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In industrial furnace systems for waste‑to‑energy incineration, metallurgical sintering, coal gasification, and related fields, grate bars are the core components of the grate system. They perform multiple functions including ash support, air distribution, ash breaking, and slag discharge. Though they are individual bar elements, collectively they are the key to whether the entire combustion system can operate efficiently, stably, and continuously.
What Are Grate Bars?
A grate is the component in a boiler or industrial furnace that supports solid fuel and enables its effective combustion. It mainly consists of a frame and grate bars. After assembly, necessary ventilation gaps are maintained between adjacent bars. Air passes through these gaps into the fuel bed, providing the oxygen essential for combustion.
In simple terms, grate bars are like rows of precision “ventilation skeletons” – they must support hundreds of kilograms or even several tonnes of fuel or material, ensure uniform air penetration through the fuel bed, and maintain structural integrity under high‑temperature burning without burning out or deforming.
In waste‑to‑energy incineration plants, the grate system typically consists of alternating moving and stationary grates. The grate bars are designed to the same shape and specification for easy replacement and maintenance. The width of the grate system is determined by the waste treatment capacity. In metallurgical sintering machines, the grate bar surfaces come into direct contact with sinter at about 1300°C, making the working conditions extremely harsh.
Core Functions of Grate Bars
1. Supporting fuel and material
Grate bars directly support solid fuel (coal, waste, biomass, etc.) or sintered material, holding it stably in the combustion zone within the furnace. In large industrial furnaces, the grate must support several tonnes or even dozens of tonnes of material.
2. Uniform air distribution and ventilation
The ventilation gaps between adjacent bars are the only passages for air to enter the fuel bed. Proper gap design ensures uniform and adequate air distribution across the entire fuel bed, directly affecting combustion efficiency. The void fraction of the grate is one of the key parameters determining combustion efficiency.
3. Ash breaking and discharge
Modern grates also perform ash breaking, ash discharge, and gasifying‑agent distribution functions. During combustion, the ash formed from the fuel must be crushed and discharged from the furnace through the mechanical action of the grate (such as reciprocating or rotating motion) to make room for fresh fuel.
4. Distributing the fire layer
In equipment such as coal gasifiers, the grate distributes the fire layer uniformly by delivering saturated air through the ash‑bed gaps, ensuring uniform and stable gasification reactions.
Material Selection for Grate Bars
Material selection is the primary decision in grate‑bar design. Different operating conditions demand entirely different materials – from general industrial furnaces to 1300°C sintering machines, from ordinary coal‑fired boilers to highly corrosive waste incinerators – the material must be chosen according to the furnace type and conditions.
Main Material Grades
1. Heat‑resistant steel series (high‑chromium‑nickel austenitic heat‑resistant steels)
This is currently the most widely used and best‑performing material family for grate bars:
| Material Grade | Cr Content | Ni Content | Max. Service Temp. | Key Characteristics | Typical Applications |
|---|---|---|---|---|---|
| ZG40Cr25Ni20Si2 (2520) | ~25% | ~20% | 1100°C | Excellent oxidation, thermal‑fatigue, and corrosion resistance | Waste incinerators, sintering machines, high‑temperature roasters |
| ASTM A297 HC | 26‑30% | 4‑7% | 1000°C+ | High‑temperature oxidation resistance, wear resistance, fracture resistance | Sintering furnaces, roasters, heat‑treatment furnaces |
| ZG35Cr28Ni10 | ~28% | 8‑11% | 1200°C+ | High‑chromium oxidation resistance, good high‑temperature strength | Ultra‑high‑temperature sintering conditions |
| ZG30Cr18Mn12Si2N (Cr‑Mn‑N) | ~18% | — | 950‑1000°C | Good high‑temperature strength, thermal‑fatigue resistance | High‑temperature cooler plates, incinerator grates |
ZG40Cr25Ni20Si2 is a high‑chromium‑nickel‑silicon heat‑resistant steel, strengthened by silicon addition. It can operate stably at 1100°C for extended periods, offering excellent high‑temperature oxidation resistance, thermal‑fatigue resistance, and corrosion resistance – effectively resisting acid‑gas attack and slag abrasion in waste incineration. ASTM A297 HC, with 26.0‑30.0% chromium and 4.0‑7.0% nickel, has a stable metallurgical structure, outstanding high‑temperature oxidation resistance, and is not prone to scaling or burn‑through.
