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 ID fan sits
- Burners and furnace
- Superheater
- Convection bank
- Economiser
- SCR
- Air heater
- ID fan
- Stack
The ID fan, one or two per boiler, sits at ground level after the air heater and before the stack. There is no collector in normal practice. Suction covers the furnace, superheater, convection bank, economiser, any SCR catalyst and the air heater, typically 200 to 350 mmWC. The fan stands in a classified area, so the motor, instruments and accessories must suit the area classification. Placing it at the cold end keeps the volume down, but puts it close to the acid and ammonium bisulphate deposition temperatures.
Gas at the ID fan
| Gas temperature, normal | 140 to 170°C after the air heater |
| Maximum or upset | Up to 250°C with air heater bypass or oil firing |
| Dust at the fan | None on gas; soot during fuel oil firing |
| Moisture and dew point | High water vapour on hydrogen-rich gas; acid dew point set by residual sulphur, typically 100 to 130°C |
| Corrosive species | SO₃, and ammonium bisulphate where SCR ammonia slip meets sulphur |
| Density at the design temperature | About 0.83 kg/m³ at 155°C, 69% 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 refinery fuel gas fired boilers
| Blade form | Backward curved or aerofoil, for clean gas and long running |
| Material of construction | Mild steel with insulated casing; corrosion-resistant steel where gas runs close to the dew point |
| Drive and arrangement | Direct coupled, often a medium-voltage motor suited to the hazardous area |
| Bearings and cooling | Oil-lubricated pedestal bearings with vibration and temperature monitoring |
| Flow control | VFD or inlet vanes on furnace pressure |
| Also specified | Documentation to the plant fan specification, which often refers to API 673 |
What this ID fan has to cope with
Gas volume changing with fuel composition
Hydrogen-rich fuel gas produces more water vapour, and a different flue gas volume per unit of heat, than methane-rich or heavier gas. As refinery operations change, the ID fan duty shifts with them. The fan is selected across the full range of compositions the refinery expects, with the margin applied to the worst case rather than the average.
Ammonium bisulphate fouling
Where SCR is fitted and the fuel carries residual sulphur, ammonia slip forms ammonium bisulphate, which deposits as a sticky layer in the air heater and sometimes on the fan. It collects soot and unbalances the impeller. Controlling ammonia slip and keeping the gas above the deposition range limit the fouling.
Long runs between turnarounds
Refinery boilers run for years between turnarounds, and an ID fan failure can cut steam to process units. Bearings, seals and couplings are chosen for long life, with continuous vibration and temperature monitoring so that developing faults are seen early and repaired at the next planned opportunity.
Energy and control
Refinery boilers run continuously, usually at 60 to 100% load, with swings as process steam demand changes. Fan power over years of continuous running makes efficiency important, and a VFD saves more than inlet vanes over the part-load hours. Keeping air heater leakage low and ammonia slip under control also prevents the fan duty from creeping up between turnarounds.
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 Forced Draft (FD) Fan for refinery fuel gas fired boilers.
Questions buyers ask
How does refinery fuel gas affect the boiler ID fan?
Refinery fuel gas is a mixture of hydrogen, methane and heavier hydrocarbons from process units, treated to remove most of the H₂S. Its composition changes with operations, which changes flue gas volume, moisture and dew point. The ID fan is selected for the range of compositions, not one analysis.
Is API 673 required for a refinery boiler ID fan?
Many refinery specifications call for fans to API 673 or similar requirements for continuous, critical service. The requirement comes from the plant's own engineering standards, and the purchaser's specification decides which clauses apply. The fan supplier should confirm compliance clause by clause at the enquiry stage.
Why does ammonium bisulphate deposit on my refinery boiler ID fan?
Where SCR is used and the fuel has residual sulphur, ammonia slip reacts with SO₃ to form ammonium bisulphate, which condenses as a sticky deposit within a band of gas temperatures. Controlling ammonia slip and keeping the gas above that band reduce the deposits on the air heater and fan.
Information needed to quote an ID fan for refinery fuel gas 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 gas range. Range of hydrogen content and calorific value, and any backup fuel.
- Plant specification. Applicable fan specification and area classification at the fan.
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.