Pulverised Coal Burner Nozzle

In pulverised‑coal combustion systems for thermal power generation, cement rotary kilns, metallurgy, and the chemical industry, the pulverised coal burner nozzle is the core actuating component of the entire burner. It is responsible for mixing pulverised coal with air in a specific ratio and injecting the mixture precisely into the furnace, where ignition and combustion take place. The performance of the nozzle directly determines coal burnout rate, flame shape, NOx emission levels, and the service life of the burner.

In simple terms, the pulverised coal burner nozzle is a “flame controller” that maintains precise flow channels and stable combustion under extreme conditions of high‑temperature flame radiation, high‑velocity coal‑particle erosion, alkali‑sulphur corrosion, and frequent thermal shock. This seemingly simple “nozzle” is in fact a systematic engineering integration of heat‑resistant materials science, precision casting, and aerodynamic design.

1. Core Functions and Operating Conditions of the Nozzle

Core Functions

1. Precise injection and mixing

The nozzle injects the mixture of pulverised coal and primary air into the furnace at a specific velocity and angle, ensuring thorough mixing with secondary and tertiary air to create conditions for efficient combustion.

2. Flow‑field organisation and flame stabilisation

Through structural features such as convergent‑divergent outlets and swirl vanes, the nozzle creates a high‑temperature recirculation zone near the outlet, providing a continuous heat source for timely ignition of the coal particles and ensuring flame stability.

3. Flame‑shape control

The structural parameters of the nozzle (such as swirl angle and outlet geometry) directly determine the flame length, diameter, and stiffness, allowing adaptation to different kiln types and process requirements.

Severe Service Environment

The operating environment of a pulverised coal burner nozzle is among the most extreme in industrial equipment:

• High temperature: Main nozzle continuous service temperature 1050–1150°C, with short‑term tolerance up to 1200°C.
• Severe erosion: High‑velocity coal‑air flow (≤30 m/s) continuously scours the nozzle, causing significant wear.
• Heavy corrosion: Sulphur, chlorine, and other components in the flue gas chemically attack the nozzle material.
• Frequent thermal shock: Repeated heating and cooling during unit start‑up, shutdown, and load changes.

2. Common Failure Modes of the Nozzle

1. High‑Temperature Oxidation and Burn‑Through

At temperatures above 1100°C, materials that are not adequately heat‑resistant will rapidly oxidise, burn through, or even melt and deform. Excessive wall temperature at the nozzle outlet can also cause slagging, blocking the flow passages.

2. Erosive Wear from Coal Particles

The high‑velocity coal‑air flow causes continuous erosive wear on the inner walls and outlet edges of the nozzle. Wear is most severe on the side with higher coal concentration, and over time, distinct erosion grooves form at the bottom of the nozzle outlet.

3. Thermal‑Fatigue Cracking

Frequent start‑up, shutdown, and load changes subject the nozzle to repeated severe temperature fluctuations. When the accumulated thermal stress exceeds the material’s fatigue limit, thermal‑fatigue cracks develop, which in severe cases can lead to spalling or fracture of the nozzle.

4. Deformation and Failure

At high temperatures, material strength decreases. Combined with thermal and mechanical stresses, the nozzle may undergo plastic deformation, altering the flow‑channel geometry and reducing air‑distribution accuracy, ultimately compromising combustion stability.

3. Material Selection for the Nozzle

Material selection is the primary decision in nozzle design. Temperature, wear, and corrosion levels vary greatly across different nozzle parts, so the material must be chosen according to the specific location and conditions.

