What Is the Difference Between Chain Grates and Biomass Reciprocating Grates?
Chain grates and biomass reciprocating grates differ in structural form, fuel conveying and combustion mechanism, fuel adaptability, operation control, wear life and sealing performance. The first suits steady coal firing, while the second suits low-grade agricultural and forestry biomass.
Structural form and conveying mechanism
A chain grate is a continuous endless chain structure, and the whole grate surface travels in step at a constant speed. The relative position of the grate bars stays basically fixed, so the bars do not slide against each other. The main parts are the driving sprocket, the chain, the driven roller and the continuous grate bar assembly, which circulates as a whole and is classed by load capacity as light, medium or heavy chain grate. Segmented air chambers sit underneath, but the fuel bed itself is not turned actively, and the layout is basically horizontal or slightly inclined with no stepped offset structure.
Fuel creeps forward with the whole grate, and the bed stays essentially static and burns passively with no active turning. The combustion sequence is drying at the front, burning in the middle and burn-out at the tail, which is set by air distribution, so the bed structure does not change. Air reaching the bottom of the bed is still acceptable, while the surface layer tends to slag and cake and lose permeability, so high-moisture and long-fibre fuels smoulder, slag and fail to burn out.
A biomass reciprocating grate uses alternately arranged fixed bars and movable bars in a stepped layout. It relies on a crank or a hydraulic drive to make the movable grate slide back and forth. The main parts are the fixed frame, the reciprocating push rod, segmented grate bars, independent multi-compartment air chambers and the reciprocating drive, and the bars keep shifting against each other in a stepped ripple; the whole unit is an inclined stepped structure split into a drying zone, a main combustion zone and a burn-out and ash removal zone.
Through the reciprocating shift it pushes fuel slowly backwards while stirring, kneading and breaking the bed and the slag shell, so caking is broken up and overall permeability improves. Segmented and precise air distribution can match biomass: strong air for drying early on, air matched to volatile combustion in the middle and air that ensures residual carbon burn-out at the tail. It is not suitable for pure fine powder fuel, though, which easily causes flame blow-through and local overheating.
Fuel adaptability
A chain grate suits raw coal and clean formed fuel that is even in size, low in moisture, low in volatiles and well shaped. Its weakness is that bulky mixed biomass with high moisture and long fibres easily cakes and slags, ventilation deteriorates, burn-out is poor and air and coal leakage is serious, so slag resistance is weak and there is no self-turning ability. A biomass reciprocating grate suits high-moisture, long-fibre, irregular and low-calorific agricultural and forestry biomass waste such as wood chips, straw, bagasse and palm shells. Its weakness is that pure fine powder, fuel with too much hard impurity and fuel with a very high calorific value easily suffer local burn-out and slag jamming, while slag resistance is good because reciprocating agitation keeps breaking the slag.
| Item | Chain grate | Biomass reciprocating grate |
|---|---|---|
| Structural form | Continuous endless chain, the whole surface travels in step at constant speed | Fixed and movable bars in an alternating stepped layout, sliding back and forth |
| Drive | Driving sprocket turns the chain as a whole | Crank or hydraulic drive moves the movable grate back and forth |
| Bed agitation | Almost none, passive combustion | Continuous stirring, kneading and slag breaking |
| Fuel adaptability | Raw coal and formed fuel, even, low moisture, low volatiles | High-moisture, long-fibre, low-calorific agricultural and forestry biomass waste |
| Main wear points | Driving engagement bars, sprocket and chain | Contact faces of movable and fixed bars, push rod, pins, side seals |
| Corrosion type | Mainly high-temperature oxidation and sulphur attack | Plus alkali-metal and chloride high-temperature attack |
| Sealing performance | Continuous as a whole, air leakage controllable | Dynamic sliding gaps, hard side sealing, slag leakage and flame blow-through likely |
| Boilers used | Coal-fired industrial boilers, large and medium steady heat source boilers | Small and medium biomass boilers, thermal oil heaters, heating and drying boilers |
Operation control and maintenance
Chain grate control means the overall travelling speed, the coal bed thickness and the primary air ratio, and the bed is barely disturbed. Thermal cycling comes mainly from the furnace conditions as a whole and thermal fatigue is fairly even, so the grate suits steady continuous full-load duty, while frequent starts and stops easily deform, stretch and misalign the whole unit. Wear concentrates on the driving engagement bars, the sprocket and the chain, overall friction is relatively low and corrosion is mainly high-temperature oxidation with sulphur attack, with no continuous sliding wear.
Overall life is longer, the main failure modes are creep stretch, misalignment, thermal deformation and mismatch of the overall pitch, and maintenance intervals are longer. Maintenance mainly means truing the chain as a whole and replacing the chain and driving bars, so everyday work is relatively light.
Biomass reciprocating grate control means stroke, reciprocating frequency, independent primary air for each zone and feed rate, and the bed keeps sliding back and forth under alternating load. The grate takes mechanical friction, repeated thermal stress and alkaline-chlorine-sulphur attack from the biomass, suits variable load and the fluctuating duty of heating and drying, and can match the drying and burning rhythm of biomass. Wear concentrates on the contact faces of movable and fixed bars, the push rod, the pins and the side seals, and besides high-temperature oxidation and sulphur attack there is alkali-metal and chloride high-temperature attack, so the material degrades faster and local grate life is shorter.
Failure modes are wear, cracking, alkali attack and slag jamming, and maintenance work is clearly heavier. Grate gaps, the drive mechanism and the side seals need frequent inspection, and damaged grate bars need replacing.
Sealing performance and cost
A chain grate is continuous as a whole and seals well, and the main coal leakage points are the bar gaps and the two side edges. Overall air leakage is controllable, and side leakage and flame blow-through are relatively mild.
A biomass reciprocating grate has dynamic sliding gaps between fixed and movable bars, side sealing is hard, and slag leakage, flame blow-through and local air leakage easily follow, which hurts thermal efficiency. The side grate is also easily burnt out. On cost, chain grate technology is mature and the cost per unit output is moderate, and the type is mostly used in coal-fired industrial boilers and large and medium steady heat source boilers, with long-term steady coal firing as the core objective.
A biomass reciprocating grate carries a reciprocating drive, segmented air distribution and heat-resistant alloy grate bars, so the cost is higher, and the type is mostly used in small and medium biomass boilers, thermal oil heaters and heating and drying boilers. Its core objective is handling low-grade agricultural and forestry biomass fuel.
