How refinery fuel gas fired boilers work
Refinery fuel gas boilers raise high pressure steam for turbines, process heating and stripping. They fire the refinery's own gas, a mixture of hydrogen, methane, ethane and heavier hydrocarbons from process off-gases, after amine treating has removed most of the H₂S. Composition and calorific value swing with unit operations, and hydrogen content can range from about 10% to over 50%. Boilers are typically 50 to 250 TPH water tube units, often also fired with fuel oil or natural gas. Many run balanced draft, with an air heater and sometimes SCR, an ID fan before the stack, and FD fans supplying the windbox. Refinery specifications cover the fans in detail.
Where the FD fan sits
- Air intake and silencer
- FD fan
- Steam coil air preheater
- Air heater
- Windbox
- Burners
One or two FD fans stand at ground level and push ambient air through a steam coil preheater, used on oil firing and at start-up, then the regenerative or tubular air heater, to the windbox. The fan works against the preheaters, ducts, windbox and burner registers, typically 200 to 350 mmWC on balanced draft units. Because the fan stands within a classified area, its intake must be located away from any point where hydrocarbons could be released.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the air heater | 200 to 300°C, downstream of the fan |
| Pressure at the fan outlet | Typically 200 to 350 mmWC on balanced draft |
| Dust | Clean air; hydrocarbon vapour risk near the intake |
| Air requirement | Varies with hydrogen and heavier hydrocarbon content of the fuel gas |
| 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 refinery fuel gas fired boilers
| Blade form | Aerofoil or backward curved, double inlet on large units |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, motor suited to the hazardous area |
| Bearings and cooling | Oil-lubricated pedestal bearings with monitoring |
| Flow control | VFD or inlet vanes on air flow, cross-limited with fuel, with O₂ trim |
| Also specified | Compliance with the plant fan specification, inlet silencer, intake away from release sources |
What this FD fan has to cope with
Air requirement swinging with gas composition
Hydrogen needs noticeably less air per unit of heat than methane or heavier hydrocarbons, so when hydrogen content in the fuel gas changes, the air needed for a given heat input changes too. If the air control follows fuel flow alone, excess air drifts. A Wobbe index or calorific value signal, or O₂ and CO trim, keeps air correct.
Hydrocarbon vapour at the intake
A leak elsewhere in the refinery can carry hydrocarbon vapour into the FD fan intake, enriching the combustion air and creating a fire risk in the windbox and ducts. Locating the intake away from release points, and gas detection at the intake where the plant's safety studies require it, address this.
Single fan trip
With two fans, a trip must not take the boiler down, since loss of steam can trip process units across the refinery. Isolation dampers that close fast, runback logic in the combustion control and fans sized at about 60% each let the boiler continue at reduced load until the fan is restored.
Energy and control
Continuous operation with process-driven swings makes VFD control the usual choice on new refinery boiler FD fans. Accurate air control on variable fuel gas saves fuel by holding excess air low, and it also helps keep NOx down. Air heater and windbox leaks are worth surveying before any fan replacement, since they raise FD fan duty without adding anything to combustion.
ID fan and FD fan in refinery fuel gas fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After SCR and air heater, before the stack | Ahead of the air heater and windbox |
| What it handles | Flue gas with residual sulphur | Ambient air in a classified area |
| Temperature at the fan | 140 to 170°C | Ambient, 20 to 45°C |
| Main design concern | Dew point and deposits | Air control on variable fuel |
| Control | Furnace pressure | Air flow with O₂ trim |
See also the Induced Draft (ID) Fan for refinery fuel gas fired boilers.
Questions buyers ask
How does fuel gas composition affect FD fan control in a refinery boiler?
Hydrogen-rich gas needs less air per unit of heat than methane-rich or heavier gas, so the air needed for a given fuel flow changes with composition. The control uses O₂ trim, and sometimes a calorific value or Wobbe index measurement, to keep excess air correct as the refinery fuel gas changes.
Why is the FD fan intake location important in a refinery?
Refineries can have hydrocarbon releases. If vapour reaches the FD fan intake, it is carried into the windbox and furnace. The intake is placed away from likely release points and may be fitted with gas detection, following the plant's own safety requirements and area classification.
Do refinery boilers need two FD fans?
Many do, so that a single fan trip runs the boiler back instead of tripping it and cutting steam to the process. The fans are sized at about 60% each, with fast isolation dampers and control logic for single-fan running, and each fan is checked for stable operation alone.
Information needed to quote an FD fan for refinery fuel gas 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 range. Range of fuel gas composition and any backup fuels.
- Area classification. Hazardous area class at the fan location and the motor requirements.
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.