long-term operating cost. The actual route has steep inclines and many curves. Any future project will want to achieve a low cost per ton to transport the waste and build sufficient backup or storage volume to assure reliable operation and a system with enough capacity to assure that all waste can be moved during daylight hours.
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A pipe conveyor moves comingled CCRs to the landfill. |
Gypsum is dewatered and stored in the same silo. |
Design Overview
The Ghent CCR system had to be located across a highway from the main power station due to the footprint required for the facilities. The system is designed to move the comingled CCR to the landfill. Each waste stream can be moved independently or in comingled mode. The pipe conveyor moves the waste to the discharge tower located on the edge of the landfill. The pipe conveyor is approximately 7,800 ft long and the final elevation of the discharge is approximately 400 ft higher in elevation than the loading zone. The grade is undulating and some areas are very steep.
The plant is designed with plenty of contingency storage room for each material. The plant can simultaneously load gypsum, fly ash and bottom ash to the conveyors that feed the pipe conveyor. The capacity of the system is 1,200 tph. This peak capacity will be more likely during peak production periods when the most fly ash and gypsum is being produced.
Two feeding conveyors come into the loading tower for the pipe conveyor, one from the fly ash storage area and one from the gypsum storage building. A third conveyor leads from this tower to an emergency truck loading tower in case the conveyor is down.
The Eurosilo concept can be used to dewater and store gypsum inside the same silo, which dramatically reduces the site footprint required for the storage of this CCR. This has been done in Europe for many years where the area available at the sites is generally limited. When doing this, the gypsum slurry is pumped to the top of the Eurosilo where dewatering takes place.
Alternately, as with the layout at Ghent, the dewatered gypsum must be conveyed to the gypsum storage building where it is stacked with a tripper conveyor in the building and then reclaimed with a portal reclaimer and conveyed on a separate conveyor to the tail end of the pipe conveyor. This requires a fair amount of site civil and mechanical work.
In some cases, operators put a water drainage system below the gypsum storage level in the silo to pull a vacuum below the gypsum to pull excess moisture out of the gypsum before transport. This is easily done with the Eurosilo.
The load out area of the Eurosilo can be designed to load trucks or a conveyor. It can be designed to load the conveyor during conventional operation but load trucks when the conveyor is down. Using the Eurosilo to store and dewater the gypsum is a concept that should be investigated in the early stages of the design procedure to reap capital cost savings.
Richard Munson is product manager for Beumer Kansas City.
