How coal rotary dryers work
Coal rotary dryers reduce the moisture of washed coal, lignite or wet imported coal from about 15 to 35% to the level needed by the kiln, boiler or briquetting plant. Hot gas from a coal-fired furnace, often burning a share of the product, or from a waste heat source, enters a co-current drum at a limited temperature, typically 350 to 600°C, to avoid igniting the coal. Many plants recirculate part of the exhaust to lower the oxygen content of the drying gas, holding it typically below about 10 to 12% for lignite and high-volatile coals. The exhaust leaves at about 70 to 95°C, humid and laden with fine coal dust, and passes cyclones and a bag filter with explosion protection, or a wet scrubber, before the ID fan and the stack.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Coal-fired furnace
- Mixing chamber with recirculated gas
- Rotary drum
The fan sits at the coal-fired furnace or hot air generator and supplies undergrate or burner air. Hot furnace gas meets recirculated exhaust in a mixing chamber, which lowers both temperature and oxygen before the drum. The FD fan pressure covers the grate or burner and the furnace; it does not handle the recirculated gas, which comes from the ID fan discharge through a separate duct and damper.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Furnace gas | Mixed with recirculated exhaust to 350 to 600°C before the drum |
| Dust | Ambient air in a coal yard; filtered intake advised |
| Pressure | Undergrate or burner pressure and furnace resistance |
| Oxygen | Excess air kept low to limit oxygen in the drying gas |
| 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 coal rotary dryers
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven, variable speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with oxygen trim |
| Also specified | Intake filter and flow measurement |
What this FD fan has to cope with
Too much excess air
Extra combustion air raises oxygen in the drying gas and the risk of a coal dust fire or explosion. The fan is controlled with oxygen trim so combustion is complete with minimum excess, and fresh dilution air is avoided.
Coal dust in the intake
Coal yards and crusher houses are dusty, and coal dust drawn into the combustion air fan fouls the burner or grate and wears the wheel. The intake is filtered and placed upwind of coal handling.
Furnace response to moisture swings
A wet batch of coal raises heat demand quickly, and a dry one lowers it. The fan and furnace must follow without overshoot, which would raise the inlet temperature and risk ignition. A speed-controlled fan with a stable curve follows smoothly.
Energy and control
The combustion air fan is small relative to the ID fan, because recirculated gas does most of the dilution. Speed control on firing rate with oxygen trim saves fuel and power, and limiting excess air keeps the drying gas inert enough for safe operation. Fuel savings from recirculation are usually larger than any fan saving.
ID fan and FD fan in coal rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After bag filter, feeding recirculation and stack | At the coal-fired furnace |
| What it handles | Humid gas with explosive coal fines | Ambient combustion air |
| Temperature at the fan | 70 to 95°C | Ambient |
| Main design concern | Explosion safety and condensation | Low excess air |
| Control | Drum suction and oxygen | Firing rate and oxygen |
See also the Induced Draft (ID) Fan for coal rotary dryers.
Questions buyers ask
Does a coal rotary dryer use fresh dilution air?
Ideally very little. Fresh dilution air adds oxygen to the drying gas, so coal dryers cool the furnace gas with recirculated exhaust instead. Fresh air is used only where the coal is low in volatiles and the oxygen limit allows.
What is the FD fan on a coal rotary dryer?
It is the combustion air fan of the coal-fired furnace or hot air generator that makes the drying gas. It handles ambient air and supplies only what combustion needs, with minimum excess air, while recirculated exhaust provides the dilution.
How is oxygen controlled in a coal dryer?
By minimising excess air at the furnace, recirculating exhaust to dilute the furnace gas and keeping drum seals tight. An oxygen analyser at the drum outlet trims the combustion air fan speed and the recirculation damper.
Information needed to quote an FD fan for coal rotary dryers
- 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.
- Furnace. Furnace type, fuel and combustion air flow and pressure.
- Oxygen control. Target oxygen and how it is measured and trimmed.
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.