When the Coal combustion takes place in the boiler, some part of the coal remains unburnt and that un-burnt particles are called the Ash, which is approximately 26% to 40% of the total coal undergoing combustion. Basically this is nothing but the residual part of the coal. This ash will be extracted out in the form of Bottom Ash & Fly Ash according to the places of collection after the combustion.
If we calculate the ash percentage in any plant, then it will be as following:
- Total Ash = 40% of the coal combusted
- Bottom ash = 20% of the total ash
- Fly ash = 80% of the total ash
Below mentioned values are the ash consumption in a 135 MW Sub-critical Boiler in terms of Metric Tons (MT). The coal consumption in a 135 MW Sub-critical boiler is around 2000 MT (Approx.) and compared to that the ash generation will be as follows.
|
For a 135 MW Bottom Ash Disposal (in 24 Hours) |
134 MT (Approx.) |
|
For a 135 MW Fly Ash Disposal (in 24 Hours) |
537 MT
(Approx.) |
So whatever the Ash generated during the combustion process has to be sent out of the plant as it is of no use in the plant processes. Also Now-a-days the Government Norms are very strict as far as the environmental issues are concerned. So we have to handle the Ash in a proper manner in order to avoid any environmental issues. Generally most of the power plants have a tie-up with the Cement making companies and the power plants supply the Fly Ash to those Cement making companies for making cement from the Fly ash and coming to the bottom ash, which is not having any fine particles as compared to the Fly Ash. So we use it for road filling and for making Fly Ash bricks.
Ash Handling System mainly consists of the transportation of Fly ash and Bottom ash from boiler to the respective storing points and the system adopts transporting methods such as
Systems For Bottom Ash:
Bottom Ash Transportation with SSC (Submersed Scrapper Conveyor)
Bottom Ash Transportation with High Pressure Water or Jet Pumps
Systems For Fly Ash:
Fly Ash Slurry Transportation with the bottom ash
Fly Ash Dry Pneumatic Transportation
Fly Ash Handling System
In Fly Ash Handling System, the major objective is to collect and transport the fly ash from the ash hoppers of the ESP to the Fly ash silo or to the Ash Slurry making tank. Generally the power plants prefer Dry Fly ash Disposal instead of making Ash Slurry from the Fly ash and the reason is that the fly ash collected from the ESP Bottom Hopper is very fine and suitable for Cement making, if we are making slurry of it then it will not be suitable for Cement making and economically also it is very good if you are selling the Ash to the Cement Plants. So these are the reasons why most of the plants are going for Fly ash Disposal instead of Slurry Disposal (Ash Water).
Slurry Type Fly Ash Disposal System: This is one of the simplest system for ash disposal from ESP hopper to the slurry pond.High Pressure water and fly ash is mixed below the ESP hopper and below in the diagram you can see that tapping for mixing of water is provided.
(Fly Ash Slurry System)
Dry Type Fly Ash Disposal System: Fly Ash from the ESP Hoppers is collected in the Ash Vessels and from there it is transported to the Fly Ash Silos by the help of Compressed air and from the Ash Silos, the ash is transported to the Bulkers(Sealed Vessel Trucks).
Flow Path of Fly Ash Dumping
ESP Hopper: Hopper is a large conical type container used for dust or ash collection. After the field charging in ESP we go for hammering of collecting plates and the fly ash deposited on the collecting plates gets stored in the hopper. To ensure free flow of ash into the ash vessels from the hopper, the lower portions of the hoppers are provided with electric heaters. Because if the temperature of the ash falls below the ash fusion temperature then the ash will form big clusters and may choke the entire conveying system.
Dome valve: It is situated between the ESP hopper and Ash vessel; it is a special type of valve which is highly leak proof. It consists of a dome type structure with a rubber seal which is continuously getting supply from the compressed air.
