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 ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Air heater
- Multiclone
- Bag filter or ESP
- ID fan
- Chimney
The fan is placed after the final collector, and the air heater outlet temperature is set to keep the gas above its acid dew point all the way to the chimney. Suction at the fan inlet is typically 170 to 250 mmWC on a 10 to 20 TPH unit. Placing the fan downstream of the collector keeps sticky ash out of the casing; keeping the gas warm keeps acid out of it.
Gas at the ID fan
| Gas temperature, normal | 165 to 185°C after the air heater |
| Maximum or upset | Up to 230°C with fouled heat surfaces |
| Dust at the fan | Fine alkali-rich ash; low after the final collector |
| Moisture and dew point | Water dew point about 50°C; acid dew point higher than for most biomass, depending on fuel sulphur |
| Corrosive species | SO₂ with some SO₃, and chlorides; aggressive in condensate |
| 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 mustard husk and stalk fired boilers
| Blade form | Backward curved, with smooth surfaces to shed deposits |
| Material of construction | Mild steel with acid-resistant coating; SS 316 at the inlet box and drain areas, or a stainless wheel where the boiler cycles often |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing insulation and cladding, a drain and an access door |
What this ID fan has to cope with
Acid attack at the cold end
Mustard residue releases SO₂, part of which converts to SO₃ and raises the acid dew point. If the fan casing or inlet duct cools below it, sulphuric acid condenses together with chlorides and corrodes steel rapidly. Insulation, a minimum gas temperature at the air heater outlet and acid-resistant material at the low points are the response.
Months at low load
Out of season many plants run their boilers at low load, and the gas temperature at the fan drops. A fan that is fine at full load can corrode quickly if it spends months below the acid dew point. An air side bypass on the air heater, or a hot gas bypass, holds the fan inlet temperature through these periods.
Sticky ash and fouling
Mustard ash melts at a relatively low temperature and forms sticky deposits on the boiler bank and economiser. System resistance rises between cleanings, so the fan duty includes a margin for fouling, and smooth blade surfaces with an access door for washing reduce build-up on the impeller itself.
Energy and control
Oil mills crushing mustard run hardest after the rabi harvest and at reduced load later in the year. A variable speed drive on the ID fan avoids throttling losses through the low-load months. Energy saving must not push the air heater outlet below the acid dew point, so the control keeps a minimum gas temperature at the fan, even if a little heat goes up the chimney.
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 Forced Draft (FD) Fan for mustard husk and stalk fired boilers.
Questions buyers ask
Why does my ID fan corrode on a mustard husk fired boiler?
Mustard residues carry more sulphur than most agro-residues, so the flue gas has a higher acid dew point. If the fan or duct cools below it during low load or shutdown, sulphuric acid and chlorides condense and attack the steel. Keeping the gas warm and protecting the low points stops most of the corrosion.
What gas temperature should the ID fan see on a mustard stalk boiler?
Usually 165 to 185°C at full load. Going much lower to recover heat risks acid condensation in the air heater, duct and fan. The safe minimum depends on the sulphur and moisture of the fuel actually fired, and is best confirmed by measuring the acid dew point at site.
Is a stainless steel ID fan needed for mustard husk?
Not always. A mild steel fan with good insulation and protective coating performs well while the gas is held above the acid dew point. Where the boiler cycles often or runs long at low load, stainless steel at the inlet box and drains, or a full stainless impeller, is worth the extra cost.
Information needed to quote an ID fan for mustard husk and stalk 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 analysis. Sulphur and chlorine content of the mustard husk and stalk, if tested.
- Low-load operation. Minimum load and how long the boiler runs at it out of season.
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.