By Richard H. McCartney and Robert L. Williams, Jr., Roberts & Schaefer
A major design factor for the coal handling facilities at both new and existing plants has been coal blending. Many new power plants are considering the ability to blend coals as an important part of the basic coal handling facility to allow for flexibility. The continuing modification or upgrade of many existing plants has been based on the increased use of Powder River Basin (PRB) coal.
Red Hills generating facility coal storage silos
Many methods exist to accomplish coal blending at a new or existing power plant. These range from a basic use of the secondary (emergency) stockout/reclaim system to totally automated coal handling facilities with segregated areas for two or more coals. Each power plant’s coal handling facility is based on a unique set of conditions for that plant with considerations for future coal source changes.
Coal Blending Requirements
The primary coal handling systems affected are the stockout and reclaim systems. Transportation and unloading systems are also affected based on coal sourcing. The rising cost of Eastern and Midwestern coals and increasing environmental regulations have caused existing power plants to consider switching to PRB coal. Many of these existing power plants are not able to switch from all high sulfur Eastern coal to PRB coal without excessive modification costs or derating the unit(s) due to the lower Btu/lb value of the PRB coal or other boiler issues. These power plants are electing to blend with PRB at the maximum reasonable percentages.
Other existing power plants are able to switch to 100 percent PRB coal but are selecting an option for future blending. For new power plants this blending option is a major design factor. For blending with two or more coals, the active coal pile size should be compatible with the burn rate of each coal type but flexible for future changes in blend percentages. This will keep the coal turn over rate in the active coal storage piles approximately equal. This is most important for PRB coal, which is subject to spontaneous combustion. The active coal storage pile size should also have the capacity to stockout one complete coal shipment without dozing. This is required so that the coal delivery schedule is not affected by the stockout system.
Many coal stockout systems will accommodate stockout and storage of a second coal type for blending and still others can be modified to accommodate the second coal type. The stockout systems that will initially accommodate blending are based on the ability to stockout two or more coals in segregated piles. These stockout systems are as follows.
- Two stacking tubes (lowering wells) or fixed stockout conveyors normally produce two equal size storage piles, (based on pile height) for a near equal blending ratio.
- One radial stacker can produce two storage piles of any size for any blending ratio.
- One traveling stacker, which can produce two or more storage piles of any size for any blending ratio.
- Overhead traveling tripper, which can produce two or more storage piles of any size for any blending ratio.
- Two stacker/reclaimers, which normally produce two equal size storage piles (based on equal travel length) for a near equal blending ratio.
- Two or more coal yard silos normally produce two equal-sized storage capacities for a near equal blending ratio or additional silos can produce varying blending ratios.
- One barn with a traveling stacker can produce two or more storage piles of any size for any blending ratio.
- Two barns or domes, each with a stacker normally produce two equal size storage piles (based on size) for a near equal blending ratio.
The stockout systems that can be modified to accommodate the second coal type in a segregated pile for blending are as follows. These systems have the same stockout pile capacities and blending ratios as described above.
- One stacking tube or fixed stockout conveyor: add a second stacking tube or fixed stockout conveyor. This can be a separate system from a transfer point location or a modification to the discharge of the existing system to feed the second system.
- One stacker/reclaimer: use existing stacker/reclaimer for a minimum/maximum blending ratio by stockpiling two unequal size piles and use a secondary reclaim method for the minimum pile. Add a second stacker/reclaimer for a near equal blending ratio.
- One coal yard silo: add a second silo for a near equal blending ratio or additional silos for varying blending ratios.
