How coffee husk fired boilers work
Coffee husk comes from curing works that hull dry-processed cherry and parchment coffee in Karnataka, Kerala and Tamil Nadu, mostly between December and April. Cherry husk is the dried pulp and skin; parchment husk is the thin papery layer around the bean. As fired the fuel has 10 to 15% moisture, 3 to 8% ash and a calorific value of about 3,800 to 4,200 kcal/kg. Its ash is rich in potassium, which lowers the ash fusion temperature and causes clinker and sticky deposits. Boilers of 1 to 10 TPH on fixed or travelling grates supply steam to curing works, instant coffee plants and nearby industry, many of them in the hill districts at 800 to 1,200 m. Gas passes the boiler, economiser, multiclone and bag filter to the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air duct and damper
- Undergrate plenum
- Grate
The FD fan blows ambient air into the plenum under the grate; small units often have no air heater, so the air reaches the bed at ambient. Part of the air may be taken above the grate to burn volatiles. The fan works against a moderate resistance from the duct, dampers and a light husk bed, typically 60 to 120 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 35°C in the hill districts |
| Air heater | Often none on small units |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Site altitude | Often 800 to 1,200 m |
| Dust | Clean air; chaff from hulling near the fuel store |
| Density at the design temperature | About 1.16 kg/m³ at 30°C, 97% 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 coffee husk fired boilers
| Blade form | Backward curved, sized for mass flow at site density |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Impeller on the motor shaft or direct coupled |
| Bearings and cooling | Standard bearings; no cooling |
| Flow control | Variable speed drive on steam pressure |
| Also specified | Inlet screen and a rain cowl on outdoor intakes |
What this FD fan has to cope with
Short of air at altitude
Combustion needs a mass of oxygen, and in the hills each cubic metre of air holds less of it. A fan sized on sea level volumes leaves the husk short of air, with smoke, unburnt carbon and a sooty boiler as the result. The fan volume is raised to deliver the required mass flow at site density.
Clinker from uneven air
Potassium-rich husk ash fuses wherever the bed runs hot. Uneven undergrate air, for example from a fan that cannot hold plenum pressure when the bed thickens, creates exactly those hot spots. A fan with a stable rising curve and simple zone dampers spread the air and keep the bed below its clinkering point.
Turndown at night
Curing works often cut steam at night, but a fixed-speed FD fan with a damper keeps pushing more air than the reduced fire needs. The bed cools, husk smoulders and smoke rises, and the boiler loses heat up the chimney. A variable speed drive holds the right air at low load.
Energy and control
The FD fan on a curing works boiler spends many hours at low load overnight and between drying batches. A variable speed drive on steam pressure cuts power at those times and keeps excess air under control. Because small husk boilers rarely have oxygen measurement, a simple portable flue gas check once a season is a cheap way to see whether the fan is supplying far more air than needed.
ID fan and FD fan in coffee husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Under the grate |
| What it handles | Flue gas with fine potassium-rich ash | Ambient air at altitude |
| Temperature at the fan | 160 to 190°C | Ambient, 15 to 35°C |
| Main design concern | Draft at site density and sticky ash | Mass of air at site density |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for coffee husk fired boilers.
Questions buyers ask
How much combustion air does a coffee husk boiler need at altitude?
The mass of air per kg of husk is the same as at sea level, but at 1,000 m that mass occupies about 11% more volume. The FD fan must therefore move more cubic metres per hour. The duty should be worked out at site altitude and the warmest expected ambient temperature.
Does a small coffee husk boiler need an air heater?
Not always. Coffee husk is fairly dry, so it ignites without preheated air on most grates. An air heater still improves efficiency and helps when wetter cherry husk is fired, but it adds resistance that the FD fan must overcome, so the fan is checked if one is added.
Why does my coffee husk boiler smoke at night?
When steam demand drops, a fixed-speed FD fan with a part-closed damper often still supplies too much air for the smaller fire, cooling the bed so the husk smoulders. Reducing fan speed with a variable speed drive, rather than throttling, usually cures it and saves power.
Information needed to quote an FD fan for coffee husk fired 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.
- Site altitude. Elevation of the plant above sea level and the ambient temperature range.
- Fuel type. Cherry husk, parchment husk or a blend, and any other fuel fired with it.
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.