STOKER FIRING Presented By Naveen .E

Stoker Boiler Purpose of the Stoker

Can fire low calorific coal and bio-mass fuels

Using chain grate we can remove ash continuously

Operate in balanced draft

Difference Between Stoker boiler to other boilers Combustion(Stoker) Bed Combustion Air Supply System Fuel Preparation Fuel Feeder Ash Content Ash Removal

Fuel TypesCOAL A wide range of coal types having either high or low fusion temperatures can be burned. Coal sizing affects stoker operationThe normal size of the coal particles are 3/4" Using spreader feeders we can adjust coal sizing.When coal fines is smaller than 16 mesh (0.10"), it increases fly ash. But using precipitators or bag houses can handle the fly ash from spreader stokers

Refuse Fuels

A spreader stoker is an excellent combustor of cellulose waste such as:Wood(Shredded Trees to Sawdust)Garbage (Refuse Derived Fuel)Bagasse (Sugar Cane Residue)Industrial Residue (Paper, Plastics, Wood)

GRATE Purpose of grateTo burn high ash coalBaggase & Bio-mass fuels

Selection of grateDepends on coal properties and its performance

Fuel FeedersTo approach even energy release, it is necessary to have fuel feeder/distributors which will evenly feed the fuel over the entire grate surface.These feeder/distributors can be mechanical, pneumatic or a combination of both. They must be placed across the width of the front of the stoker in sufficient quantity to achieve even distributionThe fuel feeder/distributors plays a major role to operate at lowest emissions and highest combustion efficiency.

CLASSIFICATION OF STOKER FEEDERS OVERFEED STOKERS: The coal is fed into the grate above the point of air admission, mainly two types:Traveling grate stoker Spreader stoker.

UNDERFEED STOKERS: The coal is fed into the grate below the point of air admissions, and suitable for non-clinkering, high volatile coals.

TRAVELING-GRATE STOKER BOILERCoal is burnt on moving steel grateCoal gate controls coal feeding rateUniform coal size for complete combustion

CHAIN AND TRAVELLING GRATE STOKER

It consists of an endless chain which supports the fuel bed.

The chain travels over two sprockets

Coal is fed on one end of the chain grate through coal hopper.

The air required for combustion enters through the air inlets situated below the grate

Coal must be uniform in size otherwise large lumps will not burn completely.

Stokers are used for burning high volatile ash coals.

SPREADER STOKER

SPREADER STOKERS

Coal is first burnt in suspension over the coal bed and spreading the coal in entire bed area

Flexibility to meet load fluctuations

Mostly used in many industrial applications

Types of Feeder: Chain Type-OT

The motor can be either DC utilizing an SCR control system or AC utilizing a variable frequency device. The overthrow rotor which is used to distribute the coal in the furnace from the feeder is driven by a rotor drive through a variable speed drive. This variable speed drive is manually controlled and is used to adjust the longitudinal distribution of coal over the grates. The functions of the metering drive and the rotor drive are not interrelated. The function of the metering drive is to deliver a regulated supply of fuel to the furnace in accordance to boiler load while that of the rotor drive is to maintain good distribution of the fuel over the grates.

Chain Type - UT

The difference between the two is the method of distributing the coal in the furnace.

Rotor speed is not enough when the percent of coal smaller than 1/4" is very high.

Pneumatic assist has been found to be helpful in distributing fine coal to the rear of the furnace without excessive rotor speeds.

Air Swept Spout

Both mechanical rotor devices and air swept spout type devices have been utilized to feed refuse fuels into a combustion chamber.Today the air swept spout is used almost universally for this purpose. They are lower maintenance with higher availability than the mechanical distributor. Good distribution of the fuel over the grates can be achieved.

Vibrating GrateVibrating grate application has been for small and medium sized spreader stoker fired boilers.

A timing device creates dwell time and vibrating time changing with boiler load.

Units having more than one module in width are vibrated separately rather than in unison.

Refuse such as wood or bagasse is burned successfully on an air cooled vibrating grate stoker.

The low ash in wood or bagasse means that the grate needs to vibrate infrequently.

The fuel bed is quiescent without slag so the vibrating action readily moves the ash.

Coal Combustion Refuse Combustion

STOKER FIRING : ADVANTAGESGenerally less building space is necessary.

Can be used for small or large boiler units.

Very reliable and maintenance charges are reasonably low.

Cheaper grade of fuel is used.

Minimum auxiliary plants.

Less smoke produced.

Higher efficiency attained.

STOKER FIRING : DISADVANTAGE With very large units, initial cost may be higher than pulverized fuel firing system.

Construction is complicated.

Sudden variations in the steam demand cannot be met.

Bowing and clinkering of water walls are experienced.

Thank You.

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