As with the stockout systems, many coal reclaim systems will accommodate reclaim of a second coal type for blending. Other systems can be modified to accommodate the second coal type. The reclaim systems that will initially accommodate blending are based on the ability to reclaim two or more coals at specified rates and at the same time. The arrangement of the stockout piles and the type of reclaim equipment used will dictate the percentage of the active stockpile that will be reclaimed automatically without the use of mobile equipment. The blending proportion achieved and accuracy is based on the type of reclaim equipment used. These reclaim systems are as follows:
- Two or more individual reclaim hoppers with belt or vibratory feeders: this hopper/feeder arrangement will reclaim an inverted cone shape from the active storage pile above the hopper. For a coal yard silo, the hopper/feeder arrangement will reclaim all of the silo capacity (active storage). The feeders will normally be of the variable capacity type with a 5-to-1 range. This would produce a blend from 20 to 100 percent or 10 to 50 percent depending on the feeder specifications. For PRB coal, belt feeders will typically produce a more accurate blend than vibratory feeders.
- Two or more rotary plow feeders in a reclaim tunnel: this tunnel/feeder arrangement will reclaim an inverted triangular cross-section from the active storage pile above the tunnel for the full length of the plow travel. The rotary plow feeder will normally be of the variable capacity type with a 5-to-1 range.
- Traveling or radial scraper (drag chain) reclaimers: this barn, dome, or outside arrangement will reclaim the complete active storage pile. The scraper reclaimers will normally be of the variable capacity type. Scraper reclaimers will also have a blending accuracy and percent increment (ratio) of about +/- 5 percent.
- Two stacker/reclaimers: this arrangement will reclaim the full trench shaped active storage pile. The active pile trench is bounded by the reserve pile and the stacker/reclaimer berm. Due to varying active coal pile flowability, it is difficult to reclaim at a low percentage rate using only a partially filled bucket.
- Unloading: one coal type could be directly from unloading. Barge delivery works best for this application.
Figure 2 Stockout and Reclaim System: Kingston Fossil Plant
Click here to enlarge image
The reclaim systems that can be modified to accommodate reclaiming the second coal type from the second storage pile for blending are as follows. These modified reclaim systems have the same blending ratio accuracies as described above.
- One reclaim hopper with belt or vibratory feeder: add a second hopper system at a location such that the two stockout piles can be segregated. This can be a continuation of the reclaim conveyor tunnel or a separate reclaim conveyor meeting the existing system at a transfer house or the crusher house.
- One rotary plow feeder in a reclaim tunnel: the most economical method is to add a second rotary plow feeder in the same tunnel. This is based on the existing tunnel and stockout system being long enough to accommodate two segregated piles. If not, the existing tunnel may be able to be extended and the new plow added in this new tunnel section.
- One traveling scraper reclaimer: the most economical method is to add a second reclaimer in the same barn or outside area. This is based on the existing stockout system being long enough to accommodate two segregated piles.
- One stacker/reclaimer: add a second stacker/reclaimer on an extension of the berm/trackage or a second parallel berm/trackage for a near equal blending ratio. For a minimum/maximum blending ratio, stockpile the minimum coal type at one end of the existing stacker/reclaimer travel, near the secondary reclaim hopper. Use the stacker/reclaimer to reclaim the maximum portion of the blend and the reclaim hopper to reclaim the minimum portion.
- One silo add additional silo(s). The addition is based on the arrangement of the stockout and reclaim conveyors being able to integrate the new silo(s).
- One barn or dome each with a single reclaimer: add a second barn or dome. The addition is also based on the arrangement of the stockout and reclaim conveyors being able to integrate the second barn or dome.
Coal Blending Methods
For both existing and new power plants, the coal handling facility can accommodate coal blending with a range of new or modified system components. These vary from using the existing facility with a major use of mobile equipment to new or upgraded facilities for automated coal blending. The extent of initial or future coal blending at the power plant will be a major factor in the initial/modified coal handling facility.
For existing power plants a manual method of coal blending can usually be devised using the existing coal handling equipment for a blended coal test burn. If coal blending is selected for plant fueling, the existing facility can be modified for a basic blending system, or there can be a major upgrade to a fully automated blending system.
A small power plant (under 500 MW) may have a stockout system consisting of a single stacking tube or fixed stockout conveyor. The reclaim system would probably consist of a single or duel reclaim hopper. For a coal blending test burn, the second coal type could be stocked out and dozed to a separate pile.
