How coal gasification and syngas plants work
Coal gasification converts coal or coke into a fuel gas of carbon monoxide, hydrogen and methane by partial oxidation with air or oxygen and steam. Large IGCC and chemical plants use pressurised entrained flow or fluidised bed gasifiers, where the syngas is moved by the gasifier pressure and by compressors, not by fans. Fans appear at lower pressure: on fixed bed and atmospheric gasifiers that supply fuel gas to kilns and furnaces, where an air blower supplies the gasification air and a gas booster or exhauster draws raw gas through the cyclone, cooler, tar removal and ESP; and on syngas-fired boilers, tail gas incinerators and heat recovery units, which use conventional ID and FD fans.
Where the FD fan sits
- Inlet filter
- FD fan
- Steam saturation
- Air blast main
- Gasifier grate
- Coal bed
The fan draws ambient air and pushes it into the air blast main, where steam is mixed in to control the bed temperature and the hydrogen in the gas. From the main the air passes the rotating grate at the base of the gasifier and up through the coal bed. The fan works against the grate and bed resistance, which varies with coal size and ash, typically 300 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after steam injection | 55 to 65°C saturated, downstream of the fan |
| Dust | Clean air, but coal dust near the gasifier house |
| Bed resistance | Rises with fine coal and clinker |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| 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 coal gasification and syngas plants
| Blade form | Backward curved, with a steep pressure curve for stable flow against changing bed resistance |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow or gasifier outlet pressure |
| Also specified | Non-return damper at the outlet to stop gas flowing back on a fan trip, and an inlet filter |
What this FD fan has to cope with
Back-flow of gas on fan trip
If the air blower stops, gas from the gasifier can flow back into the air main and the fan. A non-return damper, steam purge and interlocks prevent this. The non-return damper is tested at each shutdown, and the fan is never opened until the air main has been purged and gas-tested.
Air flow and gas quality
Too much air raises the bed temperature, burns part of the gas and causes clinker; too little reduces output and leaves unconverted coal. Air flow is the main control of the gasifier, so the blower needs a stable characteristic, accurate flow measurement and a speed control loop that responds smoothly rather than in steps.
Coal size and clinker
Fine coal and clinker raise bed resistance and can cause channelling, where air finds a path through the bed and burns gas in it. The fan holds flow as resistance rises only if it has pressure margin, so it is selected for the worst coal size and ash fusion behaviour the plant expects, not the design coal.
Energy and control
Gasifier air demand follows gas demand from the users. A variable speed blower saves energy compared with a throttled damper, and gives finer control of the air and steam ratio, which also improves gas quality and coal consumption. Where several gasifiers share a blower header, speed control holds header pressure as units come on and off line.
ID fan and FD fan in coal gasification and syngas plants
| ID fan | FD fan | |
|---|---|---|
| Position | After gas cooling and tar removal | Ahead of the gasifier air main |
| What it handles | Raw syngas, toxic and flammable | Clean ambient air |
| Temperature at the fan | 35 to 60°C | 25 to 45°C |
| Main design concern | Gas tightness and explosion safety | Stable air flow and no back-flow |
| Control | Gasifier or main pressure | Air flow |
See also the Induced Draft (ID) Fan for coal gasification and syngas plants.
Questions buyers ask
What pressure does a coal gasifier air blower need?
Fixed bed atmospheric gasifiers typically need 300 to 700 mmWC, depending on the bed depth, coal size and grate. Pressurised gasifiers need compressors, not fans. The figure rises as the bed fouls, so the blower is selected with pressure margin. Pressurised gasifiers need compressors instead.
Why is steam added after the air blower?
Steam controls the bed temperature, prevents clinker and adds hydrogen to the gas. Adding it after the fan keeps the fan on dry ambient air. Injection points are placed after the blower so its casing stays dry and free of condensate. It also keeps steam condensate out of the fan.
Do syngas-fired boilers need FD fans?
Yes. A boiler or incinerator burning syngas needs a combustion air fan like any gas-fired unit, selected for the air needed by the lower calorific value gas. Syngas has a low calorific value, so it needs less air per cubic metre of fuel but produces more flue gas per unit of heat.
Information needed to quote an FD fan for coal gasification and syngas plants
- 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.
- Gasifier type. Gasifier type, coal size and the air and steam flows at design output.
- Safety interlocks. Required non-return, purge and interlock arrangement on fan trip.
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.