Steam Boiler: Ash Handling System

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Ash Handling Equipment in Stoker Boiler

Ash handling equipment in stoker boiler is one of parts on ash handling system in a power plant unit. Ash is the product of coal combustion in stoker boiler. It should be discharged to maintain well combustion in the furnace.

There are some ash handling equipment are installed in stoker boiler such ash chute hopper, air plenum hopper, ash cooler, ash conveyor, and the equipment of ash handling can be mechanically, hydraulically, and pneumatically.

Bottom ash from the travelling grate is discharged through the ash chute hoppers installed at the end side of travelling grate as shown in Figure 1. The manual sliding damper is installed at each bottom of the air plenum hoppers. The air plenum damper should be opened minimum once per day to exhaust the riddling ash. And opening interval should be adjusted according to the condition of the quantity of riddling ash. Before open the damper, wheelbarrow should be prepared under the damper.

Figure 1: Traveling Grate Ash Handling Equipment

Fly Ash Handling Equipment

Fly ash collected at fly ash hopper at the bottom of tubular air pre-heater and multi cyclone that discharge by butterfly damper and rotary damper accordingly as shown in Figure 2.

Figure 2: Fly Ash Handling Equipment

Fly ash re-injection system

Fly ash from boiler bank hopper and economizer hopper that contains un-burned coal dust is blown in by secondary air fan into the furnace to burn completely as shown in Figure 3.

Figure 3: Fly Ash Re-Injection System

Hydraulic & Mechanical Ash Handling Equipment

There are three types of ash handling system; pneumatic, hydraulic and mechanical ash handling system. Hydraulic ash handling equipment is usually used to convey bottom ash. Illustration below is sample of working principal on hydraulic ash handling equipment.

Figure 1: Hydraulic Ash handling Equipment
Source: Book - Central Boiler Plants - Headquarter Department of The Army

The hydraulic system applies high pressure water jet to take bottom ash from hopper which was located under steam boiler or under stoker. The bottom ash can be taken out from the hopper by instruments such as water jet nozzle and spray nozzle. The bottom ash is then distributed by water jet through sluice way to wet storage sump (see Figure 1).

The treatment to dispose ash is different for fly ash. Fly ash from the hoppers is disposed pneumatically by water jet exhausters then distribute to air separator to separate air from fly ash and then air will be discharged to atmosphere. The end of process, the mixture fly ash and water are conveyed to wet storage sump (see Figure 1).

Another type of ash handling system is mechanical ash handling equipment. This equipment can be employed for small scale boiler which uses stoker type. This type can be also used to convey both for fly ash and bottom ash. The system use mechanical equipments such as bucket conveyor, screw, drag to convey ash from steam boiler to container or silo or storage bin.

Pneumatic Ash Handling Equipment

There are three methods of ash handling equipments. One of them is pneumatic ash handling equipment. Pneumatic system is rarely used to transport bottom ash; it is only used for small scale boiler. Pneumatic ash handling system is often employed for carrying out fly ash.

For more detail please see figure 1 below:

Figure 1: Pneumatic Ash Handling Equipment
Source: Book - Central Boiler Plants - Headquarter Department of The Army

At the figure above explain pneumatic ash handling system with vacuum type. Vacuum pump which is driven by motor is installed into exhauster to generate vacuum pressure. Ash which had been collected by the hopper will be distributed to the pneumatic ash handling system. The exhauster generate suction to make air flow in the line pipe rapidly, so dry ash will be induced and flow to primary collector and secondary collector. The collector is completed by counter balance drop door which allow the ash put down into storage bin firmly and periodically.

The counter balance drop door is normally sealed and closed to maintain the system in the negative pressure. The system is running cycle and periodically, to open the drop door the pressure will be made similar, so the ash can drop into storage bin.

Ash Handling Equipments in Steam Boiler

Steam boiler need to manage not only about coal handling but also about ash handling. Ash is a remaining product of combustion. Ash will be many found in steam boiler which uses coal as its fuel. Steam boiler needs some places to collect ash and then remove and dispose it together in a system which is used to take care about ash.

From the place collection, ash can be classified into two types; bottom ash and fly ash. Ash which be collected in the furnace or under stoker is called bottom ash, while the ash which be collected in the other areas such as boiler bank, air heater, economizer, and multi cyclone is called fly ash. Bottom ash tends to agglomerated clinkers and hard. Fly ash is often not heavy, soft and easy flow.

Handling management for bottom ash and fly ash are to be better if performed separately. Therefore steam boiler must be equipped for bottom ash handling equipments and fly ash handling equipments respectively. Bottom ash can be re-used for treatment in winter road, etc and fly ash can be re-used to be sold again to concrete manufacturer. This condition will be great advantage on economic side for the owner.

Large scale boiler usually applies more complete ash handling equipments than small scale boiler. Ash handling equipments are generally classified into three types as the following below:

1. Mechanical ash handling equipment

2. Hydraulic ash handling equipment

3. Pneumatic ash handling equipment