SYSTEM DESCRIPTION

The ash handling system of a power station normally consists of the following:

3.2.1 Bottom Ash (BA) System

i) Bottom ash of each boiler is collected in a water impounded, storage type, double V /“3-V” shaped hopper. It is pertinent to note that shape of hopper depends on amount of bottom ash generation, which varies from project to project, based on source of coal.

Each Bottom Ash hopper has provision for continuous make-up and overflow. Two (2) discharge outlets are provided under each V-section. Each outlet is fitted with a feed gate and clinker grinder. The ash at the exit of the clinker grinder gets mixed with the water to form slurry while pumping with the jet pump. Each jet pump is provided with independent bottom ash slurry transportation pipeline upto common ash (fly ash &

bottom ash) slurry pit for pumping finally to the ash pond.

ii) Alternatively, dry type hopper with submerged scrapper conveyor system is provided which evacuates bottom ash on continuous basis. The bottom ash slurry in this case may be sent to a bottom ash slurry sump, but the use of gravity flow is used to the extent possible. The system consists of dry type bottom ash hoppers, submerged scrapper conveyors, clinker grinders, bottom ash slurry sump / trench upto common slurry pit etc.

iii) The coarse ash collected from the economizer hoppers is connected to the bottom ash hopper top (above the maintained water level) by means of an adequately sized sloping pipe (for transporting slurry by gravity) in case of jet pump system. However if calcium content is high in economizer ash or bottom ash hopper storage capacity is not adequate, then economizer ash is disposed off separately. For submerged scrapper conveyor (SSC) system, normally the ash from economiser, is evacuated and conveyed continuously in wet form and the ash slurry is led to ash slurry pump house, through trenches.

3.2.2 Fly Ash System (ESP, Air preheater, Stack)

i) Evacuation and transportation of dry fly ash is done in two stages, i.e. from ESP collection hoppers to the intermediate surge hoppers by vacuum conveying system/

pressure conveying system and from the intermediate surge hoppers to storage silos near plant boundary by pneumatic conveying. Air preheater and duct hoppers ash can be conveyed pneumatically, and connected to buffer hopper/collector tanks of ESPs.

Alternatively, ash from air preheaters and duct hoppers can be evacuated and conveyed continuously in wet form and the ash slurry led to ash slurry pump house, through trenches.

Standard Design Criteria/Guidelines for Balance of Plant of Thermal Power Project 2 x (500MW or above)

Section- 3 (Ash Handling Plant)

ii) In addition, wet disposal system (to be operated during initial period of plant operation or during emergency when dry disposal is not possible) for fly ash is also provided which directs the fly ash to wetting units to form the slurry and thereafter pumping the same to common slurry pit using jet pumps. Wet disposal system can be medium slurry type or high concentration slurry type.

3.2.3 Ash Water System

i) The entire water requirement of the ash handling system is met from cooling tower blow down of the station and decanted recovery water from the ash pond. A connection from raw water is also provided for fast fill and emergency makeup purposes. Clear water as necessary for equipment sealing and cooling is provided from station clarified water.

In case of once through system in coastal stations, water for ash handling is tapped from return header of CW system.

ii) Ash water system consists of ash water sump, HP water pumps, LP water pumps, economiser ash water pumps etc.

BAHP (bottom ash high pressure) pumps are used to extract bottom ash from both units sequentially in case of jet pump system and continuously in case of SSC system. In case of jet pump system, BAHP pump supply water for jet pumps, BA hopper flushing, seal trough & gate housing flushing etc. In case of SSC system, BAHP pumps supply water for quenching, BA trench jetting, seal trough flushing, gate cooling, BA sump agitation etc.

In case of jet pump system, BALP (bottom ash low pressure) pumpsl supply water for refractory cooling, BA hopper cooling water to maintain hopper water at 60 deg.C, BA hopper fill, seal trough make up/fill, slurry sump hopper make up water etc.

In case of SSC system, BALP pumps supply water for refractory cooling, cooling water for upper trough of SSC to maintain water temp. at 60 deg.C, seal trough make up, cooling water to inspection windows, wash water to grinder, BA sump make up, ash slurry sump make up etc.

