f

steam generators has increased, ash handling systemshave also grown in capacity. Increased environ-mental awareness has also caused major changes inash handling techniques. Among the most notable arethe use of sophisticated air filtration equipmentin pneumatic systems and the increased use of closedloop recirculating hydraulic systems to constantlyrecycle conveying water.

In a modern, pulverized-coal burning boiler, the ashhandling system normally collects, transports and storesall of the solid by-products of the combustion process.These typically include:

. Bottom ash: the material that falls to the bottomof the boiler; possibly a heavy slag.

. Economizer and air heater ash: the coarseparticles in the flue gas which drop out whenthe gas stream changes direction.

. Fly ash: the fine ash particles in the gas streamwhich are removed by the dust collectionequipment.. Mill rejects: the coarse, heavy pieces of stone,slate and iron pyrites discharged from the coalpulverizer.

An ash handling system must collect and transportlarge quantities of hot, extremely abrasive material. Andit must do so reliably and efficiently. Successfullymeeting these requirements demands a broad spec-trum of expertise encompassing materials engineering,equipment design and systems integration.

Since Allen-Sherman-Hoff began supplying ashhandling systems to electric utilities in 1917, thetechnology has advanced dramatically. As the size of

~

Bottom Ash. Mill Rejects

incorporates full closed cycle water recirculation anddewatering bins to drain water from the accumulatedash. Here, after passing through the dual roll clinkergrinders, the ash is pumped by centrifugal slurry pumpsto A-S-H Hydrobin@ dewatering bins. Two Hydrobins arerequired for each system. One receives ash slurry whilethe other is draining or decanting. The dewatered ashis discharged from the bottom of the bin directly intotrucks or railcars for off-site disposal. Each Hydrobinis normally sized for 36 hour storage, with the combinedtotal of 72 hours providing adequate capacity for longweekends when truck or rail service is not available.From the dewatering bins, the decanted water flowsby gravity into a settling tank to remove fines andcarry-over, and then into a surge or storage tank. Sludgepumps under the settling and surge tanks return anysettled ash back to the dewatering bin. Recirculationpumps return the water for reuse in the system.

The full recirculation system has the advantage ofrequiring little make-up water, little land area and itproduces a relatively dry ash (about 15% water by weight)easy to transport by rail or truck. Also, there is no waterdischarged into streams or rivers.

Because of the high furnace temperatures of pulverizedcoal fired boilers, bottom ash must be water quenchedas it is collected. Consequently, most utility plants in theUnited States have used hydraulic conveying of bottomash, although submerged mechanical drag conveying isgaining in popularity.

The system in diagram 1 illustrates a basic hydraulicconveying technique where bottom ash falls from theboiler into a water-filled ash hopper. Normal designcriteria suggests 12 hours of storage in the ash hopper.This provides a convenient procedure where ash istransported once during each eight hour shift. Fromthe hopper, the ash passes through a large gate andinto dual roll clinker grinders which reduce clinkers toan appropriate size and feed the ash to a hydraulic ejector.The resultant ash-water slurry is pumped to a largeretaining pond. Make-up water, seal water and impulsewater are supplied by medium and high pressurecentrifugal pumps. Once considered the basic bottomash conveying method, these systems are now limitedby environmental restraints, the need for large landareas for settling/retention ponds, and lack ofabundant water supplies.

A more modern practice is shown in diagram 2. This

4

the ash slurry and then drain or decant the conveyingwater from the solid material. Two dewatering bins areused in each installation, one to receive ash while theother is dewatering or on standby.

Initially, the Hydrobin is partially filled with water.Incoming ash slurry is discharged into the center of theHydrobin where a bar screen classifier diverts coarsermaterial to the sides of the bin while finer particles dropinto the center. The coarser particles then act as a filterto trap fines before they reach the decanting elements.

An underflow baffle directs all incoming materialdownward and helps to prevent the fines from reachingthe overflow baffle. When the bin fills, the water overflowsa serrated weir. This weir is used to insure a uniformflow pattern and to simplify leveling during construction.

Bottom ash collection. I n conventional systems, bottomash dropping out of the furnace is collected in awater-impounded hopper installed directly under theboiler. Depending on the particular boiler, fuel andplant configuration, the hopper may have one, ormore pant legs.

An independent water-filled seal trough surroundsthe boiler seal plate, which provides an atmosphericseal to the furnace, and allows for expansion of theboiler. A continuous curtain of cooling water flowsover the refractory lining of the hopper.

Allen-Sherman-Hoff bottom ash hoppers are designedwith sloped sides to permit ash to enter the conveyingsystem at high rates. The hoppers also normally includeinspection windows, access doors and lancing doorsto accommodate individual requirements.