2. High‑chromium cast iron series (Cr26Ni1, etc.)
High‑chromium cast iron is an important material choice for sintering‑machine grate bars:
Cr26Ni1 high‑chromium cast iron forms a dense chromium‑oxide protective film on the bar surface, effectively preventing further oxidation. The service life of ordinary grate bars is typically only 90‑180 days, but Cr26Ni material can extend this to more than 2 years. Nickel improves hardenability and corrosion resistance.
3. Heat‑resistant cast iron series
| Material | Key Characteristics | Max. Service Temp. |
|---|---|---|
| RQT‑Al‑12 | Much higher heat resistance than ordinary cast iron | ~950°C |
| RT‑Cr‑2S | Excellent heat resistance, long average life | ~950°C |
| RQTSi5 | Higher heat resistance | Medium‑high temperature |
| Ordinary cast iron | Low cost, suitable for general conditions | Medium‑low temperature |
4. ZG35 heat‑resistant cast steel
Specific structures such as pagoda‑shaped grates are made from ZG35 material. This material offers both heat resistance and wear resistance, suitable for coal gasifiers and similar equipment.
Material Comparison Overview
| Material Category | Temperature Resistance | Wear Resistance | Oxidation Resistance | Cost | Suitable Applications |
|---|---|---|---|---|---|
| High‑Cr‑Ni heat‑resistant steel (2520/HC) | ★★★★★ | ★★★★ | ★★★★★ | High | Waste incineration, high‑temperature sintering |
| High‑chromium cast iron (Cr26Ni1) | ★★★★ | ★★★★★ | ★★★★ | Medium‑high | Sintering‑machine grate bars |
| Heat‑resistant cast iron | ★★★ | ★★★ | ★★★ | Medium | Medium‑temperature industrial furnaces |
| Ordinary cast iron | ★★ | ★★ | ★★ | Low | General industrial furnaces |
Material Development Trends
Domestic grate‑bar development is moving towards multi‑alloyed high‑chromium cast iron. By adding alloying elements such as Ni, Mo, and V, heat‑resistant, high‑toughness, high‑strength grate bars can be developed, significantly extending service life compared with heat‑resistant ductile iron parts. Increasing chromium content to 28% and optimising production processes can greatly reduce grate‑bar consumption.
Structural Design and Specifications of Grate Bars
Common Structural Types
Flat‑type grate bars: Traditional straight bars arranged on a single plane, mounted on a grate base. Suitable for vertical boilers and early scraper‑type coal gasifiers.
Pagoda‑type grates (irregular shape) : Multi‑layer stacked structures, made of heat‑resistant and wear‑resistant cast steel, often integrated with ventilation systems and slag‑breaking rings. The pagoda‑shaped grate combines ash breaking, ash discharge, and gasifying‑agent distribution functions.
Modular grate‑bar assemblies: Composed of 2‑10 bars connected by tie‑bars, with sufficient ventilation holes between each pair of bars. The modular design offers high ventilation area and is easy to install and replace.
Dovetail‑groove combination: The upper surface of the ordinary grate bar is connected to a heat‑resistant bar via dovetail grooves that accommodate thermal expansion.
Easy‑clean grate bars for sticky materials: Features such as concave sliding fixing bars and tapping bumps address blockage and cleaning difficulties in fixed beds, grate furnaces, and sintering machines.
Specifications and Size Selection
Grate‑bar dimensions should be determined based on application, working environment, furnace structure, and fuel type:
- Small grate bars: Length 500‑600 mm, width 100‑150 mm, suitable for small boilers or industrial furnaces.
- Large grate bars: Length 1000‑5000 mm, width 100‑300 mm, suitable for large boilers or industrial furnaces.
- Common cross‑sections: 35×35 mm, 40×40 mm, 50×50 mm, and other square sections.
Manufacturing Processes
Main Casting Processes
1. Lost‑foam casting (EPC)
This is an advanced and efficient process for producing grate bars. The steps are: making a foam pattern → applying a special coating → drying → placing in a flask → filling with dry sand → three‑dimensional vibration compaction → pouring under vacuum. This process is particularly suitable for parts like grate bars that have high technical requirements and must be free of casting defects on working surfaces.
2. Investment casting
Investment casting ensures high dimensional accuracy and a smooth surface finish. Complex features (such as ventilation slots) can be formed in one piece. The cast structure is dense, free of porosity and blowholes, with uniform wall thickness, rational rib placement, and balanced stress distribution.
3. Centrifugal casting and sand casting
Depending on the product requirements, centrifugal casting and sand casting may also be used.
