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 FD fan sits
- Inlet screen
- FD fan
- Air heater
- Hot air duct
- Grate zones or air box
- Furnace
The fan sits at ground level and pushes ambient air through the air heater, which on these boilers is sized to deliver hotter air than on a bituminous coal grate. From the air heater the air goes to the undergrate zones, or to the air box on a CFBC. Fan outlet pressure is typically 120 to 200 mmWC on grate units, higher because of the larger air heater, and 1,000 to 1,500 mmWC on CFBC primary air duty.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 180 to 250°C, downstream of the fan |
| Pressure at the fan outlet | Typically 120 to 200 mmWC on grates; much higher on CFBC |
| Dust | Clean air; anthracite dust near the bunker |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for anthracite and low volatile coal boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 8 or 4 |
| Bearings and cooling | Pedestal or motor bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure, trimmed by oxygen |
| Also specified | Inlet screen, zone dampers, and an outlet damper to isolate the air heater |
What this FD fan has to cope with
Ignition loss from cold or unsteady air
Anthracite ignites only with enough heat. If air heater outlet temperature falls, or air flow to the ignition zone swings, the fire front moves back along the grate and burnout suffers. A fan with a stable curve and smooth speed control holds the air steady, and the air heater must be kept clean.
Air heater resistance and leakage
The large air heater adds pressure drop and, as it ages, leakage. Leaking air bypasses the grate, so the FD fan works harder for the same combustion air. The duty includes expected leakage, and oxygen readings before and after the air heater show when it needs repair.
Zone air imbalance
Low volatile coal needs air concentrated where burning is most active, with less at the ignition and burnout ends. A single fan with poorly set zone dampers wastes air at the ends. Zone pressure measurement and damper settings checked against ash carbon keep the distribution right.
Energy and control
Anthracite grates run best at steady load, so the FD fan works near one point. Excess air is often set high to protect burnout, and every extra percent costs fan power and fuel. A variable speed drive trimmed on oxygen, with zone air properly balanced, usually lets the plant lower excess air without losing burnout, which is worth more than the fan power saving itself.
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 Induced Draft (ID) Fan for anthracite and low volatile coal boilers.
Questions buyers ask
Why does an anthracite boiler need hot combustion air?
Anthracite has little volatile matter, so it needs more heat to ignite. Preheated air at 180 to 250°C, together with refractory arches, helps the coal reach ignition quickly on the grate. The FD fan still handles cold air; the air heater does the heating downstream.
What pressure does the FD fan need on an anthracite chain grate?
Typically 120 to 200 mmWC, which is higher than on a bituminous coal grate of similar size because of the larger air heater. The exact figure depends on grate resistance, ducts and air heater design, and should include margin for fouling. Zone dampers set the split along the grate.
Can a forced draft fan be shared between grate air and overfire air?
It can, with a branch duct and damper, but the overfire air usually needs higher pressure. On anthracite boilers, which rely on carefully controlled air distribution, a separate overfire air fan often gives better control of both streams. The choice depends on the overfire air pressure the grate maker specifies.
Information needed to quote an FD fan for anthracite and low volatile coal boilers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Air heater. Air side pressure drop and hot air temperature at full load.
- Grate zones. Number of zones and the air distribution set by the grate supplier.
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.