How methanol and alcohol fuel fired boilers work
Methanol and alcohol fuel boilers are usually three-pass fire tube or small water tube units fitted with a pressure-jet or rotary cup burner adapted for a low-viscosity, low-lubricity liquid. Methanol has a net calorific value of about 4,700 to 4,800 kcal/kg, roughly half that of furnace oil, and ethanol about 6,400 kcal/kg, so the fuel flow is large for the heat released. Neither carries sulphur or ash. The flame is pale and weakly radiating, the furnace runs at a slight positive pressure, and the flue gas, rich in water vapour, leaves through an economiser to a self-supporting stack. A single forced draft fan, built into the burner or mounted beside it, supplies all the combustion air. There is no ID fan.
Where the FD fan sits
- Boiler house air inlet
- FD fan
- Air damper
- Burner register
- Pressurised furnace
- Economiser
- Stack
The fan sits on the burner front or on a skid beside the boiler, drawing air from the boiler house or through a duct from outside. It delivers air to the burner wind box against the pressure of the furnace, the fire tube passes, the economiser and the stack, typically 80 to 200 mmWC on package units. Methanol storage and day tanks are normally kept outside the boiler room, but leaks at the burner gun and pump set can release vapour near the fan, so the air inlet is placed away from fuel handling areas.
Air at the FD fan
| Air temperature at the fan | Ambient boiler house air, 25 to 45°C |
| Pressure at the fan outlet | Typically 80 to 200 mmWC, including furnace back pressure |
| Air quantity | Methanol needs about 6.4 kg of air per kg of fuel at stoichiometric, much less per kg than oil, but a similar amount per unit of heat |
| Flue gas moisture | About 20% water vapour by volume, dew point near 60°C; affects the economiser and stack, not the fan |
| Hazardous area | Fuel vapour is heavier than air and flashes at about 11°C; the area around the burner pump set may be classified |
| 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 methanol and alcohol fuel fired boilers
| Blade form | Backward curved, for a stable rising pressure curve at the burner |
| Material of construction | Mild steel, with spark-resistant inlet ring or non-sparking pairing where the area classification calls for it |
| Drive and arrangement | Direct mounted on the motor shaft for compact burners; direct coupled on a skid for larger separate fans |
| Bearings and cooling | Motor bearings or standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or servo-driven air damper, linked to the fuel valve through the burner management system |
| Also specified | Air pressure switch for purge proving, flameproof motor if the zone requires it, inlet silencer |
What this FD fan has to cope with
Purge must be proven every start
A methanol leak during a standby period can leave vapour in the furnace. The fan must deliver the full pre-purge air flow every time, proven by a pressure or flow switch, before the ignition sequence begins. A fan that has lost pressure through a slipping belt or a choked inlet will hold the burner in lockout.
Air-fuel ratio on a low-radiance flame
The methanol flame is nearly invisible and emits little soot, so poor combustion is not obvious to the operator. Excess air drifts upward without notice, cooling the furnace and wasting fuel. A fan with smooth, repeatable control and an oxygen trim loop keeps excess air in the right band.
Electrical equipment near flammable vapour
If the boiler room is classified as a hazardous zone, the fan motor, pressure switches and junction boxes must suit that zone. The decision belongs to the plant's area classification study; the fan is then supplied with the motor and accessories it calls for. Motor certification is stated in the enquiry.
Energy and control
Alcohol fired boilers in pharmaceutical and chemical plants often follow batch steam demand, swinging between low and high fire through the day. A variable speed drive on the combustion air fan reduces power at low fire and, with oxygen trim, holds excess air close to target across the range. Because methanol is costly per unit of heat, the fuel saved by tight air control is worth far more than the fan power saved.
Questions buyers ask
Does a methanol fired boiler need an ID fan?
Usually not. Methanol and alcohol fired package boilers run with a pressurised furnace, and the FD fan on the burner provides enough pressure to push the flue gas through the boiler, economiser and stack. An ID fan is only added where a long flue, a heat recovery unit or a common stack adds more resistance than the burner fan can overcome.
Can an oil burner fan be used when converting a boiler to methanol?
The air quantity per unit of heat is similar, so the existing fan is often adequate in volume. The burner gun, pump, seals and controls must change for methanol, and the fan motor and switches must be checked against the area classification. The pressure duty should be confirmed against the new burner head.
Why is purge time important on a methanol boiler?
Methanol vaporises readily, so any leak can fill the furnace with a flammable mixture during a shutdown. The combustion air fan sweeps the furnace with several volume changes before ignition. The fan duty and the proving switch must make sure this happens every start. Shortening purge to save time is a false economy.
Information needed to quote an FD fan for methanol and alcohol fuel 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.
- Fuel and blend. Whether the fuel is methanol, ethanol, a blend or a dual-fuel arrangement with oil or gas.
- Area classification. The hazardous area zone, if any, at the burner and fan location.
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.