How ammonia plant primary reformers work
In an ammonia plant the primary reformer converts desulphurised natural gas or naphtha and steam to synthesis gas in catalyst tubes, with the process gas leaving at about 780 to 830°C. Reforming is completed in the secondary reformer, where process air is added to supply the nitrogen. The primary reformer is a large top-fired or side-fired box burning natural gas, often with purge gas from the synthesis loop. Flue gas leaves the radiant section near 1,000°C and passes a long convection section with coils for mixed feed preheat, process air preheat, steam superheat, feed and boiler water heating, and combustion air preheat. An ID fan draws the gas to the stack at 140 to 180°C. In many older plants the ID and FD fans are driven by steam turbines as part of the plant steam balance.
Where the ID fan sits
- Radiant box
- Convection coils: process air, steam, feed
- Combustion air preheater
- ID fan
- Stack
The fan is at the cold end of the convection section, at grade, after the combustion air preheater. Upstream are many coil banks, each adding resistance, so the total suction is typically 200 to 350 mmWC. The firebox is held at a small negative draft, a few millimetres of water at the arch. The ID fan is often turbine-driven, using steam from the plant's own boilers, with a motor-driven or second turbine fan in some plants for redundancy.
Gas at the ID fan
| Gas temperature, normal | 140 to 180°C after the air preheater |
| Maximum or upset | 250 to 350°C with the preheater bypassed |
| Dust at the fan | Negligible; refractory dust after repairs |
| Moisture and dew point | Water dew point 55 to 65°C from natural gas and purge gas firing |
| Corrosive species | Very little sulphur; trace ammonia from purge gas firing |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 ammonia plant primary reformers
| Blade form | Backward curved or aerofoil, for efficiency on a large clean flow |
| Material of construction | Mild steel or boiler plate rated for the bypass temperature |
| Drive and arrangement | Steam turbine or motor, direct coupled; double inlet on large plants |
| Bearings and cooling | Oil-lubricated sleeve bearings with a lube oil console, temperature and vibration monitoring |
| Flow control | Turbine speed governor or variable speed drive on firebox draft |
| Also specified | Design for the turbine's overspeed, API 673 or equivalent, and hot standby where fitted |
What this ID fan has to cope with
A fan trip trips the front end
Losing the ID fan forces the reformer burners off, and with them the steam generation and process air preheat in the convection section. The whole front end, and often the ammonia loop, goes down. Reliable lube oil systems, governor maintenance and careful trip settings avoid a large production loss from a minor fault.
Steam turbine drive and speed range
A steam turbine drive gives speed control at no extra cost, but the fan must be designed and balanced for the full speed range, including turbine overspeed trip speed. Critical speeds must lie outside the operating range, and the rotor is checked for stress at the trip speed.
Throughput increases beyond the fan
Many ammonia plants have been uprated in capacity over the years. The ID fan, sized for the original duty, often becomes the limit. A new impeller, a higher speed or a replacement fan, checked against the existing casing, foundation and drive, can release the extra capacity.
Energy and control
Ammonia plants run continuously at high load, and the primary reformer ID fan is a large power consumer, whether through electrical power or steam. With a steam turbine drive, fan efficiency affects the steam balance; with a motor drive, a variable speed drive saves power at reduced rates. A more efficient impeller on an uprated plant can lower both power and steam use.
ID fan and FD fan in ammonia plant primary reformers
| ID fan | FD fan | |
|---|---|---|
| Position | End of convection section, before stack | Before the combustion air preheater |
| What it handles | Clean flue gas | Ambient air |
| Temperature at the fan | 140 to 180°C | Ambient |
| Main design concern | Reliability and turbine drive | Air distribution and gas turbine standby |
| Control | Firebox draft | Air flow with oxygen trim |
See also the Forced Draft (FD) Fan for ammonia plant primary reformers.
Questions buyers ask
Why are ammonia reformer ID fans driven by steam turbines?
Ammonia plants produce surplus high-pressure steam from waste heat. Using steam turbines to drive large machines such as the reformer ID and FD fans fits into the plant's steam balance and gives variable speed. Some plants now use motors with variable speed drives instead, depending on the steam balance and reliability needs.
What happens when the primary reformer ID fan trips in an ammonia plant?
The reformer burners trip because the firebox can no longer be held under draft. With them, the convection section's steam generation and process air preheat are lost, so the front end of the ammonia plant shuts down. Restarting takes several hours, which is why fan reliability is critical.
Can the ID fan be upgraded when an ammonia plant is uprated?
Often yes. A new impeller designed for the higher flow and pressure, a speed increase within the drive's capability, or a replacement fan on the existing foundation can be used. The casing, shaft, bearings, drive and foundation must all be checked for the new duty.
Information needed to quote an ID fan for ammonia plant primary 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.
- Drive. Steam turbine or motor, speed range and the turbine overspeed trip speed.
- Uprating. Original design duty, present operating point and the target duty after uprating.
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.