Technical Analysis of Active and Passive Chain Grate Bars
A chain-type chain grate is formed by active grate bars and passive grate bars linked alternately in series. The two differ completely in structure, load path and function, yet they are clearly divided in role and work together to convey fuel, supply combustion air, seal against leakage and discharge slag.
Division of active and passive grate bars
The active bar (driving bar) is the load-carrying part of the chain grate; it has its own pin hinge structure, is linked into a continuous chain by round steel tie rods and meshes with the front sprocket, forming the tension frame of the whole grate. It carries tension, friction and mechanical impact throughout, and pulls all passive bars along at the same uniform speed so the grate circulates as a whole. Active bars are few in number and each carries a large load, so they are the main loaded part of the grate’s mechanical structure.
The passive bar (driven bar) has no independent drive structure and follows entirely on the pull of the active bars, packed densely between them and covering most of the grate area. It spreads and supports the fuel, forms even air gaps and seals the wind box, preventing cross-flow, coal leakage and air leakage. Passive bars are numerous and cover a wide area; they are the high-temperature combustion parts in direct contact with flame and ash.
Differences in structure and shape
Active bars are thicker in section and high in strength, with double-lug hinge pin holes that suit tie-rod linking and sprocket meshing. The face is narrow, the section is thick and rigidity is high, so resistance to tension, twisting and mechanical fatigue is good. The air gaps are small, because structural strength and drive stability come first and ventilation is a secondary function. The bar is robust and resists deformation and fracture, which suits long continuous mechanical operation.
Passive bars have a wide, thin face and no drive hinge structure; their shape follows the sealing design and is mostly a flat lap or an S-shaped lap. Adjacent bars interlock tightly to form a labyrinth seal that blocks cross-flow in the wind box and reduces coal and fly-ash leakage. The air gaps are even and regular, so the primary air ratio can be matched and the fuel bed gets enough oxygen. They cover the main combustion zone of the grate and directly withstand high-temperature flame, ash erosion and heat radiation.
Material selection
The main requirement for active bars is mechanical performance, with heat resistance as a secondary requirement. Industry practice takes malleable cast iron KTH350 as standard; for high-strength duty, ductile iron QT450 and QT500 are chosen, and for heavy-load high-temperature duty carbon cast steel ZG230-450 can be used. These materials have good toughness and high tensile strength, so they resist brittle fracture and deformation under reciprocating pull and mechanical vibration, and suit 24-hour continuous drive operation.
Passive bars work in a high-temperature combustion environment for long periods, so the main requirement is high-temperature stability. Grey cast iron HT200 is standard for normal duty; for mainstream coal-fired and biomass boilers, silicon 5 heat-resistant cast iron RTSi5 is preferred, and this material performs steadily in 750-850℃ duty. For high-sulphur fuel and hot furnace chambers, chromium-series heat-resistant cast iron RTCr2 and RTCr16 is chosen to resist high-temperature oxidation, ash corrosion and thermal-shock cracking.
| Item | Active grate bar | Passive grate bar |
|---|---|---|
| Role | Load-carrying and tension frame, few in number, large load per bar | Core of sealing and combustion, many in number, wide coverage |
| Structure | Thicker, narrow face, double-lug hinge pin holes, small air gaps | Wide and thin, flat or S-shaped lap, even air gaps |
| Common materials | KTH350, QT450, QT500, ZG230-450 | HT200, RTSi5, RTCr2, RTCr16 |
| Main wear | Pin hole wear, tensile deformation, brittle fracture; mechanical wear | High-temperature oxidation, scaling, corrosion, thermal-shock cracking; high-temperature wear |
| Normal life | 2-3 years | 12-24 months, 8-12 months under high-sulphur high-temperature duty |
Functions and service life
The active bar has three functions. First, it meshes with the sprocket and carries the tension of the whole grate belt, driving the grate to circulate and moving fuel backwards. Second, it holds the overall spacing and shape of the grate, preventing the belt from running off, going slack or jamming. Third, it withstands reciprocating pin friction and sprocket meshing impact, keeping the drive system stable.
The passive bar takes four functions. First, it spreads and supports the coal bed and biomass layer over a large area, keeping fuel distribution even. Second, even air gaps provide staged air distribution, so drying, ignition and burnout all burn steadily.
Third, it blocks cross-flow in the wind box and leakage of air and coal through the grate, cutting fan power and fuel loss. Fourth, it faces high-temperature flame and ash erosion directly and keeps operation stable at high temperature.
The wear patterns of the two also differ. Active bars rarely burn out; their failures are mostly wear, loosening, deformation and fracture, and normal life is 2-3 years, so frequent replacement is not needed. Passive bars have little mechanical wear; their main damage is high-temperature oxidation, scaling, corrosion and thermal-shock cracking, and normal RTSi5 material lasts 12-24 months. Under high-temperature, high-sulphur and wet-fuel duty, life shortens to 8-12 months, and passive bars then become the main wearing part of the grate.
Selection can combine materials by duty. For ordinary dry and wet fuel at normal furnace temperature, KTH350 active bars with RTSi5 passive bars give good value. For high-sulphur, high-temperature or poor fuel, the passive bars are upgraded to chromium-series heat-resistant cast iron, and for heavy-load duty the active bars are upgraded to cast steel.
