How agro-Residue pulp mill boilers work
Agro-residue pulp and paper mills make pulp from wheat straw, rice straw, bagasse or sarkanda grass instead of wood. Their steam and power boilers, usually AFBC or travelling grate units of 10 to 60 TPH, burn the mill's own residues: bagasse pith from depithing, straw dust and rejects, rice husk bought in, and coal for support. These fuels differ sharply. Wet pith can be 50% moisture, straw ash is rich in potassium and chloride and melts at low temperature, and husk ash is mostly silica. Flue gas passes the boiler bank, economiser and air heater at 150 to 180°C, then an ESP or bag filter before the ID fan. Chemical recovery from silica-rich agro black liquor is a separate and harder problem, often handled by a separate boiler.
Where the ID fan sits
- Furnace or bed
- Superheater and boiler bank
- Economiser
- Air heater
- ESP or bag filter
- ID fan
- Chimney
The fan sits after the ESP or bag filter and handles 150 to 180°C gas with residual fly ash. Suction at the inlet is typically 200 to 300 mmWC on a 20 to 40 TPH AFBC. On agro mill boilers the duct and collector are often added to over the years, so actual suction at the fan can be well above the original design. A measured draft survey is the starting point for any replacement.
Gas at the ID fan
| Gas temperature, normal | 150 to 180°C after the air heater |
| Maximum or upset | Up to 220 to 240°C with fouled superheater and air heater |
| Dust at the fan | Mixed ash: silica from husk, alkali-rich straw ash, coal ash; 50 to 150 mg/Nm³ after the collector |
| Moisture and dew point | High with wet pith; water vapour up to 20% by volume, water dew point near 55 to 65°C |
| Corrosive species | Chlorides and alkali salts from straw; mild acid dew point with coal |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 agro-Residue pulp mill boilers
| Blade form | Backward curved with wear liners, sized for the wettest fuel mix |
| Material of construction | Mild steel casing; impeller in wear-resistant plate or with hard-faced leading edges |
| Drive and arrangement | Direct coupled, arrangement 8, impeller speed kept moderate for erosion |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection door, casing drain, and wear liners at the cut-off |
What this ID fan has to cope with
Gas volume from wet pith
Pith from wet depithing can reach the boiler at 50% moisture, and the evaporated water adds a large volume to the flue gas. A fan sized on dry coal or husk runs out of draft and the boiler derates. The fan is sized on the wettest pith share the mill intends to burn, with drive margin.
Alkali chloride deposits
Straw ash rich in potassium and chloride forms sticky deposits on superheaters, air heaters and, at lower temperature, on the fan impeller. They build unevenly and cause vibration. Smooth blades, inspection access and balancing at each shutdown keep the fan in service. Soot blowing upstream helps.
Silica erosion from husk
When husk forms a large part of the fuel, abrasive silica reaches the fan whenever the collector is bypassed or leaking. Wear plate on the impeller and lower tip speed make the fan tolerant of fuel changes. Liners at the cut-off and blade leading edges are designed for replacement at site.
Low end corrosion during stops
Chlorides and moisture condense on cold casings during shutdowns and start corrosion that continues on restart. Insulating the casing and running the fan briefly with warm air before shutdown limit this. A casing drain removes condensate, and the inlet box, where water collects first, is inspected at every shutdown.
Energy and control
Agro mill boilers follow paper machine steam demand, and fuel mix changes with the season as straw, bagasse and husk availability shifts. Gas volume per tonne of steam can vary by a third between a dry coal-heavy mix and a wet pith-heavy one. A variable speed drive on furnace draft handles both, where a fixed-speed fan sized for the wet case wastes power on the dry one.
ID fan and FD fan in agro-Residue pulp mill boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the chimney | At the air heater inlet, feeding the air box |
| What it handles | Flue gas with mixed agro ash | Ambient air with fibre risk |
| Temperature at the fan | 150 to 180°C | Ambient |
| Main design concern | Wet fuel gas volume and alkali deposits | Fluidising pressure and clean intake |
| Control | Furnace draft | Steam pressure and oxygen |
See also the Forced Draft (FD) Fan for agro-Residue pulp mill boilers.
Questions buyers ask
Why does my pulp mill boiler lose draft when firing pith?
Wet pith adds a lot of water vapour, which increases flue gas volume and the duty on the ID fan. If the fan was sized for coal or dry fuel, it reaches its limit and the boiler must cut load. A larger impeller or new fan sized for the wet mix solves it.
What causes deposits on the ID fan of a straw-fired boiler?
Straw ash contains potassium and chloride, which form sticky salts that condense on cooler surfaces, including the fan. Deposits build unevenly and cause vibration. Regular cleaning, smooth blades and good dust collection reduce it. Keeping the fan inlet warm prevents condensation that makes the deposits harder and more corrosive.
How should an ID fan for a multi-fuel agro mill boiler be sized?
On the fuel mix that gives the largest gas volume and highest suction, usually the wettest pith or straw share, with margin for air ingress and fouled heat surfaces. Control on furnace draft with a variable speed drive covers the drier fuels efficiently. Measure first.
Information needed to quote an ID fan for agro-Residue pulp mill 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 mix. Fuels fired, their moisture and share, now and planned.
- Draft survey. Measured suction and temperature at the fan inlet at full load.
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.