f extremely abrasivematerial. Increasedenvironmental awarenesshas also caused major changes in ash handling techniques.And it must do so reliably and efficiently. Since Allen-Sherman-Hoff began supplying ash handling systemsto electric utilities in 1917. technology has advanced dramatically. Fly ash:the fine ash particles in the gas stream which are removedby the dust collection equipment.In a modern. Among the most notable are the use of sophisticated air filtration equipment in pneumatic systemsand the increased use of closed loop recirculating hydraulic systems to constantly recycle conveying water. . An ash handling system must collect and transport large quantities of hot. Economizerand air heaterash:the coarse particles in the flue gas which drop out when the gas stream changesdirection. slate and iron pyrites dischargedfrom the coal pulverizer. heavypiecesof stone. ~ Mill Rejects Bottom Ash . Bottom ash: the materialthat falls to the bottom of the boiler. As the size of . equipment design and systemsintegration.transportsand stores all of the solid by-productsof the combustionprocess.ash handling systems have also grown in capacity. . Thesetypically include: . pulverized-coal burning boiler. Mill rejects:the coarse. . possibly a heavyslag. Successfully meetingthese requirementsdemandsa broad spectrum of expertiseencompassingmaterialsengineering.the steam generators has increased. the ash handlingsystemnormallycollects. . these systems are now limited by environmental restraints. The system in diagram 1 illustrates a basic hydraulic conveying technique where bottom ash falls from the boiler into a water-filled ash hopper. the need for large land areas for settling/retention ponds. The resultant ash-water slurry is pumped to a large retaining pond. bottom ash must be water quenched as it is collected. most utility plants in the United States have used hydraulic conveying of bottom ash. Consequently. with the combined total of 72 hours providing adequate capacity for long weekends when truck or rail service is not available. and then into a surge or storage tank. 4 . Recirculation pumps return the water for reuse in the system. the decanted water flows by gravity into a settling tank to remove fines and carry-over. Normal design criteria suggests 12 hours of storage in the ash hopper. Sludge pumps under the settling and surge tanks return any settled ash back to the dewatering bin. The dewatered ash is discharged from the bottom of the bin directly into trucks or railcars for off-site disposal. little land area and it produces a relatively dry ash (about 15% water by weight) easy to transport by rail or truck. Two Hydrobins are required for each system. This incorporates full closed cycle water recirculation and dewatering bins to drain water from the accumulated ash. Also. One receives ash slurry while the other is draining or decanting. the ash is pumped by centrifugal slurry pumps to A-S-H Hydrobin@ dewatering bins. The full recirculation system has the advantage of requiring little make-up water. the ash passes through a large gate and into dual roll clinker grinders which reduce clinkers to an appropriate size and feed the ash to a hydraulic ejector. From the dewatering bins. although submerged mechanical drag conveying is gaining in popularity.Because of the high furnace temperatures of pulverized coal fired boilers. and lack of abundant water supplies. after passing through the dual roll clinker grinders. This provides a convenient procedure where ash is transported once during each eight hour shift. there is no water discharged into streams or rivers. From the hopper. A more modern practice is shown in diagram 2. Once considered the basic bottom ash conveying method. Make-up water. Here. seal water and impulse water are supplied by medium and high pressure centrifugal pumps. Each Hydrobin is normally sized for 36 hour storage. . Fittings have integral wear backs. the valve gate is retracted completely out of the flow path of the erosiveash slurry. Allen-Sherman-Hoffbottom ash hoppers are designed with sloped sides to permit ash to enter the conveying system at high rates.the ash slurry is normally transported through Ashcolite@pipe. Bottom ash disposal. Alternate pipe materials. for durability. The choice between the two types of pumps is a factor of plant layout. and allows for expansion of the boiler. In the open position. fuel and plant configuration. An independent water-filled seal trough surrounds the boiler seal plate.The gate forms a seal when closed and is hydraulically opened for feeding ash into the conveying system. Depending on the particular boiler. or more pant legs. the Hydrobin is partially filled with water. Two dewatering bins are used in each installation. access doors and lancing doors to accommodate individual requirements. Mounted on the sloping side of each pant leg is an integral discharge gate and housing assembly. Initially. close grained structure. one to receive ash while the other is dewatering or on standby. with maximum thickness in the area of greatest wear. Hydrobin@ dewatering bins receive~ 6 . the hopper may have one.gatesize all ash clinkers and slag into small particles for conveying. Knife-gate AshfloTM valves are used for isolation and shut-off. which provides an atmospheric seal to the furnace. the water overflows a serrated weir. equipment. distances and the specific gravity of the particular ash being handled. When the bin fills. eliminating the need to fabricate and stock odd lengths. a hard iron alloy pipewith uniformwall thicknessand dense. are