03-01-2013, 09:58 AM
ASH HANDLING SYSTEM
1ASH HANDLING.docx (Size: 2.8 MB / Downloads: 70)
INTRODUCTION
Ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal. Fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys of coal-fired power plants, and together with bottom ash removed from the bottom of the furnace is in this case jointly known as coal ash. Depending upon the source and makeup of the coal being burned, the components of fly ash vary considerably, but all fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO), both being endemic ingredients in many coal-bearing rock strata.
What is Ash Handling?
Ash handling refers to the method of collection ,conveying, interim storage and load out of various types of ash residue left over from solid fuel combustion processes .The most common types of ash include bottom ash, bed ash, fly ash and ash clinkers resulting from the combustion of coal, wood and other solid fuels. Ash handling systems may employ pneumatic ash conveying or mechanical ash conveyors. A typical pneumatic ash handling system will employ vacuum pneumatic ash collection and ash conveying from several ash pick up stations-with delivery to an ash storage silo for interim holding prior to load out and transport. Pressurized pneumatic ash conveying may also be employed. Coarse ash material such as bottom ash is most often crushed in clinker grinders (crushers) prior to being transported in the ash conveyor system .Very finely sized fly ash often accounts for the major portion of the material conveyed in an ash handling system. It is collected from bag house.
Ash Handling Plant (AHP):-
The ash produced on the combustion of coal is collected by ESP. This ash is now required to be disposed off. This purpose of ash disposal is solved by Ash Handling Plant (AHP). There are basically 2 types of ash handling processes undertaken by AHP:
(1)Dry ash system
(2)Ash slurry system
Dry ash system
Dry ash is required in cement factories as it can be directly added to cement. Hence the dry ash collected in the ESP hopper is directly disposed to silos using pressure pumps. The dry ash from these silos is transported to the required destination.
Ash slurry system
Ash from boiler is transported to ash dump areas by means of sluicing type hydraulic system which consists of two types of systems:
• Bottom ash system
• Ash water system
Bottom ash system
In this system, the ash slag discharged from the furnace is collected in water impounded scraper installed below bottom ash hopper. The ash collected is transported to clinkers by chain conveyors .The clinker grinders churn ash which is then mixed with water to form slurry.
Ash water system
In this system, the ash collected in ESP hopper is passed to flushing system. Here low pressure water is applied through nozzle directing tangentially to the section of pipe to create turbulence and proper mixing of ash with water to form slurry. Slurry formed in above processes is transported to ash slurry sump. Here extra water is added to slurry if required and then is pumped to the dump area.
DIFFERENT TYPES OF ASH HANDLING SYSTEM
TRADITIONAL TECHNOLOGY (TO DISPOSE OFF LARGE AMOUNT OF ASH EFFICIENTLY)
HYDRO-SLUICING:-
• Water impounded bottom ash hoppers with jet pump systems followed by lean phase slurry pump system up to ash dyke.
• Dry type Bottom Ash Hopper with submerged scraper chain conveyor system followed by lean phase slurry pumps system up to ash dyke.
• Flushing Apparatus for Fly Ash
PNEUMATIC HANDLING:-
•Negative pressure type
•Lean Phase
1. Negative pressure (vacuum) type operate essentially by Lean Phase fluidizing method. There is a material handling valve below each hopper to segregate the hopper from transport line. The water operated hydro vector or mechanical vacuum pump mounted on the top of the silo exhaust air from the transport line and the resultant rise in vacuum operates high vacuum switch. Solenoid valve is energized and material handling and associated branch segregating valves are opened. Ash in the hoppers is fed into the vacuum lines and is conveyed to a fabric filter which separates the ash from the air. The ash is discharged into the storage silo and air is drawn through the vacuum pump via a line filter and exhausted to atmosphere. This system operates on the lean phase principle with a high air to ash ratio and high conveying velocities.
2. Lean phase systems rely on fluidization of the material in the air stream to achieve conveying of material. The material particles are carried by a high velocity air stream. The required velocity is dependent mainly upon the size and density of the material particles. In Lean Phase system, the velocity is in the range of 15 m/sec to 30 m/sec. And in Medium Phase, it is in the range of 10-20 m/sec. The material/air ratio in the lean phase is in the range of 10:1 and in medium phase, the range is 20:1.