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 FD fan sits
- Air intake
- FD fan
- Air damper
- Biogas burner
- Furnace
- Boiler passes
The FD fan is either the burner's own fan or a separate fan feeding a windbox on larger and co-fired units. It takes ambient air and delivers it to the burner register. Biogas arrives at low pressure from the gas holder or a gas blower, so the burner relies heavily on air velocity for mixing. Fan pressure covers the burner, the furnace and, on pressurised shell boilers, the gas passes; on balanced draft units the ID fan takes the gas side.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Air requirement | About 5.2 to 6.2 m³ of air per m³ of biogas, varying with methane content |
| Dust | Clean; traces of H₂S if the intake is near the digester |
| Turndown | Limited by gas pressure and burner design, typically 3:1 to 5:1 |
| 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 biogas fired boilers
| Blade form | Backward curved, stable at low flow |
| Material of construction | Mild steel; coated or stainless if the intake is near the digester or scrubber |
| Drive and arrangement | Direct mounted or direct coupled |
| Bearings and cooling | Motor or pedestal bearings |
| Flow control | VFD with O₂ trim, compensating for methane variation |
| Also specified | Inlet screen, air pressure switch, intake away from digester vents |
What this FD fan has to cope with
Methane content swings
Digester methane can drop from about 65% toward 50% during feed or temperature upsets. With a fixed air setting the burner then runs with far more excess air than intended, the flame cools and weakens, and it can lift off. O₂ trim on fan speed, or a methane analyser feeding the air-fuel ratio, keeps combustion stable through these swings.
Corrosive air near the digester
Air drawn near digesters, gas holders or scrubbers can carry H₂S, which slowly corrodes the impeller, casing and motor windings. An intake taken from the clean side of the boiler house, and coated or stainless parts where that is not possible, extend the life of the fan and motor.
Flame-out at low gas pressure
When the gas holder runs low, pressure at the burner falls and the flame becomes unstable. The FD fan must reduce air quickly enough to hold the ratio, or the burner trips. Fast VFD response, a low gas pressure interlock and an automatic changeover to backup fuel are the usual design response.
Energy and control
A biogas boiler's load follows gas availability, so it modulates through the day as the holder fills and empties. The FD fan on VFD follows the firing rate and saves power over damper control. More important, holding excess air at the right level for the changing gas keeps more of the limited biogas available as steam, which is the real economic value of the 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 Induced Draft (ID) Fan for biogas fired boilers.
Questions buyers ask
How much combustion air does biogas need?
Each cubic metre of biogas at 55 to 65% methane needs about 5.2 to 6.2 m³ of air for complete combustion, plus excess air, typically 15 to 25% on simple burners. Because methane content varies, the FD fan and its control must cover the whole range, not one fixed figure.
Why does my biogas boiler flame lift off or go out?
Common causes are a drop in methane content with too much air, low gas pressure from the holder, or a fan whose air setting is fixed. Stable combustion needs air to follow the actual gas quality through O₂ trim or a gas analyser, and a burner designed for low calorific gas.
Can a natural gas burner fan be used for biogas?
The fan may be suitable, but the burner head and gas train must be designed for biogas, which has roughly half the calorific value of natural gas and arrives at lower pressure. Air per cubic metre of gas is lower, but the gas flow is about double for the same heat, so mixing changes.
Information needed to quote an FD fan for biogas 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.
- Gas pressure at the burner. Minimum and normal biogas pressure from the holder or gas blower.
- Methane range. Lowest and highest methane content seen in operation.
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.