How sponge iron kiln waste heat boilers work
In a coal based sponge iron plant, iron ore and non-coking coal are fed into an inclined rotary kiln and reduced to DRI at about 1,000 to 1,050°C. Kiln off-gas, carrying CO, volatiles and char, passes to a dust settling chamber and an after-burning chamber, where air burns out the combustibles and the gas leaves at about 900 to 1,050°C. The waste heat boiler cools it to about 150 to 180°C, then an ESP removes the dust, mostly char, coal ash and iron oxide fines, before the ID fan and chimney. An emergency cap at the top of the after-burning chamber opens to vent hot gas if the boiler or ID fan trips. Each 100 TPD kiln typically supports about 8 to 10 MW of power.
Where the ID fan sits
- Rotary kiln
- Dust settling chamber
- After-burning chamber
- Waste heat boiler
- ESP
- ID fan
- Chimney
The fan sits after the ESP and before the chimney. It holds a small suction at the kiln feed end, typically a few mmWC, and must overcome the settling chamber, after-burning chamber, boiler, economiser and ESP, often 250 to 400 mmWC in total. The ESP takes out most of the dust, but when fields trip or the ESP is overloaded during kiln upsets, the fan sees a large increase in char and oxide dust.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the boiler and ESP |
| Maximum or upset | Up to 250°C or more when boiler surfaces foul or the economiser is bypassed |
| Dust at the fan | Char, coal ash and iron oxide fines; low after a healthy ESP, several g/Nm³ when ESP fields are down |
| Moisture and dew point | Moderate water vapour from coal moisture and combustion; low sulphur coal keeps acid dew point below gas temperature |
| Excess air | High, from the after-burning chamber and air ingress at the kiln seals |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 sponge iron kiln waste heat boilers
| Blade form | Radial tipped or backward curved with wear liners, depending on ESP reliability |
| Material of construction | Mild steel casing with wear liners at the scroll; impeller in wear-resistant plate or hard-faced blades |
| Drive and arrangement | Direct coupled, arrangement 8, moderate tip speed |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and vibration monitoring |
| Flow control | Variable speed drive on kiln inlet draft |
| Also specified | Interlock with the after-burning chamber emergency cap; inspection door and casing drain |
What this ID fan has to cope with
Erosion when the ESP is weak
Many kiln ESPs lose fields during upsets or run overloaded, and then the fan takes char and iron oxide fines at high velocity. These erode blade leading edges and the scroll quickly. Wear plate, replaceable liners and a lower tip speed buy time, but the lasting cure is keeping the ESP healthy and monitoring its outlet dust.
Kiln draft is the process
Too little suction at the kiln feed end pushes gas and dust out at the seals; too much draws in air that burns DRI and disturbs the reduction profile. The fan must hold kiln inlet pressure steadily as gas volume changes with coal feed and accretion. A variable speed drive with a responsive loop does this better than a damper.
Emergency cap openings
When the boiler or the fan trips, the emergency cap opens and hot gas vents to atmosphere. Frequent openings cause emissions complaints and stress the boiler. A fan with high reliability, vibration monitoring and a quick, safe restart sequence keeps cap openings rare. Each opening also costs steam and export power.
Build-up from kiln accretion fall-off
When accretions break off in the kiln, gas volume and dust surge for a while. The fan needs margin in volume and pressure for these surges, so the kiln is not starved of draft at the moment it most needs it. A margin of 10 to 20% on volume over the steady design flow is common practice for this duty.
Energy and control
Sponge iron kilns run continuously for long campaigns, so the ID fan runs at nearly constant duty for months. A variable speed drive set on kiln draft avoids the large throttling losses common on damper-controlled fans, and an efficient impeller adds directly to the power available for export, since the fan is a large auxiliary load on the captive plant.
Questions buyers ask
Why does the ID fan on a sponge iron WHRB wear so fast?
Usually because the ESP lets too much dust through. Char, coal ash and iron oxide fines erode the blades and scroll. Restoring ESP performance is the first step; wear-resistant plate, liners and a lower tip speed on the fan extend impeller life further. Tracking vibration shows when wear needs attention.
What draft does a DRI kiln need at the feed end?
Usually a small suction of a few mmWC, enough to prevent gas leaking out of the kiln inlet seal without drawing excess air in. The ID fan holds this set point through a variable speed drive or damper, so the fan duty must include all resistance from kiln to chimney.
Can the ID fan be shared between two sponge iron kilns?
It is possible with a common boiler, but each kiln needs its own draft control, which becomes harder with a shared fan. Most plants use one fan per kiln and boiler, so one kiln can run while the other is stopped for relining or accretion removal.
Information needed to quote an ID fan for sponge iron kiln waste heat boilers
- 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 boiler. Kiln capacity, gas flow and temperature after the boiler, and the ESP type and design outlet dust.
- Draft control. Kiln inlet pressure set point and the present control method.
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.