How mustard husk and stalk fired boilers work
Mustard husk and stalk are residues of mustard harvesting and seed processing in Rajasthan, Haryana, Madhya Pradesh and Uttar Pradesh, where they fuel boilers in oil mills, solvent plants and textile units. The fuel has 10 to 15% moisture, 4 to 8% ash and a calorific value of about 3,500 to 4,000 kcal/kg. Unlike most agro-residues it carries noticeable sulphur, along with potassium and chlorine, so the flue gas holds SO₂ and the ash fuses at a relatively low temperature. Firing is on travelling or reciprocating grates, and in AFBC units on larger plants. Gas passes the boiler, economiser and air heater, then a multiclone and bag filter or ESP to the ID fan. Ambient temperature at site ranges from near freezing in winter to above 45°C in summer.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater with bypass
- Undergrate plenum
- Grate zones
The FD fan takes ambient air near the air heater and pushes it through the air heater tubes into the undergrate plenum. Many plants route part of the cold air around the air heater through a bypass at low load, to keep the flue gas outlet warm. The fan works against the air heater or bypass, the ducts, zone dampers and the husk and stalk bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 5 to 45°C across the year |
| Air after the air heater | 130 to 180°C, downstream of the fan |
| Pressure at the fan outlet | Typically 100 to 180 mmWC |
| Density range | Cold winter air is about 12% denser than summer air |
| Dust | Clean air; husk dust near the fuel yard |
| 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 mustard husk and stalk fired boilers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling |
| Flow control | Variable speed drive on steam pressure, with a modulating air heater bypass damper |
| Also specified | Inlet screen, and a motor sized for winter air density |
What this FD fan has to cope with
Clinker from an over-hot bed
Mustard ash fuses at a low temperature. Too little undergrate air concentrates combustion and raises bed temperature; too much in one zone overheats that part of the grate. Zone dampers, a fan that holds steady plenum pressure, and overfire air to burn volatiles above the bed keep the bed below its clinkering point.
Winter air density
In north Indian winters the ambient can fall close to 5°C, making the air about 12% denser than in summer. A fan sized on summer air can overload its motor in winter at the same speed. The motor is sized for the coldest air, and the variable speed drive limits current.
Air heater bypass and leakage
Bypassing air around the air heater at low load protects the gas side from acid but changes the resistance the FD fan works against. Leaking air heater tubes, common once acid corrosion has started, pass air straight into the flue gas. Both shift the FD fan duty and are included in its selection.
Energy and control
Mustard processing has a strong season after harvest and lighter loads afterwards. A variable speed drive on the FD fan saves power through the long part-load period, while oxygen trim stops excess air creeping up as operators try to avoid clinker. Lower excess air also reduces the volume of SO₂-bearing gas the ID fan must handle.
ID fan and FD fan in mustard husk and stalk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or ESP | Ahead of the air heater |
| What it handles | Flue gas with SO₂ and alkali ash | Ambient air |
| Temperature at the fan | 165 to 185°C | Ambient, 5 to 45°C |
| Main design concern | Acid dew point corrosion | Clinker control and winter density |
| Control | Furnace draft with a minimum gas temperature | Steam pressure with oxygen trim |
See also the Induced Draft (ID) Fan for mustard husk and stalk fired boilers.
Questions buyers ask
Why does my mustard stalk boiler form clinker?
Mustard ash contains potassium and melts at a fairly low temperature. If the bed runs too hot, because of uneven undergrate air or too little overfire air, the ash fuses into clinker. Balancing air across the grate zones and adding overfire air to burn volatiles above the bed usually reduces it.
Does the FD fan need a bigger motor in winter?
Cold air is denser, so at the same speed the fan takes more power in winter. In north India the difference between summer and winter air can be about 12%. The motor should be sized for the coldest expected air, or the variable speed drive set to limit current.
What is the air heater bypass for on a mustard husk boiler?
At low load the air heater can cool the flue gas below its acid dew point. Sending part of the FD air around the air heater reduces the heat it extracts, keeps the flue gas warmer and protects the air heater, ducts and ID fan from corrosion.
Information needed to quote an FD fan for mustard husk and stalk 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 climate. Minimum and maximum ambient temperature at site.
- Air heater bypass. Whether an air side bypass exists and how it is controlled.
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.