also available. bottom ash dropping out of the furnace is collected in a water-impounded hopper installed directly under the boiler. The coarser particles then act as a filter to trap fines before they reach the decanting elements. the sized ash passesinto a piping system for transportation to disposal or storage. An underflow baffle directs all incoming material downward and helps to prevent the fines from reaching the overflow baffle. Bottom ash conveying. This weir is used to insure a uniform flow pattern and to simplify leveling during construction. A-S-H grinder rolls are constructed of manganesesteel. Incoming ash slurry is discharged into the center of the Hydrobin where a bar screen classifier diverts coarser material to the sides of the bin while finer particles drop into the center. A continuous curtain of cooling water flows over the refractory lining of the hopper. Double-roll clinker grinders at each disqharge. The hoppers also normally include inspection windows. The housing is provided with both vacuum and pressure relief the ash slurry and then drain or decant the conveying water from the solid material. In conventional systems. From the grinders. required conveying rates. The plain-ended Ashcolite pipe is assembledwith sleeve couplings. such as ceramic-lined fiberglass reinforced pipe. with integrally cast teeth.Bottom ash collection. The motive force can be supplied either by an A-S-H Hydro-EjectorTMjet pump or a centrifugal slurry pump. In Allen-Sherman-Hoff systems. This feature eliminates the tendency for the hydraulic head to force fines through the stationary decanting elements. .Once the bin has received its load of ash. ash is discharged easily and quickly through a wide. The A-S-H Hydrobin discharge gate is provided with heating elements to prevent freeze up. and air-operated seal tube to prevent water leakage. floating decanter rapidly drains the standing water above the ash down to the level of solid material. The floating element. in combination with the stationary lower decanting screens and valves. vibrators to clean the side walls.is complete. allow the Hydrobin to dewater the ash to a commercially dry state (approximately 15% water by weight). horizontal bottom opening gate into trucks or rail cars for disposal. an upper. Once dewatering. and improved upon. This compaction. Allen-ShermanHoff has adopted. During operation.Mechanical drag conveying of bottom ash has been used extensively in Europe. bottom ash falls through a transition chute or retention hopper into the water trough of the conveyor. Normally. 8 . The storage bin may be equipped with decanting elements to drain excess retained water from the ash during storage and r:eturn the decanted water back to the submerged system. and the minimal amounts lost through evaporation. many of the features of proven European designs and incorporated them into an original system designed for the ash and slagging characteristics of various fuels. the ash is relatively dry and is discharged into one or more of a series of drag or belt conveyors to a storage or dewatering bin. the ash is picked up by the flight bars and moved toward the dewatering incline. Since the system is continuous. surge tanks are comparatively small. The water recirculation includes a continuous flow of water into and out of the submerged trough to maintain a bath temperature appropriate for quenching the ash. drag units are susceptible to damage from slag falls accounting for their popularity in installations burning non-slagging fuels. Overflow water from the trough is filtered in tanks or gravity settlers before it is recirculated back to the system. As the flight bars move up the dewatering incline. the ash is compacted ahead of the bars. The chute or hopper includes a seal trough to provide a seal against the boiler pressure and the sloped sides direct the fall of slag and ash into the center of the conveyor. Make-up water is only needed to replace retained water discharged with the ash. At the end of the incline. but the availability of truck and railcars requires a storage bin with a capacity of three days. ash removal from the boiler is continuous. However. plus gravity. Upon entering the wet trough. dewaters the ash and the excess water drains back into the wet trough. including lower operating costs and the need for less clearance under the boiler. Because of the inherent advantages of the drag technology. This technology offers ~everal advantages. . Vacuum systems. The bag filter may be continuous or intermittent. the headroom required under the fly ash hoppers is minimized and operation is somewhat cleaner since any leaks are into the system. Collection and filtration equipment are located on the silo roof. However. However. Because fine particles do not settle out of suspension easily. 