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 FD fan sits
- Inlet filter and silencer
- FD fan
- Combustion air preheater
- Hot air duct
- Burner manifolds
- Reformer burners
The fan is at grade near the convection section. It delivers ambient air to the combustion air preheater, where it is heated to 250 to 400°C, then through insulated ducts to burner manifolds. Pressure needed is typically 150 to 300 mmWC. In gas turbine integrated plants, the turbine exhaust, at about 450 to 550°C with 13 to 15% oxygen, is ducted directly to the burners, and the FD fan stands by to supply ambient air if the turbine trips.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 250 to 400°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Gas turbine integration | Where fitted, the fan is a standby for gas turbine exhaust |
| 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 ammonia plant primary reformers
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Steam turbine or motor, direct coupled |
| Bearings and cooling | Oil-lubricated bearings with monitoring |
| Flow control | Turbine speed governor or variable speed drive on air flow with oxygen trim |
| Also specified | Inlet filter and silencer, fast start capability where it is a gas turbine standby |
What this FD fan has to cope with
Fast takeover when the gas turbine trips
In gas turbine integrated plants, the FD fan must start and deliver full air quickly when the turbine trips, or the reformer loses its combustion air and trips too. The fan, drive and dampers are designed for a fast start, and the switchover is tested regularly so it works when needed.
Air distribution to burner rows
Primary reformers have many burners, and the air reaching each depends on the manifold design. Uneven air gives uneven tube wall temperatures, and reformer tubes are expensive and sensitive to overheating. Enough fan pressure to make the burner the main resistance, with balancing dampers, evens out the air.
Steam turbine drive reliability
Turbine-driven fans depend on steam quality and on the condition of the governor and trip valve. Wet or dirty steam, or a sticking governor, causes speed swings that show up as unstable combustion air flow and swinging excess oxygen in the firebox. Regular governor maintenance, steam quality control and a periodic trip test keep the fan steady and safe.
Energy and control
The FD fan runs continuously with the reformer. With turbine drives, efficiency feeds into the steam balance. With motor drives, a variable speed drive keeps power matched to load. Low excess air, achieved with good air flow control and oxygen trim, saves reformer fuel and reduces the flue gas flow handled by the ID fan.
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 Induced Draft (ID) Fan for ammonia plant primary reformers.
Questions buyers ask
What is gas turbine exhaust used as combustion air in an ammonia plant?
Some ammonia plants drive the process air compressor with a gas turbine and send its hot, oxygen-rich exhaust, typically 450 to 550°C with 13 to 15% oxygen, to the reformer burners as combustion air. This saves fuel because the heat in the exhaust is used. The FD fan then stands by to supply ambient air if the gas turbine trips.
Does the FD fan on a primary reformer handle hot air?
No. The FD fan on a primary reformer pushes ambient air into the combustion air preheater, and the air is heated after the fan, so the fan is a cool, clean air duty. In gas turbine integrated plants, the hot turbine exhaust goes directly to the burners through its own ducting and does not pass through the FD fan at all.
How much pressure does a reformer combustion air fan need?
Typically 150 to 300 mmWC, to overcome the preheater air side, the hot air ducts, the burner manifolds and the burner registers. The burner supplier's required air pressure at full firing sets the figure, with a margin for preheater fouling. On uprated plants the original fan pressure is often the limit and should be rechecked.
Information needed to quote an FD fan for ammonia plant primary 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.
- Gas turbine integration. Whether gas turbine exhaust is used as combustion air, and the fan's standby duty and start time.
- Drive. Steam turbine or motor, and the speed range.
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.