Fly ash HP water pumps (FAHP) supply water to wetting heads, air washers, F.A.

slurry/trench jetting, combined ash slurry sump make up, combined ash slurry sump agitation etc.

Alternatively common HP water pumps can be used for fly ash and bottom ash requirements in place of dedicated BAHP and FAHP.

Seal/cooling water pumps are provided for gland sealing of slurry pumps, vacuum pumps and cooling of compressors.

In order to conserve water used in wet ash disposal, an ash water recovery system is provided to recirculate the decanted water from the ash pond and re-using this water for ash handling purposes.

Standard Design Criteria/Guidelines for Balance of Plant of Thermal Power Project 2 x (500MW or above)

Section- 3 (Ash Handling Plant)

3.2.4 Ash Disposal System Dry disposal

The dry fly ash from ESP hoppers is conveyed to intermediate surge hoppers, which are located as close to the ESP as possible. Fly ash collected in intermediate surge hoppers is pneumatically conveyed to storage silos in a separate area near the plant boundary, with independent access from where it is unloaded into the open trucks or bulkers.

Wet disposal

While bottom ash handling and disposal shall be in wet mode, wet disposal of fly ash is to be resorted during initial period of plant operation or during emergency when dry disposal is not possible.

Fly ash and bottom ash slurry is led to common ash slurry pit. PT plant clarifier sludge is also discharged into common ash slurry pit. This combined slurry is then pumped to the ash pond through ash slurry pipelines by centrifugal type low speed ash slurry pumps The ash slurry pumps may be required to be placed in series (maximum three) for meeting high head requirement while pumping to long distances and higher elevations. In case of excessively high head requirement of ash slurry pumping a booster station may also be required between ash slurry pump house and ash pond. Ash slurry is discharged in the ash dyke at strategic locations to ensure even filling of ash pond using pipe garlanding around the dyke.

Alternatively, high concentration slurry disposal (HCSD) system is employed, which uses the slurry concentration between 55 % - 70 % depending on specific slurry rheology.

3.2.5 Ash Handling System Operation

The MOE&F notification dated 03.11.2009, stipulates for a 100% ash utilization within four years of commissioning for new plants and reduced land area (50 hectares for a 500MW unit) for emergency ash pond. The ash handling plant should therefore, adopt the following modes (option I & option II) of operation:

Option I

1. Fly ash disposal: Dry mode (normal continuous operation) and in wet slurry mode (initial operation period and emergency operation when dry disposal is interrupted )

2. Bottom ash disposal: Wet mode

Standard Design Criteria/Guidelines for Balance of Plant of Thermal Power Project 2 x (500MW or above)

Section- 3 (Ash Handling Plant) Option II

1. Fly ash disposal: Dry mode (normal continuous operation) and in HCSD mode (initial operation period and emergency operation when dry disposal is interrupted )

2. Bottom ash disposal: Wet mode

Alternatively, bottom ash can be disposed in semi- wet mode.

3.2.6 Typical scope of work for 2x500 MW thermal power project is attached at Annexure 3A

Typical flow diagrams (as listed below) of ash handling system for 2 x 500 MW are enclosed :

1. Drawing no: CEA-TETD-AHP-001 (Typical flow diagrams for fly ash handling system - 2 x 500MW coal based thermal power plant (vacuum system))

2. Drawing no:CEA-TETD-AHP-002 (Typical flow diagrams for fly ash handling system- 2 x 500MW coal based thermal power plant (pressure system)

3. Drawing no:CEA-TETD-AHP-003 (Typical flow diagrams for bottom ash handling, ash disposal - 2 x 500MW coal based thermal power plant (submerged scrapper chain conveyor system))

4. Drawing no:CEA-TETD-AHP-004 (Typical flow diagrams for bottom ash handling, ash disposal - 2 x 500MW coal based thermal power plant (jet pump system)

3.3 DESIGN CRITERIA AND BROAD FEATURES