Underfeed Stokers and its Types

Underfeed Stokers and its Types

• The principle of operation of underfeed stokers is shown in Figure A.

Figure A

• In case of underfeed stokers the air and fuel move in the same direction . Air is supplied through the holes in the grate and it comes in contact with raw coal.
• Then it passes through the layer of incandescent coke.
• The gases produced due to combustion pass through the layer of ash and then it meets with the secondary air for complete combustion. These are suitable for burning the bituminous coals.
• Typical underfeed stoker firing as shown in Figure receives fresh coal from the retorts between two tuyers. Ignition plane lies between the raw or green coal and incandescent coke.

Types of Underfeed Stokers :

These are basically two types as follows :

1. Single retort underfeed stokers
2. Multi-retort underfeed stokers

1. Single Retort Type Underfeed Stokers :

• The cross-section of a single retort underfeed stoker is shown in Figure B.
• The grate is usually inclined. The fuel placed in large hoopers on the front of furnace is fed by reciprocating ram conveyor on to the grate (not shown).
• The air required for combustion is supplied through tuyers situated along the upper edge of the grate, as shown in Figure B.

Figure B

• The ash from the grate falls and it is collected in the ash pit. Some of the unburned coal may also pass through the grate. Fusion of ash or clinker formation may take place due to uneven distribution of air.

2. Multi-Retort Underfeed Stoker :

• The schematic diagram of a multi-retort underfeed stoker is shown in Figure C.

Figure C

• It consists of a number of underfeed retorts usually 3 to 16 in numbers. Each retort is fitted with pushers connected with reciprocating ram for feeding. 
• The coal falling under gravity from hooper is pushed forward during inward stroke of the ram and then the distributing rams (pushers) slowly move the entire coal bed down the length of stoker. 
• Tuyers and air passages are provided between the retorts. 
• Primary air from the main wind box enters the fuel bed from bottom. Partially burnt coal moves on to the extension grate. Also, the low pressure air enters on the thinner fuel bed on the extension grate. 
• These stokers give high combustion efficiency at large variations of load. The slope of stokers provided and due to fuel bed movement, the formation of clinkers is minimised. These stokers can burn large variety of coals efficiently except coals with high ash content.

Advantages of Multi-Retort Underfeed Stoker :

1. It can use directly the coal from mines. However, in some cases the crushing of coal is required according to the design of furnace. 
2. It has high combustion efficiency compared to chain grate type stoker.
3. It can be used for medium sized power plants. 
4. Space requirement is less compared to pulverized coal system.
5. Variation in load can be met.
6. Overload capacity of boiler is high since a large amount of coal is stored on the grate. 
7. Dust collection problem is less severe compared to pulverized coal system. 
8. It is free from danger of explosion.

Disadvantages of Multi-Retort Underfeed Stoker :

1. It has high initial cost but it is less than pulverized coal system . 
2. Wear and tear is high to moving parts. 
3. Low grade fuels cannot be burnt efficiently.
4. Clinker formation cannot be avoided.
5. The loss of coal takes place. It reduces the heating capacity of the system.