Heat Treatment and Quality Control
High‑quality grate bars undergo high‑temperature solution heat treatment after casting to eliminate casting stresses and further improve high‑temperature strength, creep resistance, and fracture resistance. Material analysis reports are essential for verifying composition compliance.
Selection Guide – Four Steps to the Best Solution
Step 1: Identify the Equipment Type and Operating Conditions
| Equipment Type | Operating Characteristics | Recommended Material |
|---|---|---|
| Waste incinerator | High temperature + strong corrosion (acidic gases) | ZG40Cr25Ni20Si2 (2520) |
| Metallurgical sintering machine | 1300°C hot sinter scouring | Cr26Ni1 high‑chromium cast iron, ASTM A297 HC |
| Coal gasifier | Medium‑high temperature + gasification environment | ZG35 heat‑resistant cast steel |
| General industrial boiler | Medium‑low temperature | Heat‑resistant cast iron, ordinary cast iron |
Step 2: Determine the Temperature Range
- ≤950°C → Heat‑resistant cast iron (RQT‑Al‑12, RT‑Cr‑2S)
- 950‑1100°C → ZG30Cr18Mn12Si2N, ZG40Cr25Ni20Si2
- 1100‑1300°C → ASTM A297 HC, ZG35Cr28Ni10
- 1300°C+ → Cr26Ni1 high‑chromium cast iron
Step 3: Consider Corrosion and Wear
- Severe corrosion (acidic gases, molten slag) → High‑Cr‑Ni heat‑resistant steel (2520)
- Severe wear (material scouring, particle abrasion) → High‑chromium cast iron (Cr26Ni1)
- General conditions → Heat‑resistant cast iron or ordinary cast iron
Step 4: Determine the Structural Form and Specifications
- Provide parameters such as furnace structure dimensions, fuel type, and ventilation requirements
- Choose flat‑type, pagoda‑type, or modular structures
- Confirm length, width, and cross‑sectional dimensions
Usage and Maintenance Highlights
Common Failure Modes
1. Burn‑through and oxidation
The bar surface is in prolonged direct contact with high‑temperature materials, making it susceptible to oxidation and burn‑through, leading to premature failure. Countermeasure: Choose high‑chromium materials; the dense oxide film on the surface prevents further oxidation.
2. Fracture
Thermal stress, mechanical impact, or structural defects can cause bar fracture. Countermeasure: Choose materials with excellent thermal‑fatigue and impact resistance.
3. Blockage and clogging
Excessively wet material, improper operation, or poor design can block the bar gaps. Countermeasure: Stabilise mix moisture, increase the bed‑thickness of base material, and perform regular concentrated cleaning.
Routine Maintenance Points
- Regular inspection: Check for burn‑through, cracks, and deformation; pay special attention to uniform ventilation gaps.
- Timely cleaning: Regularly remove ash and sticky deposits from the bar gaps.
- Standardised operation: Avoid over‑temperature operation and frequent start‑stop cycles that cause thermal‑fatigue damage.
- Timely replacement: Replace bars promptly when severe burn‑through or fracture occurs to avoid affecting the entire grate system.
Measures to Extend Service Life
- Select high‑quality heat‑resistant materials: Cr26Ni1 high‑chromium cast iron can extend service life from 90‑180 days to more than 2 years.
- Optimise casting processes: Use lost‑foam or investment casting to eliminate defects.
- Rational structural design: Use modular, dovetail‑groove, and other designs to improve thermal‑expansion accommodation.
- Standardised operation management: Control fuel moisture and optimise operating parameters.
- Regular maintenance: Establish a system of regular inspection, cleaning, and replacement.
Conclusion
Grate bars – individual elements forming a “ventilation skeleton” – are the foundational core of solid‑fuel combustion systems. From ZG40Cr25Ni20Si2 to Cr26Ni1 high‑chromium cast iron, from flat‑type to pagoda‑type grates, from lost‑foam casting to investment casting – every material choice, every structural design, and every manufacturing step directly affects combustion efficiency, equipment life, and operational safety.
Choosing the right material, optimising the structure, operating according to standards, and maintaining regularly – these are the four pillars for maximising the performance and extending the service life of grate bars. For waste‑to‑energy incineration, metallurgical sintering, and coal‑gasification enterprises pursuing high combustion efficiency and low downtime losses, the scientific selection and meticulous management of grate bars is a systematic engineering task that delivers big benefits from a small component.