With only one reclaim hopper/feeder, the two coals could be carefully dozed to the hopper at the same time, trying for a near equal blend. A more accurate method would be to use a temporary dozer trap and conveyor to the reclaim system for the second coal type. With a dual hopper/feeder arrangement a temporary barrier could be placed between the two hopper sections to keep the two coals segregated. Each feeder then could withdraw the correct blend ratio.
A large power plant (over 500 MW) would probably have primary and secondary coal stockout and reclaim systems. The primary stockout system may consist of a radial stacker, traveling stacker, or stacker/reclaimer either enclosed or outside. The primary reclaim system would consist of one or more hopper/feeders, rotary plow feeders, stacker/reclaimer, or scraper reclaimer. The secondary stockout/reclaim system would probably be a stacking tube or fixed stockout conveyor with a hopper/feeder. For a coal blending test burn, the major coal portion would be stockpiled and reclaimed with the primary system and the minor coal portion would be stockpiled and reclaimed with the secondary system. This would allow for an accurate blend of any ratio.
If coal blending is selected as the fueling method, the existing power plant coal handling facility would be modified to accommodate the stockout and reclaim of two or more fuels. The extent of the modifications/upgrades would depend on the existing equipment, coal blending ratio and the required amount of system automation.
For new power plants, coal blending may be either an initial design consideration or an option for future coal source flexibility. Depending on the criteria, the blending capacity of the coal handling facility may be a fully automated system for two or more coals or only have provisions for a second coal source for future blending.
A small power plant (under 500 MW) could initially be designed with a stockout system with two stacking tubes or fixed stockout conveyors. If blending was a future consideration, the second system could be only planned for on the site arrangement and at the stockout and reclaim transfer points. An economical option is to use a radial stacker for the stockout and then the number of segregated piles can be varied as required. The reclaim system would consist of two separate reclaim hopper/feeder arrangements. Again, the second reclaim hopper could be installed initially or in the future with preplanning, depending on the initial or future blending requirements.
A large power plant (over 500 MW) would initially be designed with a primary and secondary coal stockout and reclaim systems. For initial blending, the primary stockout system may consist of a radial stacker, traveling stacker, or stacker/reclaimer. The radial stacker and traveling stacker are compatible with stockpiling two or more coals for blending. This equipment is suited for barns, domes, or outside applications. The stacker/reclaimer is only suited for blending if there are two machines or if a second reclaim method is used for the second coal. The primary reclaim system would consist of hopper/feeders, rotary plow feeders, or scraper reclaimers. These are compatible with reclaim for blending.
For future blending with an even blending ratio, the number of hopper/feeders, rotary plow feeders, or scraper reclaimers could be increased to match the number of coals in the blend if the equipment was not initially installed. For future blending with an uneven blending ratio, the primary systems could handle the major coal portion of the blend and the secondary systems could handle the minor coal portions.
The active coal pile is normally defined as a three-day supply of coal at the maximum burn rate that can be reclaimed without the use of mobile equipment. This coal is non-compacted, so if it is PRB coal it must be reclaimed on a first-in, first-out basis to avoid spontaneous combustion. A traveling stacker for stockout and rotary plow feeders for reclaim could be a fully automated system with an active coal pile trench arrangement. A traveling or radial stacker and a traveling or radial scraper reclaimer could also be a fully automated system. Silos with feeders are a fully automated system. These systems would only require the use of mobile equipment to handle the variances between coal delivered and coal usage (coal into/out of reserve storage). A traveling stacker for stockout and hopper/feeders for reclaim would be a semi-automated system and would require mobile equipment to reclaim the coal that does not flow by gravity into the hoppers. The amount of this coal is dependent on the hopper spacing.
Many components of the coal handling facility are affected by switching to or blending with PRB coal. These factors apply to a new coal handling facility as well as modifications to an existing facility. The major components of the coal handling facility affected by PRB coal are discussed briefly in this section.
