Case Study – Bottom ash (SSCC) & Fly ash (Dense Phase) Handling system for PF Boiler

Bottom Ash Or Fly Ash Handling system for PF Boiler-

PROJECT SYNOPSIS

Bottom Ash Or Fly Ash Handling system for PF Boiler-

BOTTOM ASH HANDLING SYSTEM:

Macawber Beekay successfully installed Submerged Scrapper Chain Conveyor (SSCC) system, followed by a combination of Belt conveyors system to handle semi-wet Bottom Ash from Boiler furnace to Bottom Ash Silos, equipped with truck Unloading system below the silo.

Furnace ash is discharged on a continuous basis into SCC of each unit, via a refractory lined steel construction bottom ash hopper installed below furnace. The furnace seal plate supplied by others dips into the seal trough filled with water around top periphery of bottom ash hopper for the sealing of furnace.  The necessary clearance is provided between seal plate and seal trough bottom to take care of expansion of furnace.

Bottom Ash Hopper is provided with 4 (four) ash outlets, each having respective hydraulically operated discharge gates. These discharge gates are operated through a common hydraulic power pack. A dip box is provided below each hydraulic discharge gate and bottom portion of the dip box is dipped into the water filled in the top trough of the SCC in order to have a gas seal at all times of plant operation.

Spray nozzles are provided in the bottom ash hopper for quenching of ash and seal trough flushing pipe and cleaning of inspection window.

Economizer ash also flows into respective SSCC in slurry form for disposal.

Suitable LP water pumps are provided to meet the system requirement.

Structural Steel platforms have been provided to gain access to the hopper rodding points (poke holes) and observation windows.

PRINCIPLE OF OPERATION:

Hot Ash/Clinkers fall from furnace through bottom ash hopper into the horizontal trough (slag bath) of the SSCC from where it is continuously dragged away by scraper bars connected to two endless alloy steelround link chains and discharge the same into the clinker crusher through chute and onto the bottom ash belt conveyor for further disposal to bottom ash silo.  Temperature and level of the water in the slag bath is maintained by continuous circulation of water supply.  The trough water overflows through parallel plates to restrict ash particles within the trough itself and is collected in an RCC tank. This contaminated water from RCC Tank could be re-cycled back into the trough through heat exchanger and clarifier.

The bottom ash shall be directly disposed to open Truck from silos through discharge gate and vibratory feeder.

FLY ASH HANDLING SYSTEM:

System consisting of Dense Phase Pneumatic Conveying System.

Fly Ash from APH and ESP hoppers shall be conveyed through Dense Phase Pneumatic Conveying System to Fly Ash Silos

Brief write-up for System at APH:

There are 4 Nos. of outlet points for APH. The Ash Vessels below each APH hopper are clubbed together in series (Master + Slave) through a common conveying line. The fly ash shall be transferred to the FA Silo through a terminal box provided on its top.

Brief write-up for System at ESP:

There are nine fields in ESP with four passes. Each pass shall have 36 Hoppers.

Beneath each ESP hopper, an Ash Vessel system has been provided. Total 6 sets of fly ash conveying pipes have been provided for handling ESP ash for each boiler.

Each Ash Vessel is equipped with unique DOME VALVE at inlet along with necessary instrumentation.

System at Fly Ash Silo:

Total of 4 sets of fly ash silos have been provided for collection of fly ash from two boilers.

At top of each silo, 1 No. Vent Filter (reverse pulse jet type) is provided on top of FA Silo to vent out clean air to atmosphere. 1 No. Level probe for high level indication and 1 No. continuous level indicator is provided in FA Silo. Silos are equipped with Pressure relief valve and Manhole at silo top.

At conical portion of fly ash silos, Fluidizing pads have been provided.3 Nos. (2W+1S) Roots Blower and 3 Nos. heaters are provided for fluidization of set of two fly ash silos.

Conveying air requirement for both the units is met from a common bank of CA compressors. Separate Instrument Air compressors have also been provided dedicated to fly ash handling system.

System for Silo 1 & 2: The FA Silo 1 & Silo 2 shall have three outlet points. First and second outlet point shall be provided with fly ash unloading spouts for unloading of fly ash into closed tankers and also ash conditioners for moist fly ash disposal into open trucks. The third outlet is provided with just isolation valve for any emergency unloading of fly ash.
System for Silo 3 & 4: The FA Silo 3 & Silo 4 shall have four outlet points. The Fourth Outlets points are connected withHCSD system for disposal of fly ash in slurry form.

HIGH CONCENTRATION SLURRY DISPOSAL SYSTEM:

System Description
HSCD system is envisaged below FA Silo-3 & Silo-4 for disposal of Fly ash in concentrated slurry form to ash disposal area.

There are two streams of HCSD Pumps. Each HCSD pump will be connected to an independent pipeline which will convey concentrated slurry form Silo to ash disposal area / Pond area.

The high concentration fly ash slurry preparation system consists of suitable ash feeding system, loss-in-weight type weighing system, dosing screw feeder, and followed by an ash mixer.

The dosing and mixing unit would create a pre-set ash/water mixture and feed this into the Mixing Tank, where through a control loop and water addition the right rheology is created. This way the proper flowing behaviour to spread out in layers on the disposal site at an angle of repose, wherein virtually nominal amount of water release is ensured. The control loop continuously monitors the density of the slurry and automatically provides for correction, if required, through the addition of water. The HCSD pump will develop the pressure required to transfer the concentrated slurry to the other end of the pipeline for discharging into the ash pond.

There shall be two independent streams each consisting of 1 No. Mixing Tank, 1 No. HCSD Pump and 1 No. slurry disposal pipeline.