How imported coal AFBC boilers work
Many AFBC boilers in textile, paper and process plants now fire imported coal, mostly Indonesian sub-bituminous grades with 4,200 to 5,500 kcal/kg, 20 to 35% total moisture, low ash of 3 to 12% and low sulphur, sometimes blended with South African or Australian coal of higher calorific value. Crushed to below about 6 mm, the coal is fed overbed into a bubbling bed at 800 to 880°C. The low ash cannot sustain the bed on its own, so sand is added as make-up, and the high volatile content means much of the coal burns in the freeboard above the bed. Flue gas passes the boiler bank, economiser and air heater, then a multiclone and ESP or bag filter, before the ID fan.
Where the ID fan sits
- Bubbling bed and freeboard
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan follows the ESP or bag filter and holds the freeboard at slight suction. Suction at the fan inlet is typically 200 to 300 mmWC. Many of these boilers were originally designed for Indian coal, and the switch to imported coal changed the gas: less ash but more water vapour, and a higher volume per tonne of steam because the coal moisture has to be evaporated and carried out.
Gas at the ID fan
| Gas temperature, normal | 140 to 165°C after the air heater |
| Maximum or upset | Up to 200°C with a fouled air heater |
| Dust at the fan | Light, fine ash with bed sand fines; 30 to 100 mg/Nm³ after the collector |
| Moisture and dew point | Water vapour of 12 to 18% by volume; water dew point 50 to 60°C |
| Corrosive species | Low sulphur, usually below 0.8%; acid dew point rarely limiting at normal gas temperature |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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 AFBC boilers
| Blade form | Backward curved, with leading edge protection for sand fines |
| Material of construction | Mild steel casing and impeller; liners only at the inlet edges and cut-off |
| 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 volume margin for the wettest coal cargo |
What this ID fan has to cope with
Fan short of volume after the switch from Indian coal
A boiler designed for Indian coal may produce 10 to 20% more flue gas per tonne of steam on wet Indonesian coal, because of the extra water vapour. The existing ID fan reaches full speed or full damper opening before full load. A new impeller sized for the actual gas volume, or a speed increase within the casing's limits, restores capacity.
Sand fines from bed make-up
Low-ash imported coal cannot maintain bed height, so plants add river or silica sand regularly. The sand attrits in the bed into fines that elutriate with the gas and are harder than the coal ash. With a good collector the quantity is small, but the fan inlet edges and scroll cut-off wear first and benefit from liners.
Wet coal cargoes and condensation
Imported coal stored in the open picks up rain, and total moisture can exceed 35%. The gas dew point rises, and at start-up or low load cold casing surfaces can run wet. Ash wetted on the casing hardens into deposits. Insulation, a drain and a warm-up before full flow reduce both deposits and corrosion.
Energy and control
Imported coal AFBC boilers in textile and process plants often follow production shifts and run at part load at night. A variable speed ID fan saves considerably over damper control. Reducing coal moisture by covered storage lowers gas volume, and the fan saving is matched by a gain in boiler efficiency because less heat goes into evaporating water.
ID fan and FD fan in imported coal AFBC boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the chimney | Before the air heater, feeding the plenum and overbed air |
| What it handles | Wet flue gas with light ash and sand fines | Clean ambient air |
| Temperature at the fan | 140 to 165°C | Ambient |
| Main design concern | Gas volume from coal moisture | Bed and overbed air split |
| Control | Furnace draft | Bed temperature, steam pressure and oxygen |
See also the Forced Draft (FD) Fan for imported coal AFBC boilers.
Questions buyers ask
Why does my AFBC boiler ID fan run at full speed after switching to imported coal?
Indonesian coal carries much more moisture than Indian coal, and that water leaves as vapour in the flue gas. The gas volume per tonne of steam rises, often by 10 to 20%, so the fan must move more gas for the same output. If the fan was sized for Indian coal, it may need a larger impeller or a speed increase.
Does imported coal cause less ID fan wear than Indian coal?
Usually yes. Imported coal has far less ash, and its ash is generally less abrasive than the quartz-rich ash of Indian coal. The main wear source on an imported coal AFBC boiler becomes the bed sand fines, which are smaller in quantity but hard. Light liners at the inlet edges are normally enough.
Is sulphur a concern for the ID fan on an imported coal AFBC boiler?
Not usually. Indonesian coals are low in sulphur, often below 0.8%, so the acid dew point is low and the gas after the air heater stays above it in normal running. Higher sulphur blends or petcoke co-firing change this, and the casing should then be insulated and the gas kept above the acid dew point.
Information needed to quote an ID fan for imported coal AFBC 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 analysis. Total moisture, ash, sulphur and calorific value of the coals fired, including the wettest cargo.
- Original design fuel. The fuel the boiler and present fan were designed for, if different from today's 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.