When a plant switches fuel, starts blending fuels, or changes transportation modes, big changes are needed in coal handling and receiving equipment.
Although few new coal-fired plants have been built in recent years, a combination of factors have driven power plant owners to upgrade their coal handling facilities. Older conveyors and equipment, for example, have been rehabilitated to become more efficient. But of equal or greater significance has been the need to upgrade and modify coal handling capabilities to accommodate fuel switches or blending schemes. Such switching and/or blending is commonly required to meet new environmental demands, and sometimes to lower operating costs. Recent coal yard upgrades have also been initiated by changes in the way coal shipments are received or by the need for additional coal receiving modes. And while each plant that has rehabilitated its coal handling facilities did so based on a unique set of plant-specific conditions, many conditions are common among all plants.
All Conveyors Are Not the Same
A common driver behind modifications at coal-fired power plants has been the need to handle alternate types of coal due to fuel switching or blending. These conveyor modifications include increased capacity requirements requiring higher drive horsepower. Belt conveyor capacity needs differ for eastern/mid-western coal and western Powder River Basin (PRB) coal. 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. Eastern/mid-western (bituminous) coal has a 50 lb/ft3 density and a 25 degree angle of surcharge. Western PRB (sub-bituminous) coal has a 45 lb/ft3 density and a 20 degree angle of surcharge. For example, a 42-inch belt conveyor with a belt speed of 500 ft/min has a 1,105 tph capacity using Eastern/mid-western coal and a 920 tph capacity using Western PRB coal.
The Sammis plant’s unloading and initial stackout arrangement.
Fuel changes can also increase horsepower requirements. Evaluating a belt conveyor for increased capacity or the same capacity of western PRB coal requires a review of the conveyor drive. Normally, the existing conveyor is evaluated for both the current capacity and the revised capacity for the alternate coal type, based on current Conveyor Equipment Manufacturers Association (CEMA) design criteria. CEMA design criteria have been modified over the years based on experience and newer conveyor components. It is not uncommon to find a conveyor drive designed 40 plus years ago that will not meet the current CEMA standards. Current CEMA standards have resulted in higher belt tensions and therefore, higher horsepower requirements. Belt cleaners have increased tension from 2 lb/inch to 5 lb/inch of belt width. Belt plows have changed tension from 1.2 lb/inch to 5 lb/inch of belt width. Pulleys have changed their tension from an average of 40 to 150 pounds.
An example of the conveyor drive horsepower requirements from a recent study describes a plant switching from eastern coal to blended coal and then to western PRB coal. The 54 inch conveyor with 20 degree idlers transfers coal from the car dumper to the stockout pile. The conveyor was originally designed for eastern coal with a density of 55 lb/ft3.
This conveyor drive sizing condition was typical for about 60 percent of the coal belt conveyors at this facility. For handling eastern/mid-western coal, the belt capacity is above the design capacity, but the drive motor is undersized. When switching to all western PRB coal, the capacity is reduced, but the drive motor is adequate due to the lighter coal and reduced capacity. To handle 2,000 tph of western PRB coal, this conveyor’s belt speed would have to be increased to 815 ft/min. An alternative would be to replace the 20 degree trough idlers with 35 degree idlers. Then, a belt speed of 645 ft/min would produce the 2,000 tph capacity. The current belt conveyor at 815 ft/min would require a 450 hp motor, or a revised belt conveyor with 35 degree idlers at 645 ft/min would require the current 400 hp motor. Therefore, changing the idlers would allow the current drive and belt speed of 705 ft/min to be used.
Lower Heat Value = Higher Conveyor Use
At most coal plants, coal switching or blending means increased use of western PRB coal. PRB coal has a lower heating value, meaning an increased volume is needed to achieve equivalent boiler heat input. That places higher demand on the existing handling systems and the conveyors connecting them. Take, for example, a two-unit station rated at 230 MW. In the late 1990s the maximum burn rate of midwestern coal was 117 tph. In 2000 the plant started blending PRB and the maximum burn rate increased to 141 tph. With the final switch to all PRB, the maximum burn rate increased to 155 tph. This meant handling 32 percent more coal per hour. During the conversion, the station capacity factor also increased. The station had previously operated as a peak load unit with a 35 percent capacity factor. With PRB, it became a baseload facility with a 70 percent capacity factor. This doubling of capacity factor, combined with the increased maximum burn rate due to coal switching, resulted in yearly coal usage jumping from approximately 359,000 tons per year to 950,000 tons per year – a 265 percent increase.
