How anthracite and low volatile coal boilers work
Anthracite and other low volatile coals, with volatile matter below about 10 to 15%, are hard to ignite and slow to burn out. In India they are fired only in a minority of boilers, usually with imported anthracite or semi-anthracite where it is available at a good price. On chain or travelling grates, long refractory arches reflect heat back to the incoming coal, and the undergrate air is preheated to about 180 to 250°C to help ignition. CFBC boilers handle these coals well because of their long residence time. Unburnt carbon in the fly ash is usually higher than on bituminous coal. The gas passes the boiler, economiser and air heater, then a multiclone and an ESP or bag filter before the ID fan.
Where the ID fan sits
- Furnace and arches
- Boiler bank
- Economiser
- Air heater
- Multiclone
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the final collector. On grate-fired units the multiclone takes the coarse char, which is sometimes returned to the grate, and the ESP or bag filter takes the fines. Suction at the fan inlet is typically 150 to 250 mmWC on grate units and 250 to 400 mmWC on CFBC units. The large air heater that low volatile coals need adds its own resistance on the gas side.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 220°C with fouled air heater |
| Dust at the fan | Fine ash with hard unburnt char; 30 to 100 mg/Nm³ after the final collector |
| Moisture and dew point | Low fuel moisture; water dew point about 40 to 50°C |
| Corrosive species | Depends on coal sulphur, often 0.5 to 1%; acid dew point 110 to 130°C |
| Excess air | Often higher than on bituminous coal grates, raising gas volume |
| 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 anthracite and low volatile coal boilers
| Blade form | Backward curved with wear liners behind an ESP or bag filter; radial tipped after a multiclone |
| Material of construction | Impeller in wear-resistant plate or mild steel with hard-faced leading edges; liners at the cut-off |
| Drive and arrangement | Direct coupled, arrangement 8, at moderate tip speed |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection doors, drain, and vibration monitoring |
What this ID fan has to cope with
Hard char erosion
Anthracite char keeps much of the hardness of the parent coal and burns out slowly, so a larger share reaches the collectors than with bituminous coal. Where it passes the collector, it erodes blade leading edges. Wear-resistant plate, lower tip speed and liners at the most exposed points extend impeller life.
Higher gas volume from excess air
Operators often run anthracite grates with more excess air to keep burnout acceptable. A fan sized on theoretical air for bituminous coal is then short of volume. The duty should be based on the excess air actually used, confirmed by oxygen readings at the economiser outlet.
Air heater resistance
Low volatile coals need hotter undergrate air, so the air heater is larger and its gas-side resistance higher. As it fouls with fine ash, resistance and gas temperature rise together. Fan pressure margin and regular cleaning prevent the air heater becoming the limit on output.
Energy and control
Anthracite boilers usually run at steady load, because frequent load changes disturb ignition on the grate. The ID fan therefore operates near one point for long periods, and efficiency at that point matters most. A backward curved fan with liners behind a good final collector draws noticeably less power than a radial fan. A variable speed drive handles the remaining load changes and start-up.
ID fan and FD fan in anthracite and low volatile coal boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the final collector | Ahead of the air heater |
| What it handles | Flue gas with hard unburnt char | Clean ambient air |
| Temperature at the fan | 150 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Char erosion and excess air volume | Steady hot air for ignition |
| Control | Furnace draft | Steam pressure and excess oxygen |
See also the Forced Draft (FD) Fan for anthracite and low volatile coal boilers.
Questions buyers ask
Why is unburnt carbon high in fly ash from anthracite?
Anthracite has little volatile matter to start and sustain combustion, and its char is dense and burns slowly. Particles that leave the grate or furnace before burning out carry unburnt carbon. Hot air, refractory arches and longer residence time help, but carbon in fly ash is still higher than with bituminous coal.
What ID fan is suitable for an anthracite fired boiler?
Usually a backward curved induced draft fan with wear liners behind an ESP or bag filter, or a radial tipped fan where only a multiclone is fitted. The impeller is protected against hard char, and the duty is calculated on the actual excess air the boiler runs at.
Is a CFBC better than a grate for low volatile coal?
For ignition and burnout, often yes, because the hot circulating bed and long residence time suit slow-burning fuel. A CFBC needs higher pressure air fans and handles more fly ash at its ESP. The choice also depends on size, fuel price and the existing plant.
Information needed to quote an ID fan for anthracite and low volatile coal 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. Volatile matter, ash and fixed carbon of the coal, and the measured carbon in fly ash.
- Excess air. Measured oxygen at the economiser outlet at normal load.
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.