How imported coal travelling grate boilers work
Travelling grate boilers firing imported coal, typically 6 to 40 TPH in food, beverage and process plants, burn Indonesian, South African or Australian coal on a moving grate. The coal is sized, usually 6 to 32 mm, and fed by a hopper and gate or by spreaders. Indonesian grades are high in volatiles and moisture, low in ash and often low in ash fusion temperature. The low ash leaves a thin protective layer on the grate, and the volatiles release quickly, so overfire air carries a large share of combustion. Flue gas passes the boiler bank, economiser and air heater, then a multiclone and bag filter or ESP, before the ID fan and chimney.
Where the ID fan sits
- Travelling grate furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan follows the bag filter or ESP, after the air heater has cooled the gas to 150 to 175°C. Suction at the fan inlet is typically 180 to 280 mmWC. Because imported coal has little ash, the collector inlet load is far lower than with Indian coal, and the fan sees fine, light ash and soot. Volume rises with coal moisture.
Gas at the ID fan
| Gas temperature, normal | 150 to 175°C after the air heater |
| Maximum or upset | Up to 210°C with sooted boiler tubes |
| Dust at the fan | Fine, light ash with soot; typically below 50 mg/Nm³ after a bag filter |
| Moisture and dew point | Water dew point 50 to 60°C with Indonesian coal |
| Corrosive species | Low sulphur; corrosion mainly from condensation at cold spots |
| Density at the design temperature | About 0.81 kg/m³ at 165°C, 68% 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 imported coal travelling grate boilers
| Blade form | Backward curved or aerofoil for efficiency on low-dust gas |
| Material of construction | Mild steel throughout; light liners optional |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing insulation, drain and inspection door |
What this ID fan has to cope with
Gas volume from coal moisture
Indonesian coal moisture varies by cargo and by how it was stored, from about 20% to over 35%. The water leaves as vapour, so gas volume per tonne of steam rises with every wet consignment. A fan sized on the dry design coal runs out of capacity on wet coal. Selecting at the wettest expected coal with speed control covers the range.
Soot from rapid volatile release
Volatile matter in Indonesian coal is released quickly at the front of the grate. Without enough overfire air and mixing, it forms soot that coats the boiler tubes and reaches the fan. Soot deposits on blades cause unbalance. Good overfire air control and a smooth backward curved impeller keep the fan clean.
Oversized fans on converted boilers
Boilers converted from Indian coal to imported coal see much less ash, and the original heavy radial ID fan, chosen for erosion, now runs inefficiently. A backward curved or aerofoil impeller, suited to the cleaner gas, in the existing casing can cut fan power noticeably.
Energy and control
Imported coal travelling grate boilers in food and beverage plants often follow production demand closely. A variable speed ID fan saves most during low demand. Because the gas is clean, an efficient backward curved or aerofoil wheel stays efficient over time, and the savings from replacing an old radial wheel can be large.
ID fan and FD fan in imported coal travelling grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP, before the chimney | Before the air heater, feeding the grate zones |
| What it handles | Humid flue gas with light ash and soot | Clean ambient air |
| Temperature at the fan | 150 to 175°C | Ambient |
| Main design concern | Gas volume with coal moisture | Grate cooling with thin ash cover |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Forced Draft (FD) Fan for imported coal travelling grate boilers.
Questions buyers ask
Can I use a more efficient ID fan on an imported coal boiler?
Yes, usually. Imported coal has far less ash, so the gas at the fan is much cleaner than with Indian coal. A backward curved or aerofoil impeller, which would wear too fast on high ash coal, gives good life and higher efficiency on imported coal behind a bag filter or ESP.
Why is there soot in the ID fan of my imported coal travelling grate boiler?
Indonesian coal releases its volatile matter quickly. If overfire air or mixing is short, the volatiles crack into soot before they burn. The soot coats tubes and reaches the fan. Improving overfire air and combustion tuning removes the cause; cleaning the fan at shutdowns removes the deposits.
Does wet imported coal affect ID fan sizing?
Yes. Each extra percent of coal moisture adds water vapour to the gas, and wet cargoes can raise the gas volume by 10% or more. The ID fan should be sized on the wettest coal the plant expects to buy, not on the design analysis.
Information needed to quote an ID fan for imported coal travelling grate 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.
- Coal sources. Coal origin, total moisture range and ash content of the coals bought.
- Existing fan. Present fan type, speed and whether it was selected for Indian coal.
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.