Delivery to the Plant
Major methods of transporting coal to power plants continue to be trains, barges/ships, trucks, and overland conveyors – in that order – with 50 percent delivered by train. Power plant coal handling facilities can be modified to accept an alternate method of coal transportation for various reasons: the need for increased coal quantities, coal switching, coal blending, and/or controlling coal delivery costs. Historically, the choice of transportation method has been based on the location of the plant relative to the source of the coal. Power plants in the midwest and west received their coal by train from western mines. Power plants along major rivers like the Mississippi and Ohio received coal by barge from mines in the region. Plants in Pennsylvania and West Virginia received coal by truck from local mines. And mine-mouth power stations received their coal by conveyor.
As the Powder River Basin area was developed, a large source of low sulfur, low cost coal became available. Over time, trains and shipping on the Great Lakes have moved this coal further and further east. Meanwhile, more and more eastern mines have shut down due to depleting coal reserves, increased mining costs and environmental regulations. As a result, a major shift has occurred from delivery by barge, truck and conveyor to delivery by trains, requiring plants designed only for barges and trucks to undertake significant modification for receiving coal. Furthermore, many coal handling facilities initially designed for train delivery were modified for improved train delivery.
To meet increased demand for PRB coal and to reduce delivery costs, railroads have increased both the railcar capacity and the number of railcars per train. Unit trains in the 1980s and early 1990s consisted of 100 cars, each with 100 tons of coal, meaning a unit train capacity of 10,000 tons. Since then, car capacity has steadily increased to as much as 120 tons, and a train can now have 125 cars or more. As a result, 15,000 ton unit trains are common today. To accommodate the increased capacity, railcars have evolved from the steel car with a tare weight of 63,000 lbs and a capacity of 200,000 lb for a gross weight of 263,000 lb, to the high-sided aluminum cars with a tare weight of 46,000 lb and a capacity of 240,000 lb for a gross weight of 286,000 lb.
Power plants have had to adapt to accommodate these larger capacity trains. Heavier car loads, longer trains and increased total capacity have affected coal handling facilities, requiring modifications from the unloading system through the stockout system. Modification of unloading systems have included either switching from a rotary car dumper or a conventional bottom dump hopper, to a high capacity rapid discharge bottom dump system. Additionally, older rotary car dumpers have been modified and/or replaced with new ones for increased capacity. The stockout system modifications have included replacing a small pile capacity fixed stockout conveyor with either a longer fixed stockout conveyor with a greatly increased pile capacity or a radial stacker with a large capacity single pile or two separated piles for blending.
Many plants have also modified and upgraded their reclaim coal handling system – all the way to the plant bunkers or silos – to accommodate fuel switching or blending. The reclaim, crushing, and plant supply system modifications have included increasing conveyor and equipment capacities to handle the increased use of PRB without increasing bunker or silo fill times. Conveyor modifications have included higher belt speeds, increased troughing idler angle, and/or increased belt widths. Most modifications have required increased conveyor drive horsepower. p
Richard McCartney, P.E., is Senior Material Handling Engineer for Roberts & Schaefer Company in Chicago, Illinois.
Four Rehabilitation Projects
W. H. Sammis Plant – Rehabilitation of the Sammis plant, owned by First Energy, included upgrading conveyors and capacities as well as an alternate transportation method. The project also allowed for the future handling of PRB. The Sammis coal handling facility historically received coal by barge that was unloaded by a continuous bucket ladder unloader. To greatly increase the unloading capacity, Roberts & Schaefer (R&S) furnished a rapid discharge bottom dump train unloading system rated at 4,000 tph. The 110 to 115 car unit trains are unloaded in three to four hours. The coal source remains Pennsylvania coal, but the delivery cost has been greatly reduced due to decreased unloading time compared with the barge system. The railcar unloading hopper discharge uses a single 96-inch wide belt feeder. A third-rail system for automatic in-motion unloading and a roller railside car shaker were also furnished.
Waukegan station’s rail-mounted radial stacker for active storage.
