Coal

Optimizing CCR Waste Management through a Eurosilo Application

As utilities consider ways to store and transport coal combustion residue (CCR) materials, a new approach to consider is the Two-Eurosilo concept, which can significantly reduce operating costs and simplify the movement of CCR to a landfill.

By Richard Munson

For plants that need to move bottom ash, fly ash and gypsum to a landfill, the two-silo concept offers the flexibility to split the storage of the material to the plant and landfill and then move it rapidly to the landfill with no restriction on rate. Photo courtesy: BEUMER

As utilities consider ways to store and transport coal combustion residue (CCR) materials, a new approach to consider is the Two-Eurosilo concept, which can significantly reduce operating costs and simplify the movement of CCR to a landfill. The new EPA CCR rule focuses more on the integrity of impoundments, but how this material is moved and stored is important too. More operators are confronting the question of how to store and move their CCR waste because the volumes are enormous, and over time, small efficiencies add up to big savings.

The challenges of moving and storing CCR

No creative thinking has been applied to new CCR transport designs because moving this material is complicated. Handling three or four different types of CCR material requires complex storage and handling equipment designed to hold, and then transport, large amounts of each CCR constituent material. This drives the construction of more conservative and costly designs, while creating high capital costs to build and high operating costs to operate and maintain these systems. Also, CCR is not an ideal material to handle so engineers concentrate on traditional methods to do so. New concepts are considered speculative. The traditional way to store these materials includes, for example a concrete silo for fly ash, covered storage for FGD Gypsum, and a steel silo or concrete pad for bottom ash.

Innovative concepts for efficient CCR handling

When considering the large volumes of waste – the cost per ton to move this waste should be scrutinized. An innovative approach is to minimize the footprint and complexity at the power plant end and create surge storage at the landfill end, which provides flexibility in the plant operation. This is a departure from traditional methods because the CCR material does not lend itself to storage in a traditional mass flow silo. This concept offers flexibility for handling the CCR material which is critical. Being able to “push” the material quickly out to the landfill to a receiving vessel reduces pressure on the plant storage vessels. Having an efficient truck loading capability at the landfill contributes to the overall efficiency.

This concept uses a pipe conveyor to connect the power plant storage to the landfill storage.

The two Eurosilo approach

A Eurosilo, specifically designed to store highly cohesive material, is used at the plant to store the gypsum as well as the gypsum dewatering equipment. This creates a very small footprint.

A similar Eurosilo is also used at the landfill to store the comingled CCR waste. Trucks are loaded under this landfill silo with high loading efficiency. The pipe conveyor will carry the comingled CCR at a high tonnage rate from the plant to the landfill silo. This can be achieved at the maximum conveyor capacity.

A pipe conveyor is a great solution for transporting CCR to a dry landfill. The pipe conveyor can negotiate vertical and horizontal curves and it encloses the material in the direction of transport. On the return strand, the belt is closed with the dirty side facing in so no spillage occurs. This ensures compliance with EPA regulations.

Having a Eurosilo at the landfill allows the plant to quickly expel the daily CCR production to the landfill at a high rate per hour in a matter of a few hours. This relieves pressure on the plant operations and creates flexibility by allowing the plant operation to disengage from the landfill operation.

The plant portion of this concept envisions a feeder hopper to receive the bottom ash adjacent to the gypsum Eurosilo and then two fly ash silos to store the dry fly ash. The fly ash is conditioned under each fly ash silo while depositing it onto the pipe conveyor. All three pieces of equipment simultaneously reclaim their respective material directly onto the pipe conveyor.

Being able to reclaim in close proximity to the pipe conveyor and at a high rate allows the conveyor to operate at peak capacity saving significant power cost per ton because the large motors are operating at peak efficiency. The pipe conveyor can negotiate sharp curves and hills on the way to the landfill which eliminates transfers and additional conveyors.

Benefits of two-silo concept

The two silo concept impacts all aspects of the operation. Taking all subtle improvements into consideration helps illustrate how the landfill Eurosilo drives efficiencies.

Each constituent of the CCR waste stream has vastly different handling characteristics. The fly ash is dry and dusty and needs to be conditioned before transporting. Conditioning the fly ash is critical and has to be carefully done. The gypsum needs to be dewatered to between 10% and 15% moisture so that it is dry enough to transport but it can become sticky and can set up to some extent under pressure while being stored. Mixing these two constituents creates a better blended material for transport than either one on its own.

