Burning Solid Fuel: Grate Firing and Fluidized Bed Combustion
Solid fuel cannot be sprayed into a furnace like oil. It is burned either on a grate, a bed of bars or links that holds the fuel while air passes up through it, or in a fluidized bed, a layer of hot sand kept boiling by air from below. The choice decides which fuels a boiler can burn and how well.
You'll learn
- Describe fixed, reciprocating, travelling and chain grates
- Explain how a fluidized bed works
- Match firing method to fuel and size
- Know the operating issues of each
Before this: Air–fuel ratio and excess air, Biomass fuels, moisture and ash
Holding the fuel while it burns
- Fixed grate: stationary bars on which fuel burns; ash is raked out by hand or periodically. Simple and cheap; suited to small boilers.
- Reciprocating (pusher) grate: inclined steps that move back and forth, pushing fuel down the grate as it burns and discharging ash at the end. Handles wet, mixed biomass well.
- Travelling grate and chain grate: a continuous moving belt of grate links carries fuel slowly through the furnace and drops ash at the far end. Good for consistent coal and some biomass at larger sizes.
- Fluidized bed: fuel is fed into a bed of hot sand or ash that air from below keeps in a boiling, fluid-like state. Combustion is spread evenly through the bed.
Matching method to fuel
| Method | Suits | Watch for |
|---|---|---|
| Fixed grate | Small boilers, wood and similar fuels | Manual ash removal; uneven burning |
| Reciprocating grate | Wet and mixed biomass, husk, wood chips | Grate wear; air distribution across the grate |
| Travelling / chain grate | Graded coal, some biomass, steady loads | Fines falling through; fuel bed depth control |
| Fluidized bed (AFBC) | Low-grade, high-ash coal, washery rejects, mixed fuels | Bed material management; tube erosion in the bed |
General fit. A specific boiler’s fuel range is set by its design.
Grates are divided into zones with separately controlled undergrate (primary) air, and air is also injected above the fuel bed (secondary or overfire air) to burn the volatile gases released from the fuel. Getting the split between them right matters as much as the total air.
Why fluidized beds burn poor fuels well
A fluidized bed holds a large mass of hot inert material at a fairly low, even temperature, typically well below the ash softening point of most coals. Fresh fuel entering the bed is heated almost instantly by contact with that mass, so even wet, high-ash fuel ignites and burns steadily. The low temperature avoids slagging and limits NOx formation, and limestone added to the bed captures sulfur.
In an atmospheric fluidized bed combustion (AFBC) boiler, tubes immersed in the bed pick up heat at high rates directly from the bed material. The cost is erosion: the moving sand wears in-bed tubes and refractory, so bed velocity, tube protection and bed material size have to be controlled. Fuel must be crushed to a size the bed can fluidize and burn out, which adds crushing to the fuel system.
On grates, the fuel bed itself is the thing to manage. Too thick and air cannot pass evenly; too thin and air rushes through bare patches, raising excess air and blowing fines unburnt into the gas path. Uneven fuel spreading, clinker formation and air leakage at the grate sides are the usual causes of poor grate combustion.
Key takeaways
- Grates hold solid fuel while air passes up through it; fluidized beds suspend it in hot, boiling sand.
- Reciprocating grates suit wet mixed biomass; travelling and chain grates suit graded coal at size.
- Fluidized beds burn low-grade, high-ash and mixed fuels well at low, even temperature, but erode in-bed surfaces.
- Primary and secondary air split, and fuel bed depth, govern grate combustion.