Main Material Grades

Material Grade	Continuous Service Temp.	Key Advantages	Typical Applications
ZG40Cr25Ni20Si2 (2520)	1050–1150°C	Oxidation resistance, coking resistance, strong erosion resistance	Main burner nozzles, burner tips in utility boilers
ZG35Cr24Ni7SiNRe	1000–1100°C	Thermal‑shock resistance, creep resistance, cost‑effective	Secondary‑air nozzles, flow‑guide stabilisers, frequent start‑stop conditions
ZG40Cr9Si2	850–950°C	Wear resistance, low cost, easy machinability	Auxiliary nozzles, low‑temperature air‑duct nozzles
Ceramic composites (SiC/Al₂O₃)	1100–1300°C	Wear resistance, high‑temperature resistance	High‑wear locations, ultra‑supercritical units
ZG40Cr28Ni48W5Si2 (2848W5)	1200°C+	Ultra‑high‑temperature wear resistance	Extremely severe positions at kiln inlets

Cr25Ni20 or Cr25Ni20Si2 is widely recognised internationally as a high‑quality heat‑resistant steel, with a documented service temperature of 1200°C. This material has a high nickel content and is expensive – nickel‑iron prices once reached RMB 450,000/tonne and, though they have come down, remain above RMB 280,000/tonne. Consequently, some manufacturers substitute cheaper steels to reduce costs, leading to significantly shorter nozzle service life.

Rare‑earth high‑chromium‑nitrogen heat‑resistant steels are a newer class of materials that not only offer good high‑temperature oxidation resistance, deformation resistance, and erosion resistance to coal‑air flows, but also possess good weldability and machinability.

Three‑Step Material Selection Guide

Step 1: Determine the operating temperature

Temperature Range	Recommended Material
850–950°C	ZG40Cr9Si2 (auxiliary nozzles)
950–1050°C	ZG35Cr24Ni7SiNRe (medium‑temperature secondary nozzles)
1050–1150°C	ZG40Cr25Ni20Si2 (high‑temperature main nozzles)
1100–1300°C	Ceramic composites (SiC/Al₂O₃) or 2848W5

Step 2: Evaluate the operating conditions

• Frequent start‑stop (frequent thermal shock) → Prioritise ZG35Cr24Ni7SiNRe (excellent thermal‑shock resistance).
• Severe erosive wear → Prioritise ceramic composites or 2848W5.
• Severe high‑temperature oxidation → Prioritise ZG40Cr25Ni20Si2 (2520) (oxidation and coking resistance).

Step 3: Balance performance and cost

2520 offers excellent performance but at high cost; ZG35Cr24Ni7SiNRe stands out for cost‑effectiveness; ceramic composites offer top‑tier performance but involve complex manufacturing.

4. Structural Design and Classification of the Nozzle

Classification by Air‑Duct Configuration

In multi‑channel pulverised‑coal burner nozzles, the air‑duct arrangement from outside to inside generally follows one of two patterns:

Straight air → Swirl air → Coal air → Central air (represented by the Rotaflam four‑channel burner from Pillard, France).

The nozzles of each air duct are arranged concentrically, and the flame shape and length are controlled by adjusting the air‑flow ratio of each duct.

Classification by Nozzle Function

Nozzle Type	Function	Typical Application
Primary‑air nozzle	Conveys coal and provides ignition oxygen	Core flame‑stabilising component
Secondary‑air nozzle	Supplies supplementary combustion air, enhances mixing	Improves combustion efficiency
Swirl / guide nozzle	Organises swirling flow, stabilises flame	Reduces NOx emissions
Central‑air nozzle	Adjusts flame centre temperature	Multi‑channel burners

Key Structural Designs

Convergent‑divergent double‑bevel guide structure: The nozzle section features a double‑bevel guide structure with alternating convergent and divergent guide vanes arranged circumferentially. When the coal‑air mixture is discharged, it mixes intensively with the external hot flue gas, ensuring strong ignition and flame‑stabilisation performance.

Composite nozzle: The front end of the nozzle uses high‑temperature‑ and wear‑resistant ceramic materials, while the connecting section uses conventional metal materials. A high‑temperature step is formed at the interface between the two materials, preventing burn‑through of the metal section. This design allows the burner to operate at lower cooling‑air flow when out of service, which is beneficial for NOx control.

Removable nozzle: Consists of an outer nozzle and an inner nozzle, with the inner nozzle detachably connected via slides and positioning blocks. This design allows rapid replacement of worn parts, reducing maintenance costs.