Conveyor Rated Capacities: There is a difference in the belt conveyor capacity between Eastern/Midwestern coal and PRB coal. The conveyor design capacity is based on both the coal density and the angle of surcharge, which is the angle to the horizontal that the surface of the coal assumes while at rest on a moving conveyor belt. These design factors are as follows.
- Eastern/Midwestern (bituminous) coal has a 50 pounds per cubic foot (pcf) density and a 25-degree angle of surcharge.
- PRB (sub-bituminous) coal has a 45 pcf density and a 20-degree angle of surcharge.
Therefore, the same conveyor would have a capacity decrease of 17 percent when handling PRB coal. Additionally, the recently published sixth edition of the Conveyor Equipment Manufacturers Association (CEMA) Handbook recommends a slower belt speed and a lower capacity design factor when handling PRB coal.
Chutework: PRB coal is friable and therefore dustier than other coals. Directional flow type transfer points have been developed to reduce the dust and spillage problems related to PRB coal. The conveyor discharge chute contains an adjustable curved deflector plate to change the direction of the coal stream. This channels the coal trajectory to reduce the coal stream expansion, air entrainment, and dust generation. The load chutes above the skirtboard of the receiving conveyor are also curved plate construction. This “spoon” loading chute places the coal on the conveyor belt at the proper speed and direction to reduce impact, turbulence, belt abrasion and expelled air.
Dust Control: The use of PRB coal in any blend will require dust control by dry dust collection and/or wet suppression. A coal dust collection system would minimize the escape to the atmosphere of dust particles generated at the various transfer locations in the coal handling system. Dust collected in the induced draft filter bag units would be collected in the hopper that forms the unit housing bottom. The collected dust would be returned to the conveying system chutes, hoppers, or to an active silo/bunker in the power plant. Treating the returned dust not sent directly to the plant silo/bunker would be recommended. The dust should not be returned directly to a conveyor loadskirt if at all possible.
Wet suppression could provide dust control at the unloader hopper during unloading and for coal stockout. A water wagon should be used during the construction of the reserve coal storage pile. Chemical sprays or crusting agents should be used for the reserve coal storage pile after construction. The water-only system requires 10 to 20 gallons of water per ton of coal treated. The chemically treated (wetting agent) water system requires 1 to 2 gallons of water per ton of coal treated. Additionally, the chemically treated wet suppression system would have a carry over effect and help reduce the dusting at the downstream transfer points.
Wash Down: Despite the best efforts to have a clean coal handling system, some dust would escape from conveyors and equipment that would have to be cleaned up. Good dust control provisions would minimize the need for such cleanup. The three most common methods of such cleanup are broom and shovel, vacuum cleaning and water wash down. The preferred method depends upon the area of the plant.
Fire Protection: The use of PRB coal, in any blend, would require fire protection for every area where coal is handled. Critical to fire protection systems for coal handling facilities would be proper fire detection devices and adequate fire suppression equipment. For coal handling building and substructures, fire protection should consist of an automated dry pipe deluge system to cover each floor level with heat sensing detectors. Additionally, these areas should have fire hoses and other fire fighting components located in hose cabinets on each floor. For coal conveyors an automated dry pipe system should cover the full length of both the carrying belt and the return belt. The spray heads, spaced at about 10-foot centers, should be controlled by heat sensing detectors. The number of fire hydrants in the coal yard should also be reviewed for PRB coal use.
Electrical Devices: PRB coal presents an even greater need for intrinsically safe and explosion-proof electrical devices and components. Depending on the actual classification of the area, many electrical items for an existing plant may need to be upgraded to satisfy either Class II, Division 1 or Class II, Division 2 requirements. It is strongly recommended that all enclosed areas be classified as Class II, Division 1, due to the great potential for coal dust accumulation with PRB coal.
Authors: Richard H. McCartney is senior material handling engineer with Roberts & Schaefer based in Chicago. Robert L. Williams, Jr. is vice president of sales and marketing for Roberts & Schaefer.