The train unloading system was located approximately 2,350 feet from the coal yard reclaim area due to the site conditions. The unloading conveyor discharges to a stacking tube to form a 30,000-ton storage pile near the train unloading system. Four vibratory feeders reclaim this pile and feed onto the 1,000 tph transfer conveyor (Photo 1). The 1,900-foot transfer conveyor moves the coal to a stacking tube to form a 15,000-ton pile in the coal yard reclaim area. This dual staking tube system allows for rapid unloading of the railcars and then a reasonable conveying rate to the coal yard area, on an as-needed basis. The ancillary systems included wet suppression for dust control, tunnel ventilation, and wash down.
Seward Repowering Project – The Seward Repowering Project included upgrading conveyors and capacities as well as equipping the plant to accommodate an alternate source of coal. The original Seward plant required only a small coal handling facility. The Seward Repowering Project has added two new units to the site fueled by 100 percent local gob coal, a refuse from bituminous coal processing. The coal handling facility for the new units is completely new, from the truck delivery system to the plant silo fill system. The delivered gob coal includes rock of all sizes. The six truck dump receiving hoppers are equipped with hydraulic tilting grizzlies. Rock larger than 12 inches remains on the grizzly and is discharged off the back of the grizzly when tilted. The gob coal is conveyed to the screen house where double deck screens remove the remaining rock larger than 2 inches. The rock is discharged to an automated truck loadout system.
The conveyor system is a dual belt system rated at 1,800 tph each from the truck dump hoppers to the storage barn. The gob coal is stored in an enclosed A-frame storage barn with a capacity of 44,000 tons. Dual traveling trippers distribute the gob coal within the 550-foot long barn. The coal is reclaimed with a 1,000 tph capacity portal reclaimer. The reclaimed coal is then conveyed to the crusher house where two coal pactors produce a product size of 3/8 x 0 inch. The crushed coal is conveyed to the boiler building where a 48-inch traveling tripper distributes the gob coal to the four silos in each of the two units. The ancillary systems included truck weigh scales, sampling systems, belt scales, magnetic separators, a coal analyzer, and bag filter dust collectors.
Kingston Fossil Plant – The rehabilitation at TVA’s Kingston Fossil Plant included upgrading conveyors, upgrading train unloading capacities, alternate coal sources/coal switching, and coal blending. The project also facilitated the future handling of PRB. The existing Kingston coal handling facility received coal by train, which was unloaded at a rotary car dumper on ladder tracks. To modernize and increase the unloading system capacity, R&S furnished a rapid discharge bottom dump unloading system on a loop track, rated at 4,400 tph. To stockout and store regional coals for blending, the fixed stockout conveyor was replaced with two stacking tubes with provisions for a third stacking tube (Photo 2). Each stacking tube provided for a 62,000-ton capacity segregated pile.
The Kingston plant’s stockout/reclaim systems with stacking tubes and rotary plow feeders.
Reclaim for blending from the piles is by two variable capacity rotary plow feeders. The reclaim capacity was increased from 2,000 to 2,400 tph. At the new crusher building two 1,200 tph crushers processed the coal, and it is then transferred to the dual plant supply conveyors. These conveyors were modified with new drives and increased belt speed for the increased capacity. The ancillary systems included fire protection, wet-dust suppression, sampling systems, and ventilation systems. All process equipment is controlled by a distributed control system.
Waukegan Station – Coal yard modifications at Midwest Generation’s Waukegan Station included upgrading conveyors and capacities, upgrading train unloading capacity, alternate coal sources/coal switching. The Waukegan coal handling facility historically received coal by train, unloaded by a 1940s rotary car dumper. When the station converted to PRB, R&S furnished coal handling facility modifications for increased PRB capacity. The modifications increased the system capacity from a maximum rate of 1,400 tph (normally 1,000 tph or less) to a consistent rate of 1,800 tph. Capacity for all conveyors was increased with higher belt speeds, new drives, 35-degree idlers, and other conveyor components. New chutes and skirtboards were also furnished to improve the coal flow and reduce spillage. The short fixed stockout conveyor was replaced with a new fully automatic radial stacker to form a 16,000-ton pile for stockout (Photo 3). The breaker house originally included two Bradford Breakers and three small capacity reversible hammermill crushers. These were removed and replaced with two ring granulators and three larger capacity reversible hammermill crushers for the increased capacity. The ancillary systems included a new as-received sampling system and spray deicing systems for the belt conveyor load zones.