Stoker Firing vp5a

STOKER FIRING Presented By Naveen .E

Stoker Boiler Purpose of the Stoker •

Can fire low calorific coal and bio-mass fuels

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Using chain grate we can remove ash continuously

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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 Types COAL

• A wide range of coal types having either high or low fusion temperatures can be burned. • Coal sizing affects stoker operation • The 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

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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 •

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Purpose of grate • To burn high ash coal • Baggase & Bio-mass fuels Selection of grate • Depends on coal properties and it’s performance

Fuel Feeders To 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 distribution The 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 ission, mainly two types: Traveling grate stoker Spreader stoker.

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

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TRAVELING-GRATE STOKER BOILER Coal is burnt on moving steel grate Coal gate controls coal feeding rate Uniform coal size for complete combustion

CHAIN AND TRAVELLING GRATE STOKER

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

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The chain travels over two sprockets

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

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

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

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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

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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 Grate 

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

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

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

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

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

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

Coal Combustion

Refuse Combustion

STOKER FIRING : ADVANTAGES  Generally  Can

less building space is necessary.

be used for small or large boiler units.

 Very

reliable and maintenance charges are reasonably low.

 Cheaper

grade of fuel is used.

 Minimum  Less

auxiliary plants.

smoke produced.

 Higher

efficiency attained.

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

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Construction is complicated.

Sudden variations in the steam demand cannot be met.

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Bowing and clinkering of water walls are experienced.

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Thank You….