Coal Burning System

Coal Burning System: In case of coal fired plants, following factors affects the combustion: Fuel Size Rate of firing Supply of air Moisture content in the coal

Coal Burning System: Depending on the type of equipment, boilers may be classified as: Solid Fuels fired (a) Hand fired (b) Stoker fired Overfeed Stokers Underfeed Stokers (c) Pulverised (Powdered) fuel fired Unit System Central or Bin system Combination of (a) and (b) Liquid Fuel fired (a) Injection System (b) Evaporation System (c) Combination of (a) & (b) Gaseous Fuel fired (a) Atmospheric pressure system (b) High pressure system

Hand Firing System Suitable for small power plants Low combustion efficiency Grate is used to support the fuels and the openings are provided for air to enter on the grate for the combustion of fuel.

Stoker Firing System A stoker is a power operated fuel feeding mechanism and grate. The term stoker implies a boiler that automatically feeds (or " stokes”) the boiler. Stoker coal size is typically 1.25 inches.

Stoker Firing System Advantages: Low grade fuel can be used. High efficiency Smokeless combustion. Less number of labours required so labour cost reduces. System reliable. Maintenance charges are reasonably low. Capital investment relative to pulverised fuel system is less.

Stoker Firing System Disadvantages: Construction is complicated. For very large units the initial investment may be higher than the pulverised fuel system. Loss of coal through the grates. Excessive wear of moving parts due to abrasive action of coal.

Overfeed Stokers Mechanism In Overfeed Stokers the coal is fed into the grate above the point of air admission.

Types of Overfeed Stokers Travelling Grate Stoker Chain Grate Stoker Bar Grate type Stoker Spreader Stoker

Chain Grate Stoker

Chain Grate Stoker Traveling or chain grate stokers feed coal out onto a rotating metal belt that supports the fire. Coal is fed from a hopper. Grate speed is automatically controlled to maintain desired steam pressure. Burning progresses as the belt moves from front to back of furnace. Combustion is essentially complete at the back end of belt, and ash is dumped off into an ash pit there.

Chain Grate Stoker Advantages: Simple construction Low cost Self cleaning stoker Maintenance cost is low It is reliable Heat release rate can be controlled by speed of stoker High heat rate release per unit volume of furnace.

Chain Grate Stoker Disadvantages: Temperature of preheated air is limited to 180 0C. Loss of final coal which drops along with ash. Quantity of coal carried on grate is small due to which it cannot be used for high capacity boilers (200000 kg/hr). Clinker formation are common.

Spreader Stoker

Spreader Stoker

Spreader Stoker Advantages: Wide variety of coal can be burnt. Clinker difficulties are reduced even with the coal having high clinkering tendency. Preheated air with high temperature can be used. Volatile matter is easily burnt. It is simple to operate with low operation cost. It can response quickly with load changes on power plant since it has a small depth of bed on grate at any time.

Spreader Stoker Disadvantages: Difficulty in operating spreader with variable sizes of coal and varying moisture content. Fly ash is more, so dust collector is necessary. Clinker troubles cannot be avoided.

Water cooled vibrating grate Boiler

Water cooled vibrating grate Boiler Vibrating grate stoker is similar to a traveling grate, except that instead of being on a continuous loop, grate sections are sloped downward and periodically vibrate to cause fuel particle movement from front to back. Vibration frequency is controlled to obtain desired steam pressure/ heat output.

Underfeed Stokers Mechanism In Underfeed Stokers the coal is admitted into the furnace below the point of air admission. Actually both air and fuel entering in the same direction. These are suitable for burning Bituminous coal.

Types of Underfeed Stokers Single retort underfeed Stoker Multi retort underfeed Stoker

Multi retort Underfeed Stokers

Multi retort Underfeed Stokers Advantages: It can use the coal directly from mines. However, in some cases coal crushing is required as per the furnace design. High combustion efficiency compared to chain grate stoker. Less space requirement compared to pulverised fuel firing. Self cleaning grate. Grate, bars, tuyeres and retorts are not subjected to high temperature as they remains always in contact with fresh coal. It can be used with all refractory furnace because of non exposure of stoker mechanism to the furnace. Dust collection problem is less compared to pulverised coal system. Variation in load can be met.

Multi retort Underfeed Stokers Disadvantages: It has high initial cost but less than pulverised coal system. High wear and tear due to moving parts. Low grade fuel cannot be burnt efficiently. Loss of coal may take place. It reduces the heating capacity of the system. Clinker formation cannot be avoided. Fly-ash is much more.

Pulverised Fuel Firing System Pulverised system is generally used now a days for big power plant whose capacity exceeds 100 MW. In which, coal size reduced to fine powder with help of grinding mill. These powder is than projected into the combustion chamber with the help of hot air current. Air required (i.e. secondary air) for the combustion is supplied separately. Air required to carry the coal, to dry it before entering into combustion chamber and for creating the turbulence is called as Primary air.

Pulverised Fuel Firing System Efficiency of a Pulverised fuel firing system depends on the size of powder. The fineness of the coal should be such that 70 % of the coal would pass through a 200 mesh sieve and 90 % through 50 mesh sieve.

Pulverised Fuel Firing System Advantages: Any type of coal can be used since it is used in powdered form. High combustion efficiency and high rate of steam generation. It can generate 200000 kg/he of steam. Free from clinker and slag formation. Response to variation in load is fast. No ash handling problem. Furnace volume required is considerably less. It is also possible to use highly preheated air (350 0C) which helps for rapid flame propagation. No moving parts in the furnace subjected to high temperature. System can work with slurry type of fuels. Greater surface area per unit mass of coal allows faster combustion. It also reduces the air requirement.

Pulverised Fuel Firing System Disadvantages: Capital cost is high compared to stoker firing. Operation cost is high. Special and costly equipment are required for removing fine dust called as fly ash. Furnace temperature is high so it needs cooling arrangement. Maintenance cost is high. Skilled labours are required. High air pollution due to emission of fine dust particles. Danger of explosion because of powder coal burns like a gas. Special starting equipment are required. Separate coal preparation plant is required. Achieving fine powder and distribute it to the burners is usually difficult task.