Air Vent Line: To remove the trapped air from the vessel, we use vent line and due to this line air from the vessel is transported to Hopper and ash come down. It basically does two things, first of all by removing the air from the vessel, it is removing the back-pressure from the Vessel and simultaneously it is pressurizing the ash hopper.
Compressor: A compressed air station is set up in the plant. The compressed air station provides air for the pneumatic conveying system and purging of fabric filters as we already explained in ESP. After compressing the air, we have to remove all the moisture content from the air. To remove the moisture from air we use Adsorbent Air Drier (AAD) and Refrigeration Air Drier (RAD).The pressure of the compressor is depended on the system design.
Ash Vessels: Ash vessels are present just below the ESP hoppers with the Dome Valve assembly.They are supposed to contain the fly ash for a certain amount of time which will be carried to the fly ash silos.Their ash holding capacity is depended on the conveying capacity of the ash line to the Ash Silos.
(Fly Ash Vessels)
Fly Ash Silo: Fly Ash Silos store the fly ash generated by the Boiler in the maximum continuous operating conditions (BMCR).The bottom of each fly ash silo is equipped with two ash discharging chutes. One ash discharging chute is used for discharging the comprehensively used dry fly ash and the other one is connected with a wet mixer, discharging the wet fly ash. The wet ash mixer is just a back-up for the dry ash disposal system. Each fly ash silo is equipped with the bag filters and bag filter cleaning facilities with exhaust fans.
(Fly Ash Silos)
- Air extraction fan: It is used to create a negative pressure inside the vessel of the silo and the air goes out through the bag filters.
- Extractor: It is used to evacuate the Air from the Bulkers (Closed Vessel Truck), which is connected to the ash disposal chute and the discharge is connected to ash vessel.
- Diverting and dump valve: If one ash silo will not work we divert the line into another silo with the help of diverting valve and to dump the ash into ash silo dump valve is used.
Wet Fly Ash Disposal System:Up to the hopper part it is same as that of Dry Fly Ash handling system. After the hopper instead of going into the vessel, the ash gets mixed with a high Pressure water and this mixture goes to a slurry tank for further pumping. The slurry formed is further pumped through a series of pumps or a single GEVO pump for dumping in the Ash yard. For mixing of fly ash with water, a tapping is provided for High Pressure water below the hopper.
Bottom ash handling System
The bottom ash quantity is around 20% of the total Ash generation and there are mainly two types of bottom ash disposal systems.
- Dry Type Bottom Ash Disposal
- Wet Ash Slurry Disposal
Dry Type Bottom Ash Disposal: First of all we will discuss about the Dry type bottom ash disposal system.
Submerged Scrapper Conveyor: Submerged Scraper Conveyor is used for cooling and transmitting the hot bottom ash. The scraper conveyor used to send the bottom ash to the bottom ash silo. The Bottom ash hoppers are partially filled with water in order to avoid the direct impact of the Clinkers due to free fall. Whatever the ash, falling on the SSC conveyor is getting scrapped by the scrappers to the clinker grinder.
(Submersed Scrapper Conveyor)
Clinker Grinder: A clinker grinder is provided at the outlet of SCC to control the bottom ash size as in the bottom ash there will be some big clinkers and to make the ash of even size we use a clinker grinder.The output of the bottom ash after the clinker grinder is around 25mm to 50mm,it is adjustable.
Drag Conveyor: It transmits the crusher output to the bottom ash silo. It is same as that of scrapper conveyor. Just for conveying purpose for the bottom ash.
Bottom ash silo: Each boiler is equipped with a bottom ash silo. It is used for storing the bottom ash for a period of time and then it is dumped in the trucks for disposal.
Slurry Type Bottom Ash Disposal: Here the bottom ash from the 2nd pass of the Boilers goes to the Clinker Grinder in the 1st pass by the help of high pressure water and from the Clinker Grinder all the ash goes to a slurry sump for further pumping.
(Bottom Ash Slurry System)