10 .precipitators or baghouses. In a vacuum system. In addition. various arrangements of dust collectors separate the fly ash from the conveying air. Automatically controlled segregating valves isolate each branch line. Vacuum.Combination Vacuum/Pressure Air preheater ash is generally handled as part of these systems. some residual ash passes through the vacuum producer. Vacuum conveying generally provides the lowest initial cost for a fly ash system. The system is normally arranged into branches. Dry mechanical vacuum pumps simplify venting systems and do not produce waste fluids. With the intermittent arrangement. the system is shut down periodically and the bag house storage hopper is discharged into the silo. Even with this filtration equipment. Air inlet check valves at the heads of the branch lines provide additional conveying air. allowing each to be activated independently. each pickup point is equipped with a materials handling valve and air inlets to smoothly feed ash into the conveying line. these systems generally fall into one of three general types. At the silo. . The primary collector is equipped with separate chambers so it can operate continuously. feeding ash into the silo while the top of the collector is still under vacuum. with each branch containing a number of pickup points. Depending on site-specific considerations. fly ash is almost always collected and transported pneumatically rather than hydraulically.Most of the solid residue from a pulverized coal boiler is carried away by the flue gas. vacuum systems provide limited conveying capacities and distances. Coarser particles will drop out of the gas stream at changes in direction such as in the economizer and air preheater. the largest volume of ash in today's power plants is composed of fine particles of fly ash removed by the air cleaning devices . The conveying vacuum can be produced either hydraulically (with a Hydrovactorl@> exhauster) or jet mechanically (by a vacuum pump). Pressure . In either arrangement the bags are continuously cleaned by pulse jet action. The most common arrangement uses a cyclone collector as the primary separation device with a combination cyclone/bag filter for secondary collection. However. ash is forced down into the conveying line through an intake tee that imparts a swirl to the ash for better pick up performance. The airlock valve transfers fly ash into the pressurized conveying pipe. while preventing backflow by means of three sequential operations. The upper chamber is pressurized to a level slightly greater than the conveying line pressure and when the lower gate opens (c).Pressure systems. 11 . Pressurized conveying of fly ash offers some definite advantages over vacuum systems. Positive displacement blowers provide the air flow and pressure for conveying the fly ash. pressure systems require more headroom under each hopper and have a higher initial cost in installations with a multitude of collection points. (50m). At the end of the loading cycle the upper gate valve closes to isolate the chamber from the hopper. and simplify the dust/air separation equipment needed at the silo. This dwell time allows a measured amount of fly ash to flow by gravity into the primary chamber. Initially both gates in the valve are closed (a) and the pressure of the primary chamber is equalized with that of the overhead hopper. Automatic butterfly valves at the heads of the branch lines provide branch isolation while manually operated knife gate valves at the ends of the branch lines serve as maintenance cut off valves. Because the feeding device at each pickup point is larger. Pressurized systems provide greater capacity. Then the gate valve above the primary chamber (b) is opened for a preset period of time. can convey over longer distances. Allen-ShermanHoff airlock valves allow the fly ash to be transferred from the collection hopper into the pressurized conveying pipe. their operational costs are often less than a vacuum system's if conveying distances are much over 500 feet. Sometimes a combination system is most practical since it uses a materials handling valve at each collection point but can transport over much longer distances than a conventional vacuum system. the transfer station can have either a compact vacuum/pressure transfer tank or a surge bin which also provides intermediate storage. Ash is Sherman-Hoff Fluidizing Transporters. Pressure line upper gate is closed and the middle chamber is pressurized to slightly above the conveying line pressure. fly ash drops into the middle chamber. The surge bin allows the pressuresystem to have a lower conveying capacity than is neededwith the transfer tank system. Periodically. and fly ash falls into the bottom chamber. separated by dump gate valves. Dense-phasepressureconveying can also be used by equipping each storage bin with a pair of Allen- Fabric filter VACUUM-PRESSURE SYSTEM-TRANSFER TANK Vacuum pump Pressure blower I . The higher capacity usually allows smaller conveying pipe and the lower velocity often allows the use of standard piping materials. . In addition. and the top gate reopens for another cycle. This may lower operation costs. The vacuum line from the transfer tank discharges into a secondary collector. A second transfer method uses a surge bin. Each tank has three chambers. With the top gate open and the bottom gate closed. which has the same basic collection equipment as used on the vacuum system silo.The surge bin is equipped with one or more airlock valves which feed the fly ash into the pressurizedconveying line. I n dilute phase systems. The fly ash is transported to an intermediate vacuum/ pressure transfer station. the ~ Vscl!l!m line I Fly ash hoppers Materials handling valve separatedfrom the conveying vacuum just as in a conventional vacuum system. 