Mounted on the sloping side of each pant leg is anintegral discharge gate and housing assembly. The gateforms a seal when closed and is hydraulically openedfor feeding ash into the conveying system. The housingis provided with both vacuum and pressure relief

equipment.Bottom ash conveying. Double-roll clinker grindersat each disqharge .gate size all ash clinkers and slag intosmall particles for conveying. A-S-H grinder rolls areconstructed of manganese steel, with integrally castteeth, for durability.

From the grinders, the sized ash passes into a pipingsystem for transportation to disposal or storage. Themotive force can be supplied either by an A-S-HHydro-EjectorTMjet pump or a centrifugal slurry pump.The choice between the two types of pumps is a factorof plant layout, required conveying rates, distances andthe specific gravity of the particular ash being handled.

In Allen-Sherman-Hoff systems, the ash slurry isnormally transported through Ashcolite@ pipe, a hardiron alloy pipe with uniform wall thickness and dense,close grained structure. The plain-ended Ashcolitepipe is assembled with sleeve couplings, eliminatingthe need to fabricate and stock odd lengths. Fittingshave integral wear backs, with maximum thicknessin the area of greatest wear. Alternate pipe materials,such as ceramic-lined fiberglass reinforced pipe, arealso available.

Knife-gate AshfloTM valves are used for isolation andshut-off. In the open position, the valve gate is retractedcompletely out of the flow path of the erosive ash slurry.Bottom ash disposal. Hydrobin@ dewatering bins receive~

6

Once the bin has received its load of ash, an upper,floating decanter rapidly drains the standing waterabove the ash down to the level of solid material. This fea-ture eliminates the tendency for the hydraulic head toforce fines through the stationary decanting elements.The floating element, in combination with the stationarylower decanting screens and valves, allow theHydrobin to dewater the ash to a commercially dry state(approximately 15% water by weight). Once dewatering,iscomplete, ash is discharged easily and quickly througha wide, horizontal bottom opening gate into trucks orrail cars for disposal, The A-S-H Hydrobin dischargegate is provided with heating elements to preventfreeze up, vibrators to clean the side walls, andair-operated seal tube to prevent water leakage.

the ash is compacted ahead of the bars. This compaction,plus gravity, dewaters the ash and the excess waterdrains back into the wet trough. At the end of the incline,the ash is relatively dry and is discharged into oneor more of a series of drag or belt conveyors to a storageor dewatering bin.

Normally, ash removal from the boiler is continuous,but the availability of truck and railcars requires astorage bin with a capacity of three days. The storagebin may be equipped with decanting elements to drainexcess retained water from the ash during storage andr:eturn the decanted water back to the submerged system.The water recirculation includes a continuous flow ofwater into and out of the submerged trough to maintaina bath temperature appropriate for quenching the ash.Overflow water from the trough is filtered in tanks orgravity settlers before it is recirculated back to thesystem. Since the system is continuous, surge tanksare comparatively small. Make-up water is only neededto replace retained water discharged with the ash, andthe minimal amounts lost through evaporation.

Mechanical drag conveying of bottom ash has beenused extensively in Europe. This technology offers~everal advantages, including lower operating costsand the need for less clearance under the boiler. However,drag units are susceptible to damage from slag fallsaccounting for their popularity in installationsburning non-slagging fuels. Because of the inherentadvantages of the drag technology, Allen-Sherman-Hoff has adopted, and improved upon, many of thefeatures of proven European designs and incorporatedthem into an original system designed for the ash andslagging characteristics of various fuels.

During operation, bottom ash falls through a transitionchute or retention hopper into the water trough of theconveyor. The chute or hopper includes a seal trough toprovide a seal against the boiler pressure and the slopedsides direct the fall of slag and ash into the center ofthe conveyor. Upon entering the wet trough, the ash ispicked up by the flight bars and moved toward thedewatering incline.

As the flight bars move up the dewatering incline,

8

At the silo, various arrangements of dust collectorsseparate the fly ash from the conveying air. The mostcommon arrangement uses a cyclone collector asthe primary separation device with a combinationcyclone/bag filter for secondary collection. The primarycollector is equipped with separate chambers so it canoperate continuously, feeding ash into the silo whilethe top of the collector is still under vacuum. The bagfilter may be continuous or intermittent. With theintermittent arrangement, the system is shutdown periodically and the bag house storage hopper isdischarged into the silo. In either arrangement thebags are continuously cleaned by pulse jet action.

Most of the solid residue from a pulverized coal boileris carried away by the flue gas. Coarser particles willdrop out of the gas stream at changes in direction suchas in the economizer and air preheater. However, thelargest volume of ash in today's power plants is composedof fine particles of fly ash removed by the air cleaningdevices - precipitators or baghouses.

