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 ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Multiclone
- Bag filter
- ID fan
- Chimney
The fan is after the bag filter. Coffee husk ash is light and fine, so most of it reaches the final collector rather than dropping out in the boiler. Suction at the fan inlet is typically 150 to 220 mmWC on a small unit. Where the boiler is at 800 to 1,200 m, the air and gas are about 10 to 13% less dense than at sea level, so the fan needs more speed or a larger wheel to develop the same draft.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C at the fan |
| Maximum or upset | About 240°C with sooted surfaces |
| Dust at the fan | Fine, potassium-rich ash; low after a bag filter |
| Moisture and dew point | Water dew point about 45 to 55°C |
| Site altitude | Often 800 to 1,200 m, lowering gas density by about a tenth |
| Density at the design temperature | About 0.79 kg/m³ at 175°C, 66% 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 coffee husk fired boilers
| Blade form | Backward curved with smooth surfaces, selected at site density |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled; belt driven on the smallest units |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Access door, casing drain, and the duty stated at site altitude and temperature on the datasheet |
What this ID fan has to cope with
Altitude not allowed for
A fan chosen from a catalogue at sea level density develops less pressure in the hills. At 1,000 m the shortfall is about 11%, enough to leave the boiler short of draft at full load. The selection is corrected for site altitude as well as for gas temperature, and the motor is checked at the coldest morning conditions.
Sticky potassium deposits
Coffee husk ash carries a high share of potassium, which forms low-melting compounds. Deposits build on the economiser tubes, and fine sticky ash that passes the filter can coat the fan inlet and blades. Smooth blades and a cleaning schedule matched to the curing season keep the impeller in balance.
Seasonal lay-up
Many curing works run their boilers hard for four or five months and leave them idle through the monsoon. Damp air and residual ash corrode the casing, and bearings standing still develop false brinelling and rust. Cleaning out ash, turning the shaft periodically and protecting the bearings prevent trouble at restart.
Energy and control
Curing works boilers run in a short, intense season, often near full load by day and much lower at night when drying slows. A variable speed drive on the ID fan suits this pattern well. Because many of these fans are belt driven, checking belt tension and pulley alignment before each season is a simple step that keeps transmission losses down.
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 Forced Draft (FD) Fan for coffee husk fired boilers.
Questions buyers ask
How does altitude affect an ID fan on a coffee husk boiler?
At 1,000 m air and gas density is about 11% lower than at sea level. A fan at the same speed moves the same volume but develops proportionally less pressure, and takes less power. The selection must allow for this with a larger impeller or a higher speed so that the boiler still gets its full draft.
Why does a coffee husk boiler form clinker?
Coffee husk ash is rich in potassium, which lowers its melting point. Hot spots on the grate fuse the ash into clinker that blocks the air spaces. Even undergrate air, a moderate bed temperature and regular grate cleaning reduce it. Blending with a higher-fusion fuel such as wood also helps.
How should an ID fan be kept during the off-season?
Remove ash from the casing and inlet box, keep the bearings greased, and turn the shaft by hand every few weeks so the rolling elements do not sit in one place. Cover the inlet and outlet against rain. This prevents corrosion and bearing damage from monsoon damp before the next season.
Information needed to quote an ID fan for coffee husk fired 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.
- Site altitude. Elevation of the plant above sea level.
- Season. Months of operation each year and how the boiler is laid up.
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.