1? . Because the transporters fluidize the ash. which includes a bag filter. Now the lower gate opens. Material from the bottom chamber is continuously fed into the transport piping. When the bottom gate is closed. they convey large amounts of ash with less air and at a lower velocity than dilute phase systems. Normally this collector has an airlock valve and is discharged into the pressurized conveying line at appropriate intervals.Combination vacuum/pressure systems. the top chamber includes a cyclone collector. The vacuum/pressure transfer tank transfers ash between unequal pressure zones and is the primary ash/air separation device. the pressure in the middle chamber is equalized to the the vacuum system. in addition to conveying and displaced air. A-S-H dust conditioner/unloaders will be supplied to continuously feed and moisten the discharged ash. and their design is predicated on the overall system dynamics including the conveying. If fly ash is to be discharged into open trucks. The vent itself may be either a bag filter. the silo vent system must be sized to handle this volume. clearance beneath the silo floor for rail or truck access. or vent fans to exhaust air back into the precipitator (or baghouse) inlet breeching. Since Allen-Sherman-Hoff dry unloaders provide for displaced air to be vented back to the silo.Fly ash storage silos are an integral part of the fly ash handling system. 13 ~ . The silo system is normally designed to accommodate a 3-day storage capacity. wind and seismic considerations. venting. aerating. Other design parameters include the aeration stone geometry (and the venting of this aeration air). and unloading subsystems. The ash will then be discharged through the rotary feeder into a single storage tank which will feed an airlock valve. Mechanical drag technology can also be used for economizer ash. The ash can then be transported conventionally. and a more usual technique is to include economizer ash collection as part of the fly ash sytem and handle it pneumatically. However. But some types of economizer ash are difficult to decant or remove from the conveying water. One method is to handle this ash hydraulically and discharge it into the Hydrobin@ dewatering bin along with the bottom ash and mill rejects. In this application.The coarser ash in the gas stream which falls out under the economizer may be handled in a number of different ways depending on the overall system configuration. In this case each economizer hopper is usually equipped with a secondary storage hopper to continuously receive this ash from the hot gas stream and reduce its tendency to sinter. A less costly alternative is to feed several economizer hoppers into a single secondary hopper through the use of downcomers. the physical layout of equipment in the economizer area often precludes this arrangement. or materials handling valve. the drag unit is dry and sealed. 14 . A single drag unit will normally collect from all the hoppers on the economizer. with each hopper equipped with an airlock valve or materials handling valve to feed the material into the transport piping. These optional erection management services can be supplied on either of two levels of activity. Regional Customer Servicemen regularly visit the plants in their assigned areas to discuss ongoing system and equipment operation with plant personnel and provide suggestions for improvements in equipment and maintenance procedures. the company has designed and supplied thousands of systems for utilities and a variety of industrial installations both in the United States and abroad. the Engineering Department is deeply involved in producing systems and equipment to meet the increasing challenges of environmental protection and gr~ater energy efficiency. oil and refuse fired boilers. an A-S-H construction supervisor advising the customer and his construction organization. The A-S-H Engineering Department has contributed numerous advances which have been adopted as industry standards. iron and steel. a multi-divisional organization manufacturing engineered capital equipment for the power. Allen-Sherman-Hoff's Field Service Department offers complete project management services for the erection. Today. High speed computers and automated drafting equipment are used to increase the accuracy. the parts control system provides for faster parts deliveries and the incorporation of system improvements. The resources of the corporation have further strengthened Allen-ShermanHoff's position as a leading supplier of ash handling. or a complete turnkey installation by Allen-Sherman-Hoff. check-out and start-up of ash handling systems. A multi-million dollar inventory of components. PA. custom generated manuals that are provided for all customers. Over the decades. pipe and fittings is maintained at a modern warehouse facility in Honey Brook. A-S-H has a complete line of equipment for industrial plants and also sells systems worldwide through Ecolaire's extensive sales office network. pollution control and other industries. and alerts the replacement parts department to any Allen-Sherman-Hoff engineering changes since the original system installation. In terms of customer benefits. Allen-Sherman-Hoff is an operating company within Ecolaire Incorporated. monitors inventory. chemical processing. and decrease preparation time of contract documents and drawings. A computerized system expedites parts shipment.Allen-Sherman-Hoff has been a pioneer in ash handling systems since 1917. Allen-Sherman-Hoff dedicates major resources to its renewal parts and service operations. A-S-H offers specialized training programs for operations and maintenance personnel. Allen-Sherman-Hoff's research and development activities are geared to developing new technologies and equipment for tomorrow's fossil fueled boilers. Due to the severity of ash handling service. These programs gre individually structured to address the requirements of each specific installation and are based on the detailed. 15 . subassemblies. and the resulting demands on equipment. equipment for all types of coal.