How steam methane reformers work
A steam methane reformer produces hydrogen by passing natural gas or naphtha with steam through catalyst-filled tubes in a large firebox. Process gas leaves the tubes at about 850 to 900°C. Most hydrogen plant reformers are top-fired, with burners in the roof between tube rows and flue gas collected in tunnels at the floor; others are side-fired or terrace-fired. The main fuel is PSA tail gas, rich in CO₂ and low in heating value, topped up with natural gas. Flue gas leaves the radiant box at about 1,000°C and passes a convection section that preheats feed, superheats steam, generates steam and preheats combustion air. An ID fan draws the gas to the stack at 120 to 160°C, and an FD fan supplies combustion air through the preheater.
Where the FD fan sits
- Inlet filter and silencer
- FD fan
- Combustion air preheater
- Hot air ducts
- Burner manifolds
- Roof or wall burners
The fan sits at grade near the convection section. It delivers ambient air to the combustion air preheater, where it is heated to 300 to 450°C by flue gas near the cold end of the convection section, and then through insulated ducts to manifolds feeding each row of roof or wall burners. Pressure needed is typically 150 to 300 mmWC, covering the preheater, ducts, manifolds and burner registers.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 300 to 450°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Turndown | Hydrogen demand sets the load, typically 50 to 100% |
| 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 steam methane reformers
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Oil-lubricated bearings on large fans |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet silencer and filter, API 673 or equivalent |
What this FD fan has to cope with
Uneven air across burner rows
The many burners in a top-fired reformer must each get similar air, or the tube rows next to starved burners run cooler while others overheat. Tube life depends strongly on tube wall temperature, so uneven air shortens tube life. Manifold design, balancing dampers and enough fan pressure to make the burner the main resistance even out the distribution.
Preheater bypass and tube temperature
When the combustion air preheater is bypassed for maintenance, cold air reaches the burners and the fuel demand rises to hold the same process duty. The flue gas flow rises with it. The FD fan must still supply the needed flow, and the burners and the ID fan are both checked for this case so that the plant can run without the preheater.
Air flow and tail gas swings
PSA tail gas flow pulses with each PSA cycle, and the trim fuel controller responds, so the air demand changes continuously. A fan with fast speed control and oxygen trim lets the air follow the fuel closely, keeping combustion stable and excess oxygen low without the operator having to leave a large safety margin of air.
Energy and control
The FD fan runs continuously with the reformer, at a load set by hydrogen demand. A variable speed drive, with oxygen trim, keeps excess air low and matches fan power to load without damper throttling. Low excess air also reduces the flue gas volume, which cuts ID fan power, and raises the radiant efficiency, so less fuel is fired for the same hydrogen output.
ID fan and FD fan in steam methane reformers
| ID fan | FD fan | |
|---|---|---|
| Position | End of convection section, before stack | Before the combustion air preheater |
| What it handles | Large, clean flue gas flow | Ambient air |
| Temperature at the fan | 120 to 160°C | Ambient |
| Main design concern | Reliability; no natural draft fallback | Air distribution to many burners |
| Control | Firebox draft | Air flow with oxygen trim |
See also the Induced Draft (ID) Fan for steam methane reformers.
Questions buyers ask
How hot is the combustion air on a steam methane reformer?
The combustion air on a steam methane reformer is typically preheated to 300 to 450°C in the convection section, downstream of the FD fan. Preheating the air reduces the fuel needed in the radiant box. The burners must be designed for this hot air, and the ducts after the preheater are insulated and allowed to expand.
Does the FD fan on a reformer handle hot air?
No. The FD fan on a reformer pushes ambient air into the combustion air preheater, and the air is heated after the fan. This keeps the forced draft fan small, cool and efficient, built in mild steel with standard bearings. Only in a few plants with gas turbine exhaust as combustion air does the fan arrangement differ.
How many burners does a reformer FD fan supply?
Large top-fired reformers can have many dozens of burners, arranged in rows between the tube rows in the roof, all fed from manifolds supplied by the FD fan. Side-fired reformers have many small wall burners at several levels. In both cases the fan supplies a common duct, and distribution depends on manifold and burner design.
Information needed to quote an FD fan for steam methane reformers
- 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.
- Burner arrangement. Number of burners and rows, and the burner air pressure required.
- Preheater. Air side pressure drop and air temperature.
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.