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 ID fan sits
- Radiant box
- Flue gas tunnels
- Convection section coils
- Combustion air preheater
- ID fan
- Stack
The fan sits at the end of the convection section, after the combustion air preheater and often after an SCR for NOx control. Gas temperature at the fan is low, 120 to 160°C, because the natural gas and tail gas fuels have little sulphur and the heat recovery can go deep. Suction is typically 200 to 350 mmWC, covering the many convection coils, preheater and SCR. The fan is large relative to the hydrogen output.
Gas at the ID fan
| Gas temperature, normal | 120 to 160°C at the convection outlet |
| Maximum or upset | 250 to 350°C with preheater bypass or coil fouling |
| Dust at the fan | Negligible; refractory and catalyst dust |
| Moisture and dew point | High water vapour from hydrogen-rich fuel; water dew point 55 to 65°C |
| Corrosive species | Little sulphur; ammonia slip and ammonium salts where an SCR is fitted |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 steam methane reformers
| Blade form | Backward curved or aerofoil, for efficiency on large, clean flows |
| Material of construction | Mild steel or boiler plate, rated for the upset temperature |
| Drive and arrangement | Direct drive, double inlet for large flows, with large motor or turbine drive |
| Bearings and cooling | Oil-lubricated sleeve bearings with temperature and vibration monitoring |
| Flow control | Variable speed drive or inlet vanes on firebox draft |
| Also specified | API 673 or equivalent requirements, and redundancy in lube and control systems |
What this ID fan has to cope with
Fan trip trips the plant
A top-fired reformer depends on the ID fan to pull flue gas down through the floor tunnels and along the convection section. Losing the fan makes the firebox go positive within seconds, and the burners must be cut. Reliable bearings and lube systems, redundant instruments, and trip logic that avoids spurious trips keep the hydrogen plant on line.
Fuel composition swings
PSA tail gas composition and flow change with the PSA cycle and with hydrogen demand, and the natural gas trim fuel changes to match. Flue gas volume per unit of heat changes with the fuel mix, since tail gas carries a lot of CO₂. The fan should be checked across the full range of tail gas and natural gas mixes.
SCR fouling
Where an SCR is fitted in the convection section, ammonia slip can form ammonium salts that deposit on the downstream coils and preheater, raising pressure drop between cleanings. The ID fan must be selected with a pressure margin for this fouling, or the reformer will be draft-limited before the scheduled cleaning comes round.
Energy and control
A hydrogen plant reformer runs continuously at a load set by hydrogen demand. The ID fan is one of the largest motors in the plant. Variable speed drives are used for efficiency at partial loads. Excess oxygen control in the firebox, with stable draft, saves more fuel than fan efficiency improvements.
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 Forced Draft (FD) Fan for steam methane reformers.
Questions buyers ask
Can a steam methane reformer run without the ID fan?
Top-fired steam methane reformers generally cannot run on natural draft, because the flue gas must be pulled down through floor tunnels and along a long convection section with many coils. Losing the ID fan usually means cutting firing quickly or tripping the reformer, which is why reliability dominates the selection of a hydrogen reformer ID fan.
What temperature does the ID fan on a hydrogen reformer handle?
Typically 120 to 160°C after deep heat recovery in the convection section. Natural gas and PSA tail gas are low in sulphur after feed desulphurisation, so the gas can be cooled further than on oil fired heaters without acid corrosion. The fan should still be rated for the upset case with the preheater bypassed or the coils fouled.
Why is the ID fan on a reformer so large?
The reformer fires a large heat input for the hydrogen it makes, and the tail gas fuel adds CO₂ and water to the flue gas, increasing its volume. The convection section, the air preheater and any SCR also add significant resistance. Together these give a large flow at a high pressure, so the reformer ID fan is one of the biggest drives in the plant.
Information needed to quote an ID fan for steam methane reformers
- 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.
- Firing arrangement. Top-fired, side-fired or terrace-fired, and the firebox draft set point.
- Fuel mix. Tail gas and trim fuel composition at minimum and maximum hydrogen output.
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.