Because fine particles do not settle out of suspensioneasily, fly ash is almost always collected and transportedpneumatically rather than hydraulically. Depending onsite-specific considerations, these systems generallyfall into one of three general types.. Vacuum. Pressure .Combination Vacuum/PressureAir preheater ash is generally handled as part of these

systems.Vacuum systems. Vacuum conveying generally providesthe lowest initial cost for a fly ash system. In addition,the headroom required under the fly ash hoppers isminimized and operation is somewhat cleaner sinceany leaks are into the system. However, vacuum systemsprovide limited conveying capacities and distances.

The conveying vacuum can be produced eitherhydraulically (with a Hydrovactorl@> jet exhauster) ormechanically (by a vacuum pump). Collection andfiltration equipment are located on the silo roof. Evenwith this filtration equipment, some residual ash passesthrough the vacuum producer. Dry mechanical vacuumpumps simplify venting systems and do not producewaste fluids.

In a vacuum system, each pickup point is equippedwith a materials handling valve and air inlets to smoothlyfeed ash into the conveying line. The system is normallyarranged into branches, with each branch containinga number of pickup points. Automatically controlledsegregating valves isolate each branch line, allowingeach to be activated independently. Air inlet checkvalves at the heads of the branch lines provide additionalconveying air.

10

of three sequential operations. Initially both gates inthe valve are closed (a) and the pressure of theprimary chamber is equalized with that of the overheadhopper. Then the gate valve above the primary chamber(b) is opened for a preset period of time. This dwell timeallows a measured amount of fly ash to flow by gravityinto the primary chamber. At the end of the loadingcycle the upper gate valve closes to isolate the chamberfrom the hopper. The upper chamber is pressurizedto a level slightly greater than the conveying linepressure and when the lower gate opens (c), ash is forceddown into the conveying line through an intake teethat imparts a swirl to the ash for better pickup performance.

Automatic butterfly valves at the heads of the branchlines provide branch isolation while manuallyoperated knife gate valves at the ends of the branchlines serve as maintenance cut off valves.

Pressure systems. Pressurized conveying of fly ashoffers some definite advantages over vacuum systems.Pressurized systems provide greater capacity, canconvey over longer distances, and simplify the dust/airseparation equipment needed at the silo. Because thefeeding device at each pickup point is larger,pressure systems require more headroom under eachhopper and have a higher initial cost in installationswith a multitude of collection points. However, theiroperational costs are often less than a vacuum system'sif conveying distances are much over 500 feet. (50m).

Positive displacement blowers provide the air flowand pressure for conveying the fly ash. Allen-Sherman-Hoff airlock valves allow the fly ash to be transferredfrom the collection hopper into the pressurized conveyingpipe.

The airlock valve transfers fly ash into the pressurizedconveying pipe, while preventing backflow by means

11

Combination vacuum/pressure systems. Sometimesa combination system is most practical since it usesa materials handling valve at each collection point butcan transport over much longer distances than aconventional vacuum system.

The fly ash is transported to an intermediate vacuum/pressure transfer station, which has the same basiccollection equipment as used on the vacuum system silo.I n dilute phase systems, the transfer station can haveeither a compact vacuum/pressure transfer tank or asurge bin which also provides intermediate storage.

The vacuum/pressure transfer tank transfers ashbetween unequal pressure zones and is the primaryash/air separation device. Each tank has three chambers,separated by dump gate valves. In addition, the topchamber includes a cyclone collector.

With the top gate open and the bottom gate closed, flyash drops into the middle chamber. Periodically, the separated from the conveying vacuum just as in a

conventional vacuum system. The surge bin is equippedwith one or more airlock valves which feed the fly ashinto the pressurized conveying line. The surge bin allowsthe pressure system to have a lower conveying capacitythan is needed with the transfer tank system. This maylower operation costs.

Dense-phase pressure conveying can also be used byequipping each storage bin with a pair of Allen-

Sherman-Hoff Fluidizing Transporters.Because the transporters fluidize the ash, they convey

large amounts of ash with less air and at a lower velocitythan dilute phase systems. The higher capacityusually allows smaller conveying pipe and the lowervelocity often allows the use of standard pipingmaterials.

upper gate is closed and the middle chamber ispressurized to slightly above the conveying line pressure.

Now the lower gate opens, and fly ash falls intothe bottom chamber. When the bottom gate is closed, thepressure in the middle chamber is equalized to thethe vacuum system, and the top gate reopens for anothercycle. Material from the bottom chamber is continuouslyfed into the transport piping.

The vacuum line from the transfer tank dischargesinto a secondary collector, which includes a bag filter.Normally this collector has an airlock valve and isdischarged into the pressurized conveying line atappropriate intervals.

