Pulverized coal firing system is the most widely used method for firing coal in modern utility boilers of coal based thermal power plant. In this system, coal is ground into fine powder, mixed with air and fired into the furnace through specially designed burners. Because of the small particle size, pulverized coal burns rapidly and efficiently, delivering high boiler performance.

Components Integrated in a Pulverized Coal Firing System in Boiler

The pulverized coal firing system in boilers integrates:

• Coal bunkers (raw coal storage)
• Rotary feeders
• Coal Pulverizing mills (Ball Mill / Bowl Mill)
• Primary air system
• Pulverized Fuel (PF) piping and ejectors
• Burner nozzles
• Secondary & tertiary air
• Fine coal burners
• Heavy oil startup system

This combination ensures efficient, reliable and clean combustion in thermal power plants.

Coal Bunkers (Raw Coal Storage)

Coal received from the track hopper or wagon tippler is crushed to 20–30 mm size and stored in the coal bunkers.

Rotary feeders

Coal is discharged from the bunker through a rotary/compound feeder. Its function is to control the flow of raw coal into the pulverizing mill.

Coal Pulverizing Mills

The crushed coal from the bunker is fed to the pulverizing mills, such as:

• Ball mills
• Bowl mills (or vertical spindle mills)

Inside the mill:

• Coal is ground to fine powder (typically 70–80% passing 200 mesh)
• Mixed with hot primary air

• Hot primary air dries the coal & Carries the pulverized coal out of the mill toward the burners through pipes

Primary Air

Primary air is the first stage of air used in a pulverized coal firing system. It is heated and supplied by the Primary Air Fan (PA fan) to dry the coal inside the mill and to carry the pulverized fuel from the mill to the burners. Primary air also helps maintain proper PF velocity, improves flame stability, and supports efficient combustion.

Key Functions of Primary Air

• Dries raw coal inside the mill.
• Transports pulverized coal to the burners.
• Provides 15–25% of the total combustion air.
• Maintains correct PA-to-coal ratio.
• Ensures proper PF velocity in the pipes.
• Supports stable flame in the furnace.
• Prevents mill choking and pipe blockage.
• Reduces unburnt carbon by improving combustion.
• Helps achieve smooth load changes in the boiler.

Pulverized Fuel (PF) Piping and Ejectors

Pulverized Fuel (PF) piping and ejectors form the transport system that carries finely ground coal from the mill to the burners. PF pipes maintain the required velocity to prevent coal settling, while ejectors (venturi tubes) improve mixing of coal with air before combustion. Together, they ensure uniform coal distribution, stable firing, and safe operation of the boiler.

Key Functions of PF Piping and Ejectors

• Transport pulverized coal from mill outlet to burners.
• Maintain high velocity to prevent PF deposition or choking.
• Ensure uniform coal distribution to all burner lines.
• Mix PF with additional air in the ejector/venturi section.
• Improve the stability and intensity of the flame.
• Reduce pipe erosion by keeping velocity in controlled range.
• Provide correct pressure and flow for steady combustion.
• Prevent backflow or reverse flow in burner lines.
• Support reliable ignition and consistent heat release.

Burner Nozzles

Burner nozzles are the final point where the pulverized coal–air mixture enters the furnace. They control the direction, velocity, and shape of the flame. Proper burner nozzle design ensures stable ignition, complete combustion, and efficient heat transfer inside the boiler. Burner nozzles also play a key role in reducing emissions and maintaining safe furnace operation.

Key Functions of Burner Nozzles

• Inject pulverized coal and air mixture into the furnace.
• Control flame shape, length, and direction.
• Maintain correct PF velocity for stable combustion.
• Create proper swirl and mixing for efficient burning.
• Ensure uniform heat distribution across the furnace.
• Support safe ignition of pulverized coal.
• Reduce unburnt carbon by improving combustion quality.
• Help in NOx control through staged combustion.
• Prevent flame impingement on water walls.

Secondary Air

Secondary air is the main air used for combustion in a pulverized coal boiler. It enters through nozzles around the burners and mixes with the PF–primary air mixture in the furnace. Secondary air shapes the flame, completes combustion of volatile matter, and ensures stable, efficient burning.

Key Functions of Secondary Air

• Supplies major portion of combustion air.
• Mixes with PF and primary air at the burner.
• Shapes and stabilizes the flame.
• Helps complete the combustion of volatiles.
• Controls flame length and furnace temperature.
• Supports NOx reduction when used in staged firing.
• Improves heat transfer to furnace walls.
• Ensures cleaner and more efficient combustion.

Tertiary Air

Tertiary air is supplied below the furnace arch to improve mixing of fuel and air inside the combustion chamber. It increases turbulence, reduces unburnt carbon, and helps achieve staged combustion, resulting in better efficiency and lower emissions—especially for large utility boilers.

Key Functions of Tertiary Air

• Enhances fuel–air mixing in the furnace.
• Increases turbulence for efficient burning.
• Reduces unburnt carbon in the bottom ash.
• Supports staged combustion for lower NOx.
• Helps maintain flame shape at higher loads.
• Improves burnout time of coal particles.
• Balances air distribution between front and rear arches.
• Contributes to stable furnace temperature.

Extremely Fine Coal Burner System

Each bunker has one rotary feeder connected to the extra-fine coal bin. This bin collects ultra-fine coal from the pulverizing system’s gas filter.

This superfine coal is fired through special burners to:

• Support partial load operation
• Achieve quick ignition due to high fineness
• Maintain a hot temperature zone for stable combustion

At high load, wall-mounted burners distribute the flame more widely for better NOx control.

Heavy Oil Firing System (For Startup & Stabilization)

Although coal is the main fuel, boilers require Heavy Fuel Oil (HFO) for:

• Startup
• Furnace warm-up
• Secondary ignition
• Stabilizing the flame when coal feed is low

Ignition Sequence

1. High-energy electric ignitor sparks
2. Oil gun atomizes HFO using steam
3. Stable oil flame is formed
4. Pulverized coal flame is ignited from the oil flame

Global Advantages of Pulverized Coal Firing System

• High combustion efficiency
• Faster boiler response to load
• Uniform flame distribution
• Lower unburnt carbon
• Compatible with low-NOx and staged combustion
• Works with different grades of coal
• Lower operating cost per MW

For these reasons, PF firing is the dominant technology in utility boilers worldwide