5. Manufacturing Processes

Core Process Flow

The manufacture of high‑quality pulverised‑coal burner nozzles typically follows this sequence:

1. Silica‑sol investment casting (lost‑wax process)

Using high‑temperature alloy steel (such as Cr25Ni20) as the raw material, the nozzle blank is produced by silica‑sol investment casting. This process ensures high dimensional accuracy, a smooth surface finish, and a dense structure free of porosity and blowholes.

2. Flow‑channel precision machining

After blank forming, the flow channels are machined using five‑axis CNC machines to achieve an inner‑wall roughness of ≤Ra1.6 μm, preventing coal particles from depositing inside the channels.

3. Hardfacing reinforcement

The inlet and outlet ends of the nozzle are hardfaced with a chromium‑carbide alloy layer (5‑8 mm thick), significantly improving wear resistance and extending service life. The bluff‑body windward surfaces and support rib plates can also be hardfaced with high‑chromium‑molybdenum wear‑resistant alloy or protected with ceramic tiles, with a hardfacing hardness of HRC ≥60.

4. High‑temperature solution heat treatment

After casting, the nozzle undergoes high‑temperature solution heat treatment to eliminate casting stresses and stabilise the internal structure, ensuring no deformation or sagging during long‑term service.

5. Non‑destructive testing

Finished products are released after passing non‑destructive testing (UT/PT) .

6. Selection Guide – Four Steps to the Best Solution

Step 1: Identify the Equipment Type and Operating Conditions

Equipment Type	Operating Characteristics	Recommended Material
Utility boiler main burner	High‑temperature main nozzle (1050–1150°C)	ZG40Cr25Ni20Si2 (2520)
Cement rotary kiln burner	High temperature + alkali‑sulphur corrosion + frequent thermal shock	ZG35Cr24Ni7SiNRe or 2848W5
Secondary‑air nozzle / flow‑guide stabiliser	Medium temperature (1000–1100°C)	ZG35Cr24Ni7SiNRe (cost‑effective)
High‑wear locations / ultra‑supercritical units	Ultra‑high temperature + severe wear	Ceramic composites (SiC/Al₂O₃)

Step 2: Determine the Nozzle Type and Function

• Primary‑air nozzle (core flame stabilisation) → Prioritise 2520, focusing on oxidation and thermal‑shock resistance.
• Secondary‑air nozzle (combustion support) → Choose ZG35Cr24Ni7SiNRe for cost‑effectiveness.
• Swirl nozzle (flow‑field organisation) → Focus on precision casting accuracy and flow‑channel smoothness.

Step 3: Confirm the Structural Form

• General requirements → Standard multi‑channel nozzle structure.
• Frequent replacement and maintenance → Choose removable nozzle design.
• Ultra‑high temperature and severe wear → Choose composite nozzle (ceramic + metal).

Step 4: Confirm Manufacturing Process and Quality

• Prioritise silica‑sol investment casting.
• Confirm five‑axis CNC flow‑channel precision machining (Ra ≤ 1.6 μm).
• Confirm high‑temperature solution heat treatment and non‑destructive testing.
• Require a material analysis report.

7. Usage and Maintenance Highlights

Failure Prevention

Burn‑through prevention:

• Burners taken out of service must have cooling air supplied to prevent nozzle deformation from lack of cooling.
• Control the furnace flame‑centre temperature to avoid excessive levels.

Wear prevention:

• Keep primary‑air duct velocity within the reasonable range.
• Regularly inspect nozzle wear, paying special attention to the side with higher coal concentration.

Slagging and blockage prevention:

• Maintain stable primary‑air duct velocity.
• Regularly clean slag from the nozzle.

Maintenance Points

• Regular inspection: Check for burn‑through, cracks, deformation, and wear.
• Timely cleaning: Regularly remove slag and ash deposits from inside and outside the nozzle.
• Monitor cooling air: Ensure cooling air is properly supplied to out‑of‑service burners.
• Timely replacement: Replace promptly when severe wear or deformation is found.