Overfeed stokers and its Types

• The principle of operation of overfeed stoker is represented in Figure A.

• In this case, the coal is fed on the grate and the primary air is supplied from the bottom to initiate combustion.
• This primary air is heated up and cools the grate while passing over it. For complete burning of gases, the secondary air is admitted above the level of fuel bed.
• The region above the ash contains the mixture of incandescent (glowing) coke where the combustion reactions take place. Oxygen reacts with carbon to form CO. Water vapour entering with air reacts with incandescent coke to form CO₂, CO and free H₂,.
• Upon further travel through the incandescent region, some of this Co, reacts with coke to form CO. Therefore, the gases leaving the incandescent region consist of N₂ of air, CO, CO₂, H₂, and no oxygen is present.
• The raw coal is continuously supplied on the surface of the bed. Here it looses the volatile matter by distillation.
• Heat required for the distillation and burning of coal comes from the burning of incandescent coke below the fresh fuel, hot gases diffusing through the surface of bed, by radiation from flames and the hot gases.
• The ignition zone lies directly below the raw fuel undergoing distillation. The gases leaving the upper surface of fuel bed are volatile matter, CO, CO₂, N₂, and H. To burn the combustible gases, secondary air is supplied above the raw fuel at high speed to create turbulence.
• It helps in completing the combustion at rapid rate. During combustion, the incandescent coal continuously looses its carbon and only the ash remains which falls through the grate into the ash pit.

Types of Overfeed Stokers :

Following are the types of overfeed stokers :

1. Travelling grate stoker

  (i) Chain grate stoker

  (ii) Bar grate type

2. Spreader stoker.

These stokers are described below.

(i) Chain Grate Stoker :

Schematic diagram of a chain grate stoker is shown in Figure B. A chain grate stoker consists of an endless chain which travels over the sprocket wheels as shown in Figure B. The coal hopper supplies the coal under gravity at its front end and the stoker takes it to the furnace. The speed of the chain can be changed according to the combustion requirements.

• Primary air is supplied through the air inlets situated below the grate. The secondary air also called as over fire air is supplied from the airports made in the furnace wall for combustion of gases.
• In order to control the depth of fuel bed on stokers, a hand adjustable gate is provided in the hooper. The speed of the stoker can be varied between 15 cm to 30 cm per minute according to combustion requirements. The variable speed is achieved by connecting the front sprocket to a variable speed drive mechanism.

1. It is simple in construction and has low cost.
2. It is self cleaning stoker.
3. Maintenance cost is low.
4. It is reliable.
5. The rate of heat release can be controlled by controlling the speed of stoker.
6. It gives high heat rate of energy release per unit volume of furnace.
7. Ash crushers are not required. 

1. The temperature of preheated air is limited to 180°C.
2. There is a loss of fine coal which drops along with ash.
3. Quantity of coal carried on the grate is small due to which it cannot be used for high capacity boilers (200 tonnes tonnes/hr or more). 
4. The clinker troubles are very common.

2. Spreader Stoker :

• A schematic diagram of a spreader stoker is shown in Figure C. 
• Coal from hooper is fed on to a feeder which is a rotating drum with vanes on it. The speed of the feeder can be changed as per the load on the plant. 
• From feeder the coal falls on to the spreader which is used to throw the coal uniformly on the grate.

• Primary air is supplied from the bottom of the grate. 
• A part of air is used to burn the fuel on the grate and remainder is used to burn the volatile matter.
• The secondary air is supplied by the nozzles at high velocity in the furnace over the fuel bed. 
• Secondary air creates turbulence which helps in completing the combustion process.
• The unburnt coal and ash are deposited on the grate which is removed periodically, Spreader stokers can burn variety of coal with high rate of combustion efficiency.
• These can be used to generate steam in boiler from 80 to 150 tonnes per hour.

1. A wide variety of coal can be burnt.
2. Clinker difficulties are reduced even with the coal having high clinkering tendency.
3. Coking tendency of coal is reduced.
4.  Preheated air with high temperatures can be used.
5. Volatile matter is easily burnt.
6. It is simple to operate with low operation cost.
7. It can response quickly with load changes on power plant since it has a small depth of bed on grate at any time.

Disadvantages of Spreader Stoker :

1. It is difficult to operate spreader with variable sizes of coal and varying moisture content. It can only use coal of sizes 40 mm and below.
2. Fly ash is more, therefore a dust collector is necessary.
3. Clinker troubles cannot be avoided.