How grate kiln pellet plants work
A grate kiln pellet plant splits induration across three machines. A travelling grate dries and preheats green pellets until they are strong enough to tumble; a rotary kiln, fired by a coal, gas or oil burner at the discharge end, hardens them at 1,250 to 1,320°C; and an annular cooler cools them. Hot air from the first cooler section enters the kiln as secondary air, and the kiln off-gas at about 1,000 to 1,100°C flows to the preheat zone of the grate. Gas drawn down through the preheat bed is returned by a windbox fan to the drying zones, and the final gas from the drying windboxes passes an ESP to the main waste gas fan. Most Indian grate kiln plants fire coal, which adds ash and sulphur to the gas.
Where the ID fan sits
- Rotary kiln
- Preheat zone
- Windbox recirculation
- Drying zone windboxes
- ESP
- Waste gas (ID) fan
- Stack
The fan is after the ESP, drawing on the drying zone windboxes at the feed end of the grate. Ahead of it the gas has passed the kiln, the transfer chute, the preheat bed and, via the windbox recirculation fan, the drying beds. Its suction is typically 500 to 800 mmWC. Because the kiln draught depends on this fan, a change in its flow changes the kiln burner flame, the secondary air and the pellet quality.
Gas at the ID fan
| Gas temperature, normal | 110 to 160°C after the ESP |
| Maximum or upset | Up to 250 to 300°C on a grate stop with the kiln firing |
| Dust at the fan | Fine iron oxide with coal fly ash, typically 50 to 100 mg/Nm³ after the ESP |
| Moisture and dew point | High water vapour from drying; water dew point 55 to 65°C |
| Corrosive species | SO₂ from coal firing; acid dew point often 120 to 140°C |
| Density at the design temperature | About 0.86 kg/m³ at 140°C, 72% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for grate kiln pellet plants
| Blade form | Backward curved with wear liners, or radial tipped where ESP performance is poor |
| Material of construction | Mild steel with wear-resistant plate on blades and scroll liners; SS 316 inlet box where gas is near acid dew point |
| Drive and arrangement | Arrangement 3 or 8, double inlet on large plants |
| Bearings and cooling | Oil-lubricated sleeve or roller bearings with a cooling disc and monitoring |
| Flow control | Variable speed drive on kiln inlet draught or grate hood pressure |
| Also specified | Insulated casing, drains, cleaning doors and a rotor designed for site replacement |
What this ID fan has to cope with
Coal ash and erosion
Coal ash from the kiln burner adds to the fine iron oxide passing the ESP. The mix is abrasive and wears blade leading edges. Wear liners, moderate tip speed and attention to ESP performance keep the rotor running between shutdowns.
Kiln draught swings
Rings and accretions in the kiln, or changes in pellet bed permeability on the grate, change the system resistance. The fan must hold kiln inlet draught steady across these changes, which needs margin on pressure and responsive speed control. Kiln inlet draught is the control signal.
Acid and moist deposits
Coal sulphur raises the acid dew point, while drying gas carries water. On start-ups and at low output the casing runs near dew point, and acid mud forms. Insulation, warm-up on hot gas and stainless parts at the cold points prevent rapid corrosion.
Energy and control
The waste gas fan runs continuously and its flow is set by kiln draught and grate permeability. A variable speed drive on draught pressure adapts to bed changes and production without throttling. Reducing air leakage at the kiln seals, transfer chute and grate side seals lowers both the gas volume and the fuel needed to heat it.
ID fan and FD fan in grate kiln pellet plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP at the grate feed end | Below each annular cooler section |
| What it handles | Moist gas with coal ash and ore dust | Ambient cooling air |
| Temperature at the fan | 110 to 160°C | Ambient, 25 to 45°C |
| Main design concern | Erosion and acid near dew point | Secondary air and section balance |
| Control | Kiln inlet draught | Section air flow and hood pressure |
See also the Forced Draft (FD) Fan for grate kiln pellet plants.
Questions buyers ask
What is the ID fan in a grate kiln pellet plant?
It is the main waste gas or process exhaust fan, after the ESP. It draws the kiln off-gas through the preheat and drying zones of the travelling grate, so it sets the draught through the kiln and the grate, and discharges to the stack. It is one of the largest fans in the plant.
Why does a grate kiln waste gas fan wear faster than on a straight grate?
Most grate kiln plants in India fire coal in the kiln, so coal fly ash joins the iron oxide dust reaching the fan. That mix is more abrasive, and the sulphur raises the acid dew point, adding corrosion to erosion. Wear liners are therefore standard.
How does kiln ringing affect the ID fan?
A ring or accretion in the kiln adds resistance. The fan speed rises to hold kiln draught, and the duty moves up the fan curve. The fan must have pressure margin, or draught is lost and pellet quality falls. Removing the ring restores the duty.
Information needed to quote an ID fan for grate kiln pellet plants
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Kiln and fuel. Kiln size, burner fuel, coal ash and sulphur content.
- Gas system. Duties of the windbox recirculation fan and the waste gas fan, and the kiln inlet draught set point.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.