2.3.1 Design Data
The bidder shall base the design of the ash handling plant on the ash densities and production rates given below, the predicted ash distribution, the requirements of this specification, and the normal safety margins which the Bidder includes in his design of ash handling plant.
(a) Ash Densities
(b) Ash Distribution in the boiler/ Ash Collection System
* Bottom Ash - 20%
* Economiser Coarse Ash - 5%
* Air pre-heater/ duct Ash - 5%
* ESP ash - 90%
(c) As the ESP is designed to operate satisfactorily with one or more fields de-energised if it is assumed that no more than two fields will be out of service at any one time.
(d) Even when any ESP field is not in operation then also mechanical collection of ash will take place due to sudden drop in velocity experienced by ash particles. This aspect shall also be considered by Bidder while designing the system.
(e) Electrostatic Precipitator Design
Each unit is provided with four (4) nos. of Electrostatic precipitators with a total of 128 ash collection hoppers per boiler. Each ESP has eight (8) fields each having four (4) nos. collection hoppers. These ash collection hoppers are located directly beneath the fields and receive the fly ash dislodged from the collecting electrodes and emitting systems. The collecting hoppers are of inverted pyramidal shape with an outlet opening size of 400 mm x 400 mm (in side) and are designed to have a minimum valley angle of 60 deg. This valley angle is provided to ensure free flow of fly ash. Each of the first field hoppers of each ESP has a capacity of twelve (12) hour ash collection.
All other hoppers have same volumetric capacity as that of the first field hoppers.
Collection hoppers heaters are provided, to avoid condensation of water vapour on the inside surface of the collection hopper. The Bidder shall ensure a gas-tight joint at each collection hopper chute, to avoid admission of moisture into the collection hoppers, which may interfere with the free flow of ash from the ash collection hoppers, due to ash depositing on the collection hopper inner surfaces.
The rapping mechanism for the collecting electrodes has adjustable duration. The first field of the ESP which collects the maximum amount of fly ash is rapped most frequently (for example approximately 20 times per hour, that is once in every 3 minutes). The amount of ash that has been collected during the above period will be rapped off in a short duration of one minute, with the result that the ash handling system will have to cope with sudden surges in the pattern of ash collection in the collection hoppers.
When the first field is rendered in-effective due to any reason the second field which was earlier collecting ash at a lower rate, will now be collecting at an increased rate equal to that in first field. The same logic applies for the subsequent fields in series. The ash handling system shall be designed to cater for the switching out of any two fields with the consequent increase in removal rates in the remaining fields.
2.3.2 Ash Removal Rates, Slurry disposal and dry ash Storage
2.3.2.1 Ash Removal Rates
The proposals shall be based on the following ash removal rates:
(a) The bottom ash removal system employing W-type water impounded hoppers and jet pumps, the system shall be capable of removing the accumulated bottom ash of four (4) hours production at the specified rates within 1hrs. 30 min. (90 minutes) period including the time required for starting, stopping, sequential changeover from one unit to second unit and water flushing of the system for the following two conditions.
(1) Maintain water level by allowing the make up valve to remain open.
(2) Complete draw down by totally emptying the hopper of all water and ash with the make-up valve closed.
Bidders may assume that the bottom ash from each individual hopper section will be removed in parallel. Bottom ash from each boiler shall be removed in series/sequence.
(b) Economiser and air preheater ash shall be removed continuously at the collection at specified rate. The economiser and air preheater ash removal of two boilers will be done in parallel and continuously.
(c) (i) The fly ash handling plant capacity shall be based on removing the total fly ash (from ESP and duct collection hoppers) produced by each boiler continuously operating during period of 8 hrs. within a 6 hrs. period. The fly ash handling plant associated with each of the two (2) boilers. shall operate in parallel.
ii) The capacity of second stage, conveying (i.e. transportation of dry ash upto main silos outside/near the plant boundary) shall match the capacity of extraction system.
2.3.2.2 Ash Slurry Disposal Rates
The ash slurry disposal system shall pump the resultant ash slurry generated from both the bottom & fly ash handling system continuously from the slurry sump to the disposal area through pipe lines at guaranteed rate with two (2) pump series in operation and both the units working.
2.3.2.3 Dry Ash Storage
Bidder shall provide two (2) nos. dry fly ash storage silo to collect dry fly ash from the Electrostatic precipitators as and when required.
The minimum effective capacity at each fly ash storage silo shall be 1000 m³.
shivajichoudhury
