How biogas fired boilers work
Biogas from anaerobic digesters treating distillery spent wash, sugar factory effluent or food waste is fired in boilers to raise process steam. It is typically 55 to 65% methane and 35 to 45% CO₂, saturated with water, with H₂S from a few hundred ppm up to 1 to 2% unless scrubbed. Its calorific value is about 4,500 to 5,500 kcal/Nm³. Many biogas boilers are shell boilers with a gas burner and burner fan; others are solid fuel boilers co-firing biogas, or water tube boilers with balanced draft. In those, the flue gas passes the economiser to an ID fan, and the FD fan supplies the burner air.
Where the ID fan sits
- Gas burner and furnace
- Boiler passes
- Economiser
- ID fan
- Stack
On balanced draft units the ID fan sits after the economiser at the foot of the stack. On biogas alone there is no collector; on co-fired units the fan follows the solid fuel dust collector. Suction is modest, typically 60 to 150 mmWC on gas alone. Because the flue gas carries a lot of water and sulphur, the fan is the coolest point in the gas path, and any drop in gas temperature at low load brings acid condensate onto the casing and impeller.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C after the economiser |
| Maximum or upset | 220 to 250°C with the economiser bypassed |
| Dust at the fan | None on biogas alone; fly ash on co-fired units |
| Moisture and dew point | High water vapour; acid dew point often 120 to 150°C with unscrubbed gas |
| Corrosive species | SO₂ and SO₃ from H₂S; siloxane-derived silica on some sewage and food waste digesters |
| 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 biogas fired boilers
| Blade form | Backward curved, suited to clean gas |
| Material of construction | SS 304 or 316 impeller and wetted parts where gas runs near the dew point; insulated mild steel where it stays well above |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | VFD on furnace draft |
| Also specified | Corrosion-resistant casing drain, insulation and cladding, shaft seal |
What this ID fan has to cope with
Acid condensation at low load
Digester output varies with effluent flow, so the boiler often runs at low load. Gas temperature at the fan then falls toward the acid dew point and sulphuric acid forms on the coolest metal, usually the casing and the impeller back plate. Stainless construction and a minimum gas temperature interlock with economiser bypass protect the fan.
H₂S scrubber outages
When the biogas scrubber is out of service or its media is spent, sulphur in the flue gas rises sharply within hours and can stay high for days. A fan designed only for scrubbed gas corrodes quickly in that period. Material selection should assume the unscrubbed case for at least part of the year.
Large flue gas volume per unit of heat
The CO₂ and moisture in biogas pass through the boiler unchanged, so flue gas volume per unit of heat is higher than on natural gas. An ID fan sized by comparison with a natural gas boiler of the same steam rating is undersized. The fan is selected on the actual biogas composition and its expected range.
Energy and control
Biogas output follows effluent flow, so boiler load changes with plant operation and season; in sugar plants the digester and boiler may stop outside the crushing season. An ID fan on VFD follows these changes without throttling. Where biogas supply is short and the boiler switches to a backup fuel, the fan control must handle both gas volumes, which favours a VFD over damper control.
ID fan and FD fan in biogas fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser, before the stack | Burner fan, or ahead of the windbox |
| What it handles | Moist, acidic flue gas | Ambient air |
| Temperature at the fan | 160 to 190°C | Ambient, 25 to 45°C |
| Main design concern | Acid corrosion | Air-fuel ratio with variable methane |
| Control | Furnace draft | Firing rate with O₂ trim |
See also the Forced Draft (FD) Fan for biogas fired boilers.
Questions buyers ask
What material should the ID fan be for a biogas boiler?
It depends on the H₂S content and the gas temperature at the fan. With scrubbed gas and a stack temperature well above the acid dew point, insulated mild steel can work. With unscrubbed gas or frequent low-load running, SS 316 for the impeller and wetted parts is the safer choice for a biogas fired boiler.
Why does a biogas boiler produce more flue gas than a natural gas boiler?
Biogas carries 35 to 45% CO₂, which passes through the flame unchanged, and it is saturated with water. Both add to the flue gas volume without adding heat, so a biogas boiler of the same steam output has a larger flue gas flow and needs a larger ID fan than its natural gas equivalent.
Can an existing coal or biomass boiler ID fan be used when biogas co-firing is added?
Usually yes, if the fan has spare capacity for the extra moisture and CO₂. The bigger concern is corrosion: biogas sulphur and moisture can push the gas at the fan nearer the acid dew point, especially when solid fuel firing is reduced and gas temperature falls.
Information needed to quote an ID fan for biogas 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.
- Biogas analysis. Methane, CO₂ and H₂S content, and whether a scrubber is installed.
- Fuel mix. Whether biogas is fired alone or with coal, biomass or oil, and in what proportion.
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.