The benefits of this approach include capital cost savings; operating cost savings; easier operating methods; and environmental responsibility.

Capital cost savings can be achieved in the following ways:

  • Consolidating the gypsum handling equipment footprint at the plant can result in an estimated savings of $8,000,000 to $10,000,000 depending on storage sizes needed.
  • Much faster schedule to construct, the slip forming of all silos can be done one after another.
  • A version of the concept uses no CCR silo at the landfill and in this case the capital savings are even higher (e.g., more like $15,000,000) but with this case the operating costs will be higher.

Operating cost savings can be achieved in the following ways:

  • If the gypsum Eurosilo has the ability to consolidate the gypsum dewatering equipment on top of the Eurosilo at the plant, a substantial amount of equipment will be eliminated – such as the gypsum storage and dewatering buildings, transfer towers, additional conveyors, portal reclaimer, and even truck-loading bays,
  • The gypsum dewatering and silo operation can be automated,
  • Being able to move all CCR to the landfill at the maximum capacity of the pipe conveyor allows the conveyor to operate fewer hours per day and to operate at peak power efficiency while in operation. This saves the hours of operating the belt and allows the motors to run at the highest efficiency on their efficiency curves. This also pushes the belt replacement years further into the future and reduces idler replacements.
  • The truck loading operation under the landfill silo is more efficient than traditional loading designs because two trucks can be loaded simultaneously with efficient queuing. The truck loading operation at the landfill can be done by the truck operators themselves if that is desired.

The two-silo approach allows for easier operating methods. Examples include:

  • With this concept the bottom ash, gypsum and fly ash can be reclaimed simultaneously onto the pipe conveyor for transport to the landfill. As a result, there will be much less power required and fewer transfers required to get the material onto the pipe conveyor.
  • Once at the landfill there would be a single conveyor discharge into the silo. Alternatively, the CCR can be conveyed to an emergency pile.
  • Once the truck loading operation starts at the landfill the material will be loaded into trucks by a series of double augers that eliminates the need for any conventional conveyor transfers thus reducing the need for bin vibrators and air cannons that can be disturbing to local landowners.
  • The material is transferred to the landfill silo in a matter of hours at close to the capacity of the pipe conveyor. This is more efficient because fewer people have to be involved or stand by waiting for the daily CCR production to transfer.
  • It is easier to design redundancy into a silo than to other types of storage. For example, in the plant gypsum silo design from Eurosilo there is an emergency bypass of the silo in case the silo internals are being maintained and there is an optional truck loading spot under the silo in case the pipe conveyor is down for maintenance.

The following examples illustrate how the two-silo approach is more sustainable and environmentally friendly than alternative options:

  • The gypsum is directly put onto the pipe conveyor using far fewer conveyors and motors.
  • Fewer foundations are needed to construct the facilities at the plant.
  • Construction time is significantly reduced.
  • Less power is required per ton of gypsum moved.
  • The pipe conveyor motors are run at closer to peak efficiency reducing wasted power.
  • The loading operation at the landfill can be done underground or partially underground where the operation is sheltered from the elements and where it is quieter.
  • Comingling the CCR materials provides a better handling and more consistent sand like material that is more consistently packed into place in the landfill.
  • The fly ash does not need to be conditioned as much and the gypsum does not need to be dewatered as much since they will be comingled in the CCR silo at the landfill. While more testing may be needed to verify the limits of this, initial tests show that the blended material is like sand and behaves predictably with less sticking.

Conclusion

For plants that need to move bottom ash, fly ash and gypsum to a landfill this concept offers the flexibility to split the storage of the material to the plant and landfill and then move it rapidly to the landfill with no restriction on rate. This will save a significant amount of operating time just to transfer the material to the landfill. Once stored in the landfill silo the truck loading operation is faster.

For large volumes of CCR to be moved over a 20 or 30-year period, this truck loading efficiency gain adds up to sizable dollar savings. The two-silo concept offers clear capital and operating cost benefits at the plant end that should be carefully considered. When the operating costs are modeled and projected over a long term project life the savings are significant for plants generating significant volumes of CCR waste. The environmental benefits are also obvious.

 

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