A second transfer method uses a surge bin. Ash is

1?

Fly ashhoppers

Materialshandlingvalve~ -

Vscl!l!m line IVACUUM-PRESSURESYSTEM-TRANSFERTANK

Fabric filter

Vacuumpump

Pressureblower

I

..

Pressure line

Fly ash storage silos are an integral part of the fly ashhandling system, and their design is predicated on theoverall system dynamics including the conveying,venting, aerating, and unloading subsystems. The silosystem is normally designed to accommodate a 3-daystorage capacity. Since Allen-Sherman-Hoff dryunloaders provide for displaced air to be vented backto the silo, the silo vent system must be sized to handlethis volume, in addition to conveying and displacedair. The vent itself may be either a bag filter, or vent fansto exhaust air back into the precipitator (or baghouse)inlet breeching.

Other design parameters include the aeration stonegeometry (and the venting of this aeration air), clearancebeneath the silo floor for rail or truck access, wind andseismic considerations. If fly ash is to be dischargedinto open trucks, A-S-H dust conditioner/unloaders willbe supplied to continuously feed and moisten thedischarged ash.

13

~

The coarser ash in the gas stream which falls out underthe economizer may be handled in a number of differentways depending on the overall system configuration.

One method is to handle this ash hydraulically anddischarge it into the Hydrobin@ dewatering bin alongwith the bottom ash and mill rejects. But some types ofeconomizer ash are difficult to decant or removefrom the conveying water, and a more usual techniqueis to include economizer ash collection as part of thefly ash sytem and handle it pneumatically. In this caseeach economizer hopper is usually equipped with asecondary storage hopper to continuously receivethis ash from the hot gas stream and reduce its tendencyto sinter. The ash can then be transported conventionally,with each hopper equipped with an airlock valve ormaterials handling valve to feed the material into thetransport piping. A less costly alternative is to feedseveral economizer hoppers into a single secondaryhopper through the use of downcomers. However,the physical layout of equipment in the economizer areaoften precludes this arrangement.

Mechanical drag technology can also be used foreconomizer ash. In this application, the drag unit is dryand sealed. A single drag unit will normally collect fromall the hoppers on the economizer. The ash will thenbe discharged through the rotary feeder into a singlestorage tank which will feed an airlock valve, or materialshandling valve.

14

manufacturing engineered capital equipment for thepower, iron and steel, chemical processing, pollutioncontrol and other industries. The resources of thecorporation have further strengthened Allen-Sherman-Hoff's position as a leading supplier of ash handling.equipment for all types of coal, oil and refuse fired boilers.A-S-H has a complete line of equipment for industrialplants and also sells systems worldwide throughEcolaire's extensive sales office network.

Allen-Sherman-Hoff has been a pioneer in ash handlingsystems since 1917. Over the decades, the companyhas designed and supplied thousands of systems forutilities and a variety of industrial installations bothin the United States and abroad.

The A-S-H Engineering Department has contributednumerous advances which have been adopted asindustry standards. Today, the Engineering Departmentis deeply involved in producing systems and equipmentto meet the increasing challenges of environmentalprotection and gr~ater energy efficiency. High speedcomputers and automated drafting equipment areused to increase the accuracy, and decrease preparationtime of contract documents and drawings.

Allen-Sherman-Hoff's research and developmentactivities are geared to developing new technologiesand equipment for tomorrow's fossil fueled boilers.

Allen-Sherman-Hoff's Field Service Department offerscomplete project management services for the erection,check-out and start-up of ash handling systems. Theseoptional erection management services can be suppliedon either of two levels of activity; an A-S-H constructionsupervisor advising the customer and his constructionorganization, or a complete turnkey installation byAllen-Sherman-Hoff.

Regional Customer Servicemen regularly visit theplants in their assigned areas to discuss ongoing systemand equipment operation with plant personnel andprovide suggestions for improvements in equipmentand maintenance procedures. A-S-H offers specializedtraining programs for operations and maintenancepersonnel. These programs gre individually structuredto address the requirements of each specific installationand are based on the detailed, custom generated manualsthat are provided for all customers.

Due to the severity of ash handling service, and theresulting demands on equipment, Allen-Sherman-Hoffdedicates major resources to its renewal parts andservice operations. A multi-million dollar inventory ofcomponents, subassemblies, pipe and fittings ismaintained at a modern warehouse facility in HoneyBrook, PA. A computerized system expedites partsshipment, monitors inventory, and alerts the replacementparts department to any Allen-Sherman-Hoff engineeringchanges since the original system installation. Interms of customer benefits, the parts control systemprovides for faster parts deliveries and the incorpora-tion of system improvements.

Allen-Sherman-Hoff is an operating company withinEcolaire Incorporated, a multi-divisional organization

15