How tea waste fired boilers work
Two kinds of tea waste are fired in boilers. Spent tea leaves from instant tea and tea extract plants leave the extractors at 70 to 80% moisture and are pressed to 55 to 65% before firing. Factory tea waste, the fibre, stalk and fluff removed during sorting, is dry at about 10% moisture. Both carry 4 to 8% ash and a dry calorific value of about 4,000 to 4,500 kcal/kg, but pressed spent tea yields only about a third of that as fired. Spent tea is burnt on a reciprocating or travelling grate with hot undergrate air, often blended with wood, factory waste or coal. The flue gas passes the boiler, an air heater and economiser, and a multicyclone or bag filter before the ID fan.
Where the ID fan sits
- Reciprocating or travelling grate
- Boiler
- Air heater
- Economiser
- Multicyclone or bag filter
- ID fan
- Chimney
The fan follows the multicyclone or bag filter. Because so much of the gas is water vapour, the volume per tonne of steam is far higher than on a dry fuel boiler, and an ID fan sized by dry-fuel rules runs out of capacity before the boiler reaches rating. Suction at the fan is typically 150 to 250 mmWC. The water dew point is high enough that an uninsulated casing or a bag filter at low load can fall close to condensation.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the economiser |
| Maximum or upset | Up to 230°C with dry factory waste or fouled tubes |
| Dust at the fan | Fine, light ash with some char; low after a bag filter |
| Moisture and dew point | 20 to 30% water vapour by volume with wet spent tea; water dew point about 60 to 70°C |
| Corrosive species | Low sulphur; the risk is condensate wetting ash in the casing, where it hardens into a cement-like layer |
| 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 tea waste fired boilers
| Blade form | Backward curved, sized on wet gas volume |
| Material of construction | Mild steel with full casing insulation; SS 304 for drains and the inlet box where condensate collects |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain at the lowest point, insulation and cladding, and an inspection door |
What this ID fan has to cope with
Fan too small for wet fuel
A boiler changed from coal or wood to spent tea needs much more flue gas volume per tonne of steam, because the water in the fuel leaves as vapour. If the existing ID fan is kept, the boiler cannot reach rated output however well the grate burns. The fan is checked on the wet gas volume from the actual fuel moisture, not the old nameplate.
Condensation and hard deposits
At start-up or low load the gas cools, and with a dew point of 60 to 70°C water condenses on the casing and impeller. Wet ash then sets hard and unbalances the wheel. Insulation, a drain, warming the system on dry fuel before feeding spent tea, and an air heater bypass that holds the gas temperature prevent it.
Moisture swings in the fuel
Spent tea moisture varies with the press condition and with the blend coming from the extraction plant. Wetter fuel means more gas and a cooler furnace for the same steam. The fan needs volume margin above the design moisture, and a drive that lets it follow the change without starving the grate of draft.
Energy and control
Tea extraction plants run continuously through the processing season, and the ID fan carries a large share of the boiler auxiliary power because of the wet gas. Pressing the spent tea harder, or drying it with flue gas, cuts the gas volume and the fan power directly. A variable speed drive then matches the fan to the moisture actually being fired instead of the worst case.
ID fan and FD fan in tea waste fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multicyclone or bag filter | Before the air heater, feeding the grate |
| What it handles | Wet flue gas with fine ash | Ambient air |
| Temperature at the fan | 150 to 180°C, up to 230°C | Ambient, 15 to 35°C |
| Main design concern | Wet gas volume and condensation | Hot drying air and site altitude |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Forced Draft (FD) Fan for tea waste fired boilers.
Questions buyers ask
Why does my boiler lose output when firing spent tea leaves?
Wet spent tea produces much more flue gas per tonne of steam than wood or coal, because its water leaves as vapour. If the boiler ID fan was sized for dry fuel, it reaches its limit before the boiler reaches full output. A fan sized on the wet gas volume at the real fuel moisture solves this.
How wet is spent tea used as boiler fuel?
Spent tea leaves the extractors at 70 to 80% moisture and is usually pressed to 55 to 65% before firing. Every point of moisture removed reduces the flue gas volume, the ID fan power and the hot air needed to dry the fuel on the grate, so press condition matters to the whole boiler.
Should the ID fan on a tea waste fired boiler be stainless steel?
Normally the impeller and casing are mild steel, insulated so they stay above the dew point in running. Stainless steel is used for drains, inlet boxes and other parts that stay cold or wet during start-up and shutdown. A full stainless fan is rarely justified unless the plant cycles daily.
Information needed to quote an ID fan for tea waste 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.
- Fuel moisture. Moisture of the spent tea as fired, and the share of other fuels blended with it.
- Fuel drying. Whether a screw press or flue gas dryer is installed or planned.
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.